[Introduction]In the IGBT era, the selection of gate voltage is relatively uniform, nothing more than Vge=+15V/-15V or +15V/-8V or +15V/0V. In the emerging field of SiC MOSFETs, there is no conventional gate voltage specification. This article would like to provide some useful references for the confusion on gate voltage selection of SiC MOSFETs.
The following description is mainly based on the M1H series products and applications of Infineon’s industrial 1200V SiC MOSFETs. Other SiC products of different voltage levels or different manufacturers are not the same.
In the specifications of SiC products, there will be SiC Vgs voltage range and recommended voltage range (as shown in Figure 1) for your reference in practical applications. However, it is recommended that it is not mandatory. Regarding the turn-off voltage of Vgs, it can be either 0V or negative voltage.
Figure 1. IMBG120R030M1H Specification Book Vgs Description
Remark 1: For the M1H single-tube series newly released this year (2022), the negative voltage extreme value of the gate Vgs voltage is further extended from the above -7V to -10V, which makes the customer’s negative voltage selection more flexible, and the gate voltage is more flexible. AC BTI characteristics have also been greatly improved.
In fact, no matter how the gate voltage range of the device changes, it is implemented at the application level: how to choose the SiC gate voltage? What aspects should be considered? How to choose between performance and reliability? After reading this article, it is natural to clear the clouds and see the sun.
01 Influence of Vgs positive voltage of SiC MOSFET on Rdson and Esw
Taking Infineon’s industrial 1200V/M1H series SiC single tube as an example, as shown in Figure 1, the maximum positive voltage of Vgs is 23V. Considering the 5V margin, 15V or 18V can be selected as the turn-on voltage in practical applications.
1.1 The higher the positive pressure of Vgs, the smaller its Rdson
The Rdson calibration value of the industrial 1200V/M1H specification book is obtained by Vgs=+18V. If Vgs=+15V is appropriate, then Rdson will increase. Example:
1.2 The higher the positive pressure of Vgs, the smaller the Esw (Eon)
In order to more intuitively explain the influence of different Vgs positive voltage on Esw, a simple simulation circuit (800V/25A/25C/Rg=10/6Ohm) was built using the SiC SPICE model (IMBG120R030M1H) on the official website, as shown in Figure 2 and Figure 3 Shown are the simulation results of Vgs=18V/0V and Vgs=15V/0V. The higher the positive pressure Vgs, the smaller its Eon. Therefore, it is still necessary to pay attention to the soft switching occasions with high switching frequency or only Eon or Eoff.
Figure 2. Different Vgs positive voltage (Vgs=18V and 15V), the effect on SiC switching characteristics (25C)
Figure 3. Different Vgs positive voltage (Vgs=18V and 15V), the effect on SiC switching characteristics (25C)
Therefore, if a positive voltage 15V drive is selected, compared with 18V, not only the conduction loss (Rdson) will be sacrificed, but also some switching losses (Eon) will be increased. Of course, 15V driving is also beneficial. Benefiting from the reduction of the turn-on speed, the turn-on overshoot has been improved, which will help the parasitic conduction of Vgs.
02 Influence of Vgs negative pressure of SiC MOSFET on Rdson and Esw
2.1 Vgs negative pressure is different, its Rdson remains unchanged
2.2 The lower the negative pressure of Vgs, the lower its Esw (Eoff)
Similarly, let’s look at the simulation results of different Vgs negative pressures, as shown in Figure 4 and Figure 5:
Figure 4. Effect of different Vgs negative voltage (Vgs=0V and -3V) on SiC switching characteristics (25C)
Figure 5. Effect of different Vgs negative voltage (Vgs=0V and -3V) on SiC switching characteristics (25C)
Therefore, if Vgs=-3V is selected to be turned off, it will not help the Rdson of SiC, but the reduction of the turn-off loss Eoff is still relatively obvious, especially for some occasions where only the turn-off loss Eoff is required. At the same time, choosing Vgs=-3V can also immediately reduce the risk of parasitic conduction at the turn-on time.
Then, we directly choose Vgs=+18V/-3V,
Isn’t everyone happy?
There is actually no standard answer to this question: different applications, different answers, different designs, different answers…
Because, in the design and application of SiC devices, in addition to the above performance (Rdson, Esw) considerations, reliability and robustness are also a part that cannot be ignored. Especially the effect of Vgs voltage on short circuit characteristics and gate reliability.
03 Influence of Vgs voltage of SiC MOSFET on short-circuit characteristics
To undertake the above question, why can’t Vgs=+18V/-3V be used directly, “one size fits all”.
First of all, Vgs positive voltage of 18V or 15V is directly related to the short-circuit characteristics of SiC, and some applications just need the short-circuit capability of the device, so only 15V can be selected, and choosing Vgs=18V means that the device loses the short-circuit withstand capability.
At the same time, similar to the short-circuit withstand of IGBT, different bus voltages will also affect its short-circuit withstand time.
In a word, the short-circuit withstand capability of SiC MOSFETs is much worse than that of IGBTs with similar current specifications. In fact, it is not difficult to understand. After all, in the Si era of MOSFETs, such as the CoolMOS family that Infineon is proud of, there is no short-circuit withstand capability. The so-called short-circuit withstand capability is only the continuation and legacy of IGBT applications.
Figure 6. Short-circuit characteristics of SiC MOSFETs
As to why the short-circuit tolerance of SiC MOSFET is shorter than that of IGBT, in short, the short-circuit energy density caused by large current, small area, and thin thermal layer is much higher than that of IGBT short-circuit (about 20 times). For details, please refer to the following. of Figure 7:
Figure 7. Comparison of short-circuit energy density between SiC and IGBT
04 Influence of Vgs voltage of SiC MOSFET on gate reliability
Continuing with the above questions, after talking about the Vgs positive voltage selection and short-circuit characteristics, let’s look at the Vgs turn-off negative voltage and gate reliability. Regarding this hot issue, Infineon has previously released a white paper with detailed content that can be regarded as a model in the industry, and has made in-depth analysis and explanations for you on different occasions and platforms. The core content, as shown in Figure 8:
Figure 8. SiC Gate Oxide Reliability Challenge and Introduction to Infineon SiC Reliability White Paper
The specifics will not be repeated here, please refer to the following links:
“How Infineon controls and guarantees the reliability of SiC-based power semiconductor devices”
We continue to talk about the selection of Vgs negative pressure. In common half-bridge topologies, there are mostly Vgs spikes generated by Miller capacitance and source inductance, which are the overshoot at the turn-on time and the undershoot at the turn-off time.
Figure 9. Schematic diagram of overshoot and undershoot of SiC MOSFET gate Vgs
The negative pressure selection of Vgs should not only overshoot not exceed the threshold voltage Vgs.th, but also undershoot should not exceed the Vgs.min limit.
To sum up, in order to select a suitable Vgs voltage, in addition to the recommended value of the reference specification, not only the change to Rdson, but also the impact on Esw should be considered, and the reliability of the application and design should be considered. The relevant requirements, the final Vgs voltage value, must be the result of compromise and optimization, as shown in Figure 10.
Figure 10. Comprehensive reference for selection and evaluation of SiC drive voltage Vgs
The ancients said: Water has no constant form, and people have no constant form. It is better to teach a man to fish than to give him a fish.
Mastering the selection and evaluation method of Vgs voltage is far better than the so-called empirical Vgs voltage value.
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