UCC28600: TINA example model setup, changing the transformer to a coupled inductor

Hey,

First, I really appreciate your answer, thank you very much! I took your advice and I was trying to simulate the circuit only with one output. Right now, all of the component values have been calculated but the circuit either works in a really weird way, or I get the infamous convergence problem. For example, the weird thing is that voltage top of the current sense resistor has a waveform which is far from ideal, it doesn't have the ramp increase, instead the voltage changes instantly. If the current source is around 4 A, than the duty cycle of the OUT pin looks strange as well. It has a kind of a normal cycle, after that some really thin pulses and it keeps repeating. I attached the current version. Do you have an idea why is it working like this?

Edit.: I left the given transformer model in the simulation, I just changed its values based on my calculations.

mine.TSC

Hi,

Since then I did what you suggested, and the main reason why the circuit was not working is the incorrect secondary diode I've chosen.

My problem now is the following: The attached simulation works as expected until 1.4A of load. The problem is, if I go higher than this value, the controller is unable to hold the output voltage on the desired 12V, even though all the values (OVP, current sense etc) have been calculated for 197W output power with 85% efficiency, therefore if the output current would be around 16A, it still should be able to work. Now I'm a bit stucked. I think the chosen OVP resistor values and current sense protection (Rcs and Rpl) have been calculated correctly, I don't really see why the controller stops the operation. Unfortunately, I don't know TINA that well, but my first thought was maybe the diode on the output causes some problems, as the current one is able to handle 1A maximum forward current, but changing that value doesn't help.

Maybe somehow the state machine starts to work incorrectly? May I ask your help in this regard?

Thank you in advance!step_by_step.tsc

12V slowly drops, it is quite clear that the controller doesn't put energy from the primary side, so the capacitor on the secondary side just loses its charges and the voltage drops in the same ratio.

This is how it looks the drop after the load reaches the problematic level.

And there are weird spikes in the current signal after the 2ms mark if I zoom in.

This happens at 2A of load. This thing goes even more weird, if I change the load to 16A. In that case the controller keeps the primary side transistor open, and although the voltage on the output drops, the waveforms look really strange after the load goes on.

I added the loop compensation as you suggested by following the steps in the "Compensation Design With TL431 for UCC28600" (SLUA671) document. The circuit unfortunately still doesn't work as expected. I can go a bit higher with the current up to around 4A (you can see a picture about a simulation with 1.6A load below), but it is still far from the final 16A. A was also trying to add the Zener network later before Rled, but it didn't help either. Based on the transfer characteristics, the compensation network should do the work, I changed the crossover frequency to 3kHz of Vo/Vcomp, it also meets with all the stability conditions.

The other really weird thing for me (although maybe I just misunderstood something) is the switching frequency. Shouldn't it work with at least 40kHz bursts? Right now, depending on the load, the switching frequency varies between ~3kHz - 7kHz.

And the pictures:

Unfortunately it is quite hard to figure out if it reaches 12V or not, because the simulation stops with convergence problem a lot of times. Sometimes pressing 'retry' works, but I don't know how accurate the result going to be after this error. Also if I don't change anything in the example simulation, but I change the simulation time to 30ms, it doesn't run. It quits with convergence error.

So back to my circuit: It reaches the target output voltage @1.6A, but during the simulation I had to press 'retry' on the convergence error once.

At 3.5A at the start it looks like the system operates in CCM, but the controller is a DCM one, so it shouldn't happen right?

The Bode of Vo/Vcomp

I also attached the updated simulation circuit.

5824.step_by_step.tsc

How did you decide the loop compensation?

How did you design your converter parameter values?

For the loop compensation as I said I followed the method presented in SLUA671.

The following MATLAB code shows my implementation

I need your converter parameter design too. Since you mentioned your converter entering CCM, this indicates some parameters may need adjustment. One possibility is your transformer may need to adjust.

If you designed your converter using the Excel tool on ti.com, please forward your excel design file to me. If you did not use that tool, please first check your design based on that tool, if you cannot find anything wrong, then please forward your excel design file to me. 

Thank you very much. I don't want to look ungrateful, but if I change the load to 16A, it doesn't work. Unfortunately the goal is to have 200W power output.
Also, could you please explain why did you change the reference voltage and leave C1 from the compensation network?

In the meantime, I will compare the excel results to my calculation results (I don't want to use the excel, because I'd like to understand what and how one can calculate the necessary values).

Because the simulation clearly doesn't work based on my calculations, I thought I give the excel a try, so I can see how a 200W design should look like. I attached the excel file. I also attached the TINA model which has the components values calculated by the excel given by TI. For the compensation network, I used the following MATLAB code:

Please provide your excel design sheet for review.

On the simulation, as I have proved the model can work with parameter value adjustment. The model is ok. But simulation has its own setup. You would need to make adjustment sometimes it is a trial and error. For example, the step size etc can affect convergence. You need to solve these mainly on yourself. Basically the model is proved ok so if your simulation cannot get through you need spend time to make try. If you do need simulation support or need to make simulation for you, please contact your local TI sales office to get help.

I provided the excel sheet in my last comment.

I see that with some adjustment you can end up with a working model, the problem is that it doesn't prove that there is any correlation between the expected behavior and the simulation results. In my opinion, the only point of the simulation is to legitimize the used calculations. Thank you for suggesting TI sales, but I don't need someone to make the simulation for me, I would like to do it myself, I don't need a complete solution, I would like to know what kind of mistakes have I done and learn from it.

Should I ask for the sales' help regarding to my previous asked questions as well?:

"could you please explain why did you change the reference voltage"
"...why you say in the excel that the calculated Np/Ns is a minimum recommended value. If that number is higher than the calculated one, the flyback voltage will be too high and it destroys the primary switch."

The simulation mistakes and experience would come from your own trial and error. If you just ask why, we would have to spend time to make trouble shooting for you that is equivalent to say we are doing simulation for you.

Please indicate which sheet, and which rows, the below come from.

"could you please explain why did you change the reference voltage"
"...why you say in the excel that the calculated Np/Ns is a minimum recommended value. If that number is higher than the calculated one, the flyback voltage will be too high and it destroys the primary switch."

"could you please explain why did you change the reference voltage" : The comment you made on Apr 19, 2020 7:03 PM contained a TINA model about the circuit we are talking about. In that, you changed the voltage divider for TL431's reference voltage, so it is not 2.5V anymore.

"...why you say in the excel that the calculated Np/Ns is a minimum recommended value. If that number is higher than the calculated one, the flyback voltage will be too high and it destroys the primary switch." : Excel document I attached before, QR Design Tool sheet, B87 (the explanation I am asking about is in the I-J-K-L-M87)