Page 181 - Power Electronics Handbook

Integral half-cycle regulation 173

main cause of interference is removed and no suppression filters are
required. The power supplied to the load is controlled by regulating whole
half cycles of the supply voltage, there being two main methods by which
this is done, as illustrated in Figure 8.18. For burst firing the power
switches are either fully on or fully off for the whole duration of the sensing
period. For instance, if the temperature of an oven is being controlled, the
power switches will be on when the oven cools below a reference setting,
and power will be supplied to the heaters causing the oven temperature to
rise. When this temperature exceeds the desired value the power switches
turn off and remain off until a lower temperature limit is again reached.
Burst firing or odoff control systems are suitable for high-inertia loads,
such as heating, but are not suitable for lighting or motor control since the
operating frequency is too low. In these instances proportional control,
also known as cycle syncopation, is used, the system working on a fixed
period, measured in numbers of cycles or half cycles. Figure 8.18(b)
illustrates a system in which this period is five cycles. To regulate the power
the on-to-off duty cycle within this periodic time is varied. Once again this
change can be in half cycles or in cycles, the important consideration being
that the power device turns on as close to the start of the half cycle as
possible.

60% on

Fully off
(b)
Figure 8.18 Operating modes for zero-voltage switching control