Most regulator circuits
require at least 3 to 5 volts between input and output to operate reliably.
This regulator features a P-Channel MOS-FET pass element that will allow
for a very low drop out voltage. In my particular application reliable
operation was noted down to nearly 0.1 volt between input and output at
nearly 0.8 amperes of load current. Useful applications include automotive
and battery applications were regulation is desired and available input-output
differential is less than 0.5 V. By using the LM324 Op-Amp
and the LM336 Precision Reference Diode, both single supply operation and
precision output regulation were easily achieved.
I originally designed this
for a 12V camera. The camera was a surplus property item and contained
only the camera and a small adapter box. The 1" x 2" 1" adapter box
contained a connector from which video, audio and power adapter connectors
were attached. My intent was to use a 12V 1 amp adapter from
Radio Shack to power the camera. Unfortunately the adapter provided
an unregulated 15V under no load and slightly more than 12.3V under full
load. This necessitated the use of a very low drop-out regulator
circuit. By using a MOS-FET I could minimize the drop-out voltage
and have the added benefit of very low power dissipation. This design
allowed for minimal heatsinking and could be packaged very tightly in nearly
any small enclosure. In my final design the entire regulator circuit
fit easily in the 1" x 2" x 1" adapter case.
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Reference Section:
The reference section is
formed by Vr1 (LM336BZ-2.5) and one op-amp section of IC1 (LM324A).
Vr1 is a precision 2.5 V reference diode. With a nominal operating
current of 1 mA Vr1 will typically produce an output voltage of 2.49 V.
When coupled with a non-inverting amplifier circuit and a bias resistor
( R1) a precision self-regulating reference source can be created.
The resultant circuit is self regulating and programmable.
Through the selection of R2 and R3 the output can be program to nearly
any output reference voltage. In this circuit the combination of
R2 and R3 produces an output of 6 volts.
Regulator Section:
The output of the reference circuit is compared with the output through the second op-amp of IC1. The output of the second op-amp drives a common source P-channel MOS-FET. Q1 provides both voltage and current gain. One of the primary features of Q1 is the low RDSon that allows for near zero input-ouput voltage differential regulation. When combined with single supply operation, low input offset, features of IC1, we are able to achieve near perfection in a power supply regulation.
Ac Adapter Notes:
I chose to use an adapter
to supply filtered, unregulated dc voltages to the regulator circuit.
Radio Shack adapter 273-1653B provides a loose 12.5V to 15V dc output.
My tests indicated 15V open circuit and slightly more than 12.8 volts dc
with 1V ripple at 0.8 amperes of load current. Under normal conditions
this should be sufficient, however under low line and high load conditions
regulator drop-out could occur. This is primarily due to power supply
ripple and the marginal output of the adapter ( less than 12V at full load
conditions). I found that under normal line conditions the
adapter provide about 11.5 V under full load. Note: loose filtering
and fairly wide output voltage swing tolerances may be the norm.
When possible I would suggest that you test the adapter prior to use.
This regulator will provide nearly zero input-ouput regulation characteristics
however it cannot compensate for low voltage, high ripple inputs.
Quite simply the regulators can only work with what you give it, it can't
make up for voltage that is below what is expected on the output.
Investigate a little before you assume that the regulator is at fault.
You may alternatively chose to
build your own bulk power supply and the minimum part specifications have
been included. The schematic for this section is shown below.
Subsequent sections are based on this assumption.
Transformer Notes:
When selecting a transformer some substitutions may be necessary. Center Tapped transformers may be used if rated as 12.6 VCT or 15 VCT. In the case of center tapped transformer use simply use the outer two leads and insulated the center tapped lead. Connect the outer two leads to the diode bridge.
Diode Notes:
Substitutions on diodes are allowed as long as the PIV (or PRV) rating is equal to or greater than listed. This is true of the current rating as well. 1 amp diodes may fail under extended use or transient conditions. Diodes with a rating of 50 PIV may fail under transient conditions and should be avoided. When installing diodes avoid excessive heat during soldering, also leave at least 0.5" of lead length from body to attachment point. Diode bridge assemblies may be used as a substitute, above ratings should apply. It is recommended that the diode bridge be lightly heat sinked for maximum protection against thermal overload.
Capacitor Notes:
Capacitors C2 is required for proper operation. Avoid using output capacitor values of less than 25 uF for C2 or output oscillation may result. If you plan on using this regulator with large capacitor loads, place a reverse bias diode across the drain to source leads of Q1. This will prevent reverse current failures during shut-down. C1 may be increased, however this will increase diode and regulator heat dissipation. You may substitute a combination of 2 - 2200 uF capacitors or 4 -1000 uF capacitors for C1 if the 4700 uF is not available. The capacitors must be placed in parallel with each other. That is to say that the positive leads of the capacitors must be tied together and likewise the negative leads must all be tied together.
Fuse Notes:
Fuse selection is based on a conservative 400% overload. Use of a larger fuse may result in component failure. The use of circuit breakers is not recommended.
The power supply should be assembled with a three prong cord for maximum safety. Ground prong should be attached to either transformer or project box (if metal). Length of ground wire should be at least 1" longer than wires going to switch and transformer. The MOS-FET should be heat sinked to back of case (if metal) or attached to metal heat sink. A heat sink of approximately 3" x 2" high with at least 0.5" fin depth is recommended. Insulate the MOS-FET transistor from the metal case or heat sink with thermal insulators. The use of a light film of thermal compound between the heat sink and the transistor is strongly recommended. Be sure to use either a nylon attachment screw or plastic shoulder washer to completely insulate transistor from case or heat sink.
It is recommended that the project is tested in stages during assembly. Adult supervision is strongly advised. Follow established safety practices and ensure that during repair that the power supply cord is removed from the electrical outlet before proceeding. Words of Wisdom :
Avoid testing the primary
side of the circuit unless absolutely necessary. With a transformer
rated at 12.6 VAC you should measure about 12.6 VAC or more with no load.
After the diodes you should measure about 15 VDC with no load and about
14 VDC under full load conditions. The ripple voltage under full
load should be about 2 volts peak to peak. The output voltage of
the regulator should be 12 volts +- 0.6 volts (5%). This variation
may be trimmed by placing various high values across either R5 or R6.
A 330K resistor across either resistor will raise (across R6) or lower
(across R5) by about 0.1V. Between no load and full load the output
voltage should not vary more than about 0.1 volt. If you find
that it does, make sure that R5 is tied close to the output as close as
possible to the output terminal of the power supply. Use at least
18 gage wire for the output and ground connections. Keep all connections
short and direct to avoid ground loops and excessive voltage drops.
Under full load conditions the Q1 may be warm, if you can measure it, be
sure that it operates at temperature of less 180 degrees Fahrenheit.
This translates into about 80 degree Celsius which should provide a reasonable
margin of safety. If the MOS-FET runs hotter than this you will need
to provide extra heat sinking. Either increase the metal radiating
surface (increase the size of mounting surface) or use a larger heatsink.
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In this photo the heat sink is shown. |
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| Here the assembled PC board is shown connected to the various connectors inside of the adapter box. | Finally the completed assembly is shown. For this particular project all photos were taken with the power supply operating as shown. |
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| Harris Semiconductor Product Data Sheets |
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| International Rectifier |
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| Motorola Semiconductor Product Data Sheets |
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| National Semiconductor Product Data Sheets |
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| DigiKey Corporation |
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| Disclaimer: |
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