Power supply design from EMC test rectification to product production details

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Power supply design from EMC test rectification to product production details [Copy link]

How to be comprehensive during actual product production testing, and what needs to be done before EMC testing?

What are the conditions for the EMC test rectification process? How to modify the PCB after the EMC test is completed? The processes involved in the product production line are helpful for improving production knowledge.

1. Preparation before EMC rectification test

1. Temperature rise test, 45℃ oven environment, input 90,264, transformer core, wire package does not exceed 110℃, PCB is within 130℃. For the specific values of other components, refer to the safety requirements. The most difficult to adjust the temperature is usually the transformer.

2. Insulation withstand voltage test DC500V, resistance greater than 100MΩ, primary and secondary voltage AC3000V time 60s, less than 10mA, production line can be AC3600V, 6s. It is recommended to use DC4242 voltage withstand voltage. The withstand current is set to 10mA. During the test, the test instrument alarms. Check the primary and secondary distances, the distance from the primary to the shell, and the distance from the secondary to the shell. It is better to draw the curtains in the test room so that the electric spark at the discharge position can be quickly found.

3. The impedance to ground should generally be less than 0.1Ω, and the test condition current is 40A.

4. ESD generally requires 4K for contact and 8K for air, and there is a problem with the resistance and capacitance model. Generally, the level will be increased to the highest 8K for contact and 15K for air. When applying ESD, if there is a discharge needle under the common mode inductor, the discharge needle will discharge. The ESD of the power supply will also spark between the heat sink and different components, which is generally a distance problem and PCB layout problem. If the power supply is damaged by ESD at 15K, you will know how much voltage your power supply can withstand, and you will have a clear idea when doing safety certification. If the customer requires a higher voltage, you will also know how to deal with it. Refer to EN 61000-4-2.

5. EFT has no problem with 2KV. Refer to EN 61000-4-4.

6. Lightning strike, differential mode 1K, common mode 2K, use varistor 14D471, with input large electrolytic, basically pass if there is no major problem with the wiring. I have encountered a situation where I could not pass the lightning strike, and the low-power 5W and 10W were broken. Electrolytic capacitors that can resist lightning strikes were used. The MOSFET was broken during flyback in the unipolar PFC. A diode was added in series with the electrolytic capacitor after the input bridge stack, and the capacitor absorbed the energy. In the case of 2K and 4K in the LED power supply, 4KV must use the form of varistor + GDT. Refer to EN 61000-4-5.

EFT, ESD, and Surge have levels A, B, and C. Generally, level A is required: interference has no effect on the power supply.

7. Low temperature start. Generally, the temperature range of cheap power supplies is 0-45℃, while the temperature range of expensive, industrial, or LED power supplies is -40℃-60℃, or even 85℃. At -40℃, the input NTC increases by N times, and the input electrolytic capacitor is obviously not enough. The ESR is very large, and it is also a bit dangerous to use a 500V MOSFET for PFC (the withstand voltage of the MOSFET decreases at low temperatures). Before, when the input was 90V, the output voltage jumped, or the LED flashed several times before it became normal. Increase the input capacitor capacity, reduce the NTC, increase the VCC capacitor, lengthen the soft start time, and increase the primary current limit (the input capacity is not enough, resulting in very low voltage and high current, triggering protection) from 1.2 times to 1.5 times. The VCC winding of the IC increases the auxiliary voltage by 2T; check whether the protection circuit is too extreme and is triggered by low temperature (such as PFC overvoltage is easily triggered).

After the basic performance and safety issues are resolved, the remaining issues are conduction and radiation. At this time, you can discuss the subsequent price with the customer and optimize the circuit yourself.

Confirm safety issues with safety engineers, and confirm with production line engineers whether the components on the subsequent PCB need to be repositioned, whether the production line is convenient to operate, etc. If there are problems with AI, reflow soldering, etc., adjust the components in time.

2. Suggestions for rectification of conduction and radiation

Conduction rectification details:

1. People have read a lot about conduction and radiation testing, and they have also talked a lot about it in forums. In fact, this is a money-making thing. If you spend a lot of money, you will naturally get it right, and the rectification will be quicker. There are only a few things that can be changed.

What is invisible here, and is particularly important, is the PCB. If you are skilled enough, you can find the wires on the PCB, cut them, and change the routing method to remove the 3dB margin.

2. Generally, when you see a laptop power adapter, there is an ugly lump at the part that connects to the computer. This is an EMI filter. The long distance from the outlet of the adapter to the laptop can be regarded as an antenna. Adding a filter can filter out the loss. Therefore, there is a filter inductor at the output end of the general switching power supply, and the effect is the same.

3. For input filter inductors, UU type is very useful for low power, and ring type and ET type are basically used for high power. If the company has a conduction laboratory or conduction instrument, you can go there to try it out if you have an idea. If you want to go to a third-party laboratory, it will be more painful, and you have to bring a lot of rectification materials. It is better to use high-conductivity 10K material for filter inductors, which has a good effect on conduction and radiation suppression. If the conduction is poor, you can change to 12K or 15K. If the radiation is poor, you can change to 5K or 7K.

4. The input X capacitor should be as small as possible, mainly because it takes up space. This needs to be adjusted in conjunction with the filter inductor.

5. Y capacitors. If there is no Y capacitor installed in the primary and secondary, or the Y capacitor is very small, the value of 150K-30M is usually floating, or it is out of the limit. Installing a 471-222 capacitor will be enough. The connection method of the Y capacitor directly affects the test data of conduction and radiation. Generally, the primary ground is connected to the secondary ground. There is also a primary high voltage connected to the secondary ground, or two Y capacitors are placed with the primary high voltage and the primary ground connected to the secondary ground. No one can tell before it is adjusted. Stringing magnetic beads on the Y capacitor is effective for above 10MHz, but not all. Everyone has different methods and ways to debug conduction and radiation. The models are different and the problems are different, so maybe my method is only suitable for me. Most of the non-Y solutions are made by changing the transformer, and the power is not easy to increase.

6. For MOSFET absorption, DS can be directly connected to a 221 at most, otherwise the temperature will be too high, generally 47pF, 100pF. For RCD absorption, a 10-47Ω resistor can be connected in series on C to absorb the spike. A 10-100Ω resistor can also be connected in series on D, and the MOSFET drive resistor can also be changed to within 100Ω.

7. For the absorption of the output diode, RC absorption is generally sufficient.

8. Transformer. Transformer has copper foil shield and wire shield. Copper foil shield is good for conduction, wire shield is good for radiation. As for primary wrap secondary, secondary wrap primary, and some other winding methods are all for better conduction and radiation.

9. For PFC flyback power supply, the input part needs to add differential mode inductor. Generally, rod inductor or yellow and white ring of iron powder core is used.

10. When rectifying conduction, try to reduce the 10-30MHz part to a 15-20dB margin, so that the radiation is easier to rectify.

The switching frequency is generally 65KHz. When looking at the conduction, you can see the 65K multiple position, which generally has a very high value.

In short: the conduction phenomenon can be regarded as the oscillation caused by the switching of the power device being amplified and displayed on the input line. To avoid the oscillation signal from going out, high-frequency oscillation must be avoided, or the high-frequency oscillation must be absorbed and dissipated so that it does not exceed the standard when displayed.

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led2015

Thank you for sharing. I feel that it is still not enough to do all of them in my experience.

秦天qintian0303

One is the plan, the other is experience. Safe distance is very important.

zgy_chenxin

Thank you for sharing!