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Short circuit detection is a sensitive topic throughout all the power electronics community. The below groups are just examples for the whole community:
- SiC MOSFET users fear damaging their precious devices if a short circuit is not detected and the power transistors are shut down not fast enough
- IGBT development engineers are forced to go with compromises regarding the VCE(sat) or the switching energies
- SiC MOSFET technologists strive for the best Rds(on) per unit area, but short circuit requirements lead also here to compromises
It is therefore time to highlight some facts about short circuit detection and the DESAT function.
As shown in Fig 1, the DESAT function bases on a collector-emitter voltage VCE monitoring (or VDS for SiC MOSFETs) during on-state. Once vCE reaches the value of VDESAT, the IC turns off the power transistor. The so-called “short circuit blanking time” is an adjustable, external filter and takes care, that the function works properly. Key parameters for the performance of the DESAT function are the trigger threshold VDESAT, the DESAT current IDESAT, the short circuit blanking time, and the tolerances of these three parameters as listed in table 1.
Table 1: DESAT parameters and their tolerances of selected enhanced gate driver ICs
|
X3-family |
1ED-F2 family |
ISO5852 |
ACPL-352J |
|
|
IDESAT |
500 µA +25 µA / -30 µA |
500 µA ± 50 µA |
500 µA ± 80 µA |
1000 µA ±150 µA |
|
VDESAT |
9.18 V ± 0.3 V |
9 V +0.5 V / -0.7 V |
9 V +0.5 V / -0.7 V |
9 V ± 0.5 V |
Let's do an exercise for worst-case scenarios of the slowest IC reactions on a hard short circuit event. Figure 2 gives examples of EiceDRIVER™ products and selected competitor parts for two cases. The first case has no external filtering at all and represents the fastest reaction of the DESAT function assuming worst-case timings (Min and Max) shown left in fig. 2. The second case is the same as the previous plus an additional external filter of 1 µs using a DESAT capacitor of 56 pF depicted right in figure 2.
We can see, that the DESAT function can react in principle fast, even below 1 µs. Please note, that the left diagram without external filtering still includes IC-internal noise filtering.
Nevertheless, EiceDRIVER™ isolated gate driver ICs support the trend for lower short circuit times of power switches already today. The key is the precision of all DESAT parameters according to the grey bars in fig. 2. It is much easier to find a safe DESAT detection window with more precise DESAT parameters in an application so that DESAT does neither react too sensitive nor exceeds the short circuit capabilities of a power switch.
Infineon’s EiceDRIVER™ Enhanced isolated gate drivers, from the industry-standard 1ED-F2 and 2ED-F2 device to the new X3 Analog and Digital family (1ED34xx and 1ED38xx), with best-in-class tight tolerances, can be used for accurate and fast short-circuit protection of SiC MOSFETs (like CoolSiC™ MOSFET) and IGBTs (like TRENCHSTOP™ IGBT).
So where are the limitations of the DESAT function in applications? We see two critical points in general. The first point is the IGBT´s voltage tail at turn-on. The DESAT function should not react too fast here, because it could trigger a DESAT event even though there is a regular turn-on, but the IGBT's collector-emitter voltage vCE does not collapse fast enough. Erroneous triggers are the consequence. SiC MOSFETs do not show the same behavior as IGBTs. Thus, DESAT can be used with much shorter blanking times. The second point is capacitive or capacitive-inductive loads for power switches with low current ratings up to a collector current of e.g. IC = 50 A. Such load cases occur with longer cables, or even the winding capacitance of motors can be big enough to let IGBTs desaturate temporarily. The DESAT function should wait until the temporary desaturation vanishes for safe detection.
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