I am studying a step-down chopper with RL load. I see that the output current waveform starts at a non-zero value (say Imin).

Shouldn't it start from zero? When the switch is closed, how does the current decrease from Imax to Imin? When I searched I found the term "continuous current". Can anyone help me?

waveform for step down chopper

  • \$\begingroup\$ When the switch is closed (as in the contact is closed) current into the inductor increases. Yes, it does start at zero current but, the graph is showing the waveform after the initial transients have long passed. \$\endgroup\$
    – Andy aka
    Feb 8, 2023 at 16:59
  • \$\begingroup\$ Thanks for your reply. \$\endgroup\$ Feb 8, 2023 at 17:04
  • 1
    \$\begingroup\$ An even better search term for you would be continuous conduction mode (CCM). \$\endgroup\$
    – winny
    Feb 8, 2023 at 17:21

2 Answers 2


If you let toff be a large enough time, then the current would decay to zero and the next cycle would start from zero. But this is not good if you are trying to make an SMPS because for a power supply you need continuous current (or close to continuous), this is why we also have to choose ton and toff so that the output can be positive and then use filtering to give near continous current.

  • \$\begingroup\$ I got it. Thanks \$\endgroup\$ Feb 8, 2023 at 18:23

Before diving into topology specifics it would help you to understand modes of switch mode power supplies (SMPS's). There are a lot of great resources out there, one of them is Colorado boulders online Power Electronics Specialisation course. It's free, and I think pretty amazing. It helped me immensely, as, I self studied to get into this industry.

Definitely take that course, and, all your questions, initially, will be answered. If for some reason you don't feel like that, key words you can look up are Continuous Conduction Mode (CCM), and Discontinuous Conduction Mode (DCM), boundary (Or critical) Conduction Mode (CrM or BCM). Look these terms up, and reply back here which you think it is and why, we can go from there.


But, because I'm new to this I have to give an answer here...

CCM is what you are seeing because the instantaneous current never reaches zero as you point out. The instantaneous current is continuously above zero, meaning, during the switching cycle, Ton (switch ON time) and Toff (switch OFF time), the instantaneous current does not touch zero. That's the basic idea of CCM.

CrM, is where the instantaneous current touches zero, but almost immediately increases back to some non-zero level. So, goes from zero to some non-zero value, never staying at zero.

DCM, is where the instantaneous current stays at zero for some amount of time.

They each have there pro's and con's, different modes work practically better for different topologies and control types. But, these questions are answered by long books. Hopefully that helps


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