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Offline flyback with on time less than 1us is OK?

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treez

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Hello,

we are designing a flyback (35W) for vin = 85-265VAC.

Vout = 25V
F(sw) = 70KHz.

We wish to be in DCM across the mains range, and also be dcm across the range.

This means that at the peak of high mains (374VDC) the on time of the fet is just 1us.

is this too short a time for a fet to be on?

If not, what woudl be the limit of fet on time?......would 300ns be ok?

I thought that if the switching transition times become a significant portion of the on time then control problems start to occur and oscillations are seen in the output voltage?
 

is this too short a time for a fet to be on?
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No, but transistor switching times and related switching losses are giving a practical lower limit.
 
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You need a very effective gate drive (30nS transitions) to keep the losses in the mosfet down for short on pulses (<1uS), better to go for a resonant flyback (sometimes called quasi-resonant)
 
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Thanks Orson, i see what you mean, but from one viewpoint, even with longer on times, the fet still has to switch on and off, so surely the switchin glosses will be no worse with short on times?
 

the fet still has to switch on and off, so surely the switchin glosses will be no worse with short on times?
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Losses will be surely worse with short on time. Switched current will increase inversely proportional to on-time. In a rough estimation, switching losses are equal to trf*Usw*Isw.
 
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Ah, here we see that experience counts, if you have a very fast, low Z, gate driver then you can achieve short on times with similar switching losses - but if your gate drive is a bit soft then the fet will not be fully on before you start to order it off again - result = fet operating in the linear range with high losses.
 
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