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Overvoltage adjustment in TNY286

Posted by: Motiontech on Tue, 04/20/2021

A flyback power supply based on TNY286 was tested and worked properly.
However ,overvoltage protection got activated at about 360VDC.
For test purposes bias winding was bypassed and problem was solved.
However, we need the OVP to be activated at 410Vdc.
Voltage on capacitor (10uf) on bias winding = 5.2V
What would be the best method to increase OVP threshold?
Sch is attached .

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Submitted by PI-Salt on Tue, 04/27/2021


Could you also provide the PIXLS pdf? This would help in understanding your specific applications' input/output specifications and trf turns ratio, etc.

Thank you.

Submitted by Motiontech on Sat, 05/22/2021


Please see attached sch and PI design. 

Power supply start up is in light load condition. This works as expected. Problem free startup tested till 400VDC.

Load is applied to the power supply when a 'RUN' push button is pressed.

This load can be applied only till 360V DC.

At a voltage > 360VDC, the power supply completely shuts down( latched shut down) when load is applied.

It restarts after power is completely removed.

However if load is applied before hand , say at 330VDC, the input dc voltage can be increased till 400VDC.

Bias winding has been opened out to disable OVP.

What is this latched shut down?

There are no auto restart pulses on drain to indicate an overcurrent condition.


Attachment Size
TinySwitch-4_PIDesign3_0.uds 2.64 MB
IPMS_00_1.pdf 28.02 KB
Submitted by PI-Salt on Mon, 05/31/2021



The answer to your questions will be based on the latest schematic you attached using TNY287DG from PI XLS Design. 

To summarize your questions:

- Startup at light load condition works as expected until 400VDC.

- Load is applied using a push button and only applied until 360VDC (I assumed this is on full load startup)

Vin > 360 VDC, latched shutdown at full load (FL) startup. (Again I assume it is on FL startup)

- Restarts after power is completely removed (AC recycle)

- Works FL normal operation at 330VDC and can be increased until 400VDC. 

- Bias winding was disconnected to disable OVP.

- No AR pulses to indicate OCP.



Output Overvoltage Shutdown

Latch Shutdown is a feature of TinySwitch-4 which implements PI's proprietary primary overvoltage sensing via the bias winding voltage during MOSFET off time. 

Latch shutdown is triggered when a current (I_SD) > 6.5 mA flow into BP/M pin. 

Latch Shutdown: Isd > 6.5 mA

How to reset latch shutdown? 

1. EV/UV pin current fall below UV threshold typical current (25uA)

2. BP/M pin voltage falls below 4.9V (happens usually when AC is removed, bulk capacitor discharges = AC recycle)


Since you disabled the OVP, you could check if at FL operation if the duty cycle exceeds the maximum duty cycle (DCmax). 

If bias winding is connected and OVP is enabled, you could check the startup waveforms on Vds and Vbias and compute if it reaches the OVP latch trigger. You could also check the Vds,pk that could trigger OVP when reflected on Vbias (for this you could check if the snubber circuit is working properly).

In summary, if the injected current at BP/M pin exceeded 6.5mA OVP is triggered and you need to AC recycle to remove the latch shutdown. 


We could also further debug this issue through analyzing the waveforms of Vds, Vbias, Ids at full load, no load, in different input voltage conditions. 


For reference you also could look at the following resources: