Push-Pull Power Converter Topologies

This course is an introduction to a family of DC-DC power converters often referred to as "Push-Pull" topologies. We will explore some of theadvantages and tradeoffs made as compared to the more common "Single-Ended" converters. Our main interest will be several topologies whichapply to isolated DC to DC converters.

You will see how a conventional Forward converter design is transformed into the Push-Pull converter and then get an introduction to the partsthat National offers for these application.

Push-Pull converters get their name from the fact that the transformer windings get used in a bi-directional manner (two quadrant operation),unlike Forward converters which operate in a single magnetic quadrant.

Course Map/Table of Contents1. Course Navigation

1.1 Course Navigation1.

2. Single-Ended Review

2.1 Common One-Switch Power Converter Topologies1.2.2 Buck Regulator Basics2.2.3 Buck Converter Characteristics3.2.4 Forward Converter4.2.5 Forward Diode Currents5.2.6 Forward Converter Characteristics6.2.7 Common Two-Switch Power Converter Topologies7.

3. Push-Pull Topology

3.1 Push-Pull Topology1.3.2 Push-Pull Switching Waveforms2.3.3 Push-Pull Diode Currents3.3.4 Core Utilization: Forward & Push-Pull Converters4.3.5 Push-Pull Characteristics5.

4. Push-Pull Controller

4.1 LM5030 Push-Pull Controller1.4.2 LM5030 Push-Pull Demo Board2.4.3 LM5030 Push-Pull Demo Board Schematic3.4.4 LM5030 3G Base Station RF Power Supply4.4.5 LM5030 3G Base Station RF Supply Schematic5.

5. Cascaded Buck/Push-Pull

5.1 Cascaded Buck & Push-Pull1.5.2 Cascaded Voltage-Fed Converter Benefits2.5.3 Current-Fed Push-Pull Concept3.5.4 Cascaded Current-Fed Converter Benefits4.5.5 Current-Fed Switching Voltages5.5.6 Current-Fed Push-Pull Switches6.5.7 Current-Fed Switch Waveforms7.5.8 Why is it important to reduce secondary rectification losses?8.5.9 Comparison of Rectifier Stresses9.5.10 Sync Rectifier Waveforms10.

6. Cascaded PWM Controller

6.1 LM5041 Cascaded PWM Controller1.6.2 LM5041 Block Diagram2.6.3 LM5041 Current-Fed Push-Pull Demo Board3.6.4 LM5041 / LM5100 Demo Board Schematic4.

7. Half-Bridge Topology

7.1 The Basic Half-Bridge1.

8. Half-Bridge Controller

8.1 LM5035 Half-Bridge Controller1.8.2 LM5035 Demo Board Schematic2.8.3 Cascaded Half-Bridge Concept3.8.4 Cascaded Half-Bridge Characteristics4.

9. Full-Bridge

8.1 LM5035 Half-Bridge Controller1.8.2 LM5035 Demo Board Schematic2.8.3 Cascaded Half-Bridge Concept3.8.4 Cascaded Half-Bridge Characteristics4.

9. Full-Bridge

9.1 Full-Bridge Concept1.9.2 Full-Bridge Current Doubler2.9.3 Cascaded Full-Bridge Concept3.9.4 Cascaded Full-Bridge Characteristics4.

Course Navigation

1.1 Course Navigation

Course Navigation

This course is organized like a book with multiple chapters. Each chapter may have one or more pages.

The previous and next arrows move you forward and back through the course page by page.

The left navigation bar takes you to any chapter. It also contains the bookmarking buttons, 'save' and 'go to.' To save your placein a course, press the 'save' button. The next time you open the course, clicking on 'go to' will take you to the page you saved orbookmarked.

The 'Go to Final Test' button on the left navigation bar takes you back to the Analog University course listing, where you started.Take the course final test by clicking on 'Test Yourself.'

The top services bar contains additional information such as glossary of terms, who to go to for help with this subject and anFAQ. Clicking home on this bar will take you back to the course beginning.

Don't miss the hints, references, exercises and quizzes which appear at the bottom of some pages.

Single-Ended Review

We will start off with a brief review of common DC to DC power converter topologies.

2.1 Common One-Switch Power Converter Topologies

2.2 Buck Regulator Basics

2.3 Buck Converter Characteristics

2.4 Forward Converter

2.5 Forward Diode Currents

2.6 Forward Converter Characteristics

2.7 Common Two-Switch Power Converter Topologies

Common One-Switch Power Converter Topologies

Shown here are the power stage arrangements for several popular power converter topologies which use a single primary switching element.

The Buck and Buck-Boost are among the simplest and apply to non-isolated power converters.

The Forward and Flyback topology are used in isolated converters where it is desirable to electrically isolate the Primary andSecondary grounds.

A Forward converter is simply a Buck regulator with a transformer inserted between the buck switch and the load. The input to outputtransfer function is the same as a Buck regulator if the transformer's turns ratio is one.

The Flyback regulator is derived from the Buck-Boost regulator and the same logic applies with regards to transfer functions.

The advantage of the transformer coupled designs is that any output voltage can be produced from any input voltage with the properchoice of turns ratio.

The one-switch Forward and Flyback are both examples of single-endedconverters.

Buck Regulator Basics

A more detailed look at the anatomy of a Buck regulator shows a switching section, comprised of Q1 and D1, and an output filtercomprised of L1 and C1.

The Buck regulator is used to efficiently step down voltages.The output voltage is given as Vin·D, where D is the duty cycleof the main switch Q1.All of the transfer functions we will show assume the inductorcurrent does not return to zero during the switching cycle, this iscalled "Continuous Conduction Mode" or CCM operation.The rising inductor current is driven by Q1 during its on time,and then "freewheels" through the rectifier, D1 while Q1 is off.

For continuous conduction mode, the inductor current is alwaysflowing.

For continuous conduction mode, the inductor current is alwaysflowing.

Find the Output VoltageA buck regulator operates at a duty cycle of 0.3 with 5V at the input. What is the output voltage?

1. 0.5V

2. 1.5V

3. 2.5V

4. 3V

1 Answer: Vo=Vin·D=5V·0.3=1.5V

Buck Converter Characteristics

Non-Isolated GroundsVoltage Step-Down OnlySingle Output OnlyVery High EfficiencyLow Output Ripple CurrentHigh Input Ripple CurrentHigh Side (Isolated) Gate Drive RequiredLarge Achievable Duty Cycle RangeWide Regulation Range (due to above)

We will be able to eliminate many of the drawbacks associated with non-isolated Buck regulators by adding a transformer in the power path.That result is what is classically known as the "Forward" converter. This addresses the need for isolation, allows step-up or step-downoperation, allows a more favorable duty cycle for high step down ratios and allows the main switch to be ground referenced for simpler drive.All of this however, comes with a price.

Forward Converter

The first isolated topology we will look at is the Forward converter.

A Forward converter is basically a transformerisolated Buck regulator.The output inductor current is still thecomposite of two alternating switch currents,in this case D1 and D2.D1's current is the secondary current from thetransformer, which equals I(Q1) divided by theturns ratio (Ns/Np).The transfer function is the same as the Buckregulator with an additional transformervoltage gain term of Ns/Np.One problem with the Forward topology is thatthe primary switch voltage can rise essentiallyunconstrained. When the switch turns off,energy stored in the transformer primarywants to cause current to continue to flowtoward the FET drain. Since the FET has beenturned off, this causes the voltage on the drainto rise rapidly. The good news is this tends tohelp reset the transformer (more on this later). The bad news is it will radically increase the required breakdown voltage of the FET.The reset winding Nr helps keep this reset voltage under control.

Forward Diode Currents

This plot shows each of the rectifier

Forward Diode Currents

This plot shows each of the rectifierdiode currents which sum together toform the inductor current. Theinductor current looks exactly thesame as in an equivalent Buckregulator.The need to reset the transformertends to limit the maximum duty cyclethat a Forward converter can operateat. There is a requirement that in anyinductor in steady state operation, theaverage DC voltage across theinductor must be zero. From this it'seasy to conclude that the inductor'sVolt·Second product must be equalduring each half cycle. Thetransformer's primary winding is aninductor and so this principle appliesto it as well. During the time the primary switch is on the voltage across the primary winding is Vin. So the Volt·Second product duringthe "on time" is Vin·Ton. If the on time becomes large relative to the off time, the voltage Voff, must get large relative to Vin. Simply put,the larger the duty cycle, the higher the switch voltage during the off time must get. This need to reset the transformer is a seriouslimitation for the Forward topology.

To maintain Volt·Second balance, Vin·Ton=Voff·Toff.

Forward Converter Characteristics

A Forward Converter is a Buck type converter with an added Isolation Transformer.Grounds are Isolated.Voltage Step-down or Step-up.Multiple Outputs Possible.Low Output Ripple Current.High Input Ripple Current.Simple Gate Drive.Limited Achievable Duty Cycle Range.

Note that the transformer is only driven in one direction in a Forward converter. This means that the transformer core is not being utilized to itsfull capability. There is also the matter of resetting the core each cycle. These issues tend to limit the power level that a Forward converter isuseful for. In general the topology is best used for power levels between 30W and 150W although they have been used successfully to over1kW. Above the 150W level, it's usually more economical to look at Push-Pull topologies that better utilize the magnetic core's power handlingability.

Common Two-Switch Power Converter Topologies

Here are several popular isolated powerconverters which use two or more primaryswitches. The Push-Pull and Half-Bridgerequire two switches while the Full-Bridgerequires four switches. Generally, the powercapability increases from Push-Pull toHalf-Bridge to Full-Bridge.Note that these are all derived from the basicBuck architecture and share the fundamentaltransfer function that the output voltage isequal to the input voltage multiplied by theswitch duty cycle, assuming a 1:1 transformerturns ratio. So for the general case, thetransfer function becomes: Vout=Vin·D·Ns/Np.In all cases the primary switches are driven insuch a sequence that the transformer

transfer function that the output voltage isequal to the input voltage multiplied by theswitch duty cycle, assuming a 1:1 transformerturns ratio. So for the general case, thetransfer function becomes: Vout=Vin·D·Ns/Np.In all cases the primary switches are driven insuch a sequence that the transformerwindings get driven in alternate directions oneach half cycle. In the case of the Push-Pulland Half-Bridge this means one switch at atime. For the Full-Bridge the transistors are driven in pairs across the diagonals, i.e. top right/bottom left followed by top left/bottomright.

The Push-Pull, Half-Bridge and Full-Bridge are all Buck derivedtopologies.

Push-Pull Topology

3.1 Push-Pull Topology

3.2 Push-Pull Switching Waveforms

3.3 Push-Pull Diode Currents

3.4 Core Utilization: Forward & Push-Pull Converters

3.5 Push-Pull Characteristics

Push-Pull Topology

The Push-Pull topology is basically a Forwardconverter with two primaries. The primaryswitches alternately power their respectivewindings. When Q1 is active current flowsthrough D1. When Q2 is active current flowsthrough D2.The secondary is arranged in a center tappedconfiguration as shown.The output filter sees twice the switchingfrequency of either Q1 or Q2.The transfer function is similar to the Forwardconverter, where "D" is the duty cycle of agiven primary switch, that accounts for the "x2" term. When neither Q1 nor Q2 are activethe output inductor current splits between thetwo output diodes.A transformer reset winding shown on theForward topology is not necessary, thetopology is self-resetting. Keep in mind thatwith any transformer, the number of volts/turn is a constant across the entire structure, both primary and secondary windings.Therefore, when Q1 is on, Vin appears across ½ of the primary winding. As such, Vin will also appear across the other half of theprimary that connects to the drain of Q2. That forces the drain of Q2 to 2·Vin.

Push-Pull Switching Waveforms

Shown here are oscilloscope waveforms forthe drain voltages of the two primary switchesand the output inductor current.You can see the off switch drain voltage beingdriven to twice the input as expected. Whenneither switch is active then both drainvoltages are at the input voltage potential.Note that like the Forward converter, the drainvoltage is inherently unconstrained. There'snothing related to the basic topology that actsto clamp the FET drains and limit theinstantaneous voltage. As a result the FET'sneed to be over rated beyond the theoretical

voltages are at the input voltage potential.Note that like the Forward converter, the drainvoltage is inherently unconstrained. There'snothing related to the basic topology that actsto clamp the FET drains and limit theinstantaneous voltage. As a result the FET'sneed to be over rated beyond the theoreticallevel that would be expected. It can be seen inthe photo above that there can be significantovershoots beyond the expected 2·Vin. Thesespikes are cause by the transformer's primaryreferenced leakage inductance. This inductance is cause by uncoupled flux linkages. This is magnetic field energy that does not getcoupled into one of the windings of the transformer. This energy is released in the form of primary current continuing to flow for a timeafter the switch is turned off. The leakage energy in the form of L·I2/2 is transferred into the FET's output capacitance in the formC·V2/2. This can be limited with snubbers or Zener clamps, but there's the obvious cost of additional parts and board real estate, not tomention efficiency losses due to these efforts.

The uncoupled transformer leakage inductance contributes to drain voltagespikes.

Push-Pull Diode Currents

Shown here is the current for each of the twooutput diodes.These two currents sum to form the outputinductor current shown on the previous page.Note that as discussed previously whenneither of the primary switches are active, theoutput inductor current has a negative slopeand flows half in each of the two secondarydiodes. In theory, this current should splitequally through the secondary windings andtherefore the resulting magnetic fields shouldexactly cancel. In practice, due to slightdifferences in diode forward voltage, windingresistance, and layout related parasitics, it'spossible to see small imbalances.There's also a chance for slight differences in primary volt·second inputs to the two winding halves. If these offsets are systematic, aphenomenon know as "flux walk" can occur. Over time, the magnetic flux being developed in the transformer's core material will bebiased slightly in one direction. After some number of switching cycles, the peak magnetic flux can increase beyond the core'ssaturation limit and cause the primary current to spike up rapidly. The use of current-mode control prevents this from ever getting out ofhand and provides good, inherent flux balancing. Voltage-mode control can prove problematic if this issue isn't carefully addressed.

For the Push-Pull, current-mode control provides good fluxbalancing.

Core Utilization: Forward & Push-Pull Converters

Shown here are the transformer B-H curves for the Forward and the Push-Pull topology.

The "X" axis represents Magnetic Field Intensity (Coercive Force) which is proportional to the Ampere·Turns.

The "Y" axis represents Flux Density which is proportional to the Core Area and the Volt·Seconds applied to the primary winding whenactive.

The lines are the core's complete B-H loop.In normal operation the converter operatesthrough a minor loop as shown in red. Notethat the area enclosed in the loop isproportional to the product of current,voltage and time and as such representsenergy. This is the "Core Loss" of everyswitch cycle.The primary magnetizing inductance isproportional to the slope of the curve. This

that the area enclosed in the loop isproportional to the product of current,voltage and time and as such representsenergy. This is the "Core Loss" of everyswitch cycle.The primary magnetizing inductance isproportional to the slope of the curve. Thisslope is known as the permeability of thecore material, µ. Note that when the fluxdensity reaches B

SAT the slope decreases

drastically. This corresponds to a significantreduction in the transformer's primaryinductance.The Forward converter operates in a singlequadrant of the B-H curve, moving up theright side of curve when the switch is activeand resetting during the OFF time. Since there is no active reset the Coercive Force falls to zero and the residual flux drops to theRemanent Flux, B

R.

The Push-Pull converter operates in two quadrants of the B-H curve, see-sawing back and forth as each primary is activated.

This important fact allows the maximum power capability of a Push-Pull transformer to be more than twice that of a Forwardtransformer.

Core UtilizationWhich of the following are true statements?

1.The B-H curve represents stored energy.

2.The forward converter operates in one quadrant of the B-H curve.

3.At saturation, the core has reached it's maximum magnetization point.

1 Answer: They are all true.

Push-Pull Characteristics

A Push-Pull Converter is a Buck type converter with a bi-directionally driven Isolation Transformer.Push-Pull transformers and filters are much smaller than comparable Forward converter filters.Voltage Stress on the Primary Switches is > Vin·2.Voltage Step-down or Step-up.Multiple Outputs Possible.Low Output Ripple Current.Lower Input Ripple Current.Simple Gate Drive (dual) .Large Achievable Duty Cycle Range.

The better core utilization and the lack of a realistic duty cycle limit in the Push-Pull architecture allows them to operate at significantly higherpower levels. For moderate input voltages (Telecom, 36-72 for instance) Push-Pull converters are useful to 500W and beyond. Above thatpower level the unclamped FET drains become a significant problem and force the FET breakdown voltage rating to be much higher thanwould be desirable. At that point, things like the Half-Bridge and Full-Bridge converters start to look more attractive.

Let's look at a National implementation of a Push-Pull controller and a few typical applications built on that design.

Push-Pull Controller

4.1 LM5030 Push-Pull Controller

4.2 LM5030 Push-Pull Demo Board

4.3 LM5030 Push-Pull Demo Board Schematic

4.4 LM5030 3G Base Station RF Power Supply

4.5 LM5030 3G Base Station RF Supply Schematic

LM5030 Push-Pull Controller

National Semiconductor has developed a controller designed specifically for the Push-Pull topology.

LM5030 Push-Pull Controller

National Semiconductor has developed a controller designed specifically for the Push-Pull topology.

This controller is designed for current-modecontrol, so the flux walk issue discussedearlier is of no concern.The LM5030 controller has many innovativefeatures.Although designed for the Push-Pull topologythis versatile controller can be used for mostcommon power converters.

100V Push-Pull Current Mode PWM Controller: Net Links

LM5030 Push-Pull Demo Board

Shown here is a demo board utilizing theLM5030 controller in a Push-Pull topology.The power level is on the low side for aPush-Pull implementation. The purpose is todemonstrate the operation of the controller.The waveforms shown earlier were taken fromthis board.

LM5030 Evaluation Board

LM5030 Push-Pull Demo Board Schematic

36V-75Vin to +3.3V @ 10A

Shown here is the schematic for the 33Wdemo board.Note the controller connects directly to theinput voltage to provide the initial bias poweron Vcc. Once operational, the winding on the

36V-75Vin to +3.3V @ 10A

Shown here is the schematic for the 33Wdemo board.Note the controller connects directly to theinput voltage to provide the initial bias poweron Vcc. Once operational, the winding on theoutput inductor provides the bias power.There are very few components neededaround the actual controller. A significant levelof complexity gets added to the design wheninput-output isolation is required. The voltageloop's feedback signal needs to getcommunicated across the isolation barrier andis usually done with an optocoupler as isshown here. A significant portion of the controlcircuitry is related to the opto driver andcontrol.

Application Note 1305 LM5030 Evaluation Board

LM5030 3G Base Station RF Power Supply

Shown here is an actual application at thehigher end of the Push-Pull power capability.This unit is designed to power a telecom BaseStation RF Power Amplifier.

LM5030 3G Base Station RF Supply Schematic

-48Vin to +27V @ 30A

Shown here is the schematic for the 810W design.

The schematic although somewhat more complicated then the 33W design, still has all of the same basic functional blocks.

Cascaded Buck/Push-Pull

5.1 Cascaded Buck & Push-Pull

5.2 Cascaded Voltage-Fed Converter Benefits

5.3 Current-Fed Push-Pull Concept

5.4 Cascaded Current-Fed Converter Benefits

5.5 Current-Fed Switching Voltages

5.6 Current-Fed Push-Pull Switches

5.7 Current-Fed Switch Waveforms

5.8 Why is it important to reduce secondary rectification losses?

5.9 Comparison of Rectifier Stresses

5.10 Sync Rectifier Waveforms

Cascaded Buck & Push-Pull

Cascaded Buck & Push-Pull Power Converter (Voltage-Fed)

Now let's combine a Buck Regulator stage and a Push-Pull stage.

The first thing to note here is that each switchof the Push-Pull stage is set to operatealternating at 50% duty cycle. This essentiallyconfigures the Push-Pull stage as an ideal DCtransformer. A voltage presented to the Vppnode will be transferred to the output dividedby the transformer turns ratio.It is the Buck stage that is actually used toregulate the output.If we combine the Buck stage transfer functionand the Push-Pull stage transfer function weget the overall transfer function as shown inthe lower right corner.The Push-Pull stage is said to be"Voltage-Fed" since the Vpp node contains theoutput capacitor from the Buck stage.The Push-Pull switches actually operateslightly less than 50% duty cycle such thatthere is no overlap during the switching transitions. Since the dead-time is very short the rectified secondary voltage is almostcontinuous. Therefore, only a very small inductor is required on the secondary side to filter this output waveform.

A Cascaded converter consists of more than one power stage inseries.

Cascaded Voltage-Fed Converter Benefits

A Voltage-Fed Push-Pull Converter is a Buck type converter consisting of a Buck Regulation Stage followed by (cascaded by) aPush-Pull Isolation Stage.

The Push-Pull stage FET voltage stresses are reduced to Vout·N·2 over all line conditions.

The output rectification can be easily optimized due to reduced and fixed voltage stresses.

A Voltage-Fed Push-Pull Converter is a Buck type converter consisting of a Buck Regulation Stage followed by (cascaded by) aPush-Pull Isolation Stage.

The Push-Pull stage FET voltage stresses are reduced to Vout·N·2 over all line conditions.

The output rectification can be easily optimized due to reduced and fixed voltage stresses.

The output rectification is further optimized since the power is equally shared between the rectifiers over all load and line conditions.

Favorable topology for wide input ranges.

We will spend more time explaining the important features in blue on the following pages.

Current-Fed Push-Pull Concept

The Cascaded "Voltage-Fed" Buck and Push-Pull is a viable design approach, however there are several large components which canbe removed, while still maintaining all of the performance benefits of the cascaded approach. On the previous Voltage-Fed page, notewe had 2 complete L-C filters. The Buck stage capacitor and the Push-Pull stage inductor can be removed and actually provide severalbenefits.

Shown here is a Current-Fed Cascaded Buckand Push-Pull stage. The Push-Pull stage issaid to be Current-Fed since only the Buckinductor, which acts a current source feeds thePush-Pull.In this case the Push-Pull switches need tohave a very small overlap at the switchingtransitions to maintain the inductor currentpath. In the Voltage-Fed a small dead-time isrequired.

An example which we will look at next is a 2.5 Volt output, which has been designed with an 8 to 1 transformer turns ratio. Working from theoutput back from right to left yields a voltage at the Push-Pull primary of 20 Volts.

Removing the output capacitor from the Buck stage changes the Push-Pull from voltage-fed tocurrent-fed.

Cascaded Current-Fed Converter Benefits

A Current-Fed Push-Pull Converter is a Buck type converter consisting of a Buck Regulation Stage followed by (cascaded by) aPush-Pull Isolation Stage.

There is no high current output inductor!

Reduced switching loss in Push-Pull stage.

Favorable topology for multiple outputs since all outputs are tightly coupled.

Favorable topology for wide input ranges, since the Buck stage pre-regulates while the Push-Pull and secondary operateindependently of the input voltage level.

Current-Fed Switching Voltages

Shown here are scope plots of the Push-Pullstage drain voltages and the voltage at thecommon junction of the Buck stage switches.Note that the Buck stage operates at twice thefrequency of either the Push or Pull switch.

Shown here are scope plots of the Push-Pullstage drain voltages and the voltage at thecommon junction of the Buck stage switches.Note that the Buck stage operates at twice thefrequency of either the Push or Pull switch.This ensures that the Push-Pull stage currentsare symmetrical. Each half cycle sees thesame input conditions this way.Also note the overlap of the of the Push-Pullstage. This is done to ensure that the buckinductor current always has a path to ground.If there was any dead time where both FETswere off, the drain voltage would fly todamaging levels.

Current-Fed Push-Pull Switches

Shown here are scope plots of the Push-Pulldrain voltages and Push-Pull switch currents.The drain currents sum to form the inductorcurrent.

On the next page we will take a more detailed look atthe switching transitions of these waveforms.

Current-Fed Switch Waveforms

Expanded Scale

One of the many advantages of the cascadedapproach is a reduction in switching losses inthe Push-Pull stage switches.You can note during the overlap time whenboth switches are ON the Buck inductorcurrent divides equally between the twoswitches. At the conclusion of the overlap timethe drain voltage is already at zero andtherefore the switching losses are cut in half.Also remember that the voltage stress on thePush-Pull stage switches are reduced by theBuck stage and held nearly constant.

Why is it important to reduce secondary rectification losses?

Why is it important to pick a topology whichoffers the best opportunities to reduce lossesin the secondary synchronous rectifiers?A look at a typical power loss budget of a 3.3Vpower converter shows approximately 40% of

Why is it important to pick a topology whichoffers the best opportunities to reduce lossesin the secondary synchronous rectifiers?A look at a typical power loss budget of a 3.3Vpower converter shows approximately 40% ofthe overall power conversion losses occur inthe secondary rectification. The Cascadedtopology provides for lower peak voltages andcurrents in the inductors. Since this area is thelargest loss contributor, anything that can helpreduce these losses will have a significantimpact on the total system efficiency.

For low voltage outputs, the rectifier represents the largest losselement.

Comparison of Rectifier Stresses

This chart compares secondary rectifier stresses for three of the topologies we have seen so far. The comparison example is for atypical 3.3 Volt output with a 35 to 80 Volt input.

On the top chart voltage stresses arecompared. As you can see for the Forwardand the Push-Pull the voltage stresses areproportional to the input voltage. At high linethe calculated stresses are much higher thanthe Cascaded topology whose rectifierstresses are only proportional to Vout.All of the compared topologies have twosecondary rectifiers. The lower chartcompares the ratio of ON times for eachtopology. The Push-Pull and the Cascadehave balanced loading on the two secondaryrectifiers. The loading ratio on the rectifiers for a Forward topology vary in proportion to the input voltage. Optimized and reliabledesigns are more readily accomplished with balanced loading.

Sync Rectifier Waveforms

At low output voltages, the output rectifier' s forward drop becomes the biggest impediment to achieving high efficiency. A typicalSchottky diode in a high current application will have a forward voltage drop of roughly 500mV to 700mV. When the output voltage is3.3V, and since one diode is always in series with the output power path, the maximum achievable efficiency is 3.3V/3.8V= 87%(assuming a 500mV diode). And that assumes the rest of the circuit is 100% efficient, something we know to be unachievable. Ofcourse as the output voltage is further reduced, this problem gets worse.

One thing that can be done to radically improve efficiency with low output voltages is to replace the secondary rectifiers with lowon-resistance MOSFETs. The forward drop can be made arbitrarily low by selecting the appropriate transistors.

This scope plot shows the drain voltagewaveforms of the two synchronous rectifiers ina 2.5 Volt output. Excluding the switchingspikes, the voltage stress is as expected, 5volts. Therefore, extremely low voltageMOSFETs can be used in these applications.

a 2.5 Volt output. Excluding the switchingspikes, the voltage stress is as expected, 5volts. Therefore, extremely low voltageMOSFETs can be used in these applications.

Cascaded PWM Controller

6.1 LM5041 Cascaded PWM Controller

6.2 LM5041 Block Diagram

6.3 LM5041 Current-Fed Push-Pull Demo Board

6.4 LM5041 / LM5100 Demo Board Schematic

LM5041 Cascaded PWM Controller

National Semiconductor has developed a controller designed specifically for Cascaded topologies.

The LM5041 controller has many innovative features:

Internal 100V Capable Start-up Bias Regulator.Programmable Line Under Voltage Lockout with Adjustable Hysteresis.Current-Mode Control.Internal Error Amplifier with Reference.Dual Mode Over-Current Protection.Internal Push-Pull Gate Drivers with Programmable Overlap or Dead-Time.Programmable Soft-Start.Programmable Oscillator with Sync Capability.Precision Reference.Thermal Shutdown (165°C).

Packages:

TSSOP 16LLP 16 (5 x 5 mm)

LM5041 Cascaded PWM Controller: Net Links

LM5041 Block Diagram

Shown here is the block diagram for the LM5041 Cascaded Controller.

Note that on the right are the 4 switch controloutputs. Gate drivers are included within thedevice for the Push and Pull outputs. Aresistor connected to the TIME pin is used toset either overlap or dead-time of thePush-Pull outputs. Connecting the TIMEresistor to ground sets the overlap time.Connecting the time resistor to REF setsdead-time.The Buck stage outputs are logic levelcontrols which work with the LM5100 family ofBuck stage gate drivers.The bias, control and protection circuits usedin this controller are very similar to theLM5030 controller, which is current-mode

dead-time.The Buck stage outputs are logic levelcontrols which work with the LM5100 family ofBuck stage gate drivers.The bias, control and protection circuits usedin this controller are very similar to theLM5030 controller, which is current-modecontrol.A unique LM5041 feature is a line undervoltage lockout (UVLO) with adjustablehysteresis.

LM5041 Current-Fed Push-Pull Demo Board

The demo board shown here is an example of a typical application of the LM5041:

4-Layer Board.Planar Magnetics. (Coilcraft standard productsfor Transformer and Inductor).100V Chipset:

LM5041 Cascaded ControllerLM5101 Synchronous Buck Driver

LM5041 / LM5100 Demo Board Schematic

2.5V @ 50A Cascaded DC-DC Converter

Shown here is the schematic for the LM5041 demo board.

The right side of the schematic contains the secondary synchronous rectifiers and the output capacitors.

The left side contains the primary circuits: the Buck stage, Push-Pull primary and the controller. The gate driver shown for the Buckstage is the LM5101.

Application Note 1299 LM5041 Evaluation Board

Half-Bridge Topology

7.1 The Basic Half-Bridge

The Basic Half-Bridge

This is a basic Half-Bridge architecture circuit.

One advantage of the Half-Bridge comparedto the Push-Pull is that the FET drain voltagesare inherently limited to the input voltage. Ifthe center tap of the input stage tries to pullabove Vin or below ground, the FET bodydiodes act to clamp the switch node at therails. So unlike the Push-Pull design, we canuse FETs rated at slightly above the maximuminput voltage as compared to significantlygreater than twice Vin. The downside for thisis the need to have a pair of relatively largeinput caps to act as the right side of the bridgeand split the input supply in half. There is alsoa blocking capacitor shown in series with thepower transformer primary to prevent the fluxwalk problem discussed for the Push-Pull fromcausing transformer saturation. In mostdesigns this capacitor proves to be unnecessary.Note also that the transformer primary is now a single winding rather than a center tapped winding. It sees true 100% utilization sincethe entire winding is driven each half cycle.Historically, Half-Bridge circuits have used voltage-mode control. There's an inherent instability in the system if current-mode control isused that causes the capacitor center tap voltage to walk off to a rail. It takes active control of the center tap voltage to be able to usecurrent-mode control.

For the Half-Bridge, voltage-mode control keeps the capacitor center tap voltage from walkingoff to a rail.

Half-Bridge Controller

8.1 LM5035 Half-Bridge Controller

8.2 LM5035 Demo Board Schematic

8.3 Cascaded Half-Bridge Concept

8.4 Cascaded Half-Bridge Characteristics

LM5035 Half-Bridge Controller

National has developed the LM3035 Half-Bridge Controller.

This part uses voltage-mode control to avoidthe instabilities discussed above. It

LM5035 Half-Bridge Controller

National has developed the LM3035 Half-Bridge Controller.

This part uses voltage-mode control to avoidthe instabilities discussed above. Itincorporates input voltage feed-forward tokeep the loop gain under control as the inputvoltage varies and also provide good linevoltage rejection.Another significant feature of the LM5035 isthat it contains on board control for a pair ofsynchronous rectifiers. This feature greatlysimplifies the overall circuit complexity. Asoutput voltages continue to fall, the ability todrive synchronous rectifiers becomes critical.Be aware that the high-side driver of theLM5035 can easily be ground referenced tobe used to drive a conventional Push-Pulldesign as well. Simply ground the HS pin andconnect the HB pin to the VCC rail.

LM5035 PWM Controller with Integrated Half-Bridge and SyncFET Drivers: Net Links

LM5035 Demo Board Schematic

The design shown here accepts an input of 36V to 72V and provides a 3.3V, 30A output.

Note that in the example below, the primary side MOSFETs are only rated at 100V for a 72V input. Had this been a Push-Pull design,the switches would need to be rated at a minimum of 200V and possibly higher. Of course, since the transformer primary voltage is ½the input voltage the current is twice what would be seen in a Push-Pull design. As a general rule this is a beneficial trade-off since aFET's on-resistance will vary approximately to the 3/2 power of its rated breakdown voltage for a given area of silicon.

The synchronous rectifier drivers simplify the implementation of the secondary side MOSFET drives. The correct timing relationship tothe primary side switches is ensured without adding a great deal of discrete circuits to introduce needed time delays. Also, a uniqueoptocoupler feedback interface is included to help speed up the loop response and minimize phase delays caused by the optocoupler.

Application Note 1435 LM5035 Evaluation Board

Cascaded Half-Bridge Concept

Cascaded Half-Bridge Concept

The Cascaded approach can be extended to many other configurations.

Here a Buck stage is cascaded with aHalf-Bridge stage. In this case the Half-Bridgeis said to be voltage-fed, since the splittercapacitors are necessary for proper operation.This approach offers the benefit of furtherreduced voltage stresses on the primary sideswitches of Vout·N, where N is the turns ratio,and a single primary winding.

Cascaded Half-Bridge Characteristics

A Cascaded Half-Bridge Converter is a Buck type converter consisting of a Buck Regulation Stage followed by (cascaded by) aHalf-Bridge Isolation Stage.The Isolation Stage is Voltage-Fed.Voltage splitter capacitors and a small output stage inductor are required.Dead-time is required for Half-Bridge switches.The Half-Bridge Stage FET stresses are reduced, to Vout·N. (2x less than the Push-Pull).

Since the Half-Bridge stage operates at very nearly 50% duty cycle, the output inductor requirements are minimal. A very small inductor is allthat's required to prevent large currents spikes from trying to drive the output capacitor at each switch transition. This helps improve overallefficiency. Probably the biggest drawback of this architecture is the need to be voltage-fed due to the rail splitter caps. This is slightlydetrimental to overall loop stability since it is a two pole system and also provides a fairly large energy reservoir to drive a shorted output.

Both of these issues can be resolved by using the current-fed, Cascaded Full-Bridge design. Before we discuss the cascaded bridge, let'slook briefly at the basic bridge architecture and examine its behavior.

Full-Bridge

9.1 Full-Bridge Concept

9.2 Full-Bridge Current Doubler

9.3 Cascaded Full-Bridge Concept

9.4 Cascaded Full-Bridge Characteristics

Full-Bridge Concept

Here is a basic Full-Bridge concept.

Note that the transformer primary is insertedacross the mid points of both sides of abridge-configured set of MOSFETs, hence thename of the architecture. The drives to PhaseA and Phase B will be 180 degrees out ofphase and driven alternately. The FETs aredriven across the diagonals of the bridge; inother words, Q1 and Q4 driven togetherfollowed by Q2 and Q3. Transformer primarycurrent flows in alternate directions so the

name of the architecture. The drives to PhaseA and Phase B will be 180 degrees out ofphase and driven alternately. The FETs aredriven across the diagonals of the bridge; inother words, Q1 and Q4 driven togetherfollowed by Q2 and Q3. Transformer primarycurrent flows in alternate directions so thecore is fully utilized. The drain voltages of allfour FETs get clamped between Vin andground by the FET's body diodes. The FET'sbreakdown voltage rating does need to beraised to handle spikes or overshoots. Allturn-off transitions are clamped. Note also thatthe transformer primary is a single windingwith no center tap. This topology is particularlywell suited for very high output power levelsand is most economical above about 1kW.The cost of 4 primary switches and driversmakes this design less desirable at low powerlevels, but it will still work quite well. There's also the issue of having two FETs in series. This doubles the conduction losses but sincethe breakdown rating of the switches can be relatively low, the on-resistance can be made proportionally lower than would be possiblewith a Push-Pull design.The output rectifiers are synchronously driven MOSFETs for low losses. Note that the cross coupled drive really only works well for a3.3V output. For lower output voltages the drive voltage gets pretty limited and for much higher voltages the gates are overdriven athigh input lines.One disadvantage of the Full-Bridge is the center tapped secondary winding. High currents are forced to flow through only ½ of thewinding at a time, so the transformer secondary isn't optimally utilized. One solution to this is what's know as the current doublersecondary which will be seen next.

For the Full-Bridge, the MOSFET drain voltage is approximately equal toVin.

Full-Bridge Current Doubler

The current doubler configuration requires theuse of a second inductor on the secondaryside. However, each inductor only has tohandle ½ the total load current and can bequite low in value. The ripple currents throughthe two inductors are 180 degrees out ofphase and so tend to cancel to a large degree,allowing a larger ripple in each side while stillproducing a relatively low net output ripplecurrent.Operation is as follows: When the dotted endof T1 is positive, the Phase A switch is on andcurrent ramps positive in the upper inductor.When the transformer voltage switches overthe Phase B switch turns on and currentramps up in the lower inductor while the upperinductor's current freewheels though thePhase B switch. When a sync switch iscarrying the transformer current it also carriesthe freewheeling current from the un-driveninductor. As such the FET sees the full loadcurrent during this condition. During theperiods where all four primary switches are off, both of the sync rectifiers will be on, with each now carrying ½ the total load current.This architecture is good for high current outputs but must have the secondary switches actively driven as opposed to being self drivenby the secondary winding.The current doubler can be used with all of the double ended architectures discussed here.

The current doubler requires two output inductors.

Cascaded Full-Bridge Concept

Cascaded Full-Bridge Concept

Here's an example of a Buck stage cascaded with a Full-Bridge stage.

The benefit here is:

Reduced primary FET voltage stress ofVout·N.Reduced switch current relative to theHalf-Bridge.A single primary winding.

One thing to note is that the bridge is no longerclamped by a large capacitor. So the bridge voltage isno longer absolutely constrained as in the voltage-feddesign. Some care must be taken when usingsynchronous rectifiers on the secondary since theregulator is capable of sinking output current andtrying to boost the primary voltage. This is only apotential issue at very light loads and should be of noconcern as long as the output is heavily loaded.

Cascaded Full-Bridge Characteristics

A Cascaded Full-Bridge Converter is a Buck type converter consisting of a Buck Regulation Stage followed by (cascaded by) aFull-Bridge Isolation Stage.

The Isolation Stage is Current-Fed.

No voltage splitter capacitors or output stage inductor are required as in the Cascaded Half-Bridge.

Overlap time is required for Isolation Stage switches.

The Full-Bridge Stage voltage stresses are Vout·N, similar to the half-bridge.

Full-Bridge Stage current levels are half that of a Half-Bridge.

This completes our course on Push-Pull switching regulators. As you have seen there are a number of possible variations on the basic theme,all of which carry advantages and disadvantages.

B-H

Characteristic curve for a magnetic material, representing Flux Density B on the y-axis and Magnetic Intensity H on the x-axis.

BRRemanent Flux

BSATSaturation flux density.

Buck

A voltage regulator used to step down a higher input voltage to a lower output voltage.

Buck-Boost

A voltage regulator used to invert a voltage, such as positive input to negative output.

Cascaded

Buck-Boost

A voltage regulator used to invert a voltage, such as positive input to negative output.

Cascaded

Term used for a circuit that is made up of more than one circuit in series.

CCM

Continuous Conduction Mode, referring to an inductor where current is always flowing.

Coercive Force

The magnetizing force H which is required to reduce the remanent flux BR

to zero is called the coercive force.

Core Loss

Usually expressed in watts per pound or mW per cubic centimeter. The area enclosed by the B-H curve is proportional to the core loss.

Current-Fed

Circuit where the input is fed by a current source, usually with a inductor at the input.

Current-Mode

A dual loop control method with an inner loop that servos on the inductor current, and an outer voltage control loop.

D

Duty cycle. The ratio of on-time to period, t(on)/T.

DC-DC

A converter which accepts a DC input voltage and produces a DC output voltage.

Dead-Time

The time when actively driven switches are off.

FET

Field-Effect Transistor.

Flux Density

The flux density of a magnetic field B is expressed as volt·second (weber) per square meter (tesla) or lines per square centimeter (gauss).There are 10,000 gauss per tesla.

Flyback

A transformer isolated Buck-Boost regulator. The transformer is also the energy storage inductor.

Forward

A transformer isolated Buck regulator.

Freewheel

Refers to the current in a rectifier which is allowed to "freewheel" when the rectifier is forward biased by the inductor voltage.

Full-Bridge

An isolated converter where the primary is diagonally driven between the input voltage and ground by four switches.

Half-Bridge

An isolated converter where one end of the primary is alternately driven to the input voltage and ground by two switches.

An isolated converter where the primary is diagonally driven between the input voltage and ground by four switches.

Half-Bridge

An isolated converter where one end of the primary is alternately driven to the input voltage and ground by two switches.

Magnetizing Inductance

Generally referenced to the transformer primary, which must be magnetized in order to transfer energy.

MOSFET

Metal-Oxide Semiconductor Field-Effect Transistor.

OV

Over-Voltage

Permeability

Permeance is an expression of the ease with which a magnetic field is conducted. Permeability is represented by µ, where µ=B/H.

Push-Pull

A Forward converter with two primary switches and transformer windings. The primary switches alternately power their respective windings.

PWM

Pulse Width Modulation

Remanent Flux

The residual flux left in the core when the magnetic intensity H is returned to zero. This is the residual magnetism of the core after beingdriven into saturation, or maximum magnetization.

RF

Radio Frequency

Single-Ended

A converter which only drives one end of the primary or energy storage inductor. The term is usually applied to transformer isolatedconverters.

Synchronous

Used to describe a converter which has active devices for the output rectifiers which are synchronously driven.

UVLO

Under-Voltage Lockout

Voltage-Fed

Circuit where the input is fed by a voltage source, usually with a capacitor at the input.

Voltage-Mode

A control method with a single control loop which regulates the output voltage.

Volt·Second

Proportional to the flux density of a magnetic field B which is expressed as volt·second (weber) per square meter (tesla) or lines per squarecentimeter (gauss). There are 10,000 gauss per tesla.

Frequently Asked Questions

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Questions

Proportional to the flux density of a magnetic field B which is expressed as volt·second (weber) per square meter (tesla) or lines per squarecentimeter (gauss). There are 10,000 gauss per tesla.

Frequently Asked Questions

Do you have a question? We may have already answered it. Check below to see if you can find the answer to your question.

Questions

Answers

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