Let’s discuss the power generation mode of electric vehicles

When the power module works inside an electric vehicle, there are two modes;

One is the electric mode, where current flows from the battery at the DC end to the motor at the AC end, converting electrical energy into mechanical energy.

One is the power generation mode, which is just the opposite. At this time, the electric motor becomes a generator, and the current flows from the motor at the AC end to the battery at the DC end, converting mechanical energy into electrical energy.

Both modes require IGBT/SiC MOSFET modules because their core circuit is a three-phase full-bridge inverter circuit (rectifier circuit when generating electricity)

The discussion a few days ago was all about the electric mode. Today, let’s discuss the power generation mode.

This diagram separates the generator and the motor. In fact, the motor can be used as a generator. Therefore, they can share a motor and a full-bridge module.

Well, consider the power generation mode, then the current must flow from the load to the DC bus, that is to say, for the bus, the electricity generated by the generator flows to the battery after rectification, and the direction is from DC- to DC+.

Since DC- flows to DC+, the potential of DC- must be higher. From the figure, the current flows through the diode.

So what is the main energy loss of the circuit at this time? Obviously, it is the current I multiplied by the voltage drop VF. In other words, the voltage drop VF of the freewheeling diode (actually it should be called the rectifier diode at this time) determines the efficiency of the rectifier circuit.

So for power generation circuits, which one is more efficient, IGBT or SiC MOSFET?

Simple and crude, directly compare with the data sheet:

The above picture shows the freewheeling diode VF of BYD BG820 IGBT module, which is about 1.6V

The above picture shows the body diode Vsd of BYD BM840 SiC MOSFET module. Yes, you read it right, 4.6V. The body diode of SiC MOSFET is blown into slag in front of the additional anti-parallel diode of IGBT module.

In the previous article, we mentioned that SiC MOSFET is very efficient in the electric mode of electric vehicles. However, today, the VF in the power generation mode is beaten by Si IGBT. Is the SiC module naturally unsuitable for energy recovery in electric vehicles?

SiC module smiled and said, "I have a hidden trick up my sleeve. Today I will show you that its efficiency can surpass IGBT in electric mode, and its efficiency is also unmatched in power generation mode."

The unique skill of SiC MOSFET is: Third Quadrant conduction

Looking at the picture, if the MOSFET is turned on during rectification, then at this time, the MOSFET can be regarded as a resistor. At this time, the current flows from Source to Drain, and the Rsdon and Rdson of the reverse flow are approximately the same (even when the channel is fully turned on, due to the shunting of the body diode, the impedance is lower than the forward conduction!)

After calculation, Vsd≤Rdson multiplied by Id=2.7mΩ×360A=0.972V!!!

At this time, the voltage drop of IGBT is 1.4V!

In other words, SiC MOSFET utilizes the characteristics of conduction in the third quadrant to achieve a voltage loss far lower than that of the IGBT module and a rectification efficiency far higher than that of IGBT+FWD.

Why? To understand this, you need to understand the structure of DMOS:

We can see that when DMOS is fully turned on, the on-resistance of the entire MOSFET is completely determined by the various parts in the figure, and each part has a resistance characteristic. Therefore, you can regard the fully turned-on MOSFET as a small resistor (Rdson=Rsource+Rchannel+Rjfet+Rdift+Rdrain). Therefore, its voltage drop is only related to the current. Under low current, this advantage is huge.

Let's look at the diode again

Due to the existence of the built-in electric field, there will be a minimum forward bias voltage Vbi even in the forward biased case. That is to say, this voltage must be exceeded to form current. Therefore, the output characteristic curve of the diode is particularly disadvantaged under small current conditions. Even if the current is close to 0, VF is still that much.

As shown in the figure, when the current is close to 0A, the VSD of the MOSFET is almost 0 (Vsd≤I×Rdson), while the VF of the diode is close to 0.7V.

When the current is 200A, the Vsd of the SiC MOSFET is less than Vds by less than 0.5V, while the VF of the diode exceeds 1V.

Only when the current reaches 600A, the VF of the diode is almost the same as the Vsd of the SiC MOSFET.

For electric vehicles, the rated power generally does not exceed 200kW. Even if it is equipped with a 400V platform, the current generally does not exceed 600A. If it is an 800V platform, the current basically does not exceed 300A. In other words, in the field of electric vehicles, the third quadrant conduction characteristics of SiC MOSFET completely crush the VF characteristics of the diode.