Design of wireless LED lighting drive system

1 Overall design of the system

This system mainly includes: wireless power supply module, constant current drive source module, active power filter (APF) module, control circuit, and the overall block diagram of the system is shown in Figure 1. Among them, the inverter and rectifier filter 2 constitute the wireless power supply module; the forward conversion circuit and rectifier filter 3 constitute the constant current source drive module. The MCU collects light through the photocell and performs feedback regulation on the output current to ensure output voltage stability and constant current. The APF outputs current through the main controller to offset the current harmonics injected into the grid by the wireless drive module to improve the power quality at the input end.

2 System Circuit Design

2.1 Wireless Power Supply System Design

The wireless power supply system consists of a control end, a transmitter, and a load rectification circuit. It realizes wireless power supply through electromagnetic coupling (short-distance transmission method) and electromagnetic resonance (long-distance transmission method). The system structure diagram is shown in Figure 2.

The transmitter is mainly composed of an inverter and a transmission channel. The inverter is responsible for converting direct current (DC) into alternating current (AC), and it consists of an inverter bridge, control logic, and filter circuit. The short-range inverter frequency is 200~500 kHz, and it is transmitted by electromagnetic coupling, using a U-shaped loosely coupled ferrite core isolation to achieve wireless power supply, as shown in Figure 3.

The long-distance inverter frequency is 1 MHz, and it is transmitted by electromagnetic resonance. It uses air-core transformer coupling, and the primary and secondary are respectively wound on cylinders as transmission media to achieve long-distance power transmission, as shown in Figure 4.

This system achieves stable and efficient transmission of electric energy by intelligently switching the transmission mode. The relationship between the secondary equivalent resistance and the distance to the primary is shown in Figure 5, where z is the impedance and L is the distance. The equivalent impedance is the smallest only at the position of L0. According to this principle, when the receiving distance is far away from L0, the impedance increases and the primary current decreases. The Hall current sensor is used to collect the primary input current to determine the mode switching.

2.2 Constant current source circuit

The constant current source system is mainly composed of a DC/DC forward converter circuit and an MCU control circuit. The forward converter circuit is a constant current drive source for LEDs, as shown in Figure 6. After rectification and filtering, the inverter output is stabilized by the chip L7824ACV to provide +24 V input for the DC/DC converter circuit. The constant current source of this structure has the characteristics of high-precision output. Its output power depends on the selection of transformer parameters and can generally reach more than 32 W, meeting the power requirements of most LED lighting applications.

This circuit uses TL494 as a constant current control chip. The switching frequency fosc is set by the formula fosc=1.1/(RTCT). The dead time is adjusted by potentiometer R4, and the current output error can be <1%. In Figure 6, RS is the current sampling resistor; R3 is the feedback voltage sampling resistor, which is used to limit the maximum output voltage. When the output current changes, the potential of pin 2 (1IN-) also changes accordingly. After the internal error amplification comparison of TL494, the duty cycle of the PWM drive signal is changed to achieve negative feedback adjustment of the output current. The microcontroller collects the photocell voltage through A/D, and after judgment, the output PWM is converted into a reference voltage proportional to the duty cycle through a low-pass filter to change the output current size, so as to achieve the purpose of automatically adjusting the LED luminous intensity. The voltage follower of the operational amplifier plays the role of isolation and enhancing the driving ability. Adjusting R2 can change the proportional relationship between the reference voltage and the output current. L1 and D3 are magnetic discharge windings to prevent magnetic saturation of the transformer primary coil, so that magnetic reset can be provided for the primary coil during the switch off period. Since the driving capability of TL494 is limited, the push-pull output of the transistor is used to increase the driving capability of TL494.

Table 1 shows the actual test results of the DC/DC conversion circuit with an output of 16 W, which proves that the constant current source can achieve high efficiency and high precision current output.

2.3 Harmonic compensation system

2.3.1 System Hardware Structure

The hardware of the harmonic compensation system is mainly completed by the active power filter (APF), and the structure is shown in Figure 7. This paper uses the TMS320F2812 digital signal processor (DSP) as the core control and signal processing unit. The conditioning circuit mainly includes current/voltage sensor signal amplification, rectification, and anti-aliasing filtering. Current sensor 1 samples the three-phase current at the load end, and sends it to the DSP through the signal conditioning circuit. After A/D acquisition, it is processed by the harmonic current extraction algorithm and control algorithm, and drives the inverter to make corresponding offsets to the harmonic current, and uses the compensation current sampled and output by current sensor 2 for feedback regulation. The inverter DC bus voltage is converted by the Hall voltage sensor and supplied to the internal A/D acquisition of the DSP, and the DC side capacitor voltage is controlled to be stable through the algorithm. The zero-crossing point of the three-phase voltage signal is used as the zero-crossing trigger signal, as the clearing and starting signal of each cycle software processing.

2.3.2 Harmonic current extraction algorithm

1) Three-phase instantaneous reactive power principle

The software part of the compensation system mainly includes the harmonic current extraction algorithm, which adopts the currently commonly used three-phase instantaneous reactive power theory (also known as ip-iq algorithm). This detection method organically combines the three-phase current with the ip and iq current components decomposed based on the theory through a certain transfer matrix, and based on this as the starting point, the three-phase current harmonics and reactive current can be obtained respectively, and the expressions are:

The corresponding fundamental current components can be separated by a low-pass filter (LPF). Since ipf and iqf can be obtained by transforming the fundamental components iaf, ibf, and icf, ipf and iqf can be transformed into iaf, ibf, and icf in the three-phase current by inverse transformation, that is:

When it is required to detect harmonics and reactive current at the same time, just ignore the channel for calculating ip, and calculate the fundamental active components iapf, ibpf, icpf of the detected current from ipf, that is:

Subtracting ia, ib, ic from iapf, ibpf, icpf can give the harmonic components and fundamental reactive components of ia, ib, ic.

2) Simulation results

This paper simulates the algorithm based on the simulation mode of DSP compilation environment CCS3.3, assuming that the original three-phase input current is a 50 Hz sine wave with a phase difference of 120°, and takes single-phase clipping distortion as an example. Figure 8(a) is a single-phase clipping distortion waveform. According to the total harmonic distortion (THD) calculation formula:

Where Ih is the effective value of each harmonic current, I1 is the effective value of the fundamental current, and spectrum analysis of the waveform shows that the initial current of this phase is 12.2%. Figure 8(b) shows the fundamental waveform after the harmonics are extracted by the algorithm. After filtering by the three-phase instantaneous reactive power algorithm, the harmonic current is basically eliminated and the original waveform is restored.

3 Conclusion

The system is not limited by wires. LEDs can be installed anywhere within the wireless power supply acceptance range, and the LED brightness can be adjusted through automatic light sensing by photoelectric sensors. At the same time, the active power filter harmonic compensation technology is used to design a harmonic elimination device that matches the wireless drive module to filter out high-order current harmonics in the system operation and reduce the total harmonic distortion of the transmission network. This design can be applied to home, vehicle, scene or landscape LED lighting, effectively improving the flexibility of lighting distribution, saving energy and reducing light pollution, and improving power quality, and has a wide range of application prospects.