Page 158 - Power Electronics Handbook

D.C. contacton 151
Figure 7.2(a) shows a simple d.c. contactor circuit in which thyristor TH1
is the main power semiconductor, which is fired to COM& the load across
the supply. This also permits capacitor C to charge through resistor R with
plate 1 positive. To turn the contactor off thyristor TH2 is fired, which
connects C across thyristor THl, reverse biasing it. In effect, C provides an
auxiliary path for the load current and consequently commences charging
with plate 2 positive. Provided the time during which THl is reverse biased
is greater than its turn-off time it will remain off. When TH1 next fired
is
thyristor TH2 turns off since C is connected across it. This contactor circuit
has two major limitations:

(i) It is inefficient, since thyristor THz is conducting during the off period
and energy is lost in resistor R. This resistor can be made large
valued, but then the operating frequency is reduced.
(ii) After thyristor THl is turned on sufficient time must be allowed for C
to charge to the supply voltage, through R, before thyristor THz can
be fired to turn it off. Therefore the contactor has a top limit to its
operating frequency, and the greater the value of R, the lower this
frequency.
The contactor shown in Figure 7.2(b) is capable of much higher
operating frequencies, even under conditions of high load impedance.
Initially C charges through D3 to the d.c. supply with plate 1 positive.
Thyristor TH, is now turned on to supply load current. During this period C
would discharge, owing to the leakage currents of TH2 and D3, had this not
been compensated for by the current through R1. Therefore this resistor can
have a large value since it has only to pass the leakage current of two devices.
To turn TH, off, TH, is fired. Capacitor C resonates with L through TH, and
TH, and the volage on C reverses, plate 2 being positive. Once again C
resonates with L through D2 and D,, turning TH, and TH, off. Figure 7.2(c)
shows the voltage on capacitor C assuming negligible resonant losses.
The mean current of thyristor TH, is given by the maximum load current
Ipk, with a peak current, due to resonant discharges, given by equation (7.4)
where V is the supply voltage.

The peak voltage of thyristor TH, is given by equation (7.5)

vpk = v -k lpk J(LI/c) (7.5)
,/($)I
The maximum turn-off time seen by TH, is given by equation (7.6)
tom = J(L,C) [; - sin-' [$ + cos-' [: ,/($-)]I (7.6)

The mean current rating of thyristor TH2 is low, the peak value being
given by VV(C/&). Its voltage rating must exceed V.
Diode Dl passes a peak current of Z,,k and sees a voltage of
2V+ZP~V(L1IC).