M2 3 Introduction to Switching Regulators. 1. What is a switching power supply? 2. What types of switchers are available?

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1 M2 3 Inroducion o Swiching Regulaors Objecive is o answerhe following quesions: 1. Wha is a swiching power supply? 2. Wha ypes of swichers are available? 3. Why is a swicher needed? 4. How does a swicher operae in general? 5. How does a buck converer operae? Ouline 1. Swiching Regulaor Overview Wha is a Swiching Regulaor? Why is a swicher needed? Wha are he main differences beween a swiching and linear regulaor? Buck, Boos, Buck Boos (Invering) 2. Swiching Regulaor Componens 1

2 Wha is a Swiching Regulaor? Swiching Regulaor Convers an inpu volage ino desire oupu volage. The power ransisor operaes as a swich, compleely on or off. An energy sorage par (inducor) is used in he archiecure Choosing Beween Linear and Swiching Regulaors When possible, mos designers would prefer o use a linear volage regulaor raher han a swiching volage regulaor Linear regulaors are usually lower in price Linear regulaors are usually simpler o implemen Linear regulaors l do no have associaed id noise/ripple problems apparen in swiching regulaors 2

3 Choosing Beween Linear and Swiching Regulaors When o use a swiching regulaor #1: When he minimum inpu volage is a or below he desired oupu volage Linear regulaors canno provide an oupu volage greaer han he inpu volage < Choosing Beween Linear and Swiching Regulaors When o use a swiching regulaor #2: The hea sink of a linear regulaor is prohibiive in price or space 3

4 Choosing Beween Linear and Swiching Regulaors When o use a swiching regulaor #3: The efficiency of a linear regulaor canno mainain he juncion emperaure below he specified maximum The maximum juncion emperaure is usually 150C The efficiency of linear regulaors ofen prohibi heir use in high volage, high curren applicaions Why are swiching regulaors needed? The power dissipaion is oo high for a linear regulaor The efficiency of a linear regulaor canno mainain he juncion emperaure below maximum (150 C) The hea sinking of a linear regulaor is prohibiive in price or space Oupu Power Swiching Regulaor Linear Regulaor Linear Regulaor Maximum Power Dissipaion 4

5 Why are swiching regulaors needed? The desired oupu volage is greaer han he inpu volage 1.5 Baery Power Supply 5 Required The desired oupu volage is opposie polariy han he inpu volage 12 Baery Power Supply -12 Required Types of Swiching Regulaors AC DC, AC AC, DC AC, and DC DC Converers AC-DC AC-AC DC-AC DC-DC 110 ac 110 ac 12 dc 12 dc 12 dc 220 ac 110 ac 5 dc 5

6 Types of DC DC Converers Sep Down, Sep Up and Invering in = 12 Sep Down Buck ou = 5 in = 5 Sep Up Boos ou = 12 in = 5 Invering Buck-Boos ou = -10 Basic Circui Configuraion Buck > Boos < Buck-Boos < - < I SW GATE I L L I L GATE I SW L C C C GATE I I L SW L All opologies consiss of he same basic componens bu are arranged differenly 6

7 Buck Configuraion GATE I SW I L ime L C ime The inpu volage is always greaer han he oupu volage Boos Configuraion ime L GATE I L I SW C ime The inpu volage is always less han he oupu volage 7

8 Buck Boos Configuraion IN 20 GATE I SW 0 ime L I L C ime The inpu volage is no consrained by he oupu volage Oher Swiching olage Regulaor Topologies SEPIC Push Pull and Forward Converer Flyback Converer 8

9 2. Swiching Regulaor Componens Swiching Power Supply Nework Swich Nework PWM Conroller Error Amplifier Bandgap Reference Swiching Power Supply Block Diagram Error Amplifier The error amplifier deermines if is valid is divided down and compared o he reference volage OUT R2 R R 1 2 R 1 DI R 2 REF PWM Conroller 9

10 PWM Conroller In a swiching volage regulaor, he pass ransisor is used as a swich i is eiher on or off The oupu volage, however, is an analog value PWM conroller senses error in via he error amplifier PWM conroller updaes he duy cycle of he of ransisor adjusing he oupu volage Error Amplifier PWM Conroller 0-100% Swiching Transisor Bipolar and MOSFET Bipolar Base Collecor Gae MOSFET Drain Emier Swich Speed Slow Fas Drive Mehod Curren olage Source Drive Circui Complex Simple ESD Robusness High Low 10

11 Exernal Nework An exernal nework (consising of an inducor, capacior, and diode) ransforms he energy from he PWM conrolled power swich ino a desired oupu volage Swich Nework = 12 = 5 Recirculaion Diode S IN S GND 11

12 Recirculaion Diode A Schoky diode s reverse recovery ime (swiching from a forward dbias sae o a reverse bias sae) is very fas This fas reverse recovery ime minimizes he power loss in he recirculaion pah The recirculaion diode mus have a sand off volage (break down volage) higher han he maximum posiive volage seen a Efficiency of Sep Down Swiching Regulaors Calculaing he efficiency of sep down swiching regulaors is raher complex Power loss occurs in he swiching regulaor, inducor, recirculaion diode, inpu capacior, and oupu capacior Mos swiching volage regulaor daashees have ypical efficiency curves o illusrae heir expeced efficiency P = P +P OUT OUT DISSIPATED 12

13 Example Efficiency Curve 95% 90% 85% 80% = 5 I LOAD = 120mA 75% 80 ma 70% P = P +P OUT OUT DISSIPATED 40 ma 65% Inpu olage () Summary Swiching volage regulaors conver a non useful inpu volage o a useful oupu volage When in regulaion, he power ransisor is eiher compleely on or compleely off The duy cycle of he power ransisor is varied o regulaed he oupu volage Sep down swiching volage regulaors are used for high h efficiency i conversion Sep up swiching volage regulaors are used because linear regulaors canno perform a sep up conversion 13