[Introduction]With the advancement of fabrication technology, the cost of silicon carbide (SiC) devices and modules has been reduced year by year, driven by continuous demand. The development and application of related products have also been greatly accelerated. Especially in the development of new energy vehicles, renewable energy and energy storage applications, it should not be underestimated.
Future Electronics has been committed to creating personalized solutions for customers and shortening product design cycles with professional technical services. In the field of practical application of third-generation semiconductors, Future Electronics combined its own technology accumulation and project experience to write a series of articles on SiC-related design. I hope this will give you some design reference, and look forward to further communication with you.
As the first part of a series of articles, this article will first analyze and discuss the driving voltage of SiC MOSFETs.
Common Vgs and Vgs(th), and the impact on SiC MOSFET applications
The driving voltage Vgs and gate voltage threshold Vgs(th) are related to the reliability of the SiC MOSFET in the application process, the power loss (on-resistance), and the compatibility of the driving circuit. This is a very critical parameter for SiC MOSFETs and needs to be considered during the design process. In different designs, it is more cost-effective to set different driving voltages. Figure 1 below lists the Vgs and Vgs(th) values of some SiC MOSFETs from several common manufacturers for comparison.
Discussion on the setting of driving voltage of SiC MOSFET
1. Does the driving voltage high level Vgs_on select +12V, +15V, +18V or +20V?
As shown in Figure 1, the maximum forward voltage of SiC MOSFET is about 22V~25V. The recommended working voltage is mainly +20V and +18V. For specific applications, please refer to the DATASHEET of different SiC MOSFET models. As shown in Figure 2 below, when Vgs exceeds 15V, both the on-resistance and the on-current gradually tend to be flat (the reference standards given by the DATASHEET of various SiC MOSFETs are different, and some are the curves of Rds(on) and Vgs, Some are the curves of Id and Vgs). Of course, the higher the driving voltage Vgs, the smaller the corresponding Rds(on) and the smaller the loss.
Future design tips: Vgs cannot exceed the maximum value given by DATASHEET when setting Vgs, otherwise it may cause permanent damage to the SiC MOSFET.
(1) For SiC MOSFETs with +18V or +20V high-level drive voltage recommended
As shown in Figure 1, due to the improvement of the new generation of SiC MOS process, the recommended high-level driving voltage of some SiC MOSFETs is +18V. As shown in Figure 2 below, the improvement of the process makes the Rds(on) of Vgs change little from +18V to +20V, and the difference in conduction loss is not obvious.
Future design tip: +18V drive voltage is recommended for the latest generation of SiC MOSFETs. It is more beneficial to reduce driving loss and reduce Vgs overshoot damage.
(2) Can SiC MOSFET be driven for +15V high level
Under normal circumstances, there is no recommendation on DATASHEET and its use is not recommended. However, considering that it is compatible with 15V-driven Si IGBT, it is necessary to calculate the increase in conduction loss, design with sufficient heat dissipation conditions and consider the overall loss of the device, it can also be used. The relationship between Vgs and Rds(on) is shown in Figure 2 below. It can be seen that the higher the gate voltage is, the smaller the Rds(on) is. If it works at +15V, the Rds(on) will be larger than the nominal value.
Future Design Tip: When Vgs is set to +15V, the SiC MOSFET losses will be larger than the nominal value.
(3) Can SiC MOSFET be driven for +12V high level
The working principle is the same as the +15V drive voltage, but there will be fewer applications, and it is generally not recommended. However, some special application scenarios, such as low-power high-voltage auxiliary power supply applications, may need to be compatible with Si MOSFET control ICs currently on the market, and need to use 1700V SiC MOSFETs. After comprehensive consideration, customers may accept a slightly higher Rds(on). case, it can be used.
Future Design Tips: When Vgs is set to +12V, the SIC MOSFET loss will far exceed the nominal value, and the Rdson when Vgs=+12V should be referred to when calculating the loss.
2. Is the drive voltage low level Vgs_off 0V, -3V or -5V?
The selection of the low level of the driving voltage is much more complicated than that of the high level, and false turn-on needs to be considered. The false turn-on is caused by the high-speed changing dv/dt, which is coupled to the gate through the Miller capacitor Cgd to produce a gate voltage change, which causes ΔVgs to exceed the threshold voltage during turn-off. Therefore, the false turn-on is not only related to the threshold voltage Vgs(th), but also related to the voltage change caused by dv/dt.
(1) How to choose -3V or -5V shutdown voltage
First refer to the recommended shutdown voltage on the DATASHEET of the SiC MOSFET.Then consider the gate voltage threshold margin as
ΔVgs_th=Vgs(th)-Vgs_off, when dv/dt tends to infinity, the gate voltage change generated by dv/dt is:
ΔVgs=Vbus*Crss/Ciss. It can be seen that when the gate voltage threshold margin ΔVgs_th is larger than the gate voltage change ΔVgs caused by dv/dt, the device Vgs_off safety margin is larger and the risk of false turn-on is smaller. However, the smaller the Vgs_off, the greater the drift of Vgs(th) is caused, and the conduction loss is increased.
Future design tips: After comprehensively calculating ΔVgs_th, measuring ΔVgs during the experiment can further improve the stability and performance of practical applications.
(2) Discussion on 0V turn-off voltage
Although the SiC MOSFET can be turned off when the driving voltage Vgs is 0V, the ΔVgs caused by dv/dt may cause the SiC MOSFET to be misconnected and damage the device, so it is not recommended. Of course, if the designed dv/dt is very small, the Crss/Ciss ratio is large enough, and fully considering the influence of ΔVgs on the misdirection of SiC MOSFET, customers can decide according to their own design.
Future design tips: Focus on the ΔVgs caused by dv/dt and the equivalent inductance of the loop, which will affect the misconnection. Only when Vgs_off=0V can the system be more stable.
Influence of Vgs(th) Drift, and Factors Affecting Vgs(th)
Due to the inherent characteristics of the wide bandgap semiconductor SiC and the interface characteristics of the semiconductor oxide layer different from Si materials, threshold voltage changes and drift phenomena will be caused. In order to understand these differences, explain the relationship between these differences and the properties of the material itself, and evaluate its impact on applications and systems, more research and exploration are needed.
(1) Influence of Vth drift on application
In the long run, for a given Vgs, the main effect of threshold shift is to increase Rds(on). In general, increasing Rds(on) increases conduction losses, which in turn increases junction temperature. This increased junction temperature needs to be taken into account when calculating power cycling.
Future design tip: If the switching loss accounts for a high proportion of the total loss, the turn-on loss caused by Vgs(th) drift can be ignored.
(2) Vth drift will not affect the basic functions of the device, mainly including:
● The withstand voltage will not be affected;
● The reliability level of the device, such as the ability to resist cosmic rays, the ability to resist moisture, etc. will not be affected;
● Vth drift will have a slight effect on the total loss;
(3) Parameters affecting Vth drift mainly include:
● Switching times, including switching frequency and operating time;
● Drive voltage, mainly Vgs_off;
(4) The following parameters have no effect on Vth drift caused by switching operations:
● Junction temperature;
● Drain-source voltage, drain current;
● dv/dt, di/dt;
This article mainly discusses the influence of the driving voltage Vgs and the gate voltage threshold Vgs(th) itself on the use of SiC MOSFETs.
In the actual application process, the set Vgs voltage is a comprehensive consideration of the reliability of the device, the power loss and the compatibility of the driving circuit. The theoretical calculation is only a part of the design reference, and the actual measurement can also be considered to obtain real data to revise the design parameters. The actual measured ΔVgs will have more reference value for setting Vgs_off, and will make the SiC MOSFET application design more stable and make full use of its performance. At the same time, the setting of the driving voltage Vgs will also be affected by the driving resistances Ron and Roff, the driving current and the driving loop, etc., which will not be discussed here.
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