What is a universal HID lamp ballast?
In order to light a discharge lamp, an impedance element called an electronic ballast is required to regulate the current through the arc tube. This is the same reason for inserting a resistor in series with the LED. For the arc tube, it is driven by AC power, so the impedance element uses a choke (inductor) as shown in Figure 1a to reduce resistive losses and stabilize the arc. Electronic ballasts replace the current control function of the choke with a compact and lightweight electronic circuit.
Figure 1b shows a typical configuration of an electronic HID ballast. A step-up converter boosts the power input voltage to 300V or higher, enough to drive HID lamps. Boost converters usually have PFC (power factor correction) functionality. Below is the buck converter used to control the lamp current constant. Since HID lamps for general lighting are all driven by AC, the output current of the buck converter is converted into a square wave AC current of 100 to hundreds of Hz by the inverter, and is provided to the HID lamp. The igniter produces a high-voltage pulse of several thousand volts when started, which is necessary to start the lamp discharge.
There is a typical simple high-frequency driving method in fluorescent lamp ballasts, but HID lamps with high arc tube pressures have problems with acoustic resonance phenomena
. To avoid this problem, HID lamps need to be driven at low frequency or DC below 1kHz.
wKgaomW8sDeAHNT3AAF-8gjnsXY822.pngThe lamp types in brackets are not HID lamps and have very different characteristics from HID lamps, but since the ballast is designed to drive many types of discharge lamps, I added it to the supported lamp types. Since there is an ignition in the ballast, the accompanying lamps must not have a built-in starter. The electronic ballast in this project is divided into three functional blocks (boost converter (PFC control), buck converter (current control) and inverter (DC-AC conversion)). Since PFC characteristics are not the essence of electronic ballasts, general PFC ICs can be used, but the processing resource space is large, so we decided to implement PFC characteristics into MCU to study PFC. PFC and other functions are completely independent of each other and are therefore explained in the next two chapters.
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