Switching power supply ripple suppression - Database & Sql Blog Articles

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For switching ripple, both theoretically and practically there must be. There are three ways to suppress or reduce it:

(1) Increase inductance and output capacitance filtering

According to the formula of the switching power supply, the current fluctuation in the inductor is inversely proportional to the inductance value, and the output ripple is inversely proportional to the output capacitance value. Therefore, increasing the inductance value and output capacitance value can reduce the ripple.

Similarly, the relationship between the output ripple and the output capacitance: vripple=Imax/(Co×f). It can be seen that increasing the output capacitor value can reduce the ripple.

As a general rule, for output capacitors, aluminum electrolytic capacitors are used to achieve high capacity. However, the electrolytic capacitor is not very effective in suppressing high-frequency noise, and the ESR is also relatively large, so a ceramic capacitor is connected in parallel with it to compensate for the shortage of the aluminum electrolytic capacitor.

At the same time, when the switching power supply is working, the voltage Vin at the input terminal does not change, but the current changes with the switch. At this point, the input power supply does not provide a good current, usually near the current input (in the case of the BucK type, which is near SWITcH), and a shunt capacitor is used to supply current.

The above approach has a limited effect on reducing ripple. Because of the volume limitation, the inductance will not be very large; the output capacitance will increase to a certain extent, and there is no obvious effect on reducing the ripple; increasing the switching frequency will increase the switching loss. So when the requirements are strict, this method is not very good. Regarding the principle of the switching power supply, etc., reference can be made to various switching power supply design manuals.

(2) Secondary filtering is to add a first-order LC filter

The LC filter suppresses the noise ripple significantly. Choosing the appropriate inductor and capacitor according to the ripple frequency to be removed constitutes a filter circuit, which generally reduces the ripple.

The sampling point is selected before the LC filter (Pa) and the output voltage is reduced. Because any inductor has a DC resistance, when there is a current output, there will be a voltage drop across the inductor, causing the output voltage of the power supply to drop. And this voltage drop is a function of the output current.

The sample point is selected after the LC filter (Pb) so that the output voltage is the voltage we want. However, this introduces an inductor and a capacitor inside the power system, which may cause system instability. Regarding the stability of the system, a lot of information has been introduced, and it is not written in detail here.

(3) After switching power supply output, connect to LDO filter

This is the most effective way to reduce ripple and noise. The output voltage is constant and there is no need to change the original feedback system, but it is also the most costly and power consuming method. Any LDO has an indicator: the noise rejection ratio. It is a frequency-dB curve, as shown on the right is the curve of the Linear Technology LT3024.

For reducing ripple. The PCB layout of the switching power supply is also very critical, which is a very difficult problem. There are special switching power supply PCB engineers. For high-frequency noise, due to the high frequency and large amplitude, the post-stage filtering has a certain effect, but the effect is not obvious. There is a special study in this area. The simple method is to use a capacitor C or RC or a series inductor on the diode.

(4) On the diode and capacitor C or RC

Parasitic parameters should be considered when the diode is turned on and off at high speed. During diode reverse recovery, the equivalent inductance and equivalent capacitance become an RC oscillator, producing high frequency oscillations. In order to suppress this high frequency oscillation, a capacitor C or RC buffer network is connected in parallel across the diode. The resistance is generally 10Ω-100Ω, and the capacitance is 4.7pF-2.2nF.

The capacitance C or RC connected in parallel on the diode is determined by trial and error. If improperly chosen, it will cause more serious oscillations.

For high frequency noise requirements, soft switching technology can be used. There are many books devoted to soft switches.

(5) Diode rear inductor (EMI filter)

This is also a common method of suppressing high frequency noise. Selecting a suitable inductive component for the frequency at which noise is generated can also effectively suppress noise. It should be noted that the rated current of the inductor should meet the actual requirements.