Switch Mode Power Supply (SMPS) Topologies (Part II) - Microchip

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AN1207FIGURE 5:BUCK CONVERTER WAVEFORMSQ1 Commandt(A)VDC + VD, onVQ1t(B)I2I1IQ1t(C)VD1t(D)(-VDC + VQ, on)I2I1ID1t(E)VDC - VOUTVLAt(F)B-VOUTI2I1ILt(G)TONTOFFT(A) = Command signal and MOSFET gate(B) = Voltage and MOSFET(C) = Current flowing into MOSFET(D) = Voltage on D1 diode(E) = Current in D1 diode(F) = Voltage on LO inductor(G) = Current in LO inductorDS01207B-page 4© 2009 <strong>Microchip</strong> Technology Inc.
AN1207Supposing the output load RO (connected in parallel tothe output capacitor CO) changes by increasing, thischange has the effect of reducing the average outputcurrent. As shown in Figure 6, current moves from lineA for the nominal load, to line B for a larger load. Whatshould be noted is that the slopes of the two ramps,both during TON and TOFF, do not change because,they only depend on VDC, VOUT and L, and they havenot been changed. As a consequence, increasing theload results in RO becoming greater. Since VO equalsconstant (the control loop explained earlier handlesthis) and RO increases, the current diminishes.FIGURE 6:VLINDUCTOR CURRENT AT DIFFERENT LOADSTTONTOFFIncreasing load(reducing IO, av)ABCDCONTINUOUS MODEOperating in the Continuous mode is so named sincethe current in the inductor never stops flowing (goes tozero).As shown in Figure 6, if the load continues to increase(reducing IO, av), at some time the inductor current plotwill touch the x-axis (line C). This means the initial andfinal current (at the beginning and the end of the switchingperiod) in the inductor is zero. At this point, theinductor current enters what is considered as Criticalmode.If the load is further increased, the current during thedown-ramp will reach zero before the end of the periodT (line D), which is known as Discontinuous mode.Note:In Discontinuous mode, the only way tofurther decrease the inductor current is toreduce the ON time (TON).One key point is that the inductor current at the end ofthe TOFF period must equal the inductor current at thebeginning of the TON period, meaning the net change incurrent in one period must be zero. This must be trueat Steady state, when all transients have finished, andthe circuit behavior is no longer changing.TONUsing the value of IL(TON) derived from Equation 3 andEquation 5 creates the relationship shown inEquation 8.EQUATION 8:ΔI LV DC– V Qon ,– V OUT∝ ( )T =( ONV OUT+ V )T D,on OFFNeglecting VD, on and VQ, on, Equation 8 can besolved for VOUT, as shown in Equation 9.EQUATION 9:V OUT= V DCDwhere D = Ton / T (duty cycle), orV OUTD = -------------V DCThe maximum duty cycle is achieved when the inputvoltage is at its minimum, as shown in Equation 10.EQUATION 10:D max=V-------------------- OUTV DC,mintTherefore, D must obviously be between ‘0’ and ‘1’.© 2009 <strong>Microchip</strong> Technology Inc. DS01207B-page 5
Page 1 and 2: AN1207Switch Mode Power Supply (SMP
Page 3: AN1207FIGURE 4:VLINDUCTOR BEHAVIORI
Page 8: AN1207OUTPUT CAPACITORThe current r
Page 13 and 14: AN1207MOSFET selectionThe key param
Page 15 and 16: AN1207EQUATION 50:V OUTwhere D is t
Page 19 and 20: AN1207Q1 ON (INTERVAL 0 - TON)For t
Page 21 and 22: AN1207Input Circuit BehaviorBefore
Page 23 and 24: AN1207TRANSFORMER: PRIMARYThe core
Page 25 and 26: AN1207FIGURE 22:FORWARD CONVERTER W
Page 27 and 28: AN1207EQUATION 96:( - )------------
Page 29 and 30: AN1207FIGURE 24:FORWARD CONVERTER W
Page 31 and 32: AN1207Q1 ON, Q2 ON (INTERVAL 0 - TO
Page 33 and 34: AN1207Q1 OFF Q2 OFF (INTERVAL (TON
Page 35 and 36: AN1207FIGURE 30:TWO-SWITCH FORWARD
Page 37 and 38: AN1207DIODESTable 2 provides calcul
Page 39 and 40: AN1207HALF-BRIDGE CONVERTERDesign E
Page 41 and 42: AN1207Q1 OFF, Q2 ONIn this configur
Page 43 and 44: AN1207FIGURE 39:HALF-BRIDGE CONVERT
Page 45 and 46: AN1207Design Equations and Componen
Page 47 and 48: AN1207PUSH-PULL CONVERTERThe Push-P
Page 49 and 50: AN1207Q1 OFF, Q2 ONIn this configur
Page 51 and 52: AN1207TRANSFORMER: PRIMARY, NUMBER
Page 53 and 54: AN1207TRANSFORMER: SECONDARY, NUMBE
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AN1207Switching LossesFigure 48 plo
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AN1207FULL-BRIDGE CONVERTERA Full-B
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AN1207Q1 ON, Q4 ON; Q2 OFF, Q3 OFF(
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AN1207Design Equations and Componen
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AN1207TRANSFORMER: SECONDARY, NUMBE
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AN1207OUTPUT INDUCTORThe minimum in
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AN1207Q1 ON (INTERVAL 0 - TON)Figur
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AN1207Design Equations and Componen
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AN1207FIGURE 61:FLYBACK CONVERTER T
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AN1207Topology Equations - Continuo
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AN1207TRANSFORMER WINDINGS TURN RAT
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AN1207FIGURE 64:BUCK CONVERTER - BA
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AN1207FIGURE 67:VOLTAGE MODE CONTRO
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AN1207ADVANTAGES AND DISADVANTAGES
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AN1207PROBLEMSAs seen in Figure 71,
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AN1207Other Current Mode Techniques
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AN1207Control TheoryUp to this poin
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AN1207The GM(s) transfer function i
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AN1207bus and the address generatin
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AN1207The block diagram now is as s
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AN1207Behavior of the PIDAs can be
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AN1207The voltage from the ADC is s
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AN1207TABLE 5:FrequencyFIGURE 90:FR
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AN1207FIGURE 91:PROGRAM FLOWMAINIni
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AN1207ADC Interrupt Service Routine
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AN1207APPENDIX A:SOURCE CODESoftwar
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Note the following details of the c
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Switch Mode Power Supply (SMPS) Topologies (Part II) - Microchip

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