How to successfully and reliably implement short-circuit protection in industrial motor drives

Industrial motor drives operate in a relatively harsh environment, where high temperatures, AC line transients, mechanical overloads, wiring errors, and other unexpected situations may occur. Some of these events may cause large overcurrents to flow into the power circuit of the motor drive. Figure 1 shows three typical short-circuit events.

Figure 1. Typical short-circuit event in an industrial motor drive.

Three typical short-circuit events in industrial motor drives:

1. Inverter shoot-through. This can be caused by incorrectly turning on both IGBTs in one of the inverter legs, which can be caused by electromagnetic interference or a controller failure. It can also be caused by wear/failure of one of the IGBTs in the leg, while the normal IGBT keeps switching.

2. Phase to phase short circuit. This can be caused by insulation breakdown between the motor windings due to degradation, excessive temperature, or an overvoltage event.

3. Phase short to ground. This can also be caused by insulation breakdown between the motor windings and the motor housing due to degradation, excessive temperature, or an overvoltage event.

Generally speaking, motors can draw very high currents for relatively long periods of time (milliseconds to seconds, depending on motor size and type); however, IGBTs – the main component of the inverter stage of industrial motor drives – have a short-circuit withstand time of microseconds.

The short-circuit withstand time of an IGBT is related to its transconductance or gain and the thermal capacity of the IGBT chip. Higher gain results in higher short-circuit currents in the IGBT, so it is obvious that IGBTs with lower gain have lower short-circuit levels. However, higher gain also results in lower on-state conduction losses, so a trade-off must be made.

Developments in IGBT technology are contributing to a trend towards increasing short-circuit current levels but decreasing short-circuit withstand times. In addition, advances in technology have led to the use of smaller chip sizes, which reduces module size but reduces thermal capacity, resulting in a further reduction in withstand time. Also, it has a lot to do with the IGBT collector-emitter voltage, so the parallel trend of industrial drives towards higher DC bus voltage levels has further reduced the short-circuit withstand time. In the past, this time ranged from 10 μs, but the trend in recent years has been towards 5 μs and as low as 1 μs under certain conditions. In addition, the short-circuit withstand time of different devices also varies greatly, so for IGBT protection circuits, it is usually recommended to build in additional margin beyond the rated short-circuit withstand time.

IGBT protection against overcurrent conditions is key to system reliability, both for property damage and safety reasons. IGBTs are not fail-safe components and their failure could cause the DC bus capacitors to explode and cause the entire drive to fail. Overcurrent protection is typically implemented through current measurement or desaturation detection.

Figure 2 shows these techniques. For current measurement, measurement devices such as shunt resistors are required on both the inverter arm and phase outputs to account for shoot-through faults and motor winding faults. Fast-acting trip circuits in the controller and/or gate driver must shut down the IGBT in time to prevent the short-circuit withstand time from being exceeded. The biggest benefit of this approach is that it requires two measurement devices on each inverter arm, along with all associated signal conditioning and isolation circuitry. This can be mitigated by simply adding shunt resistors on 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 that the response time of the signal conditioning is fast enough to protect the IGBT within the required short-circuit withstand time.

Figure 2. Example of IGBT overcurrent protection technology

Desaturation detection uses the IGBT itself as the current measurement element. The diode in the schematic ensures that the IGBT collector-emitter voltage is only monitored by the detection circuit during the on-time; in 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 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 above normal voltage levels can be used to indicate the presence of a short circuit, while the desaturation trip threshold level is typically in the 7 V to 9 V region. Importantly, desaturation can also indicate that the gate-emitter voltage is too low and the IGBT is not fully driven into the saturation region. Desaturation detection needs to be implemented carefully to prevent false triggering. This is particularly likely to occur during the transition from the IGBT off state to the IGBT on state when the IGBT has not yet fully entered saturation. A blanking time is usually included between the turn-on signal and the moment the desaturation detection is activated to avoid false detection. 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 trips caused by noise pickup. When selecting these filter components, there is a trade-off between noise immunity and reacting within the IGBT short-circuit withstand time.

After an IGBT overcurrent is detected, 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 through low driver impedance and gate drive resistance. If the same gate turn-off rate is applied for an overcurrent condition, the collector-emitter di/dt will be much larger because the current changes more in a shorter time. The parasitic inductance of the collector-emitter circuit due to wire bonds and PCB trace stray inductance can cause large overvoltage levels to reach the IGBT instantly (because VLSTRAY = LSTRAY × di/dt). Therefore, it is important to provide a higher impedance turn-off path when turning off the IGBT during a desaturation event to reduce di/dt and any potentially damaging overvoltage levels.

In addition to short circuits caused by system faults, transient inverter shoot-through can also occur under normal operating conditions. In this case, IGBT turn-on requires the IGBT to be driven into the saturation region, where conduction losses are minimized. This usually means that the gate-emitter voltage in the on-state is greater than 12 V. IGBT turn-off requires the IGBT to be driven into the cut-off region of operation in order to successfully block the reverse high voltage across it when the high-side IGBT is turned on. In principle, this can be achieved by reducing the IGBT gate-emitter voltage to 0 V. However, the side effects of turning on the low-side transistor on the inverter leg must be considered. The rapid change in the switch node voltage at turn-on causes a capacitive induction current to flow through the parasitic Miller gate-collector capacitance of the low-side IGBT (CGC in Figure 3). This current flows through the low-side gate driver (ZDRIVER in Figure 3) turn-off impedance, creating a transient voltage increase at the low-side IGBT gate emitter terminals as shown. 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 leg shoot-through—because both IGBTs are briefly turned on. This generally does not damage the IGBT, but it can increase power dissipation and affect reliability.

Figure 3. Miller induction inverter direct pass

Generally speaking, there are two ways to address the inductive turn-on problem of the inverter IGBTs—using bipolar supplies and/or additional Miller clamps. The ability to accept bipolar supplies on the isolated side of the gate driver provides additional margin for induced voltage transients. For example, a –7.5 V negative rail means that an induced voltage transient of more than 8.5 V is required to induce a spurious turn-on. This is sufficient to prevent a spurious turn-on. Another approach is to reduce the off impedance of the gate driver circuit for a period of time after the turn-off transition is completed. This is called a Miller clamp circuit. The capacitive current now flows through a lower impedance circuit, which then reduces the magnitude of the voltage transient. Using asymmetrical gate resistors for turn-on and turn-off provides additional flexibility in switching rate control. All of these gate driver features have a positive impact on the reliability and efficiency of the entire system.

The experimental setup uses a three-phase inverter powered by the AC mains through a half-wave rectifier. Although the system can be used with a DC bus voltage of up to 800 V, the DC bus voltage in this example is 320 V. In normal operation, a 0.5 HP induction motor is driven by an open-loop V/Hz control. The IGBTs are 1200 V, 30 AIRG7PH46UDPBF from International Rectifier. The controller is an ADSP-CM408F Cortex®-M4F mixed-signal processor from Analog Devices. Phase current measurements are made using isolated Σ-Δ AD7403 modulators, and isolated gate drive is achieved using the ADuM4135, a magnetically isolated gate driver product that integrates desaturation detection, Miller clamping, and other IGBT protection functions. Short circuit tests are performed by manually switching shorts between motor phases or between a motor phase and the negative DC bus. Short circuits to ground are not tested in this example.

Figure 4. Experimental setup

The controller and power board are shown in Figure 5. They are ADSP-CM408FEZ-kit® and EV-MCS-ISOINVEP-Z isolated inverter platform from Analog Devices.

Figure 5. ADI isolated inverter platform with full-featured IGBT gate driver

In the experimental hardware, IGBT overcurrent and short-circuit protection is implemented in a variety of ways. They are:

• DC bus current sensing (inverter shoot-through fault)

• Motor phase current detection (motor winding fault)

• Gate driver desaturation detection (all faults)

For the DC bus current sensing circuit, a small filter must be added to avoid false triggering because the DC bus current is discontinuous due to potentially high noise currents. An RC filter with a 3 μs time constant is used. After the overcurrent is detected, the remaining delay for the IGBT to turn off is through the operational amplifier, comparator, signal isolator, the trip response time in the ADSP-CM408F, and the gate driver propagation delay. This adds an additional 0.4 μs, making the total fault-to-turnoff time delay of 3.4 μs—well below the short-circuit time constant of many IGBTs.

Similar timing also applies to motor phase current sensing using the AD7403 and the integrated overload detection sinc filter on the ADSP-CM408F processor. A sinc filter with a time constant of around 3 μs works well. In this case, the only remaining system delays are due to the internal routing of the trip signal to the PWM unit and the gate driver propagation delay, since the overload sinc filter is internal to the processor. Together with the reaction time of the current sensing circuit or the fast digital filter, the ultra-short propagation delay of the ADuM4135 in both cases is important to achieve effective fast overcurrent protection, regardless of the method used.

Figure 6 shows the delay between the hardware trip signal, the PWM output signal, and the actual gate-emitter waveform of the upper IGBT of one of the inverter legs. It can be seen that the total delay after the IGBT starts to turn off is about 100 ns.

Figure 6. Overcurrent shutdown timing delay

Channel 1: Gate-emitter voltage 10 V/div;

Channel 2: PWM signal 5 V/div from the controller;

Channel 3: Low level active transition signal 5 V/div; 100 ns/div

Gate driver desaturation detection is much faster than the overcurrent detection method described above and is important in limiting the upper limit to which the short-circuit current is allowed to rise, thereby improving the overall stability of the system beyond what can be achieved even with fast overcurrent protection. This is shown in Figure 7. When a fault occurs, the current rises rapidly - in fact, the current is much higher than shown in the figure because it is measured with a bandwidth-limited 20 A current probe for reference only. The desaturation voltage reaches the 9 V trip level and the gate driver begins to shut down. It is clear that the entire duration of the short circuit is less than 400 ns. The long tail of the current represents the inductive energy caused by freewheeling in the anti-parallel diode of the lower IGBT. The initial increase in the desaturation voltage at turn-on is an example of a stray desaturation detection emf due to the collector-emitter voltage transient. It can be eliminated by increasing the desaturation filter time constant, thereby adding additional blanking time.

Figure 7. IGBT short circuit detection

Figure 8 shows the collector-emitter voltage across the IGBT. Due to the large impedance of the turn-off during desaturation protection, the initial controlled overshoot is about 80 V above the 320 VDC bus voltage. Current flows in the downstream anti-parallel diode, and circuit parasitics actually cause the voltage overshoot to be slightly higher, up to about 420 V.

Figure 8. IGBT short circuit shutdown

Figure 9 shows the value of the Miller clamp to prevent inverter shoot-through during normal operation.

Figure 9. Miller clamp at turn-on

Channel 1: Gate-emitter voltage 5 V/div;

Channel 2: PWM signal 5 V/div from the controller;

Channel 3: Collector-emitter voltage 100 V/div; 200 ns/div

As the short-circuit withstand time of IGBTs drops to the 1 μs level, it is becoming increasingly important to detect and shut down overcurrent and short circuits in a very short time. The reliability of industrial motor drives has a lot to do with the IGBT protection circuit. This article lists some methods to deal with this problem and provides experimental results, emphasizing the value of stable isolated gate driver ICs (such as the ADuM4135 single-channel gate driver).