DP9900M PoE module getting hot using with W5500

I am using a DP9900M module and PoE with a W5500 using the reference design. The transformer of the DP9900M module is getting very hot (>50 degrees Celcius), so I suspect the reference design is drawing too much power? The DP9900M delivers 5V, so I use a SY8089 to get a stable 3.3V (checked) that I use to feed the W5500. I made one mistake, R3 is 49R9 instead of 10R but that does not case the problem I guess? If I cut off R1, R2 and R3 from the PCB the heating stops (but I can not connect anymore). If R1, R2 and R3 are pull ups, I think their value is rather low. Might that be the cause of the problem? It is difficult to experiment because my PCB has 0402 sized resistors. Any suggestion is very welcome!

Hii, :wave:

First up, you mentioned that resistor R3 is a whopping 49R9 instead of 10R. That difference could definitely be causing problems. Even though it might not seem like the main culprit, resistors are like tiny traffic cops for current, and the wrong value can lead to unexpected power draw.

Interestingly, when you cut off R1, R2, and R3, the overheating stops. That strongly points to these components being involved. Now, if R1, R2, and R3 are pull-up resistors, their low values could absolutely be causing the excessive current draw and heat. Normally, pull-up resistors have much higher values, like between 4.7kΩ and 10kΩ, to keep the current flow in check. Values as low as 49.9Ω and 10Ω are super low for pull-ups and would allow way too much current to flow, which explains the heat buildup.
You can also check this: Faults in W5500-based prototype boarsnowflake

I hope this will help you.

Respected community member :innocent:

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Hi Ben,

Thanks for your kind reaction! Unfortunately it does not help yet…

The point is that these resistor values come from the official Wiznet reference design! So my question to Wiznet is: are they correct since they are so low?

In other situations I recognize the pullup values that you mention. And for the mistake a made: I used a higher resistance than the reference design. Taking the reference design value lowers the resistance and increases the current even more.

BTW, the circuit works perfectly, only the PoE to 5V module DP9900M is getting very hot!

Best, Jeroen

Desolder the module and measure the current being consumed. Publish your design with part numbers, maybe you have made other mistakes causing these symptoms.

Hi @Eugeny, thanks for your advice. I borrowed a thermal imaging camera and found out that the 3 resistors are not hot at all, so not a lot of current is flowing though these resistors. Also, I found an almost identical PoE module, the AG9905-MTB of Silver Telecom. This module has a “thermal datasheet” stating that at an environmental temperature of 30 degrees Celcius and a power delivery of 9 Watts at 5 Volts (mine is about half I guess), the temperature of the transformer will be 50 degrees Celcius, and of one of the diodes even higher!! So to my surprise these high temperatures are normal. So closing the topic for now :).

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Hello there
Thanks for sharing these valuable information this is helpful for my new projects :heart_eyes:

Hi Helenne, if you are going to use a PoE module I suggest the AG9900-MTB series (they come in different output voltages as indicated by the last two digits) instead of the DP9900M series. The latter is cheaper but mine stopped working after four days. My best guess is that it’s thermal protection kicked in. The AG9905-MTB I use is also hot but not so extreme. It is about 20 degrees Celcius cooler (but more expensive)! Also, under low loads, you can hear it. This is also stated in the datasheet. Both modules have the same functionality and are pin compatible. Best regards, Jeroen

Hello, I’m WIZnet Alan.

What reference circuit diagram are you looking at?

Hi Alan, I have used these .

Hello @Jeroen88
Excessive heat in your DP9900M module is likely due to high power consumption. Removing R1, R2, and R3 resolves the issue, suggesting they are the cause. Try replacing them with higher-value resistors (e.g., 1 kOhm) to reduce current draw. Verify your voltage regulator’s performance and measure current consumption to identify potential excessive power draw.

R1, R2 are Ethernet termination resistors.
R3 is the damping resistance of the offset value of the trans.

These values will never cause the PoE Module to be heated.

Never remove or change these values.

The reason why there is no heat when removed may be because the current value decreases due to the lack of Ethernet communication.

I don’t think the problem will be with the reference circuit. Does PoE output the voltage well without noise?

Isn’t there a fine frequency sound by any chance?

Thanks for the solution , this is very helpful for me.

Hi Alan, thank you for your insights! Now we know that R1 and R2 are not pull ups buts termination resistors. I think the problem is in the module itself. After the PoE module that outputs 5 Volts in my case, I have a small circuit aroud a SY8089 that brings that back to the required 3.3 Volts, so I think the output voltage is stable. Also the consumption of the W5500 itself is low, in combination with R1, R2 and R3 not causing a lot af extra load, I believe it must be the module itself. I de-soldered it and replaced it with a pin compatible other module and that one stays at least 20 degrees cooler under the same load.

I’m glad it worked out well.

What module did you replace with on the DP9900M and did it get better?

Hello,
Can you please explain this in more.