EPC2201 Power Electronic Devices Tutorial Sheet

Transcription

1 EPC2201 Power Electronic Devices Tutorial heet 1. The ON state forward voltage drop of the controlled static switch in Figure 1 is 2V. Its forward leakage current in the state is 2mA. It is operated with a switching frequency of 1 khz and a duty cycle of 30%. Neglect the switching transition times and determine: (a) the peak and average power dissipations in the switch; (b) the proportion in which this power dissipation is shared between the ON state dissipation and the state dissipation. [(a) 28W, 8.54W; (b) 98.4%, 1.6%] 100V = 7Ω Figure 1 2. The ON state voltage drop of each controlled switch in Figure 2(a) is 2V. The forward leakage current of each is 5mA. They are operated at a repetitive switching frequency of 500Hz according to the timing diagram shown in Figure 2(b). Neglect switching transition times. (a) ketch the waveforms of the load voltage V AN and the switch voltage V s1. Determine the peak blocking voltage capability needed for the switch. (b) Determine the average and the peak power dissipations in a switch. (c) Determine the overall efficiency of power conversion. [(a) 120V; (b) 1.5W, 4.01W; (c) 95.9%] N 60V 1 = 29Ω A v 1 1 ON 60V 2 v 2 2 ON 0 0.6ms 1.0ms 1.6ms 2.0ms (a) Figure 2 (b)

2 3. The turn-on and turn-off switching transition times for the controlled switch in Figure 3 are respectively 2 and 4 μs. Assume linear variation of current and voltage during the transitions. Neglect ON state voltage drop. (a) Derive the expression for the average switching power loss in terms of voltage, current, the switching transition times and the switching frequency. (b) Determine the average switching power loss at switching frequencies of 1, 10 and 100 khz. [(a) ( ) 1 VI t + 6 on t off f ; (b) 2.25W, 22.5W, 225W] sw 150V = 10Ω Figure 3 4. The in Figure 4 is used to determine the switching characteristics of the MOFET device with an inductive load. (a) Assuming that the MOFET switching transients take place with linear voltage and current variation, sketch waveforms for the MOFET drain-souce voltage and the MOFET drain current during one complete switching period given that the load inductance is large leading to a constant load current I. The duty cycle of the gate signal applied to the MOFET is equal to 0.5. abel the current and voltage magnitudes as well as the turn-on and turn-off times in your diagram. D V ignal MOFET Figure 4 (b) Deduce expressions for the turn-on energy loss, the turn-off energy loss and the average switching power dissipation in terms of the turn-on and turn-off times t on and t off, the supply voltage V, steady-state load current I, and the switching frequency f sw. For the switching energy loss calculations, you can assume that the on-state voltage drop of the MOFET is negligible. 1 =, E VI on 2 toff 1 [ E VI on 2 ton =, P sw VI( t on + t off ) f sw = 2 1 ]

3 5. (a) The in Figure 4 is operated with a switching frequency of 20kHz and a supply voltage of 100V. The steady-state load current is 10A, the MOFET onstate resistance is 0.2Ω, and the turn-on and turn-off times are 0.2μs and 0.3μs respectively. Given that during each switching cycle the MOFET is fully on for exactly half the switching period, determine the average static power dissipation and the average switching power dissipation in the MOFET. (b) Determine the maximum heatsink thermal resistance allowable for a maximum junction temperature of 100 C given a MOFET junction-to-case thermal resistance of 0.4 C/W, a case-to-heatsink thermal resistance of 0.2 C/W, and an ambient temperature of 40 C. [(a) 10W, 5W; (b) 3.4 C/W] 6. The four static switches in the of Figure 5 (a) are operated according to the timing diagram shown in Figure 5 (b). The forward ON state voltage drop of each switch is 1.5V. Each switch has a thermal resistance from junction to casing equal to 1.2 C/W. The four switches are mounted on a single heat sink. The thermal resistance from device casing to heat sink surface is 0.8 C/W while that of heat sink surface to ambient is 0.5 C/W. The switching frequency is small, and therefore the switching power loss can be neglected. (a) Determine the peak and average power dissipations in each switch; (b) Determine the junction temperature in the device and also that on the surface of the heat sink for an ambient temperature of 45 C. [(a) 29.55W, 17.73W; (b) T = C, T J = C] 200V 1 10Ω ON 2 3 ON 0 6ms 10ms 16ms 20ms 2 4 Figure 5 (a) Figure 5 (b) 7. In the of Figure 6 (a) assume that is very large and that the controlled switch has been ON for a long time initially, so that the current I can be treated as constant at 20A. The switch is now turned for a short time, so that the current freewheels through the diode D. During the short time of the switch assume that the current in the inductance is unchanged. is turned ON again. The waveform of the current through the diode from the instant is turned ON is sketched in Figure 6 (b). For the values of the parameters indicated in the figure and the basis of the waveform given, determine the following: (a) The initial rate of decay of the forward current through the diode when is turned ON; (b) the turn time of the diode;

4 (c) the approximate amplitude of the reverse voltage spike that is expected across the diode during its recovery; (d) the maximum instantaneous magnitude of the current spike to be expected in the switch during its turn ON transient. [(a) 20A/μs; (b) 3.4 μs; (c) 600V; (d) 36A] 20μΗ i D 20Α 400V D i D 20Α 20Ω 0 16Α lope = 10A/μs Figure 6 (a) Figure 6 (b) 8. A diode has ratings of 1000V, 200A and the maximum junction temperature is not to exceed 155 C. The diode forward characteristic can be modelled by the linear expression: v i V + r i d ( d ) = F 0 d d where V F0 is 1.2V and r d is 3mΩ. The thermal resistance, junction-to-case, is θjc = 0.1 C/W. For a given heatsink the thermal resistance, sink-to-ambient, is θa = 1 C/W and it is known that for this arrangement the thermal resistance, caseto-sink, θc can be kept below 0.05 C/W. Estimate the maximum direct current that the diode can tolerate safely if the ambient temperature is maintained at 40 C. [70.8A] 9. A thyristor unit has the following data. The maximum forward voltage drop V TH(ON) is 1.5V while the maximum continuous current I A is 50A. The maximum values of thermal resistance, junction-to-case θjc is 0.55 C/W, and case-to-sink θc is 0.11 C/W. If the junction temperature is not to exceed 120 C and if the maximum ambient temperature is 60 C, determine the maximum value of the thermal resistance, sink-to-ambient θa that is permitted for long thyristor conduction periods. [0.14 C/W]

5 10. In the of Figure 7, the power BJT is being used as a static switch to switch a resistance load of 10Ω in a 200V DC. Its parameters are h FE = 15, V CE(sat) = 2V and V BE = 0.7V. (a) Determine the minimum value of the driver output voltage labelled V B in the figure necessary to drive the switch to obtain the maximum current in the load. Determine also the power dissipation in the switch under this condition. (b) If V B falls to 80% of the value determined in (a), what will be the change in the load current and the change in the power dissipation in the transistor? [(a) 7.3V, 40.52W; (b) ΔI = -4.38A, ΔP D = W ] 10 Ω 5 Ω B C V B E Figure In the of Figure 8, the transistor switch has been ON for a long time. It is turned by removing the gate drive. Assume that the inductance is very large. What will be the maximum voltage that the collector will have to withstand during the turn switching? [180V] 4 Ω 8 Ω D F 120 V Base drive Base drive Figure 8 Figure 9 C 12. For the transistor switch in Figure 9, the storage time and the fall time during turn switching are 1.8 and 1.5μs respectively. The ON state current is 20A. Determine suitable values for the snubber capacitance C and the snubber resistance that will ensure the following: (a) the voltage rise across the transistor will not exceed 100V at the instant when current falls to zero; (b) the peak discharge current of the capacitor at a turn-on switching will not exceed 8A. (c) Determine also the snubber power loss at a switching frequency of 10 khz for the chosen snubber parameters. [(a) 0.15 μf; (b) 25 Ω; (c) 30W]

6 13. The thyristor in Figure 10 is used as a static switch to control current in the highly inductive. The latching current of the thyristor is 250mA. What is the minimum duration of the gate drive pulse required for the thyristor to latch ON? [2.5 ms] 14. The thyristor in the switching of Figure 11 is in the ON state. Its holding current rating is 150mA. An additional resistance is introduced in the by opening the switch labelled across this resistance. What is the lowest value of that will cause the thyristor to turn off when is opened? [656.7 Ω] 2 Ω 2 H 10 Ω 100 V Figure 10 Figure The thyristor in the switching of Figure 12 has a di/dt rating of 50A/μs. Determine the minimum value of needed to protect the thyristor from failure due to excessive di/dt. [8 μh] 16. The dv/dt rating of the thyristor in the of Figure 13 is 100V/μs. Determine the minimum value of the capacitance C necessary so that no erratic turn ON due to dv/dt will occur when the power is turned ON by closing the switch. [0.5 μf] 8 Ω 400 V 400 V C Figure 12 Figure 13

7 17. The thyristor illustrated in Figure 14 uses an C snubber to protect against dv/dt turn-on. Consider that the thyristor has just been turned off. Calculate the minimum value of C so that the thyristor will not turn on again due to dv/dt breakdown. The junction capacitance of the thyristor is 20 pf and the minimum value of the charging current required to turn on the thyristor is 4mA. [25 nf] = 20 Ω 100 V s 10 Ω C s Figure In the of Figure 15, a thyristor controls power from a 200V dc to a resistive load of 4Ω. For this condition the thyristor has a delay time t d of 0.5 μs and a current rise-time t ri of 3 μs at turn-on. The thyristor leakage current is I A leak = 2mA and the on-state voltage drop is V TH(ON) = 1.5V. Estimate: (a) the components of energy loss incurred in the thyristor during turn-on; (b) compare the turn-on loss with the thyristor loss during the on-state over an equivalent interval of time t ri. [(a) t d : 0.2 μj, t ri : 0.5 mj; (b) 0.22 mj] 19. In the of Figure 16, a thyristor controls power from a 200V dc source to an load with a freewheeling diode connected across it. The value of the inductance is high enough to consider the load current to be constant at 48A for steady-state operation as a chopper. For this condition the thyristor has a delay time t d of 0.5 μs, a current rise-time t ri of 3 μs and a voltage fall-time t fv of 2 μs at turn-on. What is the energy loss in the thyristor during turn-on? [24 mj] G V G G V G i A i A i i 4 Ω Figure 15 Figure 16