Over voltage (OV) protection triggers at different values

Posted by: Radko Stoyanov on 02/27/2018

Dear All,

Our team is developing a 30W offline power supply, intended for operation within an extended voltage range of 230VAC to 530VAC. The design we are testing is similar to the solution presented in DI-124, with the core difference being the use of TOP series (more specifically TOP264KG), rather than the LinkSwitch series. So far, the circuit operates mostly as intended. Regardless, there is an occurring problem that I was not able to resolve.
The problem is related to the Overvoltage (OV) Protection of the TOP264KG. Currently the V pin is connected to the input DC voltage with five series connected resistors with a total value of 9MΩ – the protection should be triggered at around 1000VDC. During test however, the protection triggers at around 950V when the circuit is at about 5% load and at around 1050V when the circuit is at about 100% load. Although the design operates even at the upper voltage limit the drain source voltage of the cascode transistor is already too close to its breakdown point.
Based on the datasheet and more specifically the internal structure and the description of the operation of TOP264KG, I was not able to find relationship between the OV protection the load current. I would like to keep the OV feature and fix it at a rather stable point.
Could you please help me to figure out a reasoning and solution to this predicament?

Thank you all in advance!

Best Regards,
Radko

Comments

Hi Radko,
Regarding triggering at 950 and 1050V, I think this is pretty close since you are still within 5% of your OVP setting. Usually for the Vpin, this is programmed to be in the worst case meaning in the maximum allowable Vdc input that would ensure your Vds is still below the rated voltage. So, if 1050V is close to the rating of the Vds then you set Vpin resistor lower than 9Mohms. I suggest you program this 20% of your maximum input which is 900Vdc.

Regards.

you have a 700V rated fet in the TOPswitch use a 700V fet to give you closer to 1400V total capability...

I think Radko is using a lower voltage rating fet due to cost.

Thank you both for your responses, I am currently considering both – adjusting the voltage protection for a lower value and/or updating the transistor for higher voltage.