LNK304 Power Supply

The need for a MOV depends on the differential surge level you are trying to meet. In my experience a surge level above 1kV one should be included with the input capacitance shown. At 2kV it is essential in design with a full bridge because the input diodes are then stressed at or slightly above their rating.

For the schematic posted as it has only a single rectifier diode then for surge levels above 500 V a MOV is a good idea.

I noticed 450 V capacitors so I'm assuming 265 VAC maximum input. That means 275 VAC MOV rating. As these clamp at ~700 V DC a second MOV (or TVS) may be used to ensure the DC bus voltage doesn't exceed the VDS spec of the LNK304. However I'd only expect to see a second clamping element at 2 kV surge levels or above (below this the energy is integrated by the input capacitors which limits the maximum voltage). For reference with no MOVand 47 uF input capacitor the peak voltage capacitor voltage with a 2kV surge is ~700 V for a 230 VAC input.

As for placement in the circuit. The MOV should be protected by a fuse as they fail short circuit. In the example the fuse would also need to be a fusible type as in the event of failure of the MOV the line voltage would appear across the 22 ohm resistor.

Hope that helps

Cheers PI-Chekov

Thanks a million.
The information you provided are really useful.

Hi I am having surge failure in my design at 1KV differential. i have attached the circuit. i am using LNK 306PN and 320V MOV. during test microcontroller is failing because. and ZCD I/O pin in the microcontroller also failing. kindly help me to solve this problem.

Hi rajakumar,

You may want to use a 10/20ohm resistor in series with the fuse to reduce the impact of surge voltage. You have to start for lower surge voltage level, let say 500V and increase by 100 to see if there is any effect on your circuit. if 20ohms is not enough until 1K level then you should increase this up until to hundreds ohm value.

Regards

Thanks for your help. in the same circuit i have connected a motor. which will take 3.5 Amps in normal condition. worst case it will take 6Amps. so i cant connect such high resistor. power rating of resistor also problem. i have done 500V surge there is no problem. during 1KV only i have problem. ist ok if i connect 320V mov for LNK 306? or i need to use 275V mOV?. or any other circuit is having problem.

Hi rajakumar,
275V mov would be more effective since it would clip the input voltage surge to a lower level. Just make sure surge energy rating is sufficient. You may also want to add output capacitor incase the controller protects and can still provide enough power. You may also want to add a high frequency filter to the input of your microcontroller just incase if microcontroller is sensitive to noise.
Regards.