The working principle of LLC resonant circuit, do you really know

Do you really know the working principle of LLC resonant circuit? When it comes to LLC, it is favored by many engineers for its high efficiency and high power. The internal structure of LLC contains a single-port network of inductance, capacitance and resistance elements, and at some operating frequencies, when the phase of the port voltage and current waveforms are the same, it is also called circuit resonance. In this article, we will talk about those things about LLC resonant circuits. Interested engineers should not miss it!

What is an LLC and what makes it unique?

In an AC circuit with resistive R, inductive L, and capacitive C elements, the voltage across the circuit is generally out of phase with the current in it. If the parameters of the circuit elements (L or C) or the frequency of the power supply are adjusted so that they are in the same phase, the entire circuit appears purely resistive. The circuit reaching this state is called resonance. In the resonance state, the total impedance of the circuit reaches an extreme value or approximately reaches an extreme value. The purpose of studying resonance is to understand this objective phenomenon, make full use of the characteristics of resonance in science and application technology, and at the same time prevent the harm it produces. According to the circuit connection, there are two kinds of series resonance and parallel resonance.

series resonance

At series resonance, the equivalent impedance is the smallest, and the impedance is pure resistance. Although the size of the series resistance does not affect the natural frequency of the series resonant circuit, it can control and adjust the current and voltage amplitudes during resonance.

parallel resonance

is another typical resonant circuit. The definition of parallel resonance is the same as that of series resonance, that is, the working condition when the voltage U on the port is in phase with the input current I is called resonance. Since it occurs in a parallel circuit, it is called parallel resonance.

What value can an LLC have in those areas?

The application of resonant circuit in Electronic technology is very extensive. Because of its selectivity to frequency, it is often used as the load of high-frequency and intermediate-frequency amplifiers in transmitting and receiving equipment; the resonant circuit is an important part of the oscillator; the resonant circuit is used as an absorption circuit in the electronic circuit to filter out interference signals, etc. , here are some examples.

choice of signal

The input loop circuit of an AM radio is shown in Figure 6-16. In the circuit, L1 is the receiving antenna of the radio input loop, L2 and C are the resonant circuit to form a radio frequency selection circuit, and L3 sends the selected radio signal to the radio receiving circuit.

The radio antenna receives electromagnetic waves from different radio stations in the air, and adjusts C to make L2 and C resonate on the carrier frequency of a desired radio station. At this time, the maximum current flows through L2, and the radio signal is selected. Adjust C to make L2 and C resonate on the carrier frequencies of different radio stations, so that programs from different radio stations can be received.

filtering of the signal

The TV outputs 38MHz IF signal after being mixed by the high-frequency tuner. If there is a 38MHz signal in the external signal entering the TV, it will cause serious interference to the TV’s IF signal, so the external 38MHz signal must be filtered. In addition, as shown in Figure 6-17, the LC series resonant circuit is connected in parallel with the input end of the TV, and the LC circuit resonates at 38MHz. According to the characteristics of series resonance, it presents a small resistance to the 38MHz signal, which is equivalent to short-circuiting the 38MHz signal to the ground, preventing external intermediate frequency signals such as other TV sets from entering the TV and interfering with the work of the local intermediate frequency amplifier. Prevent the IF signal of the machine from radiating out through the antenna to interfere with the work of other machines. Since the LC loop presents a high impedance to the TV signal, it will not affect the normal operation of the TV.

Component measurement

The Q meter is an instrument used to measure parameters such as quality factor, inductance, capacitance, etc. It works by using the characteristics of the resonant circuit, and its principle can be illustrated by Figure 6-18.

The signal source uses a high-frequency signal generator with variable frequency and output voltage. During measurement, change the frequency while keeping the output voltage of the power supply constant.

Measure the Q value of the Inductor coil Lx, connect Lx in series with a standard capacitor Cb and then connect it to the output of the high-frequency signal generator, adjust the capacitance of Cb or the frequency of the high-frequency signal generator, so that the circuit reaches resonance, at this time Cb The voltage across it reaches a maximum value and is equal to Q times the supply voltage. Both ends of the standard capacitor Cb can be connected to a voltmeter or a Q meter in parallel, and the Q value should be read out. Generally, the output frequency of the Q meter and the capacitance value of the standard capacitor Cb are marked on a scale.

Using the series resonance, the known resonance frequency f0 and the standard capacitance Cb, the inductance of the coil Lx under test can be obtained. The above is the analysis of the working principle of LLC resonant circuit, I hope it can help you.

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