I believe the original problem was that the raw unregulated voltage
may be marginally too high for a conventional three-terminal to take
safely. I have often encountered this problem, which is due to the
wide input range possible considering the worst-case line voltage
tolerance, transformer regulation, transformer selection limits, and
possible surge voltages. If you drop the voltage with extra stages or
series devices, you may run out of headroom, but if you don't, then
it may run dangerously close to the maximum input rating of the
regulator. If adding to, or reusing existing power circuits, there's
often already some degree of protection from MOVs or gas-tubes, but
these are very coarse, so are unlikely to be effective.
Whenever I run a three-terminal regulator from raw DC, I put in a
series fuse or PTC, and an over-voltage clamp ahead of it to assure
that the input rating will never be exceeded. Also, the load may need
to be protected - it depends on its characteristics. If the raw
voltage is too high, the first step is to add some series diodes that
can drop it some - you just have to make sure there's enough
remaining headroom at lowest line and maximum load, etc conditions.
Be sure to bypass the regulator input with as much extra C as
possible to stabilize it, and provide more filtering. In the other
extreme, when high-line occurs, there should be a comfortable
distance from the raw voltage to the input rating. Then, only
transient protection should be needed - heavy zeners or transorb type
devices should be enough. If facilities are available, it's best to
do this design part empirically, with actual parts, variac, and curve
tracer - you can put real conditions on the real stuff. I also always
add the usual reverse-protection diode across (O-I) the regulator,
even if this fault seems unlikely - I can't count the number of times
I've accidentally shorted out the regulator input nodes during design
and construction - this would normally take out the regulator if it
has lots of output C (almost always).
If the peak at high-line is too close to the input rating, then I use
an amplified zener clamp, with a big bipolar transistor driven by a
big zener of the proper voltage. Alternatively, a traditional SCR
crowbar circuit could be used, but I prefer a more transparent,
self-resetting solution. Depending on the particular application, my
favorite, and the most durable version is the "substrate" style,
where the supply is referenced to a chassis ground or board ground
plane. For example, to protect a positive supply, a large PNP
transistor is anchored directly to the chassis - I'm talking about
using only power devices, like TO-220 and larger - with no isolation,
to provide maximum cooling from the collector tab into the substrate
(the collector lead should go to the supply common too). The emitter
then goes to the +supply, and the zener (typically 1W size) goes from
the base (K) to ground (A). A small resistance around a couple of
hundred ohms from B-E will make sure it doesn't turn on unless
needed. For a negative supply, an NPN would be used, etc. If the
supply common is not the substrate, then the same circuit topology
can be used, except that galvanic isolation from the transistor tab
to the substrate will be needed. Cooling won't be quite as good, but
since the circuit floats, either type transistor can be used for any
polarity. This same type of circuit can of course be used to protect
the load side - this is even easier since the voltage is usually
fixed, and the regulator (even if failed) adds to the series
impedance limiting the fault current. I also put a large
reverse-protection clamp diode on any supply that is exposed to the
outside world, or is within a multiple-supply environment, where a
fault between any two is possible - during design or testing too, as
above, even if unlikely in operation.
The transistor SOA rating should be sufficient to trim the peaks
during possible high-line and transient conditions, and clear the
series fuse if necessary, depending on the situation. Bigger is
better for this purpose.
The degree of protection and complexity, of course, depend on the
criticality of failure, and value of the load. Not much is needed for
routine or low power circuits, but for very important stuff, these
things can make it nearly indestructible from the powering perspective.
Ed
>>Bob Stewart wrote:
I also wanted to reduce the amount of power wasted through passive
devices. As it turned out, though, I had more tolerance for heat
waste than I had thought. But, the general discussion this has
become is also good.
<<
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