Page 289 - Power Electronics Handbook

Design of chopper circuits 279

The peak load current (IL(pk)) is shown in Figure 12.16 to increase as the
frequency of the chopper reduces, that is, as the time given by equation
(12.19) increases. This is the value of the current which has to be carried
and commutated by the main thyristor, and it should be as low as possible,
so a high chopper operating frequency is desirable. The maximum value of
this load current equals (VB-VF)/R when the power switch, which is a
thyristor in Figure 12.14, is continuously on.
Figure 12.20 shows that the ripple current in the load also increases as
the chopping frequency is reduced, which is expected due to the lower
smoothing effect of circuit components. The ripple current reaches a peak
at an equal mark-space ratio, which is the setting for which any filters need
to be designed, and is independent of V,. The mean load current, shown in
Figure 12.19, varies linearly with the output voltage, again as expected
since the load is assumed to be linear.
The rating of the power switch (thyristor) and free-wheeling diode are
given by Figures 12.17 and 12.18 respectively. The average current rating
of the thyristor decreases with frequency, and although the rating of the
diode now increases, this is usually acceptable since the thyristor is the
more expensive component. The peak rating of the thyristor is reached at
maximum output voltage, when the device is fully on, whilst that of the
diode occurs at half voltage, when the mark and space periods in the
chopped waveform are equal.
So far the discussions have ignored the effects of any commutation
circuit used in the design of the chopper circuit. Although the effect of
commutation on the design of the circuit will be largely determined by the
commutation method, an example is given here using the circuit of Figure
12.4. In this circuit the mean current rating of the free-wheeling diode D1 is
still given by equation (12.17), its peak rating (lypk)) being the same as
that of the thyristor, since it carries this current at the instant that the
thyristor is commutated, assuming the commutation interval to be short
relative to the chopping period. The voltage rating of the diode must
exceed 2vB7 which occurs at the instance of commutation, when the
voltage of capacitor C is added to that of the supply.
The mean current rating of the thyristor, in Figure 12.4, is increased due
to commutation capacitor reset by a value given by equation (12.20). This
total current is as in equation (12.21). The peak current is either ZL(pk), as
in equation (12.15), or the resonant value given by equation (12.22),
whichever is greater. Its voltage rating is not affected by the commutation
circuit and must exceed VB. Assuming that commutation capacitor C
discharges at constant load current, thyristor TH1 must have a turn-off
time shorter than the value given by equation (12.23).

(12.20)
ITH(av) = IT(av) + IC (12.21)

(12.22)

(12.23)