The difference between undervoltage protection and loss of voltage protection. The design and function of undervoltage protection circuit.

The difference between undervoltage protection and loss of voltage protection

Under-voltage protection and voltage loss protection are two different concepts in the power system. Their main differences are as follows:

1. Definition: Undervoltage protection means that in the power system, when the voltage drops below the set reasonable working range, the protection device will be triggered and take corresponding measures to avoid equipment damage or failure. Loss of voltage protection means that in the power system, when the voltage of the power supply drops severely or disappears completely, the protection device will isolate or cut off the power supply to protect the safe operation of the equipment and system.

2. Triggering conditions: Undervoltage protection is usually triggered when the voltage drops below a preset threshold, which is determined according to the working requirements and standards of the equipment and system. Loss of voltage protection is triggered when the power supply voltage drops to a certain level or disappears completely.

3. Purpose: The main purpose of undervoltage protection is to protect the equipment from damage caused by too low voltage to ensure the normal operation of the equipment. The main purpose of loss of voltage protection is to protect the equipment from abnormal operation or system crash caused by loss of power supply voltage.

4. Reaction measures: After the undervoltage protection is triggered, a series of measures can be taken, such as warning the equipment operator, sending alarm notifications, automatically switching to backup power, etc., to protect the safe operation of the equipment and system. After the undervoltage protection is triggered, the most common reaction measure is to cut off the power supply to prevent the equipment from continuing to operate or suffering greater damage.

In summary, undervoltage protection and loss of voltage protection have different meanings and applications in power systems. Undervoltage protection is intended to protect equipment from damage caused by voltage drop, while loss of voltage protection is intended to protect equipment from abnormal operation or system crash caused by loss of power supply voltage. They differ in triggering conditions, purposes, and response measures.

Undervoltage protection means that when the input voltage of an appliance does not reach the working voltage, the input voltage will be cut off, effectively protecting the appliance from damage caused by repeated starting due to low voltage. When the input voltage returns to normal, the appliance will automatically resume normal operation.

Due to short circuit faults and other reasons, the line voltage will drop significantly or even disappear in a short period of time. It will cause damage to the line and electrical equipment. For example, it will cause the motor to fail or stall, thereby generating an overcurrent several times the rated current and burning the motor; when the voltage is restored, the self-starting of a large number of motors will cause the voltage of the motor to drop significantly, causing harm.

The voltage that causes the motor to fail is called the critical voltage. When the line voltage drops to the critical voltage, the protective device is activated, which is called undervoltage protection. Its main task is to prevent the equipment from burning due to overload. When the line voltage is lower than the critical voltage, the protective device is activated, which is called loss of voltage protection. Its main task is to prevent the motor from starting automatically.

What is the reason for undervoltage protection?

Brown-out protection is a protective measure used to prevent the voltage in an electrical system from dropping to unreasonable or dangerous levels. Here are some common causes that may lead to brown-out conditions:

Overload or short circuit: When the load in the power system is too large or a short circuit occurs, the current will increase, causing the voltage to drop. This may be caused by a failure of the power equipment, overload of the cable or relay, current overload, etc.

Grid failure: When an accident or failure occurs in the power grid, such as generator failure, transformer failure, transmission line failure, etc., it will cause voltage drop. This may be caused by power network supply interruption, power transmission system failure, power equipment damage, etc.

Long-distance transmission loss: In long-distance transmission, the power will have a certain resistance loss in the transmission line, resulting in voltage drop. Especially when the load is far away from the power source, the voltage drop will be more obvious.

Power system adjustment and switching: When the power system is adjusted, switched or loads are transferred to backup power sources, it may cause a transient voltage drop. This is because the stability of the power supply may be affected during the switching process.

High-frequency equipment interference: Some high-frequency equipment, such as frequency converters, arc furnaces, industrial furnaces, etc., may introduce high-frequency noise into the power system, causing voltage drops.

The purpose of undervoltage protection is to protect equipment and ensure the normal operation of the power system by monitoring voltage levels and triggering protection devices when they drop to a certain threshold. Preventing these undervoltage conditions and taking appropriate protective measures can reduce the risk of equipment failure, grid collapse, and other adverse consequences.

Design and Analysis of Undervoltage Protection Circuit

The following is a schematic diagram, marked with wide voltage input through capacitors, common mode inductors, and TVS tubes. This is a common input power supply processing, of course, there will be anti-reverse protection, the role of D9 is to ensure that the MOS tube GS voltage is not too high, generally up to about 25 volts, the critical conduction voltage is 4 volts, that is, 9.1V is definitely in a saturated conduction state.

1. When the voltage rises from zero volts to about 12V, Q2 is turned on, D9 is turned on, causing Q5 to turn on and the power supply is turned on. When the voltage continues to rise, the states of Q2 and D9 remain unchanged. However, when D6 rises with the voltage, the on-current reaches the saturation current of Q4, the on-bias voltage of Q5 is pulled down, and the MOS tube is turned off. The power supply has no output.

Second, when the voltage returns to 12V to 60V, Q2 is turned on normally, Q4 is turned off, and MOS is turned on normally. Of course, we should pay attention to the power of R17 and R18 resistors. As the input voltage increases, its power will also increase.

Near the critical point of the protection voltage, repeated oscillations between protection and normal operation will occur.

For example, if the protection voltage is 12V, when the voltage rises to 12V, the protection circuit turns on the output. Here, due to the increase in load, the 12V voltage drops slightly, perhaps by tens of mV, and the protection circuit turns off the output again.

As a result, rapid oscillations of turning off/on occur near the critical point, which may cause damage to the equipment.

To solve this problem, we need to design a protection circuit with hysteresis effect.

It can be realized by using a hysteresis comparator, which can be realized by positive feedback of the comparator output. For example, the comparison reference voltage of the comparator is 2.5V and the hysteresis range is 0.5V.

Then when the input voltage rises from 0V to 2.5V, the comparator output is low and the comparison reference level drops to 2.25V.

At this time, if the input voltage drops slightly, as long as it is not lower than 2.25V, the output will no longer be high.

When the input voltage drops from high level to 2.5V, the comparator output is high and the comparison reference level rises to 2.75V.

At this time, the input voltage rises slightly, and as long as it is not higher than 2.75V, the output will not be low.

According to the above analysis, the following undervoltage protection circuit is designed using the hysteresis comparator:

D1 is a 2.5V voltage regulator diode and can be replaced by a high-precision TL431.

U1A is an op amp LM339, which is open drain when the output is high, so the output is pulled up to the power supply through R4.

R3 is a positive feedback resistor used to achieve hysteresis effect.

The adjustable resistor R8 is used to adjust the protection voltage. If the protection voltage is 12V, the ratio of the adjustable resistor can be set to about 20%.

Q1 is a PNP transistor used for switch control. If the load current is large, it can be replaced by a PMOS.

If we set the ratio of the adjustable resistance to 20%, the reference voltage is 2.5/20%=12.5V

According to the voltage superposition principle, when the comparator output is low, the voltage division of the 2.5V regulated voltage at the in-phase end is 2.47V.

The corresponding power supply voltage is 2.47/20%=12.35V.

When the comparator output is high, the power supply voltage is 12V and the voltage at the non-inverting end is approximately 2.585V.

The corresponding power supply voltage is 12.9V.

That is, when the power supply voltage rises from 0V to above 12.9V, Q1 is turned on, the power is turned on, and the load starts to work. When the power supply voltage drops from above 12.9V to below 12.35V, Q1 is turned off, the power is turned off, and the load stops working.

Therefore, the hysteresis range of this circuit is 12.9-12.35=0.55V.