Robust Automotive 40V Power MOSFETs Improve Automotive Safety

“ST’s state-of-the-art 40V power MOSFETs fully meet the mechanical, environmental and electrical requirements of automotive safety systems such as EPS (Electric Power Steering) and EPB (Electronic Parking Brake). These electromechanical systems must meet the automotive AEC Q101 specification, specifically, the low-voltage MOSFETs must withstand high temperatures and high current spikes.

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Summary

ST’s state-of-the-art 40V power MOSFETs fully meet the mechanical, environmental and electrical requirements of automotive safety systems such as EPS (Electric Power Steering) and EPB (Electronic Parking Brake). These electromechanical systems must meet the automotive AEC Q101 specification, specifically, the low-voltage MOSFETs must withstand high temperatures and high current spikes.

1 Introduction

Both EPS and EPB systems consist of two main components: electric servo unit and mechanical gear unit. The electric servo unit transmits the rotational motion of the motor to the mechanical gear unit for torque amplification and mechanical action. Electric servo units are two-phase or three-phase inverters implemented with power MOSFETs, as shown in Figure 1.

Figure 1. Servo Pack Topology for EPS and EPB Systems

The load in the picture is a motor, usually a permanent magnet brushless DC motor (BLDC), powered by a 12V battery.

2. Automotive requirements for power MOSFETs

40V power MOSFETs used in EPS and EPB inverters must meet all of the following requirements in order to qualify for AEC Q101 automotive qualification:

1. Very low switching loss and conduction loss

2. Large output current

3. The ratio of Ciss/Crss is small, and the EMI immunity is strong

4. Excellent avalanche resistance

5. Excellent overcurrent and short circuit protection

6. High thermal management and heat dissipation efficiency

7. Adopt stable SMD package

8. Excellent resistance to load dump and ESD

2.1. Parameter measurements of AEC Q101 power MOSFETs

We selected some competing products that meet the EPS and EPB system requirements and compared them with ST’s 40V automotive power MOSFETs. Table 1 lists key parameter measurements for ST’s STL285N4F7AG automotive 40V power MOSFET and its competitors.

Table 1. STL285N4F7AG and competitor’s parameter measurement comparison table

Since the operating voltage of the two safety systems is in the range of 12V-13.5V, and the nominal voltage of the power MOSFET is 40V, as long as the breakdown voltage (BVdss) is close to 46V, the Overvoltage due to parasitic inductance. In order to suppress the voltage drop during turn-on, the static on-resistance (RDSon) is preferably lower than 1mΩ. Only with small intrinsic capacitance and Rg can switching losses be minimized, enabling fast switching operations. The Crss/Ciss ratio is a very sensitive parameter that helps prevent any abnormal turn-on due to the Miller effect and allows better control of the di/dt and dV/dt rates, in conjunction with the body-drain diode Qrr reverse recovery charge and reverse recovery softness, which can significantly reduce the susceptibility of the device to EMI.

To meet the requirements of low power dissipation and electromagnetic interference, the STL285N4F7AG optimizes the capacitance ratio (Crss/Ciss). Figure 2 is a comparison chart of the capacitance ratio between STL285N4F7AG and competing products.

Figure 2. Comparison of Crss/Ciss Capacitance Ratio Measurements of STL285N4F7AG and Competitors

In addition, Figure 3 shows a comparison graph of the performance measurements of ST’s STL285N4F7AG body-drain diode and competing products.

Figure 3: Comparison of body-drain diode performance measurements for the STL285N4F7AG and the competition

Measurement parameters show that for a fixed di/dt value, the reverse recovery charge (Qrr) and recovery time (Trr) of STL285N4F7AG are both smaller than those of competing products. The benefits of this feature are summarized as follows:

– Low Qrr reduces the dynamic losses of the inverter at turn-on and optimizes the EMI characteristics of the power stage;

– Better Trr improves dynamic peaking of diode recovery voltage rise rate (dv/dt). Trr is a common cause of bridge failure when current flows through the body-drain diode during freewheeling.

Therefore, dv/dt is an important parameter to ensure latch-up tolerance, and the measurement results show that the dv/dt performance of ST’s products (Figure 4) is better than that of competing products (Figure 5).

Figure 4. dv/dt t measurements for STL285N4F7AG

Figure 5. Competitor dv/dt measurements

2.2. Short-circuit experimental performance test

We use a short-circuit experiment to measure and verify the stability of ST’s 40V automotive power MOSFETs in automotive safety applications. Electronic systems can short-circuit for a variety of reasons, such as the presence of moisture, lack of insulation protection, accidental contact of electrical components, and excessive voltage. Because short circuits are usually accidental, short circuits are rarely permanent, typically lasting a few microseconds. During a short circuit, the entire system, especially the power stage, must withstand multiple high current events. We did a short-circuit experiment with STL285N4F7AG and the test board, and the measurement results are shown in Figure 6:

Figure 6: Test Board

Follow the steps below to complete the experiment:

1) Pre-test the main electrical parameters with a curve measuring instrument;

2) The test board was heated to 135°C and a short-circuit pulse of 10 μs was applied twice with an interval of less than 1 s. The current limiter protection function is activated to do an experiment, and it is not activated to do an experiment.

3) De-solder the device and measure the main electrical parameters again to check the integrity or performance degradation of the power MOSFET.

The measurement results are shown in Figure 7.

Figure 7: STL285N4F7AG short circuit test

The actual current value measured during the short circuit event was in the range of 2000A with a pulse duration of 10μs. We performed ten tests with Tperiod = 5s. The STL285N4F7AG successfully withstood the short-circuit shock without any failure; however, when the current value was greater than 2400A, a failure occurred (Figure 8).

Figure 8. Current measurement when STL285N4F7AG fails (Id > 2400A)

3. Conclusion

Experimental data shows that ST’s state-of-the-art AEC-Q101 40V power MOSFET can easily meet the stringent requirements of automotive safety systems. Therefore, ST’s new trench N-channel device is the best choice for automotive EPS and EPB systems.