Download Building a Better Voltage Regulator for Your Test Fixtures

Building a Better Voltage Regulator for Your Test Fixtures
For those of you who roll your own test fixtures here is a suggestion for an improved
Voltage Regulator for powering the Device Under Test (DUT). The voltage is easily set
by selecting one resistor. Output current is designed for one Amp or less at a maximum
of around 30 Volts. Specific maximum would depend on current being delivered and
whether or not you put a heat sink on Q7. As shown it is designed for +13 Volts out at
500 mA with no heat sink on Q7. It does not do current limiting. That is handled by a
separate stage of the power supply. For that purpose the test fixture has a Control input
(Testing) that is used to turn the regulator on or off or turn it off when an over-current
condition is sensed.
Differences between this design and others.
In most of my previous voltage regulator designs current limiting would have been done
by sensing emitter current of Q7 and using that signal to bypass the Emitter-Base current
of Q7 turning it off. That is the usual method used by most regulators. This puts the
current sense resistor between the regulator and the DUT. Any voltage we drop across the
sense resistor degrades our voltage regulation to the DUT. In extreme cases of over
current we actually get current through the bypass transistor. Sometimes this can be
enough to stress the bypass transistor to the point of failure. So let’s move the current
sense circuit to the other side of the voltage regulating circuit in a pre-regulator design.
Output voltage is usually set by a Zener diode. This requires a stuck of Zener diodes
being on hand for different designs. This has been replaced by a TL431. This is an easily
programmable voltage reference set by a single resistor. The setting resistor is R23 in this
circuit.
Q2 and ISO1
This circuit is intended to come from a microcontroller or a current sense circuit. It is
powered by an isolated power supply (VCC) and ground (GND TF). When “Testing” is a
positive voltage Q2 turns on, ISO1 turns on and the voltage regulator turns on.
ISO1, Q6, and U4
When ISO1 turns on Q6 turns on. U4 is the TL431. It gets its current source through Q6
and R21 from the V IN power line. When Q6 turns off we lose current to U4 and our
output voltage drops to 0 Volts. When Q6 turns on we develop a voltage across U4 that is
about 1.2 Volts higher than our desired output voltage.
Q7
Q7’s emitter is the output of our voltage regulator. The voltage here will be about 1.2
Volts less that the Base voltage. U4 regulates the voltage on the Base and puts Q7 into a
conduction state to give us the output voltage we need. We turn Q7 on and off through
Q6 and Iso1. Q6 and ISO1 are in the Base – Collector circuit of Q7. As we turn ISO1 and
Q6 on and off we also turn Q7 on and off.
The Base current of Q7 is minimal. With a current gain of around 1,000 and an emitter
current of around 500 mA we have a base current of less than a milli-Amp. For good
regulation the current through U4, Q6 and R21 should be five to ten times that amount.
As designed there should be about three to five volts difference between V IN and V
OUT. These characteristics give us a value for R21 if you should want to make changes
to the circuit.
Putting the over-current sense and control circuits under the control of a microcontroller
we have the prerogative of making it Slow-Blow, Fast-Blow or even Self-Resetting by
software control.