Wide ouput range power supply Armond Gauthier Pierre Yves Droz
I Introduction I
Goal / Constraints of the project Offline power supply. Constraints: - cheap - wide output range application : Power supply compatible with a wide range of devices circuit good point : power loss is not a big issue for offline applications, it is just limited by the heat that the box can dissipate. I
Specifications DC Input Voltage Max Power Output Current Limit Output Voltage Output Ripple Ratio (I and V) PWM Frequency 260 V 390 V 0 W 10 A 2. V 30 V ±1% 200 khz 10 9 I circuit I = D 1 V out 1 L f We want to limit L to 1mH f = 200 khz Iout 8 7 6 4 3 2 Output range 1 0 0 2. 7. 10 12. 1 17. 20 22. 2 27. 30 Vout max power
Exploring the different topologies Assumption : Minimizing the range in which D varies will help us to reduce power dissipation. 2 1.8 1.6 1.4 Vout/Vin 1.2 1 0.8 0.6 D (Buck) D2 (Buck/Buck) D/(1-D) () D2/(1-D) (/Buck) I 0.4 0.2 0 0 0.1 0.2 0.3 0.4 0. 0.6 0.7 0.8 0.9 1 D
D 2 0.010 < D < 0.3 D 2 D 1 0.26 < D < 0.93 with N=7 0.010 < D < 0.30 0.36 < D < 0.77 with N=20 I
Buck I
power dissipation Transistor Diodes Transformer Inductors Capacitor 2 W (23 W switching) 6 W 40 W 2 W neglectable I Transistor P diss = t sw V ds I sw f C V 2 f gd ds 2 Transformer P diss = 1 2 L mag I 2 f Diodes P diss = V drop I
Metrics of Comparison Assume: Cost follows power dissipation Package and component size Cost follows switch stress Why? I Small parts count, single transistor Cheap! DC isolation DCM operation for low power application
Switch Utilization Starting Methodology Switch stress: S = V I Center D variation around max U(D) Switch utilization: U(D) = P load S Bias voltage associated with max U(D) towards lower end of output range Governing formula: U(D) = 1 D D I
converter Governing equations: Vout Vin = n D 1 D Iout Iin = 1 D n Transistor on Transistor off I
Implementation 1 n.049(20 turns on primary side) D range.12 -.70 U(D) range.30 (low D) -.21 (high D) Transistor stress I MAX = 681mA, V MAX = 990V, S = 67 W Diode Stress I MAX = 23.9A, V MAX = 49.1V, S = 1173 W Required Inductance 4.73 mh High V MAX : I Look at flyback input º Transistor must block stacked voltage V in + V out / n Conclusions : Look at n equation N proportional to V D,opt To decrease transistor blocking voltage, increase V D,opt and n. n= V D,opt V i,nom. 1 D opt D opt
Implementation 2 n.07(13 turns on primary side) D range.079 -.606 U(D) range.260 (low D) -.306 (high D) Transistor stress I MAX = 942mA, V MAX = 790V, S = 744 W Diode Stress I MAX = 22.6A, V MAX = 63.8V, S = 1438 W Required Inductance 3.24 mh Overall max power dissipation 43.0 W Conclusions: Sacrificing?S in favor of reducing peak transistor voltage advantageous Smaller, cheaper device with lower loss U(D) just a general metric, not an end in itself I How to further push down peak transistor voltage and power dissipation? Cannot reduce D MIN too far º too much current stress Try to reduce?d...
Multiple secondary windings Each winding will supply a portion of total output range Will reduce D, but will require switching between windings Questions: How to divide output range among secondary windings? How to optimize number of secondary windings? I
Optimizing ranges Will minimize D if:? M D =Constant n If output range divided equally, low secondary has largest? D Constant { I Advantages? D equal for all ranges U(D) held closer to optimum Reduce peak transistor voltage Disadvantage Look at transformer inductance required for current ripple versus D º
Implementation 3 Range 1: 2.V < Vout <.V n=.027 Range 2:.V < Vout < 12.V n=.061 Range 3: 12.V < Vout < 30.0V n=.147 D range.179 -.440 U(D) range.347 (low D) -.371 (high D) Transistor stress Diode Stress * Required Inductance I MAX = 780mA, V MAX = 9V, S = 464 W I MAX = 27.1A, V MAX = 87.3V, S = 2366 W 8.14 mh Overall max power dissipation 23.8 W I * Using single diode and capacitor at output high diode stress Pdiss 11 10 9 8 7 6 4 3 2 1 0 0 0.2 0. 0.7 1 1.2? 1. 1.7 M(D) / n 2 2.2 2. 2.7 3 3.2 3.
Switches on DC side Advantages: Smaller diodes required º cheaper Less power lost per diode Disadvantages: Multiple diodes and output capacitors Possible problem with unloaded output I Switches on transformer side Advantages: Single diode and output capacitor required Output never unloaded Disadvantages: Large required diode Switches must block AC voltage