2016-10-26 Why Separate Power Supplies Are Needed

Peter asked a question about why is there a need for two separate power supplies for the same project.  I guess it never occurred to me to explain this simply, so everyone can understand it.

I have often had to use a separate power supply for one part of a circuit, with a second supply for the rest of the same circuit.  When I built both parts using a single PS, I found that the circuit had a tendency to act unstable or sometimes it would “motorboat.”  This is caused by feedback from one part of another through the power supply.  In order to quickly eliminate the problem, I used two separate power supplies while I was working on the circuit.  This completely eliminated any coupling between parts of a circuit.

As an example, suppose we have an audio amplifier with three stages of amplification. The first stage is a high gain microphone preamplifier. The second stage is a voltage amplifier, and the third stage is the power out to drive a speaker.

Each stage inverts the signal so that a positive going signal at its input is negative going at its output. So the microphone input is positive going, the preamp output is negative going, which is also the input to the second amplifier stage. And the output of the second amplifier stage is positive going, which is the same as the positive going microphone. The result is that the output of the second stage can go back through the power supply line and cause the two stages to oscillate. This is usually at a low frequency, because the power supply has large value capacitors. Thus it can sound like a motor boat putt-putting. The oscillations can be in bursts, and can be heard and seen on an oscilloscope.

An easy solution is to isolate the power to the stages by using separate power supplies. If this fixes the problem then the solution is to use a resistor and capacitor in the power supply line between the first and second stages. If it already has the resistor and capacitor, then this low pass filter is not low enough, and should be lower.

The third stage is the power amplifier stage and draws heavy current to drive the speaker. The power supply may consist of a transformer secondary winding, a bridge rectifier and a large filter capacitor of several thousand uF. When the amp puts out high power to the speaker, it draws high current from the transformer, rectifier and capacitor. The supply voltage drops, and the supply to the first and second stages also drops. The signal of the first stage is inverted compared to the power amp, so probably it won’t cause motorboating. But the second stage is in phase with the power amp, so it could be the cause of motorboating. We could put a resistor and capacitor between the second stage and power amp, be the supply voltage to this second stage must be high enough to drive the power amp to full power. If the resistor drops too much voltage, the drive won’t be high enough. One solution may to use a diode in place of the resistor, so the voltage drop will be less than 1 volt. And the filter capacitor for this second stage should be large enough to hold the voltage high during the time the power amp is sucking a lot of current from the power supply. A separate supply would be the best choice for this second stage. But designers will not be able to spend the extra cost to do this, so they will compromise and settle for less power, and use the resistor.

I have only discussed the feedback through the power supply. There are other ways the signals can leak out of a high power stage and get back into the earlier stages. One is by induction. The high currents generate magnetic fields that in turn induce currents in other components, especially if they are electromagnetic. These could be chokes, transformers, or even long leads. If the high power stage has high voltages then the stray currents might be induced electrostatically. The small capacitances between wires can be enough to cause feedback, especially at high or ultrasonic frequencies. This is why low level signals are run through shielded cable.

Also I haven’t mentioned ground loops. When the high power signals return from the speaker, they may go through the same ground wiring that is used by low level signals. The induced currents can cause major problems. This is where a separate power supply may help isolate the ground currents. The usual solution is to run separate grounds for the low level signals all the way back to the rectifier and first filter capacitor.

The schematic may not show all the design changes that were made to keep interference from causing these problems. The troubleshooter may see these modifications in the circuit board when he traces down the circuit. Sometimes pieces of metal shielding cover parts of a circuit board. And for shock and safety protection, the circuit board may have slots cut between parts of a power supply to isolate the high voltage from other circuits.

Separate power supplies are a quick way to get a device running. In a piece of commercial equipment, the designer may use them at first but eventually the design will have to be changed to operate from a single well isolated supply.

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