Lecture # 12&13 SWITCHING-MODE POWER SUPPLIES 1. Switching-Mode Power Supplies 2.

Lecture # 12&13

SWITCHING-MODE POWER SUPPLIES

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Switching-Mode Power Supplies

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Introduction to Switching RegulatorsObjective of topic is to answer the following questions:

1. What is a switching power supply?2. What types of switchers are available?3. Why is a switcher needed?4. How does a switcher operate in general?5. How does a buck converter operate?6. How does a boost converter operate?7. How does a buck-boost converter operate?8. How many design topologies of a switching power supplies?

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Introduction to Switching RegulatorsIntended Audience:• Electrical engineers with limited power supply

background• A simple, functional understanding of inductors and

capacitors is assumed• A simple, functional understanding of transistors is

assumed

Expected Time: • Approximately 120 minutes

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Outline of the presentation1. Switching Regulator Overview

What is a Switching Regulator? Why is a switcher needed? What are the main differences between a switching and linear

regulator? Buck, Boost, Buck-Boost (Inverting)

Five designing topologies.. Flyback converter. Forward converter. Push-pull converter. Half Bridge converter. Full-Bridge converter.

3. Switching Regulator Operation . How does a Switching Regulator works? Switching Regulators components.

2. Switching mode DC power supplies

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What is Switching Power Supply?

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What is Switching Power Supply?

The advantage of a switching-mode power supply is that the relatively high frequency oscillator allows the use of small, lightweight and low-cost transformers.

This makes them considerably smaller and lighter than linear power supplies. Almost all modern powers supplies, including those in PCs, are switching mode power supplies. Their disadvantages are complexity and RF egress (interference).

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What is a Switching Regulator?Switching Regulator

• Converts an input voltage into desire output voltage.

• The power transistor operates as a switch, completely on or off.

• An energy storage part (inductor) is used in the architecture

12 V 5 VControl

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Choosing Between Linear and Switching Regulators• When possible, most designers would prefer to use a

linear voltage regulator rather than a switching voltage regulator

• Linear regulators are usually lower in price

• Linear regulators are usually simpler to implement

• Linear regulators do not have associated noise/ripple problems apparent in switching regulators

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Choosing Between Linear and Switching Regulators

• When to use a switching regulator :

When the minimum input voltage is at or below the desired output voltage

• Linear regulators cannot provide an output voltage greater than the input voltage

VIN < VOUT

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Choosing Between Linear and Switching Regulators• When to use a switching regulator:

• The efficiency of a linear regulator cannot maintain the junction temperature below the specified maximum

• The maximum junction temperature is usually 150C

• The efficiency of linear regulators often prohibit their use in high voltage, high current applications

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Why are switching regulators needed?

• The power dissipation is too high for a linear regulator– The efficiency of a linear regulator cannot maintain the junction

temperature below maximum (150 °C)– The heat sinking of a linear regulator is prohibitive in price or space

OutputPower Switching Regulator

Linear Regulator

Linear RegulatorMaximum Power Dissipation

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Why are switching regulators needed?

• The desired output voltage is greater than the input voltage– Linear regulators cannot provide an output voltage greater than the input

voltage

The desired output voltage is opposite polarity than the input voltage– Linear regulators cannot invert an input voltage

1.5 VBattery Power Supply 5 V

Required

Power Supply12 VBattery

-12 VRequired

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Types of Switching Regulators AC-DC, AC-AC, DC-AC, and DC-DC Converters

AC-DC DC-AC DC-DC

12 Vdc

t

110 Vac

t

12 Vdc

t

12 Vdc

t

5 Vdc

t

AC-AC

110 Vac

t

220 Vac

t

110 Vac

t

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Types of DC-DC Converters Step Down, Step Up and Inverting

Step Down Buck

V

t

V

t

Vin = 12 VVout = 5 V

Step Up Boost

V

t

Vin = 5 VV

t

Vout = 12 V

InvertingBuck-Boost

V

t

Vin = 5V

V

tVout = -10 V15

Basic Circuit Configuration

VOUT

VIN

VM

VGATE

LC

ISW

ILVOUT

VIN

VM

CVGATE

LIL

ISW

VOUT

VIN

VM C

VGATE

LIL

ISW

Buck-Boost VIN < -VOUT < VIN

Boost VIN < VOUT

BuckVIN > VOUT

• All topologies consists of the same basic components but are arranged differently 16

Buck Configuration

• The input voltage is always greater than the output voltage

VOUT

VIN

VM

VGATE

LC

ISW

IL

VIN

time

20V

15V

10V

5V

0V

VOUT

time

7.5V

5V

2.5V

0V

10V

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Boost Configuration

• The input voltage is always less than the output voltage

VOUT

VIN

VM

CVGATE

LIL

ISW

VIN

time

20V

10V

5V

VOUT

time

10V

0V 0V

20V

5V

15V 15V

24V

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Buck-Boost Configuration

• The input voltage is always not constrained by the output voltage

VOUT

VIN

VM C

VGATE

LIL

ISW

VIN

time

20V

15V

10V

5V

0V

VOUT

time

-10V

-20V

0V

-15V

-5V

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Switched-Mode DC Power Supplies

• Five Designing Topologies.– Flyback converter.– Forward converter.– Push-pull converter.– Half Bridge converter.– Full-Bridge converter.

• Operate at high frequencies– Easy to filter out harmonics

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Flyback Converter

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Mode 1 Operation -- Q1 ON• Current builds up in the primary winding• Secondary winding has the opposite polarity D1 OFF• C maintains the output voltage, supplies load current

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Mode 2 Operation -- Q1 turned OFF• Polarity of the windings reverses• Diode D1 conducts, charging C and providing current to

the load RL

• Secondary current falls to 0 before the next cycle begins

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Waveform Summary

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Double-ended Flyback Converter

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Forward Converter

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Features• Includes a “reset” winding to return energy.• Secondary “dot” so that D2 forward biased when Q1 is

ON – no energy stored in the primary.• Operates in continuous mode.

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Mode 1 Operation -- Q1 ON• Current builds up in the primary winding• Energy transferred to the load

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Mode 2 Operation -- Q1 turned OFF• Polarity of transformer voltages reverses• D2 turns OFF, D1 and D3 turn ON

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Waveform Summary

Vo

D3

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Double-ended Forward Converter

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Push-Pull Converter

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Push-Pull Operation• Q1 ON, Vs across the lower primary winding

• Q2 ON, Vs across the upper primary winding

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Half-Bridge Converter

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Mode 1 Operation • Q1 ON, D1 conducting

• Energy transferred to the load

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Mode 2 Operation• Both transistors are OFF• D1 continues to conduct due to current in L1

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Mode 3 Operation• Q2 ON, D2 conducting

• Energy transferred to the load

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Mode 4 Operation• Both transistors OFF• D2 continues to conduct due to current in L1

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Full-Bridge Converter

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Mode 1 Operation• Q1,Q4 ON, Q2,Q3 OFF

• D1 conducting, energy transferred to the load

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Mode 2 Operation• All transistors are OFF• D1 continues to conduct due to current in L1

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Mode 3 Operation• Q2,Q3 are ON, Q1,Q4 OFF

• D2 conducting, energy transferred to the load

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Mode 4 Operation• All transistors are OFF• D2 continues to conduct due to current in L1

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VIN

Switching Regulator

Duty Cycle Controller

OutputMonitor

VOUT

time

5V

VoltageOK50%

Filt

er N

etw

ork

VOUT

How a Switching Regulator Works

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VIN

Voltage Regulator


OutputMonitor

VOUT

time

5V

VoltageOK50%

Filt

er N

etw

ork

VOUT


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VIN

Voltage Regulator


OutputMonitor

VOUT

time

5V

VoltageOK50%

Filt

er N

etw

ork

VOUT


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VIN – 1V

Voltage Regulator


OutputMonitor

VOUT

time

5V

VoltageLow60%

Filt

er N

etw

ork

VOUT


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VIN – 1V

Voltage Regulator


OutputMonitor

VOUT

time

5V

VoltageLow60%

Filt

er N

etw

ork

VOUT


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VIN

Switching Regulator


Output Monitor

VOUT

time

5V

VoltageOk50%

Filt

er N

etw

ork

VOUT


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Switching Regulator Components

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VIN VOUT

Switching Power Supply

Switching Power Supply Block Diagram

Switch

Error Amplifier

Bandgap Reference

PWMController

Network Network

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PWM Controller• In a switching voltage regulator, the pass transistor is used as

a switch - it is either on or off• The output voltage, however, is an analog value• PWM controller senses error in VOUT via the error amplifier

• PWM controller updates the duty cycle of the of transistor adjusting the output voltage

ErrorAmplifier

PWMController

0-100%VOUT

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Switching TransistorBipolar and MOSFET

Bipolar MOSFET

Switch Speed Slow Fast

Drive Method Current Voltage

Drive Circuit Complex Simple

ESD Robustness High Low

Collector

Emitter

BaseDrain

Source

Gate

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VIN VOUT



Switch

Error Amplifier

Bandgap Reference

PWMController

Network Network

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External Network• An external network (consisting of an inductor, capacitor, and

diode) transforms the energy from the PWM controlled power switch into a desired output voltage

NetworkSwitchVIN VOUT

VIN = 12 VVOUT = 5 V

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VIN VOUT



Switch

Error Amplifier

Bandgap Reference

PWMController

Network Network

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VOUT

VIN

VM

VGATE

+ VL -

COUT

ISW

IL

VGATE goes high

VM ~ VIN

VL = VM – VOUT

t

VM

t

VGATE

t

IL

VOUT

t

ISW

t

RLOAD

-VF

Step Down Switching RegulatorSteady State Operation

-VF

+

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VOUT

VIN

VM

VGATE

COUT

ISW

IL

VL Constant

t

VM

t

VGATE

t

IL

VOUT

t

ISW

L LdI V= Constant

dt L

IL and ISW increase

t

RLOAD

COUT is charged by IL

and

VOUT increases

-VF


-VF

+

+ VL -

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VOUT

VIN

VM

VGATE

COUT

ISW

IL

VGATE = 0V

The pass transistor

is turned off

ISW = 0At

VM

t

VGATE

t

IL

VOUT

t

ISW

t

RLOAD

LV

=dtdI LL

IL cannot go to 0A instantly:

VM goes negativeVL = VM – VOUT

L LdI V= < 0 A/s

dt L

-VF

-VF

+

+ VL -


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VOUT

VIN

VGATE

COUT

ISW

IL

But, VM is clamped

to -VF

and IL decays

through the diode t

VM

t

VGATE

t

IL

VOUT

t

ISW

t

RLOADCOUT stabilizes

the output voltageso VOUT will only

slowly decay

-VF


VM = -VF

-VF

+

+ VL -

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VOUT

VIN

VGATE

COUT

ISW

IL

The MOSFET is

turned on and off

to repeat

the sequence

RLOAD

t

VM

t

VGATE

t

IL

VOUT

t

ISW

t

-VF


VM = -VF

-VF

+

+ VL -

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For student study :

Read Chapter 21 discrete & integrated voltage regulators topics (21.5) from the book introductory electronic devices and circuits by (Robert T .paynter)conventional flow version.

Wikipedia & world wide web.

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REFERENCES

THANK YOU!

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Lecture # 12&13 SWITCHING-MODE POWER SUPPLIES 1. Switching-Mode Power Supplies 2.

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Transcript of Lecture # 12&13 SWITCHING-MODE POWER SUPPLIES 1. Switching-Mode Power Supplies 2.