Voltage applied to semiconductors can push them into reverse bias, which can then damage inputs on an integrated circuit. In an extreme case of high overvoltage under reverse polarity, an integrated circuit could be destroyed and would need to be replaced. A common reverse polarity limit on single-ended integrated circuit I/Os built with the standard CMOS process is -0.3 V, and your reverse polarity protection measures should protect against this level of reverse overvoltage.
In this article, we’ll briefly examine some simple options for protecting integrated circuits from reverse polarity. These options apply to batteries, power supplies, and regulator circuits providing DC power to integrated circuits or other loads sensitive to overvoltage.
Reverse Polarity Protection Circuits
The purpose of reverse polarity protection components and circuits is to drop excess voltage such that it does not reach a protected load once the polarity of the applied voltage is reversed. Reverse polarity components are normally placed as a series element before the protected circuit. When operating normally (positive voltage), the device has low impedance and minimal voltage drop. During reverse polarity, the device should have high impedance so that the voltage reaching the protected circuit is dropped across the protection circuit.
There are three principle components used for reverse polarity protection:
- Silicon diode
- Schottky diode
- P-type MOSFET
The simplest method for adding DC reverse polarity protection to a circuit is to place a series diode in front of the protected load or circuit. The circuit diagram below shows the correct placement.
A simple series diode can provide significant reverse polarity protection.
Once a reverse polarity event occurs, the diode will be driven in reverse bias. This forces the diode to limit the reverse polarity voltage to a very low value (less than the -300 mV limit on CMOS I/Os). The corresponding current will also be very low. This is because the only current being given to the circuit is the leakage current from the diode.
Schottky diodes perform the same function as a basic semiconductor diode. These components have higher leakage currents in reverse bias, but they may also have much lower forward voltage. For the Schottky diode and the silicon diode, there are two important specifications on these components that have to be considered:
- Leakage current
- Breakdown voltage
Leakage currents can bias the component or circuit being protected during the reverse polarity event; some circuits or components could have low current ratings when driven with a negative voltage, so this should be checked to ensure the component provides full protection. If the reverse polarity drives a diode too deep into reverse bias, the reverse voltage could exceed the diode’s breakdown voltage, and the diode will begin conducting again. This defines the maximum reverse voltage that can be protected.
P-Type MOSFET Circuit
A p-type depletion mode MOSFET can be used as a toggleable reverse polarity protection element placed in series with the protected circuit in reverse. An example is shown below. In this circuit, the FET is ON and the body diode is oriented to allow forward-bias operation in positive polarity. When reverse polarity occurs, the FET now switches off and the body diode is in reverse bias, which blocks conduction and only permits leakage current to reach the load.
Reverse polarity protection with a p-type MOSFET.
The Zener diode here also plays an important role in protecting the MOSFET from damage. The MOSFET has a maximum gate-source (Vgs) voltage that it can sustain. The Zener diode limits Vgs within these levels, so it essentially provides some overvoltage protection. It does this by allowing current to flow in reverse when polarity switches and exceeds breakdown, which can be set below the MOSFET’s gate voltage limit.
The diagrams shown here illustrate the basic strategies for applying reverse polarity protection. More elaborate versions of these circuits can be constructed for targeted cases, such as battery charging and higher voltage/current allowances.
Simulating Reverse Polarity Protection
Simulation of reverse polarity protection is simple, and it can be performed with DC analysis or transient analysis. In DC analysis, the applied overvoltage can be swept through a range of possible reverse polarity values, and the resulting current and voltage at the protected circuit’s input is monitored. Common diodes and MOSFETs will have DC models that can be used for this purpose.
To examine a dynamic reverse polarity event, a transient analysis simulation is needed to check the voltage and current at the I/O over time. This would allow a designer to see how the voltage passed to the protected circuit and the resulting bias current vary over time. Typically, because of the protection provided by series overvoltage circuits, the rate of change in the voltage/current at the I/O pins on protected components will be very slow.
When you need to design and evaluate reverse polarity protection circuits and components, use the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.
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