The difference between active and passive filters

Any electronic system has its own frequency bandwidth (limit to the highest frequency of the signal), and the frequency characteristics reflect the essential characteristics of the electronic system. The filter is an engineering application circuit designed according to circuit parameters’ influence on the circuit’s frequency bandwidth. Everyone knows that there are many filters, and the easiest way to distinguish between active and passive filters is to see if a power supply is needed. The most significant difference in function is that active filters can have gain, and passive filters can have gained. No gain is attenuated.

  The active filter is called active because, as the name implies, the device needs to provide power (to compensate for the harmonics of the primary circuit). Its application can overcome the shortcomings of traditional harmonic suppression and reactive power compensation methods such as LC filters (traditional harmonic suppression and reactive power compensation methods). Only fixed compensation), realizes dynamic tracking compensation, and can compensate for both harmonics and reactive power.

  Passive filters are composed of passive components such as LC, designed to have extremely low impedance at a specific frequency and shunt the harmonic current of the corresponding frequency. Its behavior mode is to provide a passive harmonic current bypass channel. At the same time, the active filter is composed of The power conversion equipment composed of power electronic components. DSP detects the load harmonic current and actively provides the corresponding compensation current. The compensated source current is an almost pure sine wave, and its behavior mode is active current source output.

different filtering principles

  Active filtering (APF) and passive filtering (FC) differ in filtering principles. Passive filtering mainly uses the LC resonance characteristics of RC components to form a low-resistance channel for a specific frequency in the system. This low-impedance channel forms a parallel shunt relationship with the system impedance, allowing harmonic components to flow through the filter system. That is to say, the passive filter uses capacitors and reactors to form an LC resonant circuit to filter one or several harmonics in the power grid system to achieve the effect of filtering the system.

  Active filter APF uses modern power electronic devices to actively generate a harmonic with the same magnitude and opposite phase as the system harmonic to “cancel” the harmonics generated by the system. In a nutshell, FC is a parallel shunt, and APF is an active cancellation.

the difference in power consumption

  Due to the impedance of the resistance and the inductance of the passive filter, the power consumption is still higher than that of the active filter under the same conditions, and the delay of the circuit is more considerable. The power consumption of the active filter will be relatively small, and there will be no attenuation in the passband. The amplification factor can be changed by setting the Q value of the filter. The general analog low-pass filter uses a lot of active filters of Butterworth type, and the effect is still excellent. But the problem still follows. The bandwidth can generally be improved, which is limited by the op-amp selected by the circuit. Once the characteristic frequency range of the op-amp is exceeded, the circuit will easily self-oscillate, and the output is all noise.

the difference in application fields

  The passive filter circuit has a simple structure and is easy to design. Still, its passband magnification and cut-off frequency vary with the load, so it is unsuitable for occasions with high signal processing requirements. Passive filter circuits are usually used in power circuits, such as filtering after rectification of DC power supplies or LC (inductance, capacitor) circuit filtering when large current loads are used.

  The load of the active filter circuit does not affect the filter characteristics, so it is often used on occasions with high signal processing requirements. The active filter circuit is generally composed of an RC network and an integrated operational amplifier, so it must be used under the condition of a suitable DC power supply, and it can also be amplified. But the composition and design of the circuit are also more complicated.

  Active filter circuits are not suitable for high voltage and high current occasions but only for signal processing.

  1. The capacity of a single set of active filters does not exceed 100KVA, and there is no such limit for passive filters.

  2. When active filtering provides filtering, it cannot or rarely provides reactive power compensation because it takes up capacity, while passive filtering provides reactive power compensation at the same time.

  3. The current maximum applicable grid voltage of an active filter does not exceed 450V, while the maximum applicable grid voltage of a low-voltage passive filter can reach 3000V.

  4. Passive filter is widely used in electric power, oil field, steel, metallurgy, coal mine, petrochemical, shipbuilding, automobile, electric railway, new energy, and other industries because of its price advantage and not limited by hardware; The hardware problem solved cannot be used in large-capacity occasions. It is suitable for units with low power consumption and high harmonic frequencies, such as telecommunications and hospitals, and is superior to passive filtering.

Active Filter has a rapid response and high price.

  Active filtering is itself a source of harmonics. It relies on power electronic devices to generate a set of harmonic vectors with the same amplitude and opposite phase to the system when detecting the system harmonics, which can cancel the system harmonics and make it a sine waveform. In addition to filtering out harmonics, active filtering can also dynamically compensate for reactive power. The advantage is that it reflects the action quickly, filters out harmonics up to 95%, and compensates the reactive power meticulously.

  Compared with the passive filter, the active filter has a good treatment effect, and it can mainly filter out multiple and high-order harmonics simultaneously without causing resonance.

  The disadvantage of an active filter is the high price and small capacity. Due to the immature technology of large-capacity silicon valves worldwide, the current typical active filter capacity does not exceed 600 kilowatts. Its operational reliability is also less than passive.

  Generally, ordinary op amps can still be used for low-frequency signal filtering, but once the frequency of the signal increases, especially when the signal reaches MHz, it is still difficult for ordinary op-amps to meet the requirements, and high-speed op amps need to be selected. The price is really high. Once a high-cost high-speed op amp is added to the active filter, the cost is significantly increased.

low cost of the passive filter, low harmonic filtering rate

  Generally, passive filtering refers to a low-resistance (tuning filter) state of a particular harmonic in parallel through the matching of inductors and capacitors, forming a low-resistance path for a certain harmonic current. This way, harmonic currents do not flow into the system. The advantages of passive filtering are low cost, stable operation, relatively mature technology, and large capacity. The disadvantage is that the harmonic filtering rate is generally only 80%, and the reactive power compensation for the fundamental wave is also unavoidable. Currently, where the capacity is large and the compensation is required to be meticulous, the active and passive hybrid type is generally used. The passive filter compensation is performed for a large capacity, and the action is used for fine-tuning.

  The passband change of the passive filter is also affected by input and output resistance. That is, the internal resistance of the input signal source will affect the filter network, and secondly, the load change will also affect the filter network. Therefore, in the design, take this into consideration when.

Impedance effect

  Passive filters are severely affected by system impedance, and there is a danger of harmonic amplification and resonance; active filters are not affected.

frequency effect

  The resonance point of the passive filter is shifted, and the effect is reduced; the active filter is not affected.

load impact

  Passive filters may be damaged due to overloading; active filters are not in danger of damage. Only the compensation effect is insufficient when the amount of harmonics exceeds the compensation capacity. The compensation effect of the passive filter varies with the change of load; the active filter is not affected by the change of load.

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