Simple and reliable two-piece electronic ballast design-EEWORLD

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1 Introduction

It is an indisputable fact that the lighting efficiency of incandescent lamps is far lower than that of fluorescent lamps. For the early inductive fluorescent lamp ballasts, in addition to their low power factor, I am afraid that many people have had the experience of standing on benches, tables, wooden beds or other bedding that can raise the body to twist the starter back and forth. Therefore, some developed countries such as the United States have begun to restrict the production and use of incandescent lamps and inductive ballasts as early as the 1990s, which provides a broad space for the research, production, innovation and development of electronic ballasts. In fact, semiconductor giants such as IR, Samsung, ST, Siemens, Motorola, and Philips have never stopped the development and production of electronic ballast control ICs. This article will give a design scheme for a simple, reliable and low-cost new electronic ballast using Fairchild Semiconductor's FAN7527 as a power factor correction controller and KA7541 as a fluorescent lamp electronic ballast controller.

Simple and reliable two-piece electronic ballast design

2 PFC Controller FAN7527 Introduction

2.1 Main features of FAN7527

FAN7527 is a simple and efficient dynamic power factor correction controller specially produced by Fairchild Semiconductor for PFC application operation in critical conduction mode. It is also a low-power PFC controller. The internal threshold of the error amplifier and multiplier in the device can open the output for current limiting operation when the circuit is overloaded, and can also prevent the load from disconnecting. Since this output drive limiting circuit limits the overload of the power MOSFET gate drive circuit at the midpoint of the power supply voltage, the reliability of the entire electronic ballast can be greatly improved. The main features of the FAN7527 PFC controller are as follows:

*Built-in start timer;

* Contains overvoltage comparator, which can effectively prevent the output voltage from running away;

*With zero current detection function;

*With a finely tuned internal reference;

*In the high clamp state, the device adopts the totem pole output form;

*The startup and operating current of the device is very low;

* Available in 8-pin DIP and 8-pin SOP packages.

Simple and reliable two-piece electronic ballast design

2.2 Functional Structure of FAN7527

FAN7527 has 8 pins, and its pinout is shown in Figure 1. The functions of each pin are as follows:

INV (pin 1): The conversion input of the on-chip error amplifier. Connecting a resistor divider to this end can set the voltage at 2.5V.

EA OUT (pin 2): On-chip error amplifier output. A feedback compensation network is usually connected between this terminal and the INV terminal during design.

MULT (pin 3): The input terminal of the on-chip multiplier. The voltage at this terminal can be limited to below 3.8V by connecting a resistor divider between the full-wave rectifier output and ground.

CS (pin 4): PWM comparator input, which can detect the operating current of the device during the boost process through a current detection resistor connected to the source of the external MOSFET of the chip; because the device contains a rising edge interval circuit, it can filter out high-frequency noise on the current waveform;

Idet (pin 5): zero current detection input, used to detect the inductor current to prevent the voltage on the boost inductor auxiliary winding from falling below 1.8V;

GND (pin 6): ground terminal;

OUT (pin 7): high current power drive output, when in use, it should be connected to the gate of the power MOSFET through a current limiting resistor;

Vcc (pin 8): power supply terminal.

Simple and reliable two-piece electronic ballast design

The FAN7527 dynamic power factor correction controller can be used to design electronic ballasts with high reliability, low power consumption and high power density. It is a complete dynamic PFC controller. Figure 2 shows the internal structure principle block diagram of FAN7527.

3 Introduction to Electronic Ballast Controller KA7541

3.1 Structural features of KA7541

Figure 3 shows the internal structure schematic of KA7541. It is also a simple and reliable electronic ballast dedicated control chip produced by Fairchild Semiconductor. It is also the smallest control unit of the most optimized, low-power, and highly reliable electronic ballast. KA7541 has a soft start function inside, so there is no need to design an external soft start circuit. The soft start switching frequency and soft start time of the electronic ballast circuit designed with KA7541 can be adjusted according to the type of fluorescent lamp. Since KA7541 has a no-light protection circuit inside, it can prevent damage to the circuit caused by excessive switch output current under no-light conditions. In addition, the output gate drive circuit in the circuit can clamp the gate voltage of the power MOSFET, so the gate voltage is independent of the power supply voltage.

The main features of KA7541 are as follows:

*The device has a soft start function inside;

*The soft start frequency can be adjusted according to the type of fluorescent lamp;

*With no-light protection function;

*5% on-chip bandgap reference has been adjusted;

*With undervoltage lockout output function;

*High-end clamping can be used to achieve totem pole output;

*Has very low starting and operating current.

Simple and reliable two-piece electronic ballast design

3.2 Pin Functions of KA7541

KA7541 adopts 8-pin DIP or SOP package, and its pin arrangement is shown in Figure 4. The specific functions of each pin are as follows:

CS (pin 1): Circuit soft start capacitor connection port. In normal operating mode, the voltage on this pin can be used to determine the soft start phase of the circuit.

CT (pin 2): Timing capacitor connection port. The sawtooth wave generated by the charging and discharging of this capacitor can determine the oscillation frequency of the device's internal oscillator.

RS (pin 3): soft start resistor connection port. The structure of this resistor can be used to determine the initial switching frequency of the circuit in soft start mode.

Ldet (pin 4): Circuit protection input. If the voltage on this pin is lower than 2V, the gate driver output of the circuit will be disabled.

GND (pin 5): Ground pin.

OUT2, OUT1 (pins 6, 7): high current power drive output, used to drive the gate of external power MOSFET.

VCC (pin 8): device circuit pin.

Simple and reliable two-piece electronic ballast design

4 Simple and reliable electronic ballast circuit

Figure 5 shows the specific circuit of a double-tube fluorescent lamp electronic ballast that uses FAN7527 for power factor correction and KA7541 as the electronic ballast controller. In the figure, L1 and C1-C4 form an electromagnetic interference (EMI aluminum foil device), the varistor V1 is used to simulate transient overvoltage, and the thermistor NTC is used to simulate the surge current impact that occurs when the circuit is turned on. FAN7527, L2, D5 and switch tube Q1 together form a boost type active power factor correction pre-converter. KA7541, T1, Q2, Q3, L3 and C15, L4 and C16 form a half-bridge inverter series resonant electronic ballast circuit.

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