Methods to Reduce DC-DC Converter Output Ripple

I am Steve Butler. Today we'll be discussing DC-DC converter output ripple, its characteristics, and how it may be reduced. This is a basic diagram of a switching topology we use in our DC-DC converters. This is a forward topology. 

We also use forward, and we use flyback, in a lot of our converters. This topology will create output ripple in its output capacitor. So the current in this inductor is a triangle wave current, and that current will flow in this capacitor and create some voltage ripple.

Separately, we also use a flyback topology. Flyback topology is slightly different in that it creates a pulsating current – something like this – which also flows in the output capacitor and creates voltage ripple on that capacitor.

Internal to most all of VPT's DC-DC converters, we include a second stage output filter, along with low ESR (equivalent series resistance) output capacitors; this gives very low output ripple for the basic DC-DC converter by itself. 

Output ripple of the DC-DC converter can include both differential mode and common mode components. 

Usual waveform will look something like this: it will include a sinusoidal component, which is due to this current flowing in this capacitor. It will also include some higher frequency switching spikes due to switching events occurring on the diode’s rectifiers. 

All this needs to be filtered, and is filtered, by this LC here. But this is an approximation of the waveform you might see at the output of the converter. 

The output ripple of the DC-DC converter is fairly well filtered with the converter standalone. It may be in the range of 20 millivolts (mV) to 50 mV peak-to-peak.

For applications which require a much lower output ripple, the first thing we always recommend is adding an output capacitor. 

This is by far the simplest way to reduce output ripple and output noise, and it is the most effective. Use a low ESR ceramic output capacitor. In fact, we would recommend a small capacitor, one microfarad (μF) to 10 μF, for any application.

For ultra-low ripple requirements, you might push that as far as 100 μF, and this can get your ripple down into the 10 mV peak-to-peak range. 

For super sensitive applications where you need ripple even lower than that, you might add an LC filter. So that includes an inductor and a second capacitor. 

This adds an LC, an extra roll-off in the output, and so it can give you a lower output ripple noise. There is some danger here; if this converter has remote sense lines, you have to be careful. 

The remote sense lines need to be connected to this side of this extra LC. Otherwise, the LC can affect the internal control loop of the DC-DC converter. 

The corner frequency of the LC added on the output, also, you need to be careful – it cannot be too low. If it is too low, it may still affect the internal control loop. 

So usually we recommend a fairly small L out there, a few microhenries (μH), and keep that corner frequency maybe 50 kilohertz (kHz) or so. If you bring the corner frequency too low, you're going to need to damp that LC, so it does not have a high Q factor. 

For other applications which may require low ripple, you might choose a common mode (CM) filter. The common mode filter will help to reduce common mode noise generated by the DC-DC converter, as well as reduce the α-ripple, some of which is common mode. 

It may also improve your EMI, and it may do a better job filtering high-frequency noise spikes. But you have to make sure to be careful of the same danger of the LC, just have to watch out for the resonance of the common mode filter, that it’s not too high-Q and not too low frequency, such that it interferes with the internal control loop bandwidth. 

So that's a few ways to deal with output ripple. Now, let's go in the lab and take a look at some of this in practice. 

Now we'll take a look at the output ripple here in the lab. This is the DVHF2805S DC-DC converter. It's 15 W, 5 V at 3 A output. The setup here, I'm measuring the output ripple simply with an oscilloscope probe. We've removed the ground clip and we've plugged the probe directly into a socket at the output of the converter. 

Now let me turn the converter on. 

Now, here on the oscilloscope, we can see the output ripple, and we can see about 30 mV peak-to-peak. This is measured with a 20 MHz bandwidth on the oscilloscope. 

The specification for this part is 40 mV peak-to-peak. Now, if I turn the bandwidth limit off on the oscilloscope, I can see there's quite a bit more ripple, especially high frequency. 

Now, the best way to filter this is to add an additional output capacitor to the converter. So we will do that very quickly. 

We'll use a low ESR ceramic capacitor – remove the oscilloscope probe to measure that. Now we're looking at the output ripple with a low ESR ceramic capacitor. We have 20 μF added to the output. 

Again, this is full-bandwidth on the oscilloscope, and you can see that we are about 10 mV peak-to-peak. Now, if we change the bandwidth back to 20 megahertz, you can see we have less than 5 mV of ripple at the output. 

Once again, the best way to reduce the output ripple on your DC-DC converter is simply with a low ESR ceramic capacitor added to the output. That can get you very low ripple, and it's very easy to do with few adverse effects. 

This concludes our video on DC-DC converter output ripple. Thank you.