Figure 1 shows a typical block diagram of a power converter in an AC motor control unit. It consists of an inverter that converts the DC bus voltage to alternating current at a variable frequency to drive the motor. IGBTs are the expensive power switches that form the heart of the inverter. These power devices must operate at a high frequency and be able to withstand high voltages.
Isolation amplifiers, such as the ACPL-C79A shown in Figure 1 (see inset), work in conjunction with shunt resistors to provide accurate current measurements in power converters, even in the presence of high switching noise. When used with a resistive divider, the isolation amplifiers function as precision current sensors to monitor the DC bus voltage. The current and voltage information from the isolation amplifiers is collected by the microcontroller, which uses the data to calculate the return values and output signals necessary to provide effective control and fault handling of the power converters.
Fault Protection Requirements:
In an inverter, IGBTs are the most expensive components, so it makes sense to provide the strongest possible protection against potential damage. Isolation amplifiers provide rapid detection of fault conditions, and the algorithms executed by the microcontroller can prevent these conditions from causing the IGBTs to fail. Furthermore, optical isolation in the amplifiers prevents fault conditions from overloading the microcontroller and causing it to fail.
However, IGBT protection must be cost-effective—the market continues to demand sufficient protection for IGBTs against fault conditions at a price that does not significantly impact the overall cost of the motor control system. To meet this demand, IGBT gate circuit drivers (such as the ACPL-332J [1]) and current sensors with protective functions for essential fault detection, in addition to their control and detection functions, have been introduced. These products provide a cost-effective scheme for implementing IGBT protection and eliminate the need for separate detection and feedback components. See References [1], [2] for details on the protection features integrated with gate circuit drivers and how to implement those protection functions. The remainder of this article will focus on some of the fault protection functions that can be implemented with current/voltage sensors, as listed in Table 1.
Overcurrent Detection:
Overcurrent conditions in an IGBT can occur due to a phase-to-phase short circuit, a line-to-ground short circuit, or short-circuit current. Isolation amplifier shunt current detection devices on the output phases and DC bus provide fault detection in addition to current measurement (see Figure 1). The short-circuit survival times of common IGBTs have been rated up to a limit of 10 ms [3, 4]. To ensure effective protection, this limit must not be exceeded. Within this limit, the fault must be detected, fed back to the controller, and the shutdown procedure must be completed within this timeframe. To meet this requirement, isolation amplifiers use different methods.
For example, the ACPL-C79A has a fast response time of 1.6 µs for a stepped input. This allows the amplifier to capture transients during short-circuit and overload conditions (see Figure 2) [5]. The midpoint propagation delay from the input signal to the output is only 2 µs, while it takes only 2.6 µs for the output signal to catch up with the input signal, reaching 90% of the final levels.
In addition to the fast response time, the ACPL-C79A provides a gain accuracy of ±1%, excellent nonlinearity of 0.05%, and a signal-to-noise ratio (SNR) of 60 dB. The ACPL-C79B, which offers higher gain accuracy of ±0.5%, and the ACPL-C790, which has a gain tolerance of ±3%, are also available. All devices in the ACPL-C79A family are certified to withstand a peak insulation voltage of 1230 V and can reject common-mode transient noise up to 15 kV/µs. These features are delivered in an SO-8 package that is 30% smaller than the standard DIP-8 package.
Another example is the HCPL-788J, which takes a different approach to achieving a fast response in overcurrent detection (Figure 3) [6, 7]. In addition to the output signal stem, it features a fault stem that rapidly switches from high to low to indicate an overcurrent condition. This isolation amplifier provides a measurement accuracy of ±3%.
Within fault feedback design, one issue that must be addressed is interference tripping. Interference tripping is a false activation of fault detection in the absence of any apparent fault condition that could damage the IGBTs. To prevent false alarms, the HCPL-788J features a pulse discriminator circuit to effectively inhibit the influence of di/dt and dv/dt faults. The advantage of this method is that the rejection is amplitude-independent, meaning the fault threshold can be set at a much lower level without increasing the risk of interference tripping.
In the implementation of the circuit for rapid fault detection, two comparators are used in the fault detection block to detect negative and positive fault thresholds. The switching threshold is equal to the 256 mV sigma-delta modulator reference. The outputs of these comparators are connected to suppression filters with a suppression period of 2 ms and are then sent to the encoder block.
To ensure that the fault state is transmitted across the isolation boundary as quickly as possible, two unique digital encoding sequences are used to represent the fault condition: one code for the negative level and another for the positive level. When a fault is detected, the normal data transfer through the optical channel is interrupted, and the bit stream is replaced with the fault code. These two fault codes deviate significantly from the normal encoding regime, so on the detector side, the decoder immediately recognizes the codes as fault conditions [7].
The time required by the decoder to detect and communicate the fault condition across the isolation boundary is approximately 1 ms. Adding the 400 ns delay to the antialiasing filter results in a propagation delay of 1.4 ms. The delay between the fault event and the fault output signal is the sum of the propagation delay and the suppression period (2 ms), resulting in an overall fault detection time of 3.4 ms (see Figure 4).
The fault output stem allows the fault signals from multiple devices to be linked together, enabling multiple parts to be wired (or wired) to create a single fault signal (see the upper right of Figure 5) [6]. This signal can then be used to directly disable the PWM inputs via the controller.
Overload detection
An overload condition refers to a situation where the motor current exceeds the unit's rated current, but not so much as to put the inverter or motor in immediate danger of failure, for example, if the motor is mechanically overloaded or a motor stall situation as a result of a bearing failure.
Inverters are typically specified with an overload capacity in addition to their nominal rating. The duration of this allowable overload capacity depends on the time interval before overheating becomes a problem. A typical overload capacity is 150% of the nominal load for up to one minute.
The ACPL-C79A accepts a full input range of ±300 mV, and the datasheet specifications are based on a nominal input range of ±200 mV. A designer has the flexibility to choose the overload threshold at either of these values, or somewhere in between. If the accuracy of the overload current measurement with the threshold near 300 mV is less critical compared to that of the normal operating current, which is often the case, this is a good choice, as it allows full use of the isolation amplifier's input dynamic range. However, setting the threshold at 200 mV ensures the accuracy of the overload current measurement. Once the voltage levels have been determined, the designer must choose the appropriate value of the sensing resistor according to the corresponding current level.
The HCPL-788J includes an additional feature, the ABSVAL output, which can be used to simplify the overload detection circuit. The ABSVAL circuit rectifies the output signal, providing an output signal proportional to the absolute level of the input signal according to the formula:
ABSVAL = |ENT V| *REF V,EXT / 252mV
This output can also be wired (wire OR-able). When three sinusoidal phases from the motor are combined, the rectified output (ABSVAL) is essentially a DC signal representing the motor's RMS current. This DC signal and a threshold comparator can indicate motor overload conditions before damage occurs to the motor or controller (see lower right of Figure 5).
Overvoltage Detection:
The DC bus voltage must also be kept under constant control. Under certain operating conditions, a motor can act as a generator, returning a high voltage to the DC bus through the inverter's power supply and/or recovery diodes. This high voltage adds to the DC voltage and forms a very high spike applied to the IGBTs. This spike can exceed the IGBT's maximum collector-emitter voltage and cause damage.
The miniature isolation amplifier (ACPL-C79A) is often used as a voltage sensor in DC bus control applications (Figure 6). A designer must reduce the DC bus voltage to match the input range of the isolation amplifier by choosing the values of R1 and R2 according to the appropriate ratio.
In conclusion,
Avago Technologies' isolation amplifiers provide an effective method for protecting IGBTs against overcurrent, overload, and overvoltage conditions, in addition to their current/voltage sensing function. Using these devices, along with multi-function gate drivers such as the ACPL-332J, a cost-effective yet comprehensive IGBT protection scheme can be easily implemented.
Author:
By: Hong Lei Chen, Product Manager, Avago Technologies
References
1. ACPL-332J Data Sheet, Avago Technologies, AV02-0120EN.
2. Hong Lei Chen and Chun Keong Tee, “Using gate drive Optocouplers in IGBT protections,” EETimes Europe, December 2010, pp. 28-31.
3. Terje Rogne, “Short-Circuit Capability of IGBT (COMFET) Transistors,” IEEE, 1988.
4. J. Li, R. Herzer, R. Annacker, B. Koenig, “Modern IGBT/FWD Chip Sets For 1200V Applications,” Semikron Elektronik GmbH, 2007.
5. ACPL-C79B, ACPL-C79A, ACPL-C790 Precision Miniature Isolation Amplifiers Data Sheet. Avago Technologies, AV02-2460EN.
6. HCPL-788J Data Sheet, Avago Technologies, AV02-1546EN.
7. Patrick Sullivan, Denis Kobasevic, “Smart Current Sensor For Motor Drive Control.” PCIM Europe, 1998, Vol.10 no.4, pp. 182-186.
