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Design Concept of Output Overvoltage Protection Circuit for Switching Power Supply

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.05.08
1.Introduction

Switching power supply may experience high or low output voltage during use: There is a rated voltage in the switching power supply. If the rated voltage is exceeded, it may exceed the withstand voltage value of the output capacitor, causing the power supply to heat up, breakdown, burn or even catch fire. Therefore, different types of protection circuits are designed. When the control circuit fails or other faults cause a voltage increase, the output of the power supply is turned off to protect the load and improve the reliability of the system.

2. Common methods

The following introduces several common principles and advantages of overvoltage protection circuits:

(1) Output overvoltage protection circuit one

This scheme uses a combination of a voltage regulator and an optocoupler to control the primary control IC to stop working and achieve overvoltage protection by relying on the conduction of the optocoupler; When an external voltage higher than the normal output voltage range is applied to the output terminal or a fault occurs in the circuit itself causing an increase in output voltage, the circuit will clamp the voltage to the set value.



Working principle analysis:

When the output overvoltage occurs and the voltage applied to D730 is greater than its stable value, D730 will conduct and the output voltage will be clamped. At the same time, the overvoltage signal will be fed back to the primary side through OC730, causing the primary side control IC to pull down or raise the pin used for overvoltage protection (as shown in the figure: pulling down the SNSBOOST pin) and stop working.

Analysis of circuit advantages and disadvantages:

Advantages:

The circuit form is simple and the cost is low.

Disadvantages:

The accuracy is not high, and due to differences in the batch and temperature characteristics of the stabilizing tube, the overvoltage clamp position will fluctuate up and down, resulting in differences in batch shipments.

(2) Output overvoltage protection circuit two

This scheme is a series of modifications based on the first circuit, removing the original voltage regulator diode and using TL431 to detect the output voltage of the circuit, improving the sampling accuracy.
Figure 2

Working principle analysis:

When overvoltage occurs, the output voltage is divided by resistors R730 and R731//R732, causing VA>Vref and U730 to conduct. At the same time, the overvoltage signal is fed back to the primary side through OC730, causing the primary side control IC to pull down or raise the pin used for overvoltage protection (as shown in the figure: pulling down the SNSBOOST pin) and stop working.

Analysis of circuit advantages and disadvantages:

Advantages: The output overvoltage protection value can be accurately set.

Disadvantage: The cost of using a relatively stable pressure pipe clamping method is slightly higher.

There is an optocoupler in both of the above solutions because our power supply needs to be isolated. However, the price of optocouplers itself is not cheap. Therefore, we are considering whether it is possible to detect the output voltage while removing the optocoupler, without the need for isolation and optocouplers. Naturally, we will think of a series of magnetic core devices such as transformers that we commonly use. However, adding devices violates the principle of wanting to be cheaper, so overvoltage protection needs to be achieved without adding other devices.

And isolated power supplies all have an isolation transformer, which is common in every switching power supply. Therefore, we can use this transformer to achieve primary and secondary side isolation, because the primary side of the switching power supply has VCC windings. Therefore, we can use VCC windings to achieve output overvoltage protection. The third type of protection circuit has emerged.

(3) Output overvoltage protection circuit three

This scheme adopts the primary auxiliary winding VCC, which couples the secondary output voltage. The increase in output voltage leads to an increase in VCC voltage, thereby achieving output overvoltage protection.



Figure 3

Working principle analysis:

When overvoltage occurs, when the output voltage Vo2 increases, the auxiliary winding voltage PAUX voltage increases. The voltage is supplied to the DEM pin of the IC through the up and down resistors R812 and R813//R814. When the voltage of the DEM pin exceeds the OVP voltage threshold, the IC will enter the output voltage overvoltage protection state and stop working.

Analysis of circuit advantages and disadvantages:

Advantages: The cost is relatively lower.

Disadvantages: It is greatly affected by the coupling degree of the transformer, resulting in poor accuracy and consistency.

Among the above three schemes, both Scheme 1 and Scheme 2 can be used to work when there is a problem with the self feedback loop or when the output voltage is forced to increase by external voltage, while Scheme 3 only works when there is a problem with the self feedback of the power supply.

There are also two feasible solutions for dealing with anomalies caused by external voltage forced increase in pure output voltage:

(4) Output overvoltage protection circuit four

Add a clamp diode at the output end, as shown in Figure 4:


Figure 4

Working principle analysis:

When the reverse voltage is injected into the switching power supply at the output end, the output voltage regulator will conduct to prevent damage to internal components of the power supply caused by voltage injection. At the same time, the disadvantage is relatively obvious, as the voltage regulator has a short clamping time and is prone to damage if the time is too long.

Analysis of circuit advantages and disadvantages:

Advantages: Lower cost.

Disadvantage: The voltage clamping time is relatively short.

(5) Output overvoltage protection circuit five

To solve the problem of long backflow voltage time at the output end, we can use the method of connecting diodes in series at the output end, so that the backflow voltage cannot enter the internal power supply, and can only be output from the power supply, rather than from the external input. However, this will cause other problems. Due to the conduction voltage drop of the diode, when the output circuit is large, the diode heats up sharply, increasing power loss, and the accuracy of the output voltage is reduced due to the conduction voltage drop. The principle is shown in Figure 5:


Figure 5

Therefore, this scheme is only used for products with small output current and low output voltage accuracy.

Summary

From the analysis of the principles and advantages and disadvantages of the five commonly used circuits mentioned above, Scheme 4 and Scheme 5 only focus on overvoltage protection in the form of external injection voltage, and are not suitable for different scenarios. The accuracy issues of Scheme 1 and Scheme 3 are also more obvious. Compared to Scheme 2, the additional costs only due to optocouplers and 431 have little impact on medium and high-power products. The market is more concerned about the stability and safety of the product in use.

At present, Kinri Energy's LM series of shell switch power supplies and LI series of rail power supplies with a power output of over 120W all use the output overvoltage protection circuit of Scheme 2 to improve the reliability of the entire system.

In the future, Kinri will continue to respond to market trends, consistently cultivate power supply technology innovation, and down-to-earth promote national industrial brands, providing customers with higher quality products and contributing to the rise of domestic power supply.
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