InnoSwitch3 max. on-time limit

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Hi.

Our projects have to work from 36 Vdc to 265 Vac input voltage.

Even InnoSwitch3 line haven't been projected for such a low input voltage, we prefer it because of its great technology and the very low total pcb space required (as I'd written in the "https://ac-dc.power.com/forum/low-power-design/2015/innoswitch-minimal-vdrain/" topic).

We've overcome the minimum Vds voltage by external start-up biasing (LR645N8-G) and some Vbps start-up issues by suitable transformer turns ratios or some very simple biasing solutions.

My concern at this moment is that, under very low input voltages, the the InnoSwitch3 max. on-time limit (12,4 us) is reached. From this moment, the controller needs to increase the switching frequency in order to keep the output voltage constant. In principle, it doesn't seem to be a problem, but I wish to make sure about that.

In some testing designs using PI Expert, it reports an "info" type notification about this condition (# 703). Should I avoid it? I would NOT like to decrease the VOR, as suggested, because it would lead to a higher secondary max. Vds mosfet (considering the higher positive forward voltage at 265 Vac input).

Thanks in advance.
Christos

Hi Christos,

The behavior you described is as expected since the on time cannot be lower than the minimum threshold, the frequency has to be increased to deliver the desired power. This should not be a problem. If your efficiency is within your desired requirements then this should not be of a concern.

Best Regards,
PI-AP