Choosing right varistor

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We are using LYT4227 IC for 50watt and 30watt street light led drivers.
However Power fluactions are normal in India. Our  driver stop working after some of days. We have analyzed the driver and found that. Varistor and Fuse are blown in 9 out of 10 faulty driver. Other all parts are working well.

So we checked the competitors’ LED driver to check out what kind of technology they are using. And we have found that they using 385V varistor with 2A fuse. That’s why they are working good in worst power conditions.

So my question is that can I use 385V Varistor with 3A fuse with LYT4227 IC? Can LYT4227 IC handle the surge 385VAC ?

Let me know the possible the maximum voltage rating of Varistor.

I am using following components in 50W driver.

  • 3.14 A Slow Blow Fuse (Series)
  • 14mm 80J 300V Varistor
  • 1.5M + 1.5M Resistors
  • 305V 220nF X Cap
  • 12mH Filter Chawk (U10)
  • DB207S (2A 1000V Bridge Rectifier)
  • 2 x 4.7uF 450V Capacitor for 10KV surge protection (With series of US1M & 470K)
  • 220nF 400V DC Cap (105C)



Just for clarification, is this the one failing - 14mm 80J 300V Varistor? If you have to use a 385VAC varistor, then please verify its clamping voltage.

It might have a 1kV clamping voltage and you may need to use a larger bridge rectifier.

The 2x 4.7uF capacitor will help clamp the voltage during surge. But you have to verify if LYT4227 can survive surge.

The breakdown voltage rating of LYT4 is only 725V. With a 385VAC varistor with 1kV clamping voltage, it won't help the device during surge. Only the e-cap will prevent the voltage.

As I mentioned earlier mine drivers blow up with only two components (Fuse and Varistor). It is because some time driver has to handle above 385VAC for some time. So It is not about surge.

I found following things

  • There is no surge or spike problem. However  driver can handle it easily (320VAC 14mm Varistor, 15mH Filter Choke, 2 x 4.7uF 450V Capacitor) 
  • It might be floating neutral or Faulty neutral problem because of this some driver has to handle 400VAC for some times (15 minutes or more).
  • I have checked some application of LYT4227 (DER-396, DER-350) and found that they are using 400V MOSFET to in dimming application. This MOSFET cut off the power supply when it needed. 
  • In some of LNK419 applications (DER-318) they are using MOSFET to protect from surge also. This MOSFET cut off the power supply at over voltage condition.
  • Some of competitors are using 420V Varistor in their circuit and then succeeds proving warranty. (Some of them are using main 900V MOSFET, one of them is using main 625V MOSFET-Checked twice). 
  • One of competitor is using only 625V MOSFET with 420VAC Varistor. (No Input filter choke, One 100nF X Cap, No Input Inductors, No other protection).Still this driver are performing better than mine. 

So My question is that 

Can I use 420V Varistor into driver with extra 400V MOSFET cutoff protection?  

Extra circuits deactivate the 400V MOSFET when input voltage is more than 310 V. And cutoff the total supply to IC. Which provides protection upto 1125V DC (400V MOSFET + 725 V IC MOSFET).


Using the protection circuit implemented in DER-396 will definitely help because during surge, it puts a series resistance to the line which limit the surge current.

However, please consider alternative approach as well and decide which makes more sense to you.

RDR-290 has an RCD clamp circuit for surge protection up to 2 kV.


Thanks for your quick answer.

I have checked DER-396 and seen that it can protect from only surge but not from continuous 400V-420V.


In mine condition there is no issue of surge. But it is issue of overvoltage protection up to 420V.
In short, I do not want to blow any part(FUSE or MOV) of my driver during overvoltage (continuous 400V-420V).
Driver must be work again when voltage comes to normal (continuous 180-320V).
I just want put extra circuit which can cut the power supply to IC during overvoltage.

I have attached my current circuit layout.


Please let me know If any changes required.
Also I want to change Varistor to 420V ac. So please suggest me the extra circuit with MOSFET

I 75W 36V 2.png259.05 KB

For the schematic your provided, if the 320VAC-rated MOV does not get shorted when it is subjected to 420VAC input, then the circuit looks fine, subject to surge verfication.

You can utilize the circuit used in DER-396 for surge protection. You can remove the active damper control circuit (Q1,R6) and short D1.