How to Measure the Ripple of DC Power Supply or SMPS or LED Driver
What is Ripple? Why would you like to measure it?
Ripple is the small unwanted residual periodic variation of the direct current (dc) output of a power supply which has been derived from an alternating current (ac) source.
This ripple is due to incomplete suppression of the alternating waveform within the power supply.
In AC to DC converters, AC voltage is apparent.
But in DC-DC converters, we are switching the input DC. This results in the ripple being introduced.
Waveform Analysis
The below waveform shows the DC Supply and Ripple or AC Component in a AC-DC Supply.
Looking at this level, it looks like a constant DC is achieved.
Looks like we have a constant DC 5.0V here
Looking closely, we can see a distinct voltage ripple
How you can measure ripple by Instrumentation available on Production Shop Floor?
We needs to test a SMPS with the following output ratings:
5V, 1A. Tolerance 2%
We are going to test this SMPS at no load condition.
I am using a Fluke 115 Multimeter. Note that the Selector is on DC Volts
When we check DC Voltage it shows 5.040V
This is within the claimed tolerance level of 2%.
The question still remains, what is the Ripple it is giving at No load?
Next we select the AC Voltage on the Multimeter.
In above image you can see the AC Voltage present with DC.
The ripple voltage is 0.021 V. That is, 21mV.
This is the way ripple is measured.
And according to us, this is the way ripple is measured.
How to Calculate Ripple Percentage
Here we are going to calculate ripple percentage.
Ripple percentage in DC supply is calculated by below formula:
Percentage Ripple = (output AC component/Output DC Component) * 100
For above example:
Ripple Percentage = (21mV / 5.040V) * 100 = 0.41%
We have seen is testing laboratories also, that this is the accepted method.
If you find any difference, or you can send feedback, please do so at leddrivertesting@gmail.com
More Resources
Video on Ripple Power Supply
Another Video of interest, explaining ripple voltage is by EEVBlog
Watch VideoThis video shows how to measure and analyse the ripple in any SMPS.
Ripple is generated due to charge and discharge of capacitor. Noise is everything else on high frequency.
This videos shows the difference between Analog and digital oscilloscope. It shows the waveform in both oscilloscopes.
Digital oscilloscope may be slow due to high calculations and analog is fast.
Ripple measurement is also depend on connection type between scope and supply.
There is an internal resistance presence on probes. due to which waveform may be looking distorted more.
This video also stated the bandwidth of ripple voltage present in SMPS. This Video shows the analysis of 5V 2A power supply. It contains 2mV ripple of 20MHz Bandwidth.