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How Are MOSFETs Used in Circuit Protection?

MOSFET circuit protection

MOSFETs are one of those “wonder” components that seem to find a home in just about every circuit you can imagine. From power regulators to logic circuits, their switchable 2-state characteristic and rectification capability enables many diverse circuits. MOSFETs can also act as replacements for other parts as long as they are properly modulated between ON and OFF states, and one of these potential uses is in circuit protection.

Circuit protection generally involves protection from one of four possible events while a circuit is in operation:

  • Voltage surge (overvoltage)
  • Current surge or inrush (overcurrent)
  • Strong voltage transients or pulses

This article will show some basic uses of MOSFETs for circuit protection in an electronic device. Most MOSFETs can be used for these purposes, depending on a few important specifications.

Options For Circuit Protection With MOSFETs

MOSFETs can be used in several ways for circuit protection due to their many advantages over other components. The main specifications that matter for circuit protection include:

  • ON-state resistance (R-ON)
  • Switching time (related to terminal capacitances)
  • Peak current handling
  • Breakdown voltage handling (BVDSS)

MOSFETs can be seen as general-purpose components for a variety of circuit protection schemes. In some cases, MOSFETs may provide insufficient protection compared to more targeted circuit protection solutions, such as TVS diodes or circuit breakers. MOSFETs might also be used alongside these devices to provide additional circuit protection within specific voltage and response time ranges, depending on the standards level or compliance metric you need to hit.

Overcurrent Protection

MOSFETs can be used as electronic switches to protect circuits from excessive currents. This can be done in two ways:

  1. As a shunt switching element that diverts current from a protected high-Z load
  2. As a series element that reduces the total current reaching a low-Z load

An example for Option #2 with an NMOS transistor is shown below; this is a typical method to deal with moderate power surges in power systems that may need to handle 10’s of Amps of current. The current is determined by measuring across a current-sense resistor (CSR).

MOSFET overcurrent protection

When the current through a MOSFET exceeds a certain threshold, the MOSFET can be configured to quickly turn ON (shunt element) or OFF (series element), which will interrupt the current flow and protect the circuit from damage up to some prescribed current level. This requires using some external circuit to measure the current in the protected circuit, and then it will drive the gate terminal of the MOSFET if the current exceeds some threshold.

Overvoltage Protection

MOSFETs can be used as voltage clamping devices to protect sensitive components from overvoltage conditions, which might be produced by surges or switching. This involves a variation on the two circuit options listed above. Instead of measuring the current with a CSR, the sense and gate drive element measures the voltage reaching the load. The MOSFET can then provide a low-resistance path for current to protect a high-Z load, or a high-resistance clamp to protect a low-Z load.

MOSFET overvoltage protection

Reverse Polarity Protection

MOSFETs can also be used to protect circuits from reverse polarity connections, such as might occur in a battery that does not have a polarized connector. By placing a MOSFET in series with the power supply, it can be configured to block current flow if the input voltage polarity switches, or if power source terminals are connected backwards. When the polarity is correct, the MOSFET is turned on, and current will flow normally. This works thanks to the body diode across each FET.

MOSFET reverse polarity

Reverse polarity configuration with a push-pull arrangement of MOSFETs.

In each of the above cases, MOSFETs provide a fast response time, low on-resistance, and efficient power handling capabilities, making them suitable for circuit protection applications. However, it's important to note that MOSFETs are often used in conjunction with other components and circuitry to implement comprehensive protection schemes tailored to specific needs. To ensure adequate circuit protection, your MOSFET-based protection circuit needs to be simulated.

How to Simulate in Circuit Protection With MOSFETs

SPICE simulations of circuit protection with MOSFETs are primarily time domain simulations that involve examining the time-domain response to a change in the operating conditions. Some of these responses can include:

  • Fast step changes in applied voltage
  • Polarity reversal in the applied voltage
  • Exposure to fast pulses, like ESD or switching events
  • Fast DC current rises in various parts of the system (such as short circuits)
  • Fast and slow voltage or current surges

Obtaining accurate results requires using a MOSFET model that captures the circuit characteristics of the component (most importantly the capacitances and R-ON). As long as the MOSFET model is accurate, it’s possible to examine the circuit’s transient response to the input changes listed above. These responses allow you to directly quantify the circuit behavior just by looking at the time-domain curve.

Whenever you need to design circuit protection using MOSFETs, make sure you 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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