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TOPSwitch Control

Posted by: so on 10/29/2021

I am attempting to use the TOPSwitch as a variable PWM mosfet for design of an AC-DC converter. From the data sheets I was under the impression that after the soft start up varying the input current to the Control pin would vary the duty cycle, but when I tested it I was unable to get the output to switch at all. I am wondering if I'm not properly setting up the soft start, I provided the control pin with up to 100mA of current (in small increments) but nothing seemed to be happening. Any insight into how the control pin functions would be much appreciated.

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Testing diagram, probably not too useful 149.57 KB

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Submitted by PI-Wrench on 11/02/2021

Which TOPSwitch are you trying to use for this application?  From your diagram, it looks like you are trying to use one of the old 3-pin devices, which are no longer supported. Data sheets for earlier revisions of TOPSwitch are available on the internet using a search engine. It would be a good idea at this point to download the data sheet for your particular device and familiarize yourself with its functions and modes of operation.

Submitted by PI-Wrench on 11/03/2021

You might want to try a test setup similar to this one for initial investigations.

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TOPSWITCH_TEST.png 9.33 KB
Submitted by so on 11/07/2021

Sorry I should of specified, we've been using the TOP262EN, we were disabling the pins and I neglected to include them in my sketch. Since then we have ran some more tests. We did observe the control pin rising to 5.8V when Vds was 100V but when we sourced current (0-6mA) to the control pin we still didn't see any switching. Attached is a more detailed wiring schematic. 

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Test Setup TOP262EN_0.jpg 56.56 KB
Submitted by so on 11/07/2021

I am also curious about the diode in your schematic, is this necessary for testing purposes?

Submitted by PI-Wrench on 11/08/2021

You should try shorting the V pin to source to allow the TOPSwitch to start at a lower voltage. Also, you absolutely need the 47uF capacitor at the control pin. This is present for every TOPSwitch model (from the beginning) and stores energy to allow the TOPSwitch to boot up. It also serves as frequency compensation for the low impedance control pin. The diode in my test schematic is present to isolate the control pin from the power supply used to drive the control pin, allowing the 47uF energy storage cap to charge via the control pin from current derived from a tap on  the power fet inside the TOPSwitch.

Configuring the control pin the way it was shown in your latest schematic does not provide any impedance to limit the current into the control pin, which may cause abnormal behavior or actual damage. If things go to plan, the TOPSwitch should operate in autorestart mode when you apply voltage from drain to source, and you should see the control pin voltage vary for ~5.7V to ~4.7V. It should then be possible to drive sufficient current into the control pin to allow the device to run continuously at max duty cycle, assuming you have a resistor to limit current into the control pin.

Submitted by PI-Wrench on 11/12/2021

I think at this point it would be best to step back to the beginning and ask what you ultimate goal is for your use of TOPSwitch. If your ultimate goal is to design a flyback power supply using TOPSwitch, there are reference designs available that you can use as a guide in the design and construction of same. Here is a link to several reference designs utilizing TOPSwitch-HX:

https://www.power.com/solutions/designs?segment=ac-dc-conversion

Submitted by so on 11/15/2021

Thanks for your responses, I'll get back to you if we decide to do further testing with the TOP262.

Submitted by PI-Wrench on 11/15/2021

I tried some additional measures suggested by our IC design team on a TOPSwitch set up on my lab bench. Use the attached setup shown in the picture, that disables most of the extra features of the IC so that it emulates a 3-terminal TOPSwitch. Using the supply attached to the control pin resistor network, raise the voltage until the control pin voltage reaches 6.2V, then decrease the voltage until the control pin reaches 5.3V. At this time, apply the drain voltage. The TOPSwitch should then start switching continuously, and you can then use the voltage applied to the control pin network to vary the duty cycle. 

If the TOPSwitch is used in a standard flyback power supply application similar to those shown in the reference design, these special measures are not necessary, and it will function and regulate the output voltage.

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TOPSwitch_test.png 10.1 KB