Author Topic: Chinese LX-2BUPS UPS module EMI issues  (Read 3885 times)

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Offline radiolistenerTopic starter

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Chinese LX-2BUPS UPS module EMI issues
« on: February 26, 2024, 11:36:51 am »
I bought this Chinese LX-2BUPS which seems very popular:
https://www.aliexpress.com/item/1005005195434497.html

See PCB picture below. Here is details:
U1 = TP4056 - 1A Li-lon Battery Charger
U2,U3 = XB7608 - One Cell Lithium-ion/Polymer Battery Protection
U4 = XR2981 - 24W Output Step-up DC/DC Converter

The instance that I got has 4.95V on the output with no load and 4.92 with 1A load, but it seems that its easy to fix to get 5.1V by replacing R7 with formula from datasheet:
R7 = (5.1/1.24-1)*10k = 31 kΩ

In my instance R7=30 kΩ which should produce 4.96V, which well corresponds with measured 4.95V (with take into account resistor tolerance).


It appears that this UPS produce a lot of EMI noise at VHF band (near 145 MHz). Any idea how to reduce it?

At first look, it don't satisfy XR2981 PCB guide, which require to minimize path from IC to inductor and capacitors. Also it has too large surface for inductor pads which increase parasitic capacitance.

At a glance I think about cut-off excess conductors surface for the inductor and add ferrite beads on the output. What is your thoughts?

Also I notice a huge noise appears when I connect external power, so it seems that TP4056 layout also has some EMI issue, but I'm not sure what is wrong?
« Last Edit: February 26, 2024, 12:01:59 pm by radiolistener »
 

Offline radiolistenerTopic starter

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Re: Chinese LX-2BUPS UPS module EMI issues
« Reply #1 on: March 11, 2024, 07:07:22 am »
I tried to add two large ferrite beads on output (GND and +5V), it looks like noise at 145 MHz is reduced a little bit, but still present
 

Offline shadow.dark

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Re: Chinese LX-2BUPS UPS module EMI issues
« Reply #2 on: March 11, 2024, 07:48:40 am »
I observed the PCB layout and was concerned that the designer may not have considered EMC as he violated many power supply design rules, such as lack of low equivalent resistance ceramic capacitors, too large hot loop area, etc.
There are no easy solutions to EMC problems; once the noise is created, it is very difficult to eliminate, and there is no one-size-fits-all device that will solve the problem quickly. If it is affecting your VHF communications, my advice is to just throw this product away.

Slava Ukraini
 
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