Re: Transformless Transformers


The problem is that with most MOSFETs "off" is only off to a few
hundred volts, then it turns on again. You need kV for safety. If you
try to do that with semiconductors a transformer starts to look very,
very attractive in almost every way. A transformer can get kV with a
bit of tape and insulated wire. And it will pass safety agency
mandated hipot tests with ease. I can't think of a single place I've
seen a semiconductor allowed between hot and ground, but maybe someone
else can.

Imagine the hot is connected to S1, and your toddler is sitting in a
grounded metal bathtub chewing on S3/S4. Then the fat mechanical
contactor in your beer fridge switches off (the microcomputer
determines Tb <= icy_cold) and the compressor motor inductance
superimposes a 1.5kV spike on the hot. Do you feel comfortable with
this situation?



yes, as I mentioned the circuit senses the current through each branch so
that no current can be between both. As I mentioned in my original post, if
S1 is shorted for some reason then current will always be flowing through
that branch which will preclude S3 and S4 from being open.

So even if S1 and S3 were both shorted, as long as the "saftey circuit" was
functioning then everything would be ok. If that circuit is well designed
then it should allow the circuit to be completely isolated. i.e., assume
the saftey circuit is ideal... then in no case will any significant amount
of current flow.

Now I do not know if one can practically implement the saftey circuit to
approximate the ideal case enough for your example but it is a fairely
unreasonable one too. You could easily come up with off the wall cases for
transformers too.

Of course what you are really asking is, if S1 or S3 fails then will it
still be isolated.. This is much more reasonable because S1 and/or S3 will
eventually fail hence the "saftey circuit".

e.g., the saftey circuit might control a circuit breaker before the
isolation that is tripped when current flows through both branches which
will shut off the circuit. One could add multiple switches in series to
significantly decrease the chance of failure. (this doesn't avoid
catastrophic failure though). e.g., if p is the probability a switch will
fail then p*p is the probability of 2 switches failing. (of course p is
really a distribution)

I do agree that transformers are not useless(I never said this would replace
them for all applications) but transformers are not cheap and do not out
perform this topology in many regards. One has to look at the complete
picture. We already see non-transformer based power supplies all the time
now. My "idea" only adds to that by adding more isolation.

Computer SMPS use transformers for isolation after the switching circuit
because it can be made smaller. By adding my isolation method the isolation
is increased and potentially the transformer can be removed. Of course this
might not be a great idea but might be applicable in some cases.

On only needs to design the safety circuit to have a failure rate that of
transformers and ability to sense accurately enough. This may or may not be
impossible.



.

Relevant Pages

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