High-Voltage Regulator With Short Circuit Protection

The
circuit is a positive series regulator, using a pnp transistor (T2) to
keep the voltage drop as low as possible. The operation of the circuit
is very straightforward. When the output voltage drops, T4 pulls the
emitter of T3 lower. This drives T2 harder, which causes the output
voltage to rise again. R4 restricts the base current of T2. C1 and C2
have been added to improve the stability of the circuit.

These
are connected in series so that the voltage across each capacitor at
switch-on or during a short circuit doesn’t become too large. You should
use capacitors rated for at least 100 V for C1-C3. D1 protects T2
against negative voltages that may occur when the input is
short-circuited or when large capacitors are connected to the output. We
use two zener diodes of 39 V connected in series for the reference
voltage, giving 78 V to the base of T3.

Because R6 is equal to R7
the output voltage will be twice as large, which is about 155 V. T4
acts as a buffer for potential divider R6/R7, which means we can use
higher values for these resistors and that the voltage is not affected
by the base current of T2 (this current is about the same as the emitter
current of T3). This is obviously not a temperature compensated
circuit, but for this purpose it is good enough.

The
current limiting section built around T1 couldn’t be simpler. When the
output current rises above 30 mA the voltage across R1 causes T1 to
conduct. T1 then limits the base-emitter voltage of T2. R2 is required
to protect T1 against extremely fast peak voltages across R1. R3 is
needed to start the regulator. Without R3 there wouldn’t be a voltage at
the output and hence there wouldn’t be a base current in T2. R3 lets
T2 conduct a little bit, which is sufficient for the regulator to reach
its intended state.

During normal operation with a voltage drop
of 15 V across T2 and a current of about 30 mA there is no need for
extra cooling of T2. The junction temperature is then 70 °C, which means
you can burn your fingers if you’re not careful! The lower the input
voltage is, the more current can be supplied by this regulator. This
current is determined by the SOAR (Safe Operation ARea) of T2. During a
short circuit and an input voltage of 140 V the current is about 30 mA
and T2 certainly requires a heatsink of at least 10 K/W in those
conditions.

To increase the output voltage you should use a
larger value for R6. If you want to use a higher reference voltage, you
should replace T4 with a MJE350. If you only ever need to draw a few
milli-amps there is no need to include T4 and R4. The potential divider
(R6/R7) can then be connected directly to the emitter of T3. The ripple
suppression of the circuit is about 50dB. The quiescent current is 2.5
mA and for small currents the dropout voltage is only 1.5V.