summary
The overall market trend for industrial motor drives is a continuous increase in the demand for higher efficiency, reliability, and stability. Power semiconductor device manufacturers are constantly seeking breakthroughs in conduction losses and switching times. Some trade-offs related to increasing the conduction losses of insulated-gate bipolar transistors (IGBTs) include: higher short-circuit current levels, smaller chip size, and lower thermal capacity and short-circuit withstand time. This highlights the importance of gate driver circuitry and overcurrent detection and protection functions. This article discusses the issue of successfully and reliably implementing short-circuit protection in modern industrial motor drives.
Short circuits in industrial environments
Industrial motor drives operate in relatively harsh environments, and may encounter high temperatures, AC line transients, mechanical overloads, wiring errors, and other unforeseen events. Some of these events can cause significant overcurrents to flow into the motor drive's power circuitry. Figure 1 illustrates three typical short-circuit events.
Figure 1. Typical short-circuit events in industrial motor drives.
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1 indicates inverter shoot-through. This could be due to incorrect switching of two IGBTs on one of the inverter arms, which could be caused by electromagnetic interference or controller malfunction. It could also be due to wear/failure of one of the IGBTs on the arm, while the normal IGBT remains switched on.
2 indicates a phase-to-phase short circuit. This could be caused by insulation breakdown between motor windings due to performance degradation, excessive temperature, or overvoltage events.
3. A phase-to-ground short circuit. This can also be caused by insulation breakdown between the motor windings and the motor housing due to performance degradation, overheating, or overvoltage events. Generally, motors can absorb extremely high currents for relatively long periods (milliseconds to seconds, depending on motor size and type); however, IGBTs—a key component of industrial motor drive inverter stages—have short-circuit withstand times in the microsecond range.
IGBT short-circuit withstand capability
IGBT short-circuit withstand time is related to its transconductance or gain and the thermal capacity of the IGBT chip. Higher gain leads to higher short-circuit current within the IGBT, so obviously, IGBTs with lower gain have lower short-circuit levels. However, higher gain also leads to lower on-state conduction losses, so a trade-off must be made. The development of IGBT technology is driving a trend towards increasing short-circuit current levels while reducing short-circuit withstand time. Furthermore, technological advancements have led to the use of smaller chip sizes, reducing module size, but also decreasing thermal capacity, further shortening withstand time.
Furthermore, it is also closely related to the collector-emitter voltage of the IGBT. Therefore, the parallel trend of industrial drives moving towards higher DC bus voltage levels has further reduced the short-circuit withstand time. In the past, this time range was 10μs, but the recent trend is towards 5μs3 and, under certain conditions, as low as 1μs.
In addition, the short-circuit withstand time varies considerably among different devices. Therefore, for IGBT protection circuits, it is generally recommended to have an additional margin beyond the rated short-circuit withstand time.
IGBT overcurrent protection
Whether for reasons of property damage or safety, IGBT protection against overcurrent conditions is crucial to system reliability. IGBTs are not fail-safe components; their failure can cause the DC bus capacitors to explode, leading to a complete drive failure. Overcurrent protection is typically achieved through current measurement or desaturation detection. Figure 2 illustrates these techniques.
For current measurement, both the inverter arms and phase outputs require measuring devices such as shunt resistors to handle shoot-through and motor winding faults. The fast-acting switching circuits in the controller and/or gate driver must promptly shut down the IGBTs to prevent exceeding the short-circuit withstand time. The greatest advantage of this approach is that it requires two measuring devices on each inverter arm, along with all necessary signal conditioning and isolation circuitry. This can be mitigated simply by adding shunt resistors to the positive and negative DC bus lines. However, in many cases, either arm shunt resistors or phase shunt resistors are present in the drive architecture to serve the current control loop and provide motor overcurrent protection; they may also be used for IGBT overcurrent protection—provided the signal conditioning response time is fast enough to protect the IGBTs within the required short-circuit withstand time.
Figure 2. Example of IGBT overcurrent protection technology
Desaturation detection utilizes the IGBT itself as a current measurement element. The diode in the schematic ensures that the IGBT collector-emitter voltage is monitored only by the detection circuit during conduction; under 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 a rapid rise in the collector-emitter voltage. The aforementioned normal voltage level can be used to indicate the presence of a short circuit, while the desaturation threshold level is typically in the 7V to 9V range. Importantly, desaturation can also indicate that the gate-emitter voltage is too low and the IGBT is not fully driven into the saturation region. Care must be taken when deploying desaturation detection to prevent false triggering. This is especially likely to occur during the transition from the IGBT off state to the IGBT on state before the IGBT has fully entered saturation. The blanking time is typically between the turn-on signal and the desaturation detection activation moment to avoid false detections. A current source charging capacitor or RC filter is also usually added to create a short time constant in the detection mechanism to filter out spurious filter transitions caused by noise pickup. When selecting these filter components, a trade-off must be struck between noise immunity and IGBT short-circuit withstand time.
After detecting IGBT overcurrent, the further challenge is turning off the IGBT when it is in an abnormally high current level state. 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 through low 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 changes more rapidly in a shorter time. Parasitic inductance in the collector-emitter circuit due to wire bonding and PCB trace stray inductance can cause large overvoltage levels to instantaneously reach the IGBT (because VLSTRAY = LSTRAY × di/dt). Therefore, during desaturation events, it is important to provide a high-impedance turn-off path when turning off the IGBT to reduce di/dt and any potentially damaging overvoltage levels.
Besides short circuits caused by system faults, instantaneous inverter shoot-through can also occur under normal operating conditions. In this case, IGBT turn-on requires the IGBT to be driven to the saturation region, where conduction losses are minimized. This typically means that the gate-emitter voltage is greater than 12V when on. IGBT turn-off requires the IGBT to be driven to the off region to successfully block the reverse high voltage across it when the high-side IGBT is on. In principle, this can be achieved by reducing the IGBT gate-emitter voltage to 0V. However, the side effects of the low-side transistors on the inverter arm being on must be considered.
The rapid change in the switching node voltage during turn-on causes a capacitive induced current to flow through the parasitic Miller gate-collector capacitance (CGC in Figure 3) of the low-side IGBT. This current flows through the turn-off impedance of the low-side gate driver (ZDRIVER in Figure 3), creating a transient voltage increase at the gate-emitter junction of the low-side IGBT, as shown in the figure. If this voltage rises above the IGBT threshold voltage VTH, it can cause a brief turn-on of the low-side IGBT, resulting in a transient inverter arm shoot-through—because both IGBTs are briefly turned on. This generally does not damage the IGBT, but it can increase power consumption and affect reliability.
Figure 3 Miller induction inverter direct circuit
Generally, there are two methods to address the inductive turn-on problem of inverter IGBTs—using bipolar power supplies or additional Miller clamping. The ability to accept bipolar power supplies at the gate driver isolation terminals provides additional margin for inductive voltage transients. For example, a -7.5V negative rail indicates that an inductive voltage transient greater than 8.5V is required to induce stray turn-on. This is sufficient to prevent stray turn-on. Another method is to reduce the turn-off impedance of the gate driver circuitry for a period of time after the turn-off transition is complete. This is called Miller clamping. Capacitive current now flows through the lower impedance circuitry, subsequently reducing the magnitude of the voltage transient. Using asymmetrical gate resistors for turn-on and turn-off provides additional flexibility for switching rate control. All these gate driver features have a positive impact on the overall system reliability and efficiency.
