High power factor, high efficiency electronic ballast controller ML4831 - Power Circuit - Circuit Diagram

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1 Overview

The ML4831 is a high power factor, high efficiency electronic ballast controller with adjustable brightness. It is mainly composed of power factor controller, oscillator, preheat shutdown sequence, control strobe logic, output driver and overvoltage and overtemperature protection (see Figure 1). The restart and lamp re-output are controlled by external timing, and considering the integrated control of different lamp characteristics, the ballast control device can adjust the lamp power by frequency modulation (FM), and adjust the lamp through compensation programming. Power, through the compensation programming to adjust the operating frequency of the voltage controlled oscillator (VCO). These functions are integrated on a single chip and can be used in different situations.

The main features of the chip are as follows:

Power factor correction and power adjustment functions are performed by an integrated circuit.

● Low distortion, high efficiency, continuous boost, average current detection power factor.

● Quick start or instant start, adjustable start time.

● Lamp current feedback controls brightness.

● Variable frequency dimming and starting.

●When the lamp is off, it can be adjusted to restart to avoid the ballast heating.

● Internal over-temperature shutdown protection replaces external sensing protection to ensure the safety of the equipment.

● The overvoltage comparator in the PFC can eliminate the loss of control caused by the open circuit of the load.

●Adopting large amplitude oscillator and standard current multiplier to improve noise immunity.

2. Pin function

The ML4831 is available in a dual-in-line 18-pin package with pinouts as shown in Figure 2. The function description of each pin is as follows:

● EA OUT: PC error amplifier output and calibration terminal.

● IA OUT: The output and adjustment node of the PFC average current transconductance amplifier.

● I (SINE): PFC current multiplier input.

● IA: PFC average current transconductance amplifier non-inverting input and PFC cycle-by-cycle current limit comparator peak current detection point.

● LAMP FB: The lamp arcing current samples (or adjusts) the inverting input of the error amplifier and is also the brightness control input node.

●LFB OUT: Lamp current error transconductance amplifier output.

● R (SET): An external resistor that sets the maximum frequency of the oscillator, Fmax and R(X)/C(x).

●INTERRUPT: Used for the detection and restart of the lamp. Start again after the time interval of the foot.

● R (X) / C (X): warm-up timing, brightness lock and interrupt.

●GND: ground

● P GND: Chip power ground.

● OUT B: The MOSFET of the ballast drives the output B.

● OUT A: The MOSFET of the ballast drives output A.

● PFC OUT: PFC MOSFET drive output.

● VCC: Chip positive power supply.

● VREF: 7.5V reference voltage buffer output.

● EA_/OVP: PFC error amplifier inverting input and OVP comparator input.

3. Main parameters

●Power supply ICC: 75mA

● Output current. DC current flowing into or out of 13, 14, 15 feet: 250mA

● 3-pin multiplier input I (SINE): 10mA

● Pin 5, 9, 18 input voltage: 0.3VCC ~ -2V;

● Pin 4 input voltage: -3 ~ 2V;

● Maximum overload voltage of pins 1, 6: -0.3~ 7.7V;

● Maximum overload current of pins 1, 2, and 3: ±20mA;

● Pin 2 maximum overload voltage: -0.3 ~ 6V;

● Operating temperature range: -65 ~ 150 °C

4. ML4831 main unit function introduction

Figure 1 shows the internal block diagram of the ML4831. The functions of several main units are as follows:

4.1 Power factor section

The power factor correction circuit in the ML4831 uses an average current detection boost type power factor control circuit. The working principle of this circuit can refer to relevant information.

4.2 Multiplier

The ML4831 multiplier is a linear current input multiplier that is part of the power factor controller. It has a strong ability to suppress interference caused by high power conversion. The sinusoidal voltage is input to the rectified DC voltage, which is converted into a current by a step-down resistor. Thus, a small ground noise is generated on the reference of the PWM (Pulse Width Modulation) comparator. The output voltage of the multiplier is applied to the non-inverting input of the current amplifier to form the reference for the current error amplifier. It is calculated as follows.

VMUL≈[I(SINE)×(VEA-1.1V)]/4.17mA

Among them, I (SINE) is the current on the step-down resistor; VEA is the output voltage of the error amplifier. The maximum output voltage of the multiplier is 1V.

4.3 Average current and output voltage stability

The pulse width modulator (PWM) of the PFC control section suppresses positive voltage interference caused by the multiplier output and negative voltage interference on the current sampling resistor connected to pin 4 and negative voltage interference on the current sampling resistor connected to pin 4. The MOSFET is protected when the current-limiting current is used for high-speed current transients. When the voltage at pin 4 is below -1V, the period of the pulse width modulator is terminated.

4.4 Overvoltage Protection (OVP)

Overvoltage protection (OVP) is used to protect the power supply circuit from excessive voltages when the load is abrupt (the lamp is broken open). Overvoltage protection value after DC high voltage is divided. When the voltage at pin 18 is above 2.75V, the PFC transistor turns off and the ballast will continue to operate. The overvoltage protection value must be set so that the power supply part can operate safely and not too low to affect the up-correction loop.

4.5 Oscillator

The oscillator control principle is shown in Figure 3. The oscillation frequency range of the oscillator is controlled by the LFB (lamp feedback) amplifier output (6-pin). When the lamp current decreases, the voltage of pin 6 rises, the control signal increases, and the charging current of capacitor C(T) decreases, thereby reducing the oscillation frequency. Since the ballast output network attenuates a higher frequency, the lamp power is increased.

4.6 Undervoltage lockout and over temperature shutdown

The integrated circuit (IC) contains a shunt regulation that limits the voltage VCC to 13.5V (VCCZ). The IC requires a small supply current, which is primarily supplied by the auxiliary group of the ballast transformer. When VCC is lower than VCCZ-0.7V, the quiescent operating current of the IC is lower than 1.7mA, and the output is turned off. Here, the IC can be directly powered by the step-down resistor after being rectified by the AC mains. Figure 4 shows the undervoltage lockout and power supply current waveforms.

In order to reduce the cost, the ML4831 contains a temperature sensor. When the junction temperature of the IC is higher than 12 °C, the ballast will stop working. In order for the internal sensor to safely replace the external sensor, the IC must be placed in the appropriate position in the ballast to ensure accurate detection of the ballast temperature. The temperature (Tj) of the ML4831 chip can be calculated by the following empirical formula:

(Tj)=TA PD×65°C/W

Where TA is the ambient temperature; PD is the dissipated power of the chip.

4.7 Start, Restart, Warm-up and Shutdown

With the ML4831, the start of the fluorescent lamp does not affect the lamp life, and the heat generated by the ballast is small when the fluorescent lamp is extinguished.

The fluorescent lamp preheating and interrupt timer circuit is shown in Figure 5. C(X) is translated by current IR(set)/4 and then discharged by R(X). The initial voltage across C(X) is 0.7V (VBE), and the time required to rise from 0.7V to 3.4V is the filament warm-up time. During this time, the oscillator's charge current (ICHG) is 2.5V/R(set). This will generate a high current for the filament to warm up, but will not produce a high voltage that will cause the lamp to start.

After the cathode filament is preheated, the frequency of the inverter is reduced to Fmin and a high voltage is generated to activate the lamp. After the lamp is started, the voltage of the two commercials is no longer lowered. When the feedback voltage applied to the lamp of the 9-pin is raised above the reference voltage Vref, the charging current of C(X) is interrupted and the inverter is blocked. At this time, C(X) discharges through R(X) until the voltage across C(X) drops to the threshold of 1.2V, and the inverter starts operating again. Turning off the inverter in this way prevents the inverter from generating excessive heat.

Selecting a larger resistance value of R(X) allows the warm-up time to be long enough. Before the voltage across C(X) reaches the 6.8V threshold, LFB OUT has no effect on the oscillator, so all power is applied to the lamp. Then adjust the brightness and the voltage across C(X) is clamped at 7.5V.

The operating frequency of the inverter in various states is listed in the table to the right.

5. Typical applications

Figure 6 is a high power factor, high efficiency fluorescent lamp ballast principle circuit realized by ML4831. The input AC voltage (120V) is rectified to obtain DC high voltage. Three sets of outputs are obtained through the switching tubes Q2, Q3 and the switching transformers T2B, C1, C2, and the three-way daylight (fluorescent) lamp can be started and controlled, and the brightness is adjustable. R16～R19, D10, D11, C16, C17 form a rectification sampling circuit, and send the feedback signal and brightness control voltage of the lamp to the 5th pin of the chip, and output the pulse frequency through the control 13 and 14 feet to realize the brightness adjustment and automatic Stable control. Q2 and Q3 are alternately turned on, and Q3 acts as a freewheeling, thereby improving power efficiency.

The main loop current sampling circuit is composed of R1 and D10, D11, and the sampling current signal is applied to the pin 4 of the chip as a PFC input signal. R12, R13 and D19~D22 form an overvoltage protection sampling circuit, and the sampling signal is applied to the chip 18 pin. Drive Q1 through a 15-pin output for power factor correction and overvoltage protection. The power factor correction circuit is mainly composed of Q1, L3, C7, and D7.

Figure 6 component parameters:

C1, 13, 33: 0.22 μF, 50 V, 10%;

C2, 3: 3.3nF, 125VAC, 10%;

C4: 0.33 μF, 250 VAC;

C28: 2.2nF, 50V, 10%;

C6: 47 μF, 25 V, 10%;

C9: 10nF, 25V, 10%;

C7: 120μF, 250V, 20%;

C10, 5: 1.5nF, 50V, 2.5%;

C11, 16: 0.0047μF, 50V, 10%;

C12, 20: 220pF, 2kV, 10%;

C14: 0.068 μF, 160 V, 5%;

C15: 4.7 μF, 50 V, 20%;

C17: 2.2nF, 50V, 2%;

C18: 0.22 μF, 63 V, 10%;

C19: 0.068 μF, 250 V, 5%;

C21: 6.8nF, 630V, 5%;

C22: 22nF, 630V, 5%;

C23: 330pF, 50V, 10%;

C26: 0.1 μF, 250 V, 5%;

C29: 1μF, 25V, 10%;

C30: 0.1 μF, 25 V, 10%;

C32: 4.7nF, 25V, 10%;

R1: 0.5 Ω, 5%, 1 W;

R2: 36kΩ, 1/2W, 5%;

R3: 33 Ω, 1/4 W, 5%;

R4: 360kΩ, 1/4W, 5%;

R5: 17.8kΩ, 1/4W, 1%;

R6: 35.7kΩ, 1/4;

R7: 110kΩ, 1/4W, 1%;

R8: 51.1kΩ, 1/4W, 1%;

R9: 1.96kΩ, 1/4W, 1%;

R10: 681kΩ, 1/4W, 5%;

R12: 5.49kΩ, 1/4W, 1%;

R16: 100kΩ, 1/4W, 1%;

R17: 412 kΩ, 1/4 W, 1%;

R28: 100kΩ, 1/4W, 5%;

R19: 9.09kΩ, 1/4W, 1%;

R21, 22: 22 Ω, 1/4 W, 1%;

R23: 475kΩ, 1/4W, 1%;

R27: 100Ω, 1/4W;

R29: 100kΩ;

R30: 12kΩ, 1/4W, 5%;

R31: 4.3kΩ, 1/4W, 1%;

D1-4: 1A, 600V;

D19-22: 1A, 600V, fast diode;

D6: 20V, 5%, 1W;

D7: 3A, 400V, fast diode;

D10-13: 0.1A, 75V;

D15, 18: 0.1A, 75V;

D16, 17: 1A, 50V;

Q1-3: 5.5A, 400V;

T1: Inductance 20mH, taps 1 and 4 are 150匝; taps 2 and 3 are 75匝.

T2A: Inductance 3.87mH, taps 1 and 3 are 200匝, taps 5 and 6 are 200匝.

T2B: Inductance 1.66mH, taps 1 and 2 are 200匝, taps 5 and 6 are 3匝, taps 7 and 8 are 3匝.

L3: Inductance 1mH, taps 1, 4 and 195 匝.

T4: Inductance 1.38mH, tap 1 and 3 190 匝, tap 4, 6 70 匝.

The L1 and L2 inductors are 600μH and the DC resistance is 0.45Ω.

F1: 2A.