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What specifically causes the Tinyswitch 4 to go into autorestart?

Posted by: willieb3 on Fri, 04/30/2021

I am trying to do some troubleshooting/debugging on a power supply built using a Tinyswitch 4. I have removed the tinyswitch from the board and isolated it in it's own circuit, and basically apply 30VDC at the drain pin. The BP/M min has a 0.1uF cap to source, and for the EN/UV pin I have placed a potentiometer to adjust the current leaving between 25 uA to 120 uA. My hopes were that I could use the potentiometer to adjust the current, and there by change the duty cycle.

The problem is, the TNY is going into restart mode! From the TNY datasheet:

"An internal counter clocked by the oscillator is reset every time the ENABLE/UNDERVOLTAGE pin is pulled low. If the ENABLE/UNDERVOLTAGE pin is not pulled low for 64 ms, the power MOSFET switching is normally disabled for 2.5 seconds (except in the case of line undervoltage condition, in which case it is disabled until the condition is removed)."

I don't understand this. The internal counter will reset when the EN/UV is pulled low (from what I understand below 25uA), does this mean it will enter Auto-restart mode? It also says if it is not pulled low for 64ms, then it will enter Auto-restart. So in which case will it not enter auto-restart? Does the EN/UV pin need to have a fluctuating current? How would a fluctuating current be maintained in a regular power supply?

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Submitted by PI-Kryten on Fri, 05/21/2021


Thank you for choosing Power Integrations for your solution.

The TInySwitch operates on the principle of ON/OFF control (cycle skipping). The EN pin is like an input to a comparator which only enables or disables switching. Change of current from the pin therefore will not allow you to control the duty cycle.


TinySwitch will stop switching when the current drawn out of the EN pin exceeds the EN/UV pin turn-OFF threshold current as indicated in the datasheet. In a real power supply, as the output voltage rises, the EN/UV pin will be pulled low by the feedback circuit which will cause the next switching cycle to be inhibited. As soon as the switching is inhibited, the output voltage will start dropping and it will cause the feedback circuit to stop pulling current out of the EN/UV pin and will allow the switching to resume. The drop in voltage at the output is very small and corresponds to the output ripple.


As indicated in the datasheet, in the event of a fault condition such as output overload, output short-circuit, or an open loop condition, the EN/UV pin will not be pulled low by the feedback circuit and if it is not pulled low for 64 ms, the power MOSFET switching is normally disabled for 2.5 seconds. During normal operation of the power supply, the EN/UV pin is being periodically pulled low (current drawn out of the EN pin exceeds the EN/UV pin turn-OFF threshold current) by the feedback circuit.


I will be happy to help with the debug of your power supply. Kindly provide me the details of the design including a schematic and transformer design and the details of the problem you have and we will help you with the root cause investigation.


Best Regards,