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Keeping inverters safe

Keeping inverters safe

The circuits that connect solar arrays and other renewable resources to the grid are subject to overloads and unstable conditions that can damage power electronics. Optocoupling can help isolate problem areas.

The converters normally employed with solar and wind power generation systems must work reliably under a wide variety of operating conditions. It is customary to protect the IGBTs 9 insulated gate bipolar transistors) and other power devices used in these applications so they can continue to function despite somewhat unpredictable conditions that characterize renewable energy scenarios. In particular, designers try to build in protection to avoid damage resulting from conditions such as under voltage, desaturation of the power device, miller effect, overloads and short circuits.

Gate drive optocouplers are widely used in motor drive applications because they provide high-output current and fast switching speed. They increasingly show up in renewable energy converters as well because they offer IGBT-protection features, including under-voltage lock-out (UVLO), desaturation (DESAT) detection, and an active miller clamp. Besides using gate drivers, designers employ other methods of IGBT protection that include current sensors on the output phases and on dc buses. To detect over-currents and overloads, they may use isolation amplifier (iso-amp) current sensors featuring fast fault detection in conjunction with current measurement.

A typical block diagram of a power conversion stage might include an inverter which converts the dc bus voltage to ac power, either to drive a load such as a motor or to be connected to the grid in a renewable energy system. Costly power devices such as IGBTs and Power MOSFETs are the heart of the inverter, which operates at high frequency and must withstand a dc bus load normally measuring in the hundreds of volts.

Optocouplers are commonly used in such systems to galvanically isolate the control circuits and help protect against damage caused by high voltages in the dc bus. Another primary purpose of these devices is to provide a high degree of common-mode rejection (CMR) and help prevent the fast switching of the pulse-width modulation (PWM) signals from erroneously driving the IGBT. (As a quick review, CMR is defined as the fastest switching rate of common mode noise voltage the optocoupler can withstand while keeping output in the correct state.)

In particular, gate drive optocouplers are widely used to drive IGBTs because they can provide high output current for precise switching while at the same time giving a conversion efficiency. Precise switching is also necessary to keep the high and low sides of an IGBT from turning on simultaneously, thus causing a short circuit.

In motor drive systems, iso-amplifiers, working in conjunction with shunt resistors, accurately measure the current through power converters even in the presence of high switching noise. These devices incorporate short circuit and overload detection features that respond quickly to faults. When used with a resistive divider, they work as precision voltage sensors. The controller uses this current and voltage information to regulate the converter and manage faults.

The situation is similar in industrial ac and servodrives. IGBT gate drivers and current sensors now contain protection features that allow let them detect faults during their driving and sensing functions.

One of the protective features gate drive optocouplers may incorporate is an Under Voltage Lock-out feature. This comes into play when the circuit is powering up, forcing the IGBT to have a low output when its gate voltage is lower than normal operating conditions. IGBTs typically need a gate voltage of at least 15 V to hit their rated VCE(SAT) voltage. At the gate voltage below 12 V, the VCE(SAT) voltage rises dramatically, and when the device conducts higher currents, the condition leads to thermal overstress. At low gate voltage (below 10 V), the IGBT may operate in the linear region and quickly overheat.

Fault detection circuits in the optocoupler monitor the VCE(SAT) voltage of the IGBT and trigger a local fault shutdown sequence if the collector voltage goes into desaturation; that is, a voltage buildup across the collector and emitter while the device is fully on. Desaturation can be caused by phase or rail supply short circuits due to faulty wiring; control signal failures caused by computational errors; overloads induced by the load; or failures in the gate drive circuitry. During desaturation, the IGBT current and power dissipation rise drastically and the device overheats, possibly to failure. To prevent such damage, desaturation fault detection turns off the IGBT in a controlled manner.

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A typical application of an integrated gate drive optocoupler in an inverter system illustrates how these circuits check for desaturation. The IGBT collector-emitter voltage, VCE(SAT), is monitored by the DESAT pin of the coupler. In the event of a short circuit, high current flows through the IGBT, putting it into desaturation mode, and VCE rises. Once VCE goes above the internal desaturation fault detection threshold voltage of 7 V, the gate drive optocoupler decides a fault has occurred. The coupler VOUT will be slowly brought down to turnoff the IGBT. A controlled soft shut down is needed to prevent large di/dt-induced voltage spikes. At the same time, the internal feedback channel will bring a FAULT output low to notify the microcontroller of the fault condition. At this point, the microcontroller takes the appropriate action to shut down or reset the system for a failsafe system recovery.

An immediate shut down during a fault condition will induce a high-voltage across the IGBT because of parasitic inductance in the load residing in the collector circuit. The resulting overshoot might exceed the IGBT breakdown voltage. To prevent this during a fault, a weak pull-down device in the output drive stage will switch on to turn off the IGBT and prevent large di/dt induced voltages. The IGBT gate is discharged slowly to prevent fast changes in collector current and the consequent voltage spike.

This is a two-stage protection process. During the slow turn off, the main output pull-down device of the gate driver remains off until its output voltage falls below VEE + 2 V. Then, the large pull-down device (in the case of a ACPL-332J, a 50X DMOS transistor) clamps the IGBT gate to VEE.

One of the common problems when operating an IGBT is the parasitic turn-on due to a Miller capacitor, the parasitic capacitance between gate and drain. One can visualize the problem with two IGBTs arranged in a half-bridge configuration. During switching, the upper IGBT, S1, is turning on while S2 is turning off. There is a voltage change dVCE/dt across the lower IGBT, S2. This voltage change causes a current through the parasitic Miller capacitor CCG of S2. This current flows through a gate resistor RG and internal resistor, RDRIVER and causes a voltage drop across the gate and emitter of IGBT, S2. The IGBT will turn on and conduct current if this voltage exceeds its gate threshold voltage. It should be noted that rising IGBT chip temperature would lead to a slight reduction of gate threshold voltage.

To prevent the unwanted IGBT turn-on, a low resistance path from the gate to emitter can be used to shunt the parasitic Miller current, often realized by adding a transistor between the gate and emitter. This transistor shorts the gate-emitter region after gate voltage reaches a threshold. Called an Active Miller Clamp, it can be found in the ACPL-332J gate drive optocoupler. During turn-off, the gate voltage is monitored and clamps the output when the gate voltage drops below 2 V (relative to VEE). In smaller drives, Miller clamp can also be used to replace the negative power supply of the gate driver to quickly discharge the gate of the IGBT and turn it off quickly.

Detecting over currents

Inverter circuits can experience over-current conditions that include a phase-to-phase short circuit. In motor drives, for example, such a short can arise from a short between motor leads or a breakdown in motor wiring insulation. An insulation breakdown that grounds the motor wiring can also cause a ground fault, another source of over current. High current can additionally stem from a shoot-through condition where IGBTs on both sides of the inverter are on simultaneously, likely caused by false IGBT turn-on.

Over-current conditions can be detected at several points in the inverter circuit, including at the IGBT emitters, at the three output phases, and at the HV+ and HV- dc buses. Optimally, there should be over-current detection at each IGBT. However, this approach may be too expensive in some cases. Designers can sometimes reduce the number of over-current detection points through use of current-sense circuits on the phases and dc buses that double as fault detectors.

One of the main requisites of the fault detection circuit is speed. Typically, an IGBT's short-circuit survival time is rated up to 10 µsec.

On the other hand, what is also important is the absence of so-called nuisance tripping. Nuisance tripping is a false triggering of the fault detection when there isn't really a fault. Common causes of false triggering include low-energy di/dt and dv/dt glitches. To avoid their effects, a pulse discriminator circuit can effectively blank them out. The advantage of this method is that the rejection is independent of amplitude, which means the fault threshold can be set to a much lower level without increasing the risk of nuisance tripping.

This is the approach taken in the integrated fault detection circuit in isolation amps such as the HCPL-788J/ACPL-785J. This iso-amp has ±3% gain accuracy while the ACPL-785J device has a gain tolerance of ±5%. The two share the same IC and package platform. Two comparators are used to detect the negative and positive fault thresholds, and the switching threshold is equal to the sigma-delta modulator reference, in this case 256 mV. The outputs of these comparators are connected to blanking filters with blanking periods of 2 µsec.

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To ensure the fault status travels across the isolation boundary quickly, two special digital coding sequences are used to represent the fault condition: one code for the negative level, the other for the positive. The detection of a fault interrupts the nominal data transfer across the optical channel and the bit stream gets replaced with the fault coding sequence. These two fault codes deviate significantly from the normal coding scheme, so the decoder immediately notes any fault condition.

The decoder detects and communicates the fault condition across the isolation boundary in about 1 µsec. An antialiasing filter adds about 400 nsec of delay for an overall propagation delay of 1.4 µsec. The delay between the fault event and the output fault signal is the sum of the propagation delay and the blanking period (2 µsec), giving a typical overall fault detection time of 3.4 µsec.

It is customary for these circuits to provide a way to let fault signals from several devices connect together to create a single fault signal. This signal may then be used to directly disable the PWM inputs through the controller.

There can also be overload conditions in which the load current exceeds the rated current of the drive, but not enough to put the inverter or load in immediate danger of failure. For example, such conditions can arise if the inverter drives a motor that is mechanically overloaded or a bearing failure causes a motor stall.

It may not be feasible to suddenly shut down the motor in such cases. For this reason, inverters are usually specified with an overload rating in addition to the nominal rating. The time period of the allowable overload rating depends on the time that can elapse before overheating becomes an issue. A typical overload rating is 150% nominal load for a period up to one minute.

The HCPL-788J/ACPL-785J iso-amps include an additional feature, called the ABSVAL output, which can be used to help simplify the circuit requirements and detect such an overload condition. The ABSVAL circuit generates an output signal proportional to the absolute level of the input signal according to the formula: ABSVAL = |VIN | × VREF,EXT / 252 mV

This output can also be wired together to form an OR logic function. When three sinusoidal motor phases are combined, ABSVAL is essentially a dc signal representing the RMS motor current. This single dc signal and a threshold comparator can indicate motor overloads before they cause damage.

The dc bus voltage also needs to be kept under continuous control. Under certain operating conditions, a motor can act as a generator, delivering high voltage back into the dc bus through the inverter's power device and/or the recovery diodes. This high voltage is added to the dc bus voltage and forms a high surge voltage on the inverter power devices. The surge voltage may exceed the maximum blocking voltage (VCES or VRRM) of the power devices and can damage them and other components in the inverter. VCES is the maximum collector-emitter voltage of an IGBT with a short circuited gate. VRRM is the repetitive maximum reverse voltage of a diode.

Regenerative current flow is undesirable. The inverter must detect when it leads to a dc bus over-voltage. A miniature iso-amp, such as the ACPL-C78A device, is often used to sense dc bus voltage. Designers typically use a voltage divider to scale the dc bus voltage to fit the small input range of the iso-amp.

References

“ACPL-332J 2.5 Amp Output Current IGBT Gate Driver Optocoupler with Integrated (VCE) Desaturation Detection, UVLO Fault Status Feedback and Active Miller Clamping, Data Sheet,” Avago Technologies, AV02-0120EN.

“Desaturation Fault Detection, Application Note 5324,” Avago Technologies, AV02-0258EN.

“Soft Turn-Off Feature, Application Note 5315,” Avago Technologies, AV02-0073EN.

“Active Miller Clamp, Application Note 5314,” Avago Technologies, AV02-0072EN.

Terje Rogne, “Short-Circuit Capability of IGBT (COMFET) Transistors,” IEEE, 1988.

J. Li, R. Herzer, R. Annacker, B. Koenig, “Modern IGBT/FWD Chip Sets For 1200V Applications,” Semikron Elektronik GmbH, 2007.

HCPL-788J Isolation Amplifier with Short Circuit and Overload Detection, Data Sheet, Avago Technologies, AV02-1546EN.

ACPL-785J Isolation Amplifier with Short Circuit and Overload Detection, Data Sheet, Avago Technologies, AV02-1545EN.

ACPL-C78A, ACPL-C780, ACPL-C784 Miniature Isolation Amplifiers, Data Sheet, Avago Technologies, AV02-1436EN.

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