Hello,
See the figure bellow.
On the left we have my circuit, on the right a circuit found on the texas website.
Question: Some designers do not consider the TVS diode (D2), see the figure on the left.
Could you please tell me why the designers don't consider this TVS diode?
Hellow,
See the figure bellow.
This figure is the transformer we are using.
When we chose this transformer, we thought that the voltage on VDD should be: VDD=Vin*NA/NP=311*0.114=35V.
When Vin_rms=220Vac.
But in the docs, you don't use Vin, you use Vout to check VDD voltage in permanent regime: VDD=Vout*NA/NS=3.3*2.667=8.8V
Why do you use the second option?
Regards.Hi Wesley
See the figure bellow.
This figure represente the waves signal at Vaux transformer. We have three consectives pulses. When we have 13V on VDD, we will have a OVP protection. But, in this figure, we have a many oscillations. Could this be because of parasitic inductances?
Hi, Wagner:
This oscillation comes from the transformer's primary side inductance, Lm, leakage inductance, Llk and capacitance on MOSFET switching node, Csw.
It is normal phenomenon when flyback operates in DCM. As your waveform, this converter works in a very light load or no load. VDS/VAUX oscillates because there is no enough energy to keep a stable voltage on transformer side. You may get more information as below link.
https://training.ti.com/understanding-basics-flyback-converter
For OVP issue, please check if the Vs pin voltage through the equation Vs * Rs2/(Rs1+Rs2) with Vs =25V ( which I read the plateau value based on the waveform you shared, and you could measure it more accurate by using cursor function ). If Vs is over 4.52V, the Vsovp triggered and forced IC goes to protection mode.
Regards,
Wesley
Hello Wesley, thank you for your feedback.
But I have a many quetions. See the figure bellow. We have a TLK_Reset and T_OVP representations.
Figure 1; TLK_Reset and T_OVP representations.
Figure 2: Yours documents.
My circuit is not switching yet.
I designed my circuit to operate at 5V at steady state output.
When there is an overvoltage, we would have 5.75V at the output. The transformer's transformation ratio is NA/NS=2.667; NP/NA=23.33;
I designed the circuit to operate at 5V at steady state output.
I considered an OVP protection voltage at 5.75V.
Then, we have VDD=5*2.667=13V on permanent regime.
Your calculator excel doc give to me R1=59k and R2=27k.
But, look at the Figure 1. We have em T_OVP 25V on Vaux. So VS=25*27k/(59k+27k)=7.85V So VS>VS_OVP. But if I adjust the RS2 value to solve the OVP problem, I will change the protection voltage at the output. Question 1: How can I proceed in the best possible way?
I don't understand why I'm measuring 25V in Vaux.
Question 2: It should be VS Sample = (Vout+VF)*NA/NS=(5+0.55)*2.667~=15V. Is it correct? Look at next figure for it.
Question3: in figure 3 which point represents VS Sample in figure 13? Point 1 or 2?
Figure 3
Hi, Wagner:
Based on your description, this phenomenon may come from your transformer leakage inductance I guess. Your calculation is correct but the Vaux is impact by not only Ns but also Np if the winding is too closed when the transformer made. You may check your VDS waveform. To change D2 and then check if the Vaux and VDS voltage vary with different D2. If yes, to modify the transformer structure would be a way to fix the Vaux issue.
For Vs sample point, actually the sample position would be position 1. As you can see on fig 13, the IC samples the Vs based on the slope drops significantly.
So position1 matches fig13.
Now you may calculate RS1 and RS2 based on Vaux = 25V. Vs=4V= Vaux * Rs2/(Rs1+Rs2) = 25V*Rs2/(Rs1+Rs2), than you can set 5.75V by Rs1 and Rs2 setting. But the root cause should come from transformer. It is still needed to fix it.
Regards,
Wesley
Dear Wesley.
My circuit is working perfectly. All voltage levels are correct as described in the datasheet. Everything is working perfectly. Thank you for all your support during this step. Texas Instruments is amazing. Thanks a lot for all the support.
Wagner Coelho Leal Researcher in Sustainable Energy at Certi Foundation - Florianópolis Brazil.
Dear Wesley.
My circuit is working perfectly. All voltage levels are correct as described in the datasheet. Everything is working perfectly. Thank you for all your support during this step. Texas Instruments is amazing. Thanks a lot for all the support.
Wagner Coelho Leal Researcher in Sustainable Energy at Certi Foundation - Florianópolis Brazil.
See the figures bellow.
Channel 1 is Vaux and Channel 3 is Iout.
I'm noticing that the switching frequency is not constant. See the "a" and "b" points.
Question 1: Is this normal to happen? Is that correct? If it's not correct, how can I improve?
Question 2: I expect to have 2 A at the output. See that it is oscillating between 1.96A and 2.46A.
Is this normal to happen? Are these oscillations because of the switching?