Diodes with RL loads. Freewheeling Assume ideal diode (zero reverse rocvery time, zero voltage drop)

An antiparallel diode known as the freewheeling diode must be connected across the load to provide a path for the inductive current to flow.

Diode rectifiers

Learn performance parameters of rectifiers and power converters

Study effects of load inductance on load current

L

R

D1

D2

Vs

First neglect the freewheeling diode

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The diode current is found using the Laplace transform method:

Note that the diode conduction time islonger than half-period

The waveform is nonsinusoidal but periodic.

Now with the freewheeling diode

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Recommended reading

● Chapter 1● Sections 2.10-2.13 and examples in those sections, the rest are optional)● Sections 3.1 to 3.3, example 3.2

Single phase full wave rectifier

Full wave rectifier with centre tapped transformer

Full wave bridge rectifier

Single phase full wave rectifier with large inductive load

Single phase full wave rectifier with RL load

T/2

For a steady state solution, require i0(0)= i0(T)=I0. Then

I0>0 guarantees that the current flows continuously.

Download Matlab code fwr.m, (also requires fun_fwr.m).

Load, source and diode currents (E=10V).

Load, source and diode currents (E=0.6Vm)

E=10 V

E=120 V

Load voltage

Source voltage

Load current

Conclusion● An inductive load can make load current flow

continuously if voltage ratio is below the critical value

● E=0 guarantees continuous current● If L=0, any E>0 will cause current interruptions ● References: Section 3.4 and examples in that

section

Three phase bridge rectifier

(Note 30 deg phase shift).

0

Single phase rectifier 3 phase rectifier

Performance:

3phase rectifiers with RL load

E

Condition for continuous load current

Largest critical value of the EMF ratio is substantially higher (95.5% vs 63.6%)