0195136047.pdf

16.1 POWER SEMICONDUCTOR-CONTROLLED DRIVES 755

+

TRIAC

Ac
supply

Resistive

load R

G Vm

Vm

−Vm

π

π

2 π

2 π

vs = Vm sin ωt

ωt

ωt

vo

vo

vs

(a) (b)

0

0

−

+

− −Vm

α

vs = Vm sin ωt

α

Figure 16.1.7Single-phase ac–ac converter with TRIAC.(a)Circuit diagram.(b)Voltage waveforms.

Figure 16.1.7 shows a single-phase ac–ac converter with a TRIAC to obtain a variable ac
output voltage from a fixed ac source. The output voltage is controlled by changing the conduction
time of the TRIAC.
Figure 16.1.8 shows a dc–dc converter in which the average output voltage is controlled by
changing the conduction timet 1 of the transistor. The chopping period isT, the duty cycle of the
chopper isδ, and the conduction timet 1 is given byδT.
Figure 16.1.9 shows a single-phase dc–ac converter, known as an inverter, in which the output
voltage is controlled by varying the conduction time of transistors. Note that the voltage is of
alternating form when transistorsQ 1 andQ 2 conduct for one-half period andQ 3 andQ 4 conduct
for the other half.

+

DC
supply

Transistor T 1

Load

Vs

Vs

Dm

VBE

δ =

Vo^ =^ δVs

t 1 T

t 1

t

vo

vo

(a) (b)

0

−

+

−

+

−

T

T

t 1

t

VBE

0

1

Figure 16.1.8A dc–dc converter.(a)Circuit diagram.(b)Voltage waveforms.