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 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.

 

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.

looking first, This SMPS simulation shows constant dc voltage

Looks like we have a constant DC 5.0V here

 

Looking closely we can see dc ripple in the SMPS

 

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%.

.

Electronic Meter shows 5.040V dc.

 

 

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 percenatge = (21mV / 5.040V) * 100  = 0.41%


 

 

We have seen is testing laboratories also, that this is the accpeted method.

If you find any difference, or you can send feedback, please do so at

leddrivertesting@gmail.com

 


MORE RESOURCES

Another Video of interest, explaining ripple voltage is by EEVBlog

Video on Ripple Power Supply

This video shows how to measure and analyse the ripple in any SMPS.

Ripple is generated due to charge and discharge of capacitor. Noise is everyhing 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.

 

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(Updated: December 10, 2018)