[Guide]The latest wideband gap (WBG) semiconductors are moving towards the most ideal state, that is, ultra-fast switching with high voltage and low loss, and modern MOSFETs and trench IGBTs can also have high dV/dt and di/dt . However, the fast switching in the low-side circuit will couple the transient voltage to the gate drive circuit, causing confusion or damage, and the signal and power isolation of the high-side gate driver will also be affected by stress. This article will explore these effects, explain how to mitigate the effects, and evaluate the results of experiments on damage caused by stress and partial discharge (PD).

Modern semiconductor switches, MOSFETs and some IGBTs using wide band gap (WBG) technology can achieve extremely fast switching. This reduces the power consumption of switching, and at the same time performs high-frequency operation with high efficiency, high power density, smaller passive components and lower cost. However, the disadvantage of high dV/dt and di/dt is that it will increase the EMI and stress of the gate drive insulation system. Figure 1 shows a typical gate drive circuit of an IGBT. A positive voltage is applied between 5V and 20V to turn the device on, and 0V to turn it off. This circuit is also very suitable for enhanced Si MOSFET and WBG devices in SiC and GaN technology; in all cases, the device is guaranteed to be turned off when 0V is applied to the continuous gate.

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Figure 1: Basic gate drive circuit

However, as shown in Figure 2, there will be problems when the device switches quickly, and parasitic capacitance and inductance components will cause problems.

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Figure 2: Gate driver with parasitic components

If take di/dt as an example, the drain-source current is 10A/ns (this is possible in the most advanced GaN), and the source inductance is 15nH. If V = – L di/dt, 150V will appear across the Inductor. When turned off, the voltage drag source is negative, opposite to the gate drive, and the direction is positive when it is turned on, again opposite to the gate drive. This may reduce efficiency, and false turn-on may even lead to breakdown and damage. 15nH may seem large, but the corresponding PCB track is actually only 25mm. Even the PCB through-hole inductance of about 1.2nH will generate 12V transient voltage. In these high di/dt situations, only the chip size package is practical to connect to the gate and source of the gate drive in Kelvin. Applying a negative voltage to drive the gate in the off state can help the inevitable inductance.

In actual circuits, such as push-pull or full-bridge circuits in inverters or motor control, two lower-side devices usually share a common loop of source and gate drive currents, as shown in Figure 3.

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Figure 3: Common grounding for lower arm devices

Kelvin connections cannot be used now because the two drives each have their own circuits. The grounding of the two drivers and the two emitters (sources) must be connected together. If this contact is Powergnd 1 close to the left switch, the right switch will have more source connection inductance than the left, resulting in asymmetric switching , Potential EMI and damage caused by the induced voltage across the inductor. If you want to be symmetrical, Powergnd 2 is the only choice, but now the two sources have the same and high connection inductance in the gate drive loop, so this is not an appropriate compromise, especially when the high-power system equipment does not Set together.

The solution is to provide isolated signals and power for the two gate drivers, as shown in Figure 4. Now, the driver signal and power return can be directly connected to the respective device transmitter (source), excluding most of the external inductance in the driver loop.

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Figure 4: Gate drive using Kelvin connection and signal and power isolation

The challenge of the upper bridge arm switch

The configuration of Figure 4 solves the problem of gate voltage transients in the emitter (source) inductance caused by di/dt. This configuration is also usually used for the two upper-arm switches of the H-bridge, where the two gate drive returns are actually inverting switching nodes, and therefore must be isolated from each other. In the configuration of the upper leg, the high switching voltage that appears on the gate drive isolation component may cause other problems. According to I = C dV/dt, the high dV/dt may be a problem caused by the displacement current generated by the isolation capacitor. Because the edge rate can easily reach 100V/ns, a 10pF barrier capacitor may allow one ampere of current to pass and circulate in the primary circuit of the gate drive circuit, which may cause work interruption.

The gate drive signal isolation component is usually an optocoupler or a transformer, and sometimes capacitive coupling is also used. Table 1 shows the key parameters of isolated gate driver ICs. Among them are the Common Mode Transient Immunity (CMTI), which is most relevant to our high dV/dt circuit. However, this value is the data measured in the laboratory, and it is likely to be a single pulse. There is no mention of reliability under continuous high voltage and high dV/dt waveforms.

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Table 1: Key parameters of isolated gate drive

Other VIORM/VIOWM/VIOTM/VPR parameters are also very important, but they are not directly related to our switching circuit because the standard test is usually set to 50/60Hz, DC or peak. A separate gate drive transformer has similar limitations, and usually only requires a simple Hi-pot test of one second or one minute, DC or 50/60 Hz AC current. Reliability ratings of high-frequency switching voltages applied to windings or CMTIs are very rare. In the case of transformers, the method to obtain high isolation differs depending on the application; enameled wire can be tested separately for Hi-pot but it is not reliable, and it is almost guaranteed that there will be pinholes on the paint surface. Of course, safety agencies do not allow it to be used as a safety barrier for any voltage. Wires with better insulation, such as “triple insulated wires”, can be recognized by safety agencies, but they are too bulky, resulting in relatively high coupling capacitance and displacement current in the transformer. Due to the partial discharge (PD) effect between the insulating layers, triple insulated wires perform poorly at high switching voltages. If you want to meet the requirements of safety agencies, the ideal structure is that the windings are separated from each other, with sufficient space in the middle, providing low winding capacitance, and not relying on solid materials that may cause partial discharge, as shown in Figure 5.

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Figure 5: The windings of the gate drive transformer are separated from each other

The same considerations also apply to the transformer inside the isolated gate drive power supply. CMTI is often overlooked, and high-voltage isolation is often displayed in various ways.

Partial discharge effects

We have already mentioned partial discharge (PD), a phenomenon in which solid insulating materials slowly degrade after being subjected to high voltage stress. This phenomenon is caused by the continuous destruction of the micropores of the material. If it is an organic material, the plasma will cause carbonization. The void causes a permanent short circuit, which reduces the overall insulation thickness, thereby generating a stronger voltage field on the remaining insulation layer, and ultimately leading to complete failure. The PD effect starts suddenly at the “onset” voltage, which depends on the gas, pressure, and size of the gap in the Paschen curve.[1]As a feature. If it is a switching voltage, the starting voltage will be determined by the frequency.

In addition, the breakdown voltage of bulk materials should not be completely believed. For example, glass is considered an excellent insulator, with a breakdown voltage of about 60 kV/mm, but this is at a frequency of 60 Hz. If the frequency is 1MHz, the value is less than one tenth, which is about 5kV/mm. For some gate drive ICs with an insulation pitch of <10μm, high-frequency effects need to be carefully considered.

Therefore, switching voltage, dV/dt and frequency are the key parameters for evaluating insulation reliability. Transient voltage caused by overshoot and resonance of parasitic capacitance and inductance should also be evaluated and added to the system voltage.

Barrier insulation evaluation and research

Gate drive power supply manufacturer RECOM [2]It is known that the transformer of the DC-DC converter has the potential problem of high switching common-mode voltage, and it has been carried out together with Priv.-Doz. Dipl.-Ing. Dr.techn. Christof Sumereder, an insulation material expert from Technische Universität Graz and FH Johanneum University. Research. The internal code of this project is BIER (abbreviation of Barrier Insulation Evaluation and Research), which includes the evaluation of 30 half-bridge power levels, which use isolated upper and lower arm switch structures, such as Shown in Figure 6. Table 2 shows that three different configurations work for 1464 hours at 70°C ambient temperature, the DC voltage is 1000V, the switching frequency is 50kHz, and the edge rate is 65kV/μs. T1 is not included in the test

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Figure 6: PD test evaluation circuit

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Table 2: BIER test configuration

The partial discharge was measured once before and after the test, and the performance was not significantly reduced (Figure 7). The PD start-up voltage is maintained at twice the peak voltage of the applied switch, indicating that it has a good margin and good long-term reliability.For the full report, please visit the RECOM website[3].

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Figure 7: PD assessment results

in conclusion

In push-pull and bridge circuits, isolating the gate drive signal and power solves the problem of voltage transients coupled to the gate in the low-side and high-side circuits. However, at high frequencies and high edge rates, the isolation components of the upper bridge arm still bear high common-mode voltage stress. The actual partial discharge test shows that the isolation components of the gate drive DC-DC power supply can be designed to have good long-term reliability.

RECOM offers various series of DC-DC converters with output voltage and isolation ratings suitable for gate drive applications on bridge arms on IGBT, SiC and GaN technologies.





Source: RECOM

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