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Posted on 06 November 2019

Short-Circuit Detection is Possible for Power Drive Systems with Long Motor Cables

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Long motor cables add high parasitic inductive and capacitive loads to the system. These parasitics lead to reflections on the collector-emitter voltage and the collector current, which have a considerable duration due to the length of the cable.

By Marc Buschkühle and Christian R. Müller, Infineon Technologies AG and Benno Weis, Siemens AG

In this article, the influence of a long cable on the switching characteristics is investigated experimentally and it is shown that short circuits can still be detected in spite of these significant parasitic effects.

High inductive and capacitive loads in power drive systems

In power drive systems, a common way to reduce the emission of electromagnetic interference (EMI) is to use shielded motor cables. Due to its shielding, the motor cable reduces the radiated EMI but, on the other hand, adds parasitics. As a consequence, a significant additional charging current flows from the inverter into the cable during switching operations. Two major challenges are encountered, as the charging current is superimposed on the load current:

  1. Thermal management of the power semiconductors
  2. Protection circuit of the system

The article focuses on the influence of the charging current on the short-circuit detection using a 200 m long shielded cable. The target is to switch off the short circuit within 5 μs. Compared to the standard turn-off time frame of 10 μs, the system reaction must be twice as fast.

The device under test (DUT) was an 25A EasyPIM™ 2B power module. A nominal DC link voltage of VCC of 600 and an increased voltage of 850 V were selected as operating conditions. The collector current IC was adjusted in steps from low current ratings up to 100 A, which is 400% of the rated chip current.

Schematic view of the experimental setup together with the applied and determined voltages

A 4-wire cable was used to connect an inductive load between the positive DC link voltage and the power terminal of one AC output. Fig. 1 shows the schematic of the electrical setup with the relevant sixpack topology of the power module. The collector emitter voltage (VCE) and the gate voltage (VGE) of one lower system were measured directly, and the collector current (IC) was measured by a current probe integrated in the measurement setup.

Experimental results and interpretation

The measurement focused on investigating the charging current (IOsc) as a result of the cable parasitics. Due to the position of the current probe and the load inductance in the measurement setup, the charging current can only be clearly measured when the device is turnedon. Thus, the device was operated in the double-pulse mode and the charging current IOsc, which is superimposed on the common turn-on waveform of the IGBT, was determined.

Turn-on characteristics of the 1200 V IGBT, i.e. the lower system in the setup

The turn-on characteristic of the IGBT is shown in Figure 2. VCE decreases rapidly when the IGBT is turned-on. After the DUT is switched on, reflections in the range of a few volts are visible on the collectoremitter voltage. The reflections decay with the turn-on time. Reflections with an amplitude of more than 15 A occur in the collector current. These reflections are superimposed on the diode-recovery peak current (Irec).

VCE and IC differ significantly from typical waveforms and may limit the short-circuit detection of the IGBT.

A schematic drawing of a simplified equivalent circuit diagram of the measurement setup is shown in Figure 3. The cables parasitic generate characteristic impedances in parallel to the load inductance and the free-wheeling path. When the voltage drop across the load inductance Lload changes rapidly, a charging current in the characteristic impedance can be observed; in the case of a switching event, not only the charging current itself, but also its reflections after twice the propagation time are observed. Therefore, the collector current through the IGBT is given by: IC = Iload + IOsc + Irec, with load current Iload. Due to the fact that Irec is significantly lower than IOsc, it can be neglected.

Schematic drawing of a simplified equivalent circuit diagram of the measurement setup

The fast short-circuit detection is realized with a VCE-desaturation-detection circuit, the drop of VCE is monitored, and the reaction time of the system is extracted. To determine this reaction time, the time interval is measured between the turn-on pulse at the gate terminal, i.e. VGE = 0, and the instant in time that VCE drops below a certain value, e.g. VCE ≤ 10 V.

A desaturation-detection circuit can be realized if this reaction time is well below the short-circuit withstand time of the IGBT. At high currents, this reaction time is expected to be longer, as it takes longer for VCE to drop.

Reaction time of the system versus the collector current at different VCE-desaturation-detection levels for VCC = 600 and 850 V

The reaction time of the system versus the collector current is shown for VCC = 600 and 850 V is shown in Figure 4. For a desaturation-detection level of VCE = 7.5 and 10.0 V, the reaction time of the system increases from 0.8 μs - at a low collector current - to 2.6 μs at a high collector current with VCC = 600 V. At a dc voltage of VCC = 850 V and for the detection level of VCE = 10.0 V the same behavior is observed. However the reaction time is less than 0.3 μs longer. In contrast to this, the reaction time is always longer than 2 μs for VCE = 7.5 V at VCC = 850 V.

Independent of the DC link voltage, a desaturation-detection level of VCE = 5.0 V always provides reaction times longer than 2.5 μs and for 400% of the nominal current, VCE did not drop below 5.0 V at all. Therefore, VCE = 5.0 V does not allow a reliable distinction to be made between normal operation and when a short circuit occurs.

The maximum value of the collector-emitter voltage (VCE max Osc) after the turn-on process of the 1200 V IGBT has finished is shown in Figure 5. For VCC = 600 V, VCE max Osc is in the range of 6.0 V for low collector currents, and reaches a value of 7.5 V at 400% of the nominal current. At VCC = 850 V, VCE max Osc is consistently above 7.5 V; however, even for IC = 100 A, it still remains below 10.0 V.

These results show that even for very high collector currents, a reaction time of less than 3 μs can be achieved. To implement reliable short-circuit protection, the desaturation-detection level should be at least 7.5 V (VCC=600 V) or 10 V at VCC = 850 V.

The maximum amplitude of IOsc in relation to IC and maximum VCE after the 1200 V IGBT has been turned on for VCC = 600 and 850 V

The maximum amplitude of IOsc in relation to IC is also shown in Figure 5. It can be seen that for low collector currents, the charging current is at least twice the load current. With increasing collector current, the ratio between IOsc and IC decreases and the charging current becomes less dominant.

These results illustrate that the charging current leads to a massive additional load for the IGBT. Especially for low collector currents, the losses of the IGBT increase significantly. As a consequence, the thermal management of the inverter and the IGBT have to consider these additional losses.

Conclusion

The protection of fast switching IGBTs is strongly influenced by cable parasitics. Especially for very long cables, a significant charging current is added to the load current and oscillations are observed in the collector-emitter voltage. These effects can significantly limit the efficiency of protection measures, such as desaturation detection. Although the influence of the cable parasitics is considerable, it is experimentally shown and concluded that it is theoretically possible to detect a short circuit in less than 5 μs.

 

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