Design of digital controlled adjustable lighting ballast circuit with PFC

　　As energy issues become increasingly serious, dimming technology has received more and more attention in lighting applications. At present, most dimmers are based on thyristors, which can work very stably when the load is purely resistive (such as incandescent lamps) because the thyristor can be triggered to conduct at any point of the sine wave until it is turned off when the sine voltage is close to zero. In such a system, incandescent lamps can smoothly achieve dimming from almost 0 to 100%. Energy-saving lamps have long replaced incandescent lamps in many occasions due to their high luminous efficiency and no flicker. However, due to the load characteristics of energy-saving lamps, their circuits need to be adjusted when they are used in dimming systems. Therefore, a new type of dimmable electronic ballast came into being. Therefore, this paper designs a digitally controlled adjustable lighting ballast with PFC based on MC68HC908LB8, which can reduce the number of components by 50%.

　　MC68HC908LB8 is a member of the low-cost, high-performance 8-bit M68HC08 series of microcontrollers. All MCUs in this series use the enhanced M68HC08 central processor and can provide a variety of different module configurations, memory sizes, memory types and package types. MC68HC908LB8 also has dedicated peripherals for high-resolution PWM and power factor adjustment (PFC). 8MHz internal bus frequency; adjustable internal oscillator: 4.0 MHz internal bus operation; 8-bit adjustment function; 25% unadjusted accuracy; 5% adjusted accuracy 8K bytes, 10,000 typical erase and write cycles of in-circuit programmable FLASH with encryption options; 128 bytes of built-in random access memory (RAM); dual-channel high-resolution PWM with dead time insertion and shutdown input functions. The output is frequency pulsated to achieve an output resolution of 4ns; a dual-channel pulse width modulator (PWM) module provides power factor adjustment function; a 7-channel 8-bit successive approximation analog-to-digital converter (ADC); an operational amplifier/comparator for power factor adjustment function or general purpose; a 7-bit keyboard interrupt; a 16-bit, two-channel timer interface module, one of which is output to a port pin (PTA6) that can be used for input capture and PWM; 17 general-purpose input/output (I/O) pins, and 1 single input pin.

　　The digitally controlled dimmable 36W×2 fluorescent lamp electronic ballast system using MC68HC908LB8 is mainly composed of PFC boost pre-conversion stage, half-bridge inverter and lamp drive circuit.

　　PFC boost pre-converter circuit

　　The full name of PFC in English is "Power Factor Correction", which means "power factor correction". The power factor refers to the relationship between effective power and total power consumption (apparent power), that is, the ratio of effective power to total power consumption (apparent power). Basically, the power factor can measure the degree to which electricity is effectively used. The larger the power factor value, the higher the power utilization rate. The computer switching power supply is a capacitor input circuit. The phase difference between its current and voltage will cause the loss of exchange power. At this time, the PFC circuit is needed to improve the power factor. There are currently two types of PFC, one is passive PFC (also called passive PFC) and the other is active PFC (also called active PFC).

　　The PFC boost pre-converter circuit controlled by IC2 (MC68HC908LB8) is shown in Figure 2. Among them, S1 is the PFC main switch, LPFC is the PFC boost inductor, D2 is the boost diode, C4 and C10 are the PFC circuit input and output capacitors respectively, and R5 is the current sensing resistor. The PFC circuit operates in continuous conduction mode (CCM). R1, R2, R3, DZ1, D1, R4 and C3 form the AC line voltage zero-crossing detection circuit, and the zero-crossing detection signal is received by IC2 pin 5 (see Figure 5). The gate driver output signal on IC2 pin 13 is input to the gate of S4. The output signal of S4 drain is buffered and amplified by S2 and S3, and then drives the main switch S1. The output DC voltage (VDCB) of the PFC stage is detected by the voltage divider R11, R12, R13 and R14, and is fed to the pin 16 of IC2. The current sensing signal of the PFC stage on R5 is low-pass filtered by R7 and C7, and input to the current sensing input terminal (pin 12) of IC2. Since IC2 embeds the FC controller, the independent power factor control IC in the traditional solution can be omitted. The PFC circuit shown in Figure 2 can provide an input power factor higher than 0.99, a THD lower than 6%, and a stable DC output voltage of 400V.

　　Half-bridge inverter circuit design

　　Figure 4 shows the half-bridge converter circuit and lamp network of the ballast. The half-bridge driver uses IR2106 (IC1). The control inputs (high side and low side) on pins 2 and 3 of IC1 are provided by the outputs on pins 6 and 7 of IC2. D4 between pins 8 of IC1 is a bootstrap diode, C19 between pins 8 and 6 is a bootstrap capacitor, and S5 and S6 are the upper and lower switches of the half-bridge, respectively. LRES and C24, C23, etc. form an LC resonant tank circuit. R23 and R24 are the grounding resistors of the filaments of the two lamps, respectively, which are used to sense the lamp current. The detection voltage signals on R23 and R24 are rectified and filtered by D9, D10, C26, R25 and D11, D10, C27, R26, and are input to pins 17 and 18 of IC2 respectively. L3H and R21, R22, D7, R20, C25 form a voltage difference detection circuit for the two lamps, and input the detection signal to pin 20 of IC2.

　　The system uses a PWM dimming solution. The high-precision PWM (HRP) module in IC2 can continuously control the brightness of the lamp by controlling the PWM duty cycle of the half-bridge (input through IC1 pins 2 and 3). The low-cost MC68HC908LB8 digitally controlled dimmable electronic ballast can provide active power factor correction, greatly reduce the number of system components, reduce system complexity and cost, and improve system energy efficiency.