Effect of gate drive positive voltage on power semiconductor performance
Author: Zheng Ziqing, Li Xiang
For semiconductor power devices, the value of the gate voltage has a great influence on the device characteristics. We have talked about the impact of gate negative voltage on the switching characteristics of the device before, and today we will take a look at the impact of gate positive voltage on the device. The article will discuss conduction loss, switching loss and short-circuit performance respectively.
Effect on conduction loss
Both MOSFET and IGBT are gate-controlled devices. Under the same current conditions, generally the higher the gate voltage, the smaller the conduction loss. Because the higher the gate voltage means the stronger the channel inversion layer, the smaller the channel impedance generated by the gate voltage, and the lower the voltage drop for the same current flowing. However, in addition to being affected by the gate channel, the device conduction loss is also closely related to the thickness of the chip. Generally, the thinner the conduction loss, the smaller the conduction loss, so the conduction loss of a wide bandgap device under the same chip area is higher. Much smaller. The higher the withstand voltage of the same material, the thicker the device will be, and the conduction loss will become larger. This conduction loss caused by chip thickness is not affected by the gate voltage, so the higher the device withstand voltage, the contribution to conduction loss will be limited even if the gate voltage further increases.
We can easily get this conclusion from the device specification book. Figure 1 a and b are the output characteristic curves of an IGBT device IKW40N120CS7 respectively. Under the same IC current, the higher the gate voltage, the steeper the corresponding output line, and the smaller the VCE saturation voltage drop. However, after the gate voltage is greater than 15V, even if the gate voltage increases again, the VCE saturation voltage drop becomes much smaller. Therefore, it is a good choice to use 15V driver for IGBT.
Effect on switching losses
In addition, the positive voltage of the gate is also helpful to reduce switching losses. Because the turn-on process is equivalent to a process of charging the gate capacitor, the greater the initial voltage, the faster the charge, and generally speaking, the smaller the turn-on loss. The turn-off loss is affected by the negative gate voltage and is almost unaffected by the positive gate voltage. We used a double-pulse platform to test switching waveforms. Figure 4 shows the switching loss of SiC MOSFET under different gate voltages and different IC currents. Figure 5 is the turn-on loss of IGBT. Since the absolute value of the switching loss of SiC MOSFET is much smaller than that of IGBT, the effect of SiC MOSFET is more obvious in terms of the ratio of switching loss reduction.
Effect on short circuit time
Everything has gains and losses. Although a high gate voltage is good for conduction loss and turn-on loss, it will sacrifice short-circuit performance. The following formula is the theoretical formula of MOSFET short-circuit current. The short-circuit behavior of IGBT is similar to that of MOSFET. In the formula, μn is the migration rate of electrons, Cox is the capacitance of the gate oxide layer per unit area, W/L is the width-to-length ratio of the oxide layer, Vgs is the driving positive voltage, and Vth is the gate threshold voltage. It can be seen from the formula that the greater the gate positive voltage, the current will increase significantly.
For example, IGBT has a short-circuit capability of 10μs when the gate voltage is 15V, but when the gate voltage is 16V, the short-circuit capability will drop to less than 7μs, as shown in Figure 6. For SiC MOSFET, the chip area for the same current is much smaller, and it may operate at a higher bus voltage, resulting in greater short-circuit transient energy. If the gate voltage exceeds 15V, it will even lose its short-circuit withstand capability.
Whether for IGBT or SiC MOSFET, the higher the gate positive voltage used, the conduction loss and turn-on loss will be reduced, which is beneficial to the overall switching efficiency. However, it will affect the short circuit withstand capability of the device. If short-circuit capability is not required when using SiC MOSFET, it is recommended to increase the positive voltage of the gate appropriately.
The conduction loss of SiC MOSFET behaves similarly. Figure 2 shows the output characteristics of IMW120R030M1H. Compared with the abscissa in Figure 1, the voltage span in Figure 2 is larger, which means that SiC MOSFET is suitable for higher gate voltages (such as 18V), with smaller conduction losses and greater benefits. However, considering the reliability of the gate oxide layer, the voltage used generally does not exceed 20V. Infineon's 1200V SiC MOSFET recommends a voltage of 18V.
Based on the above two characteristics, the 1200V IGBT generally does not change significantly after 15V, while the 1200V SiC MOSFET changes greatly, as shown in Figure 3. This is mainly because for 1200V level SiC MOSFETs, channel resistance accounts for a large proportion, and an effective way to reduce channel resistance is to increase the gate voltage.
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