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:-

enter image description here enter image description here enter image description here

So, please answer, review and help!


  • \$\begingroup\$ You should include schematics of each of the topologies you're wondering about, as feedback mechanism is relevant in determining the answer. Also, this smells like homework, so to get the best possible result, include your own hypothesis or attempt to solve the problem. If the circuits are assumed to be non intelligent (no overvoltage/open circuit protection) you'll be able to make a hypothesis based on the fact that the controller can only send more/less pulses in order to maintain output current, and the nature of any inductance in the system. \$\endgroup\$
    – K H
    Sep 23, 2018 at 1:17
  • \$\begingroup\$ Bear in mind the principle on which the boost circuit works, that if you charge an inductor at a low impedance and discharge it at a higher impedance the output voltage will be higher than the input. For the discontinuous converters, are you assuming they are in pulse width or pulse frequency modulation? \$\endgroup\$
    – K H
    Sep 23, 2018 at 1:18
  • \$\begingroup\$ Never heard anyone include the current in the acronym. Can you stick with the TLA CCM and DCM like everyone else? \$\endgroup\$
    – winny
    Sep 23, 2018 at 11:19
  • \$\begingroup\$ Much better! Have you simulated the time domain difference between the two for your load step? \$\endgroup\$
    – winny
    Sep 23, 2018 at 12:02
  • \$\begingroup\$ Nope. I am just considering a hypothetical situation for all these choppers, nothing more \$\endgroup\$
    – mosfet
    Sep 23, 2018 at 14:44

1 Answer 1


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.

  • \$\begingroup\$ I have made few changes to the question, hopefully now some answer might come \$\endgroup\$
    – mosfet
    Sep 23, 2018 at 12:23

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