Keeping inverters safe

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 a typical inverter circuit, IGBTs drive a load (in this case, a 30 motor)and are driven by optically isolated gate drivers. Current sensors and bus voltage sensors typically are also galvanically isolated.

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.

Graphs of IGBT operating regions and saturation voltage show areas to be avoided. IGBTs should be kept out of their linear region for example, and their V _CE(SAT) rises sharply when their gate drive is too low.

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 V_CE(SAT) voltage. At the gate voltage below 12 V, the V_CE(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.

A detailed block diagram of an iso-amplifier shows the coding applied to the input voltage. The coding plays a role in falut detection: A fault causes an interruption in the data transfer across the optical channel. The bit stream is replaced with a coding sequence denoting a fault. The resulting signals and their timing are evident in a scope snapshot (right).

Fault detection circuits in the optocoupler monitor the V_CE(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.