How a digital phase locked loop works
Digital Phase Locked Loop Technology

How a digital phase locked loop works

As more and more photovoltaic grid-connected power generation systems are connected to the power grid, the power quality of the power grid has attracted more and more attention. In order to achieve reliable grid connection without current impact, the grid-connected photovoltaic power generation system must make the amplitude, phase and frequency of the filtered inverter output voltage consistent with the grid voltage. The grid-connected circulating current can even cause damage to the photovoltaic power generation system [1]. Therefore, phase-lock control must be carried out in the process of grid connection to meet the requirements of grid connection. According to IEEE Std1547-2003, the maximum phase error is 20°, the instantaneous voltage error cannot exceed 10% of the grid voltage, and the maximum frequency error cannot exceed 0.3Hz.

Phase-locked loop PLL (Phase-Locked Loop) is a phase tracking system. The basic feature of phase-locked loop is to use the external input reference signal to control the frequency and phase of the oscillating signal inside the loop. Because the phase-locked loop can automatically track the frequency of the output signal to the frequency of the input signal, the phase-locked loop is usually used in a closed-loop tracking circuit. During the operation of the phase-locked loop, when the frequency of the output signal is equal to the frequency of the input signal, the output voltage and the input voltage maintain a fixed phase difference, that is, the phase of the output voltage and the input voltage is locked, which is the name of the phase-locked loop. origin.

The basic structure of a typical phase-locked loop is shown in Figure 1, which consists of three parts: a phase detector PD (Phase Detector), a loop filter LF (Loop Filter) and a voltage-controlled oscillator (VCO) (Voltage-Controlled Oscillator) [72 ]. The phase detector in the phase-locked loop is also called a phase comparator. Its function is to detect the phase difference between the input signal and the output signal, and convert the detected phase difference signal into a vD voltage signal for output. The signal is low-pass filtered. The control voltage vC of the voltage-controlled oscillator is formed, and the frequency of the output signal of the oscillator is controlled. There are various forms of loops in practical applications, but they all evolve from this basic loop. The phase-locked loop is essentially a feedback control system, which is different from the conventional control system in that the conventional control system collects the voltage or current signal converted by the sensor or directly collected from the system, while the phase-locked loop collects the phase Signal.

Figure 1 - Basic structure of a phase locked loop
Figure 1 – Basic structure of a phase locked loop

The reference signal is formula (1), where Vim is the amplitude of the input voltage, ωi is the angular frequency of the input voltage, and φi is the initial phase angle. The output signal of the voltage controlled oscillator is formula (2), Vom is the amplitude of the output voltage, φ0 is the initial phase angle, assuming that the output voltage is the same as the input voltage angular frequency.

Assuming that the phase detector is an ideal analog multiplier, then

The loop low-pass filter is a linear low-pass filter, which filters out the high-frequency components in the output error voltage of the phase detector, and plays the role of filtering and smoothing, so as to ensure the stability of the loop and improve the loop tracking performance and phase noise characteristics. It is a very important component, which plays a decisive role in the adjustment of the loop parameters, and also has an important influence on the performance indicators of the loop. Analog loop filters are divided into passive filters and active filters. Commonly used loop filters include RC integral filters, passive proportional integral filters and active proportional integral filters, as shown in Figure 2 ( a), (b), (c).

Figure 2 - Analog Loop Filter
Figure 2 – Analog Loop Filter

After filtering by the loop filter, the components containing twice the input frequency in formula (3) are filtered out, and formula (4) can be obtained.

The voltage-controlled oscillator is a link that converts the voltage into a phase, and its oscillation frequency changes linearly with the control voltage vC, as shown in formula (5).

The voltage controlled oscillator integrates the frequency error signal, so that the output signal follows the phase of the input signal to achieve the purpose of phase locking. The mathematical expression of the voltage-controlled oscillator is equation (6). The VCO is an inherent integral link in the phase-locked loop and plays a very important role in the loop.

Phase-locked loops can be divided into four categories according to different implementations: analog phase-locked loops, digital-analog hybrid phase-locked loops, all-digital circuit phase-locked loops and digital phase-locked loops.

①Analog phase-locked loop: All are realized by analog circuit. The analog multiplier is used as the phase detector, the analog loop filter realizes the filtering, and the voltage-controlled oscillator realizes the frequency output. The main disadvantage of the analog phase-locked loop is the nonlinearity of the voltage-controlled oscillator, the operational amplifier and transistor in the loop are prone to saturation, the zero-drift of the operational amplifier and the phase detector, the temperature drift and aging of the circuit devices, etc. Performance also has a certain impact.

②Digital-analog hybrid phase-locked loop: The phase detector adopts XOR gate and JK flip-flop digital logic circuit, while other modules are composed of analog circuits. The digital-analog hybrid phase-locked loop can achieve high operating frequency and control accuracy. Its main disadvantage is that its center frequency is affected by the parasitic capacitance on the chip, and the variation range is large. In strict applications, the center frequency must be adjusted. Has the disadvantage of an analog phase-locked loop.

③All-digital circuit phase-locked loop: With the rapid development of digital circuits, all-digital phase-locked loops have appeared. It does not include any passive components, such as resistors and capacitors, and has the significant advantages unique to all digital systems, that is, the circuit is completely digital, using logic gate circuits and flip-flop circuits, and the possibility of external and power supply interference is greatly reduced. The circuit is easy to integrate, and it is easy to make an integrated monolithic all-digital phase-locked loop circuit, and the reliability of the system is greatly improved. In addition, the all-digital circuit phase-locked loop can also slow down or eliminate the influence of the nonlinearity of the voltage-controlled oscillator in the analog phase-locked loop, the saturation of the device, and the zero-drift of the operational amplifier and the phase detector on the loop performance. The disadvantage is that the realization of its hardware is limited by the logic speed of the digital integrated circuit, and the all-digital circuit phase-locked loop is not widely used. Most of the phase-locked loops currently used in high-performance applications are still digital-analog hybrid phase-locked loops.

④ Digital phase-locked loop: also known as software phase-locked loop. With the rapid development of single-chip microcomputer and DSP digital signal processor, its processing capability is getting stronger and stronger. Generally, the system no longer uses special hardware to realize phase-locked loop, but software program to complete the sampling of input signal and phase-locked loop algorithm. . It can overcome some difficulties that are difficult to overcome by hardware, such as DC zero drift, device saturation, and calibration must be initialized. It is characterized by a higher degree of intelligence, better performance, flexible control, convenient device upgrade, and even online modification of the control algorithm without changing the hardware circuit. Therefore, it is one of the current research hotspots to use a chip with high-speed data processing capability combined with a simple analog circuit to study a reliable digital phase-locked loop method to solve the grid voltage phase-locking and frequency tracking problems in the grid-connected system.

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