What Is a Short Protection Circuit?

In view of the existence of some instability factors in the power circuit, the circuit designed to prevent such instability factors from affecting the circuit effect is called a protection circuit. For example, there are overcurrent protection, overvoltage protection, overheating protection, no-load protection, short circuit protection, etc.

In power supply systems, the most prone to failures are short circuits, overloads, grounding, and lightning strikes. In order to ensure the safe and reliable operation of the power supply system, protection devices must be installed in order to monitor the working condition of the power supply system, detect faults in time and cut off the power supply of the faulty equipment to prevent the accident from expanding. The protection circuit is generally composed of various relays, signal indicating devices and other components. ^[1]

When selecting power electronic switching devices, a certain margin must be left to ensure the safe and reliable use of the devices. At the same time, there must be a protection circuit to prevent damage to the device caused by the failure. The protection circuit mainly includes overcurrent protection, overvoltage protection and overheating protection.

Overcurrent protection

Overcurrent protection is achieved by connecting fast fuses in series in the circuit, which is suitable for thyristors and GTOs because they have a higher surge current capability. For MOSFET, GTR, IGBT, because they have very low over-current capability, they must have a special over-current protection circuit, and require the over-current protection circuit to complete the over-current detection, signal transmission, and protection actions in an instant. The current is limited within the overload capacity. The commonly used methods are: monitoring method, Hall current sensor protection method.

Overvoltage protection

A variety of reasons can cause power electronics circuits to generate overvoltages, such as turning off switching devices and turning on and off power switches. Due to the presence of parasitic inductance in the circuit, a sudden change in current caused by various reasons will cause a voltage spike, which will cause an overvoltage. Varistor element is the main method to achieve over-voltage protection. It mainly includes varistor and anti-lightning transient voltage suppressor. When the terminal voltage exceeds a certain value, the varistor will decrease sharply, thereby suppressing the transient overvoltage.

Overheating protection

Power electronic switching devices consume power during operation, such as conduction losses and switching losses. These losses are converted into heat, which will increase the junction temperature of the device, cause the device to operate abnormally, and even cause damage to the device. Therefore, when the device is operating, the heat generated must be released in time to ensure that the device temperature is within a suitable range. This is the main content of the heat dissipation design and it is an important part of power electronic circuit design. ^[2]

Line protection circuit

Current protection circuit: The current protection commonly used in power supply lines mainly includes definite-time overcurrent protection, inverse-time overcurrent protection, overcurrent and quick-break protection, and differential protection.

Motor protection circuit

There are three types of faults that often occur in electric motors: short circuit, overload, and ground. Generally, fuses, automatic air switches, thermal relays, current relays, voltage relays and other components are used to form circuits such as overcurrent protection, low voltage protection, and phase failure protection to protect the motor.

Lightning protection circuit

In order to prevent lightning damage to the power supply system, lightning protection rods, lightning rods and arresters are usually used for lightning protection. ^[3]

Lithium battery protection circuit

It consists of two field effect tubes and a special protection integrated block S--8232. The overcharge control tube FET2 and the overdischarge control tube FET1 are connected in series to the circuit. The protection IC monitors and controls the battery voltage. When the battery voltage rises to 4.2V , The overcharge protection tube FET2 is turned off and stops charging. In order to prevent malfunction, a delay capacitor is usually added to the external circuit. When the battery is in the discharging state, and the battery voltage drops to 2.55V, the over-discharge control tube FET1 is turned off and stops supplying power to the load. Overcurrent protection is to control the FET1 to turn off when a large current flows through the load, and stop discharging to the load. The purpose is to protect the battery and the field effect transistor. Over-current detection uses the on-resistance of the field-effect transistor as a detection resistor, monitors its voltage drop, and stops discharging when the voltage drop exceeds a set value. A delay circuit is usually added to the circuit to distinguish between inrush current and short-circuit current.

Overcharging or overcharging Li + batteries can cause an explosion and cause personal injury, so safety is a major concern when using this type of battery. Therefore, commercial lithium-ion battery packs often include protection circuits like the DS2720. The DS2720 provides all the protection features required for a rechargeable Li + battery, such as: protecting the battery during charging, preventing circuit overcurrent, and extending battery life by limiting the discharge voltage of the battery.