Power ORing diodes are used to isolate power supplies from each other and prevent the failure of any one of the shunt-connected power supplies.
Schottky diodes are commonly used in power ORing protection circuits, as they have a fast reverse recovery time, lower forward voltage drop than conventional diodes, and are simple to use in circuits.
Power MOSFETs have a low forward voltage drop and can be used in place of Schottky diodes in power ORing diode protection circuits.
Redundant power supplies improve the reliability of a system
When designing electrical circuits, it is common practice to connect two power supplies in parallel to ensure the break-free operation of the circuit in case of a power supply failure. Redundancy, reliability, current, and power requirements of load are all factors that establish the need for paralleling power supplies.
When using redundant power supplies, it is important to be prepared to handle potential failures. When a power supply fails, its voltage will be zero, and there is a chance it could short circuit other connected supplies. This can be prevented by using diodes on the output side of the power supply. In particular, a power ORing diode can be used to isolate the power supplies from each other and prevent the shutdown of other connected power supplies in case of a failure of any shunt connected power supplies.
The Advantages of Power ORing Diode Protection
Power ORing diodes are used to protect circuits from input power supply fault conditions and to provide system redundancy by paralleling power supplies. Power supplies are paralleled using power ORing diodes in redundant power supply schemes. Generally, Schottky diodes are employed for this power ORing purpose, as they have a fast reverse recovery time, a lower forward voltage drop than conventional diodes, and are simple to use in circuits.
The figure below illustrates a power ORing diode protection scheme used in parallel power supplies.
Power supply 1 and power supply 2 are both protected by diodes
Schottky diodes are used on the output side of each power supply. Through these diodes, the supplies are connected to the load. If a power supply fails or gets shorted, the connected diode gets reverse biased and protects the other power supply from shutting down.
The load is always connected to the power supply and service is not affected by power supply fault conditions. The current drawn by the load from each power supply, otherwise called load sharing, depends on the forward voltage drop of the power ORing diodes and the voltage difference between the power supplies. More current will be drawn from the supply with higher voltage and lower forward diode voltage drop.
The Disadvantages of Schottky Diodes
In the previous section, we examined the advantages of using Schottky diodes as power ORing diodes. However, using Schottky diodes in power ORing diode modules have two distinct disadvantages worth noting:
The forward voltage drop of the Schottky diode is high. This leads to a large fraction of voltage dissipating across the Schottky diode, generating heat. The forward voltage drop of Schottky diodes is inversely proportional to temperature, and this can ultimately lead to a situation where the load is not shared between the power supplies. In certain cases, one of the power supplies might begin to deliver the entire load in the presence of other paralleled power supplies. In those cases, the amount of heat generated can become detrimental, illustrating the need for careful thermal management between the diodes used in the protection scheme.
The reverse leakage current of the Schottky diode poses a serious problem in the power ORing diode protection scheme. The reverse leakage current drastically increases with temperature, and unless careful thermal management operations are in place, the reverse leakage current of the Schottky diode used as a power ORing diode can cause a loss of efficiency and thermal run-aways. If a power ORing Schottky diode is damaged by this phenomenon, it can draw down the power supply of the entire system.
Power MOSFETs as Power ORing Diodes
Power MOSFETs have a low forward voltage drop and can be used in place of Schottky diodes in power ORing diode protection circuits. The advantage of using a MOSFET is the voltage drop across the MOSFET corresponding to full-load current in the power ORing circuit is less than the forward voltage drop of the MOSFET. The graphic below displays the operating region of power MOSFETs when used as power ORing diodes. Power MOSFETs require an active drive circuit for turning on the power ORing diode.
At full load current, a voltage drop across the MOSFET should be less than the forward voltage drop
The main advantages of using power MOSFETs as power ORing diodes are:
The turn-on and turn-off thresholds can be selected.
Engineers have control over the turn-on and turn-off speeds.
The forced gate turn off can be used for testing and blocking applications.
Protecting paralleled power supplies is of great concern to engineers and designers. Typically, Schottky diodes or power MOSFETs are used as power ORing diodes in such protection circuits. When selecting a device for power ORing diodes, parameters such as the forward voltage drop, reverse recovery current, reverse recovery voltage, and the thermal behavior of the device need to be carefully analyzed. By using these parameters as a guide, the proper power ORing diode can be employed, providing the best protection scheme for redundant power supplies.