LNK363 power supply design
I have built a 12V 200mA power supply using LNK363. The design is very similar to Figure 5 shown in LinkSwitch-XT datasheet. I have some issues when I performed some reliability tests.
Case 1:
When the bulk storage capacitor C2 is removed, the LNK363 device will blow and generate a spark. Are there solutions available to prevent the spark from occurring?
Case 2:
When the AC input is subjected to 3kV 1us pulse, the bulk storage capacitors C1 & C2 will have ringing of 300V amplitude. Is this a concern? Are there any solutions to counter this?
Thanks.
Comments
Thank you for the explanation.
Are there any ways to limit the drain-source voltage to <560V?
For universal input, what are the chances that the 700V drain-source voltage will be exceeded? Under what conditions can this happen?
Thanks.
The best way to limit the peak drain voltage is to minimize the transformer's leakage inductance and optimize the drain clamp circuit for minimum leakage spike at turn off. Depending on your design, you may be able to lower the VOR by changing the transformer's winding ratio. Use PIXLS software to determining the VOR and turns ratio. If your are concerned about surge, the best way is to use a MOV to clamp the surge voltage.
If you are using a design recommended by PI Expert or PI XLS and follow the data sheet's application section, the design will be solid. Of course if a surge voltage is high enough, would cause the drain voltage to exceed 700 Volts. If the transformer's leakage inductance greatly increases (due to manufacturing tolerances) could increase the peak drain voltage. This can be controlled by limiting the maximum leakage inductance in your Transformer Specification Drawing you provide the manufacturer.
Z. Cochrane
I am using a design that is recommended by PI Expert. Also, the transformer is made according to the specifications recommended by PI Expert. Noted on the MOV recommendation.
May I know the calculations to estimate the drain voltage?
Thanks.
The application note AN-16, starting on page 14, has a good explanation of the peak drain current calculation. You can find AN-16 on our website.
Z. Cochrane
Case 1: The failure of the device when C2 is removed is expected. If the power supply is enclosed and the spark does not exit the enclosure, this should be OK. If this application is an open frame, then you may have to add another capacitor in parallel with C2. If one opens (single point failure), the other will provide the necessary capacitance. I like to add that several manufacturers have used this design without having any safety issues.
Case 2: If the drain voltage does not exceed 700 Volts and if the voltage on C1 and C2 does not go negative, it is usually OK. Also make sure that the surge rating of the bulk capacitors is not exceeded (usually 2X the rated voltage). A MOV can be added across the line on the fused side of RF1. This will help reduce the amplitude of the surge that gets across the bulk capacitors.
Z. Cochrane