1. System structure

The hardware of digital ballast consists of EMI filter circuit, rectifier and power factor correction circuit, high frequency inverter, LCC series parallel resonant circuit and digital control circuit based on MCU.

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Figure 1 – 1 system block diagram

two . Active power factor correction circuit

3. Bridge inverter circuit and load resonance circuit

High power metal halide lamp works in high-frequency AC state, which requires a circuit to convert the DC voltage obtained from the power factor correction circuit to high-frequency AC voltage. Therefore, the bridge inverter circuit is the most basic and key part of the electronic ballast circuit.

Figure 3-3 series parallel load resonant conversion circuit

The full bridge LCC series parallel resonant circuit is shown in Figure 3-3 (a). The series parallel resonant circuit has band-pass characteristics, which can effectively attenuate the high-order harmonic component of square wave VAB, and the series capacitor CS can filter the DC component of the power supply voltage.

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Figure 3-4 working mode of the whole bridge

High power metal halide lamp works in high-frequency AC state. In order to ensure the lamp life and effective work, the two electrodes of metal halide lamp should work alternately to avoid the transition consumption of one electrode. Therefore, the working time of the two electrodes should be consistent, and there should be no DC component in the lamp current. Based on the above requirements, the full bridge inverter LCC series parallel resonant circuit is used in the rear stage of the digital high-power metal halide lamp electronic ballast. The basic full bridge inverter circuit is shown in Figure 3-4. In this design, the duty cycle of the driving signals of the upper and lower half bridges should be the same as far as possible in one working cycle of the LCC resonant converter, and the duty cycle of the driving signal should be close to 50% in order to avoid the arc extinction phenomenon when the current is zero. Assuming that all devices in LCC resonant converter work in ideal state, there are four working modes in a high frequency working cycle of full bridge inverter LCC resonant circuit, as shown in Figure 3-4. Firstly, Q1 and Q4 are on, Q2 and Q3 are off, and the resonant network and metal halide lamp are powered by the power bus. At the end of half a working cycle, the inductor is freewheeling through the parasitic diodes of Q2 and Q3, then Q2 and Q3 are on, Q1 and Q4 are off, the resonant network and metal halide lamp are powered by the power bus, and the inductor is freewheeling through the parasitic diodes of Q1 and Q4.

4. Starting mode

Based on the startup mode of LCC series parallel load resonant circuit, the starting voltage can be greatly reduced and the service life of the bulb can be prolonged. Combined with the voltage transmission characteristics of LCC resonant network, a sliding frequency soft start method can be adopted. When the working frequency of full bridge inverter is close to the inherent resonance point of LCC network, the voltage applied to both ends of metal halide lamp increases with the frequency. This method can effectively reduce the impact of starting voltage on the lamp pole and prolong the life of the bulb.

As shown in Figure 4-2, the sliding frequency soft start process is as follows: the working frequency gradually slides from the working point (1) to the working point (2), the lamp resistance changes from infinity to approximate short circuit (3) after the lamp is started, and the detection circuit directly transits to the working point (4) after judging that the lamp is on. It should be noted that when the frequency reaches ω R (off) and the lamp is not on, the voltage applied to both ends of the lamp is very large, and this voltage is only limited by the equivalent series resistance of inductance and capacitor, so it may lead to unsafe situation. Therefore, in the specific application, we need to determine the minimum value of a voltage limit and a frequency to ensure the safety of the circuit.

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As can be seen from Figure 4-3, before the metal halide lamp is on,The voltage at the lamp end increases gradually, the current is basically zero, and it is close to zeroAs if the lamp end is open circuit, after the lamp is on, the lamp will turn offThe voltage decreases rapidly while the lamp current increases rapidly,It is similar to sudden short circuit.

5. Control strategy

After start-up, the metal halide lamp will go through the preheating stage from start-up to steady state, and finally reach steady state. According to the electrical characteristics of the high-power metal halide lamp, the working process after the metal halide lamp is divided into three stages: constant frequency control stage, transition stage and steady-state working stage, as shown in Figure 5-1.

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Figure 5-1 subsection control strategy of high power metal halide lamp

6. Auxiliary control power supply

The auxiliary power supply provides + 12V DC power supply in the power factor correction circuit, and + 5V DC power supply is needed in the drive circuit of full bridge inverter. In this paper, uc3844 and peripheral components are used to form a buck circuit to provide the + 12V auxiliary power for the ballast, and the linear regulator mcp1703a is used to convert it to + 5V power supply to provide power for the control circuit of full bridge inverter.

7. Communication dimming circuit

Communication uses 485 bus, MODBUS communication protocol, by adjusting the frequency of the full bridge circuit to change the output power, to achieve the effect of dimming, through different spectra for different functions of plants, to find the adjustment point, and then to achieve different plants or different time periods of light effect. RS485 interface chip is used for 485 communication, and CRC verification is included in mosbus protocol. The operation is stable and reliable, and the remote PC can control and adjust multiple groups of metal halide lamps.

8. Software design

1. Program flow chart:

Figure 8-1 flow chart of main program figure 8-5485_ Modbus Communication photon modulation function flow chart

2. Through the micro chip pic16f18854cip peripheral NCO + CWG output complementary square wave, ADC for DC bus and load lamp current and voltage acquisition, realize the closed-loop power output control for the full bridge frequency conversion circuit.

CIP (core independent peripherals)

Why use CIP? It can reduce CPU overhead, hardware level execution speed, unload program tasks from CPU, release CPU for centralized processing of application algorithms, reduce power consumption, CPU can run at a lower speed, CPU can sometimes enter sleep mode, and reduce the complexity of system hardware.

1. The new hardware module of NCO (numerically controlled oscillator) is a timer. Under the action of the selected input clock source, the timer increases to a 20 bit accumulator with a fixed increment of 16 bits, which causes the accumulator to overflow periodically to divide the frequency of the input clock source. NCO has real linear frequency control ability.

2. The new hardware module of CWG (complementary wave generator) generates a two-way output complementary wave with dead time delay and automatic shutdown function from a selected input source. The CWG module has the characteristics of clock source selection, input source selection to generate complementary output waveform, output enable and polarity control, rising and falling edge dead time control, automatic shutdown / restart control and so on.

Figure 8-6 control mode diagram

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Microchip mplabxidemc is configured with NCO + CWG mode. Two groups of CWG are used to control two metal halide lamps. Four ADC channels are used to collect the voltage and current of DC main power supply and frequency conversion resonant circuit. After configuration, generate C language code through generate, and then program for other control programs.

Author: Tyler.Wang

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