I'm looking to replace a stack if 1N4085 low TC zeners used to make a stable
600V reference. Any ideas for a non-zener based shunt regulator that would
give a stable 600V. load is less than 1mA.
thanks
You didn't say how stable. "Back in the old days" people would run an
NPN planar silicon transistor with negative on the collector (forward
biasing the BC junction) till the BE junction zenered. The
temperature coefficient of the resulting zener+PN could be quite low.
So you _may_ be able to do well enough using something like a high-
voltage transistor with a 100:1 (nom) resistive divider,
collector:base:ground, with about a 5.6V zener emitter to ground. You
need to use a high beta transistor so your resistive divider can be
moderately "stiff" relative to the required base current. And you can
overcome that by using a darlington connection with two 5.6V zeners in
series, emitter to ground. Since high voltage transistors tend to not
have very high beta, you could use a lower-voltage transistor whose
collector is connected to a lower supply voltage. To keep transistor
dissipation lower (1mA at 600V is still 600mW) you could put a
resistor in the collector of the HV transistor; voltage divider still
must come from the regulation point, of course.
A problem with the above is that current through the zener may vary
too much to maintain the stability you want. The thermal may be OK,
but not the load regulation.
Personally, I'd look at adding a transistor and a divider to a shunt
regulator IC. I remember doing this in a design a long time ago, and
it worked well (though you do have to be careful about compensation; I
recall discovering that they didn't tell you everything you needed to
know in the data sheet for the regulator, since they hadn't
anticipated anyone would want to use it the way I was trying to... a
different shunt regulator part number solved the problem nicely
though.) You may even be able to find an ap note about doing this, or
an example circuit in a data sheet. I'm thinking you might try TI,
National, Linear Technology, Analog Devices... A TL431 looks like it
would do just fine if you simply add an NPN high voltage "cascode"
transistor between it's "cathode" lead (transistor emitter) and the
regulation point (collector) with the base running at 5V or so. Be
sure to use a high voltage resistor (or at least a string of several
resistors in series) for the feedback from the regulation point. It
would be courting disaster to use an 0805 SMT part with 600V across
it...
Cheers,
Tom