Electronic – DC choppers output voltage in ccm and dcm mode with no load

power electronics

I was wondering what would happen to the steady state output voltages and currents of the
Buck, Boost and the Buck-Boost converters, working in

1 – Continuous conduction mode (CCM),

2 – Discontinuous conduction mode(DCM),

when suddenly their load gets disconnected due to some fault.
All three converters are assumed to be in ideal conditions with all ideal components. The load (purely resistive) in all three is connected across an output filter capacitor. The switches in all three are controlled by PWM scheme.

Further assume that:

All converters are having an Input of say 20V, and a fixed duty ratio of 0.8. The resistive load is of say 20 ohms and the switching frequency is 100 kHz.

My thinking in the case of CCM is:

– For Buck converter, the capacitor will charge to full voltage and then no current will flow.

– For Boost, the inductor will keep giving its energy to the output capacitor and after a certain point when voltage limits of the capacitor exceed, burnout of switch or diode may occur.

– For Buck-Boost, similar to buck or boost scenario depending upon the duty ratio.

I have no idea about what might happen in DCM.

For reference the schematics of the three choppers are given below:-

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So, please answer, review and help!


Best Answer

What happens in each case depends on the control scheme and how fast it can react. You left out the all-important part that controls the switch. This controller essentially decides when to do a pulse and for how long.

With a slow controller, you will get more overshoot than with a fast one. There are many tradeoffs that are balanced in a controller. What you are asking about it known as the transient response. Getting good transient response often comes at the expense of higher output ripple.

For example, a pulse on demand system has fast transient response, but more output ripple than a carefully tuned "smooth" controller.

Your question therefore can't be answered without knowing the innards of the controllers, which you omitted from your descriptions.