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When less can be more – with Smart Module Design (Part 2)

Jan_B
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When less can be more – with Smart Module Design (Part 2)

How optimized module layouts can address the electrical performance challenges from chip shrinkage

In Part 1 of this article, I mentioned that size and power often seem like opposite sides of a coin. When you reduce size, you inevitably reduce power. In that article, we covered the impact of chip shrinkage on thermal performance and how to mitigate the effects through optimized chip placement and module layouts. Now, let’s look at how we can improve electrical performance. Again, we’ll be using the new 1200 V, 600 A EconoDUAL™ 3 module with TRENCHSTOP™ IGBT 7 technology as an example, which is optimized for applications like general-purpose drives (GPD), commercial, construction and agricultural vehicles (CAV), uninterruptible power supplies (UPS), and solar.

The 1200 V TRENCHSTOP™ IGBT 7 medium-power technology features a chip shrinkage of about 30% compared to the former IGBT 4 technology. Chip placement and module layout can have a positive impact on the smaller chips’ thermal performance, but they can also influence switching losses.

Electrical challenges with smaller chips

In medium-power modules like the EconoDUAL™ 3, multiple chips are connected in parallel to achieve high module currents. In order to make use of the full switching performance of the chip technology, it is crucial to have a proper module design in place, meaning that symmetry of paralleled chips, as well as the top and bottom switch, are of importance.

A limiting factor with regard to switching speed and losses can be the commutation from the IGBT to the diode during IGBT turn-on. Figure 1 illustrates the IGBT turn-on process for two different module layouts at the same di/dt switching speed and the same IGBT and diode technology and size.

crop1.PNGFig. 1: IGBT 7 turn-on of module layout V1 and V2 with the same di/dt switching speed.

When the current starts to rise, the inductive voltage drops. An obvious difference between the two different layouts is that the voltage (Vce) shows a humped curve in V1, caused by the recovery process of the diode. The diode current needs to cross the zero line in order to be able to take up voltage. From that point, the IGBT can transfer the voltage to the diode, letting its own voltage fall until it reaches saturation (Vcesat).

Due to the parallel connection, the slowest diode determines the overall switching speed. Though both layouts show an equal di/dt in the first phase, V2 has a higher reverse-recovery peak and V1 shows a higher tail for the recovery current in the last phase. This shows that the diode recovery process of both layouts is different and that it directly affects the turn-on IGBT losses and turn-off diode losses. To visualize this a bit more clearly, you can compare the simplified schematics of the module layouts for V1 and V2 (Figure 2).

Comparing module layout schematics for improved commutation

crop2.PNGFig. 2: Simplified schematic of module layouts V1 & V2. The turn-on process of the low-side IGBT and the current path of the reverse-recovery current are highlighted in red.
[LEFT] Simplified schematic of module layout V1; LD3 < LD12; LT3 < LT12; LLS>LHS>>LT12=LD12
[RIGHT] Simplified schematic of module layout V2; LLS=LHS=LAC>>LT=LD

In V1, all IGBTs and FWDs of the high side (HS) and low side (LS) are connected in parallel separately, and then connected via a common inductance (LHS). During turn-on of the LS IGBT, all HS diodes commutate with the LS IGBTs over this single common inductance, which lowers the di/dt in the phase up to the recovery current peak, and thus leads to slower extraction of charge carriers.

In V2, a different physical distribution is used. Here each HS diode can directly commutate over its own current path with the associated LS IGBT. This leads to a steeper di/dt in the phase between the zero crossing of the diode current up until the recovery peak. More charge carriers are extracted in the first phase, and the diodes can pick up voltage much faster (Figure 3).

 

crop3.PNG

Fig. 3: Diode turn-off of module layout V1 and V2 with the same di/dt switching speed. 

When the extraction of charge carriers from all diodes is synchronized, the IGBT voltage can drop faster, lowering the IGBT turn-on switching losses. The best-case scenario is when parallel IGBTs can directly commutate with their respective freewheeling diodes (FWD) on the opposite side, with all paths having ideally the same inductance. Despite the increased asymmetry of the LS and HS in V2, a huge reduction in the overall switching losses can be achieved – about 7% at the same di/dt (Figure 4).

crop4.PNGFig. 4: Relative IGBT 7 switching losses of module layout V1 and V2 at switching conditions as shown in Figure 2 and 4.

Comparing thermal and electrical performance in the 1200 V 600 A TRENCHSTOP™ IGBT 7 vs. the former generation IGBT 4

From Part 1 – and now Part 2 – of this article, it is clear that optimizing the module layout has a significant impact on both thermal and electrical performance. But how does this apply practically? For this, let’s compare the performance of the former EconoDUAL™ 3 1200 V, 600 A with TRENCHSTOP™ IGBT4 equipped with module layout V1 (FF600R12ME4_B72), and the new EconoDUAL™ 3  1200 V, 600 A with TRENCHSTOP™ IGBT7 equipped with module layout V2 (FF600R12ME7_B11).

To get a practical comparison, let’s look at the performance under typical application conditions (Figure 5). We’ve run the modules in an inverter operation mode, with forced-air heat-sink cooling. To get a full thermal image of the module, the junction temperature of the IGBTs and FWDs were measured with an infrared camera.

crop5.PNGFig. 5: Conditions for simulation of typical application

The IGBT4 module (FF600R12ME4_B72) was measured at an IGBT (du-dt)on switching speed of about 4.1 kV/μs, limited by diode snap-off. The IGBT7 modules were measured at two different switching speeds – 5 kV/μs and 6.5 kV/μs. From the results presented in Figure 6, it is possible to see that the IGBT 4 module recorded a maximum RMS current of 490 A, whereas the IGBT7 modules recorded 520 A for an IGBT (du-dt)on of 5 kV/μs and 535 A for an IGBT (du-dt)on of 6.5 kV/μs. This means that, in typical application conditions, the new EconoDUAL™ 3 1200 V, 600 A with TRENCHSTOP™ IGBT7 can provide about an 8% higher output RMS current without a limitation of switching speed.

crop6.PNGFig. 6: Measurement of the average chip temperature Tvj,avg as a function of RMS output current IRMS for different IGBT (du-dt)on.

These numbers show that even without changing the chip technology, higher output current can be achieved by module design in terms of thermal and electrical aspects. In actual measurements under application conditions, these findings were confirmed.

The overall switching losses of the newly developed EconoDUAL™ 3 1200 V, 600 A with TRENCHSTOP™ IGBT7 could be reduced by about 10% to 25% compared to its former generation with IGBT 4. In addition, its static losses are up to 20% lower. Measurements confirm the performance increase of about 7% higher output current at 150°C, but this difference would be more pronounced if the IGBT 7 overload operation of up to 175°C were used.

Through smart module design, chip shrinkage must not necessarily lead to reduced application performance.