Feature: Circuit design


Turning a thyristor off One way to turn off the main thyristor in a DC circuit is to add auxiliary thyristor, capacitor and resistor to form a circuit like the one shown in Figure 1. At turn-on of the main thyristor Tymain capacitor C charges through the added resistor Raux close to the supply voltage Vs auxiliary thyristor Tyaux


, to a value . At the Tymain Tis decreases the voltage at Tyaux


discharging. When the Tymain


value close to Vs high negative current flows through Tymain turn-off point, the


is turned on by a control pulse at its gate. ’s anode and, consequently, a due to capacitor C


turns off, capacitor C charges again to a


charged through the load resistor of Tymain point, Tyaux


, this time with opposite polarity since it has now . At the next turn-on


is turned off by sending a negative current pulse though it – a process analogous to turning off Tymain . Te benefit


of this solution is that the time interval between the on and off states of the main thyristor can be arbitrarily long. Tis is achieved by a continuous on state of Tyaux


during the off state of Tymain


which increases power consumption. On the other hand, the anode current of Tyaux


lower than that of Tymain Tyaux


of Tyaux holds true for IHaux is twice that of its hold current IHaux can be much


can be considerably lower than that of Tymain . Tus, any power loss due to Tyaux


current can be considered negligibly small. Tyaux


. A sufficient value of anode current of . Te nominal current ; the same ’s anode


’s lower current rating is enabled by the fact that the


duration of the high initial current pulse flowing through its anode is in the order of 100µs. Allowed value of an overcurrent of a thyristor for such a short time is typically 30 times the nominal anode current. Te value of resistor is:


at time t = tr


Te conclusion is that capacitor C is just partially discharged . Te instant of a sharp current peak iaux


and capacitor C is smaller than iaux anode at t = tr (tr ).


Te appearance of a negative voltage at the main thyristor’s is a consequence of a partially-discharged


capacitor C; see the yellow trace in Figure 3. For t > tr C charges through resistor Rm


capacitor, to an opposite-polarity voltage.


Figure 4 shows the circuit’s length of operation. Te yellow trace is Tymain


’s gate voltage. A “short” peak with 1.2V amplitude Capacitor C value is:


controls the current pulse, and a positive gate voltage of about 0.6V is then created by Tymain


itself due to its on state. Te


blue trace corresponds to an anode-cathode voltage of Tyaux in Figure 5, the negative


.


Te negative excursion of this voltage exceeds 10V. Contrarily, at the anode-cathode voltage of Tymain


where tqmain is the turn-off time of the main thyristor and Imain is


its anode current. Te circuit in Figure 1 was realised for testing purposes with


thyristors of the same type, the KT120A. Tis is a so-called fast thyristor, and was produced by the TESLA enterprise in the former Czechoslovakia. Te values of the passive devices were


and the supply voltage was


Operation To turn off the main thyristor, Tyaux


departure is less pronounced, having an amplitude of only about 6V. Te difference is due to the fact that capacitor C is discharged to a lesser extent at the Tyaux


’s turn-off point than when Tymain


turns off. A negative departure also appears on Tymain


’s gate, shown


in Figure 6. Its value is -1.25V, but decays very rapidly – within 400ns, resembling the reverse-recovery of a silicon switching diode. Note that the ground-level of the blue trace is shiſted downward from the centre by -30V in Figures 4 and 5 and by -1.5V in Figure 6.


is turned on with a


rectangular control pulse at its gate, shown as a yellow trace in Figure 2. A rectangular pedestal on the gate voltage represents the control pulse. Te blue trace in Figure 2 represents the current flowing through Tyaux


’s cathode. Te cathode current rises 38 February 2021 www.electronicsworld.co.uk


Applications Te circuit in Figure 1 is especially suitable for applications where two equivalent or similar loads have a common terminal but are switched on in an alternating manner. In this case, Tymain Tyaux


and become equivalent to each other and should be of equal rating. (t) is strong


evidence of the main thyristor being turned off, since the sum of currents that later flow through resistor Raux


,


Tis decrease of charge lowers the original voltage of capacitor C by a value:


Te charge decrease (DQ) of capacitor C at time tr iaux


(t) is then: of the peak of


Tis approximation is based on two measured values of current iaux


(tr ) = 0.424A and


exponentially from zero, up to a peak value within rise-time tr


=1ms. If we consider an exponential rise of Tyaux


can be modelled as: ’s cathode current, it


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