Part Number: TPS23731EVM-095
Other Parts Discussed in Thread: CSD17577Q3A
Hi team,
My customer want to konw how to design Q2 (FET, FDMS86252) , T2 (Transformer, LDT1038-50R) , Q10 (FET, CSD17577Q3A) and L4 (Inductor, 150nH).
Customer already looked TPS253731 DS, but not found these calculations. Please support this.
Best Regards, ny
Hi NY,
Usually I will use the power stage designer tool (https://www.ti.com/tool/POWERSTAGE-DESIGNER).
Put the transformer information (PRI:SEC = 6:1, L = 65 uH) into the parameters:
1. Check the peak transformer current. Make sure the transformer saturation current is higher ~120% of the peak current.
2. Check the FETs' rms current and peak Vds(off), in an example of the primary side FET.
- For current rating, the selected FET normally has >2 times of the RMS value. Too low on-resistance sometimes causes high Coss and switching loss. There could be a trade-off between the switching loss and condition loss.
- For voltage rating, the primary FET is usually 150 V. The secondary FET you need to consider the overshoot caused by leakage & stray inductance. Voff,sec = Vout + Vin/N + Vovershoot. You can assume the overshoot is ~1/3 to 1/2 of the sum of other parts at the beginning to select the FET. Then you can adjust the snubber circuit later to adjust the overshoot voltage.
3. L4 is a filer inductor. Make sure its current rating higher than the rms value and its saturation current higher than the peak value. Normally add 30-50% margin.
The filter value calculation can be also found in the user guide (https://www.ti.com/lit/ug/slvubb4b/slvubb4b.pdf)
Best regards,
Diang
