# 405_Switch_Mode_Converters

Flyback Converters Boost Converter • Store energy in an inductor (choke) before transferring the energy to the load and output capacitor • While energy is being transferred to the inductor, load energy is provided by the output capacitor • Two basic versions: – Boost (same polarity) – Buck-Boost (output V polarity inversion) 7-1 • Essentially same components as Buck – arranged differently • ‘Boost’ cause V > Vg 7-2 1 Topology BOOST CONVERTER SWITCHING ANALYSIS IN DC STEADY STATE iL iL Vo C vL p q C vL Vin Vin p q (a) Vo (b) Figure 3-7 Boost dc-dc converter. 3 7-3 • When switch closed, diode reversed, output isolated, inductor charging • When switch open, output receives energy from input and the inductor – hence Boost • Large C maintains the output V constant 7-4 2 Analysis 7-5 7-6 3 7-7 7-8 4 Steady State Inductor Current 7-9 7-10 5 Step-Up (Boost) Converter • • • • Output voltage always higher than the input voltage When the switch is ON: diode is reversed biased output circuit is thus isolated inductor is charged When the switch is OFF: the output stage received energy from the inductor as well as from the input Filter capacitor is very large to ensure constant output voltage Applications: • regulated switch mode power supplies • Regenerative braking of dc motors 7-11 7-12 6 Step-Up (Boost) Converter: Continuous current conduction mode • • Boost converter: operation and waveforms Inductor current iL flows continuously Average inductor voltage over a time period must be zero q t 0 vL Vin Vd ton Vd V0 toff 0 Vin vA 0 iL Vo vA v A Vin Vo vL A Vin t q 1 V0 T 1 s Vd toff 1 D (Vo Vin ) (a) B t 0 iL IL 0 Vo Assuming a lossless circuit Vin Vd I d V0 I 0 vL Vin Vo v A Vo idiode iL iL iL ,ripple iL Vo Vin t 0 0 Dividing both side by Ts Vo 1 Vin 1 D t I diode ( I o ) t 0 Vin V V DTs o in (1 D )Ts L L Vin I in Vo I o I L I in Vo I 1 Vo Io o Vin 1 D 1 D R iC (t ) idiode,ripple (t ) idiode I o iC t 0 and q0 I0 1 D Id ( I0 ) (b) (c) Figure 3-8 Boost converter: operation and waveforms. 7-13 7-14 7 PSpice Modeling: Concept Quiz In a Boost converter operating in a continuous-conduction mode, increasing the duty-ratio decreases the output voltage to the input voltage ratio. A. False (correct) B. True 7-15 7-16 8 Simulation Results Boost converter: voltage transfer ratio 15 10 Vo Vin 5 0 1 1 D -5 1 -10 IL 0 DCM -15 1.950ms I(L1) 1.955ms V(L1:1,L1:2) 1.960ms 1.965ms 1.970ms 1.975ms 1.980ms 1.985ms 1.990ms 1.995ms 2.000ms I L ,crit CCM Figure 3-10 Boost converter: voltage transfer ratio. Time 7-17 7-18 9 Capacitor - Output Voltage Ripple Step-Up (Boost) Converter: Effect of parasitic elements • • • Parasitic elements are due to the losses associated with the inductor, capacitor, switch and diode Figure shows the effect of the parasitic elements on the voltage transfer ratio Unlike ideal characteristics, in practice, Vo /Vd declines as duty ratio approaches unity • • • 7-19 Choose L such that ripple magnitude is obtained Choose C that desired voltage ripple is obtained In practise, capacitor equivalent series resistance leads to increased voltage ripple 7-20 10 Inductor Selection 7-21 11

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