TPS23756: TPS23756 damaged after using Adapter Option 3

Chentsu,

Put a diode on both outputs to prevent current flowing backwards into the secondary side.

- Tom

Hi Tom,

Thanks for your response, but in this schematic, it use Rectifier Diode, and also a diode right after the adapter output.
This just like what you mention "Put a diode on both outputs ".

1. If the schematic is put diodes on both outputs, how this happened? RC?
2. If the external power backward to pri. side, the voltage should be 5.1V times turn ratio, but this voltage is not too high to damage IC?

Thank you very much.

Hi Chentsu,

Can you forward the schematic that I can review? We've done this type of ORing in the past and would like to see how it is implemented by your customer. Thanks!

Regards,
Darwin

In addition, other than the IC, is any other component damaging? Thanks!

Hi Darwin,

Thanks for your responce, the device got only TPS23756 damaged, here is the impedance mesurement below:

Pin	Pass IC	Fail IC
1	70.9K	72.6K
2	70.9K	72.6K
3	333	334.4
4	1.3	1.2
5	0.814M	0.842M
6 (VC)	12.53K	69.5K
7	1.3	1.2
8	1.3	1.2
9	10.38M	10.38M
10	12.55M	12.46M
11	10.38M	10.38M
12	open	open
13	open	open
14	open	open
15	open	open
16	71.9	72.8
17	1.3	1.2
18	82K	82.1K
19	68K	67.2K
20	13.87M	14.28M

Schematic:

 

Hi Chentsu,

Thanks for this info. A couple comments:

I have not seen this type of FET ORing circuit before. It looks like it turn ON when PoE is present rather than OFF (if it was ORing). Can you try with a diode on the adapter and remove the FET?

Also if the VC pin is damaging, it could be peak charging. Can you try increasing R20. I would start with 22 ohms then 50 ohms. I've seen this resistor go up to 100 ohms in the past.

Regards,
Darwin

Hi Darwin,

I will try this with our customer, but what's the side effect if modify R20?

 
BTW, so form your perspective, does the adapter voltage possible backwark to primary side even though D1/D2 are there?
If not, so I should focus on Vc's stress rather than external adapter?

Thank you.

Hi Darwin,

Thanl you, the issue has been foxed. Customer found voltage spike at VC when adapter connected, and the voltage spike reduced after modify the resister.

BTW, they use D1 and D2 as sec. side rectifiers.