Switch Mode Power Supply (SMPS)

Power Electronics ConvertersDC-DC Converter (Chopper)Isolated DC-DC ConverterSMPS circuit TechniquesSMPS Block Diagram

Agenda

Power Electronics Converters

AC input DC output

DC input AC output

AC to DC: RECTIFIER

DC to DC: CHOPPER

DC to AC: INVERTER

DC input DC output

DC-DC Converter (Chopper)

DEFINITION: Converting the unregulated DC input to a controlled DC

output with a desired voltage level.

General block diagram:

LOAD

Vcontrol(derived from

feedback circuit)

DC supply(from rectifier-filter, battery,fuel cell etc.)

DC output

APPLICATIONS: – Switched-mode power supply (SMPS), DC motor

control, battery chargers

Isolated DC-DC Converter

Isolated DC-DC requires isolation transformerTwo types: Linear and Switched-mode

Advantages of switched mode over linear power supply-Efficient (70-95%)-Weight and size reduction

Disadvantages -Complex design-EMI problems

Linear and SMPS block diagram

6

Basic Block diagram of linear power supply

Basic Block diagram of SMPS

Base/gateDrive

ErrorAmp.

LineInput

3/1

50/60 HzIsolation

Transformer

Rectifier/Filter

+

Vd

-

Vce=Vd-Vo

C E

B

Vo

Vref

RL

+Vo

+

Vo

-

DC UnregulatedDC Regulated

EMIFILTER

RECTIFIERAND

FILTER

HighFrequency

rectifierandfilter

Base/gatedrive

PWMController

errorAmp

Vo

Vref

DCRegulated

DC-DC CONVERSITION ANDISOLATIONDC

Unregulated

SMPS – Circuit TechniquesSMPS without isolation:

Buck converter.Boost converter.Buck-Boost converter.

SMPS with isolation:Forward converter.Flyback converter.Half and Full bridge dc-dc converterPush Pull converter.

Advantages of SMPS

High efficiency& less heat generation - , Linear power supplies are typically 40- 50% efficient, while switchers have efficiencies from 60 to 90%.

Tighter regulation, and Smaller size and weight. – This makes them ideal for use in portable equipment.

Greater efficiency is the biggest advantage.

Fig. 1: The SMPS transformer (left) and the AC power transformer both provide isolation and various output voltages, but the SMPS transformer is smaller, lighter weight, and more efficient.

COMPARISON B/W SMPS AND AN AC TRANSFORMER

. One of the first switch mode power supplies was the mechanical vibrator circuit used in early automobiles. The vibrator “chopped” the 6 volt battery voltage into an AC signal that could be stepped up and down to deliver the plate and bias voltages needed to power the tube radio.

Today’s more sophisticated SMPS still employ the same basic concept used in the early vibrator supply: a DC voltage is converted to an AC signal, the AC signal is stepped up by a transformer; and the stepped-up AC is converted back to a DC voltage

Conventional, series-pass regulator

A conventional, series-pass regulator dissipates the unused power as heat.

Block Diagram of SMPS

A basic SMPS consists of a DC voltage source, a switching device, a step up/down transformer, (flyback) and a switching control stage

Regulators Types

The regulators used in consumer equipment fall into two types; 1) pulse width modulated (PWM), and 2) pulse rate modulated (PRM). Television receivers and computer monitors may use either type, while VCR commonly use PRM supplies.

The PWM regulates by varying the “on” time of the switching transistor switching transistor stays on longer, and more energy is applied to the switching transformer. Note that the frequency of the signal remains constant.

PWM Regulator

PRM Regulator

The PRM regulator varies the rate (frequency) at which the switching transistor is turned off and on. Notice, however, that as the pulse rate increases. This varies the switching transistor’s conduction time.

The four most important circuits in a switch mode power supply are: UnregulatedB+(MIC1),Startup and Drive(MIC2). Secondary Loads (MIC3), and Feedback and control(MIC4).

The blocks can be grouped in to four sections or “most important circuits” (MICS): 1) Unregulated Bt, 2) Startup& Drive, 3) Secondary circuits, and 4) Feedback& Control.

The gray shaded blocks provide the unregulatedB+ to power the switcher.

The pink shaded blocks are responsible for the signal that drives the switching transistor.

The secondary circuits are indicated by the black blocks .

Red blocks couple a sample of the output voltage back to the driver stage. Two major parts of this functional area are the feedback divider network and the opto isolator

A: input EMI filtering; : bridge rectifier;

B: input filter capacitors;Between B and C : primary side heat sink;

C: transformer;Between C and D : secondary side heat sink;

D: output filter coil;

E: output filtercapacitors.  

Thank you