ECE514 Power Electronics Converter Topologies Homework Assignment #4 Due date October 31, 2014, beginning of the lecture Part 1 [100 pts] Redo Term Test 1 (attached) Part 2 [100 pts] Design of an RDC snubber for flyback converter Your task is to design a minimum- volume minimum- power snubber for the flyback converter. The converter is designed to operate in CCM at 200 khz, for the input voltage of 200 V, Vout = 12 V, and the output power of 30 W. Data sheets of the flyback transformer and the power switch are given on the course web site. a) For the case when the 140 : 6 transformer ratio is used, i) draw equivalent circuit of the converter that includes primary side leakage inductance of the transformer Lleak and the output capacitance of the transistor Cdc. ii) In your diagram clearly label values of Lm, Lleak and Cds. b) i) Draw approximate voltage and current waveforms of the transistor and; ii) clearly label: peak inductor current and frequency of the oscillation during turn- off transition c) Now, add an RDC snubber to your circuit (draw it on your diagram) and select the snubber capacitor and its resistor such that: a. The maximum voltage stress across transistor does not exceed 90% of its maximum allowable value (for the transistor with the worst tolerance) b. The variation of the output capacitor voltage does not exceed 10% of its maximum value c. In your diagrams, clearly label the values of the snubber components and calculate power lost on the snubber.
Extra Credit Problems EC 1: For the converter operating without snubber, find expressions for accurate current and voltage waveforms of the transistor after its turn off transient and draw the same. Clearly label: peak voltage and current values, frequency of oscillations, and decay slope of the oscillations (in your calculations you can assume that the resistance of your equivalent circuit is only affected by the dc resistances of the transformer windings). EC 2: Now, assume, that the converter operates in DCM, i) find expressions for accurate current and voltage waveforms of the transistor after the soft turn off the flyback diode and draw your results. Clearly label: peak voltage and current values, frequency of oscillations, and decay slope of the amplitude of oscillations (in your calculations you can assume that the resistance of your equivalent circuit is only affected by the dc resistances of the transformer windings) Note for both EC problems: To calculate the peak values, you can assume that, for the first oscillation cycle only, equivalent resistances have no effect on the amplitude of the waveforms.
Term test 1 Redo Problem 1: The converter of Fig.1 operates in the continuous conduction mode. It operates such that during the first portion of a switching interval 0< t DTs the transistor Q is turned on and the diode D is turned off. During the second portion of the interval Ts = 1/fsw the diode conducts and Q is turned off. The magnetizing inductance Lm is very large and not used for energy transfer. You can also assume that the capacitor values are very large, i.e. that their ripples are negligible. Figure 1 a)[9 pts] i) Draw equivalent circuit of the converter for the both portions of the switching interval Ts. You can neglect all the losses except the forward voltage drop across the diode, VF. ii) ifor the both intervals, also write expressions for the inductor voltages and capacitor currents. b)[10 pts] From the obtained equivalent circuits find expressions for the capacitors voltages, inductor currents, dc conversion ratio M(D)= V/Vg, and the converter efficiency. All expressions should be written in terms of Vg, VF, D, ns/np, and R. c) i)[16 pts] For Vg = 150 V, VF = 0.6 V, and Vout =20 V, fsw= 200kH, select ns/np, L2, and D, such that: 1) the converter operates in CCM for 0.5 A < Iload < 6 A, 2) the maximum voltage at any point of the secondary circuit does not exceed 48V (maximum allowable safety level). ii)[5 pts] Select the inductance L1 value such that its peak-to-peak ripple amplitude is no larger than 20% of the maximum dc current value. iii)[15 pts] For a 50% load (3A) and your design from the previous parts, draw the current and voltage waveforms, clearly label the minimum and maximum values of the voltages and currents. iv) 5 pts] Calculate efficiency of the converter for the 50% of the load, i.e. for Iload = 3 A. How does it change with the load current? Explain your answer.
Problem 2: Your task is to design a flyback converter operating at the boundary between continuous conduction and discontinuous conduction for a wide range of input voltages. To maintain operation at the boundary, the transistor ton time is varied as the input voltage changes, meaning that the switching frequency is varied as well. Figure 2. a) [9 pts] Draw equivalent circuit of the converter for three portions of the switching interval when the converter operates in the discontinuous conduction mode. b) [11 pts] Using the DCM analysis, find the expression for Vout/Vg in terms of Lm, ton, R, and the converter switching frequency fsw, when the converter operates at the boundary between CCM and DCM. For this part, you might want to use the provided diagrams.
c) [10 pts] For 120 < Vg < 366 V, Vout = 20 V, and 0.5A < Iload < 2A, find Lm, n, and range of ton values, such that: i) the voltage on the secondary side does not exceed 48 V, switching frequency of the converter is always larger than 20 khz (above the audio range). d) [10 pts] Using the values from the previous parts, draw voltage and current waveforms for Vg = 150 V, Pout = 20 W. Clearly label the values of maximum and minimum voltages and currents as well as of ton and Ts times.