Solution Finder Get Tech Support

Tinyswitch-4 Not Switching, Problem with BP/M Pin

Posted by: bschafer08 on

I have been testing a 15V power supply developed with the tinyswitch-4  technology, and recently have uncovered a rather strange and inconsistent issue. Periodically, in a low power scenario (this is a solar application), the 15V power supply will turn on and off according to the load placed upon the supply. This makes sense, and most of the time the supply will turn back on as usual. However, and this is the weird part, after this cycle of turning on and off has been taking place for a while (still have not determined what incites the failure), the power supply will attempt to restart, and then it just stops switching despite voltage being present on the Drain of the MOSFET.

Some things to know:

  • The BP/M pin is externally supplied so should be normally clamped to 6.4V by the internal zener clamp in the IC and is when the supply is running normally.
  • When the issue occurs, at least the time I managed to get a scope shot, the BP/M pin remains in a steady state of approximatley 3.28V
  • Due to the undervoltage feature of the tinyswitch-4, the switch is supposed to remain off when the BP/M pin drops below 4.9V and will not turn back on until it rises to 5.85V
  • The Drain voltage is approximately 177V in steady state and is supplied a DC Bus of a fairly large capacitance. The BP/M supply also comes from this source.
  • The EN voltage in steady state is 1.84V.
  • In order to get the supply to start switching again, the drain voltage (analogous to the DC Bus) must be dropped to below the BP/M voltage to allow that voltage to start dropping. Once the BP/M voltage has dropped an arbitrary amount, and power is resupplied to the unit, the tinyswitch-4  will resume switching as normal. 

So my question essentially is as follows: Is there a scenario in which the BP/M pin could get latched below the 6.4V clamp, thus causing the MOSFET to stop switching? 

Note: I have attached some oscilliscope captures to help describe the situation.

Comments

Submitted by PI-LeoDavinci on 01/10/2017

Hi bschafer08,

Thank you very much for considering Power Integrations' product. We've been studying your case and rest assured we are carefully analyzing what is happening. However before proceeding, we would like to know some information about your project's issue and these are the following:

1.   Is the issue also present in other units? Or is the issue just occurs in one unit? The reason for this is such that we would like to make sure that it is happening constantly in other unit. Because if it is not then it may just be a case of unit problem. It may be just an isolated issue. 

2. Is the transformer construction correct and no other winding was winded incorrectly? 

3. Is there no wrong component that was loaded or any component that was incorrectly mounted?

We just would like to make sure that all are basically verified first. Hope for your response.

 

 

Submitted by bschafer08 on 01/10/2017

In reply to by PI-LeoDavinci

1. Yes, it happens with other units. The issue is very inconsistent, however, and seems to occur only under certain low power conditions when the unit loses supply voltage repeatedly. 

2. The power supply works under normal operating conditions, so the transformer should be wound correctly. This is more of a corner case that can cause issues in the early morning since this is a solar application. As the sun rises in the morning, the power supply will turn on and off repeatedly. Occasionaly, the supply will lockup and will draw very little power as the there is no output voltage from the power supply. Thus, the solar panels supplying the unit never drop out and the power supply stays locked out all day. 

3. All of the components are correctly mounted. These are manufactured units that have all been tested at production. 

 

EDIT: Problem solved. It was a noise issue on the BP/M pin. During shutoff, a sufficienty high noise current was driven into the BP/M pin which caused a overvoltage latchup, which cannot be resolved until power is completely removed from the circuit. The fix is to add the 100nF capacitor as used in the reference design.

Submitted by PI-LeoDavinci on 01/16/2017

Hi bschafer,

 

That's great to hear. If you have more inquiry feel free to send us some message. Have a great day! :)