Dc dc converter output noise problem

Thread Starter

ilginsarican

Joined Jul 13, 2017
96
Hello,
I use uvq-24/4.5-d24p-c DC Dc converter.
When I connect resistor to the output of converter I saw 688mV peak signal on oscilloscope.
I wonder why this happened.
I use different model UWE-24/3-Q12P-C then I saw 1.4V peak signal on output of converter
 

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ericgibbs

Joined Jan 29, 2010
9,522
hi,
I have always found that this type of modular SMPS is very noisy, they need lots of output decoupling capacitors.
Try to avoid using them on low level signal circuits.
As a test add a 100uF 10V and 100nF cap on the output and retry.
E
 

AlbertHall

Joined Jun 4, 2014
9,263
What value resistor?
How are the output and sense terminals, resistor, and 'scope connected?
Are there any other connections to the output of the supply?
How 'clean' is the input voltage?
What frequency is the signal you see on the 'scope?
 

Thread Starter

ilginsarican

Joined Jul 13, 2017
96
What value resistor?
How are the output and sense terminals, resistor, and 'scope connected?
Are there any other connections to the output of the supply?
How 'clean' is the input voltage?
What frequency is the signal you see on the 'scope?
The resistor is 47 Ohm, 11Watt.
The frequency of the signal 200khz.
The input signal has 40mV peak,there are not connections to the output of the supply.
 

AlbertHall

Joined Jun 4, 2014
9,263
I saw about 200mV peak on the supply(24V) signal and 40mV peak on the ground pin of supply.
With the resistor and the scope connected across the output you saw 688mV on the scope.
If you take that arrangement but connect the probe tip to the same supply output pin as the ground what do you see on the scope?
 

Thread Starter

ilginsarican

Joined Jul 13, 2017
96
I wrapped the converter with insulating tape card then wrapped with EMI
So most of the noise you are seeing is a measurement artifact. See post #9 link
I connected common mode choke filter and ceramic capacitor out pin of the converter.
Then I wrapped the converter with EMI Fabric.
I measured output signal then I saw 120mV peak.
 

AlbertHall

Joined Jun 4, 2014
9,263
The specification says typical 100mV, max 200mV so you are now in the ballpark.
I can tell you, having tried doing this professionally, it takes great care to get repeatable results.
 

ericgibbs

Joined Jan 29, 2010
9,522
hi il,
As I said in post #2, I have used a lot of these modules and I have found it essential to add filtering on the output lines, also keep the SMPS module away from low level signals. It also helps if you add a 100nF cap close to the module on the input rails.
For plastic enclosures you can buy conductive spray or self adhesive foil for screening.

E
 

ebp

Joined Feb 8, 2018
2,332
It looks like you are using a wirewound resistor as a test load. They make terrible test loads for this sort of thing because they are inductive, so not only do you have a load which rises in impedance with rising frequency, but also it can resonate with the output capacitance of the converter.

You cannot use an oscilloscope probe ground lead and get accurate results. The ground lead is inductive and forms a resonant circuit with the probe's capacitance. You must use a probe tip grounder right at the power supply output terminals.

You cannot use two pieces of wire draped across a bench to connect the converter to the input power supply - they are inductive (see a theme here?). The wires should be twisted into a pair so that the equal and opposite magnetic fields can at least partially cancel each other, minimizing inductance.
 
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