Analysis on Switching Power Supply Ripple Measurement and Suppression

Our ultimate goal is to reduce the output ripple to a tolerable level. The most fundamental solution to achieve this goal is to avoid the generation of ripple as much as possible. First of all, we must understand the types and causes of switching power supply ripple.

 
As the switch turns on and off, the current in the inductor L also fluctuates around the effective value of the output current. Therefore, a ripple with the same frequency as the switch will also appear at the output end. This is what is generally referred to as ripple. It is related to the capacity and ESR of the output capacitor. The frequency of this ripple is the same as that of the switching power supply, which is tens to hundreds of kHz.

In addition, SWITCH generally uses bipolar transistors or MOSFETs. No matter which type, there will be a rise time and fall time when it is turned on and off. At this time, a noise with the same frequency or an odd multiple of the rise and fall time of SWITCH will appear in the circuit, which is generally tens of MHz. Similarly, at the moment of reverse recovery, the equivalent circuit of diode D is a series connection of resistance, capacitance and inductance, which will cause resonance and the noise frequency generated is also tens of MHz. These two types of noise are generally called high-frequency noise, and the amplitude is usually much larger than the ripple.

If it is an AC/DC converter, in addition to the above two ripples (noise), there is also AC noise, the frequency of which is the frequency of the input AC power supply, which is about 50-60Hz. There is also a common mode noise, which is caused by the equivalent capacitance generated by the use of the casing as a heat sink by the power devices of many switching power supplies. Because I am engaged in automotive electronics research and development, I have little contact with the latter two types of noise, so I will not consider them for the time being.

Switching Power Supply Ripple Measurement

Basic requirements: Use an oscilloscope with AC coupling, 20MHz bandwidth limit, and unplug the ground wire of the probe

1. AC coupling is to remove the superimposed DC voltage to obtain an accurate waveform. 
2. Opening the 20MHz bandwidth limit is to prevent interference from high-frequency noise and prevent erroneous results. Because the high-frequency component has a large amplitude, it should be removed during measurement. 
3. Unplug the grounding clip of the oscilloscope probe and use the grounding ring for measurement to reduce interference. Many departments do not have grounding rings. If the error allows, the grounding clip of the probe can be used directly for measurement. However, this factor should be considered when judging whether it is qualified. 
Another point is to use a 50Ω terminal. Any oscilloscope document states that the 50Ω module removes the DC component and accurately measures the AC component. However, few oscilloscopes are equipped with such special probes. In most cases, standard 100KΩ to 10MΩ probes are used for measurement, and the impact is not clear for the time being.

The above are the basic precautions when measuring switching ripple. If the oscilloscope probe is not directly in contact with the output point, it should be measured using a twisted pair or 50Ω coaxial cable.

When measuring high-frequency noise, use the full passband of the oscilloscope, which is generally from several hundred megahertz to GHz. The rest is the same as above. Different companies may have different test methods. In the final analysis, the first thing is to know your test results. The second is to get customer approval. 
About oscilloscopes:

Some digital oscilloscopes cannot measure ripple correctly due to interference and memory depth. At this time, the oscilloscope should be replaced. In this regard, although the bandwidth of the old analog oscilloscope is only tens of megabytes, it performs better than the digital oscilloscope. 
Tektronix has software specifically for measuring the above two types of ripple (noise) separately. Similarly, you can also take a look at the relevant knowledge about the grounding and power supply testing of the oscilloscope.