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Voltage Regulator Output Reverse Diode.



Output reverse diodes and why they are required in some circuits.

Output diodes.

Basic Power Supply schematic.
Fig.1: Basic power supply.

C1, C2, D1, D2 is the rectifier. U1, U2 are the positive and negative voltage regulators.
D3, D4 are the output reverse diodes.
R1 is some load between VR+ and VR-.

IC1 is an attempt to show an IC powered by these regulators.
Most ICs require that all inputs and outputs are ≤VCC and ≥VEE.
The diodes shown inside IC1 can be input or output protection diodes or it can be the parasitic pn-junctions inherent in most IC designs.
In case of protection diodes, the maximum current handling is 1 mA to 20 mA.
Forward biasing the parasitic pn-junctions may cause the IC to latch up and potentially be damaged. Most ICs without protection diodes will handle up to 50 mA reverse current before latch-up will occur.

In case of a short circuit between VR+ and VR-, D3 or D4 will conduct, keeping VR+ ≥-0.7 V and VR- ≤0.7 V.
Without D3 and D4, the output voltages will be at same potential somewhere between VR+ and VR- ( not at GND ), depending on which regulator will supply most current.

Ideally D3 and D4 should have the lowest possible forward voltage ( schottky diodes ) and a continuous current handling larger than the worst-case short-circuit current from the regulators.
For most 1..2 A regulator applications, ordinary 1 A rectifier diodes like 1N400x are sufficient if these regulators is the only power source.

For systems using multiple supplies, D3 or D4 may be required to handle the current output from another regulator.
For example for a system running of a ±15 V, 100 mA supply and a 5 V, 10 A supply, D4 must be able to handle the current from the 5 V supply if there is any risk of a short between the 2 supplies.
For supplies where all rails have same polarity, reverse diodes are normally not required.
For low-volume designs, I usually put them anyway as it is a cheap insurance.

I have used fig.1 as an example as this is a circuit where there is another reason to use reverse protection diodes.
When the circuit is starting up, there is 0 V on VF+ and VF-.
One of VF+ or VF- will come up before the other with this rectifier circuit.
Assume VF+ comes up first. Then we have supply on VF+ and 0 V on VF-.
U1 will start to supply current, pulling both VR+ and VR- ( through the load ) above GND.
This may cause U2 to go into latch up, and you will end up with only the positive supply.
I have seen this with LM78xx/LM79xx regulators. For some reason, there are no damage to the regulators, so sometimes the circuit will start and sometimes not. It did take some time to figure this out the first time I saw it.
I have not seen this with LM3x7 type regulators, most likely because the current into the ADJ pin is limited. They can output the wrong polarity during start-up though, so reverse diodes should be included.

If your design requires reverse polarity protection in case of a regulator failure, the diodes must be large enough to clamp the rectifier circuit ( VF+ or VF- to GND ) until something ( hopefully a fuse ) gives in.

Using Transient Voltage Suppressor ( TVS ) diodes for D3 and D4, will provide over-voltage protection for VR+, VR-. This is mainly useful where VF+ >> VR+ and/or VF- << VR- as the difference between a TVS' operating and clamping voltages are quite large.

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