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Posted on 29 June 2019

IGBTs in Reverse Gear

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The complex interplay of freewheeling diodes and IGBTs

State-of-the-art power semiconductors are intended to switch quickly in order to reduce dynamic losses. Once an IGBT turns off, the load current commutates to the freewheeling diode. To avoid undesirable side-effects , these two semiconductors have be very well matched in terms of their electrical properties.

By Stefan Schuler, Development Engineer and André Müller, BA student, SEMIKRON

 

Electronic circuits are usually designed such that no IGBT has to be operated in reverse direction. The problem is, however, that situations occur in applications in which this is exactly the case, at least for a transitional period. Figure 1 shows two half bridges connected via a load inductance LL. The switches T3 and T2 are conducting; a current iL flows through the load inductance. Let us now take a closer look at the point in time at which T2 turns off: current iL begins to commute from T2 to the freewheeling diode D1. This produces a voltage overshoot induced by the diode turn-on behaviour. Depending on the current rate of rise di/dt, this voltage may briefly amount to as much as several hundred Volt; this is also referred to as the diode "forward recovery time“. At the same time, the emitter potential of T1 is higher than that of the collector – the IGBT is now in actual fact poled in reverse direction.

Two half bridges connected via a load industance LL

The question which arises in this context is whether the voltage amplitude and forward recovery duration may be dangerous for the antiparallel IGBT.

Forward recovery time

A basic high-voltage diode comprises a p+-n--n+ layered structure, where the middle zone is far bigger and often weakly doped only. In Punch-Through (PT) diodes, the width of the middle zone, as well as the doping of this region, is selected such that he field strength is only partially reduced until the n+ region is reached – here it drops continously down to zero, but it must never go beyond the edge. In order to switch these diodes from blocking to conducting state, the electron-hole plasma first has to be formed in the middle zone. To do so, electrons and holes are injected from the adjacent n+- and p+zones. This process is, however, somewhat sluggish, meaning a high forward recovery voltage Ufr is produced perpendicular to the middle zone when a sudden current is applied to the diode. This is typical of inductive loads. The amplitude of this voltage depends on the voltage class of the diode, as well as the diode technology. For higher voltage classes the middle zone is larger and thus slows down the process of plasma formation significantly. Figure 2 shows the voltage curve for various di/dt of a 1200V diode. As the current rate of rise increases, the time and amplitude of the maximum move towards earlier and higher values. A rate of rise measured in a harsh lab test and barely achievable in practice amounting to 32kA/μs even causes the forward voltage to exceed the 200V mark briefly. It goes without saying that this high value is a theoretical value more than anything else and in practice the maximum values are in the region of 8…10kA/μs.

Recovery times of a 75A/1200V power diode for various di/dt and Imax of 480A.

Shift into reverse gear

For negative collector-emitter voltage, the pn junction between the p+substrate and the n buffer blocks, thus preventing current flow. Normally, however, IGBTs are not optimised for reverse operation [1], which is why the blocking capability of this pn junction is poor and breakdown occurs at some 10V already. In this case, the IGBT is filled with electron-hole plasma due to the emitter-side pn junction and goes into an uncontrolled state [2]. As with the diode, this takes a finite time and depends on how long the charge carriers need to distribute themselves in the weakly doped n-zone.

A further factor to be born in mind in relation to IGBTs is the critical field distribution at the p-wells owing to their finite curvature [3], especially at the edge of the chip. Here, an excessive local field strength increase occurs during operation which may result in what is known as lateral breakdown rather than vertical breakdown triggered by the avalanche effect of one or more IGBT cells. A possible countermeasure are field plates. However, usually several p-doped guard rings for junction termination (at the edge zone) are implemented. This enables high blocking voltages to be achieved in forward direction without the risk of lateral breakdown.

Nevertheless, in reverse operation the conditions are different, since for the junction termination to be effective the underside of the wafer would have to be doped; for technical reasons, however, this is not done.

In conclusion, it can be said that there are two essential weakpoints for reverse operation: on the one hand, the lack of an junction termination for reverse operation and the plasma-induced instability, on the other hand.

Schematic of a Trench IGBT in Field Stop technology.

Dielectric strength

Accordingly, the external IGBT conditions are dictated by the switching properties of the antiparallel diode. For a diode with ideal behaviour, there would be no forward recovery time and thus no overvoltage on turn-on. Owing to the light doping in the middle zone and the need for electron-hole plasma formation first, real power diodes have real delay times which lead to voltage drops as a result of the externally impressed di/dt. By common definition [4], according to which time is calculated from the point at which the voltage reaches 10% of the flow voltage Uf up to the point where the voltage has dropped once again to a factor of 1.1 of the Uf, the forward recovery time tfr may amount to as much as several 100 ns, depending on the diode being used.

Put to the test

In the tests conducted, voltage pulses of various amplitudes and duration are applied to an IGBT poled in reverse direction. The chip temperature was also varied using a heatable base plate. The chosen pulse duration is 20ms, 1ms and 10μs. The first two durations may seem disproportionately long as regards the actual time needed for forward recovery. They are taken, however, in order to illustrate how much time is needed for plasma formation in the n zone of the IGBT. After all, the better reverse voltage capability for short pulses can be explained by the fact that an approximately equal number of charge carriers then have to be moved in a shorter period of time.

The voltage amplitude is then gradually increased until the protective circuit is triggered. The protective circuit is required to stop the IGBTs from being destroyed and thus enable the IGBT behaviour under different parameters to be tested. The current increases exponentially with the voltage amplitude (Figure 4). At higher temperatures additional displacement occurs, resulting in higher currents. At a chip temperature of 150°C and an applied voltage of 120V, for example, the current measured amounts to as much as 500mA.

IGBT reverse current vs. the applied voltage for various temperatures (pulse duration per 20ms reading).

In a subsequent test on numerous IGBTs, voltage pulses of gradually increasing amplitude are applied until the IGBTs are distroyed. The results of this test are given in Figure 5 in the form of horizontal lines showing the maximum possible voltage amplitude for a specific pulse duration. This figure also shows the voltage curves resulting from the forward revovery times for the diode under observation. If a horizontal line lies above the diode voltage curve for a defined di/dt, the situation can be deemed sufficiently safe for the IGBT. A remarkable fact here is that, owing to the slow rate of plasma formation, the safety margin increases as the pulse duration decreases.

Safe Operating Area (SOA) of the antiparallel IGBT4 at 150 °C for pulse widths of 20ms, 1ms and 10μs, in relation to the forward recovery time of a CAL4 power diode (75A/1200V).

For pulse durations of 1ms and 20ms commutation above 16kA/μs is still in the critical region. For a pulse duration of 10μs, however, this is clearly no longer the case. Taking the more realistic case of 10kA/μs, the safety margin increases six-fold for a defined pulse duration of 10μs.

Conclusion

In state-of-the-art low-inductance power modules, high commutation rates of rise (di/dt) to the freewheeling diode are already applicable. Here, it is imperative that the resulting forward recovery voltage does not cause breakdown of the antiparallel IGBT, since this would inevitably lead to the destruction of the module.

The extreme current rates of rise tested here are intended to demonstrate the inherent difficulty in forward recovery and the resulting load placed on the antiparallel IGBT. Having said that, the high di/dt used in the lab tests are virtually impossible to achieve in field conditions. Similarly, the constant amplitude and pulse duration used for the IGBT here are both more extreme than in actual field applications. These facts should not be forgotten when evaluating the 6-fold safety margin.

Given the development of new and fast electronic switches and layouts with short switching paths and low paristic inductances, this problem will have to be tackled sooner rather than later. Indeed, well matched IGBTs and diodes are an absolute must if the "reverse gear“ is not to fail us in the future.

 

References:

1) Dr.-Ing. Günter Schmitt: Ansteuerung von Hochvolt-IGBTs über optimierte Gatestromprofile, Dissertation 2009.
2) Prof.Dr.-Ing. N.Kaminski: Leistungselektronik und Stromrichtertechnik I, Vorlesungsskript Universität Bremen 2009.
3) Dr. Ulla Knipper: Untersuchungen zur Robustheit von IGBT-Chips im Lawinendurchbruch, Dissertation 2011.
4) Josef Lutz: Halbleiter-Leistungsbauelemente, Springer-Verlag 2006.

 

 

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