How to select filter capacitors in power supply design

Normally, the role of electrolytic capacitors is to filter out low-frequency signals in the current, but even low-frequency signals have frequencies that are divided into several orders of magnitude. Therefore, in order to be suitable for use at different frequencies, electrolytic capacitors are also divided into high-frequency capacitors and low-frequency capacitors (high frequency here is relative).

The impedance of an inductor is directly proportional to frequency, and the impedance of a capacitor is inversely proportional to frequency. Therefore, the inductor can block the passage of high frequencies, and the capacitor can block the passage of low frequencies. By properly combining the two, various frequency signals can be filtered. For example, in a rectifier circuit, connecting a capacitor to the load or connecting an inductor in series to the load can filter out the AC ripple.

Capacitor filtering is a voltage filtering, which directly stores the pulsating voltage to smooth the output voltage. The output voltage is high, close to the AC voltage peak; it is suitable for small currents. The smaller the current, the better the filtering effect.

Inductor filtering is a current filter, which relies on electromagnetic induction generated by current to smooth the output current. The output voltage is low, lower than the effective value of the AC voltage; it is suitable for large currents. The larger the current, the better the filtering effect. Many properties of capacitors and inductors are exactly opposite.

Low-frequency filter capacitors are mainly used for mains filtering or filtering after transformer rectification, and their working frequency is consistent with the mains power supply of 50Hz; while high-frequency filtering capacitors are mainly used for filtering after rectification of switching power supplies, and their working frequency ranges from several thousand Hz to several Hz. 10,000 Hz. When we use low-frequency filter capacitors in high-frequency circuits, due to the poor high-frequency characteristics of the low-frequency filter capacitor, its internal resistance is large and the equivalent inductance is high during high-frequency charging and discharging. Therefore, during use, a large amount of heat will be generated due to frequent polarization of the electrolyte. Higher temperatures will vaporize the electrolyte inside the capacitor and increase the pressure inside the capacitor, eventually causing the capacitor to bulge and burst.

The size of the power supply filter capacitor is usually used in design. The front stage uses 4.7u, which is used to filter low frequencies. The second stage uses 0.1u, which is used to filter high frequencies. The 4.7uF capacitor is used to reduce output pulsation and low-frequency interference. The 0.1uF capacitor is used to filter the low frequency. The capacitor should reduce high-frequency interference caused by instantaneous changes in load current. Generally, the bigger the front one, the better. The difference between the two capacitance values ​​is about 100 times. Power supply filtering and switching power supplies depend on how big your ESR (equivalent series resistance of the capacitor) is, and the best choice for high-frequency capacitors is at their self-resonant frequency. Large capacitors prevent surges, and the mechanism is just like a large reservoir with stronger flood control capabilities; small capacitors filter high-frequency interference, and any device can be equivalent to a series-parallel circuit of resistors, inductors, and capacitors, thus creating self-resonance. Only at this self-resonant frequency, the equivalent resistance is the smallest, so filtering is the best!

The equivalent model of the capacitor is a series connection of an inductor L, a resistor R and a capacitor C. The inductor L is connected to the capacitor lead, the resistor R represents the active power loss of the capacitor, and the capacitor C.

Therefore, the resonant frequency can be found equivalently to a series LC circuit. The conditions for series resonance are WL=1/WC and W=2PIf, thus obtaining the formula f = 1/(2pi* LC). , the reactance at the center frequency of the series LC loop is minimum and behaves as pure resistance, so the center frequency has a filtering effect. The size of the lead inductance varies depending on its thickness and length. The inductance of the grounding capacitor is generally about 1MM to 10nH, depending on the frequency of grounding.

Parameters need to be considered when using capacitor filter design:

ESR

ESL

Withstand voltage value

Resonant frequency

#So how to choose the power supply filter capacitor?

How to select power supply filter capacitors, and mastering its essence and methods is actually not difficult.

1. Theoretically, the impedance of an ideal capacitor decreases as the frequency increases (1/jwc). However, due to the inductance effect of the pins at both ends of the capacitor, the capacitor should be regarded as an LC series resonant circuit. The self-resonant frequency is FSR parameter of the device, which means that when the frequency is greater than the SFR value, the capacitor becomes an inductor. If the capacitor is filtered to ground, when the frequency exceeds SFR, the suppression of interference will be greatly reduced, so a smaller capacitor is needed in parallel to ground. .The reason is that the small capacitor and large SFR value provide a path to ground for high-frequency signals.

Therefore, in power supply filter circuits, we often understand it this way: large capacitors filter low frequencies, and small capacitors filter high frequencies. The fundamental reason is that the SFR (self-resonant frequency) values ​​are different. Think about it. If you think about it from this perspective, you can understand why. Why do the capacitors and ground pins in power supply filtering need to be as close to the ground as possible.

2. So in actual design, we often have questions, how do I know the SFR of the capacitor? Even if I know the SFR value, how do I choose capacitor values ​​with different SFR values? Should I choose one capacitor or two capacitors?

The SFR value of a capacitor is related to the capacitance value and the pin inductance of the capacitor, so the SFR values ​​of 0402, 0603, or plug-in capacitors with the same capacitance will not be the same. Of course, there are two ways to obtain the SFR value:

Device Data sheet, such as 22pf, the SFR value of 0402 capacitor is around 2G

Directly measure its self-resonant frequency through a network analyzer. Think about how to measure S21?

After knowing the SFR value of the capacitor, use software simulation, such as RFsim99, to choose one or two circuits based on whether the working frequency band of your power supply circuit has sufficient noise suppression ratio. After the simulation is completed, it is time to conduct the actual circuit test. For example, when debugging the receiving sensitivity of a mobile phone, the power supply filtering of the LNA is the key. Good power supply filtering can often improve it by several dB.

The essence of a capacitor is to pass AC and block DC. Theoretically, the larger the capacitor used for power supply filtering, the better. However, due to the wiring and PCB wiring, the capacitor is actually a parallel circuit of an inductor and a capacitor (and the resistance of the capacitor itself, which cannot be ignored sometimes). This introduces the concept of resonant frequency: ω=1/(LC)1/ 2

Capacitors are capacitive below the resonant frequency, and inductive above the resonant frequency. Therefore, generally large capacitors filter low-frequency waves, and small capacitors filter high-frequency waves.