Figure 4: Current vs frequency for a motor drive inverter with similar size 600V IGBT
example of trade-off with 4 different speeds is shown in Table 2.
While this is well characterised in the datasheets for hard switching applications, more care must be taken for soft switching applications. Comparing two devices in hard switching conditions can lead to misleading conclusions on their soft switching behavior. As shown in Figure 3, when adding a snubber capacitor to achieve soft turn off, the tail of the device plays a bigger role than in normal hard switching application.
Package selection
Packages can be divided in through-hole and surface mount types. Through hole packages offer a wider range of options for higher current ratings and can be cooled very efficiently as can be seen by their RthCS rating. These ratings are based on typical mounting method with isolation. Surface mount devices simplify assembly but are limited to lower current ratings and have much worse thermal performance, even when using thermal vias. It is important to note that through- hole devices should not be assembled using SMD
Table 2: Example of speed trade-off for a 600V IGBT Standard (S) IRG4PC40S
Recommended switching frequency range VCE(ON) ETS
< 1 kHz 1.2 V
6.95 mJ
methods as they are not meant to withstand the higher stress of those processes.
Electrical and thermal analysis
Engineers need the ability to compare different devices in terms of specific application design goals. These can typically be: efficiency, maximum current rating, maximum temperature, etc… Although Spice models are available, they offer very bad correlation when predicting switching losses. For this reason a common approach is to build behavioural models for the devices and calculate via simple formulas the overall conduction and switching losses in a specific application. For motor drives this approach can be used to calculate the maximum allowed current as a function of switching frequency with a fixed ∅TJS as seen in Figure 4: this graph shows the trade-off between conduction losses, switching losses and thermal performance for different generation IGBT with similar die size.
Fast (F, M) IRG4PC40F
1-8 kHz 1.4 V
2.96 mJ
Ultrafast (U, K) Warp (W,P) IRG4PC40U IRG4PC40W
8-30 kHz 1.7 V
1.1 mJ
>30 kHz 2.05 V
0.34 mJ
Figure 5: Example of cost/performance trade-off with IR IGBT selector too
Cost analysis
The final stage of cost analysis is an integral part of the IGBT selection process as it offers one more degree of freedom. This can be easily seen in Figure 5 in the output of IR IGBT Selector Tool where different devices are offered that meet the input parameters. They all represent different points of the cost/performance trade-off of the selection process. In particular, offering a full catalogue for
different generations allows customers to choose different cost/performance trade-offs. Planar technologies can be used for cost efficient solutions, while the latest trench IGBT offer the best performance.
International Rectifier |
www.irf.com
Andrea Gorgerino is IGBT Applications Manager, Energy Saving Products - International Rectifier
April 2012 CIE Power Supplement
17
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