TFS7708H ：The problem about soft start and LINE-SENSE (L) Pin
Hello！
I use tfs7708 to design a 24v10a power supply.
1. soft start function：the soft start function was not realized successfully. At the beginning of power on, the secondary rectifier diode overshoot was large. The regulation of R5 and C11 can improve but affect the loop stability.
2. Input the undervoltage detection function：According to the reference design, when VBUS reaches 336vdc, it can work. When the input is powered off, the threshold value of undervoltage detection is 215vdc. At this time, the duty cycle is too large, resulting in the abnormal operation of the secondary side diode.
评论
About question 2, You can see the waveform 1#“UVLO-VD.png”.When the VBUS starts to drop to a very low level, it cannot be demagnetized.Maximum duty cycle limit reached
about soft start function,I am doing the test according to your method, but the effect is not ideal. The first 1 ~ 2ms is unable to realize soft start. CH2: output voltage CH3: secondary rectifier diode
| Attachment | 大小 |
|---|---|
| tek00309.png | 21.22 KB |
Please post your TL431 secondary regulation circuit.
Use op amp instead of TL431
| Attachment | 大小 |
|---|---|
| 402084142.png | 117.83 KB |
| 20200402085058.png | 88.01 KB |
I'm going to need more detail to figure out what is going on with your application. I looks like you are trying to design a battery charger or similar CVCC circuit. Are you using PFC or rectified mains for your front end? Also, I need to know whether you are running at 66 or 132 kHz. Higher frequency operation is more problematic re high side bias and XFMR reset. Are you using a clamp-to-rail or clamp-to ground scheme?
Yes, we are using tfs7708 to design a battery charger.380vdc is provided by PFC，132kHZ,clamp-to-rail
Check this circuit out for a possible implementation of a voltage soft start for your application.
A better way to get a reasonable soft start function is to place a capacitor between cathode and anode of the TL431. While the voltage is still under regulation limit, the capacitor will act as a dV/dt limiter to slow the output voltage rate of rise. Use a general purpose electrolytic cap (UCC KMG or Nichicon VZ are typical). Start with a capacitance of 1uF and work up until you have the desired output rate of rise. The general purpose electrolytic caps have a high ESR (tens of ohms) so they have little effect on the loop response of the TL431 once it turns on and starts to regulate the output, as it has a cathode impedance of about 1 ohm when active. This solution has proved successful over years of use. Do not use a low impedance electrolytic in this position unless you put a resistor in series with it.
Re your question 2), what do you mean by "abnormal operation"?