Analog and Telecommunication Electronics

Transcription

1 Politecnico di Torino - ICT School Analog and Telecommunication Electronics G3 - Switching regulators» PWM regulators» Buck,» Boost,» Buck-boost» Flyback 30/05/ ATLCE - G DDC

2 Lesson G3: Switching power supply PWM regulators Switching PSU/regulators Buck, Boost, Buck-boost Isolated flyback power supply 30/05/ ATLCE - G DDC

3 PWM amplifiers and regulators Power delivered through a switch ON full power; OFF no power Control achieved by changing the ON/OFF duty cycle Requires low-pass filter on the load» Many loads have mechanical parts, with intrinsic lowpass response Efficiency of the control element Ideal switch no power dissipation Actual devices losses from Ron, Ioff, transients Applications Amplifiers: class D, E,. Power supply systems: switching PSU and regulators 30/05/ ATLCE - G DDC

4 Switch-mode power control Switching regulator or amplifier (class D) Output voltage is controlled by the duty-cycle of the switch Uses an averaging circuit on the load to smooth the output 30/05/ ATLCE - G DDC

5 LC storage/averaging circuit Squarewave requires two switches Replace a SW with a diode + I I The inductor L tries to keep constant current The diode provides current path when SW is open 30/05/ ATLCE - G DDC

6 Switching regulator with feedback Feedback loop: f = z(or i); f = or2/(r1+r2) o = i(r1+r2)/r2 Switching power control o is compared with reference voltage z 30/05/ ATLCE - G DDC

7 Switching voltage regulators DC DC converters, with various topologies: Buck out < in Boost out > in Buck-Boost inverted polarity Benefits No high dissipation as in linear regulators Losses only from parasitic R, L, C Usually high efficiency (close to 1) Drawbacks Output ripple EMI generation 30/05/ ATLCE - G DDC

8 Review of inductor behavior Ideal inductor No current step Rs= 0 Driven by L (t) L + I L I L I L 1 L t L L T L dt t step response I ramp L I L Periodic squarewave driving voltage (t) Triangular current I(t) L I L 30/05/ ATLCE - G DDC

9 Buck regulator Switches SW1 and SW2 complementary commands, ON duty cycle D for SW1, ON duty cycle 1-D for SW2 SW1 out < in L D 1-D in SW2 L out SW1 ON SW2 OFF SW1 OFF SW2 ON 30/05/ ATLCE - G DDC

10 Buck regulator, ON state SW1 closed, current flows from in to out through L Current in the ON state ON state D I A L I L I L L D 1-D in 1-D I B out I A I B I IN L OUT T ON Current change in the ON state 30/05/ ATLCE - G DDC

11 Buck regulator, OFF state Current change in the OFF state I OFF state OUT T L OFF D Current in the OFF state L I L I L L D 1-D in I A 1-D I B out I A I B I IN L OUT T ON OUT T L OFF Current change in ON and OFF states is balanced; the transfer ratio corresponds to the switching duty cycle OUT IN D 30/05/ ATLCE - G DDC

12 Buck regulator final diagram SW2 is replaced by a diode SW1 closed Diode has reverse bias (SW2 OFF) SW1 open The inductor keeps constant current, which can flow only through the diode (SW2 ON) SW1 out < in in D L out 30/05/ ATLCE - G DDC

13 Buck regulator efficiency Ideal SW and diode no loss Efficiency = 1 Real circuit Diode losses: d, (Is) SW losses: Ron (Ioff) Transition losses L losses (series R, magnetics) C losses (ESR) Efficiency < 1» Related with SW frequency Similar parameters for all SW regulators 30/05/ ATLCE - G DDC

14 Boost regulator Invert SW1 and inductor from Buck Similar operation and analysis Output voltage is higher than input: out > in Use diode as SW2 L 1-D in out D 30/05/ ATLCE - G DDC

15 Boost regulator, ON state ON state L Current in the ON state 1-D L I L D 1-D in I L D I A I B out I A I B I L IN T ON Inductor Current change during ON phase 30/05/ ATLCE - G DDC

16 Boost regulator, OFF state SW open: L=-(o i); IL is a ramp OFF state L Current in the OFF state 1-D L I L D 1-D in I L D I A I B out I A I B L IN T ON OUT L IN T OFF The transfer ratio is controlled by the duty cycle OUT IN 1 1 D 30/05/ ATLCE - G DDC

17 Boost - continuous current mode (CCM) No voltage drop on SW and D SW closed: L = I ; linear increase of I L SW open: current l L decreases and flows through D L I u i C i S C u R L u D 30/05/ ATLCE - G DDC

18 CCM analysis SW closed: il(t) = Ia+t I /L At T1:» Ib = Ia+T1I /L» Ib Ia=T1I /L SW open: il(t) = Ib+t (-U + I ) / L At T2:» Ia = Ib -T2 (U i ) / L Ib Ia = T2 (U i ) / L v L i i - u i L I b I a t U = (T I 1 +T 2 ) /T U / I = 1 / (1- D) T on T 1 T off T 2 t T sw 30/05/ ATLCE - G DDC

19 Input and outpu current Input current I S = I L flows when the switch SW is closed Input current I i is the average of I L Capacitor current Ic corresponds to I L Output current Current flows in the diode when the switch SW is open Output current I u corresponds to average value of diode current. 30/05/ ATLCE - G DDC

20 Buck-boost regulator Combines buck and boost topologies Generates inverted output voltage D 1-D in out L 30/05/ ATLCE - G DDC

21 Buck-boost regulator, ON/OFF state ON PHASE OFF PHASE D 1-D L I L D 1-D in I A I L L I B out I A I B L IN T ON L OUT T OFF Negative output voltage, controlled by the duty cycle OUT IN D D 1 30/05/ ATLCE - G DDC

22 Switching regulator design Select L Select C Select switches (MOS and Diode) Operating frequency Efficiency 30/05/ ATLCE - G DDC

23 EMI and noise Buck topology Input current is pulsed» Noise injected towards the mains Output current is smooth» Low EMI towards load Boost topology Input current is smooth» Low interferences at the input. Output current is pulsed» Noise injected to the load» Stress of the output capacitor 30/05/ ATLCE - G DDC

24 Protection with switching regulators Buck topology The switch can break the path from input to output Short-circuit at the output» Possible current limit protected Input overvoltage» The input switch receives overvoltage spikes no protection. Boost topology The switch can break the input mesh to GND Short-circuit at the output» Continuous path from in to out, no current limit no protection Input overvoltage» Input overvoltage spikes filtered by LC network protected 30/05/ ATLCE - G DDC

25 Discontinuous Current Mode In DCM, I = 0 during Tidle Three time slots Ton, T2, Tidle Same analysis: I Ton /L = -ut2 /L U /I = -Ton / T2 T2 unknown 30/05/ ATLCE - G DDC

26 Fly-back PSU Basically a buck-boost converter, with the inductor replaced by a transformer. Provides galvanic isolation. Regulation requires isolated feedback (photocoupler). S D C i C u R L u 30/05/ ATLCE - G DDC

27 Multiple output flyback Input switch can be low-side Multiple (isolated) outputs Feedback loop for regulation D 1 C u1 R L1 u1 i C i L p S D 2 C u2 R L2 u2 30/05/ ATLCE - G DDC

28 Actual diagram Example of switching regulator 30/05/ ATLCE - G DDC

29 Block diagram of switching PSU EMI Filter Mains to DC Switching voltage regulator DC to AC (HF small isolation transformer) AC to DC Feedback for regulation 220 SW REG out = 50 Hz 500 khz 30/05/ ATLCE - G DDC

30 Power Factor Correction 30/05/ ATLCE - G DDC

31 Lesson G3 final test Describe the PWM regulation technique Draw the block diagram of a PSU with switching regulator, and describe the function of each unit. Describe the difference between linear and switching regulators. Draw the block diagram of the two basic switching regulator topologies. Which parameters influence the output ripple in switching PSU? Which parameters influence efficiency? Describe criteria to select the active and passive devices in the power section of a switching regulator. Describe operation of the flyback regulator. 30/05/ ATLCE - G DDC