UCC28630: Safety in event of a secondary short

Hello Alex,

Thank you for your interest in the UCC28630 flyback controller.

I understand your concern with the case of a shorted output rectifier diode.  In such a case, the output capacitor would rapidly discharge through the shorted diode and secondary winding.  If the primary MOSFET were to turn on during this discharge moment, it would experience a very high peak drain current until the current sense (CS) loop can turn it off again.  If the output cap was already discharged by the time the primary side turned on, it would impress a 0-V shorted condition across the secondary winding, reflected to the primary.  In either case, the rise of primary current will be limited by the leakage inductance of the transformer, the bulk voltage applied to the primary, and amount of time it is applied.  This is very similar to a shorted-output condition except that the diode impedance will not be in series.

In the UCC28630, the CS loop (Figure 32, page 35 of datasheet) has a leading-edge-blanking (LEB) interval of 100ns (pg 34), a propagation to output delay of 100ns (tPROP(gate), pg 52), and the turn-off delay of the MOSFET itself after Vgs goes low.  The MOSFET will need to sustain the peak current (Ipk = (Vbulk/Lm)*(tLEB+tPROP+tFET)) cycle by cycle until the VDD voltage decays below the UVLO shutdown threshold of 8V.  After that, the controller will attempt to restart periodically with three exploratory gate pulses (through VDD-UVLO cycling) but fail each time as long as the shorted condition exists.

It’s a good idea to put a second diode after the main rectifier will help to reduce stress on the output capacitor from the reverse voltage which would attempted to be impressed across it during the main FET on-time.  A large cap may be able to sustain the brief reverse pulses indefinitely, but a small cap may not without the anti-parallel diode.

A fuse in parallel with the secondary rectifier would simply be a short circuit itself, and then blow open after a few pulses.  Rather a fuse in series with the output diode would open only on excessive current.  However, the fuse would need to not blow under normal rms loading and any nuisance overloads, but is expected to blow in the shorted diode condition.  Depending on leakage impedances, these may be mutually exclusive.  Depending on circumstances you may or may not be able to find a single fuse that will blow when you want it to and not blow when you don’t want it to.  Plus it will increase the total leakage inductance all the time that such protection is not needed.

A suitably rated primary MOSFET should be able to survive the shorted diode condition since it is virtually the same as a shorted output condition, which is a standard test for power supplies.   An anti-parallel diode across the output cap may be useful if that cap needs protection from reverse voltage/current pulses.  

Regards,
Ulrich