Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

With the development of automotive Electronic technology, the demand for light weight and intelligence has also driven the large-scale application of Infineon Intelligent Power Devices (IPD) in vehicle body load drives. For loads with large inductance, such as wipers, blowers, fans, relays, etc., it is necessary to consider the impact of the energy generated when the load is turned off on the system, and the driving device cannot be broken down by this energy. This paper provides methods and tools for evaluating the measurement of inductive energy. In a well-defined application scenario, the generated clamping energy (ECL) during instantaneous turn-off is compared with the energy capability of the high-side device itself to ensure long-term IPD devices. Works reliably.

This article was written by Ren Baodong of Infineon and jointly with Hong Wei, Xue Yang, and Zhang Jiuqing of United electronic Automotive, and published in the eleventh issue of “Automotive Practical Technology” in 2022.

Summary

With the development of automotive electronic technology, the demand for light weight and intelligence has also driven the large-scale application of Infineon Intelligent Power Devices (IPD) in vehicle body load drives. For loads with large inductance, such as wipers, blowers, fans, relays, etc., it is necessary to consider the impact of the energy generated when the load is turned off on the system, and the driving device cannot be broken down by this energy. This paper provides methods and tools for evaluating the measurement of inductive energy, in a well-defined application scenario, the resulting clamping energy (ECL), compared with the energy capability of the high-voltage side device itself, to ensure long-term reliable operation of the IPD device.

1 Introduction

Introduction to the demagnetization process

Automotive applications increasingly require the ability to drive high-current, large-inductance actuators. In transmission control module (TCU) applications, commonly used actuators such as motors, solenoid valves (purification, air intake), etc.; in body control In the module (BCM), commonly used actuators such as wipers, relays or fans, water pumps, and oil pumps also show inductive characteristics. The easiest and most common way to drive these loads is to connect them to the output of a high-side switch, as shown in Figure 1 (the device’s integrated diagnostic and protection features are not shown in the block diagram).

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load
Figure 1 Block diagram and equivalent circuit of the high-side control module

When charging the inductive element during the conduction phase of the switch, the stored energy is related to the load current (IL) is related to the inductance (L) as follows:

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

After the switch is turned off, the load current will drop to zero, the previously stored energy plus VBATThe energy produced will be dissipated at the same time: when the energy is small, it will be dissipated into the load itself in the form of heat (RL); when the energy is large, most of the energy will be absorbed by the built-in clamping diode of the IPD, thus protecting the IPD chip and the load. In general, engineering can implement different techniques to reduce this dissipated energy applied to the interior of the IPD, such as by using freewheeling diodes or RC parallel branches. However, in addition to increasing the cost and complexity of the system, the above method will also prolong the closing time of the actuator (tF).

In some applications, such as fuel injector drives, PWM control valves, etc., there are strict requirements for closing time. Therefore, IPD has the function of active clamping, making it a very perfect solution. Typically, the clamping voltage of the IPD (VCL), the higher its off time tFwill be shorter. Also, energy is dissipated into the IPD built-in TVS during clamping, which is called the clamping energy (ECL), for high-power application scenarios, there are usually more energy shocks, which will generate repeated thermal stress in the device silicon, thereby affecting the device life and other functions.

2 Inductive load

2.1 Inductive load Ecl energy measurement in engineering application

The best way to evaluate actual load characteristics and obtain a value for the clamping energy dissipated in the high-side switch is through actual measurements. Of course, in addition to ensuring the accuracy of the measuring instrument, it is important to reproduce the operating conditions of the actuator as much as possible, so that it is closer to the actual application situation. As shown in Figure 2, for the clamp energy test, it is recommended to keep the load at the expected operating temperature in the test chamber for measurement.

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load
figure 2

The formula for the stored energy of an Inductor is:

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

The clamping energy is expressed as:

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

where VDand ILare the switching voltage and load current, respectively, tFis the time it takes for the load current to return to zero after it is turned off.

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

1) Nominal value determined by LCR measurement
2) Device characteristics are obtained from Infineon’s official data sheet

Now let’s look at a real example: using the mathematical functions of the common digital oscilloscope, it is easy to get the measured VDILand the product of their integrals, as shown in Figure 3.

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load
Figure 3ECLmeasure (RL=0.53Ω, LL=206uH, VBAT=12V, TA=25°C)2

Remark 2 The green C4 is the current shutdown waveform, the purple C2 is the negative voltage waveform of the out pin, and the blue F2 is the integration of C2*C4 during the shutdown time.

The result is as follows:

CVCL=38.2V
CIL=11.3A
CtF=92 microseconds
CECL=6.51mJ

In engineering applications, the current is usually directly equivalent to a linear function for approximate solution. The formula is calculated as follows:

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

If we compare the approximation with the measured value, it is clear that we can see the error is higher than 100%.

2.2 ECLTheoretical Model analysis and calculation

Separate the main components integrated in the IPD, consider the two states of ON and OFF, and its equivalent circuit is shown in Figure 4.

In the ON state, the load IL current is shown in formula (1):

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

τRis the time constant for the rise of the inductor current

ILIMis the limiting current of the IPD device itself, tONis the ON duration of the actuator. In addition to including the possible intervention of switching current protection in equation (4), we also take into account the fact that switching on for a short time does not give the load sufficient time to reach its state current.

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load
Figure 4 Equivalent circuit

2.2.1 Solving homogeneous differential equations

When the output is off, the equivalent differential equation of the circuit is

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

with iL(0)=ILSolving equation (5) for the initial conditions yields the dynamic equation for the inductor current.

The general solution of the first-order linear homogeneous differential equation is that the inductor current decays exponentially, and the decay speed depends on the electromechanical constant τ of the inductor itself. VBAT-VClamp=0, the differential equation to be solved is as follows:

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

The extraction feature equation is

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

The characteristic solution is obtained as

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

Then, the general solution is obtained as

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

Obtain the general solution, that is, the transient component, and continue to find the specific solution, that is, the steady-state solution.

2.2.2 Solving inhomogeneous differential equations

The dynamic current equation is

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

The three-factor method, the general form of the solution to a next-order differential equation with constant excitation is

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

According to this, the dynamic equation of the current obtained is expressed as:

Calculation and Analysis of Safe Demagnetization Energy of Automobile Inductive Load

make

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