SiC Mosfet tube characteristics and its dedicated driving power supply

I. Introduction

Because the Mosfet tube with Si as the substrate has high input impedance, low driving power, and simple driving circuit, it has the conductive characteristics of relying on the majority carrier to work, and there is no storage time required for the minority carrier to conduct work, so the switching speed is fast. The working frequency can reach 500kHz, even above MHz. But as its reverse withstand voltage increases, the on-state resistance also rises sharply, which limits its application in high-voltage applications. IGBT has the characteristics of high reverse withstand voltage and large current, but it has strict requirements on the driving circuit and is not suitable for working in high-frequency occasions. The working frequency of IGBT is generally below 20kHz.

SiC, as a broad-based forbidden semiconductor device, has the characteristics of high saturation electron drift velocity, high electric field breakdown strength, low dielectric constant and high thermal conductivity. Mosfet tube based on SiC has the characteristics of high blocking voltage, high operating frequency and high temperature resistance, and at the same time has low on-state resistance and low switching loss. It is an application of increasing power density and efficiency in high-frequency and high-voltage occasions. trend.

2. Performance comparison between SiC Mosfet and Si IGBT

At present, the current of SiC Mosfet common on the market is not more than 50A. Taking the common 1200V/20A as an example, some electrical parameters of the SiC Mosfet tube of Cree and Rohm are listed; the 1200V/20A of Fairchild and APT are also cited. Compare electrical parameters of Si IGBT series;

Through the table performance comparison, it can be seen that SiC Mosfet has three aspects of performance that are significantly better than Si IGBT:

1. Extremely low on-resistance RDS(ON), resulting in extremely excellent forward voltage drop and conduction loss, more suitable for working in high temperature environment;

2. SiC Mosfet tube has the input characteristics of Si Mosfet tube, that is, a relatively low gate charge, resulting in excellent performance switching rate;

3. Wide forbidden band width material, with relatively low leakage current, more suitable for high voltage environment applications;

3. The drive circuit requirements

Sic Mosfet has very similar switching characteristics as Si Mosfet tube. Through the study of the characteristics of Si Mosfet, its driving circuit has the same characteristics:

1. For the drive circuit, the most important parameter is the gate charge. The gate input of the Mosfet tube is equivalent to a capacitive network, so the device does not require drive current during stable on-time or off-time of off. However, in the process of device switching, the input capacitance of the gate needs to be charged and discharged. At this time, the gate drive circuit must provide a sufficiently large charge and discharge pulse current. If the faster the device operating frequency, the shorter the charge and discharge time requirements of the gate capacitor, the smaller the input gate capacitance and the greater the pulse current of the drive to meet the drive requirements;

2. The gate drive circuit must reasonably select a certain drive voltage. The higher the gate drive voltage, the larger the Mosfet's inductive conductive channel and the smaller the on-resistance; but if the gate drive voltage is too large, it is easy Break down the insulating layer between the gate and drain, causing permanent failure of the Mosfet tube;

3. In order to increase the speed of the switching tube, it is necessary to reduce the off time of the switching tube; and in order to improve the working reliability of the Mosfet tube in the off state, the drive circuit is designed to be in the gate when the off state The reverse bias voltage is added to the pole to quickly release the charge of the gate input capacitor, reducing the turn-off time, making the drive circuit to turn off Mosfet more reliably; but the reverse drive voltage will increase the circuit loss, reverse bias The voltage should not exceed -6V;

4. When the driving object is the power Mosfet of the full-bridge or half-bridge circuit, or to improve the anti-interference ability of the control circuit, the drive circuit is designed as an isolated drive circuit at this time; the way to achieve electrical isolation can be through magnetic coupling transformer and optical coupling Devices; but no matter whether magnetic coupling transformers or optical coupling devices are used, the delay time and coupling distributed capacitance of the coupling devices must be guaranteed; the isolated power supply must also have the characteristics of high isolation, fast response time and low coupling capacitance.

4. Characteristics of isolated power supply

From the point of view of the characteristics of the driving circuit, the driving power source is required to have the following characteristics:

1. In order to adapt to the use requirements of high frequency, the driving power source is required to have the characteristics of instantaneous driving high power, that is, it needs to have a large capacitive load capacity;

2. In order to adapt to the requirements of high-voltage applications, the drive power supply is required to have high withstand voltage and ultra-low isolation capacitance to reduce the interference of the high-voltage bus part on the low-voltage control side;

3. The isolated driving power supply must have a suitable driving voltage, that is, the power supply needs to have positive and negative output voltages, and the positive and negative output voltages are not symmetrical output characteristics.

QA01C has a complete drive circuit recommendation. Asymmetrical positive drive voltage of 20V and negative bias turn-off voltage of -4V can be obtained through a dedicated power supply for SiC drive. To prevent the drive voltage from damaging the gate, add D2 and D3 to absorb the spikes Voltage is necessary. The SiC driver can use the general driving chip; in order to achieve the isolation of the control signal and the main power circuit, isolation measures need to be taken. It is recommended to use the common optocoupler isolation scheme. The optocoupler used must have a high common mode rejection ratio (30KV/us) and an isolation withstand voltage greater than that of the isolated power supply and have a very small delay time to adapt to the high frequency operating characteristics of the SiC Mosfet tube.

5. Summary

By comparing the relevant electrical parameters of SiC Mosfet tube and Si IGBT tube, we found that SiC Mosfet will become an application trend in high voltage and high frequency applications. According to the research on the switching characteristics of SiC Mosfet tube, Jin Shengyang recommended a special power supply QA01C that can simplify its isolation design, and also recommended a drive circuit based on SiC Mosfet.