SiC MOSFET replaces Si MOSFET, is the bootstrap circuit suitable?

[Introduction]The bootstrap suspension drive circuit can greatly simplify the design of the drive power supply. Only one power supply can drive the two switches of the upper and lower bridge arms, which can save the cost of the Si MOSFET power device solution. As new energy is promoted and supported by global governments, the demand for semiconductor chips related to new energy has surged, resulting in a shortage of production capacity. The innovative application of green and low-carbon technologies is an important part of achieving the goal of carbon neutrality. Silicon carbide is a common technology used in green and low-carbon fields. SiC MOSFETs have become a new choice for many manufacturers to replace Si MOSFETs. However, what is the difference between the driving of SiC MOSFET and Si MOSFET, and how to adjust the circuit design when replacing, is of great concern to engineers. Our “SiC MOSFET replaces Si MOSFET, how to achieve negative voltage when there is only a single power supply positive voltage? “The article has shared the tips of negative pressure bootstrapping. Considerations for SiC MOSFETs driving conventional bootstrap circuits in this article.

SiC MOSFET replaces Si MOSFET, is the bootstrap circuit suitable?

figure 1:

The working principle of the bootstrap circuit:

As shown in Figure 1, when the lower tube is turned on, the power supply charges the bootstrap capacitor Cboot through Rboot and Dboot. After the lower tube is turned off, Cboot provides power to drive the upper tube.

SiC MOSFET replaces Si MOSFET, is the bootstrap circuit suitable?

Vgsh is the driving waveform of the upper tube, Vgsl is the driving waveform of the lower tube, and Vgshin is the driving waveform of the input side of the upper tube. The result is that a double pulse is sent to drive the lower tube when the test board is powered on, and the upper tube is a complementary driving waveform. It can be seen from the figure that when the input driving waveform of the upper tube is “on”, the GS of the upper tube is not in time. It starts to follow the input drive signal state after a delay of about 40us. This is because in the initial state, the upper tube driver chip is not powered, and the power supply of the upper tube driver chip starts to be energized after the lower tube is turned on. After the driver chip is powered on until the chip can work normally, there is a delay of about tens of us, which leads to the phenomenon in the figure. This is also a problem with the bootstrap circuit. This problem can be increased by increasing D1 and R1 through the bus voltage to the Cboot capacitor. Perform a pre-charge solution.

SiC MOSFET replaces Si MOSFET, is the bootstrap circuit suitable?

By observing the circuit, it can also be seen that the driving power is VCC2. When the lower tube is driven, VCC2 can output at full scale. However, due to the existence of Dboot, the Cboot voltage of the upper tube will always lack a Dboot voltage drop compared to VCC, and the switching frequency and occupation of the lower tube are affected. The empty ratio also has related requirements. The lower tube must reach the Cboot of the upper tube for a fixed time in order to be fully charged every cycle and work normally.

SiC MOSFET replaces Si MOSFET, is the bootstrap circuit suitable?

It can be seen from the above figure that since the upper tube cannot reach full VCC, the turn-off negative pressure is not negative enough, and the turn-on positive voltage is not positive enough. Increasing the VCC voltage will cause the lower tube negative pressure to be too large and there will be a risk of breakdown of the SiC driver chip. , the use of bootstrap circuits requires trade-offs in this regard.

To sum up, the SIC MOSFET driver can also drive a half-bridge with a bootstrap circuit, thereby reducing one power supply to save costs.However, there are also some problems that need to be paid attention to when implementing the bootstrap circuit. The specific summary is as follows

1. Since the upper tube needs to be discharged through the bootstrap capacitor when it is turned on, in order to ensure the normal switching of the upper end, it is necessary to adjust the PWM to reserve charging time for the bootstrap capacitor

2. Regarding the choice of Dboot, since the Cboot is charged instantaneously, the current carrying capacity of Dboot needs to be considered. When the lower tube is turned on, the Dboot terminal will bear the large voltage of the busbar level, so it needs to have enough withstand voltage

3. The bootstrap capacitor Cboot needs to choose the capacitor with the smallest parasitic inductance as possible to prevent LC oscillation during charging

4. Since the driving voltage of the upper tube will decrease to a certain extent and there are high requirements for the stray parameters of the entire bootstrap circuit, it is recommended to use the bootstrap circuit at medium and low power.

The performance and reliability of SiC MOSFETs of Pinjie Semiconductor have been on par with the first-tier SiC chip factories in the world. For the third-generation semiconductor application industry, silicon carbide planar MOSFET technology is still a mainstream technology. Pincher’s third-generation planar gate SiC MOSFET technology features industry-leading HDFM specifications and low switching losses, as well as high efficiency and low emissions when operating at high temperatures. In 2021, Pincher Semiconductor already has a MOSFET product with the smallest Qgd x Rds(on) (switching figure of merit) in the world. In addition, Pinjie Semiconductor’s SiC MOSFET products have made a major breakthrough in the verification of new energy vehicle OBC applications, and have won tens of millions of orders from leading new energy vehicle companies. For new energy vehicles, IDC, photovoltaics, fans, light charging and storage and other fields, Pinjie Semiconductor has complete drive solutions and demo cases of typical applications for customers’ reference and help customers achieve rapid R&D introduction. Such as: 3000w totem pole PFC scheme, 65w fast input high voltage scheme, etc.

The Links:   QM50TX-H MG300J2YS45