In this Session • • • • • • DC-DC Switch-Mode Converters Applications: • Regulated switch mode dc power supplies • dc motor drives DC-DC power conversion Linear Vs Switch based Control of DC-DC converters Characteristics of Buck Inductor effects Design Considerations dc-dc Converters: • Step-down (buck) converter • Step-up (boost) converter • Step-down/step-up (buck-boost) converter • Cuk converter • Full-bridge converter 7-1 7-2 1 Power Conversion Functional Block Diagram of DC-DC Converter System • Analogous to Transformer: – Linear System – Switching System • Linear systems can only step down Controlled dc output at a desired voltage level Unregulated dc voltage obtained by rectifying the line voltage, and therefore will fluctuate with line voltage magnitude 7-3 – Bipolar transistor or MOSFET pass unit operated in linear mode; i.e. Drive to pass unit is proportionally changed to maintain the output voltage – Dissipates considerable power (Vdrop Iload) 7-4 2 Linear Power Supplies Linear Regulator • Headroom Loss causes linear regulator to be 35-65% efficient • Has to consider a costly heat sink: eg. If a 5V regulator has 12V input and supplying 100mA, it must dissipate 700mW to deliver 500mW to the load (42% efficient) • But still cost effective in step down applications for <10W applications (exception: low drop-out regulator) • Linear regulators are simple and cheap; considerable quieter as no HF switching 7-5 7-6 3 7-7 7-8 4 Control of DC-DC Converters In a dc-dc converter: • Average output dc voltage must be controlled to equal a desired level. • Utilizes one or more switches to transform dc from one level to another. • The average output voltage is controlled by controlling the switch on and off durations (ton and toff). • Let’s consider the following switch-mode dc-dc converter: • • • 7-9 Average output dc voltage Vo depends on ton and toff. Switching is done at a constant frequency with switching time period Ts. This method is called pulse-width modulation (PWM) in which the duty ratio D is varied to control Vo, where D=ton/Ts 7-10 5 PWM Control of DC-DC Converters (cont’d) • • • The switch control signal, which controls the on and off states of the switch, is generated by comparing a signal level control voltage vcontrol with a repetitive waveform. The switching frequency is the frequency of the sawtooth waveform with a constant peak. The duty ratio D can be expressed as D t on vcontrol ^ Ts V st 7-11 7-12 6 Switching Power Supplies Buck (Forward Mode) Converter • Two basic types of PWM switching power supplies: – Forward Mode – Boost Mode • Forward mode converter is recognised by the presence of an L-C filter on its output. • The L-C filter creates a DC output voltage, which is essentially the Volt-time average of L-C input AC rectangular waveform 7-13 • Most applications need a reasonably steady DC voltage • Linear Regulators have large fluctuations (0 - +Vin) • So need a filter for energy recovery • Buck DC-DC converter is a step-down converter with a filter (forward mode) 7-14 7 • Assumptions: – Inductor current is continuous – Average inductor voltage is zero – Average capacitor current is zero – Ideal components – Filter capacitor is large 7-15 7-16 8 Analysis 7-17 7-18 9 7-19 7-20 10 Maximum, Minimum and Average Inductor Current 7-21 7-22 11 Output Voltage Ripple 7-23 7-24 12 Step-Down (Buck) Converter - Summary • • • • • • converts dc from one level to another the average output voltage is controlled by the ON-OFF switch pulse-width modulation (PWM) switching is employed lower average output voltage than the dc input voltage Vd depending on the duty ratio, D D=ton/Ts Average output: V0 Ts 1 Ts 1 ton v0 t dt Vd dt 0 dt Ts 0 Ts 0 t on ton Vd DVd Ts • • Applications: • • regulated switch mode dc power supplies • • dc motor drives • 7-25 low-pass filter: to reduce output voltage fluctuations diode is reversed biased during ON period, input provides energy to the load and to the inductor energy is transferred to the load from the inductor during switch OFF period in the steady-state, average inductor voltage is zero in the steady-state, average capacitor current is zero 7-26 13 Concept Quiz Step-Down (Buck) Converter: Continuous current conduction mode • • Inductor current iL flows continuously Average inductor voltage over a time period must be zero Ts t on Ts v dt v dt v dt 0 L 0 L 0 L t on Area A and B must be equal , therefore , Vd V0 ton V0 Ts ton or V0 ton D Vd Ts duty ratio Assuming a lossless circuit Vd I d V0 I 0 and I 0 Vd 1 I d V0 D Buck converter is like a dc transformer where the turns ratio can be controlled electronically in a range of 0-1 by controlling D of the switch 7-27 In a Buck converter operating in steady state, the input voltage is 15 V, the output voltage is 12 V and the output power is 60 W. Calculate the average value of the current from the input voltage source. A. 5 A B. 4 A (correct) C. 3 A 7-28 14 7-29 7-30 15 PSpice Modeling: Simulation Results 16 12 8 4 0 -4 -8 450us I(C1) 455us 460us I(L1) V(L1:1,L1:2) 465us 470us 475us 480us 485us 490us 495us 500us Time 7-31 7-32 16 • Following parameters are needed to select an inductor: An Example – Max input voltage – Output voltage – Switching frequency – Max ripple current – Duty Cycle 7-33 7-34 17 Example….. Solution…. For a buck converter, R=1 ohm, Vd=40 V, V0=5 V, fs=4 kHz. Find the duty ratio and “on” time of the switch. D = V0 /Vd = 5/40 = 0.125 = 12.5% Ts = 1/fs = 0.25 ms = 250 ms Ton = DTs = 31.25 ms Toff = Ts – ton = 218.75 ms When the switch is “on”: VL = Vd - V0 = 35 V When the switch is “off”: VL = -V0 = - 5 V I0 = IL = V0 / R = 5 A Id = D I0 = 0.625 A 7-35 7-36 18 Example 2 Solution 7-37 7-38 19 Summary Buck (Step-Down DCDC) Converters Buck Disadvantages • Major disadvantage is its topology only – Buck is not an isolated topology; hence for safety reasons the forward converter (Buck) cannot be used for voltages greater than 42.5 VDC – If isolated BUCK – 60-200 VDC 7-39 • Buck (Step-Down DC-DC) Converters – Applications – Operation in Steady State 7-40 20 Quiz In a Buck converter operating in steady state, the input voltage is 20 V, the output voltage is 12 V and the output power is 60 W. The switching frequency is 200 kHz. It is designed such that the peak-peak ripple in the inductor current doesn’t exceed 2 A. Calculate the minimum value of the inductance that must be used. A. 12.0 micro-henries (correct) B. 6.0 micro-henries C. 3.6 micro-henries 7-41 21

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