TOP271 - Destroyed at Startup

Hi tobbz,

Such a dramatic failure at start-up can also be caused by an open snubber connection or open output diode. So, please check for open circuits in addition to the short circuits and polarity reversals.

To debug the cause of failure, you can monitor the drain (D) waveform of the primary FET to see if the voltage rating is exceeded. Given that something is blowing up, you may also want to start the input power supply at a lower voltage (remembering to adjust/disable any UVLO) to observe for bad behaviors.

Please also double check the transformer to make sure that all windings and polarities are proper.

-PI-ODO

Hello again,

Thanks for your quick reply and please forgive the late feedback.

In the meantime we was able to solve the previously described problem, in fact, the drain voltage was exceeded and the controller was destroyed. With appropriate adjustments to the Clamping circuit, as well as to the UV / OV programming, the problem could be eliminated.

However, we are now approached the next problem: When the module is put into operation, the voltage at the output varies between approximately 3.45V-7.3V. The voltage seems to pulsate properly. Could the problem be related to the shunt regulator used? - Do you have an idea what could cause the problem?

The appendix contains the updated circuit.

Thanks in advance for your help.

UPDATE *: The fluctuating output voltage actually resulted from the shunt regulator used, which probably had a defect. After this has been replaced, the voltage at the output remains constant. However, only to 3.98V instead of the desired 5.25V. The voltage ratio of the shunt regulator has been checked and appears to be correct. Do you have any advice on which circuits should be checked?

Thank you in advance.

Hi tobbz,

You seem to be on the right path. Definitely double check all of the feedback circuit components, making sure that reference voltage values and feedback resistor component values are all correct. Also, you can check that the bias voltages going to the opto are as expected. Finally, you can also probe the actual nodes to see if there is some large ripple riding on the signals and effectively causing an offset.

-PI-ODO

Hello,

Thanks again for the competent help. In fact, there were problems with the reference voltage of the shunt regulator. After the reference line has been re-drawn, we get a constant 5.25V at the output. The problem is thus solved.

Hi tobbz,

That's great to hear!

-PI-ODO

hello,

We are currently working on a new project in which we are using the TOP271 EG . The prototype circuit was created on the basis of PiExpert and also the layout of the prototypes was meticulously respected for the corresponding guidelines. During startup the device burnt.  The power rating of the circuit is 91 w and the output voltage is 24V@3.8A.

The transformer is also constructed as per the guidelines.  using E30/15/7.

what is the solution for this problem could you advice.

If the circuit was started up with no load, there is no load shown at the output to control its ultimate value, so the output voltage could rise to several times the rated output voltage. A suggested value for a preload resistor to use at the output would be 10% of the max load. 

I would also suggest the you monitor the voltage across R8 to determine how much current is going into the control pin at startup. This may require sacrificing a couple of TOPSwitches to get the data needed.

There is also some concern regarding the leakage inductance of the transformer - a single section primary is not a really good idea for a 91W flyback supply, as it increases the leakage inductance of the transformer. and the energy that must be absorbed by the primary snubber. Given that 2 parallel strands are being used for the primary, it may be possible to split the primary and reduce the leakage inductance of the transformer. Suggested winding order - 1/2 primary, bias, secondary, 1/2 primary.

Also, I ran a check on the design using PIXLS, which showed that the reflected voltage was very low (71V), increasing the peak primary current and not allowing the TOPSwitch to attain full rated power.  A reflected voltage of about 120V would be more appropriate for a design like this.. I also am concerned that the E20/15/7 (EF30 core) is rather small for a design at this power level.

Hi Wrench,

Thanks for your reply, I have altered the transformer(primary inductance matched) and resized from E30/15/7 to PQ26-25, now the problem is the device is not at all on and the control pin voltage is 1.56 volts.  

as per your advice the VOR also increased from 71 volts to 120v.  Added a 10 ohm resistor as a preload too.

pls guide

Since you changed the reflected voltage to 120V, you will also need to change the value of the primary zener champ to 150V, otherwise it will limit the output voltage (and possibly burn up as well). A more appropriate value of preload resistor (assuming you are using a 10% preload)  would be around 68 ohms or so. 5  pieces of 330 ohm, 2W in parallel would get the job done.

Dear Mr.Wrench, 

Thanks for your reply, I have been using RCD+Zener clamp in which  I am using P6KE 150CA TVS diode.  At the output added a preload resistance of 68 ohms as per your advice.

The transformer is constructed on PQ 26-25 core.  The circuit I am using is attached here with. The voltage at V-pin is about 6.89 V, control pin voltage is 1.54V

I didn't get output.

Kindly advice.

Please check and make sure that C5, C7, and VR2 are inserted in the right orientation. 

Dear Mr.Wrench,

Thanks for your reply, the mentioned components are correctly placed as per the polarity, kindly go through the pdf files attached and if there is any mistake in the transformer kindly revert.

Thanks

Dear Mr.Wrench,

still device is not starting after changing the transformer design as per your advice and the primary inductance is at 131uH split winded, secondary zener chosen, the problem persists. attached the files here with.

could you identify and advice pls.

Is the primary inductance 131 uH as stated above or 144uH as stated in the transformer document? 

Please probe the output with the o-scope set on a relatively long time scale (say, 50 milliseconds/div) and see if the output is trying to rise. Is the failure to start happening at full load? Can you get the supply to start at a lighter load?  Also, check the output and the bias windings to make sure the windings are properly phased. At the anode of the bias and output rectifiers, the the signal polarity should go negative during the "on" time of the switching cycle. 

In the document it is given 144uH, but with a given tolerance of 10% it can be Min 129.6 and max 158.4,  So, I left with 131uH.  after checking all the parameters will let you know.

one thing i for got to tell you that with a preload of 68 ohms only I am testing the board.

If the supply is not starting with a relatively light load of 68 ohms, I strongly suspect that one of the transformer windings is mis-phased.. A properly designed and implemented flyback will have both the bias and output windings isolated from their respective filter caps by the bias and output rectifiers, allowing the transformer to store energy in its magnetizing inductance when the primary switch is on. This stored energy is transferred to the secondary and bias outputs when the primary switch turns off.

If one of the windings is mis-phased, there is a direct path through either the bias or output winding to the filter caps (loks like a short circuit), causing high peak current on the primary side that trips the internal overcurrent protection inside the TopSwitch.

Another possibility is that the optocoupler (or some other component in the feedback chain) is perhaps miswired. If there is no feedback from primary to secondary, the TOPSwitch will sit in autorestart mode indefinitely, no matter the load.

I ran a check on the design in PIXLS, and there are some deign issues that need addressing once you get the power supply up and running.