[Tig] Spirit Shutdown
Fri Oct 12 18:19:57 BST 2001
From: "John Hinchliffe" <john-h at lineone.net>
> Power supplies, on the other hand, are not so cheap
> and tend to blow up if they are frequently turned on and off. Effectively,
> the power supply is a dead short across the mains during the first half
> cycle of the mains power - which is really quite a big hit to keep taking
> over and over.
Not necessarily the case any more, except for outdated or cheap switch-mode
The input topology of a typical half or full bridge SMPS consists of an EMI
filter, a start-up current surge limiter, an overvoltage surge suppressor, a
rectification stage and the input bulk filter capacitors. There might even
be a power-factor-correction circuit and, of course, after that you'll find
the switching mosfets, snubbers, transformer, etc.
Of the above, the start-up current limiter typically consists of one or two
Thermistors with a cold value of 6 to 12 ohms and a hot value down around
0.5 ohms. This, BTW, is one of the reasons why you shouldn't power-cycle
systems quickly, that thermistor needs to cool to it's cold value in order
to protect the input rectifiers from abnormally high currents.
What happens during startup? The goal is to charge the input bulk capacitor
so that a comparator in the control circuit can give the go-ahead and let
the oscillator through to the MOSFET's, etc.
On a good design the assumption is that the surge current could be around 5
times greater than the average DC operating current and, with this in mind,
the appropriate semiconductors are chosen. The input rectifiers are the
primary focus of this decision because the MOSFET's do not start conducting
until the right voltage appears at the bulk capacitor for a couple of
cycles. The primary failure mode of MOSFET's is magnetic core imbalance or
core saturation, with catastrophic results. The EMI filter, with its series
inductance and MOV also play a role in preventing abnormally high current
and voltage from reaching the rest of the components.
That's the basics. If you choose to turn off anything with a SMPS, keep it
off long enough to let things come back to a reasonable startup condition.
If the power supply is designed properly it will not be harmed. Of course,
we've all seen perfectly good (and expensive) transistors protect $0.50
fuses. So, there, what do I know!
eCinema Systems, Inc.
ecinema at pacbell.net
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