Page 291 - Power Electronics Handbook

The step-up chopper 28 I
transistor TR1. Comparison of this circuit with the step-down chopper of
Figure 12.l(a) shows that the main difference is that the positions of the
main switch (transistor) and the diode have been interchanged, and the
inductor is connected to the supply end rather than to the load end.
The step-up chopper works by storing energy in the series inductor L1
during the period that the transistor is on and transferring this to the load
when the transistor is off. Because the load energy is supplied as a series of
pulses, a capacitor C is usually added to give some smoothing. The
operation of the chopper can be followed by reference to the circuit
waveforms of Figure 12.21. When the power transistor is on load current
builds up in L1, reaching a peak of zs(pk) after time fc, as shown in Figure
12.21(b). The voltage across the transistor is low during this period and has
been assumed to be zero in Figure 12.21(d). The current through the
transistor builds up to the same peak value of ZS(~~), having started from a
residual value of Zscmin). The current provided from the supply to the load
(IL) during the transistor conduction period is zero, as shown in Figure
12.21(c), all the load current being obtained from the smoothing capacitor.
When the transistor goes off its collector current falls to zero, but the
current in the inductor Ll continues to flow, delivering energy to the load
and its smoothing capacitor. The voltage across the transistor now jumps to
the value of the load voltage, since diode D1 is conducting, and this voltage
has been assumed, in Figure 12.21(b), to remain constant during the whole
of the off period to. The current in L1, which started with a value of Is@k) at
the beginning of the off cycle, now starts to decay, reaching a minimum of
by the end of the off period. The load current waveform follows that
of the inductor current, since with diode D1 conducting currents Is and 1,
are equal.
Assuming a smooth input and output voltage and a linear current
waveform in the inductor, as in Figure 12.21, the value of the load voltage
is given by equation (12.24).
(12.24)

From this equation it is seen that the load voltage will vary, from a
minimum value equal to the supply voltage of VB when the transistor is
continuously off, to a maximum approaching infinity as the off period of
the transistor becomes smaller. The circuit of Figure 12.21(a) is therefore a
stepup chopper, the output varying upwards from the supply voltage. If
the losses in the circuit are ignored then power from the supply is fed to the
load, so that equation (12.25) holds true. This shows the traditional
transformer equation where a step-up change in voltage is accompanied by
a step-down change in current, and it is the same as with a step-down
chopper.
v,z, = v,1, (12.25)
Although step-down choppers are frequently used, step-up ones are less
popular due to the high smoothing requirements caused by the pulses of
energy delivered to the load. Where step-up of voltage is required it is
more usual to do this by an inverter feeding into a step-up transformer, as
described in Chapter 13.