The Circuit Designer's Companion - diagramas.diagram...

Active components 125Conduction currentno conductionconductionI HI Hgate triggerI Hnot reached by end of pulseI Hexceeded by end of pulseFigure 4.15 Effect of thyristor holding currentRDCR LFigure 4.16 The thyristor snubber circuitdevice is triggered the conduction region spreads relatively slowly through the silicon.If the turn-on current rises too rapidly then a high current flow is concentrated into asmall region near the cathode, causing localised overheating and eventual devicedestruction. Maximum di/dt is sometimes specified and can be met by incorporating asmall amount of inductance (calculated from L = −V/di/dt) into the load circuit; oftenthe load itself is inductive enough for this. Permissible di/dt is strongly influenced bygate drive level and rise time, since a higher level of gate drive will spread theconduction region faster through the device. Higher gate drive also reduces the delaytime from application of drive to turn-on, which is typically 1−2µs.Turn-offA reverse voltage cannot be used to turn off a triac, which conducts in both directions.However, thyristors can be turned off by reverse voltage, and their turn-off time has twocomponents, reverse recovery time and forward blocking recovery time. The formerhas the same mechanism as a reverse-biased diode, i.e. removal of minority carriersfrom the reverse blocking junction with application of reverse voltage. The longer timeconstant is associated with forward blocking recovery and is the time required for thecharge stored in the forward blocking junction to recombine. The total turn-off time isof the order of tens of microseconds, and is increased by increasing junctiontemperature and on-state current. Negative gate bias will decrease it, as this will speedup the removal of charge from the forward blocking junction.4.2.6 SnubbingRestrictions on dV/dt can be met by connecting a capacitor-resistor-diode network inparallel with the device. This technique is known as “snubbing”, and it can apply to anyswitching circuit, not just to thyristor/triac circuits (earlier examples have been givenfor switching inductive circuits). The basic circuit is shown in Figure 4.16.