IGBTs are used to switch power supplies in many products, including variable frequency drives (VFDs), servo drives, electric vehicles, buses and trucks, trains, medical equipment (X-ray and MRI), air conditioners, and even some professional audio systems. These products fall under the category of "high-power" applications and are easily mistaken for electrical rather than electronic components, leading to the assumption that they are not easily damaged. However, many different failure mechanisms can cause IGBT failure unless careful design is implemented to ensure proper device operation.
Like all devices, IGBTs can fail due to environmental factors (temperature, thermal shock, thermal and power cycling, and vibration). ESD (electrostatic discharge) is also a failure factor. Since IGBTs and gate drivers are often installed by cabinet installers rather than electronics professionals, it is not surprising that a large number of failures are caused by improper human handling. To prevent such failures, it is crucial to strictly follow installation guidelines and ensure the device operates under specified conditions.
Overcurrent is another potential cause of failure. Integrated solutions are available for this problem, but there are also simpler, lower-cost solutions that use a current sensor at the AC output for measurement, and most customers prefer the lower-cost option.
Other major failure mechanisms include short circuits, excessively high di/dt, excessively high dv/dt, and gate-emitter and collector-emitter overvoltages. The industry needs protection against these failure mechanisms, especially at power levels up to 100 kW and when system costs are high. Therefore, IGBT driver manufacturers such as Power Integrations have integrated innovative and reliable protection mechanisms into their products to address these issues and provide robust protection for IGBT modules.
Short circuit
Figure 1 illustrates the IGBT's performance under two different short-circuit conditions – small inductance (Case 1) and large inductance (Case 2). A common method for detecting short circuits and turning off the IGBT before it fails is to use an optocoupler IC with integrated desaturation protection. Unfortunately, this method has two disadvantages. First, the optocoupler IC with desaturation protection also requires a high-voltage diode, which is not only expensive but also has high losses. Second, and perhaps more importantly, the required desaturation monitoring electronics are often sensitive to EMI or Vce voltage spikes. This can cause the short-circuit protection to malfunction, leading to unexpected IGBT turn-off.
Figure 1
Power Integrations' IGBT drivers take a different approach. They use an ASIC chipset to reduce component count and size while improving performance, efficiency, and scalability. This chipset also features advanced monitoring and control capabilities. To address short-circuit measurement issues, the SCALE™-2 chipset and resistor strings are used to dynamically measure the IGBT's VCE (see Figure 2). This not only means that small voltage spikes will not cause false short-circuit protection triggering, but also offers other advantages. The resistor string method is less expensive than standard diode measurement methods and has no coupling capacitors, thus eliminating redundant capacitors that affect efficiency and being unaffected by dv/dt. A further advantage is that the sensitivity of the short-circuit protection can be easily adjusted using a single reference pin to suit specific applications.
Figure 2
Advanced/Dynamic Advanced Active Clamp
The SCALE™-2 chipset is also used to implement sophisticated active clamping techniques to address other IGBT failure modes mentioned earlier—excessive di/dt, excessive dv/dt, and gate-emitter and collector-emitter overvoltages.
Figure 3: Dynamic advanced active clamping with dv/dt feedback
Basic active clamping (box AC in Figure 3) limits the IGBT's VCE during turn-off. The IGBT partially turns on immediately when its VCE exceeds a preset threshold and then maintains operation within the linear region, thus reducing the rate of decrease in collector current and limiting collector-emitter overvoltage. In SCALE™-2 technology, advanced active clamping (AAC) feedback (boxes AC and AAC in Figure 3) is implemented by the secondary ASIC of the driver. As soon as the potential to the right of resistor R2 rises due to active clamping action, the turn-off MOSFET of the driver's drive stage connected to GL is gradually turned off. This reduces the charge flowing from the IGBT gate into COM, which flows through the turn-off gate resistor Rg,off. This not only reduces collector-emitter overvoltage during IGBT turn-off but also reduces TVS losses, thereby improving efficiency.
The SCALE™-2 driver also incorporates dv/dt feedback (dv/dt feedback in Figure 3). Its function is to provide highly effective turn-off overvoltage limiting during normal switching operation without causing TVS thermal overload. As the collector-emitter voltage rises, the resulting current flows into the dv/dt feedback capacitor connected in parallel with the TVS. This current further supports advanced active clamping, as it flows into the same driver terminal but precedes the TVS action of the advanced active clamp. By employing this additional driving method, VCE voltage clamping becomes more effective, and TVS losses are reduced. If set correctly, the IGBT can operate continuously in this operating mode. Therefore, IGBT modules can be switched with a larger DC bus stray inductance without exceeding the IGBT's reverse bias safe operating area (RBSOA). Furthermore, a snubber capacitor is not required.
Power Integrations has taken clamping technology to a new level: Dynamic Advanced Active Clamping (DA2C) adds an additional TVS diode (boxed DA2C in Figure 3) in series with the TVS used in advanced active clamping. For approximately 15-20µs from the start of IGBT turn-on until the IGBT issues a turn-off command, the auxiliary IGBT Q0 remains on, shorting this additional TVS to lower the active clamping threshold and ensure efficient active clamping (the additional TVS does not operate during IGBT turn-off). After this 15-20µs delay, the auxiliary IGBT Q0 turns off, the added TVS is activated, and the driver's active clamping threshold is raised, allowing the DC bus voltage to rise to a higher value during IGBT turn-off. This means that the converter system's output inductor will be demagnetized after an emergency shutdown, but there is no need to worry about the effects of an unavoidable short-term rise in DC bus voltage.
soft shutdown
Both AAC and DA2C are suitable for applications with high commutation stray inductance, which require control of di/dt during IGBT turn-off to ensure the IGBT operates within the reverse bias safe operating area (RBSOA). However, for some applications, such as those with low commutation stray inductance and where the IGBT turn-off overshoot is within the RBSOA, a simpler option is soft turn-off (SSD) because it has the advantage of not requiring active clamping by a TVS diode. SSD activates upon detecting a short circuit. It protects the semiconductor from damage by limiting the short-circuit duration and current slope, ensuring that the instantaneous VCE is always below VCES (the semiconductor's blocking voltage capability). Figure 4 illustrates the operation of the SSD function.
Figure 4
VCE desaturation is visible during time P1 (green line). Due to rail-to-rail output technology, VGE (gate-emitter voltage, pink line) remains very stable. After P1 (approximately 5 µs), VGE is limited to a lower value within a specific time (tFSSD). During tFSSD, IC (short-circuit current) is limited, initially resulting in a small VCE overvoltage. During P3, the semiconductor gate discharges further. As the discharge process nears completion, the gate connects to COM, VGE rapidly drops to the turn-off negative voltage, the remaining gate charge is removed, the short-circuit current is cut off, and then a small VCE overvoltage reappears. The entire short-circuit detection and safe turn-off time is less than 10 µs. Power Integrations achieves this with the new SCALE™-2+ technology, which enables AAC/DA2C or SSD functionality.
integrated
Power Integrations' SCALE™-2 chipset is central to all the IGBT protection mechanisms described in this article. This chipset enables the simultaneous driving and control of IGBT modules, as well as the monitoring of their performance. It eliminates the need for numerous passive and active components required in other solutions to achieve these functions. Using this chipset, drivers can offer more functionality, improved reliability, and reduced size.
Author: Michael Hornkamp, Senior Director of Regional Marketing and Systems Engineering for High Power Products, Power Integrations
