ECE320 Home Work # 8 – SOLUTIONS PROBLEM 1 Explain in your own words what Power Electronics is. Give one or two power electronics product examples that you can think of and explain why they are power electronics and what’s inside to your knowledge. SOLUTION: Power electronics is power conversion circuits and their control, from one available form of power/energy source to a desired from of power. Examples are: Computer power supply; switch-mode power supply (inside they have ac-dc rectifier and dcdc converter) Motor drive controller: Inverter inside to produce variable ac (both in frequency and in voltage amplitude) to control motor speed and torque Vdc PROBLEM 2 The figure below shows an inverter circuit using four switches to converter a dc source into ac. Draw a control sequence for each of the four switches by using “0 (for off)” and “1 for on”, respectively, to complete the waveform figure. S1 S3 0 Vac Vdc 0 Vdc S1 Load vac vdc S2 S2 S4 S3 S4 SOLUTION: Two solutions. PROBLEM 3 S In the given circuit, assume the switch is used to dim the light and turned on and off in a sequence as shown. a). Derive the power loss equation expressed in dc voltage Vdc, on-current IL = Vdc / RL, on-drop voltage Von, switching frequency fsw, turn-on time ton, and turn-off time toff (assuming that the current changes linearly during the switching and ton and toff are very small compared with the switchingcycle) . b). calculate power loss using the derived equation given that a 50% duty cycle, Vdc= 100 V, on-current IL = 10 A, on-drop voltage Von= 1 V, switching frequency fsw= 1 kHz, turn-on time ton= 1 s, and turn-off time toff = 1 s. c) Calculate the efficiency for the power conversion described in b). Vdc is vs IL Vdc ton Conduction power loss: TON TSW When the switch is in off state, current is zero, thus no off-state loss. Turn –ON power loss PT-ON : Assume VON Vdc PT ON 1 TSW = tON 0 1 (vs is ) dt TSW VON I L tON ……. (1) 2 TSW Von toff 1/fsw Derive the power loss equation PON (VON I L ) Light Bulb RL vS SOLUTION: a) iS tON 0 VON t tON I L dt Turn-off power loss PT-OFF : 1 PT-off = Tsw toff (v s . is ) dt 0 1 = Tsw = toff (v dc )( 0 t toff ) I L dt Vdc I L toff 2 TSW Total power loss Ploss PON PT ON PT off = VON I L tON V I t TON dc L off 2 TSW 2 TSW TSW VON I L calculate power loss using the derived equation b) Vdc 100V I L 10 A , TSW 1 f SW 1 ms , TON 50% TSW .5n , tON 1 s , and toff 1 s Ploss PON PT ON PT off = VON I L = = c) (1)(10)( VON I L tON V I t TON dc L off 2 TSW 2 TSW TSW 6 0.5 (100)(10) 1106 (1)(10) 110 ) 3 3 1 2 2 110 110 5 (5 103 ) (0.5) 5.505 W Calculate the efficiency for the power conversion P P POUT V I (50%) PLoss IN Loss dc L PIN PIN Vdc I L (50%) = (100)(10)(0.5) 5.505 (100)(10)(0.5) = 98.89%