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Default UCC28061 bodge

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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