TPS54140A: WHY THE FREQUENCY IS NOT WHAT I SET

Hi Xiowei,

That's definetaly an interesting use of the TPS54140A. If you don't mind me asking, why is Vsense connected the way it is in the design? How is the output feeding back to the device?

Thanks,
Andrew

Hi Andrew

Actually there is a voltage of 8 volts between GND and GND4. So Vsense does have an voltage. We can calculate that the voltage is 8/(200+1.5k+1.1k)*1.1k=3.143V.

Regarding to the reason of this design, actually I don't understand, either. This is the design from the last engineer of my company, and we cannot contact him at all. So I send this topic to you TI engineers and want to discuss with you to see if you have experience of this design.

Fortunately, we get the correct frequency that we expected after we change the resistance of R602 to 750Ω, and I calculate that at present the voltage of Vsense is about 2.449V. Would you please do me a favor to provide some explanations why the device working like this way?

Regarding to the feedback question, we actually don't have feedback, so when we have an 50mA load in the output pin of LP2951, then the voltage of VCC13 is falling from more than 20V to 15V more or less. I think it is normal because it doesn't feedback. 

Looking forward to your reply.

Thanks.

BR,

Joey

BTW, I share the normal waveform of output of TPS54140A, it should be AC signal because it need to drive the transformer.

Please help to show me in the picture which duration is soft start time that is determined by Css. Thanks very much.

BR,

Joey

Hi Joey,

I will check with my team.

Thanks,

Andrew

Hi Joey

My understanding of the circuit is that it's an inverting flybuck circuit.

May be it would be easier to understand each piece seperatly

inverting buck boost (Negative VOUT circuit)

https://www.ti.com/lit/an/snva856b/snva856b.pdf

Flybuck circuit (VOUT on primary and secondary side of the transformer)

https://www.ti.com/lit/an/snva674c/snva674c.pdf

I suspect that R171 may be causing the discrepancy on why FSW is not acting as it should. Could you try shorting it out, or trying a lower value and see if it affects the design.

Thanks,
Andrew

Hi Andrew

I did two tests, changing the resistance to 10 Ω and short R171 direstly, but neither is useful to my design. Do you have any other suggestions?

I looked through the inverting flybuck circuit, and I think this it the reason why GND is 8V high than GND4. I will read it carefully later.

Btw, I would like to know which part is soft start duration? Would you please do me a favor to show in the waveform picture?

Thanks and looking forward to your reply. And have a good weekend.

BR,

Joey

Hi Joey,

For adjusting how much high GND is from GND4, you may be able to change the value by changing the resistor value or what resistor the jumper is using.

What value do you want for GND4?

Thanks,

Andrew

Hi Andrew

Thanks for your reply.

8V is OK for my design. Otherwise LED1 will not work.

Now I would like to know how to adjust the SS time, I tried to change the value of Css (C650), but nothing had happened. Did Css not work? Or actually it worked but I didn't find. So would you please do me a favor ro point out what will happen if I change the value of Css?

Thanks.

BR,

Joey

 Hi Joey,

Does this equation not work? What is your target soft start time?

Thanks

Andrew

Hi Andrew

Actually I can't get the value of tss(mss) from the waveform picture. So I cannot tell whether this equation work or not.

I changed the value of CSS from 15nF to 100nF, but the waveform didn't change. Should I change this value more bigger, e.g. 1uf?

I'm very appreciated if you can point out which is soft start time in the picture, from where to where?

Thanks and looking forward to your reply.

BR,

Joey

Hi Joey,

Yeah we can try a larger value 500nF ~ 1uF capacitor. It could help.

Thanks,

Andrew