Short circuit in industrial environment
Industrial motors have a relatively poor working environment, high temperature, AC line transients, mechanical overload, wiring errors, and other unexpected situations. Some of these events may cause large overcurrents to flow into the power circuit of the motor driver. Figure 1 shows three typical short-circuit events.
1 is the inverter through. This may be due to improper opening of one of the two IGBTs of the inverter arm, which may be due to electromagnetic interference or controller failure. It may also be due to one of the IGBTs on the IGBT wear / failure caused by the normal IGBT holding the switch action.
2 is relatively phase short. This may be due to performance degradation, the temperature is too high or overvoltage events lead to dielectric breakdown between the breakdown caused by the insulation.
3 is a phase-to-ground short circuit. This may also be due to performance degradation, the temperature is too high or overvoltage events lead to motor windings and electrical insulation between the shell caused by the breakdown. In general, the motor can absorb very high currents in a relatively long time (milliseconds to seconds, depending on the size and type of the motor); however, IGBTs - industrial motors drive the main part of the inverter stage - short circuit Tolerance time is microsecond.
IGBT short circuit tolerance
IGBT short-circuit withstand time and its transconductance or gain and IGBT chip heat capacity. Higher gain results in higher short-circuit current in the IGBT, so it is clear that the lower-gain IGBT has a lower short-circuit level. However, higher gain also results in lower on-state conduction losses, and therefore trade-offs must be made. The development of IGBT technology is driving the trend of increasing the short-circuit current level, but reducing the short-circuit withstand time. In addition, advances in technology lead to smaller chip size, reduced module size, but reduced heat capacity, and even shorter lead time.
In addition, there is a great relationship with the IGBT collector-emitter voltage, so the industrial drive tends to a higher DC bus voltage level parallel to further reduce the short-circuit withstand time. In the past, this time range was 10 μs, but the trend in recent years was in the direction of 5 μs3 and some conditions as low as 1 μs.
In addition, the short-circuit withstand times of different devices are also quite different, so for IGBT protection circuits, it is generally recommended to have additional margins built with more than the rated short-circuit withstand time.
IGBT overcurrent protection
Whether for property damage or security considerations, IGBT protection for overcurrent conditions is the key to system reliability. IGBTs are not a fail-safe component, and if they fail, they may cause the DC bus capacitor to explode and cause the entire drive to fail. Overcurrent protection is typically achieved by current measurement or desaturation detection. Figure 2 shows these techniques.
For current measurement, both the inverter arm and the phase output require measuring devices such as shunt resistors to cope with straight-through faults and motor winding faults. The quick-execute transition circuit in the controller and / or the gate driver must be turned off in time to prevent short circuit withstand times. The biggest advantage of this method is that it requires two measuring devices on each inverter arm and is equipped with all relevant signal conditioning and isolation circuits. This can be alleviated by adding a shunt resistor to the positive DC bus line and the negative DC bus line. In many cases, however, there is either an arm shunt resistor in the drive architecture, or a phase shunt resistor to service the current control loop and provide motor overcurrent protection; they are also possible for IGBT overcurrent protection - provided The response time of the signal conditioning is fast enough to protect the IGBT in the required short-circuit withstand time.
The desensitization detection utilizes the IGBT itself as a current measurement element. The diode in the schematic ensures that the IGBT collector-emitter voltage is only monitored by the detection circuit during conduction. During normal operation, the collector-emitter voltage is very low (typically 1 V to 4 V). However, if a short-circuit event occurs, the IGBT collector current rises to the level at which the drive IGBT exits the saturation zone and enters the linear operating region. This results in a rapid increase in collector-emitter voltage. The above normal voltage level can be used to indicate that there is a short circuit and the desaturation transition threshold level is typically in the 7 V to 9 V region. It is important that the desaturation also indicates that the gate-emitter voltage is too low and the IGBT is not fully driven to the saturation region. Perform a desaturation detection deployment carefully to prevent false triggering. This may occur in particular when the IGBT has not yet fully entered the saturation state from the IGBT turn-off state to the IGBT turn-on state. The blanking time is usually between the on signal and the desaturation detection activation time to avoid false detection. Typically, a current source charging capacitor or an RC filter is added to generate a brief time constant in the detection mechanism to filter the filter spurious transitions caused by noise picking. When selecting these filter elements, a tradeoff between noise immunity and IGBT short-circuit withstand time is required.
After detecting the overcurrent of the IGBT, a further challenge is to turn off the IGBT at an abnormally high current level. Under normal operating conditions, the gate driver is designed to be able to close the IGBT as quickly as possible in order to minimize switching losses. This is achieved by a lower driver impedance and a gate drive resistor. If the same gate turn-off rate is applied to the overcurrent condition, the di- dt of the collector-emitter will be much larger because the current changes greatly in a short period of time. The parasitic inductance of the collector-emitter circuit due to wire soldering and PCB traverse stray inductance may cause a large overvoltage level to reach the IGBT (due to VLSTRAY = LSTRAY × di / dt). Therefore, it is important to provide a high-impedance shutdown path when the IGBT is turned off during the desaturation event, which reduces di / dt and all potentially destructive overvoltages.
In addition to the system fault caused by short circuit, instantaneous inverter through the same will occur under normal operating conditions. At this point, the IGBT turns on the IGBT to the saturation region, where the conduction loss is the lowest. This usually means that the gate-emitter voltage in the on-state is greater than 12 V. The IGBT turn-off requires the IGBT to drive to the operating cut-off area to successfully block the reverse high voltage at both ends when the high-side IGBT turns on. In principle, the target can be achieved by lowering the IGBT gate-emitter voltage to 0 V. However, the side effects of the low-side transistor on the inverter arm must be taken into account.
The rapid change in the switching node voltage at turn-on leads to capacitive sensing current flowing through the low-side IGBT parasitic Miller gate-collector capacitance (CGC in Figure 3). The current flows through the low-side gate driver (ZDRIVER in Figure 3) to turn off the impedance at the low-side IGBT gate emitter to create a transient voltage increase as shown. If the voltage rises above the IGBT threshold voltage VTH, it will cause a short turn on the low-side IGBT to form a transient inverter arm through - because both IGBTs are turned on briefly. This generally does not destroy the IGBT, but it can increase power consumption, affecting reliability.
In general, there are two ways to solve the inductive problem of inverter IGBT - using a bipolar power supply or an additional Miller clamp. The ability to accept bipolar power at the gate driver isolation provides additional margin for induced voltage transients. For example, a -7.5 V negative rail indicates that an inductive voltage transients greater than 8.5 V are required to sense spurious conduction. This is enough to prevent spurious conduction. Another method is to reduce the turn-off impedance of the gate driver circuit for a period of time after completion of the turn-off conversion. This is called the Miller clamp circuit. The capacitive current now flows through the lower impedance circuit, which in turn reduces the magnitude of the voltage transients. The use of asymmetric gate resistors for conduction and shutdown provides additional flexibility for switching rate control. All of these gate driver functions have a positive impact on the overall system reliability and efficiency.