In switcher parlence, the "start up" capacitor is called the
bootstrap capacitor because the supply picks itself up by its
bootstraps. It is often charged by using a high wattage resistor
connected to the power line, with a diode, capacitor, and zener
to limit its DC voltage. It supplies power to the control PWM
circuitry. When the supply starts, the bootstrap will be taken
over by voltage from a winding on the switcher transformer.
Half wave rectifying creates a lot of ripple current in these
capacitors, and they very often... and I mean VERY often go open
circuit, preventing the supply from starting.
I have fixed many tens of thousands of dollars of equipment ranging
from sewing machines to TV sets by replacing the bootstrap capacitor
and sometimes the bootstrap resistor.... Even HP supplies ;-)
Competently designed switching power supplies are a delight to
behold. They seem to protect themselves from everything, including
stupid technician mistakes.
-Chuck Harris
David Kuhn wrote:
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Switching supplies can be a problem when they fail. I recently had a
Agilent VXI E4808A chassis that the main power supply failed, at least
its 12 volts out did. That power supply is huge with a logic board mounted
about it ( I think it's a power supply monitor board). It's a lot of
physical work just to get it apart to check caps. Anyway, I doubt
HP/Agilent made that power supply themselves. I really don't know, as to
get information out of them is like pulling hen's teeth. They no longer
sell, or support, those 4 slot VXI chassis's, so it would be nice of them
to release the schematics, or service information. I think you can only
hope to buy a used chassis somewhere. I would love to have the schematics
for them.
In other switches that fail to start, I often find what I call the
"start-up" capacitor in the primary is open or leaking or has a high ESR.
I call it the "start-up cap" as I do not fully understand switching
supplies, and often if there are schematics, there is not a theory of
operation with them, but there is often a capacitor in the primary circuit
that looks to be a short to ground and then charge-up to create the initial
switch swing to get it going and then afterwards, the power supply self
sustains.
So a lot of times with a dead switcher, I have been able to fix them by
replacing the small electrolytic in the primary side of the circuit. It is
usually a 4.7uf or 10uf. Other than that, if the rest are not physically
leaking or swelled and the supply is working I don't touch it.
One instrument that I work on has an on-board +5volt switching regulator
circuit. It is very reliable. Two times (since 1999) I have seen the
switch regulator fail where the output drive to the FET shorts to ground
allowing the supply voltage (~+12volts) go through to the output with no
over-voltage protection, not even a +5.6volt zener to short. It blows
every TTL chip on the +5volt rail, and some some regulators that follow it.
Stupid, stupid German design. You have to be very urber careful probing
that +5volt switching regulator circuit. One slip of the scope probe and
you can simulate the switching regulator IC output shorting to ground
turning on the pass FET full time. So, I've learned "If it aint broke,
don't fix it"!
Sorry it that was slightly off topic.
Dave
