High side rectification with MP6908

AC-DC Power Converters
1

I am using the MP6908 controller in designing a Flyback, which output voltage is 25V.
The maximum voltage across the SR mosfet is 150V. Frequency is around 100kHz. The SR mosfet I am using is a 200V, <40mohm device.
In order to have 9V gate output I am following the datasheet indications of Fig. 6A by connecting the HVC pin to the SR mosfet drain through a 1000ohm resistor.
Despite this, the controller is overheating. If I use a higher value resistor (not reccomended by the datasheet though), the resistor itself dissipates too much. On the other hand, if I go down to the minimum recommended value (300ohm) the controller itself reaches very high temperatures.
Is this excess of heat normal in this application with this voltage level? It is still within the recommended values window. Do I need to select a high power resistor to supply safely the controller or there is any trick you can suggest me?
By the way, using a diode in series with the resistor does not help.

Thank you

2

They are also going to want to know the switching frequency of the flyback is and what mosfet it is driving. I used this part once it worked well.

3

Thank you, I edited the topic with this information.

4

Hi m.checchinato,

Welcome to the MPS Technical Forum! We will look into your issue and provide an update soon.

5

Hi M.Checchinato:

Can you please share with us the SR MOSFET that is being driven by the MP6908. Also please share with us the line voltage of the AC input Line and the turns ratio of the Flyback transformer and the Output Load current of the 25V Output rail…

Thanks,

6

Hi and thanks for your help,

the SR mosfet used in this project is an STB40NF20.
Primary side rectified voltage is variable in the range from 400 to 800V and the turns ratio of the transformer is 6.66.
The 25V output load current is about 5A.

I also tried using a different connection of the device (HVC connected to VSS), that solves the overheating issue but only enables to have 5V at the mosfet gate. This voltage level anyway is not enough to drive optimally the mosfet and the overall result is worse than connecting the pin with the mosfet drain.

Thanks
Michele

7

Hi m.checchinato:

With the 6.66 turns ratio, the secondary winding can be 120V at 800V input. There will be an additional large inductive spike that will be on top of this. It can be as high as 30-35%. SInce the output is 25V the total reverse voltage across the MOSFET will be =120+25+ (Spike = 36) = 181. This will exceed the voltage rating on the MP6908. Can you please measure the voltage across the Vd and Vs pin with a very small probe loop.

Thanks,

8

Thank you.

I did the proposed test, and indeed, by removing all filters the voltage spike is sligtly above 180V.

Is there any suggested workaround for this issue?

Otherwise I will need to revise the transformer design by adding a 20% turns at primary side, but I will have some issues when starting the SMPS with high load and low voltage.

9

Hi m.checchinato:

I suggest you to increase your turns ratio and reduce the spike by reducing the leakage inductance of the transformer. The leakage inductance can be reduced by interleaving the secondary winding between two halves of the primary winding that are connected in series. Also please reduce any stray inductance of the layout with very short primary and secondary switching loops with the positive and negative traces overlapping each other for minimum layout inductance. Also an RC snubber across the secondary mosfet will help .

Thanks,

10

All these precautions were already taken.
I will try by increasing the turn ratio by a 20% to see if we can solve this issue and keep the desired performances.
Thank you very much for your help.

Michele