Research on switching power supply protection circuit
INDUSTRIAL LCD DISPLAYS / IGBT MODULES DISTRIBUTOR

Infineon / Mitsubishi / Fuji / Semikron / Eupec / IXYS

Research on switching power supply protection circuit

Posted Date: 2024-01-16

Research on switching power supply protection circuit

1 Introduction

The quality indicators for evaluating switching power supplies should be based on safety and reliability as the first principles (1, 2, 3). Under the condition that the electrical technical indicators meet the requirements for normal use, in order to make the power supply work safely and reliably in harsh environments and sudden failures, a variety of protection circuits must be designed, such as soft start to prevent surges, and to prevent overvoltage and undervoltage. , overheating, overcurrent, short circuit, phase loss and other protection circuits. At the same time, in the same switching power supply circuit, sufficient attention must be paid to the interrelationships and issues that should be paid attention to when designing multiple protection circuits.

2 Anti-surge soft start circuit

Most of the input circuits of switching power supplies use capacitor filter rectifier circuits. At the moment when the incoming power supply is turned on, since the initial voltage on the capacitor is zero, a large surge current will form at the moment when the capacitor is charged. Especially for high-power switching power supplies, which use Larger capacity filter capacitors can increase the surge current to more than 100A. Such a large surge current at the moment when the power is turned on will often cause the input fuse to blow out or the contacts of the closing switch to burn out, causing overcurrent damage to the rectifier bridge; in the worst case, it will also cause the air switch to fail to close ( 4). The above phenomena will cause the switching power supply to fail to work normally. For this reason, almost all switching power supplies are equipped with a soft-start circuit to prevent inrush current to ensure normal and reliable operation of the power supply. Anti-surge soft start circuits usually fall into two categories: thyristor protection method and relay protection method.

(1) Thyristor protection method

Figure 1 is an anti-surge current circuit composed of thyristor V and current-limiting resistor R1. At the moment when the power is turned on, the input voltage charges the capacitor C through the rectifier bridge (D1 ~ D4) and the current limiting resistor R1 to limit the inrush current. When the capacitor C is charged to approximately 80% of the rated voltage, the inverter operates normally. The thyristor trigger signal is generated through the auxiliary winding of the main transformer, causing the thyristor to conduct and short-circuit the current limiting resistor R1, and the switching power supply is in normal operation.

Figure 1 Anti-surge current circuit composed of thyristor and current-limiting resistor

(2)Relay protection method

Figure 2 uses the anti-surge current circuit composed of relay K and current limiting resistor R1 Figure 3 replaces the R2C2 delay circuit

Figure 2 is an anti-surge current circuit composed of relay K and current-limiting resistor R1. The moment the power is turned on, the input voltage is rectified (D1~D4) and the current limiting resistor R1 charges the filter capacitor C1 to prevent the surge current at the moment of turn on. At the same time, the auxiliary power supply Vcc is connected to the relay K wire package through the resistor R2. Capacitor C2 is charged. When the voltage on C2 reaches the operating voltage of relay K, K operates, its contact K1.1 is closed and the current-limiting resistor R1 is bypassed, and the power supply enters normal operation. The delay time of current limiting depends on the time constant (R2C2), which is usually selected from 0.3 to 0.5s. In order to improve the accuracy of the delay time and prevent jitter and oscillation of the relay action, the delay circuit can use the circuit shown in Figure 3 instead of the R2C2 delay circuit.

3 Overvoltage, undervoltage and overheating protection circuit

The harm caused by overvoltage and undervoltage of the incoming power supply to the switching power supply is mainly manifested in the fact that the device is damaged because the voltage and current capacity it withstands exceeds the range of normal use, and at the same time, the electrical performance indicators are damaged and cannot meet the requirements. Therefore, the upper and lower limits of the input power supply must be limited, and overvoltage and undervoltage protection are used to improve the reliability and safety of the power supply.

Temperature is the most important factor affecting the reliability of power equipment. According to relevant data analysis (5), for every 2°C increase in temperature of electronic components, the reliability decreases by 10%. The working life at a temperature rise of 50°C is only 1/6 of that at a temperature rise of 25°C. In order to avoid overheating of power devices, If it is damaged, an overheat protection circuit also needs to be installed in the switching power supply.

Figure 4 Overvoltage, undervoltage, and overheating protection circuit

Figure 4 is an overvoltage, undervoltage, and overheating protection circuit composed of only a 4-comparator LM339 and several discrete components. The sampling voltage can be obtained directly from the auxiliary control power supply after rectification and filtering. It reflects the change of the input power supply voltage. The comparators share a reference voltage. N1.1 is an undervoltage comparator, and N1.2 is an overvoltage comparator. It can be adjusted by adjusting R1 Over and under voltage action thresholds. N1.3 is an overheating comparator, and RT is a thermistor with a negative temperature coefficient. It forms a voltage divider with R7 and is close to the surface of the power switching device IGBT. When the temperature rises, the resistance of RT decreases. Select the appropriate value of R7. resistance to make N1.3 operate at the set temperature threshold. N1.4 is used for emergency shutdown due to external faults. When its forward terminal inputs a low level, the comparator outputs a low level to block the PWM drive signal. Since the output terminals of the four comparators are connected in parallel, no matter if any fault occurs, such as overvoltage, undervoltage, or overheating, the comparator outputs a low level and blocks the drive signal to stop the power supply to achieve protection. If the circuit is slightly changed, the comparator can also be made to output a high level to block the driving signal.

4 Phase loss protection circuit

Due to the power grid itself or unreliable power input wiring, the switching power supply sometimes operates out of phase, and phase out operation is difficult to detect in time. When the power supply is in phase-loss operation, there will be no current in one arm of the rectifier bridge, while the other arms will be seriously over-current and cause damage, and at the same time, the inverter will work abnormally. Therefore, the phase loss must be protected. Current transformers or electronic phase loss detection circuits are usually used to detect phase loss in the power grid. Due to the high detection cost and large size of current transformers, electronic phase loss protection circuits are generally used in switching power supplies. Figure 5 is a simple phase loss protection circuit. When the three phases are balanced, the H potential of R1 ~ R3 nodes is very low, and the optical coupling output is approximately zero level. When the phase is missing, the potential of point H rises, and the optocoupler outputs a high level. After comparison by the comparator, it outputs a low level and blocks the drive signal. The comparator reference is adjustable to adjust the phase loss action threshold. This phase loss protection is suitable for three-phase four-wire system, but not for three-phase three-wire system. With slight changes to the circuit, the PWM signal can also be blocked at a high level.

Figure 5 Phase loss protection circuit of three-phase four-wire system

Figure 6 is a phase loss protection circuit for a three-phase three-wire power supply. If any phase A, B, or C is missing, the output level of the optocoupler is lower than the reference voltage of the inverting input terminal of the comparator, and the comparator output is low. level, blocks the PWM drive signal and turns off the power. By slightly changing the comparator input polarity, the PWM signal can also be blocked with a high level. This phase loss protection circuit uses an optocoupler to isolate strong current, which is safe and reliable. RP1 and RP2 are used to adjust the phase loss protection action threshold.

Figure 6 Phase loss protection circuit of three-phase three-wire system

5 short circuit protection

Switching power supplies are the same as other electronic devices. Short circuit is the most serious fault. Whether short-circuit protection is reliable is an important factor affecting the reliability of switching power supplies. IGBT (Insulated Gate Bipolar Transistor) combines the characteristics of high input impedance, low driving power of field effect transistors, large voltage and current capacity of bipolar transistors and reduced tube voltage. It is currently the most commonly used medium and high power switching power supply. Power electronic switching devices. The short-circuit time that an IGBT can withstand depends on its saturation voltage drop and the size of the short-circuit current, which is generally only a few μs to tens of μs. Excessive short-circuit current not only shortens the short-circuit withstand time, but also causes the current drop rate to be too large during turn-off. Due to the existence of leakage inductance and lead inductance, it leads to IGBT collector over-voltage. This over-voltage can cause IGBT lock failure. At the same time, high Overvoltage will cause IGBT breakdown. Therefore, when short circuit overcurrent occurs, effective protective measures must be taken.

In order to achieve short circuit protection of IGBT, overcurrent detection must be performed. The method suitable for IGBT overcurrent detection is usually to use a Hall current sensor to directly detect the IGBT current Ic, then compare it with the set threshold, and use the output of the comparator to control the turn-off of the drive signal; or use the indirect voltage method to detect The voltage drop Vce of IGBT during overcurrent is because the tube voltage drop contains short-circuit current information. Vce increases during overcurrent and is basically a linear relationship. The Vce during overcurrent is detected and compared with the set threshold. The comparator The output controls the shutdown of the driver circuit.

When a short-circuit current occurs, in order to avoid overvoltage caused by excessive turn-off current, resulting in IGBT locking failure and damage, and in order to reduce electromagnetic interference, soft gate voltage reduction and soft turn-off comprehensive protection technology are usually used.

When designing a gate voltage reduction protection circuit, it is necessary to correctly select the gate voltage reduction amplitude and speed. If the gate voltage reduction amplitude is large (such as 7.5V), the gate voltage reduction speed should not be too fast. Generally, a soft gate voltage reduction with a 2μs falling time can be used. , due to the large amplitude of the gate voltage reduction, the collector current is already small. It can be faster to block the gate in the fault state, and there is no need to use soft shutdown; if the amplitude of the gate voltage reduction is small (such as below 5V), the gate reduction speed can be faster. , and the speed of blocking the gate voltage must be slow, that is, soft shutdown is used to avoid overvoltage.

In order to ensure that the power supply does not interrupt operation during a short-circuit fault, and to avoid IGBT damage caused by heat accumulation caused by continuous short-circuit protection at the original operating frequency, the use of gate voltage reduction protection eliminates the need to immediately block the circuit after a short-circuit protection, and the operating frequency Reduced (for example, about 1Hz) to form an intermittent "hiccup" protection method, and normal operation will resume after the fault is eliminated. The following are several practical circuits and working principles of IGBT short circuit protection.

(1) Use the Vce of IGBT to design an overcurrent protection circuit

Figure 7 Using the principle that Vce increases when IGBT overcurrent is used for protection

Figure 7 is a circuit that uses the principle of Vce increase when IGBT overcurrent for protection, and is used in the dedicated driver EXB841. The internal circuit of EXB841 can well complete gate reduction and soft turn-off, and has an internal delay function to eliminate malfunctions caused by interference. Vce containing IGBT overcurrent information is not directly sent to the collector voltage monitoring pin 6 of EXB841, but is connected to pin 6 of EXB841 through the fast recovery diode VD1 through the output of comparator IC1. Its purpose is to eliminate the forward voltage drop of VD1 Varies with current. A threshold comparator is used to improve the accuracy of current detection. If overcurrent occurs, the low-speed cut-off circuit of the driver EXB841 slowly turns off the IGBT to avoid collector current spikes from damaging the IGBT device.

(2) Use current sensors to design overcurrent protection circuits

(a) Using current sensor for overcurrent protection circuit

(b) Output drive waveform diagram of PWM control circuit

Figure 8 Using current sensors for overcurrent protection

Figure 8(a) is an IGBT protection circuit that uses a current sensor for overcurrent detection. The primary (1 turn) of the current sensor (SC) is connected in series to the collector circuit of the IGBT. The overcurrent signal induced by the secondary is rectified and sent to The non-inverting input terminal of comparator IC1 is compared with the reference voltage at the inverting terminal. The output of IC1 is sent to the comparator IC2 with positive feedback, and its output is connected to the output control pin 10 of the PWM controller UC3525. When there is no current flow, VAVref and VB are high level, C3 charges to make VC>Vref, IC2 outputs high level (greater than 1.4V), and turns off the PWM control circuit. Because there is no drive signal, the IGBT is turned off, and the power supply stops working. There is no current flowing through the current sensor, so the VA parameter sets the PWM drive signal off time t2>>t1 to ensure that the power supply enters the sleep state. The positive feedback resistor R7 ensures that IC2 has only two states of high and low levels. D5, R1, and C3 charge and discharge circuits ensure that the output of IC2 will not change frequently between high and low levels, that is, the IGBT will not be damaged due to frequent turning on and off. .

(3) Comprehensive overcurrent protection circuit

Figure 9 is a comprehensive protection circuit that uses IGBT (V1) overcurrent collector voltage detection and current sensor detection. The working principle of the circuit is: when the load is short-circuited (or the IGBT is overcurrent due to other faults), Vce of V1 increases, and the gate of V3 The driving current passes through the R2 and R3 voltage dividers to turn on V3, and the IGBT gate voltage is reduced by being limited by VD3, limiting the IGBT peak current amplitude. At the same time, V2 is turned on through the delay of R5C3, and the soft turn-off signal is sent. On the other hand, during a short circuit, the short circuit current is detected by the current sensor, and the high level output by the comparator IC1 turns V3 on to reduce the gate voltage, and V2 is turned on for soft shutdown.

In addition, the overcurrent protection principle of detecting the IGBT collector voltage can also be applied, and the short-circuit protection circuit (7, 8) using soft gate voltage reduction, soft turn-off and reduced operating frequency protection technology will not be introduced in detail here. If you are interested Readers please refer to document (1). Although the switching power supply protection function is an additional function required by the electrical performance of the power supply device, it is crucial to the safety and reliability of the power supply device whether the protection circuit is complete and works according to the predetermined settings in harsh environments and accident conditions. When accepting technical indicators, the protection function should be verified.

The protection schemes and circuit structures of switching power supplies are diverse, but for specific power supply devices, reasonable protection schemes and circuit structures should be selected to achieve truly effective protection under fault conditions.

Figure 9 Comprehensive overcurrent protection circuit

6Conclusion

Although the switching power supply protection function is an additional function required by the electrical performance of the power supply device, it is crucial to the safety and reliability of the power supply device whether the protection circuit is complete and works according to the predetermined settings in harsh environments and accident conditions. When accepting technical indicators, the protection function should be verified.

The protection schemes and circuit structures of switching power supplies are diverse, but for specific power supply devices, reasonable protection schemes and circuit structures should be selected to achieve truly effective protection under fault conditions.

After the switching power supply protection circuit design is completed, the aging test of the switching power supply must be performed first, and then the functions of various protection circuits must be verified.

Review Editor: Tang Zihong


#Research #switching #power #supply #protection #circuit