TPS2391: Power path control of PoE rail with 48-56V

Hi Michael,

Thanks for reaching out!
Is there any power level difference between the two situations? Looks like the device is fast-trip and turning ON again. Can you help with test waveform of Vgate, Vout, input current, IRAMP voltage for the hot-swap.
Can you verify by increasing CIRAMP value.

Best Regards,
Rakesh

Hi Rakesh,

thanks for your reply. The main difference is the voltage: standard PoE will supply 45-46 VDC, while PoEplus will work at 55-56 VDC.

Yes, I will supply the waveforms. I need to get a separation transformer, as I cannot measure PoE voltages with a scope which has earth connection. So I would measure Pin 4 (IRAMP), Pin 6 (ISENSE) and Pin 7 (GATE), and eventually Pin 8 (RTN), correct?

BR,

Michael

 

Correct Michael.

Best regards, Rakesh

Hi Rakesh,

sorry it has taken me so long - I needed to get a safety transformer because of the PoE..

Blue = Vout

Pink = Iramp

Green = Gate

Ciramp = 3.3nF

Rsense = 50mohms

Cflt = 220nF

1. PoEplus - the voltage ranps up to 56V but still there is an intermediate phase before the Vout settles to the flat voltage

.

2. Standard PoE: The PoE rail voltage is smaller at 48V but the load is the same. The output is never switched fully on. 

 

Any hint is appreciated.

Best regards,

Michael

Hi Rakesh,

I made additional measurements with:

Blue = Vout

Pink = Isense

Green = Gate

1. PoE plus 

2. PoE standard

Do you need additional measurements?

Best regards,

Michael

Hi Michael,

Thanks for the measurement waveforms.
Can you please check under below two cases (do one change at a time). How much is the load capacitance?
1. Increasing CIRAMP value to 0.022uF and 0.047uF
2. Decrease RSENSE to 22mOhm

Best Regards,
Rakesh

Hi Rakesh,

thanks for looking into this. The load capacitor is 1uF. I wanted to keep this capacitor small for cost reasons, as this is a 100V type.

Please find the measurements with the different values below:

1 = yellow = Iramp

2 = blue = Vout

3 = pink = Isense

4 = green = Gate 

1. Rsense = 50mOhm, Ciramp = 22nF: It made things worse. Now neither PoEplus nor standard PoE works. I did not further increase Ciramp.

2. Rsense = 25mOhm (I did not have a 22mOhm), Ciramp = 22nF: Made also things worse: PoEplus worked in the second repeat try, but standard PoE did not work.

Best regards,

Michael

Hi Michael,

Let me work from my end on EVM. I found that I don't have this device EVM with me and I right away placed order which will reach me early next week. Excuse for any delay.

Best Regards
Rakesh

Hi Rakesh,

thanks for looking into this. 

Best regards,

Michael

Hi Rakesh,

did you get the EVM for the TPS2391?

Do you need more information on my circuit?

Best regards,

Michael

Hi Michael,

I have received the EVM. I will test and provide update by end of this week.
Apologies for the delay.

Best Regards,
Rakesh

Hi Rakesh,

any news on this?

BR Michael

Hi Michael,

My sincere apologies.. It has taken back seat due to bandwidth limitation. I have reserved my half-day tomorrow for this testing and shall provide update.

Best Regards,
Rakesh

Hi Michael,

I have configured the EVM as below and tested at two different voltages 44V, 55V. 

Cap on Faulttimer = 0.082uF

Cap on IRAMP = 0.01uF

Rsense = 20 mOhm

Cout = 100uF

CH1 GATE voltage

CH2 MOSFET drain voltage

CH3 IRAMP voltage

CH4 Input current

Vin = 44V and at no-load

Vin = 44V and 1A load

Vin = 55V and at no-load

Vin =55V and 1A load

I am able to power up the system on EVM even with 100uF output capacitance. Is there anything, I am missing Please let me know. Also, can you share your layout for review. 

Best Regards,

Rakesh

Hi Rakesh,

thanks for your results. Please find the schematic and the layout enclosed.

I am wondering whether the Cout=100uF on your board makes the difference. My board has only Cout=1uF because large high voltage capacitors are expensive.

Best regards,

Michael

Hi Michael,

I see two major differences from the EVM 

1. R64 is not kelvin sensing connection.

2. FET T2 in your circuit has lower GATE capacitance and will turn-on fast. This could lead to false trip due peak inrush currents.

Please check below on your circuit.

1. Connect the other end of R64 directly using a short wire to pin-5 of the controller

2. Check again with Rsense = 25mOhm

3. Add 4.7nF between GATE and Pin-5 to have better control on the FET and verify

Best Regards,

Rakesh

Hi Rakesh,

I did all 3 changes you suggested but it did not change the behavior:

With PoE plus it works and the load is fully switched through.

With standard PoE the load is never fully switched on and the MOSFET is running hot.

BR

Michael

Hi Michael,

Is it possible to ship a board to me? Please email me to discuss on the next steps and to setup a call for discussion.

Best Regards,
Rakesh