Using GR6953 to realize the preheating and no-light protection functions of electronic ballast

introduction

GR6953 is a half-bridge control and drive power integrated circuit (containing power field effect transistor) developed by Luda Optoelectronics specifically for electronic ballasts . It has the following features:

1) Contains two power field effect transistors 3.5Ohm/440V, which can drive 23W CFL Lamp;

2) Single power supply, built-in 20V voltage regulator circuit;

3) Low power startup;

4) R and C realize frequency control;

5) Low temperature coefficient;

6) The CT terminal can realize the shutdown function;

7) Undervoltage protection hysteresis loop;

8) Electrostatic Storage Discharge (ESD) Protection

Figure 1 is a schematic diagram of the preheating and no-light electronic ballast circuit implemented by GR6953 . The oscillation frequency of the circuit is determined by formula (1).

Figure 1: Preheating and no-lighting using GR6953 Electronic ballast Circuit schematic

f = 0.7213 / (RT * CT ) ….. (1)

Circuit Principle

Parallel switch capacitor method for preheating

The two capacitors CT1 and CT2 are connected in parallel as the timing capacitor of the output frequency, and the switching of the oscillation capacitor is controlled by the small signal MOS tube (2N7002), so that the oscillation frequency of GR6953 changes from high to low, thereby realizing the process from preheating to triggering. The effect of the circuit is delayed by Rh and Cf. When the Vcc voltage rises to the working voltage of GR6953 , Q1 and Q2 begin to turn on in sequence. The VS high voltage charges R1//R2//C3. When the voltage at point A does not reach the gate clamping voltage of 2N7002 (about 1.8V), 2N7002 does not turn on. Therefore, only CT1 works, and the oscillation frequency of GR8853 is:

f1= 0.7213 / (RT * CT1) ….. (2)

The junction capacitance of the 2N7002 MOS tube is ignored here. Because there is only CT1 capacitor, the oscillation frequency is very high, far away from the circuit resonance point, and the voltage applied to both ends of the lamp is very small. Enter the preheating process. The preheating time can be controlled by R1, R2, and C3. The preheating voltage and current can be achieved by controlling the capacitor CT1. As shown in Figure 2, the T1 time period.

When the voltage at point A continues to rise and exceeds the gate clamping voltage of 2N7002, 2N7002 is turned on, and capacitors CT1 and CT2 are connected in parallel. The oscillation frequency drops to f2. f2 is the normal operating frequency, as shown in the following formula (3):

f2= 0.7213 / (RT*(CT1+CT2)) …..(3)

The operating voltage is shown in FIG. 2 during the T2 period.

At this point, the entire process of preheating, triggering and running is realized, so that the lamp is triggered at the lowest voltage, thus increasing the service life of the lamp.

Figure 2: Preheating start voltage waveform

No-lamp protection intermittent oscillation method

GR6953 is designed for low power startup. When starting, a drop resistor is used to take the supply voltage from the DC bus. After startup, the secondary side feeding method C4 of the inductor Lr is used from the load end to maintain the power supply of GR6953 , as shown in the Lr, C4 and Rx circuit in Figure 1. When there is no light load, the induced voltage on the secondary side of the inductor Lr is zero volts, and the GR6953 cannot be powered. It can only be powered by the drop resistor. After GR6953 starts, power consumption increases, the voltage drop on Rh increases, and Vcc decreases. When Vcc drops to the lower limit of the undervoltage value Vccuv-, GR6953 stops working. At this time, the power consumption of GR6953 decreases again, the voltage drop on Rh also decreases, and Vcc starts to rise. When Vcc rises to the upper limit of the undervoltage value Vccuv+, GR6953 starts working again, and Vcc will drop to the lower limit of the undervoltage value again due to the increase in GR6953 power consumption, and stop working again. This repetition forms the intermittent working state given in Figure 3. This can reduce the power consumption of power Q1, Q2, and MOS tubes driving capacitive loads when they are unloaded, making the power tube temperature far lower than the maximum junction temperature and increasing the life of the ballast.

Figure 3. Half-bridge output voltage when no light is on

Conclusion

The series switched capacitor method and no-load intermittent oscillation method used in this article only require a few components to achieve lamp preheating and reduce the power tube switching loss when there is no lamp load, thereby greatly improving the life of the lamp and the ballast. It is a low-cost solution.

The circuit board is implemented as shown in Figure 4.

Figure 4