Sat. Dec 3rd, 2022

As cars become popular consumer goods and enter thousands of households, whether it is a traditional vehicle or a new energy PHEV, the long-term safety and reliability of the vehicle has become an important indicator to measure the quality of the car. To ensure the safety and reliability of automobiles, in addition to the need to pay attention to the certification and selection of automotive Electronic components, the protection of automotive electronic functional modules is also becoming more and more important.

As cars become popular consumer goods and enter thousands of households, whether it is a traditional vehicle or a new energy PHEV, the long-term safety and reliability of the vehicle has become an important indicator to measure the quality of the car. To ensure the safety and reliability of automobiles, in addition to the need to pay attention to the certification and selection of automotive electronic components, the protection of automotive electronic functional modules is also becoming more and more important.

In the protection of automotive electronic function modules, in addition to electrostatic protection (ISO10605), the most important thing is the protection of electrical transient interference of road vehicles, which is the test standard ISO 7637-2 and Enhanced version of ISO 16750-2 standard.
  
First, let’s take a look at the pulses defined by these two standards

“Load Dump” and Protection Selection in Automotive Electronic Design

“Load Dump” and Protection Selection in Automotive Electronic Design

Among all the pulses in the automotive electronic system, the load dump is a common and more harmful phenomenon. So what is the difference between ISO 7637-2 and ISO16750-2 for load dump testing?
  
ISO16750-2 is more stringent than ISO7637-2. In the load dump test, the new standard requires 10 tests in 10 minutes, with a 1-minute interval between each test. The old standard requires only one load dump test.
  
In most newer alternators, the voltage magnitude of the load dump has been suppressed (clamped) by installing Transient Voltage Suppression diodes (TVS).
  
For load dump protection, the most effective way is to use a transient suppression diode (TVS) for clamping protection.
  
Before discussing the protection of 5a, 5b, let’s see if there is a difference between the two pulse energies.
  
ISO 7637-2 Pulse 5a-12V TPSMC27A load dump test

“Load Dump” and Protection Selection in Automotive Electronic Design

“Load Dump” and Protection Selection in Automotive Electronic Design

ISO 7637-2 Pulse 5b-12V TPSMC27A load dump test

“Load Dump” and Protection Selection in Automotive Electronic Design

“Load Dump” and Protection Selection in Automotive Electronic Design

The above test results can be analyzed, from the load dump generator (or engine) side, in fact, there is no difference in the energy size of the 5a and 5b waveforms. When the TVS to be tested operates and the strings are under the same load condition, the current flowing through the TVS is the same, and the clamping voltage of the TVS is also the same.
  
However, in terms of load dump voltage waveform, 5b is clamped (or regulated) compared to 5a waveform, which causes the voltage on the load end to be much lower than the waveform of 5a.
  
Therefore, if 5a, 5b load dump protection is performed, from the perspective of preventing the protected device from being damaged by the load dump energy, the selection of the protection device should be consistent.
  
However, in practical applications, when load dump occurs, it is hoped that the TVS will act and discharge the energy to the ground. Then, under the different waveforms of 5a and 5b, the actual energy of the protection device TVS will be very different. Because only when the TVS is turned on, will a large current flow through the TVS, so under different load dump test waveforms, the actual energy that the TVS bears will be different.
  
Before TVS selection, first understand the experimental voltage in two modes:

“Load Dump” and Protection Selection in Automotive Electronic Design

In different experimental modes, the selection of TVS will also be different.
  
Next, we only discuss the Mode 3 12V system. In mode 3 conditions, the test voltage is up to 16V, so the Vr (reverse cut-off voltage) of TVS usually needs to be higher than 16V.

“Load Dump” and Protection Selection in Automotive Electronic Design

For ISO16750-2 12V system pulse 5a Us=101V, R1=1Ω, td=400ms
  
Choose TVS SLD8S24A, then whether TVS can meet 5a test
  
The current that the TVS withstands after it is turned on:
  
Ipp=[(Us+Ua)-Vbr]/(R1+Rd)
  
Vbr: TVS reverse breakdown voltage
  
Vc: TVS clamp voltage
  
Rd: Impedance when TVS is turned on – by translating the ordinate of the TVS V/I curve to Vbr, a linear function curve will be obtained, Rd=(Vc-Vbr)/Ipp
  
Check the Littelfuse SLD8S24A specification, calculate Rd=(38.9V-26.7V)/180A=0.07Ω
  
Calculate the current flowing after the TVS is turned on Ipp=[(101V+12V)-Vbr]/(1Ω+0.07Ω)=(112V-26.7V)/1.07Ω=79.7A 400ms
  
Calculate the energy passing through the TVS (the waveform is nearly sine wave, you can apply 1/2 x I²t x Rd) W=1/2 x(79.7A)²x td x Rd=1/2*6352.09A²x 400ms*0.07Ω=88.9J (actual turn-on time is lower than td 400ms)
  
Calculate the maximum energy that TVS can withstand, check the specification and know that 8.3ms corresponds to Ifsm is 1000A
  
TVS energy Wtvs=(Ifsm)²x tx Rf=(1000A)²x 8.3ms x 0.018Ω=149.4J
  
Rf: TVS forward conduction impedance, can pass Vf/100A=1.8V/100A=0.018Ω
  
100A is the current when testing Vf as defined in the specification
  
By comparison, it can be known that SLD8S24A can meet the pulse 5a test
  
Verified by SOA test, SLD8S24A can meet the requirements

“Load Dump” and Protection Selection in Automotive Electronic Design

In addition to TVS commonly used for pulse protection in automotive electronics, MOVs are also common protection devices.
  
Taking Littelfuse’s AEC-Q200-compliant AUMOV selection as an example, in a 12V system ISO16750-1 5a, Us=101V, Ua=12V; R1=1.5Ω (setting), td=400ms
  
1: The current that AUMOV withstands after it is turned on:
  
Ipp=[(Us+Ua)-Vnom]/(R1+Rd)
  
Vnom: The varistor voltage of AUMOV is 1mA
  
Vc:AUMOV clamp voltage
  
Rd: The impedance when the voltage is clamped at Vc when the AUMOV is turned on – you can check the AUMOV specification, take V10E17AUTO as an example; Vc=53V Ipk=5A
  
Rd=(Vc-Vnom)/Ipp
  
Check the specification and calculate Rd=(53V-27V)/5A=5.2Ω
  
Calculate the current flowing after turn-on Ipp=[(101V+12V)-Vnom]/(1.5Ω+5.2Ω)=(112V-27V)/6.7Ω≈12.7A 400ms
  
Calculate the energy passed on AUMOV (apply 1/2 x I²t x Rd) W=1/2 x(12.7A)²x td x Rd=1/2 x 187.69A²x 400ms x 5.2Ω≈167.7J (the actual on-time is low at td 400ms)
  
If the energy flowing through the AUMOV of 5a is converted into a pulse with a width of 40ms, calculate the current size
  
½x I²x 40ms x(R1+0.9Ω)=167.7J;I²=3493.75A²;I≈59.1A
  
Calculate the maximum energy that V10E17AUTO can withstand 400ms
  
1: Check the specification to know that 2ms (square wave) corresponds to Wtm=6.5J—Wtm represents the maximum 2ms Joule energy value that the MOV can withstand
  
Calculate the on-resistance Rtm of AUMOV when it is subjected to the maximum Wtm;
  
Check the specification book curve, the corresponding maximum current is 60A when the curve is 2ms;

“Load Dump” and Protection Selection in Automotive Electronic Design

Wtm=I²x tx Rtm;Rtm=Wtm/(I²x t);Rtm=6.5J/(60A²x 2ms)=0.9Ω
  
2: Check the specification book to see that the load dump energy of 40ms width is 25J;
  
Calculate the load dump current of V10E17AUTO 25J=1/2 x I²x 40ms x Rtm;I²=50J/(0.04sx 0.9Ω);I²=13.88 X10²A²;I≈37.3A
  
Under the same load dump width, it can be seen that the 5a current 59.1A that the MOV needs to withstand for conduction is much higher than the nominal load dump current 37.3A in the V10E17AUTO specification
  
So if it is replaced by the AUMOV V20E17AUTO model, can it be satisfied?
  
Calculate the maximum energy that V20E17AUTO can withstand 400ms
  
1: Check the specifications to know that 2ms (square wave) corresponds to Wtm=35J—Wtm represents the maximum 2ms Joule energy value that the MOV can withstand
  
Calculate the on-resistance Rtm of V20E17AUTO when it withstands the maximum Wtm;
  
Check the specification book curve, the corresponding maximum current is 300A when the curve is 2ms;

“Load Dump” and Protection Selection in Automotive Electronic Design

Wtm=I²x tx Rtm;Rtm=Wtm/(I²x t);Rtm=35J/(300A²x 2ms)≈0.19Ω
  
2: Check the specification book to know that the load dump energy of 40ms width is 100J;
  
Calculate the load dump current of V20E17AUTO 100J=1/2 x I²x 40ms x Rtm;I²=200J/(0.04sx 0.19Ω);I²≈2.63 X104A²;I≈162A
  
Under the same “load dump” width, it can be seen that the 5a current 59.1A that the V20E17AUTO needs to withstand to turn on is much lower than the nominal “load dump” current of the V20E17AUTO specification of 162A
  
Through experiments, it can be seen that V20E17AUTO can meet 10 Load Dump tests:

“Load Dump” and Protection Selection in Automotive Electronic Design

“Load Dump” and Protection Selection in Automotive Electronic Design

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