Electric compressor design-SiC module, compressor

The compressor is a part of the car air conditioner. It compresses the refrigerant into high-temperature and high-pressure gas, which then flows through the condenser, throttle valve and evaporator for heat exchange to achieve heat exchange between the inside and outside of the car. Traditional fuel vehicles use the engine as the power and drive the compressor through a belt. New energy vehicles are separated from the engine and powered by batteries. The inverter circuit drives the brushless DC motor, which drives the compressor to rotate and achieve the heat exchange function of the air conditioner.

The electric compressor is a core component of thermal management in electric vehicles. In addition to improving the environmental comfort (cooling and heating) in the vehicle cabin, it plays an important role in the temperature control of the electric drive system, which is crucial to the battery's service life, charging speed and cruising range.

Figure 1: The electric compressor is a core component of thermal management in electric vehicles

Electric compressors need to meet increasing demands, including low cost, smaller size, less vibration and noise, higher power levels, and higher energy efficiency. These demands are inseparable from the design of compressor drive circuits and the selection of excellent devices.

The functions of the electric compressor controller include: driving the motor (inverter circuit: including ASPM modules or discrete devices equipped with gate drive, voltage/current/temperature detection and protection, power conversion), communicating with the host (CAN or LIN, receiving start/stop and speed signals, sending operating status and fault signals), etc. ON Semiconductor has corresponding solutions in each circuit (Figure 1). In the previous chapter, we discussed the application of ON Semiconductor ASPM module solutions in electric compressors. This article mainly discusses SiC MOSFET discrete solutions.

Figure 2 Electric compressor drive circuit control block diagram

Advantages of SiC MOSFETs

In the previous chapter, we explained that ON Semiconductor ASPM power modules have great advantages over discrete devices. If SiC MOSFET can be put into ASPM modules, it is the best choice. Before the mass production of SiC MOSFET ASPM modules, SiC MOSFET discrete devices have become the choice of many electric compressor development customers due to their unique advantages.

Table 1: Comparison of physical properties of SiC and Si devices

1. Advantages of SiC MOSFET Materials

   
   

• 10 times the dielectric breakdown field strength of Si devices: smaller wafer thickness and Rsp, smaller thermal resistance

• 3 times higher thermal conductivity: lower thermal resistance and faster electron transfer

• 2x faster electron saturation speed: faster switching speed

• Better thermal characteristics: higher temperature range

2. Less loss and higher efficiency

Taking ON Semiconductor's latest generation of IGBT AFGHL40T120RWD and SiC MOSFET NVHL070N120M3S suitable for 800V platform electric compressor applications as examples, the turn-on loss is evaluated based on the I/V curve. When the current is less than 18A, the on-state voltage drop of SiC MOSFET is smaller than that of IGBT, and the operating current of the electric compressor will always be within the 18A range when it is on the road. Even when operating at the limit current (for example, when fast charging, the compressor dissipates heat for the battery), the effective value is close to 20A, and during the entire sine wave cycle of the current, the turn-on loss of SiC MOSFET is no worse than that of IGBT.

Figure 3: Comparison of SiC and IGBT turn-on characteristics

In terms of switching loss, SiC MOSFET has obvious advantages. Although there are some differences in the test conditions of the specification, it can be seen that the switching loss of SiC MOSFET is much smaller than that of IGBT.

Table 2: Comparison of switching characteristics between SiC and IGBT

We performed efficiency simulations using IGBTs and SiC MOSFETs with similar current specifications. At maximum power, SiC can also effectively improve system efficiency, especially in high-frequency applications.

Figure 4: Efficiency comparison of IGBT/SiC MOSFET with similar specifications in motor applications

3. Suitable for high frequency applications

SiC MOSFET is a unipolar device with no tail current and a much faster switching speed than IGBT. This is why SiC MOSFET is more suitable for higher frequency applications than IGBT. Higher drive frequencies (such as 20kHz or above) can effectively reduce the noise of the motor and improve the response speed and dynamic anti-interference ability of the motor system. In addition, higher frequencies will also reduce the harmonic distortion of the output current and effectively reduce the loss of the coil in the motor, thereby improving the overall efficiency of the compressor.

4. Reduce dead time

In motor applications, in order to make the switch work reliably and avoid the upper and lower bridge arms being directly connected due to the turn-off delay effect, it is necessary to set the dead time tdead, that is, the time when the upper and lower bridge arms are turned off at the same time. Since the switching time of SiC MOSFET is short, a smaller dead time can be used in practical applications to improve the problems of large dead time, large output waveform distortion, and low driver output efficiency.

Problems and solutions to be considered in the use of SiC MOSFET

1. Choice of driving voltage

From the I/V curves under different driving voltages, we can see that Rdson decreases as the driving voltage increases. This means that the higher the driving voltage, the smaller the conduction loss. However, the voltage resistance of the chip gate is limited. For example, the driving Vgs voltage range of NVH4L070N120M3S is −10V/+22V. During the SiC MOSFET switching process, Vgs will also be affected by high dV/dt and stray inductance, and some voltage glitches will be superimposed. Therefore, it is necessary to leave a certain margin for Vgs.

Figure 5: IV curves at different Vgs

2. Low threshold voltage Vth problem

SiC MOSFET (especially planar type) has a typical threshold voltage Vth in the range of 2V-4V, and Vth will further decrease with increasing temperature. On the other hand, in the half-bridge application circuit, due to the high dV/dt of the SiC MOSFET switching process, the current generated by the Cgd of the other half-bridge SiC MOSFET flows through the drive resistor, generating a voltage on Vgs. If this voltage is higher than Vth, there will be a risk of mis-conduction, resulting in direct conduction of the upper and lower bridges. Therefore, it is necessary to increase the negative voltage on the drive. As can be seen from the figure below, increasing the negative voltage can also effectively reduce the turn-off loss and further improve the system efficiency.

When using ON Semiconductor's third generation SiC MOSFET, we recommend using a +18V / -3V power supply.

Figure 6: Comparison of switching losses at different turn-off voltages

Figure 7: Vth-Temperature Characteristic Curve

3. Limited short-circuit capability

Compared with IGBT, SiC MOSFET has a very small die size and a high current density. When a short circuit occurs, it is difficult to conduct away the heat generated by the short circuit in a very short time. In addition, SiC MOSFET will not show a sharp saturation behavior when the current is too large (unlike IGBT). When a short circuit occurs, the current can easily reach more than 10 times the rated current rating, which is much higher than the operation of IGBT.

Therefore, the short-circuit tolerance time of SiC MOSFET is relatively short, and some products are less than 2us. Fast detection and fast shutdown are essential for the reliable operation and long life of SiC MOSFET. Driver chips with desaturation function (desat) can cope with this situation. By setting the response time of desat protection to less than 1us, it is possible to effectively deal with short circuits that may exist during the operation of the electric compressor.

Selection of SiC MOSFET driver chip

In electric compressor applications, it is necessary to deal with the power requirements of the lower bridge and the upper bridge of the three-way bridge, and it is not easy to add a negative power supply. In this case, it is recommended to use a dedicated SiC MOSFET driver chip NCV51705 that can generate negative pressure by itself, with desat protection, undervoltage protection UVLO and overheating protection functions. The basic functions are as follows:

• Source/ Sink Current: 6A/6A

• Desat Protection

• Adjustable negative voltage output: -3.4V / -5V / -8V

• Adjustable undervoltage lockout UVLO voltage

• 5V reference voltage output (to power other devices, such as isolation chips)

• Overheat protection

The recommended application circuit is as follows (the lower bridge does not need to be isolated)

Figure 8: NCV51705 half-bridge application circuit

ON Semiconductor's automotive-grade SiC MOSFET discrete devices

ON Semiconductor has a wide range of SiC MOSFET products that can cover all discrete electric compressor solutions on the market. The following are product models suitable for 800V platform electric compressors.

Figure 9: Some 1200V SiC products from ON Semiconductor (electric compressors)

Figure 10: Onsemi SiC MOSFET product line

Conclusion

Although there are some challenges in the application of SiC MOSFET in electric compressors, through reasonable design and technology selection, the driving frequency can be effectively increased, the system noise can be reduced and the efficiency can be improved, which will ultimately help increase the driving range of electric vehicles.