Generation, measurement and suppression of ripples in switching power supply adapters.ac power adapt

　　Our ultimate 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 know the type and cause of the ripple of the switching power supply.

　　With the switch of SWITCH, the current in the inductor L fluctuates up and down the effective value of the output current. Therefore, a ripple with the same frequency as SWITCH will also appear at the output end. Generally speaking, the ripple refers to this. 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, ranging from tens to hundreds of KHz.

　　In addition, SWITCH generally uses bipolar transistors or MOSFETs, no matter which is, there will be a rise time and a fall time when it is turned on and off. At this time, a noise with the same frequency as the SWITCH rise and fall time or an odd multiple of the frequency will appear in the circuit, generally tens of MHz. Similarly, at the moment of reverse recovery of diode D, its equivalent circuit is the series connection of resistance, capacitance and inductance, which will cause resonance and noise.

　　The frequency is also several tens of MHz. These two kinds 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 kinds of ripple (noise), there is also AC noise. 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 generated by many power devices of switching power supplies using the shell as a heat sink.

　　Unplug the ground wire of the probe

　　1. AC coupling is to remove the superimposed DC voltage to get an accurate waveform.

　　2. Turning on the 20MHz bandwidth limit is to prevent the interference of high-frequency noise and prevent the wrong results from being measured. Because the high frequency component has a large amplitude, it should be removed when measuring.

　　3. Unplug the ground clamp of the oscilloscope probe and use a ground ring to measure, in order to reduce interference. Many departments do not have a ground ring. If the error permits, use the probe's ground clamp to measure directly. But this factor should be considered when judging whether it is qualified.

　　Another point is to use a 50? terminal. According to the Yokogawa oscilloscope data, the 50? module removes the DC component and measures the AC component. However, few oscilloscopes are equipped with this kind of special probe. In most cases, the standard probes from 100K? to 10M? are used for measurement, and the impact is temporarily unclear.

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

　　When measuring high-frequency noise, the full passband of the oscilloscope is used, which is generally in the order of hundreds of megabytes to GHz. Others are the same as above. Different companies may have different testing methods. In the final analysis, you must be clear about your test results. The second is to be recognized by customers,

　　About oscilloscope:

　　Some digital oscilloscopes cannot accurately measure ripple due to interference and memory depth. The oscilloscope should be replaced at this time. In this respect, although the bandwidth of the old analog oscilloscope is only tens of megabytes, the performance is better than that of the digital oscilloscope.

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