Why does PI Expert always give DCM topswitch flybacks?

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Dear Staff of Power Integrations,

Why does PI Expert always  implement every flyback as a DCM flyback?

I  did an 8V, 6A flyback in the PI Expert software and it did it as DCM. The secondary peak current came out as 26 Amps. At peak current levels like this, surely it would be better to use a CCM design?

If it had been done with a CCM design, the secondary peak current could have easily been reduced to 9.7 Amps.

The flyback spec was:-

Vout = 8v

Iout = 6A

Fsw = 66khz

Adapter

TL431

Vin = 85-265VAC

 

Dear treez,

 

DCM flyback implementations are preferred by default (especially for low-power designs) for the following reasons:

 

1.  They are simpler to compensate for stability.

2.  By using valley switching during the turn-on of the main switching FET, efficiency can be improved substantially.

 

For heavier load currents like yours, however, other constraints come into play, as you've noted.  For equivalent output power, DCM results in higher peak currents, which impact transformer and FET selection.  Larger capacitors may also be needed to address higher ripple.

 

-PI-ODO

Thanks, but there is no feedback to the topswitch to tell it that the secondary has just discharged so as to bring about "valley switching".

As far as i know, there are no Boundary conduction mode, quasi-resonant flybacks in the product range of power integrations?

Can we use the PIXIs software to change the design to a CCM design?...and it will recalculate the phase margin and crossover frequency?

Hi treez,

 

Yes, you can use the PI Expert software tool to achieve a continuous-mode design by setting Kp to <1.0.  The phase margin and crossover frequency will automatically be recalculated.

 

-PI-ODO