AN-776 Application Note 776 20 Watt Simple Switcher Forward Converter

20Watt Simple Switcher Forward ConverterA 20W,5V at 4A,step-down regulator can be developed using the LM2577Simple Switcher IC in a forward converter topology.This design allows the LM2577IC to be used in step-down voltage applications at output power levels greater than the 1A LM2575and 3A LM2576buck regula-tors.In addition,the forward converter can easily provide galvanic isolation between input and output.The design specifications are:V i Range 20V–24V V o 5V I o(max)4A ∆V o20mVWith the input and output conditions identified,the design procedure begins with the transformer design,followed by the output filter and snubber circuit design.Transformer Designing the maximum switch voltage,input voltage,and snubber voltage,the transformer’s primary-to-clamp wind-ings turns ratio is calculated:V SW ≥V imax +V imax (N p /N c )+V snubber N p /N c ≤(V SW −V imax −V snubber )/V imax N p /N c ≤(60V −24V −5V)/24V =1.29∆let N p /N c =1.25The V snubber voltage is an estimate of the voltage spike caused by the transformer’s primary leakage inductance.2.The duty cycle,t on /T,of the switch is determined by the volt-second balance of the primary winding.During t on ;V i =L p (∆i/T ON )→∆i =(V i/L P )t on During t off ;V i =(N p /N c )=L p (∆i/t off )→∆i =(N p /N c )(V i /L p )t off Setting ∆i’s equal;(V i /L p )t on =(N p /N c )(V i /L p )t off t on /t off =N p /N cSince D=t on /T =t on /(t on +t OFF )max.duty cycle =D max =(N p /N c )/[(N p /N c )+1]D max =(1.25)/(1.25+1)=0.56(56%)3.The output voltage equations of a forward converter pro-vides the transformer’s secondary-to-primary turns ratio:V o +V diode ≤V imin x D max (N s /N p )N s /N p ≥(V o +V diode )/(V imin x D max )N s /N p ≥(5.5V)/(20V)(56%)=0.49∆let N s /N p =0.54.Calculate transformer’s primary inductance by finding the maximum magnetizing current (∆i Lp )that does not allow the maximum switch current to exceed it’s 3A limit (capital I for DC current,∆i for AC current,and lower case i for total current):i sw =i pri =i Lo’+∆i LpBasic Forward ConverterNational Semiconductor Application Note 776Frank DeStasi Tom Gross April 199820Watt Simple Switcher Forward ConverterAN-776©2002National Semiconductor Corporation Transformer Design(Continued)2Output Filter—Capacitor(Continued)∆V o=0.05Ω•1.2A)60mVTo get output ripple voltage of20mV or less(as was part of the design specs)requires a capacitor with ESR of less than 17mΩ.Snubber CircuitA snubber circuit(C S,R S,D S)is added to reduce the voltage spike at the switch,which is caused by the transformer’s leakage inductance.It is designed as follows:when the switch is off,V R=V CE−V IN−V DV LL=V D+V R−V IN(N p/N c)Substituting for V R,the voltage across the leakage induc-tance,V LL,is,V LL=V CE−V IN(1+N p/N c)Using the current-voltage relationship of inductors,t S=I PRI(L L/V LL)Substituting for V LL,t S=I PRI L L/(V CE−V IN(1+N p/N c)) Calculating for the average leakage inductance current,I L-L(AVE),I LL(AVE)=I PRI(MAX)(t S)/2T=I PRI(MAX)2L L f/2(V CE−V IN(1+N p/N c))Solving for the snubber resistor;R S=V R/I LL(AVE)Substituting I LL(AVE)and V R results in,R S=2(V CE−V IN(1+N p/N c))X(V CE−V IN−V D)/(L L(I PRI(MAX))2f)Choosing L L to equal10%of L p,R S=2(65V−24V−1V)X(65V−24V(2.25))/(7µH(3A)252kHz)=268.9Ω)270ΩUsing the current-voltage relationship of capacitors,∆V R=(T−t S)I C/C S=(T−t S)V R/R S C S)V R/R S C S f The capacitor C S equates to,C S=V R/R S f∆V RC S=40V/(270Ω)(52kHz)10V=0.28µF)0.33µF The snubber diode has a current rating of1A peak and a reverse voltage rating of30V.Other ComponentsDiodes,D R and D F,used in the secondary are5A,30V Schottky diodes.The same diode type is used for D c,how-ever a lower current diode could have been used.A compensation network of R c and C c optimizes the regula-tor’s stability and transient response and provides a soft-start function for a well-controlled power-up.The finished circuit is shown below.5V,4A Forward Converter Circuit Schematic3Other Components(Continued)01121605Vertical:1A/divHorizontal:5µs/divSwitch Current01121606Vertical:10V/divHorizontal:5µs/divSwitch Voltage01121607Vertical:1A/divHorizontal:5µs/divInductor Current01121608Vertical:20mV/divHorizontal:10µs/divOutput Ripple Voltage01121609A:Output Voltage Change,100mV/divB:Output Current,200mA/divHorizontal:10ms/divLoad Step ResponseAN-7764NotesLIFE SUPPORT POLICYNATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION.As used herein:1.Life support devices or systems are devices orsystems which,(a)are intended for surgical implant into the body,or(b)support or sustain life,and whose failure to perform when properly used in accordance with instructions for use provided in the labeling,can be reasonably expected to result in a significant injury to the user.2.A critical component is any component of a lifesupport device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system,or to affect its safety or effectiveness.National Semiconductor CorporationAmericasEmail:support@ National SemiconductorEuropeFax:+49(0)180-5308586Email:europe.support@Deutsch Tel:+49(0)6995086208English Tel:+44(0)8702402171Français Tel:+33(0)141918790National SemiconductorAsia Pacific CustomerResponse GroupTel:65-2544466Fax:65-2504466Email:ap.support@National SemiconductorJapan Ltd.Tel:81-3-5639-7560Fax:81-3-5639-7507 20 Watt Simple Switcher Forward Converter AN-776National does not assume any responsibility for use of any circuitry described,no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.。