Post on 23-Apr-2018
An Introduction to Power Supplies i
Table of Contents
4.0 Design Examples
4.1 Generic SMPS ICs
4.2 LM78S40 Universal Switching Regulator
4.3 MAX641 Step-Up Switching Regulator
Assignment Questions
For Further Research
_____ Notes _____
An Introduction to Power Supplies 4 - 1
4.0 Design Examples
Objectives
• Identify various types of switch mode converters.
• Determine the approximate current waveforms in SMPS circuits.
•
4.1 Generic SMPS ICs Of these components, only the switch and diode can be integrated; the inductor and capacitor are external components. In high power applications, even the switch and diode are discrete. It may therefore seem pointless to integrate a switching regulator. However, this is not the case. The real trick is to control the switch.
Switching converters can achieve a power conversion efficiency of 70 – 90%. However, they also create electro-magnetic interference (EMI), which can have adverse effects on other nearby electronics. Low-loss ferrite materials, high permeability magnetic shielding, and smaller semi-conductors can reduce EMI.
Bipolar switching transistors, with a gain-bandwidth product in excess of 4 MHz are often used as the principle switching element. To minimize adverse effects from inductive kickback, Schottky or fast-recovery diodes are also used.
Buck Switching Regulator
Switch Mode Power Supplies _____ Notes _____
4 - 2 An Introduction to Power Supplies
Boost Switching Regulator
Most switching regulators consist of:
Switching transistor
Diode clamp
LC filter
Control logic
It is possible to design switching power supplies to operate directly from the hydro input. This eliminates the need for a transformer, rectification, and pre-filtering.
A switching regulator IC contains the four basic components found in a linear regulator, but adds an oscillator and some control logic in order to control the transistor switch (control element).
Generic Switching Regulator IC
There are several different control methods that can be used to control the conduction in the series control element:
PWM – pulse width modulation: the frequency is held constant and the ON time is varied. This is the most common technique used.
PFM – pulse frequency modulation: the on or the off time is held constant and the frequency is varied.
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 3
PBM – pulse burst modulation: the oscillator frequency and duty cycle is held constant and oscillator cycles are gated on or off. This technique is used in simple converters below 10 watts.
As the switching frequency increases, the size of the magnetic components decreases and the switching losses increase. If the switching frequency is in the audio range, it is possible for the coil windings to vibrate, thus creating an annoying singing tone. This is a common phenomenon in TV sets, where the flyback transformer operates at 15.75 KHz.
The sampling circuit generally consists of a simple voltage divider. Under normal operating conditions, it produces an output equal to the built-in reference voltage. An op amp is used to compare the sampled and reference voltages. This creates a difference signal that used to control the series pass device (switch). A similar technique is used in linear regulators, the principle difference is that the series device is operated at the extremes of its load line instead of the linear region. This subtle difference is what gives switching converters their high power conversion efficiency.
4.2 LM78S40 Universal Switching Regulator
LM78S40 Datasheet by National Semiconductor
uA78S40 Datasheet by OnSemi
AN-711 LM78S40 Applications by National Semiconductor
Many of the functional blocks in this circuit are disconnected. This gives the designer a great deal of design flexibility. If required, the transistor switch can be used directly in low power applications, or it can be used to drive a high power series pass element.
Switch Mode Power Supplies _____ Notes _____
4 - 4 An Introduction to Power Supplies
Each SMPS IC has its own design peculiarities. In some cases, the restriction placed on the designer may preclude the use of any SMPS IC and a completely discrete circuit design must be considered. This situation however, is beyond the scope of this presentation.
Transistor Driver and Switch
The 78S40 uses a Darlington pair in the switching arrangement. The collectors of both transistors are brought out to external pins. This allows them to be connected together as is the standard configuration, or an external resistor can be placed in the driver collector to control the switch saturation current.
Oscillator
The 78S40 chip is designed to operate within a switching frequency range of 100 Hz to 100 KHz. Increasing the switching frequency increases the electromagnetic radiation and PCB layout problems but decreases the size of the inductor. Most designs based on this IC have an operating frequency of 20 - 30 KHz.
The oscillator, the charge/discharge ratio is preset to approximately 6:1. The overall switching duty cycle can be varied from approximately 17% to 50% by means of two feedback loops.
The switching frequency and duty cycle are controlled by current and voltage feedback. This will at times make triggering an oscilloscope to the switching waveforms somewhat problematic. As the load increases, the switching frequency tends to increases.
Reference Voltage
The 78S40 has an internal 1.245 volt temperature compensated, band-gap voltage reference which is available at an external pin. This reference voltage should be bypassed by a 0.1µfd capacitor to ground to help insure stability.
Current Feedback
The current feedback circuit modifies the switch ON time. A current sensing resistor RSC generates a voltage proportional to the switching current. When this potential exceeds approximately 0.3 volts, the oscillator (and hence switch) is turned OFF. This control mechanism takes priority over voltage feedback.
Voltage Feedback
The voltage feedback loop, consisting of the voltage divider and comparator controls the switch ON time. If the output voltage is too low, the ON time is increased.
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 5
Current and Voltage Limitations
The internal Darlington transistor switch can handle a maximum peak current of 1.5 amps during the ont period, and a maximum of 40 volts during the offt
period.
An external transistor switch is needed if the design requires either more current or a higher input voltage.
Voltage Sensing Resistors
The voltage divider at the output represents a minimum load. The voltage at the junction of the two resistors must equal the 1.245 reference voltage when one resistor is attached to ground and the other to the output.
Efficiency
The efficiency of a well-designed power converter can be in excess of 90%.
The output voltage divide constitutes a minimum load on the switching converter and therefore reduces the efficiency. The current drawn by the divider can be as low as 100 µa, but is more typically in the region of 1 ma. This may not be significant with high load currents, but it becomes more dominant as load current decreases.
The saturation voltage of the Darlington transistor can be as high as 1.3 volts. This decreases efficiency as load current and ont increase.
The internal Darlington transistor has a switching speed of 300 – 500 nSec. During this time, the transistor is neither ON nor OFF, and therefore dissipates power.
Any current sensing resistor in series will also dissipate power.
In the following formulas:
inV = Nominal input voltage
outV = Desired output voltage (determined by the ratio R2/R1)
satV = ON voltage drop across the switching element
DV = Forward voltage drop across the flyback diode.
outI = Desired output current.
sI = Voltage divider sampling current (~ 1 ma)
rippleV = Desired peak-to-peak ripple voltage.
SCR = Short circuit current sensing resistor.
Switch Mode Power Supplies _____ Notes _____
4 - 6 An Introduction to Power Supplies
NOTE:
When breadboarding these circuits:
It may be necessary to reduce RSC to 0 Ω.
Always keep the circuit leads as short as possible.
Always use a large decoupling capacitor at the circuit input.
Spread Sheet Design
Buck (Step down) Circuit
Simplified Circuit
Buck Design Formulas
Formula Comment
+=
1
21RR
VV refout The internal reference voltage is Vref = 1.245 volts.
(max)2 outpk II = pkI is the peak inductor current.
(max)outI is the maximum output
load current.
pkSC I
R33.0=
The value for the current sensing resistor is a given, not derived, formula.
outsatin
Dout
off
on
VVVVV
tt
−−+
= ton and toff > 10 µs ton + toff < 50 µs
offpk
Dout tI
VVL
+=
Dout
pkoff VV
LIt
+=
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 7
offT tC 51045 −×= Timing capacitor
( )ripple
offonpkO V
ttIC
8
+=
Minimum output filter capacitor
Dout
out
in
Dsatin
VVV
VVVV
+
+−
Converter efficiency.
+−+
=Dsatin
Doutpkloadavein VVV
VVII
2))(max( Input current.
Buck Design Example (AN711)
Vin = 25 volts Vout = 10 volts
Iout (max) = 500 ma Vripple < 1%
Step 1 - Calculate the peak current
(max)2 outpk II = = 1 amp
Step 2 - Calculate the current sense resistance
pkSC I
R33.0= = 0.33 Ω
Step 3 - Calculate the ton/toff ratio
8.0101.125
25.110 ≈−−
+=−−
+=
outsatin
Dout
off
on
VVVVV
tt
Step 4 - Calculate the values for ton and toff
Since offon tt 8.0= and sec50µ≤+ offon tt
Let sec22µ=offt then sec18µ=ont
Step 5 - Calculate the timing capacitor CT
fdtC offT µ01.0102210451045 655 ≈×××=×= −−−
Switch Mode Power Supplies _____ Notes _____
4 - 8 An Introduction to Power Supplies
Step 6 – Determine the inductor size
HtI
VVL off
pk
Dout µ25010221
25.110 6 ≈×
+=
+= −
Step 7 – Determine the minimum storage capacitor size
( ) ( )fd
V
ttIC
ripple
offonpkO µ50
1.08102210181
8
66
≈×
×+×=+
=−−
Step 8 – Determine the feedback resistor sizes
Ω=×
== − KI
VR
s
ref 25.1101245.1
32 (Select 1.3 K)
Ω=×−=
−= − K
I
VVR
s
refout 76.8101
245.11031 (use a 10 K potentiometer)
Final Circuit
Note: the above schematic is missing the reference and input bypass capacitors.
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 9
Basic Waveforms
Boost (Step Up) Circuit
Simplified Circuit
Switch Mode Power Supplies _____ Notes _____
4 - 10 An Introduction to Power Supplies
Boost Design Formulas
Formula Comment
+=
1
2125.1RR
Vout The internal reference voltage is 1.245 volts.
−−+
=satin
satDoutoutpk VV
VVVII (max)2
pkI is the peak inductor current.
(max)outI is the maximum output load
current.
pkSC I
R33.0=
The value for the current sensing resistor is a given, not derived, formula.
satin
inDout
off
on
VVVVV
tt
−−+
= ton and toff > 10 µs ton + toff < 50 µs
offpk
inDout tI
VVVL
−+=
inDout
pkoff VVV
LIt
−+=
offT tC 51045 −×= Timing capacitor
( )ripplepk
offoutpkO VI
tIIC
2
2−=
Minimum output filter capacitor
satDout
out
in
satin
VVVV
VVV
−+
−
Converter efficiency.
2))(max(pk
loadavein
II =
Input current.
B
refin
I
VVR
−=3
Driver collector resistor
βpk
B
II = , assume 20≈β
Boost Design Example (AN711)
Vin = 5 volts Vout = 15 volts
Iout (max) = 150 ma Vripple < 1%
Step 1 - Calculate the peak current
ampVV
VVVII
satin
satDoutoutpk 1
45.0545.025.115
15.022 (max) ≈
−−+×=
−−+
=
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 11
Step 2 - Calculate the current sense resistance
pkSC I
R33.0= = 0.33 Ω
Step 3 - Calculate the ton/toff ratio
5.245.05
525.115 ≈−
−+=−
−+=
satin
inDout
off
on
VVVVV
tt
Step 4 - Calculate the values for ton and toff
Since offon tt 5.2= and sec50µ≤+ offon tt
Let sec10µ=offt then sec25µ=ont
Step 5 - Calculate the timing capacitor CT
nfdtC offT 5.4101010451045 655 ≈×××=×= −−−
Step 6 – Determine the inductor size
HtI
VVVL off
pk
inDout µ12510101
525.115 6 ≈××
−+=
−+= −
Step 7 – Determine the minimum storage capacitor size
( ) ( )fd
VI
tIIC
ripplepk
offoutpkO µ24
15.012101015.01
2
622
≈××
××−=−
=−
Step 8 – Determine the feedback resistor sizes
Let maI s 1=
Ω=×
== − KI
VR
s
ref 25.1101245.1
32 (Select 1.3 K)
Ω=×−=
−= − K
I
VVR
s
refout 8.13101
245.11531 (use a 25 K potentiometer)
Switch Mode Power Supplies _____ Notes _____
4 - 12 An Introduction to Power Supplies
Step 9 – Determine the base drive resistor size (if desired)
B
refin
I
VVR
−=3 and
βpk
B
II = , assume 20≈β
Ω≈−= 75201245.15
3R
Final Circuit
Note: the above schematic is missing the reference and input bypass capacitors.
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 13
Basic Waveforms
Inverter Circuit In some designs (such as the one below) the switch current is extremely large. As a result, an external switching transistor may be required.
Simplified Circuit
Switch Mode Power Supplies _____ Notes _____
4 - 14 An Introduction to Power Supplies
Inverter Design Formulas
Formula Comment
+=
1
2125.1RR
Vout The internal reference voltage is 1.245 volts
−−++
=satin
satDoutinoutpk VV
VVVVII (max)2
pkI is the peak inductor
current. (max)outI is the
maximum output load current.
pkSC I
R33.0=
The value for the current sensing resistor is a given, not derived, formula.
satin
Dout
off
on
VV
VV
tt
−+
= ton and toff > 10 µs ton + toff < 50 µs
offpk
Dout tI
VVL
+=
Dout
pkoff VV
LIt
+=
offT tC 51045 −×= Timing capacitor
( )ripplepk
offoutpkO VI
tIIC
2
2−=
Minimum output filter capacitor
Dout
out
VV
V
+
Converter efficiency.
+−++
=Dsatoutin
Doutpkloadavein VVVV
VVII
2))(max( Input current.
Inverter Design Example (AN711)
Vin = 12 volts Vout = -15 volts
Iout (max) = 500 ma Vripple < 1%
Step 1 - Calculate the peak current
−−++
=satin
satDoutinoutpk VV
VVVVII (max)2
6.2212
225.115125.02 ≈
−−+−+
×=pkI
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 15
Step 2 - Calculate the current sense resistance
pkSC I
R33.0= = 0.13 Ω
Step 3 - Calculate the ton/toff ratio
625.1212
25.115=
−+−
=−+
=satin
Dout
off
on
VV
VV
tt
Step 4 - Calculate the values for ton and toff
Since offon tt 6.1= and sec50µ≤+ offon tt
Let sec10µ=offt then sec16µ=ont
Step 5 - Calculate the timing capacitor CT
nfdtC offT 5.4101010451045 655 ≈×××=×= −−−
Step 6 – Determine the inductor size
HtI
VVL off
pk
Dout µ7010106.2
25.115 6 ≈××
+−=
+= −
Step 7 – Determine the minimum storage capacitor size
( ) ( )fd
VI
tIIC
ripplepk
offoutpkO µ60
15.06.2210105.06.2
2
622
≈××
××−=−
=−
Step 8 – Determine the feedback resistor sizes
Ω=×
== − KI
VR
s
ref 25.1101245.1
32 (Select 1.3 K)
Ω≈×+=
+= − K
I
VVR
s
refout 5.17101
245.11631 (use a 25 K potentiometer)
Switch Mode Power Supplies _____ Notes _____
4 - 16 An Introduction to Power Supplies
Step 9 – Determine the switching transistor bias resistors
Select Ω≤≤Ω 300100 3R
βpk
BETsatin
IVVVV
R−−−
≈4
where vVT 3.0= (current sense threshold voltage)
4FEh=β (of the external transistor)
Ω≈−−−≈ 7201906.2
7.03.03.1124R
Final Circuit
Note: the above schematic is missing the reference and input bypass capacitors.
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 17
Basic Waveforms
Switch Mode Power Supplies _____ Notes _____
4 - 18 An Introduction to Power Supplies
4.3 MAX641 Step-Up Switching Regulator
MAX641 Datasheet
MAX641 Block Diagram
This particular IC requires the designer to only select the value of the inductor.
Boost Design Example
maIma
SectSec
KHzf
vVv
vV
vV
vV
load
on
osc
sw
D
out
in
45015
128
%2050
75.025.0
4.0
15
%105
≤≤≤≤∴±=
≤≤==
±=
µµ
To determine the value of the inductor, it is necessary to make two calculations, one for a maximum size and the other for minimum.
Case 1 Maximum L
loadswin
inDoutpk I
VVVVV
I ××−
−+= 4
mamaI pk 17415475.05.4
5.44.015 =××−
−+=
onpk
swin tI
VVL ×
−=
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 19
HSecma
L µµ 1728174
75.05.4 =×−=
Case 2 Minimum L
From the datasheet Ipk(max) = 450ma.
HSecma
L µµ 14012450
25.05.5 =×−=
A value of 160 µH would be a reasonable compromise.
Switch Mode Power Supplies _____ Notes _____
4 - 20 An Introduction to Power Supplies
Assignment Questions
On-Line Test
Quick Quiz
1.
Composition Questions To answer these questions, it will be necessary to do some research.
1.
Analytical Questions
1. Create a spreadsheet to implement the design of a Buck Regulator based on the LM78S40.
2. Create a spreadsheet to implement the design of an Inverter based on the LM78S40.
3. Create a spreadsheet to implement the design of a Boost Regulator based on the LM78S40.
4.
Switch Mode Power Supplies _____ Notes _____
An Introduction to Power Supplies 4 - 21
For Further Research