1-φ Halfwave Rectifier with R-L Load and Free-wheeling Diode
The SCR is triggered at ωt = α, the source voltage vs appears across load as vo. At ωt = π, the voltage vs becomes zero, just after this instant, freewheeling diode FD is forward biased.
Assuming waveform, we can see that SCR is reversed biased for π radian (ωt = π to ωt = 2π), so circuit turn off time
Waveforms:
The advantage by using freewheeling diode are:
(i) input power factor is improved
(ii) load current waveform is improved
(iii) as a result of (ii) load performance is better and
(iv) as energy stored in L is transferred to R during the freewheeling period, overall converter efficiency improves.
NOTE: FD prevents the load voltage V0 from becoming negative.
Principle of Operation of Step-up Choppers
When the average output voltage V0 greater than input voltage Vs , then the chopper is called step up chopper.
In this chopper, a large inductor L in series with source voltage Vs is essential as shown in figure. When the chopper CH is on, the closed current path is as shown in figure
and inductor stores energy during Ton period. When the chopper CH is off, as the inductor current cannot die down instantaneously this current is forced to flow through the diode and load for a time Toff. As the current tends to decrease, polarity of the emf induced in L is reversed as shown.
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