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Home > power-new-energy > What is the ripple and noise of the power supply?

What is the ripple and noise of the power supply?

Update Time: 2022-07-15 16:51:29

Power supply noise and ripple-related is a topic that every hardware engineer can not avoid. Then how to correctly distinguish between ripple and noise is especially important.


In this article, for the power supply ripple and noise to do a simple description, we first recognize what the power supply ripple and noise are.


1. What is ripple?


1.1 Ripple generation

Ripple is a phenomenon caused by voltage fluctuations in the DC stabilized power supply because the DC stabilized power supply is generally formed by the AC power supply through rectification, voltage regulation, filtering and other links. Due to incomplete filtering, residual AC components will inevitably be more or less in the DC stabilized amount with some AC components. This superimposed on the DC stabilized amount of the AC component is called ripple. The ripple component is more complex. Its form is generally higher than the frequency of the sine wave-like harmonics. Another is a very narrow width of the pulse wave. Generally, we see the ripple in the form of the following figure, a bit like a sawtooth wave.


image.png


From this, we know that the switching power supply's output is not pure DC voltage. There are some AC components which are caused by ripple and noise. Ripple is the output DC voltage fluctuation and is related to the switching power supply's switching action. For each on/off process, the electrical energy is "pumped" from the input to the output, forming a charging and discharging process, which causes the output voltage to fluctuate at the same frequency as the switch. The ripple voltage is the peak between the crest and trough of the ripple. Its size is related to the capacity and quality of the input and output capacitors of the switching power supply.


1.2 Ripple representation and its hazards

Representation method: It can be expressed by RMS value, peak value, or by using absolute or relative amount. Generally, the company will use the peak-to-peak value, that is, the absolute value of the voltage difference (peak-to-peak and trough subtracted), to express and, simultaneously, compare the relative amount to meet the standard requirements.

Unit: mV

An example to illustrate: If we use DC/DC output 3.3V electricity, use the active oscilloscope probe, adjust the port for DC coupling (if it is a passive probe, then use AC coupling, and adjust the bias to 3.3V), the test to get the ripple results for ± 25mV (passive probe test results correspond to 3.3V ± 25mV), then we can say that this DC/DC output The ripple of this DC/DC output is 50mV, this is the absolute amount and the relative amount that is ripple coefficient = ripple voltage/output voltage = 50mV/3.3V=1.51%.

 

The hazards of ripple.


① tends to produce harmonics on the appliance, and harmonics can produce more harm.

② Reduces the efficiency of the power supply.

③ Stronger ripple or cause the generation of inrush voltage or current, resulting in burning power-using devices.

④ can interfere with the logic relationship of digital circuits and affect normal operation.

⑤ will bring noise interference, making the imaging and audio equipment can not work properly.


1.3 Ripple classification and its suppression method

First, it should be noted that no matter what kind of ripple is involved, the most effective methods to reduce ripple are generally four kinds.


1) output filtering with the π-type circuit, LC filtering.

2) increasing the capacitance.

For output capacitance, use aluminium electrolytic capacitors to achieve high capacity. But the electrolytic capacitor is not very effective in suppressing high-frequency noise, and the ESR is also relatively large, so a ceramic capacitor will be connected in parallel next to it to make up for the lack of an aluminium electrolytic capacitor. At the same time, when the switching power supply works, the voltage Vin at the input is constant, but the current changes with the switch. At this time, the input power supply will not provide current well. Usually, near the current input (in the BucK type, for example, is near SWITcH), a capacitor is connected in parallel to provide current.

3) Increase inductance.

The selection of inductance generally also considers the size of the rated current of the inductor. Generally, the circuit design stage needs to choose a larger inductor than the rated inductance.

4) Reasonable wiring.

It should be noted that in the actual commissioning process, trying many aspects sometimes increase capacitance and inductance alone. The effect is not ideal.

Output ripple is mainly composed of low-frequency ripple, switching frequency ripple and high-frequency ripple from the frequency spectrum.


2. What is noise


2.1 Noise generation

There are two causes of noise. The switching power supply generates one; the other is the external electromagnetic field (EMI) interference, which can enter the switching power supply through radiation.


image.png


SWITCH generally uses bipolar transistors or MOSFETs. No matter which, there will be a rise time and a fall time when it is on and off. These two types of noise are generally called high-frequency noise, and the amplitude is usually much larger than the ripple. The frequency of the noise burst is much higher than the switching frequency, and the noise voltage is its peak-to-peak value. The amplitude of the noise voltage is largely related to the topology of the switching power supply, the circuit's parasitic state, and the PCB's design.


2.2 The difference between noise and ripple

In engineering, when testing power supplies, ripple and noise are generally not deliberately separated, and the synthetic interference of both ripple and noise is measured and expressed as peak-to-peak values. The above classification of ripple has included noise.


Generally, during the test, we test the output of the DC/DC power supply for ripple testing and the receiver of the most distal chip IC power pin for noise testing.


Finally 

The above is a summary of the switching power supply ripple and some of the content noise. About noise suppression, in practice, not necessarily in all applications, it is important to choose the right method according to their design requirements, such as product volume, cost, development cycle, etc..


In short, they appear in places we do not need, are harmful, and must be avoided. There are many ways and means to suppress and remove them, but eliminating them doesn't seem easy. We can only control them within a permissible range without affecting the environment and equipment even if we have achieved our goal.


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