How to prevent switching power supply ripple
Generation of Ripple in Switching Power Supply
Our goal is to reduce the output ripple to a tolerable level, and the fundamental solution to achieve this goal is to avoid the generation of ripple as much as possible. Firstly, we need to clarify the types and causes of ripple in the switching power supply.
Following the switch of SWITCH, the current in inductor L also fluctuates up and down within the effective value of the output current. So there will also be a ripple at the output end with the same frequency as SWITCH, which 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 a switching power supply, ranging from tens to hundreds of KHz.
In addition, SWITCH generally uses bipolar transistors or MOSFETs. Regardless of which one is used, there will be a rise time and a fall time when it is turned on and off. At this point, a noise with the same frequency or odd multiples of the SWITCH rise and fall time will appear in the circuit, usually in the tens of MHz range. At the moment of reverse recovery, the equivalent circuit of diode D is a series connection of resistance, capacitance, and inductance, which can cause resonance and generate noise frequencies of several tens of MHz. These two types of noise are generally called high-frequency noise, and their amplitude is usually much larger than ripple.
If it is an AC/DC converter, in addition to the two types of ripple (noise) mentioned above, there is also AC noise, which is the frequency of the input AC power supply, around 50-60Hz. There is also a type of common mode noise, which is caused by the equivalent capacitance generated by the power devices of many switching power supplies using enclosures as heat sinks. As I am engaged in automotive electronics research and development, I have less exposure to the latter two types of noise, so I am not considering them at the moment.
Measurement of Ripple in Switching Power Supply
Basic requirements: Use oscilloscope AC coupling, 20MHz bandwidth limit, unplug the probe's ground wire
1. AC coupling is the process of removing the superimposed DC voltage to obtain the correct waveform.
2. Opening the 20MHz bandwidth limit is to prevent interference from high-frequency noise and to prevent measurement errors. Due to the large amplitude of high-frequency components, they should be removed during measurement.
3. Unplug the grounding clip of the oscilloscope probe and measure with a grounding ring to reduce interference. Many parts do not have grounding rings, and if the error is acceptable, they can be directly measured using the probe's grounding clamp. But this factor should be considered when determining whether it is qualified.
Another point is to use a 50 Ω terminal. According to the information on the Yokogawa oscilloscope, the 50 Ω module measures the AC component after removing the DC component. However, few oscilloscopes are equipped with such specialized probes. In most cases, standard probes ranging from 100K Ω to 10M Ω are used for measurement, and the impact is currently unclear.
The above are the basic precautions when measuring switch ripple. If the oscilloscope probe does not directly contact the output point, it should be measured using twisted pair cables or 50 Ω coaxial cables.
When measuring high-frequency noise, the full passband of an oscilloscope is generally in the range of several hundred megahertz to GHz. Others are the same as above. Different companies may have different testing methods. Ultimately, it is important to have a clear understanding of one's own test results** To gain customer recognition.
