Posted on: July 1, 2014
What is Reverse Voltage Protection
Reverse voltage protection circuits prevent damage to power supplies and electronic circuits in the event of a reverse voltage applied at the input or output terminals. Reverse voltage protection is implemented at the input of the power supply or onboard of the custom, multiple output redundant power supplies. This is important in most electronic applications such as laptops, computers, CMOS circuits, etc.
The protection ensures that the components are not damaged by accidental swap of the power supply connections. There are various methods that differ in operation, efficiency and complexity. While some like a diode or circuit breaker provides only the reversal voltage protection, others such as the protection ICs provide the reverse voltage, over current, and overvoltage protections.
To block negative voltages, designers usually place a power diode or a P-channel MOSFET in series with the power supply. One drawback of the series diode is that it takes up board space and has high power dissipation at high load currents.
On the other hand, the MOSFET dissipates less power even though it requires an extra drive circuitry which increases the cost. Both solutions affect low power operations and especial the series diode. In addition, the solutions may not be suitable at very high load currents.
Protecting reverse voltage using a diode
The diode is connected in series with the load and only allows power to reach the load only when in forward bias. If the voltage is reversed, it blocks the voltage and the reversed power does not reach the load. Using the diode is the simplest method and has the advantage of low cost.
The disadvantages of using the diode are; the forward voltage drop which can be significant in low voltage applications, the high power dissipation in high load currents and low efficiency. A Schottky diode is sometimes used due to its fast response and low forward bias voltage drop.
Figure 1: Diode in series with load Image Credit
Using MOSFET for reverse voltage protection
A better protection uses a MOSFETS which have an advantage of very low on resistances. The method involves using a high-side P type MOSFET on the power path, or a low-side NMOS FET in the ground path.
Figure 2: Protection using PMOSFET Image Credit
In each of the MOSFET circuits, the transistor’s body diode is forward biased during normal operations. When the power is connected properly, the FET’s gate voltage is taken low for a PMOS and high if it is an NMOS, such that the channel shorts out the diode.
When the supply voltage is reversed, the PMOSFET gate voltage is high and this prevents it from turning on hence preventing the reverse voltage from reaching the load. For an NMOSFET, the gate voltage is low.
Using Circuit Breakers for reverse voltage protection
The breakers are used in high power applications of 500 W to several Kilowatts. At these high currents, it is not practical to use the diodes, or even schottky diodes due to the high power dissipation and inefficiency. The electronic circuit breakers are used together with a power shunt diode.
When the polarity is normal, and the circuit breaker is on, the current flows from the ground terminal to the –48 terminal. When the polarity is reversed, the power shut diode will conduct and create a short circuit that trips the circuit breaker.
The circuit is expensive, bulky and requires manual resetting of the circuit breaker, hence not suitable for remote installations. In addition, the accuracy of the circuit breaker may be inadequate in applications that require precise current limiting.
Figure 3: Using circuit breaker for reverse voltage protection Image Credit
Using an oRing controller
In this method, an oRing controller IC is used in conjunction with a power MOSFET to provide a simple and efficient reverse polarity protection. The oRing controller operation is automatic and as long as the polarity is correct, the IC is properly biased such that it turns on the FET. When the polarity is reversed, the IC does not have the correct bias and won’t operate to turn on the reverse biased FET. The FET remains off and prevents the reverse power from going to the load.
Figure 4: Reverse polarity protection using oRing controller Image Credit
Overvoltage and Reverse voltage protection circuits
Protection ICs such as the LTC 4365 are designed to protect sensitive circuits from reverse polarity, over current and over voltages. The ICs blocks the undesired current or voltage and only allows the safe voltages to pass through.