power electronics
1 ECE 451 Power Electronics I Tutorial questions – Chapters 1 & 2 1. The switching waveforms for a BJT with a turn -on snubber are shown below, where V CC =400 V, V CE(sat) = 2.5 V, and the collector current in the on -state is I CE = 80 A. The collector leakage current is negligible (I CEO = 0 A). Calculate the total average power dissipation. 2. The switching waveforms for a BJT with a turn -off snubber are shown below. Use the same values as in the previous problem and calculate again the total average power dissipation. 3. A single -phase diode bridge rectifier must supply an average ripple free current of 12 A to a highly inductive load. If the rectifier is supplied from an ac source of 230 V rms , determine: 3.1 the dc output voltage; 3.2 the rms output v oltage; 3.3 the ac component of the output voltage; 3.4 the rms output current (I rms ); 3.5 the the rms supply current (I s) 3.6 the apparent power of the transformer (S); 4. A single -phase diode rectifier is required to supply an average voltage of V dc = 216 V to a highly inductive load at a ripple -free current of I dc = 14 A. 4.1 Determine the rms supply voltage (V s) necessary to produce the required dc output voltage. 4.2 Calculate the rms output current (I rms ). 4.3 Find the rms current (I r) through one diod e. 4.4 Determine the average dc current (I av) through one diode. 4.5 Find the PIV across one diode. 4.6 Calculate the rms supply current (I s). 4.7 Find the apparent power of the transformer (S). 4.8 Draw the waveforms of the output voltage, output current, supply current and the current through one of the diodes. 5. Determine the average (V av), rms (V rms ), ac value (V ac), form factor (FF), ripple factor ( RF) and crest factor (CF) for the voltage waveforms below. Assume the values of A = 20 and T = 50μs. ECE 451 Power Electronics I Tutorial questions – Controlled Rectifiers & Protection of Semiconductor Devices 1A single -phase semiconverter is operated from a 240 V rms supply. The highly inductive load current with an average value of I dc = 9 A, is continuous with negligible ripple content. The delay angle is α = π/3. 1.1 the dc output voltage; 1.2 the rms output voltage; 1.3 Determine the average dc current (I av) through one thyristor. 14 Calculate the rms supply current (I s). 1.4 Find the apparent power of the transformer (S). 2. A single -phase full -converter (thyristor controlled) has a load of R = 3.14 Ohms. The ac input supply voltage is a single -phase 220 Vrms / 50 Hz. The firing angle is α = 400. 2.1 Find the dc output current (Idc). 2.2 Determine the rms output voltage (Vo(rms )). 2.3 Calculate the rms input current (Is ). Find the input power factor A three -phase rectifier is required to supply a rms v oltage of Vrms = 550 V at a ripplefree current of Irms = 2000 A. The rectifier is connected to the medium voltage supply via a Y -Y transformer (a12 = 5:1). 2.4 Determine secondary line -to-line voltage (VLL) necessary to produce the required rms output voltage . 2.5 Calculate the average output current (Idc). 2.6 Find the rms current (Ir) through one diode. 2.7 Determine the average dc current (Iav) through one diode. 2.8 Find the PIV across one diode. 2.9 Calculate the rms phase secondary current (Iph). 2.10 Find the apparent power of the transformer (S). 2.11 Determine the primary voltage (phase) of the transformer (Vp). 3. A three -phase full -converter (thyristor controlled) has a load of R = 3.14 . The ac input supply voltage is a three -phase 380 Vrms / 50 Hz. The firing angle is α = 400. 3.1 Find the dc output current (Idc). 3.2 Determine the rms output voltage (Vo(rms)). 3.3 Calculate the rms input current (Is) considering the load current is continuously and ripple free. 3.4 Find the input power factor. 3.5 4. A three -phase supply of 440 V rms / 60 Hz must supply DC power through a controlled rectifier to an industrial oven with a15 _ element. The rectifier must supply a maximum DC current (I dc_max ) of 30 A and a minimum DC current (I dc_min ) of 20 A to the load. 1 4.1 Find the firing angles (α max and α min ), for the prescribed minimum and maximum values of Idc. 4.2 Determine the maximum possible rms thyristor current I R. 4.3 Calculate the worst -case efficiency (η) and input power factor (PF) for I dc_max and Idc_min . 4.4 What will be the minimum required power rating of the transformer for the specified load currents? A parallel connected R C snubber protects a device against dv/dt. 4.5 Determine the the value of R if the discharging current must be limited to 10 A and supply voltage is V s = 240 V. (3) 4.6 Determine the dv/dt if C s = 0.1 F. (3) 4.7 Calculate the power dissipated in the snubber resistor if the device operates at 1 Hz. Consider that the power dissipated into a transistor device is 180 W and the thermal par ameters of the device are Rth -jc = 0.11 K/W, Rth -cs = 0.03 K/W, Rth -sa = 0.11 K/W and the ambient temperature is 350C. Determine the junction temperature. [ 80 ºC ] 7. Calculate the thermal resistance for a heat sink used for a transistor with the power dissipated on it of 200W, R th-jc = 0.09 K/W, R th-cs = 0.02 K/W, the ambient temperature of 45 0C and if the maximum allowable temperature of the junction is 95 0C. ] 8. A freewheeling diode is connected parallel on an inductive load of L = 250 H; the resistance is negligible (R = 0), the voltage source is V s = 200 V 8.1 Calculate the value of the current after t 1 = 100 sec. 8.2 Determine the energy E stored in the inductor. A thyristor must control a DC load of R = 10 W and L = 400 mH that is supplied from Vs =360V. The latching current of the thyristor is IL = 75 mA and the firing pulse width is 30msec. 8.3 If the thyristor is OFF find the parallel resistor R2 across the load in order to ensure the latching current after 30 msec. 8.4 Calculate the w idth of the pulse that ensures thyristor to stay ON with no other auxiliary resistor. 9. Two identical MOSFET's that share current equally are mounted on a common heat sink. The thermal resistances are as follows: Junction to Case: 1,2 °C/W; Case to Heat Sink: 0.3°C/W; Heat Sink to Ambient air: 0,75 °C/W; The maximum allowed junction temperature:
125 °C; Ambient air temperature: 25 °C Sketch the thermal equivalent circuit for the combination. (3) Determine the maximum power that can be safely dissipated by each device. (2) 9.1 Determine the heat sink temperature. (2) 2