High brightness metal halide lamp electronic ballast

HID lamp DC electronic ballast has simple circuit, easy debugging, low cost, and is suitable for amateurs to make. This circuit has excellent constant power characteristics. When the input voltage changes from 180V to 240V, the lamp power changes by less than 2%, and the image brightness and color are very stable.

1. Working Principle

The 220V mains voltage is rectified by D1-D4, filtered by C1 and C2, and generates a 310V DC voltage. This voltage is chopped by V1, and a DC voltage of about 100V can be obtained at both ends of AB, which serves as the working voltage of the HID lamp. The process of generating this voltage is: when the output pulse of lC-6 is high, the switch tube V1 is turned on, and the current generated by the 310V voltage flows through both ends of AB (HID lamp), the current-stabilizing inductor L4, the switch tube V1, the current-sensing resistor R13 and the ground to form a loop to supply power to the lamp, while storing energy in the inductor L4 to generate an induced potential with a polarity of positive on the right and negative on the left, so that the loop current increases linearly. When the current reaches the set value, the output pulse of lC-6 becomes low, so that V1 is closed, the loop current is cut off, the polarity of the induced potential of L4 is reversed (positive on the left and negative on the right), and energy is released, generating a current flowing from the positive electrode to D8, and returning to the negative electrode through the lamp, so that the lamp current continues. When the energy stored in L4 is released, the driving pulse turns on V1 again, and the above process is repeated.

The control circuit uses the excellent current-mode PWM switch power supply control chip UC3842. There is no separate power supply circuit for it in the circuit. At the beginning of startup, its working voltage is obtained by reducing the voltage from 310V by resistor R11; after startup, the voltage induced on L3 wound together with L4 is obtained through rectification by D9 and filtering by C9. This voltage is also used as the sampling voltage of the lamp voltage. After being divided by VR1 and R2, it is sent to IC-2 pin. After being amplified by the internal error amplifier of IC, it controls the pulse width output by IC-6 pin, adjusts the output voltage, and realizes the voltage stabilization function.

The lamp current is controlled by the current sampling resistor R13 to detect and take the voltage, and then sent to the IC-3 pin. After internal comparison of the IC, it controls the output pulse width of the IC-6 pin, automatically adjusts the output current size, and achieves the purpose of current stabilization.

The lamp voltage and lamp current can be set by fine-tuning VR1 and VR2.

If a short circuit or open circuit occurs at both ends of the lamp, a signal is sent to IC through the above-mentioned current feedback and voltage feedback loop, so that the output pulse is turned off, V1 is cut off, and the switch circuit stops working, thus achieving self-protection.

2. Installation and debugging

1. Installation The switching frequency of this circuit is 30kHz, the main circuit current is relatively large, and the CMOS tube works in the switching state. The existence of distributed parameters in the circuit will generate large surge currents and spike voltages. Therefore, all components should be high-quality components, and the parameters should meet the circuit requirements. The control circuit components should be installed as close to V1 as possible, and the copper sheet of the main circuit of the printed board should be as short and wide as possible. The energy storage inductor lA is wound 36 turns on the MPP magnetic powder core with a specification of φ58xφ35×15 with φ1.5 enameled wire. If EE ferrite is used, its cross-sectional area must be twice as large, and an air gap of more than 2mm should be opened. The radiator can use a fin-type radiator with a thickness of 2mm and a volume of 70x50x15.

2. Debugging When debugging, you can first connect a dummy load at both ends of AB, such as a 200W bulb, detect the 310V voltage and the voltage at both ends of the bulb, and fine-tune VR1 to make the voltage at both ends of AB 100V. Then fine-tune VR2 to see if the brightness of the bulb changes. If there is a change, it means that the control circuit is working properly. You can connect the trigger and bulb for debugging. Make the working voltage and current of the bulb meet the rated values. For a 575W metal halide lamp, its working current can be adjusted to 5.0A-5.7A.

3. Notes

1. Do not turn off the light immediately after the metal halide lamp is triggered and lit. Wait for about five minutes for it to fully heat up before turning it off. Otherwise, the bulb will turn black, making it difficult to trigger it next time. Do not turn on the light immediately after turning it off. The high-pressure gas in the lamp will make it difficult to trigger the lamp.

2. The ballast and bulb need to be cooled by fans. The fan power should be controlled separately through a time delay switch or a manual switch. After turning off the light, let the fan work for a few minutes before turning it off to prevent residual heat from burning components such as the LCD panel.