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LNK626 ON TIME EXTENSION MODE and switching frequency

Posted by: junyinsean on Wed, 06/02/2021

Hi Sir:

1\As the note in datasheet, "When the duty cycle exceeds DCMAX the LinkSwitch-CV operates in on-time extension mode." , what would the switching frequency be in on time extension mode? 

2\we found the switching frequency is around 80khz,  far below 100khz when duty cycle didn't exceed DCMAX, do you know why? 

INPUT VOLTAGE AROUND 100VDC, LP 900UH;

 

THANKS A LOT;

 

Sean

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Submitted by PI-Chloe on Mon, 06/07/2021

Hi Sean, 

Good Day ! 

Thanks for using PI products 

Please share to use your Power Supply specification and schematic. 

The LNK CV operates on ON and OFF Control. This is not the same as the traditional PWM control. 

The switching frequency varies depending on the load. From no load to higher load, frequency changes up to full load. 

The duty is controlled by the Drain Current Ids. When the Drain Current hits the Ilimit the pulse will terminate. 

The Maximum frequency of LNK CV is 100kHz. 

Thanks 

Regards 

PI-CHLOE 

Submitted by junyinsean on Mon, 06/07/2021

input voltage: 90VDC~180VDC, 4 isolated outputs ,total output power less than 5W, typical design like the one in datasheet;

I see that "The duty is controlled by the Drain Current Ids. When the Drain Current hits the Ilimit the pulse will terminate. ", while in on-time extension mode, say turn-on time longer than 10us, what would the switching frequency be during the PWM on period of ON and OFF control?

 

and the other question is we found the switching frequency is around 80khz,  far below 100khz when duty cycle didn't exceed DCMAX, is this possible and why?

 

thanks a lot;

Sean

Submitted by PI-Chloe on Wed, 06/09/2021

Hi Sean, 

 

We do not operate the power supply in max duty. We should operate the power supply below the max duty.

 

The IC is ON OFF control. Switching frequency varies with Load in order to regulate the output.

            The lesser the load, the lesser energy is require to regulate the load hence lesser switching frequency.  

            The higher the load, the higher energy is required to regulate the load hence higher switching frequency. 

 

You are seeing a 80kHz switching frequency at this point the output is being regulated at this frequency.  

Increasing the load will increase the switching frequency further.  

 

Thanks 

 

Regards

 

PI CHLOE 

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On Off Control Fs.png 54.55 KB