Overcurrent | Overvoltage Protection: Basic and Circuits Design

Circuit protection mainly refers to protecting components in electronic circuits from damage by overvoltage, overcurrent, surges, electromagnetic interference, etc. Here we will analyze the two types of overcurrent protection and overvoltage protection in detail.

Overcurrent Protection

Overcurrent protection refers to a technical measure to prevent circuit failure or damage due to current exceeding the rated value. Add appropriate protection devices and control components to the circuit, when the circuit is overloaded or short-circuited, the power supply will be blocked or cut off in time to protect the circuit, related equipment and personal safety.

1.The role of it

Overcurrent protection can prevent large currents from damaging equipment caused by circuit overloads, short circuits, or poor ground contact. It can also prevent electric shock hazards caused by circuit leakage and reduce the occurrence of accidents.

2. The way to implement it

Overcurrent protection mainly includes two types: short circuit protection and overload protection. The characteristics of the former are large setting current and instantaneous action. Electromagnetic current releases (or relays) and fuses are often used as short-circuit protection components. The characteristics of the latter are small setting current and inverse time limit action, where thermal relays and time-delay electromagnetic current relays are often used as its protection components, and fuses are also often used as overload protection components when there is not much inrush current. These protection devices will monitor the current in the circuit and automatically cut off the power supply when the current exceeds the rated value to avoid the expansion of circuit faults.

Protection Analysis

Overcurrent protective devices are sensitive to current changes when the current reaches a value that causes an excessive or dangerous increase in conductor temperature. Most of them respond to short circuit or ground fault current values as well as overload conditions.

When a phase-to-phase short circuit fault occurs in the power grid, the load increases abnormally, or the insulation level decreases, the current will suddenly increase and the voltage will suddenly drop. Overcurrent protection sets the operating current of the current relay based on the line selectivity requirements. When the current in the line reaches the action value of the current relay, the time relay acts through its contacts according to the selective requirements of the protection mechanism. During it, the time relay contacts are closed, the circuit breaker tripping coil is connected, then the circuit breaker acts to cut off the circuit, and some will also have relevant circuit fault prompts.

When unexpected situations such as load short circuit, overload or control circuit failure occur, excessive current will flow through the switching transistor in the voltage regulator tube, thereby increasing the tube's power consumption and generating heat. Without overcurrent protection, a high-power switching transistor may be damaged. Therefore, overcurrent protection is often used in switching regulators. The most economical and convenient method is to use a fuse. Due to the small thermal capacity of transistors, ordinary fuses generally cannot provide protection, and quick fuses are usually used, but the fuse specifications need to be selected according to the requirements of the safe operating area of the specific switching transistor, of course, the disadvantage of it is the inconvenience of frequent fuse replacement.

Current limiting protection and current cutoff protection commonly used in linear regulators can be applied to switching regulators. However, for switching regulator, the output of this protection circuit cannot directly control the switching transistor, and the solution is the output of the overcurrent protection must be converted into pulse instructions to control the modulator to protect the switching transistor.

In order to achieve overcurrent protection, it is generally necessary to connect a sampling resistor in series in the circuit, which will affect the efficiency of the power supply, so it is mostly used in low-power switching regulators. For high-power situation, using sampling resistors should be avoided as much as possible because of power consumption requirement. Therefore, overcurrent protection is usually converted into overvoltage and undervoltage protection.

Overcurrent Protection Circuit Examples

There are many types of overcurrent protection circuits, and the complexity of the circuit depends on how quickly the protection circuit reacts in different situations. Taking circuit board damage as an example, one of the reasons is that overcurrent causes components to burn out, and some even cause fire. Therefore, power protection design must be carried out, where fuses or thermistors are commonly used as a simple method. If targeted current detection protection is required, the circuit structure still needs to be designed.

Using a sampling resistor is our common design method, where the current is detected through it and then the control signal is output. Nowadays, there are many integrated ICs specially designed for high and low side current detection, such as LT6100, etc.

Here give some examples. The following is a sampling method using a current mirror. The internal design of the IC uses this method to set the bias current because the current of the transistor is proportional to the channel.

There are also isolated current sampling methods, such as optocouplers and current transformers.

There are many applications for current sampling using mutual inductance, such as internal detection of leakage protectors.

Overvoltage Protection

The principle of overvoltage protection is to detect whether the voltage in the circuit exceeds a set threshold in the circuit. Once an abnormality is detected, the overvoltage protection device will be triggered to protect the circuit.

Protection Measures

1. Lightning arrester: It is used to protect buildings, equipment and electrical systems from overvoltage damage caused by lightning strikes. Generally, it introduces overvoltage into the ground or other safe paths, thereby protecting the devices.

2. Discharge tube: It is also called gas discharge tube. When the voltage in the circuit exceeds a set threshold, the discharge tube directs the current, providing a lower impedance path. This helps direct overvoltages to ground or other safe paths.

3. Overvoltage protection diode: The typical one is TVS (transient voltage suppressor) diode. This diode has a fast response time and high shunt capability, and when the voltage exceeds a specified threshold, it conducts, diverting the overvoltage to ground or other safe paths.

4. Voltage limiter: It is used to limit the voltage in the circuit to not exceed a predetermined threshold. It can be an electronic component such as a voltage regulator or Zener diode, or it can be a regulating resistor or transformer, etc.

5. Fuses and circuit breakers: They are used to cut off the circuit when the current in the circuit exceeds the rated value. Although their primary purpose is overload protection, overcurrent protection can also indirectly prevent circuit damage caused by overvoltage.

6. Turbine quartz piezoelectric ceramics: Here the piezoelectric effect is used to manufacture resonators and filters, providing a certain degree of protection against overvoltage.

7. Clear circuit and control circuit design: These are all precautionary measures in advance.

Appropriate overvoltage protection measures need to be selected based on specific application conditions, circuit characteristics and equipment requirements. At the same time, relevant safety standards and specifications should also be followed to ensure the effectiveness and reliability of overvoltage protection.

Overvoltage Protection Circuit Examples

The overvoltage and undervoltage protection circuit is to protect the electronic system from being damaged by high voltage and to shut down in time when undervoltage occurs to avoid abnormal operation. The basic method is to use transistor switches to achieve control, or use corresponding clamping devices. This is a commonly used input overvoltage protection circuit.

Circuit Protection Devices

1) Varistor

It is a voltage-limiting protection device. Utilizing the nonlinear characteristics of the varistor, when overvoltage occurs between the two poles of the varistor, it can clamp the voltage to a relatively fixed voltage value, thereby protecting the subsequent circuit.

2) Transient suppression diode (TVS)

It is a voltage-limiting protection device whose function is very similar to a varistor. The nonlinear characteristics of the device are also used to clamp the overvoltage to a lower voltage value to protect the subsequent circuit.

3) Positive temperature coefficient thermistor (PTC)

PTC is a current-limiting protection device. It has an operating temperature value TS. When the temperature inside PTC is lower than TS, its resistance value remains basically constant, and the resistance value at this time is called cold resistance. On the contrary, when higher than TS, its resistance increases rapidly, so it is generally connected in series on the line for overcurrent protection of transient large currents.

4) Insurance tube, fuse, air switch

These are protective devices that are used to disconnect the short-circuit load or over-current load on the line when short circuit, over current and other faults occur inside the equipment to prevent electrical fires and ensure the safety characteristics of the equipment.

5) Inductors, resistors, wires

They are not protection devices themselves, but they can play a cooperative role in a protection circuit composed of a combination of multiple different protection devices.

6) Transformers, optocouplers, relays

They are not protection devices, but the isolation characteristics of these devices can be used in the design of port circuits to improve the port circuit's ability to withstand overvoltage.

In general, the circuit protection is to avoid overcurrent, overvoltage or physical damages, so the key is the detection of current or voltage. Whether it is a circuit built with protection devices, integrated ICs or discrete devices, the protection structure should take into account several points, such as protection threshold, response speed, hysteresis interval, etc. need to be carefully considered based on specific applications.

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