1. Problem description

Surge limit test: The product surge limit test fails at DM +6KV 90°, and the product has no output when it is powered on.

2. Reason analysis

Frist，conduct a power on test on the lightning strike limit defective products, and the products have no output. Unpack the power supply and turn it on again, check that there is no voltage across the electrolytic capacitor C1, measure the components between C1 and the AC PIN terminal, and find that the fuse F1 is open (as shown in the figure below). After replacing F1 with a good product, the product is powered on normally, so it is judged that the cause of the failure of the lightning surge limit test is the open circuit of F1.

Fuse open circuit test chart 1

Second，replace the defective product with a new fuse, simulate the test condition DM +6KV 90° and redo the surge limit test with the customer's feedback, detect the surge current flowing through the fuse, and use a series connection of about 0.1Ω resistor to detect both ends of the resistor The voltage is estimated according to I=U/R, and the waveform is shown in Figure 2 below. Estimation method: the time is taken as 25.4uS, the fuse I²t=(219/0.1)²*0.0000254=121.82A²S, after checking the fuse specification, the minimum I²t value within 10ms is 110A²S, the measured value exceeds the lower limit of the fuse specification, causing the fuse to blow.

  6KV test waveform diagram 2

3. Improvement measures

Method 1: Increase the fuse specification, increase the I²t value of the fuse, and ensure that the surge test does not blow the fuse

Method 2. During the lightning test, due to the varistor, a fuse can be placed in front of the varistor circuit and behind the circuit. A high-spec fuse is used before the varistor, and a relatively small-sized fuse can be used after the varistor. As long as the terminal fuse can meet the power-on surge of the product