introduction
With the development of power electronic device manufacturing technology, high-performance, high-capacity Insulated Gate Bipolar Transistors (IGBTs) are increasingly being used in various power conversion devices with operating frequencies below tens of kHz and output power ranging from several kW to hundreds of kW due to their characteristics such as voltage-type control, high input impedance, low drive power, low switching losses, and high operating frequency. The most crucial aspect of an IGBT inverter is the design of a high-performance overcurrent protection circuit. Dedicated drive modules all have overcurrent protection functions. Some discrete drive circuits also have overcurrent protection functions. In industrial applications, these instantaneous overcurrent protection signals are generally used to construct a memory-locked protection circuit through the memory function of the trigger-based sequential logic circuit. This avoids frequent operation of the protection circuit during overcurrent, achieving desirable overcurrent protection. This paper analyzes the dual protection circuit structure of a high-power controllable rectifier voltage-type inverter, which combines drive blocking and rectifier pull-up inverter protection.
Causes and Protection Methods of IGBT Failure
The causes of IGBT failure include:
1) Overheating damage: Instantaneous overheating caused by excessive collector current, and other reasons such as continuous overheating due to poor heat dissipation, can damage the IGBT. If the device is continuously short-circuited, the power consumption generated by the large current will cause a temperature rise. Due to the small heat capacity of the chip, its temperature rises rapidly. If the chip temperature exceeds the intrinsic temperature of silicon (approximately 250°C), the device will lose its blocking capability, and the gate control will be unable to provide protection, thus leading to IGBT failure. In actual operation, the maximum allowable operating temperature is generally around 130°C.
2) Damage caused by latch-up effect due to exceeding the safe turn-off area. Latch-up effect is divided into static latch-up effect and dynamic latch-up effect. IGBT has a PNPN 4-layer structure, and its equivalent circuit is shown in Figure 1. There is a parasitic thyristor in the body. There is a body region extension resistor Rs between the base and emitter of the NPN transistor. The lateral hole current in the P-type body will generate a certain voltage drop on Rs, which is equivalent to a forward bias voltage for the NPN base. Within the specified collector current range, this forward bias voltage is not large and has no effect on the NPN transistor. When the collector current increases to a certain extent, this forward voltage is sufficient to turn on the NPN transistor, thus causing the NPN and PNP transistors to be in a saturated state. At this time, the parasitic thyristor conducts, the gate loses its control function, and a latch-up phenomenon is formed, which is the so-called static latch-up effect. After the IGBT latches up, the collector current increases, resulting in excessive power consumption, leading to device failure. The dynamic latch-up effect mainly occurs when the current drops too quickly during high-speed turn-off of the device, resulting in a large dvCE/dt, which causes a large displacement current to flow through Rs and generate a forward bias voltage sufficient to turn on the NPN transistor, causing the parasitic thyristor to latch up.
3) Transient overcurrent: In addition to faults such as short circuits and shoot-through, the large-value overcurrents that IGBTs experience during operation also include the reverse recovery current of the freewheeling diode, the discharge current of the buffer capacitor, and the spike current caused by noise interference. Although these transient overcurrents are short-lived, if no measures are taken, they will increase the burden on the IGBT and may even lead to IGBT failure.
4) Overvoltage causes collector-emitter breakdown.
5) Overvoltage causes gate-emitter breakdown. Rectifier-inverter combined protection scheme.
IGBT Protection Methods
When an overcurrent condition occurs, the IGBT must remain within its Short-Circuit Safe Operating Area (SCSOA). The duration an IGBT can withstand a short circuit is closely related to the supply voltage, gate drive voltage, and junction temperature. To prevent IGBT damage due to short-circuit faults, a robust fault detection and protection system is essential. Common detection methods include current sensors and IGBT undersaturation protection.
1) Block drive signal
In cases of excessive load or output short circuit in the inverter power supply, a current sensor on the DC bus input to the inverter bridge is used for detection. When the detected current value exceeds a set threshold, the protection mechanism blocks the drive signals of all bridge arms. This protection method is the most direct, but the snubber and clamping circuits must be specially designed to handle short circuits. A disadvantage of this method is that it can cause excessive stress on the IGBTs during turn-off, especially when turning off very large inductive currents; the latch-up effect must be carefully considered.
2) Reduce gate voltage
The short-circuit current of an IGBT is closely related to its gate voltage; the higher the gate voltage, the greater the current during a short circuit. In short-circuit or transient overcurrent situations, if vGS can be reduced in steps or its ramp can be decreased instantaneously, the short-circuit current will decrease. When the IGBT is turned off, di/dt will also decrease. Integrated driver circuits such as the EXB841 or M579xx series have vCES detection circuits. When undersaturation is detected, the gate voltage is clamped to around 10V, increasing vCES to limit the overcurrent amplitude and extend the allowable overcurrent time.
Rectifier-inverter combined protection scheme
1. Inverter section protection
This inverter design employs a half-bridge structure with a series resonant load, driven by the IR2110 half-bridge driver chip from IR Systems. The IR2110 circuit is simple, low-cost, and suitable for medium to high power IGBTs. Experimental results also verify the feasibility of using the IR2110 to drive medium to high power IGBTs. The IR2110 chip has an SD input pin that blocks both drive outputs; when this pin is high, both outputs are immediately blocked.
The causes of short-circuit faults in voltage-source inverters include:
1) A device (including the anti-parallel diode) in a straight-through short-circuit bridge arm is damaged; or due to faults in the control circuit, drive circuit, or interference causing the drive circuit to be falsely triggered, two IGBTs in one bridge arm are turned on at the same time.
2) Load circuit short circuit: In some step-up transformer output applications, the secondary side may be short-circuited.
3) Inverter output directly short-circuited
The shoot-through protection circuit must have a very high speed. Under normal circumstances, if the IGBT's rated parameters are selected reasonably, overcurrent within 10μs will not damage the device, so the IGBT must be turned off within this time. A Hall effect sensor is used for bus current detection; its fast response speed makes it the best choice for short-circuit protection detection. An LM319 comparator is used; the detected value is compared with the set value, and if it exceeds the set value, a protection signal is immediately output to block the drive. Simultaneously, a memory-locked protection circuit is constructed using a trigger to avoid frequent activation of the protection circuit during overcurrent. An external reset circuit is also indispensable.
Rectifier section protection
For high-power voltage-source inverters, a filter inductor is typically connected in series on the DC bus to improve the input current waveform, as shown in Figure 5. Due to the presence of the inductor, if the rectifier circuit is still in rectification mode after the inverter circuit stops operating, the energy in the inductor will be released into the capacitor. At the moment the inverter protection trips, the capacitor will experience a very high overshoot voltage. If no measures are taken, this may directly lead to capacitor overvoltage damage. This is especially dangerous when the load current is very high and the energy stored in L is large.
Assume that when the inverter is turned off, all the current in the filter inductor flows through the capacitor C, while the rectifier continues to output voltage Ud. Figure 6 shows the equivalent circuit. L and C are in series and resonate. Since the rectifier bridge current can only flow in one direction, the oscillation ends at T/4.
It can be seen that the voltage across the capacitor reaches its maximum value when the resonance reaches 1/4 of the cycle, after which the resonance stops.
The final voltage across the capacitor is related to the bus current, inductance, and capacitance. In our 10kW prototype test unit, when the DC bus voltage was 200V, the inverter was momentarily shut down under protection signal, and the bus voltage suddenly rose to nearly 450V. To address this phenomenon, the rectifier circuit was switched to inverter operation (firing angle α was pulled to approximately 150°) simultaneously with the protection action, allowing most of the energy in the filter inductor to be fed back to the grid.
In practical applications, the likelihood of a shoot-through of the upper and lower IGBTs due to a fault in the drive circuit is very small. Therefore, a protection method that pulls the inverter out of the rectifier section can also be used. For load overcurrent or short circuits, the entire device can be stopped within the short-circuit current allowable by the IGBT. This protection method does not directly target the IGBT; instead, it shuts off the input of the preceding rectifier, so the IGBT remains operational during a fault. This is considered "soft protection," which does not subject the IGBT to stress shocks and also avoids the possibility of the IGBT exceeding its safe turn-off operating range and becoming latched up due to a sudden turn-off under high current.
Experimental results
This protection scheme has been successfully applied to a high-power, high-frequency, high-voltage voltage-type series resonant inverter. The medium-voltage output is stepped up to 6kV via a step-up transformer for corona treatment of materials. The prototype output power is approximately 10kW. Because the load is a high-voltage corona processor, primary and secondary breakdown can easily occur inside the step-up transformer. Tests showed that this scheme effectively protects the inverter from damage, whether caused by load short circuits, transformer breakdown-induced overcurrent, or excessively high input voltage.
in conclusion
IGBTs are the most vulnerable component in an inverter, especially in voltage-source controlled rectifier circuits. When implementing IGBT shoot-through protection, the impact of shutting down the inverter on the upstream circuitry must also be considered. The simultaneous rectifier and inverter protection scheme described in this article reliably protects the entire inverter and has achieved good results in practice.
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