Optimizing Circuit Performance with Electrolytic Capacitors

Capacitors serve as filters in electronic circuits by storing charge to smooth out direct current after rectification, blocking or allowing certain frequencies. They vary in types like ceramic, aluminum electrolytic, and film capacitors, each suitable for specific applications based on performance needs. Factors like series connection, grounding, and capacitor selection impact filter performance i

 

Aluminum ElectrolyticWhy are Capacitors Used as Filters?

1. The Role of Capacitors as Filters in Electronic Circuits

- How do filter circuits work and what is their function?

After alternating current is rectified into direct current by diodes in electronic circuits, although the negative half-wave is eliminated, the DC still has amplitude variations and is not completely smooth.

When capacitors are added, they can store charge. When the voltage fluctuates in the direction of decrease, the capacitor releases charge to maintain voltage stability. When the voltage rises, the capacitor absorbs charge to stabilize the voltage.

 

2. Capacitors can block or allow certain frequencies, thus making voltage or current output smoother.

Theoretically (assuming the capacitor is purely capacitive), the larger the capacitance, the lower the impedance, and the higher the frequency that passes through. However, in reality, capacitors exceeding 1μF are mostly electrolytic capacitors, which have significant inductive components. Therefore, the impedance may increase at higher frequencies. Sometimes, a large electrolytic capacitor is paralleled with a small capacitor. In this case, the large capacitor passes low frequencies, while the small capacitor passes high frequencies.

The function of capacitors is to allow high impedance and block low impedance, allowing high frequencies to pass and blocking low frequencies. The larger the capacitance, the easier it is for low frequencies to pass, and the larger the capacitance, the easier it is for high frequencies to pass.

Specifically used in filtering, large capacitors (1000μF) filter low frequencies, while small capacitors (20pF) filter high frequencies. Some netizens have vividly likened filtering capacitors to "ponds." Since the voltage at both ends of the capacitor does not change suddenly, it can be understood that the higher the signal frequency, the greater the attenuation. Capacitors are like ponds, which do not change in volume due to the addition or evaporation of a few drops of water.

Capacitors convert voltage fluctuations into current changes, and the higher the frequency, the greater the peak current, thereby buffering the voltage. Filtering is the process of charging and discharging.

 

3. Common Applications of Capacitor Filters, such as Power Supplies and Audio Systems

The role of filtering capacitors in amplifier power supplies includes:

1. Filtering the pulse DC obtained after rectification to reduce AC interference.

2. High-speed power supply.

3. Providing a path for audio signals.

The filtering effect of capacitors on power supplies in amplifiers only shows whether there is AC interference: the energy storage function of capacitors can provide energy for the transient large current needs of amplifiers, but if the transformer power is insufficient to charge it in time, its energy storage function is not obvious. Its role in providing a path for audio signals is also crucial for sound quality and tone.

 

What are the drawbacks of capacitor filters?

Limitations or drawbacks associated with capacitors as filters.

A filter is a device or circuit that processes signals. It primarily consists of capacitors, inductors, and resistors. Commonly used filters include digital filters, programmable filters, passive filters, and active filters. So, what are the advantages and disadvantages of filters?

Advantages of filters:

1. Operational amplifier (op-amp) has minimal impact on the RC network itself.

2. Can apply voltage for series negative feedback.

3. Can amplify signals, and amplification factor is easy to adjust.

 

Disadvantages of filters:

1. Poor load capacity.

2. No amplification effect.

3. Relatively poor reliability.

4. External DC power supply required during use.

5. Wide transition band, non-ideal amplitude-frequency characteristics, and non-steep edges.

6. Not suitable for use under high voltage or high current conditions.

2. Potential issues such as limited frequency response, distortion, and voltage limitations. How these affect circuit performance in certain situations.

 

In practical use, active filters can lead to various problems if not handled properly. What kind of problems can occur?

1. Independent devices have no electromagnetic interference issues (radiation emission and immunity are fully compliant), but interference problems arise when necessary external cables are connected.

2. The equipment's chassis or cabinet shielding is very effective, but still produces excessive radiation emissions.

3. Unable to pass radiation immunity tests.

4. Malfunction occurs when injecting rapid electrical pulses into signal cable lines.

5. Unable to pass electrostatic discharge tests.

6. Unable to pass conducted sensitivity tests on cable bundles.

7. Interference between wires within cables or between cables themselves leads to the equipment not achieving its intended functionality.

 

Which Type of Capacitor Provides Better Filtering Effects?

1. Different types of commonly used filter capacitors include electrolytic capacitors, ceramic capacitors, and film capacitors.

Ceramic Capacitors

As the name suggests, these are capacitors made with ceramic as the dielectric material and metal as the electrodes. Typically, silver layers are sprayed on both sides of a ceramic substrate, and then the silver film is fired to produce the electrodes. They come in various shapes and sizes and are generally cheaper than other capacitors. Their characteristics include small size, good heat resistance, low loss, and high insulation resistance. They can handle lower capacitance and are often used in applications with low capacitance requirements, such as noise/harmonic filtering and suppression.

 

Aluminum Electrolytic Capacitors

These capacitors use an aluminum cylinder as the negative electrode with liquid electrolyte inside, and a bent aluminum strip inserted as the positive electrode. They are characterized by good electrical performance, wide applicability, and high reliability as general-purpose electrolytic capacitors. However, it is important to note that electrolytic capacitors have positive and negative poles and should not be connected incorrectly.

 

Film Capacitors

One type is the polyphenylene sulfide (PPS) film capacitor, which uses PPS film as the dielectric material, offering high insulation, low distortion, a high-frequency range, and good temperature stability. They are usually through-hole mounted and do not provide surface mount packaging. They are widely used in audio applications, such as amateur radio for EMI/noise filtering and RF coupling.

Another type is the polyester film capacitor, known for its high heat and moisture resistance. Therefore, they can be used in more harsh environmental applications such as power converters, lighting, timing, and communication. Polyester film capacitors typically have larger physical dimensions compared to other types, occupying more valuable real estate during the process.

 

Factors Affecting Filter Performance

Today, let's discuss the factors that affect filter performance.

2.1 Series Connection of Two or More Filters

When multiple different types of filters are connected, a situation of filter series connection arises. For example, in a secure room or shielded room, the entire room is powered by a large three-phase filter. Each phase lead and neutral lead is inserted into a filter circuit capable of carrying 100A or more of current, all housed in a large enclosure. It provides power to cabinets in the room, which, in turn, supply power to 19-inch cabinets' equipment through filters installed in each rack, and individual equipment within each rack is also filtered. In this case, three filters are installed in a cascaded manner.

If the insertion loss requirement for the power line filter supplying power to the rack is 100dB at 10kHz, designers are required to achieve good attenuation below the normal recommended cutoff frequency to obtain appropriate attenuation or insertion loss under reasonable cost, size, and weight conditions.

The filter at the bottom of the rack may be a single-phase π filter, followed by another π type filter. These filters may misalign with each other, especially when some or all of the filters have circuits with Q values greater than 2. A high Q value circuit increases the likelihood of oscillation in these filters, shifting the cutoff frequency into the typical passband region. Problems caused by this oscillation often lead to reduced line voltage, resulting in equipment failure in the rack.

 

2.2 Poor Grounding of Filters

Well-designed filters would pass EMI testing in labs or EMI filter design rooms. You may have seen all the grounding methods required for testing filters in these labs. The test bench is covered with a layer of well-grounded copper plate. The tested equipment with installed filters is usually closely connected to this copper plate C-type. Most filters are designed to be directly mounted on the ground through threaded studs or connectors.

Filters are installed through holes in the bottom plate and use EMI gaskets on both sides for mounting. This way, filters are tightly mounted on the ground through nuts and washers. The gaskets used in this method can provide thousands of grounding points and can be connected to the bottom plate ground plane through hollow holes on the bottom plate. Without this ground, filters cannot achieve their designed performance.

Some claim that even in lab filter testing, there is poor grounding. When there is poor grounding, the through-hole capacitors of the filter and other ground devices cannot work properly. Additionally, tansorb tubes and MOV connected to ground cannot function properly.

If the filter housing is not grounded or has poor grounding, it is easy to see how the filter is not working properly. With poor grounding, it is equivalent to removing two through-hole capacitors and two transorb tubes from the circuit at the load end. Also, two capacitors at the input end will be removed, resulting in a non-capacitive input port.

 

3. Selecting the Most Suitable Capacitor Type for Specific Filtering Applications Based on Performance Requirements

A considerable number of designers also tend to use a microcontroller or microprocessor with selected peripheral components to create their own customized embedded controller solutions. These solutions may be implemented directly on the PCB, like ordinary SBCs, but are also subject to compressed space limitations.

Therefore, materials and packaging structures must be designed to fit a capacitor into the very small space between the CPU and chipset, without exceeding strict height limits.

 

4. Can Capacitors be Used for Signal Filtering?

How to Use Capacitors for Signal Filtering in Electronic Circuits

In high-pass filter circuits, capacitors and resistors are in series. Due to the capacitors' high-pass and low-pass characteristics, when high-frequency signals pass through the capacitor, the voltage drop is minimal, and the circuit (high-frequency) current is large. According to the voltage division in series, the voltage obtained by the resistor is large, close to the input voltage.

In other words, the high-frequency input voltage is basically equal to the resistor voltage, which is the output voltage. Similarly, when low-frequency voltage signals pass through the capacitor, the voltage drop is significant, and the voltage obtained by the resistor is very small, almost negligible. At this point, it is as if the low-frequency input voltage has no output and has been filtered out.

In high-pass and low-pass filter circuits, capacitors and resistors are also in series. Due to the capacitors' high-pass and low-pass characteristics, when low-frequency signals pass through the capacitor, the voltage drop is significant, and the voltage obtained by the resistor is very small, and the capacitor voltage becomes the output voltage. In other words, the low-frequency input voltage is basically equal to the capacitor voltage (output voltage).

Similarly, when high-frequency voltage signals pass through the capacitor, the voltage drop is small, and the voltage obtained by the resistor is large. At this point, it is as if the high-frequency input voltage has no output and has been filtered out.

 

Importance of Selecting Standard Capacitors when Using Capacitors for Filtering in Electronic Circuits

The selection of filter capacitors depends on whether you are using them in a local power supply or a global power supply. For a local power supply, it is meant to provide transient power. Why add capacitors for power supply? Because device current demands change rapidly with driving requirements (such as DDR controller), and when discussing in the high-frequency range, circuit distributed parameters must also be considered.

Due to the presence of distributed inductance, it hinders drastic changes in current, resulting in a voltage drop on the chip power supply pin, i.e., creating noise. Moreover, feedback power supplies now have a response time—waiting for a period of time (usually in ms or us) before making adjustments after voltage fluctuations occur. For ns-level changes in current demand, this delay also creates actual noise.

Therefore, the role of capacitors is to provide a low impedance route to meet rapid changes in current demand.