Over-voltage Protection Circuits

Power supply over-voltage protection is really useful - some PSU failures can put damaging large voltages on the equipment. Overvoltage protection prevents this happening.

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Although modern power supplies are now very reliable, there is always a small but real chance that they can fail.

Although they can fail in many ways, one particularly worrying possibility is that the series regulator element, i.e. transistor or FET may fail in such a way that it goes short circuit. If this happens a very large voltage often referred to as an over-voltage could appear on the circuitry that is being powered causing catastrophic damage to the whole equipment.

By adding a little extra protection circuitry in the form of over-voltage protection, it is possible to protect against this unlikely but possibly worrying possibility.

Over-voltage protection basics

There are many ways in which a power supply can fail. Normally the power supply stops working and the output falls to zero. However there are some rare cases of failures when very much higher voltages can appear at the output.

For a linear regulator, let’s take the example of a very simple Zener diode based regulator. Although more complicated supplies give better performance, they also rely on a series transistor to pass the output current. The main difference is the way in which the regulator voltage is applied to the base of the transistor.

Basic series regulator using a zener diode and emitter follower
Basic series regulator using a zener diode and emitter follower

Typically the input voltage is such that several volts are dropped across the series regulator element. This enables the series pass transistor to regulate the output voltage adequately. Usually a transistor like this would fail in an open circuit condition, but under some circumstances, the transistor may develop a short circuit between the collector and emitter. If this occur, then the full unregulated input voltage would appear in the output.

If the full voltage appeared on the output, then it could damage many of the ICs that are in the circuit being supplied. In this case the circuit could well be beyond economic repair.

The way in which switching regulators operate is very different, but there are circumstances in which the full output could appear on the output of the power supply.

Types of over-voltage protection

As with many electronic techniques there are several ways of implementing a particular capability. This is true for overvoltage protection.

There are several different techniques that can be used, each with its own characteristics. Performance, cost, complexity and mode of operation all need to be weighed up when determining which method to use.

  • SCR Crowbar:   As the name implies the crowbar circuit places a short circuit across the output if an overvoltage condition is experienced. Typically thyristors, i.e. SCRs are used for this as they can switch large currents and remain on until any charge has dispersed. Often the thyristor is linked back to a fuse which blows and isolates the regulator from having any further voltage placed upon it.
    Thyristor or SCR overvoltage protection circuit
    Thyristor crowbar overvoltage protection circuit

    Note on Thyristor Crowbar Overvoltage Protection:

    The Thyristor or SCR, Silicon Controlled Rectifier can be used to provide overvoltage protection in a power supply circuit. By detecting the high voltage, the circuit can fire the thyristor to place a short circuit or crowbar across the voltage rail to ensure it does not rise to high in voltage.

    Read more about Thyristor Crowbar Overvoltage Protection Circuit.

  • Voltage clamping:   Another very simple form of overvoltage protection uses an approach called voltage clamping. In its simplest form it can be provided by using a Zener diode placed across the output. With the Zener diode voltage chosen to be slightly above the maximum rail voltage, under normal conditions it will not conduct. If the voltage rises too high, then it will start to conduct, clamping the voltage at a value slightly above the rail voltage.

    If a higher current capability is needed then a Zener diode with a transistor buffer can be used.
    Zener diode over-voltage clamp
    Zener diode over-voltage clamp
    (a) - simple Zener diode, (b) - higher current with transistor buffer
  • Voltage limiting:   When over-voltage protection is required for switch mode or switching power supplies the clamp and crowbar techniques are less widely used because of the power dissipation requirements and the possible size and cost of the components.

    Fortunately most switch mode regulators fail in a low voltage condition. However it is often prudent to put in place voltage limiting capabilities in case of over-voltage conditions.

    Often this can be achieved by sensing the over-voltage condition and shutting down the converter. This is particularly applicable in the case of DC-DC converters. When implementing this, it is necessary to incorporate a sense loop that is outside the main IC regulator - many switch mode regulators and DC-DC converters use a chip to achieve the majority of the circuit. It is very important to use an external sense loop because if the switch mode regulator chip is damaged causing the overvoltage condition, the sense mechanism may also be damaged.

    Obviously this form of over-voltage protection requires circuits that are specific to the particular circuit and switch mode regulator chips used.

All three techniques are used and can provide effective power supply over-voltage protection. Each has its own advantages and disadvantages and the choice of technique needs to be made dependent upon the given situation.

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