IGBT overcurrent in industrial motor drives
IGBT overcurrent protection
In terms of property damage or safety considerations, IGBT protection for overcurrent conditions is the key to system reliability. IGBTs are not a fail-safe component. If they fail, they can cause the DC bus capacitor to explode and cause the entire driver to malfunction. Overcurrent protection is typically achieved by current measurement or desaturation detection. Figure 2 shows these tips.
For current measurement, both the inverter arm and the phase output require measurement devices such as shunt resistors to handle shoot-through faults and motor winding faults. The fast execution trip circuit in the controller and / or gate driver must turn off the IGBT in time to prevent the short circuit withstand time. The biggest benefit 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 simply adding a shunt resistor to the positive DC bus line and the negative DC bus line. However, in many cases, there are either arm shunt resistors in the driver architecture or phase shunt resistors to serve the current control loop and provide motor overcurrent protection; they are also possible for IGBT overcurrent protection – provided that The signal conditioning response time is fast enough to protect the IGBT during the required short-circuit withstand time.
The desaturation detection uses the IGBT itself as a current measuring element. The diodes in the schematic ensure that the IGBT collector-emitter voltage is only monitored by the sense circuit during turn-on; during normal operation, the collector-emitter voltage is very low (typically 1V to 4V). However, if a short circuit event occurs, the IGBT collector current rises to a level that drives the IGBT out of the saturation region and into the linear operating region. This causes the collector-emitter voltage to rise rapidly. The normal voltage levels described above can be used to indicate the presence of a short circuit, while the desaturation trip threshold level is typically in the 7V to 9V region. Importantly, desaturation can also mean that the gate-emitter voltage is too low and the IGBT is not fully driven to the saturation region. Be careful when performing desaturation detection deployment to prevent false triggering. This may especially occur during the transition from the IGBT off state to the IGBT on state when the IGBT has not fully entered saturation. The blanking time is usually between the turn-on signal and the desaturation detection activation time to avoid false detections. A current source charging capacitor or RC filter is also typically added to create a short time constant in the detection mechanism to filter the filter spurs caused by noise pickup. When selecting these filter components, a trade-off is required between the noise immunity and the IGBT short-circuit withstand time.
After detecting an IGBT overcurrent, 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 turn off the IGBT as quickly as possible to minimize switching losses. This is achieved by a lower driver impedance and gate drive resistance. If the same gate turn-off rate is applied for overcurrent conditions, the collector/emitter di/dt will be much larger because the current will vary greatly in a shorter period of time. The parasitic inductance of the collector-emitter circuit due to stray inductance of the wire bonding and PCB traces may cause a large overvoltage level to instantaneously reach the IGBT (because VLSTRAY=LSTRAY×di/dt). Therefore, it is important to provide a high impedance turn-off path when turning off the IGBT during a desaturation event, which can reduce di/dt and any potentially damaging overvoltage levels.
In addition to short circuits caused by system failures, instantaneous inverter through-through also occurs under normal operating conditions. At this time, the IGBT conduction requires the IGBT to be driven 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 12V. The IGBT turn-off requires the IGBT to be driven to the active cut-off region to successfully block the reverse high voltage across the high-side IGBT when it is turned on. In principle, this can be achieved by lowering the IGBT gate-emitter voltage to 0V. However, the side effects of the low-end transistor on the inverter arm when it is turned on must be considered.
A rapid change in the voltage at the switching node during turn-on causes the capacitive induced current to flow through the low-end IGBT parasitic Miller gate-collector capacitance (CGC in Figure 3). This current flows through the low-side gate driver (ZDRIVER in Figure 3) to turn off the impedance, creating a transient voltage increase at the low-side IGBT gate emitter, as shown. If the voltage rises above the IGBT threshold voltage VTH, it will cause a brief turn-on of the low-side IGBT, resulting in a transient inverter arm pass-through - because both IGBTs are briefly turned on. This generally does not destroy the IGBT, but it can increase power consumption and affect reliability.
In general, there are two ways to solve the inductive conduction problem of the inverter IGBT - using a bipolar power supply or an additional Miller clamp. The ability to accept a bipolar supply at the isolated side of the gate driver provides additional margin for induced voltage transients. For example, a –7.5V negative rail indicates that an induced voltage transient greater than 8.5V is required to sense spurious conduction. This is enough to prevent stray conduction. Another method is to reduce the turn-off impedance of the gate driver circuit for a period of time after the turn-off transition is completed. 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 transient. The use of asymmetric gate resistors for turn-on and turn-off provides additional flexibility for switching rate control. All of these gate driver functions have a positive impact on the reliability and efficiency of the overall system.
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