What can be done to prevent the ripple of switching power supply?
Following the SWITCH of the switch, the current in the inductor L also fluctuates up and down the effective value of the output current. Therefore, there will also be a ripple at the output with the same frequency as the SWITCH, which is what is generally referred to as ripple. It is related to the capacity of the output capacitor and ESR.
How to restrain the generation of switching power supply ripple, 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 this goal is:
Generation of ripple in switching power supply
Our goal is to reduce the output ripple to a tolerable level. The fundamental solution to this goal is to avoid the generation of ripple as much as possible. First of all, we must be clear about the types and causes of ripple in switching power supply.
Following the SWITCH of the switch, the current in the inductor L also fluctuates up and down the effective value of the output current. Therefore, there will also be a ripple at the output with the same frequency as the SWITCH, which is what is generally referred to as ripple. It is related to the capacity of the output capacitor and ESR. The frequency of this ripple is the same as that of switching power supply, which is tens to hundreds of KHz.
In addition, the SWITCH generally chooses bipolar transistor or MOSFET, no matter which one, there will be a rise time and a fall time when it is turned on and off. At this time, there will be a noise in the circuit with the same frequency or odd frequency multiplication as the rise and fall time of SWITCH, usually tens of MHz. Similarly, at the moment of reverse recovery, the equivalent circuit of diode D is the series connection of resistance, capacitance and inductance, which will cause resonance and the noise frequency will be tens of MHz. These two kinds of noise are generally called high frequency noise, and the amplitude is usually much larger than ripple.
If it is an AC/DC converter, in addition to the above two kinds of ripple (noise), there is AC noise, and the frequency is the frequency of the input AC power supply, which is about 50 ~ 60 Hz. There is also a kind of common-mode noise, which is caused by the equivalent capacitance produced by many power devices of switching power supply using the shell as the radiator. As I am engaged in automotive electronics research and development, I have little contact with the latter two kinds of noise, so I will not consider them for the time being.
Measurement of ripple of switching power supply
Basic requirements: use oscilloscope AC coupling, 20MHz bandwidth limitation, unplug the probe ground wire.
1, AC coupling is to remove the superimposed DC voltage and get the correct waveform.
2. Opening the 20MHz bandwidth limit is the result of preventing the interference of high-frequency noise and preventing measurement errors. Because of the large amplitude of high frequency components, they should be removed when measuring.
3. Unplug the grounding clamp of the oscilloscope probe and use the grounding ring to measure, in order to reduce interference. Many parts don't have grounding rings, so if the error is allowed, they can be measured directly with the grounding clamp of the probe. However, this factor should be considered when judging whether it is qualified or not.
Another point is to use a 50Ω terminal. According to the information of Yokogawa Oscilloscope, the 50Ω module removes the DC component and measures the AC component. However, few oscilloscopes are equipped with this kind of special probe, and in most cases, the standard probe of 100KΩ to 10MΩ is used to measure, so the influence is not clear for the time being.
The above are the basic precautions when measuring switch ripple. If the oscilloscope probe is not in direct contact with the output point, it should be measured by twisted pair or 50Ω coaxial cable.
When measuring high-frequency noise, the all-pass band of oscilloscope is used, which is generally several hundred megabytes to GHz level. Others are the same as above. Maybe different companies have different testing methods. In the final analysis, * * be clear about your test results. * * Be recognized by customers.
About oscilloscope:
Some digital oscilloscopes cannot accurately measure the ripple due to interference and storage depth. At this time, the oscilloscope should be replaced. In this respect, although the bandwidth of the old analog oscilloscope is only a few tens of megabytes, its performance is better than that of the digital oscilloscope.
