Load dump test, how does a car 24V system pass the waveform 5a test?

Whether it is a traditional fuel vehicle or a new energy vehicle (PHEV), the long-term safety and reliability of the vehicle have become an important indicator of vehicle quality. As we all know, the output of the automotive power supply system is extremely complex and is easily affected by temperature, working conditions and other conditions, and the output voltage of the battery or generator will fluctuate. In addition, automotive systems that use loads such as motors, solenoid valves, and fuel injection will introduce spike noise and several transient surge voltages into the power line or signal. Among all the transient overvoltage phenomena in the vehicle power supply environment, the load dump transient overvoltage phenomenon is an electrical hazard that requires great attention.

Differences between ISO 16750-2 and ISO 7637-2

In order to allow automobile manufacturers and their suppliers to create highly reliable in-vehicle products within the specified range, the International Standards Organization has developed a series of standards to ensure the safety of automobiles. ISO 7637-2 and ISO 16750-2 are automotive electronic testing standards often mentioned in the industry. What is the difference between them? By referring to the ISO 16750-2 (2012) and ISO 7637-2 (2004) international standard documents, as shown in the following figure:

As can be seen from the figure, the load dump test voltage required by the ISO 16750-2 standard (hereinafter referred to as the new standard) is higher than that of the ISO 7637-2 standard (hereinafter referred to as the old standard):

5a, 5b test: In the 12V system, the old standard test voltage requires 65V-87V, and the new standard test voltage requires 79V to 101V;

In a 24V system, the old standard test voltage requires 123V-174V, and the new standard test voltage requires 151V-202V;

In addition to the test voltage increase, another most significant difference is that the new standard load dump test requires a minimum of 10 pulses for 10 minutes with a 1-minute interval, while the old standard load dump test requires only one pulse. It can be seen that ISO 16750-2 is more stringent than ISO 7637-2.

On-board 24V system power supply load dump overvoltage protection solution

Maximum voltage, internal resistance and duration are the main factors affecting load dump. How to control the combined impact of these three factors is the key to load dump transient overvoltage protection. For the load dump transient overvoltage phenomenon, practice has proved that the most mainstream and effective way is to use TVS transient suppression diodes for protection. To this end, Dongwo Electronics, a professional high-quality circuit protection device manufacturer and protection solution service provider, has designed a classic protection solution for the vehicle-mounted 24V system power port load dump transient voltage phenomenon. The solution diagram is shown below:

From the diagram of the Dongwo vehicle-mounted 24V system power supply load dump overvoltage protection solution, it can be seen that the solution includes three protections: overcurrent protection, reverse voltage protection, and overvoltage protection.

1) For PTC overcurrent protection, not only the working current and withstand voltage value need to be considered, but also the temperature factor. Dongwo Electronic Technology recommends that the 24V system should use a PTC resettable fuse with a withstand voltage value of 33V. If the test load dump data is strict, Dongwo Electronic Technology recommends the use of plug-in PTC model: DWP60-300F.

2) There are different opinions on the design position of the anti-reverse connection diode. If the test surge is strict, it is recommended to place it at the back end of the TVS to avoid damage due to excessive impact current during the test. Generally, a Schottky diode is selected for a small current. The Dongwo low voltage drop series Schottky diode model SS56 can be selected.

3) Transient suppression TVS diodes are mainly used to protect against various overvoltage waveforms and load dump shocks. The selection range here is relatively large. For the protection of the vehicle's 24V system power port, Dongwo Electronic Technology recommends the use of automotive-grade TVS tube SM8S36CA, bidirectional, 36V, DO-218AB, 6600W, 114A. It is worth noting that the lower the reverse cut-off voltage of the TVS tube, the lower the clamping voltage, and the more it can protect the back-end DC-DC chip. Therefore, the selection of the back-end power chip also needs to be planned in advance. If a 60V withstand voltage power chip is selected, it can pass the test.

The above are the selection principles for the three devices. Of course, customers can also make slight adjustments based on the test indicators. For details on the specific parameters of the devices DWP60-300F, SS56, and SM8S36CA, please refer to the product specifications of the corresponding models of Dongwo Electronics.

Dongwo SM8S36CA, a TVS tube dedicated to load dump protection, protects cars

TVS tube manufacturer Dongwo Electronics has a professional EMC test laboratory. Dongwo Electronics launched SM8S36CA, a dedicated device for load dump transient overvoltage protection, which can easily pass the ISO 16750-2 24V system Pulse 5a Us=202V, Ri=3Ω, td=350ms test. The test steps are as follows:

The first step is to use an oscilloscope to capture the pulse 5a waveform:

It can be seen that the maximum output voltage Us=202V, td=350ms.

The second step is the on-site test environment, as shown below:

Step 3, test parameters: Ri=3Ω, td=350ms, Ua=28V

Step 4: First test output graph: Clamping voltage Vc=51.25V

Step 5, the last test output graph: clamping voltage Vc = 53.12V

In summary, Dongwo Electronics' automotive-grade TVS diode SM8S36CA can easily meet the needs of load dump protection under the test conditions of pulse 5a Us=202V, Ri=3Ω, td=350ms. Dongwo Electronics' SM5S series, SM6S series, SM8S series, and SM8T series TVS diodes are products specially designed for load dump protection applications, ensuring automotive safety.