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Posted on 01 August 2019

Reduced Switching Losses and Higher Operating Temperature

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IGBT4 chip generation enables efficient high current power modules

With the 1200V IGBT4 chips Infineon introduced a new generation of IGBTs. The goal of this innovative IGBT development was to reduce the saturation voltage and simultaneously decreasing the switching losses to improve the trade-off between static and dynamic losses.

By Marco Baessler, Peter Kanschat, Frank Umbach and Carsten Schaeffer, Infineon Technologies

 

The 1200V IGBT4 family offers optimized vertical structures for low, medium and high power applications. This article focuses on high power applications, namely modules with more then 1200A current rating. The related measurements were performed on a 2400A Infineon IHM single switch module.

Today state of the art IGBTs are Trench/Field-stop devices. In a Trench IGBT the MOS channel is rotated by 90° compared with a planar IGBT. Thus, a higher channel density can be realized at the chip top side which leads to a higher inundation of the top/emitter side with charge carriers. A good combination of a suitable trench density, a low backside emitter efficiency (collector doping) and a high charge carrier life time leads to a clear reduction of the saturation voltage without increase of turn-off losses [2].

In a Field-stop IGBT, an additional n+ layer is introduced close to the collector. This layer (field-stop) brings down the electric field within a very short spatial dimension. Therefore, it is possible to make the chips thinner and in such a way to reduce the static and dynamic losses. However, during switching events the silicon volume not affected by carrier extraction through the electric field is determining the amount of carriers contributing to the tail current. This tail carrier/charge is crucial for the softness of an IGBT. In case of high transient over voltages the space charge region reaches far into the field stop and the residual/tail charge is very small. For a critical voltage the tail current disappears and the current flow snaps off. Such a snap-off results in high and hardly controllable over voltages. This, so far, was a big challenge for the design-in of trench/field stop modules in high power applications.

Higher operation temperature

The new IGBT4 chip generation is balanced for the typical switching frequencies of the different power classes (low, medium and high, [1]). As a key advantage the 1200V IGBT4 chips allow a maximum operation temperature of 150°C (maximum junction temperature of 175°C) - compared to 125°C of the previous generation. The higher operation and junction temperature was introduced first by Infineon at 600V power semiconductors (IGBT3, [3], [4]).This results in the potential of a 20% higher output power, by use of the full temperature swing based on the same cooling condition. The higher temperature swing is supported by an improved power cycling reliability.

No snap-off currents

The tendency towards current snap-off is strongly dependent on the current rating of modules which are using the same IGBT chips. In today’s high current modules as much as 24 150A IGBT dices are connected in parallel. Since the switching speed of the IGBTs is almost independent from the current rating, the current gradient dI/dt increases linearly with current rating to first order. As a consequence of parasitic stray inductances this leads to high dynamic over voltages favoring current snap-off. Moreover, even though the IGBTs in Infineon’s high power modules are arranged in a highly symmetric manner any misbalance of current sharing boosts these effects. Consequently soft turnoff behavior of the IGBT is a major prerequisite for safe and controllable operation of high power modules. Due to low switching frequencies lowering conduction losses is the main development target. This also enables the optimization towards softness even if this is done at the cost of higher switching losses compared to chip optimizations for low power modules.

Investigations on the soft turn-off behaviour of the IGBT4 show the most critical case is switching of high currents at high over voltage. In contrast to the former trench/field stop generation, the new high power IGBT4 shows no snap off under operation conditions with working temperatures of 125°C or 150°C (Figure 1).

Soft turn-off behavior of an Infineon 2400A single switch IHM module with IGBT4 at 125°C

Reduced losses

In addition to softness an easy controllability of the turn off dI/dt is crucial because in this performance category an active (dI/dt) control is generally present at least to limit the over voltage peak for inverter overload conditions. A simple active clamping was used as active control for the characterization of a 2400A single switch in Infineon’s IHM module. The measurements (Figure 2 and Figure 3) show that the new chip is easily controllable which was formerly (with IGBT3) only possible with high Rg (> 1?) at the cost of drastically increased losses due to reduced dV/dt. At the nominal current of 2400A and 800Vdc, the improved softness leads to a reduction of the turn off losses of approximately 70mJ. On the other hand, there is a trade off relation between softness and current tail and therefore the optimization towards softness is linked with an increase of turn off losses at low current. At low collector currents the tail current influences the turn off losses more than the dI/dtand dV/dt, where dI/dt and dV/dt is mainly determined by the gate resistor. Generally applications are thermally limited at high current levels and moreover the overall losses throughout a whole sine wave have to be considered (Figure 4). Here the IGBT4 shows a benefit of about 5% compared to IGBT3.

Controllability of IGBT 3 and 4 (Vce=800V; Ic=2400A; Tj=25°C

Turn-off behaviour with active gate control (Vce=800V; Ic=2400A; Tc=25°C; Vbr(active clamping)=900V) for IGBT4 and IGBT3

The diode behaviour is most decisive for optimized turn-on losses at tolerable EMC (electromagnetic compatibility). The IGBT4 high power devices come with a kind of EmCon4 diode which shows an extreme soft behaviour. Therefore the IGBT can turn on much faster. Taking benefit from the diode softness the turn-on losses may be reduced by up to 20%.

Turn-off losses of a sinusoidal current

More output power

With the optimized high power IGBT4 Infineon introduced a trench/field stop component which provides an improved softness behaviour, so that large nominal currents up to 3600A can be controlled with a small gate resistor. During operation with an active gate control the total turn off losses in sum over the complete operation range decrease by 5% at a DC link voltage of 800V. Through the improved diode the turn-on losses decrease too by approximately 20%. Taking into account the 25°K higher operation temperature this means an approximately 20% higher inverter output power compared to an IGBT3 module assuming the same heat sink [1]. An optimization of the assembly technology furthermore ensures the same lifetime expectation in spite of increased operation temperature. On the other hand enhanced lifetime at comparable output power can be chosen by the customer.

 

References:

1) M. Bäßler, P.Kanschat, F.Umbach, and C. Schaeffer, “1200V IGBT4 -High Power- a new Technology Generation with Optimized Characteristics for High Current Modules,” PCIM 2006, Nürnberg.
2) T.Laska, F.Pfirsch, F.Hirler, J.Niedermeyer, C.Schäffer, and T.Schmidt, “1200V-Trench- IGBT Study with Square Short Circuit SOA,” ISPSD 1998-Kyoto.
3) P.Kanschat, T.Stolze, T.Passe, H.Rüthing, F.Umbach, and O.Hellmund, “600V IGBT³-Technology in New Low Cost Modules for Consumer Drives Applications,” PCIM 2003, Nürnberg.
4) P.Kanschat, H.Rüthing, F. Umbach, and F. Hille, "600V-IGBT³: A detailed Analysis of Outstanding Stativ and Dynamic Properties,“ PCIM 2004, Nürnberg.

 

 

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