LVPS.TXT

This document describes how the circuit shown schematically as
LVPS.GIF operates.

OVERVIEW
The power supply is best described as a "flyback switching regulator"
supply. Although such a supply can be implemented in many ways, the
one shown here is implemented with inexpensive, commonly available
parts. It also uses an off-the-shelf inductor, so no special winding
or transformer is needed.

As shown, the supply converts +12.0VDC into "high tension" voltage.
The "open circuit" output voltage is about 172VDC, and the supply
"idles" with 12V and 15 mA input. With a 75 mA load, the supply
produces about 158 volts. At this point it is drawing 1.24A from
the 12V source.


CIRCUIT
A CMOS 4049 Hex inverter package is used as the heart of the circuit.
Three of the inverter sections are connected as a Schmitt trigger
oscillator. The basic frequency is about 30kHz, but varies with
line and load changes. The oscillator is U1A,B,C. The basic frequency
is primarily controlled by R2 and C5. The "duty cycle" of this
oscillator controls the output voltage and regulation of the
output voltage. This uses one of the inverters (U1F) as a comparator.
CMOS will sort of act as an inverting op-amp, whose output is
"centered" when the input is "centered" on the supply voltage. Thus,
with a 12V input, the output (pin 12) will be 6 volts when the
input (pin 13) is 6V. The gain of this stage is relatively low (10-30)
so the output regulation is not spectacular, but, no stability
problems will be encountered. With "no load" pin 13 is about 8.2V,
and decreases towards about 5V at maximum load.

The duty cycle of the oscillator is controlled by coupling this
comparator into the oscillator via R7. R5 provides a "maximum" duty
cycle control so the circuit always oscillates. If you have trouble
with the oscillator producing too high a duty cycle, R5 may be lowered.

U1E provides a delayed "startup" of the supply, so that any circuit
hooked to the output of this supply remains unpowered for about 40
seconds. This allows tube heaters to become warm before applying HT
voltage. The delay time is controlled by R11, C7. The supply is
"enabled" when Pin 10 goes low. If this never happens, the probable
cause is leakage in C7 or D5 backwards.

The "switch" is powered by an isolating inverter stage U1D and
"totem pole" transistors Q1 and Q3. These provide adequate drive to
supply the gate capacitance of Q2. Q2 is the swtch transistor. It
"charges" inductor L1 when it is "on", and the inductor "discharges"
into D1 and the load when Q2 is "off". For output voltages of 190V
or less, an IRF640 is the best choice of part. For output voltages
greater than this, use an IRF740, which will work to almost 400V.
There is a tradeoff between efficiency (how much power is consumed
at 12V for a particular power output) and this part. For low output
powers (20 mA or so at 170 volts) an IRF630 is a better choice, as
it's lower gate capacitance provides better efficiency. The '640
allows output currents up to about 100 mA (and 150 volts), but is
slightly less efficient, as it has increased gate capacitance. The
IRF740 allows higher voltages and currents, but consumes more input
power (relatively). Only a couple of watts is dissipated in Q2, so
it doesn't need a LARGE heat sink, but does need to have some
heat sinking. If you need to provide lots of power, (higher voltages
and/or currents) use a larger heat sink.

D1 NEEDS to be a fast recovery part. A 1N4007 is doomed in this
application.

The output voltage is determined by D2, D3 and the voltage division of
R9 and R10. Note that D4 is  "never turned on". This part is used
as "overvoltage" protection. It may be successfully eliminated from
designs using the IRF740, and sufficiently high voltage part at C6.

To change the output voltage, the simplest ay is to change R9.
One mA flows in this chain (at no load) providing a relatively easy
way of calculating the open circuit output voltage (assuming you
remove D4 for voltages greater than about 180V).

Vout approximately equals 139 + R9 value in kohms. For example,
to provide 250 volts, use an IRF740, remove D4, and choose an open
circuit voltage of perhaps 259 volts. Then R9 will need to be 120k.
For 150 volts output, choose open circuit voltage of perhaps 157
volts, making the R9 value 18k.

INDUCTOR
This part NEEDS to be able to handle lots of current without
saturating. To get more power output, the inductor value needs to
be DECREASED somewhat. For the supply as shown, the 250 uH inductor
shown in the Mouser catalog as 70-IHA-103 should work. For much
higher power applications, the 70-IH3-250 or IH3-150 or IH10-100
can be used. Other suitable inductors from the Mouser catalog:
542-5254 (250 uH 2.5A) or 542-5252 (125 uH 3.5A). Notice that all
these are "solonoid" style inductors. Getting a toroid NOT to saturate
is tricky, and best avoided.

NOTES
Build the circuit with the connection going to Q2 gate missing, and
the gate of Q2 grounded instead. You should be able to confirm
oscillator etc operation. (If you don't have a 'scope, this will be
indicated by the voltage at Q1/Q3 emitters starting about 0 volts,
then after the 40 second timeout period, this voltage should go to
about 6-8 volts DC (NOT 12V, as it should be switching, not constant
DC voltage). After you get to this point, disconnect the input 12V,
and make sure there is not a short on the output, (ohming it out), and
establish the connection indicated above, completing the supply.

If the output voltage is WAY OFF, you will need to find out why,
using the circuit description above.

When you have things operating, the FET should not be HOT, not the
switching diode D1, nor the inductor L1, although they will get
WARM in normal operation. U1, Q1, Q3 should never even get warm.

This circuit is REALLY not for beginners.

There is considerable RIPPLE current at the supply frequency on
the battery. This can couple into your AUDIO circuit. Depending
on your application, you may want to put an additional LC filter
on the input 12V. (Say 200 uH and 1000 uF)

The output contains perhaps 50-100 mV of ripple on it. This can be
eliminated with traditional RC or LC filtering.