There are several instances when external diodes (or FETs) are used with power supplies:
1. Driving DC Motors
2. Series Operation
3. Redundant Operation
4. Battery Back-up

1. Driving DC Motors
There can be confusion regarding the use of external diodes when powering DC motors; mostly where to place the diode or what their purpose is. There are two types of DC motors; a brushed DC motor and a brushless DC motor.

Brushed DC motors
With this type of motor, the magnets are stationary and the coil spins – electricity is transferred to the spinning coil by the use of ‘brushes’. The advantages of this type of motor are low initial cost and easy speed control.

When the power is interrupted, the motor coil will act like an inductor and will try to continue to produce current, effectively becoming an inverted voltage source. This will apply a reverse polarity to the power supply and can cause damage. (Back EMF – Electro-Magnetic Flux)

Inserting a diode, as shown above, provides a current path for the reverse motor current and will clamp the reverse voltage to a level no greater than the forward voltage drop of the diode. This protects the power supply’s output capacitors and other components from being stressed by the reverse voltage.

Brushless DC motors
Brushless DC motors, often referred to as BDCMs or BLDC motors, have permanent magnets that rotate and the armature is fixed. Although more expensive than brushed versions, they are more reliable in the long term as there is no brush or commutator wear and position control is more accurate.

When the motor is turned off or reversed, it will act as a generator and produce a high voltage spike. This spike can cause the power supply’s overvoltage protection to trip, shutting down the unit. By using a diode in series with the output, as shown below, the spike will be blocked from interfering with the power supply.

In both cases a general purpose diode can be used, providing that the voltage and current ratings for the diode are correctly calculated.

2. Series Operation

It is quite common practice in the industry to operate power supplies in series. The benefit is that voltages greater than 60V can be obtained using off-the-shelf products.

It is possible to connect several power supplies in series, but please read the precautionary notes below:

Connect back-biased diodes across the power supply terminals as shown below.

Rate these diodes at the same output current as the power supplies.

In the event both power supplies do not turn on at the same time, or if the load becomes a short circuit, then the diodes will protect the power supplies from any applied reverse voltage.

Do not exceed the output to ground/chassis voltage rating. Inside most power supplies are noise filter capacitors connected from the output to ground. It is possible to exceed the operating voltage of those capacitors, particularly when configuring several units in series.

Avoid using ‘fold-back style’ current limited power supplies as these may lock up the power supply during initial switch on.

3. Redundant Operation

In order to achieve redundancy within a system, it is common practice to connect two power supplies together. (Please note: this is not to be confused with brute force mode current sharing)

If PSU #1 fails, theoretically PSU #2 should take over…however

If we were to look inside the power supply, the output voltage is usually monitored by an op amp and is then compared to an internal reference. If the output voltage is too high, then the comparator will lower the output voltage of the control circuit by reducing the switching converter pulse width. Likewise, if the output voltage is too low, the switching pulse width will increase to make the output voltage rise.

For example, PSU #1 is running at 24.0V and PSU #2 is set slightly higher at 24.1V. PSU #1’s control circuit will ‘see’ 24.1V as an output voltage and will turn the switching converter off believing that its output voltage is too high.

In the event of PSU #2 failing, the load demand will fall on PSU #1, which will then turn the switching converter back on and may cause a temporary loss of voltage provided to the load.

Adding a diode in series with each power supply output will stop the power supplies from “seeing” the other’s output voltage; although PSU #2 may provide the entire load, if it fails, PSU#1 will be active, ready to provide power and be able to keep a voltage available to the load.

4. Battery Back-Up

On many low cost, low wattage power supplies, overvoltage protection is provided by a Zener diode connected across the output terminals of the power supply. In the event of a control circuit malfunction causing the output to rise, the Zener will fail in short circuit mode, thereby forcing the power supply into overcurrent protection mode. In this case, it is assumed that ‘hiccup’ type current limit mode is being used.

If battery back-up is being used (or another power supply), then current will flow into the faulty power supply and cause overheating of the Zener and surrounding circuitry.

Again, a diode in series with the power supply will prevent this.

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