电力电子 总复习习题.ppt

BEIJING JIAOTONG UNIVERSITY 1896 * * * Example 7-A In the Buck dc-dc converter of following figure, L=24μH . It is operating in dc steady state under the following conditions: Vin = 20V , D=0.6 , Po =12W , and fs=200kHz . Assuming ideal components, calculate and draw the waveforms. * * Solution fs=200kHz, Ts=5μs and Ton =DTs=3μs. Vo=DVin=12V. The inductor voltage vL fluctuates between (Vin ?Vo )=8V and (?Vo) =?12V. Therefore, iL=IL+iL,ripple. When the transistor is on, iin=iL, otherwise zero. The average input currents is Iin=DIo =0.6A . * * Example 7-B In the following Boost converter, the inductor current has ΔiL= 2A. It is operating in dc steady state under the following conditions: Vin =5V , Vo =12V ,Po=10W , and fs=200kHz . Assuming ideal components, calculate L and draw the waveforms. Solution Continuous conduction mode * * Solution D=(Vo-Vd)/Vo=0.583 , fs=200kHz , Ts= 5μs and Ton=DTs =2.917μs . vL fluctuates between Vin =5V and ?(Vo ?Vin )=?7V . * * When the transistor is on, the idiode = 0, The capacitor current jumps to a value of 2.167 A and drops to （1? 0.833 ）= 0.167 A. Solution * * * Problem 2: A switch-mode power supply is to be designed with the following specifications: Vd =48V±10%, Vo=5V, fs=100kHz, Pload is 15~50W. A forward converter operating in a continuous-conduction mode with a demagnetizing winding (N3=N1) is chosen. Assume all components to be ideal except the presence of transformer magnetization inductance. (2’) (a) Calculate N2/N1 if this turns ratio is desired to be as small as possible. (b) Calculate the minimum value of the filter inductance. + Vd - + Vo - D1 L iL + vL - D2 T N1 i1 N2 N3 i3 i2 D3 Practical forward converter * Solution + Vd - + Vo - D1 L iL + vL - D2 T N1 i1 N2 N3 i3 i2 D3 Practical forward converter Vd=43.2V to 52.8V, Since N3=N1, Dmax=0.5 (a) Continuous conduction (b) N3/N1=0.232 from part (a). Pmin=15W. Io,min=IL,min=3A. At the boundary m