v D i D R Reverse blocking region Power Semiconductor Switches t rr Q rr Diodes Figure 1: A diode circuit Figure 2: i - v characteristics of a diode

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1 Power Semicductor Switches Diodes v D V rated R Reverse regi v D Figure 1: diode circuit Figure 2: i v characteristics of a diode Figure 3: Idealized characteristics of a diode v D t turn, the diode can be csidered an ideal switch because it turns rapidly compared to the transients in the power circuit. However at turn, the diode current reverses for a reverse recovery time t rr as shown in Figure 4 before falling to zero. In many circuits, this reverse current does not affect the cverter characteristic and so the diode can also be csidered as ideal during the turn transient. t rr Q rr t Figure 4: Diode turn 1

2 2 Various types of diodes are available: 1. Schottky diodes: Used where a low forward drop (typically.3 v) is needed in very low output circuits. Reverse capability is 5 1 V. 2. Fastrecovery diodes: Used in high frequency circuits where a small reverse recovery time is needed. t power levels of several hundred volts and several hundred amperes, such diodes have t rr ratings of less than a few microsecds. 3. Linefrequency diodes: The state is designed to be low. t rr is usually larger which is acceptable for linefrequency applicatis. These diodes are available with ratings of several kilo volts and current ratings of several kilo amperes. They can be cnected in series and parallel to satisfy any and current requirement. G Thyristors i G K Figure 5: thyristor thyristor can be triggered into the state by applying a pulse of positive gate current for a short durati provided that the device is in its forward state. The forward drop in the state is ly a few volts. Once the device begins to cduct, it is latched and the gate current can be removed. The thyristor cannot be turned by the gate, and thyristor cducts as a diode. The thyristor turns ly when the anode current tries to go negative under the influence of the circuit in which the thyristor is cnected. 2

3 3 i state Reverse breakdown state Reverse regi Forward breakdown Figure 6: i v characteristics of a thyristor Reverse state to Forward Figure 7: Idealized characteristics of a thyristor In reverse bias at s below the reverse breakdown, ly a negligibly small leakage current flows in the thyristor. Usually the thyristor ratings for forward and reverse s are the same. The thyristor current ratings are specified in terms of maximum rms and average currents. Turn time interval (t q ) The time which must elapse after the forward current through the thyristor has ceased before forward may again be applied without turn is called the "turn time". When cducti ceases, a high ccentrati of charge carriers still exists in the neighbourhood of the center juncti of the thyristor, and until this ccentrati has been sufficiently reduced by recombinati, it is not possible to apply a forward without cducti immediately taking place. During t q a reverse must be maintained across the thyristor and ly after this time is the device capable of a forward without giving into its state. 3

4 4 In additi to and current ratings, turn time t q, and the forward drop, other characteristics that must be csidered include the rateofrise of the current (di/dt) at turn and the rateofrise of (dv/dt) at turn. Bipolar Juncti Transistors C B i B i C v CE sufficiently large base current results in the device being fully. This requires that the ctrol circuit provide a base current that is sufficiently large so that I B > I C hfe v BE where h FE is the dc current gain of the device. Figure 8: NPN transistor i C v CE Figure 9: Idealized characteristics of a NPN transistor E BJTs are currentctrolled devices and base current must be supplied ctinuously to keep them in the state. The dc current gain h FE is usually ly 5 1 in high power transistors and so these devices are sometimes cnected in a Darlingt or triple Darlingt cfigurati to achieve a larger current gain. One of the main disadvantages of Darlingt cfiguratis are their slower switching speeds. Voltage ratings are up to 14 V and current ratings of a few hundred amperes. MetalOxideSemicductor Field Effect Transistors (MOSFETs) Nchannel MOSFET 4

5 5 G v GS D S v DS MOSFETs are ctrolled devices. MOSFETs require the ctinuous applicati of a gatesource of approximate magnitude in order to be in the state. No gate current flows except during the transitis from to or vice versa when the gate capacitance is being charged or discharged. The switching times are very short, being in the range of a few tens of nanosecds to a few hundred nanosecds depending the device type. Figure 1: n Nchannel MOSFET MOSFETs are available in ratings in excess of 1 V but with small current ratings, and with up to 1 at small ratings. v GS = 7 V 6 V 5 V 4 V v DS Figure 11: i v characteristics of an Nchannel MOSFET v DS Figure 12: Idealized characteristics of an Nchannel MOSFET 5

6 6 GTETURNOFF THYRISTORS (GTOs) The GTO can be turned by a shortdurati gate current pulse, and ce in the state, the GTO may stay without any further gate current. The GTO can be turned by applying a negative gatecarthode and therefore, causing a sufficiently large negative gate current to flow. This negative current need ly flow for a few microsecds but it must have a very large magnitude, typically as large as ethird the anode current being turned. G i G K Figure 13: Gateturn thyristor Reverse Forward Figure 14: Idealized characteristics of a GTO. i state Reverse breakdown Reverse regi Turn Turn state Forward breakdown Figure 15: iv characteristics of a GTO. 6