|
Disadvantages of passive filters: poor load capacity, no amplification effect, unsatisfactory characteristics, shallow edges, and mutual influence of various levels.
RC Filtering
1. Selection of C value: C cannot be too small, otherwise the load capacitance will have a great impact on the filter circuit. Generally, the input capacitance of IC is often 1~10pF. If the C value is too large, it will affect the high-frequency characteristics of the filter circuit, because
The high-frequency characteristics of large capacitors are generally not good.
2. Selection of R value: If the R value is too small, the load on the power supply will increase; if the R value is too large, more energy will be consumed.
The biggest drawback of the RC filter circuit is that it not only consumes the energy of the signal we want to suppress, but also consumes the energy of the signal we want to retain. In addition, due to the limitation of the high-frequency characteristics of capacitors, it cannot be used in high-frequency applications. For example, an LC filter is required for frequencies above several MHz.
1. Capacitor filter circuit
When analyzing the working principle of capacitor filter circuit, the capacitor's DC blocking and AC passing characteristics and energy storage characteristics are mainly used. The pulsating DC voltage output by the previous rectifier circuit can be decomposed into a DC voltage and a group of AC currents with different frequencies. The AC voltage part will flow from the capacitor to the ground, while the DC voltage part cannot be grounded due to the capacitor's AC blocking characteristics and flows to the next level circuit. In this way, the capacitor filters out the AC part of the original unidirectional pulsating DC voltage.
In addition, the capacitor filter circuit can also be explained by the capacitor energy storage characteristics. When the unidirectional pulsating DC voltage is at a high peak value, the capacitor is charged, and when it is at a low peak voltage, it is discharged, so that the high peak voltage is stored and then released at a low peak voltage. The uneven unidirectional pulsating DC voltage is converted into a relatively smooth DC voltage.
The capacity of the filter capacitor is usually relatively large, and it is often the largest capacitor in the entire circuit. The larger the capacity of the filter capacitor, the better the filtering effect. The capacitor filter circuit is the most commonly used one among various filter circuits.
How to select power supply filter capacitors and master its essence and methods is actually not difficult.
1) Theoretically, the impedance of an ideal capacitor decreases with increasing frequency (1/jwc), but 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 the SFR 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 the ground, when the frequency exceeds the FSR, the suppression of interference will be greatly reduced, so a smaller capacitor is needed in parallel to the ground. Can you think about why? The reason is that small capacitors have large SFR values and provide a path to the ground for high-frequency signals. Therefore, in power supply filtering circuits, we often understand it this way: large capacitors consider low frequencies, and small capacitors consider high frequencies. The fundamental reason is that the SFR (self-resonant frequency) values are different. Of course, you can also think about why? If you think from this perspective, you can also understand why the capacitor to the ground pin in power supply filtering should be as close to the ground as possible.
2) Then 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 select capacitors with different SFR values? Should I select one capacitor or two capacitors? The SFR value of the 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 value will not be the same. Of course, there are two ways to obtain the SFR value: 1) Device Data Sheet, such as the SFR value of 22pf0402 capacitor is around 2G, 2) Directly measure its self-resonant frequency through a network analyzer. Think about how to measure it? After knowing the SFR value of the capacitor, use software simulation, such as RFsim99, to select one or two circuits to see if the working frequency band of the circuit you are powering has enough noise suppression ratio. After the simulation, it is 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 capacitors is to pass AC and block DC. In theory, the larger the capacitor used for power supply filtering, the better. However, due to the lead wire and PCB wiring, the capacitor is actually a parallel circuit of inductance and capacitance (and the resistance of the capacitor itself, which is sometimes not negligible). This introduces the concept of resonant frequency: ω=1/(LC)1/2. Below the resonant frequency, the capacitor is capacitive, and above the resonant frequency, the capacitor is inductive. Therefore, generally large capacitors filter low-frequency waves, and small capacitors filter high-frequency waves.
This also explains why the capacitor filtering frequency of the STM package with the same capacitance is higher than that of the DIP package.
As for how big a capacitor to use, here is a reference:
Capacitor resonant frequency
Capacitance DIP (MHz) SMT (MHz)
1.0μF 2.5 5
0.1μF 8 16
0.01μF 25 50
1000pF 80 160
100 pF 250 500
10 pF 800 1.6(GHz)
But it is just a reference. In the words of an old engineer, it mainly depends on experience. A more reliable approach is to connect two capacitors, one large and one small, in parallel, because the large capacitor has poor high-frequency characteristics, while the small capacitor has good high-frequency characteristics. Generally, the difference is required to be more than two orders of magnitude to obtain a larger filtering frequency band.
2. Inductor filter circuit
The principle of the inductor filter circuit is similar to that of the capacitor filter, which is also due to the inductor's direct current, resistance and alternating current characteristics and energy storage characteristics. From the perspective of energy storage, it is the same principle as the capacitor. When explaining the inductor filter circuit from the perspective of direct current, resistance and alternating current characteristics, the inductor blocks the AC voltage part decomposed from the unidirectional pulsating DC voltage, while the capacitor is short-circuited to ground. The larger the inductance, the better the filtering effect. It is rare for an inductor to be used alone as a filter circuit, and it is generally used in combination with a capacitor.
3. L-shaped RC filter circuit
The L-shaped RC filter circuit is to add a resistor in front of the capacitor in the ordinary capacitor filter circuit. The resistor is connected in series in the circuit, and the capacitor is connected in parallel in the circuit. At this time, the resistor and the capacitor form an L shape, so they are called L-shaped RC filter circuits. Its filtering principle and filtering effect are similar to those of ordinary capacitor filter circuits. At this time, the capacitor and the resistor also form a voltage divider circuit. Because the capacitive reactance of the capacitor is very small, the voltage divider attenuation of the AC component is very large. In this way, the AC quantity is short-circuited to the ground through the capacitor to achieve the purpose of filtering. For the DC voltage part, since the capacitor is isolated from the DC power, the capacitor has no voltage divider effect on the resistor at this time, and the DC will not flow through the capacitor. In this filter circuit, if the resistance value of the resistor does not change, increasing the capacity of the filter capacitor can improve the filtering effect, and the larger the capacity of the filter capacitor, the better. If the capacity of the filter capacitor does not change, increasing the resistance value of the resistor can also improve the filtering effect, but the resistance value of the filter resistor cannot be too large, because if the resistance value of the filter resistor is too large, the DC output voltage will become smaller.
The main feature of LC filtering is that the inductor has a small resistance and a small DC loss. It has a large inductive reactance to AC and a good filtering effect. The disadvantage is that it is large and heavy. It is expensive. It is used in power supply circuits with high requirements.
The resistor in the RC filter consumes part of the DC voltage, so R cannot be very large and is used in circuits with low current requirements. RC is small in size and low in cost. The filtering effect is not as good as that of the LC circuit.
LC filtering is generally used in high-frequency circuits or power supply circuits, while RC is used in low-frequency circuits.
The frequency range of LC filter application is 1kHz ~ 1.5GHz. Due to the limitation of the Q value of the inductor, the cut-off region of the frequency response is not steep enough.
1. RC filters are easier to miniaturize or integrate than LC filters, and LC filters are much larger in size;
2. RC filters are lossy, while LC filters are theoretically lossless, so the power supply circuit is generally an LC circuit;
3. RC is smaller in size and lower in cost than LC;
4. RC is used in low-frequency circuits, and LC filtering is generally used in high-frequency circuits;
5. The resistor in RC filtering consumes part of the DC voltage, so R cannot be very large. It is used in circuits with low current requirements. RC is small in size and low in cost. The filtering effect is not as good as that of LC circuit. LC filtering is mainly due to the small resistance of the inductor and small DC loss. It has a large inductive reactance to AC and a good filtering effect. The disadvantage is that it is large in size, heavy and expensive. It is used in power supply circuits with high requirements.
6. The more filtering levels there are, the better the effect will be, but the loss and cost will be higher, so it is not recommended to exceed 3 levels;
7. RC filters are often used in combination with operational amplifiers to form active filters, mostly used to filter low-frequency signals. For example, they are used as loop filters in phase-locked loops.
4. π-shaped RC filter circuit
First of all, from a structural point of view, this filter circuit is composed of two capacitors and a resistor. It is actually a π-shaped RC filter circuit with a capacitor grounded in front of the resistor in the L-shaped filter circuit. The two capacitors perform filtering at the same time, and the latter filter capacitor can further filter the unfiltered DC voltage of the former capacitor. In this way, the two capacitors perform filtering at the same time, and the filtering effect is of course more ideal. The capacity of the first filter capacitor can be increased to improve the filtering effect, but the capacity of the first filter capacitor cannot be too large, because when the power is just turned on, if the capacity of the first filter capacitor is too large, the charging time will be too long. This charging current flows through the rectifier diode. When the charging current is too large and the duration is too long, the rectifier diode will be damaged. Therefore, when using this π-shaped RC filter circuit, the capacity of the first capacitor can be slightly reduced, and the filtering effect can be improved by adjusting the latter L-shaped RC filter circuit.
5. Multi-section π-shaped RC filter circuit
The multi-section π-shaped RC filter circuit is formed by connecting an L-shaped RC filter circuit after the ordinary π-shaped RC filter circuit. Its filtering principle is the same as the ordinary π-shaped RC filter circuit above, but this filter circuit has multiple DC voltage output terminals, and the DC voltage filtering effect is better at the later output terminals. The first filter output terminal has the highest voltage, and the last filter output terminal has the lowest voltage, which is mainly because each section of the resistor has a voltage drop. The multi-section π-shaped RC filter circuit is the most commonly used filter circuit in the whole circuit.
6. π-shaped LC filter circuit
This filter circuit is basically the same as the ordinary π-shaped RC filter circuit in structure, except that the resistor is replaced by an inductor. Because the resistor has the same resistance to DC and AC, and the inductor has a large inductive reactance to AC and a small inductive reactance to DC, this can improve the AC filtering effect without reducing the DC output voltage, because the inductor has no inductive reactance to DC and does not have a voltage drop to DC like a resistor. The direct-current resistance and alternating-current characteristics of the inductor are the biggest advantage of this filter circuit, but the cost of the inductor is high, so this filter circuit is not used as much as the π-shaped RC filter circuit.
| 
提升卡
变色卡
千斤顶
京公网安备 11010802033920号
