How to prevent the generation of power ripple
With the switch of SWITCH, the current in inductor L also fluctuates up and down at the effective value of the output current. So a ripple with the same frequency as SWITCH will also appear at the output end, which is generally referred to as the ripple. It is related to the capacity of the output capacitor and ESR.
How to suppress the generation of switching power supply ripple? Our goal is to reduce the output ripple to a tolerable level, and the most fundamental solution to achieve this goal is:
The Generation of Ripples in Switching Power Supplies
Our goal is to reduce the output ripple to a tolerable level, and the most 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 switching power supply ripple.
With the switch of SWITCH, the current in inductor L also fluctuates up and down at the effective value of the output current. So a ripple with the same frequency as SWITCH will also appear at the output end, which is generally referred to as the ripple. It is related to the capacity of the output capacitor and ESR. The frequency of this ripple is the same as that of a switching power supply, ranging from tens to hundreds of KHz.
In addition, SWITCHs generally use bipolar transistors or MOSFETs, regardless of which type, there will be a rise time and a fall time during their conduction and cutoff. At this point, there will be a noise in the circuit that is the same frequency as the rise and fall time of the SWITCH or odd multiples, usually several tens of MHz. Similarly, at the moment of reverse recovery, the equivalent circuit of diode D is a series connection of resistors, capacitors, and inductors, which can cause resonance and generate noise frequencies of tens of MHz. These two types of noise are generally called high-frequency noise, and their amplitude is usually much larger than the ripple.
If it is an AC/DC converter, in addition to the two types of ripple (noise) mentioned above, there is also AC noise. The frequency is the frequency of the input AC power supply, which is around 50-60Hz. There is also a common mode noise caused by the equivalent capacitance generated by the use of enclosures as heat sinks in many power devices of switching power supplies. As I am involved in the research and development of automotive electronics, I am not considering the latter two types of noise due to my limited exposure.
Measurement of Switching Power Supply Ripple
Basic requirements: Use oscilloscope AC coupling, 20MHz bandwidth limitation, unplug the ground wire of the probe
1. AC coupling is to remove the superimposed DC voltage and obtain the correct waveform.
2. Opening the 20MHz bandwidth limit is to prevent interference from high-frequency noise and prevent measurement errors. Due to the large amplitude of high-frequency components, they should be removed during measurement.
3. Unplug the grounding clamp of the oscilloscope probe and use a grounding ring for measurement to reduce interference. Many parts do not have a grounding ring, and if the error is allowed, it can be directly measured using the grounding clamp of the probe. But this factor should be considered when determining whether it is qualified.
Another point is to use a 50 Ω terminal. As mentioned in the Yokogawa oscilloscope data, the 50 Ω module measures the AC component after removing the DC component. However, there are few oscilloscopes equipped with such specialized probes, and in most cases, the standard 100K Ω to 10M Ω probes are used for measurement, which temporarily affects the clarity.
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 or 50 Ω coaxial cable.
When measuring high-frequency noise, use the full pass band of an oscilloscope, usually in the range of hundreds of megabits to GHz. The others are the same as above. Different companies may have different testing methods. Ultimately, it is important to have a clear understanding of your test results. Finally, we need to gain customer recognition.
