UCC28061 bodge

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An HPS lamp ballast on the bench came in with a  
shorted fet. This is in a UCC28061 two phase PFC  
that is designed for critical conduction.

The design seems to follow the typical guidelines  
for the application, except that the drains of the  
two fets are shorted - completely bolloxing the  
concept of dual phase interleaving.

Two fets, two boost inductors, two boost rectifiers,  
two independent zero-current-detection windings and  
drive circuits - but only one fet is driven at a time,  
with the other slogging the dv/dt and loading the node,  
with both chokes 'sharing' the current. The boost diodes  
can only share as well as such diodes might, when  
connected in parallel.

Not a wide-range input circuit (240VAC only), but  
still using the book 300uH compromise inductor value.  
Concievably it still benefits from zero current  
switching, and at this high voltage, it's unlikely  
that much benefit would result from interleaving,  
but fet conduction losses have got to be 4x that  
of a single phase controller, with the same fets  
paralleled.

Are there any other things to look out for in this  
kind of misapplication?

It ran for about an hour with both fets replaced  
before the same fet position failed (phase B).  
No evidence of gross overheating. I see nothing  
wrong in the drive circuitry.

The load is a conventional lamp inverter. I assume  
that a sudden inverter limit would produce standard  
overvoltage response in the 400V PFC circuit -  
fets are 600V.

RL

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