TPS2376-H: PD not getting past detection stage of PoE Handshake

Hi Tim,

Thank for reaching out.  

Could you point out the specific net name of your scope screenshot of PoE signal? Is it +48v_POE?

If so, the waveform is pretty strange.

Could you check the connection of front end of rectifier is correct or not?

Regards,

Ben

Hey Ben,

Thanks for response.  Yes, the signal on the scope is +48V_PoE.  I believe I have the front end of the diode bridge hooked up correctly.  It is coming from pins 9 and 10 of my RJ45 which are the spare ethernet pairs on my RJ45 (ie pin J4/J5 shorted internally and J7/J8 shorted internally) where I should be getting power and ground from my Mode B switch.  See screencap below.  Rethinking this for a minute, I am not capturing the initial plug in of my ethernet cable so perhaps I am missing the first handshake attempts and what I am capturing there is just garbage.  I will remeasure and see what I get. 

So I retested and captured the signal at startup and its exactly the same.  Some more details - that signal above is actually with the ground of the probe floating.  If I connect my probe to +48V_PoE and ground clip to PoE_RTN I get 0 volts.  If I just connect probe and float ground, I get that same signal on +48V and PoR RTN.  I tested the switch on a commercial PD device and it worked fine, so the switch doesn't appear to be the problem.  I opened the switch up and attempted to probe the correct pins on the circuit board and got a similar signal, but can't guarantee I was looking in exact right spot.  The commercial PD is also gigabit and I assume performing Mode A PoE. 

Hi Tim, as 

It will be very helpful for us to debug if you could provide the VDD_VSS start-up sequence waveform as below. It should not be as sin wave at beginning. Thanks.

Regards,

Ben

Ben,

Looks like I had my probe set to 10x accidentally.  But results are still not good.  The below screenshot is Vdd-Vss.  Just get an ugly wave that goes from 0-4V over and over even when connected from initial plug in to switch. 

Hi Tim,

Could you use single-shot triggering to capture the initial waveform? 
The waveform is pretty strange. Would like to understand what happened in the beginning? Thanks.

Regards,

Ben

Yes, I can capture initial waveform.  But it literally just goes from 0V to that waveform..  I will capture and resend. 

I guess the first three pulses are slightly different, but still does not seem like its even on the right track. 

Hi Tim,

Could you confirm the R16 is DNP or not? If possible, could you check whether is a parasitic capacitor from SGND?

Maybe use RLC meter to measure the capacitor between POE_RTN and SGND. Thanks.

Regards,

Ben

Ben,

R16 is not placed.  I do not have an RLC meter to do a direct measurement.  However, I have PoE_RTN on a roughly 200mm^2 plane on layer 2 with SGND underneath it separated by ~.127mm dielectric.  Some quick math gives me a ~56pF interplane capacitance.  Is this too much?  Should i have a keepout of SGND underneath my Poe_RTN plane? 

Tim, 

It should be fine. If possible, could you measure the P9, P10 detection waveform from PSE side before plugged- in?

Could you provide the detail circuit of D3 which is full-wave rectifier? 

Regards,

Ben

Here is link to data sheet of diode bridge - https://www.vishay.com/docs/88573/dfs.pdf

I will open up switch again and try to find good place to probe to get the signal from the PSE...

This is pin 4 referenced to pin 8 on the RJ45 from the PSE when not plugged into my device. 

Same pins monitoring as soon as I power on PSE. 

Thanks, Diang.  I will perhaps purchase the eval kit for future use.  Can you PM correct address / POC to send a PD to?

-Tim

Hi Tim,

Sure. I will reach out to you in email.

Best regards,

Diang