Switching power supply protection circuit -Lithium - Ion Battery Equipment

Introduce several protection circuits commonly used in switching power supplies. -Lithium - Ion Battery Equipment

The quality indicators for evaluating switching power supplies should be based on safety and reliability as the first principles. Under the premise that the electrical technical indicators meet the requirements for normal use, in order to make the power supply operate safely and reliably in harsh environments and sudden failures, it is necessary to design a variety of protection circuits, such as soft start to prevent surges, prevent overvoltage, and undervoltage. voltage, overheating, overcurrent, short circuit, and lack of protective circuit. Several maintenance circuits commonly used for switching power supplies are as follows:

1. Anti-surge soft start circuit

Most of the input circuits of switching power supplies use capacitor filter rectifier circuits. When the incoming power supply is turned on, because the initial voltage on the capacitor is zero, a large surge current will form during the charging moment of the capacitor, especially for high-power switching power supplies. , choose a filter capacitor with a larger capacity to make the surge current reach 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 the rectifier bridge to be damaged by overcurrent; in the worst case, it will also cause the air switch to fail to close. . The above phenomena will cause the switching power supply to fail to operate 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.

Choose an anti-surge current circuit composed of thyristor V and current-limiting resistor R1. 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 surge current. When the capacitor C is charged to about 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.

Choose an anti-surge current circuit composed of relay K1 and current-limiting resistor R1. When 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 turning on. At the same time, the auxiliary power supply Vcc is connected to the relay K1 line through the resistor R2 pair. The capacitor C2 of the package is charged. When the voltage on C2 reaches the operating voltage of relay K1, K1 operates, its contact K1.1 closes and bypasses the current limiting resistor R1, and the power supply enters normal operation. The delay time of current limiting depends on the time constant (R2C2), and is generally selected from 0.3 to 0.5s. In order to improve the accuracy of the delay time and prevent the relay from shaking and vibrating, the delay circuit can use the circuit shown in Figure 3 instead of the RC delay circuit.(Lithium - Ion Battery Equipment)

2. Overvoltage, undervoltage and overheating protection circuit

The damage caused by overvoltage and undervoltage of the incoming power supply to the switching power supply mainly manifests itself in the equipment being damaged due to the voltage and current stress it endures exceeding 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 restricted, and over-voltage and under-voltage 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 material analysis, every time the temperature of electronic components increases by 2°C, the reliability decreases by 10%. The operating life at a temperature rise of 50°C is only 1/6 of that at a temperature rise of 25°C. In order to prevent damage to power equipment due to overheating, An overheating protection circuit also needs to be installed in the switching power supply.

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 comparator shares 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 increases, the resistance value of RT decreases. Select the resistance of R7 appropriately. value to make N1.3 operate at the set temperature threshold. N1.4 is used for emergency shutdown of external faults. When its forward terminal inputs a low level, the comparator outputs a low level to turn off the PWM drive signal. Since the output terminals of the four comparators are connected in parallel, no matter whether overvoltage, undervoltage, or overheating occurs, the comparator outputs a low level and turns off the drive signal to stop the power supply and complete maintenance. If the circuit is slightly changed, the comparator can also be made to output a high-level closed driving signal.

3. Phase loss maintenance 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 arm will be seriously overcurrent and cause damage. At the same time, the inverter will operate abnormally, so the phase loss must be protected. To check the phase loss of the power grid, a current transformer or an electronic phase loss inspection circuit is generally used. Due to the high inspection cost and large size of current transformers, electronic phase loss protection circuits are generally used in switching power supplies. 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 the H point is raised, the optocoupler outputs a high level, and after comparison with the comparator, it outputs a low level and turns off the driving signal. The reference of the comparator is adjustable to adjust the phase loss action threshold. This phase loss maintenance is applicable to the three-phase four-wire system, but not to the three-phase three-wire system. With a slight change in the circuit, the PWM signal can also be turned off with a high level.

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 outputs a low level and is turned off. PWM drive signal, turn off the power supply. By slightly changing the comparator input polarity, the PWM signal can also be turned off with a high level. This phase loss protection circuit uses an optocoupler to block strong current, which is safe and reliable. RP1 and RP2 are used to adjust the phase loss protection action threshold.

4. Short circuit maintenance

Switching power supply is the same as other electronic equipment. Short circuit is the most serious fault. Whether short-circuit protection is reliable is an important factor affecting the reliability of switching power supply. 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 the most commonly used switching power supply in medium and high power. Power electronic switching equipment. The short-circuit time that the 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 acceptance time, but also makes the current drop rate di/dt during turn-off too large. Due to the existence of leakage inductance and lead inductance, the IGBT collector overvoltage is caused. This overvoltage can generate an engine inside the device. The live effect will definitely cause the IGBT to fail, and at the same time, a high overvoltage will cause the IGBT to breakdown. Therefore, when a short circuit overcurrent occurs, effective protective measures must be taken. In order to complete the short circuit protection of IGBT, it is necessary to conduct overcurrent check. The method suitable for IGBT overcurrent check is generally to use a Hall current sensor to directly check the IGBT current Ic, and 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 direct voltage method to check The voltage drop Vce of the IGBT during overcurrent is because the tube voltage drop is rich in short-circuit current information. Vce increases during overcurrent and is basically a linear relationship. Check the Vce during overcurrent and compare it with the set threshold. Comparator The output controls the shutdown of the drive circuit. When a short-circuit current occurs, in order to prevent the di/dt of the turn-off current from being too large to cause overvoltage, causing the IGBT to be ineffective and damaged, and to reduce electromagnetic interference, soft gate voltage reduction and soft turn-off comprehensive protection techniques are generally used. After detecting the overcurrent signal, the first step is to enter the gate reduction maintenance procedure to reduce the amplitude of the fault current and extend the short circuit withstand time of the IGBT. After the gate lowering action, a fixed delay time is set to determine the authenticity of the fault current. If the fault disappears within the delay time, the grid voltage will automatically recover. If the fault still exists, a soft shutdown procedure will be performed to reduce the grid voltage to Below 0V, the IGBT driving signal is turned off. Since the collector current has been reduced during the gate voltage reduction process, there will not be an excessive short-circuit current drop rate and excessive overvoltage during soft turn-off. The use of soft gate voltage reduction and soft turn-off gate drive protection can limit the amplitude and decrease rate of fault current, reduce overvoltage, and ensure that the IGBT current and voltage operating track is within the safe zone.

When designing a gate voltage reduction protection circuit, it is necessary to accurately select the gate voltage reduction range and speed. If the gate voltage reduction range 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. , because the gate voltage drop range is large and the collector current is already small, the gate can be turned off faster in a fault condition without using soft shutdown; if the gate voltage drop range is small (for example, below 5V), the gate step down speed can be faster , and the speed of turning off the gate voltage must be slow, that is, soft turn-off is used to prevent over-voltage. In order to prevent the power supply from interrupting operation in the event of a short-circuit fault, and to prevent continuous short-circuit protection at the original operating frequency from causing heat accumulation and damage to the IGBT, the use of reduced gate voltage protection eliminates the need to immediately shut down the circuit after a short-circuit protection, while increasing the operating frequency. decreases (for example, about 1Hz), forming an intermittent "hiccup" maintenance method, and normal operation will resume after the fault is eliminated.

The following introduces several useful circuits and working principles for IGBT short circuit protection.

A maintenance circuit using the principle of Vce increasing when IGBT overcurrent is used for the dedicated driver EXB841. The internal circuit of EXB841 can achieve excellent gate reduction and soft turn-off, and has an internal delay function to eliminate malfunctions caused by interference. Vce, which is rich in IGBT overcurrent information, is not directly sent to the collector voltage monitoring pin 6 of EXB841. Instead, it passes through the rapid recovery diode VD1 and is output through the comparator IC1 to pin 6 of EXB841. The purpose is to eliminate the forward voltage of VD1. The voltage drop varies with current. Use a threshold comparator to improve the accuracy of current inspection. If an overcurrent occurs, the low-speed cut-off circuit of the driver EXB841 slowly turns off the IGBT to prevent collector current spikes from damaging the IGBT device.

IGBT maintenance circuit that uses a current sensor for overcurrent check. 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 of the comparator IC1. terminal, compared with the reference voltage of 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 overcurrent occurs, the rectified voltage detected by the current sensor increases, VA>Vref, VB is high level, C3 charges to make VC>Vref, IC2 outputs high level (greater than 1.4V), and the PWM control circuit is turned off. Because there is no drive signal, the IGBT is turned off, and the power supply stops working, and no current flows through the current sensor, making VA>t1, which ensures that the power supply enters a sleep state. Positive feedback resistor R7 ensures that IC2 only has two states of high and low levels. D5, R1, and C3 charge and discharge circuits ensure that the output of IC2 will not frequently change between high and low levels, that is, the IGBT will not frequently register or turn off. damage.

The IGBT protection circuit that uses a current sensor for overcurrent detection 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), The Vce of V1 increases, the V3 gate drive current passes through R2, and the R3 voltage divider turns on V3. The IGBT gate voltage is controlled and reduced by VD3, which limits the IGBT peak current fluctuation. At the same time, V2 is turned on after being delayed by R5C3. Send soft shutdown signal. On the other hand, during a short circuit, the short circuit current is checked 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.

Using the over-current protection principle of checking the IGBT collector voltage, a short-circuit protection circuit with soft gate voltage reduction, soft turn-off and reduced operating frequency protection technology is adopted.

Under normal operating conditions, when the drive input signal is low level, the optocoupler IC4 is not conducting, V1 and V3 are conducting, and a negative driving voltage is output. When the drive input signal is high level, the optocoupler IC4 is turned on, V1 is turned off and V2 is turned on, a positive driving voltage is output, and the power switch V4 operates in a normal switching state. When a short circuit fault occurs, the IGBT collector voltage increases. Due to the increase in Vce, the comparator IC1 outputs a high level, V5 is turned on, and the IGBT completes a soft gate voltage drop. The gate voltage drop range is determined by the voltage regulator VD2, and the gate voltage is softly dropped. The pressing moment is composed of R6C1 and 2μs. At the same time, the high level output by IC1 charges C2 through R7. When the voltage on C2 reaches the breakdown voltage of the voltage regulator tube VD4, V6 is turned on and R9C3 forms a soft-off gate voltage of about 3μs. The gate voltage is softly reduced to The delay time of the soft-off gate voltage is determined by the time constant R7C2, which is generally selected between 5 and 15 μs. When V5 is turned on, V7 flows through the base current through the C4R10 circuit and turns on for about 20μs. After reducing the gate voltage for protection, the input drive signal is blocked for a period of time and no longer responds to the shutdown signal at the input end to avoid malfunctions. Hard shutdown overvoltage enables the drive circuit to perform a complete gate voltage reduction and soft shutdown protection process in the presence of a fault.

When V7 is turned on, the optocoupler IC5 is turned on, and the trigger pin 2 of the time base circuit IC2 obtains a negative trigger signal. The 555 output pin 3 outputs a high level, V9 is turned on, and IC3 is turned off. The turn-off time is determined by the timing component R15C5 ( About 1.2s), causing the operating frequency to drop below 1Hz. The output signal of the driver will operate in the so-called "hiccup" state, which prevents heat accumulation caused by continuous short-circuit protection caused by still operating at the original frequency after a short-circuit fault. The IGBT is damaged. As long as the fault disappears, the circuit can return to normal operating conditions.

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

The maintenance plans and circuit structures of switching power supplies are diverse, but for specific power supply equipment, a reasonable maintenance plan and circuit structure should be selected so that maintenance can be truly and effectively completed under fault conditions. The protection circuits described in this article can be flexibly combined to simplify circuit construction and reduce costs.



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