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Top2. Electronic Board Block Diagram
The general architecture of an electronic ballast for high intensity discharge lamps (Rashid, 2007) is shown in Figure 1.
Figure 1. General HID block diagram
An EMI filter connected to the power line is responsible for limiting electromagnetic interference. After the bridge rectifier that converts AC into DC voltage, a device called Power Factor Corrector (PFC) optimizes power consumption by the power line, avoiding unnecessary loss of power, and provides a high voltage (Vbus). The output stage, consisting of an H-bridge, is driven by two half-bridge drivers (or a full-bridge driver) and allows the lamp to be powered by an AC and low frequency (usually 200 Hz) signal. The drivers are driven by a control circuit, typically a microcontroller. It provides the PWM signals to the half-bridge drivers and avoids a short circuit situation, e.g., switching on both transistors of a branch of the H-bridge at the same time.
The HID lamp needs a high discharge voltage, provided by the so called “igniter”, in order to be started up. The igniter uses a high voltage transformer to generate the desired discharge voltage (STMicroelectronics, 2008).
For what concerns the reconfigurable output, the microcontroller drives the half-bridge driver through a programmable device (GAL). The GAL implements a logic circuit which has the aim of adjusting the current flowing in the HID/LED lamp, on the basis of signals provided by the microcontroller and the feedback from the powered lamp. The fact that this feedback does not pass through the microcontroller guarantees a certain robustness of the system.
Top3. Electrical Circuits
The designed ballast consists of three PCB (Printed Circuit Board): the communication module, the control board and power board. This section describes the main resources that constitute the control board and the H-bridge driver circuit, installed on the power board (Figure 2), designed to power a 150W HID lamp.
Figure 2. Electronic board block diagram