“The overall market trend for industrial motor drives is the ever-increasing demand for higher efficiency as well as reliability and stability, and power semiconductor device manufacturers are constantly looking for breakthroughs in conduction losses and switching times. Some of the trade-offs for increasing conduction losses in insulated gate bipolar transistors (IGBTs) are: higher short-circuit current levels, smaller die size, and lower thermal capacity and short-circuit withstand time. This highlights the importance of gate driver circuitry and overcurrent detection and protection features.
The overall market trend for industrial motor drives is the ever-increasing demand for higher efficiency as well as reliability and stability, and power semiconductor device manufacturers are constantly looking for breakthroughs in conduction losses and switching times. Some of the trade-offs for increasing conduction losses in insulated gate bipolar transistors (IGBTs) are: higher short-circuit current levels, smaller die size, and lower thermal capacity and short-circuit withstand time. This highlights the importance of gate driver circuitry and overcurrent detection and protection features.
Today we discuss the successful and reliable implementation of short-circuit protection in modern industrial motor drives, while providing experimental examples of isolated gate drivers in three-phase motor control applications.
Short circuits in industrial environments
Industrial motor drives operate in relatively harsh environments, where high temperatures, AC line transients, mechanical overloads, wiring errors, and other emergencies can occur. Some of these events can cause large overcurrents to flow into the motor drive’s power circuits. 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 pass-through. This can be caused by incorrectly turning on the two IGBTs of one of the inverter legs, which in turn can be caused by EMI or a controller failure. It can also be caused by a wear/failure of one of the IGBTs on the arm, while the normal IGBT keeps switching action.
2. Phase-to-phase short-circuit. This can be caused by insulation breakdown between motor windings due to degraded performance, excessive temperature, or an overvoltage event.
3. The phase line is shorted to ground. This can also be caused by a breakdown in insulation between the motor windings and the motor case due to degraded performance, excessive temperature or an overvoltage event.
In general, motors can draw extremely high currents for relatively long periods of time (milliseconds to seconds, depending on motor size and type); however, IGBTs—the main part of an industrial motor drive inverter stage—short-circuit The tolerance time is in the order of microseconds.
IGBT short circuit withstand capability
The IGBT short-circuit withstand time is related to its transconductance or gain and the thermal capacity of the IGBT chip. Higher gain results in higher short circuit currents within the IGBT, so obviously lower gain IGBTs have lower short circuit levels. However, higher gain also results in lower on-state conduction losses, so a trade-off must be made.
Advances in IGBT technology are contributing to the trend of 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, reducing module size, but reducing thermal capacity, resulting in further reductions in withstand time. In addition, there is also a strong relationship with the IGBT collector-emitter voltage, so the parallel trend of industrial drives towards higher DC bus voltage levels further reduces the short-circuit withstand time. In the past, this time frame was 10 μs, but in recent years the trend is towards 5 μs and under certain conditions as low as 1 μs. In addition, the short-circuit withstand time of different devices is also quite different, so for IGBT protection circuits, it is usually recommended to build in an additional margin more than the rated short-circuit withstand time.
IGBT overcurrent protection
Whether for property damage or safety reasons, IGBT protection against overcurrent conditions is the key to system reliability. IGBTs are not a fail-safe component and their failure could cause the DC bus capacitors to explode and cause the entire drive to fail. Overcurrent protection is generally implemented by current measurement or desaturation detection.
Figure 2 shows these techniques. For current measurement, measurement devices such as shunt resistors are required for both the inverter arm and the phase output to cope with shoot-through faults and motor winding faults. Fast-acting trip circuits in the controller and/or gate driver must turn off 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, 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 utilizes the IGBT itself as the current measuring element. The diodes in the schematic ensure that the IGBT collector-emitter voltage is only monitored by the detection circuit during turn-on; 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 a level that drives the IGBT out of the saturation region and into the linear operating region.
This results in a rapid rise in collector-emitter voltage. The normal voltage levels above 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. Care should be taken when deploying desaturation detection 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 yet fully entered saturation. The blanking time is usually between the turn-on signal and the activation of the desaturation detection 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 spurious filter jumps caused by noise pickup. When selecting these filter components, there is a trade-off between noise immunity and IGBT short-circuit withstand time response.
Once an IGBT overcurrent is detected, a further challenge is to turn off the IGBT that 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 by lower 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 collector-emitter circuit parasitic inductance due to wire bond and PCB trace stray inductance can cause large overvoltage levels to instantaneously reach the IGBT (since 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, which reduces di/dt and any potentially damaging overvoltage levels.
In addition to short circuits caused by system faults, transient inverter shoot-through also occurs under normal operating conditions. At this point, IGBT turn-on requires the IGBT to be driven into the saturation region where conduction losses are lowest. 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 operating cut-off region in order to successfully block the reverse high voltage across the high-side IGBT when it 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 transistors on the inverter arm must be considered. The rapid change in switch node voltage during turn-on causes a capacitive induced current to flow through the low-side IGBT parasitic Miller gate-collector capacitance (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 as shown. If this 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 shoot-through – as both IGBTs are briefly turned on. This generally does not destroy the IGBT, but it can increase power dissipation and affect reliability.
Figure 3. Miller Induction Inverter Thru
Generally speaking, there are two ways to solve the problem of inductive turn-on of inverter IGBTs – using bipolar power supplies and/or additional Miller clamps. The ability to accept bipolar power supplies on the isolated side of the gate driver provides additional headroom for induced voltage transients. For example, the C7.5 V negative rail indicates that an induced voltage transient greater than 8.5 V is required to induce stray conduction. This is enough to prevent stray conduction. Another approach 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 a Miller clamp circuit. The capacitive current now flows through the lower impedance circuit, subsequently reducing 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 overall system reliability and efficiency.
The experimental setup uses a three-phase inverter powered by AC mains through a half-wave rectifier. Although the system can use a DC bus voltage of up to 800 V, the DC bus voltage in this example is 320 V. In normal operation, the 0.5 HP induction motor is driven by open loop V/Hz control. The IGBT adopts 1200 V, 30 AIRG7PH46UDPBF provided by International Rectifier. The controller uses ADI’s ADSP-CM408F Cortex®-M4F mixed-signal processor. Phase current measurement using isolated sigma-delta AD7403 modulator and isolated gate drive using ADuM4135 (a magnetically isolated gate driver product with integrated desaturation detection, Miller clamp and other IGBT protection features) . Perform a short circuit test by manually switching shorts between motor phases, or between motor phases and the negative DC bus. Shorts to ground are not tested in this example.
Figure 4. Experimental setup
The controller and power board are shown in Figure 5. They are Analog Devices’ ADSP-CM408FEZ-kit® and EV-MCS-ISOINVEP-Z isolated inverter platforms.
Figure 5. ADI isolated inverter platform with full-featured IGBT gate driver
In the experimental hardware, IGBT over-current and short-circuit protection are realized by various methods. they are, respectively:
• DC bus current detection (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 intermittent due to potentially high noise currents. Use an RC filter with a 3 μs time constant. After overcurrent is detected, the remaining delays for IGBT turn-off are through op amps, comparators, signal isolators, transition response times in the ADSP-CM408F, and gate driver propagation delays. This adds an additional 0.4 μs, resulting in a total fault-to-turn-off time delay of 3.4 μs—much lower than the short-circuit time constant of many IGBTs.
Similar timing applies to motor phase current detection 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 reason for the rest of the system delay would simply be the internal routing of the transition signal to the PWM unit and the presence of gate driver propagation delays, since the overloaded sinc filter is an internal component of the processor. Together with the response time of the current sense circuit or fast digital filter, the ultra-short propagation delay of the ADuM4135 in both cases is important for effective fast overcurrent protection, regardless of the method used.
Figure 6 shows the delay between the hardware transition signal, the PWM output signal, and the actual gate-emitter waveform of the upper IGBT of one of the inverter arms. As can be seen in the figure, 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: Active low transition signal 5 V/div; 100 ns/div
The gate driver desaturation detection performs much faster than the overcurrent detection method described above and is important to limit the upper limit of the allowable rise of the short circuit current, thereby improving the overall stability of the system beyond what is achievable , even if the system has 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 graph, which 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 starts to turn off. Apparently, the entire duration of the short circuit is less than 400 ns. The long tail of the current represents the induced energy due to freewheeling in the anti-parallel diode of the IGBT below. The initial increase in the desaturation voltage at turn-on is an example of a stray desaturation detection EMF due to collector-emitter voltage transients. This can be eliminated by adding additional blanking time by increasing the desaturation filter time constant.
Figure 7. IGBT Short Circuit Detection
Figure 8 shows the collector-emitter voltage on the IGBT. The initial controlled overshoot is about 80 V above the 320 VDC bus voltage due to the large impedance at turn-off during desaturation protection. Current flows in the downstream anti-parallel diodes, and circuit parasitics actually cause the voltage overshoot to be slightly high, up to about 420 V.
Figure 8. IGBT Short-Circuit Shutdown
Figure 9 shows the value of the Miller clamp preventing inverter shoot-through during normal operation.
Figure 9. Miller Clamp When 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 more and more important to detect and turn off overcurrent and short circuits in a very short time. The reliability of industrial motor drives has a lot to do with IGBT protection circuits. This article outlines some ways to deal with this problem and provides experimental results that emphasize the value of stable isolated gate driver ICs such as the ADuM4135 for single-channel gate drivers.
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