Chinese LX-2BUPS UPS module EMI issues - Page 1
Forum section: electronics / general

author: radiolistener
author_rank: Super Contributor
timestamp: February 26, 2024, 11:36:51 am
content: I bought this Chinese LX-2BUPS which seems very popular:https://www.aliexpress.com/item/1005005195434497.htmlSee PCB picture below. Here is details:U1 =TP4056- 1A Li-lon Battery ChargerU2,U3 =XB7608- One Cell Lithium-ion/Polymer Battery ProtectionU4 =XR2981- 24W Output Step-up DC/DC ConverterThe 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 kΩIn 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 satisfyXR2981PCB 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 thatTP4056layout also has some EMI issue, but I'm not sure what is wrong?

author: radiolistener
author_rank: Super Contributor
timestamp: March 11, 2024, 07:07:22 am
content: 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

author: shadow.dark
author_rank: Contributor
timestamp: March 11, 2024, 07:48:40 am
content: 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

author: Dave_g8
author_rank: Regular Contributor
timestamp: August 28, 2024, 11:05:59 pm
content: Hi,On the modules I have tested, adding a ferrite bead and capacitor to GND on input and output does seem to reduce the high frequency noise.For information, a few observations on the module below.A simplified block diagram based on the PCB layout and components is shown below.- Care needs to be taken when inserting the batteries, there is no protection against battery voltage difference, which could cause high instantaneous currents.- Depending on the power supply feeding the USB-C (5V+) input, when the power source fails, if there are large reservoir capacitors on its output, the input load sharing circuit will not operate as intended. A note from the advert for the module implies it may be necessary to add a discharge resistor on the (5V+) input to ensure that the input load share switches quicker when the source power supply it turned off.- The output from the DC/DC boost converter will not regulate if the input voltage is higher than the programmed output voltage. Provided the output voltage is not set below 5V, this should not be a problem provided the (5V+) input minus the load share Schottky diode voltage drop does not exceed 5V.- Although there is protection circuitry on the battery, if the output of the boost circuit is shorted, there will be a high current path via the inductor and diode, and this will not trip the battery protection circuit. With batteries fitted in excess of 10A flows when the UPS+ output is shorted to UPS- (GND). This is an inherent issue with this type of boost converter. Adding some sort of load switch with current and short circuit detection on the output is recommend. For the 5V version, there are many devices available, generally such devices are used on USB power ports. A latched ‘OFF’ type is preferable to avoid damage to the UPS module and/or the connected circuit.73, Dave