Flyback Converter DCM/CCM selection criteria..

[ku637]

Dear experts,

I would like to know how one may select whether a DCM or a CCM mode of operation is required for a flyback converter application..

I see that DCM and CCM has some advantages and disadvantages..

like DCM peak currents high, primary inductance less,stability wise better..

In CCM currents low,big transformer, RHP zero etc..

But essentially how an expert will chose CCM or DCM for a particular application...what are the decision making factors...??

Say for 85-265VAC VIN , VOUT=12V/2A application..

Thanks for any help...

 

[BradtheRad]

DCM peak currents high

With DCM, the current waveform swings between zero and a high peak. It requires that the inductor have a higher saturation spec.

CCM has current flowing at all times. This allows you to obtain the same power from a lesser saturation spec. The transformer can be smaller, lighter, cheaper.

 

[Velkarn]

I always chose DCM, becauseof its better stability and lower EMI level (mostly). In CCM mosfet is hard switched and therefore EMI is higher.

- - - Updated - - -

i think that using DCM for flyback is common practice, is not it?

 

[BradtheRad]

i think that using DCM for flyback is common practice, is not it?

Right, 50% duty cycle is usually the goal. The idle time needs to be a sufficient portion of the cycle, so the flux field has time to collapse. So we might have to leave it running in DCM. Because if we make the duty cycle longer, that in turn shortens the time window for the flux field to collapse. Therefore there's a point where a longer duty cycle will not give us greater power output.

CCM tends to go with a longer duty cycle.

 

[schmitt trigger]

Additionally, in a DCM circuit with a low-leakage transformer, the tertiary winding's voltage (which supplies bias to the controller) is a good replica of the secondary voltage, and may be used as feedback.
This means that you can achieve reasonable regulation, without the need for a secondary side reference and an optocoupler feedback.

 

[chaitanyab]

I agree DCM is a better choice for your application and it is easy to design. I think DCM has smaller transformer, Fast transient responce, Lower Stress on Diode and Mosfet, Lower EMI on other hand CCM has lower ouput capacitor, Lower ripple current, good cross regulation for multiple ouput flyback converter.