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  • Automotive Headlamp HID Ballast

  • Created: May 23, 2016

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Summary

In automotive industry, one of the accepted lighting sources for headlight applications is the High Intensity Discharge (HID) lamp. But the process of starting up this kind of lamp is a bit complicated. Upon this process, a ballast device is needed to control the current level from going to destructive amount and then transition the HID lamp into a steady state operation. Typically HID ballasts require a large set of analog controllers to properly control the HID lamp. So by using digital control techniques through Microchip’s dsPIC device to control the HID ballast, components and cost are significantly reduced.


This design features digital HID ballast intended for interfacing with standard automotive HID lamps. An HID lamp has six stages to go through when it is lighted up. These are turn-on, ignition, takeover, warm-up, run-up and steady state. The ballast must be designed to meet this working profile of an HID lamp during its startup process; otherwise, the HID lamp will not operate as expected. The ballast in this reference design consists of four sections; the high frequency DC/DC converter, low frequency DC/AC inverter, ignition circuit and the digital signal controller. The DC/DC converter is the key stage to implement the control of the lamp voltage, lamp current, and lamp power. The DC/DC converter boosts the battery voltage (9V-16V) to a high level for the ignition circuit first, and then drops to ~85V for steady state operation. In the DC/AC circuit, a full-bridge inverter is used for this stage. The DC/AC inverter converts the DC current to a square wave current to energize the two lamp electrodes equally. The operation frequency of the inverter is dependent on the lamp state. Before ignition, the inverter runs at a frequency of 1kHz for the turn-on and ignition stages. After ignition, the operation frequency is only 20Hz for the warm-up stage. When the warm-up stage is over, the inverter operates at 200Hz. The ballast has an ignition circuit, which is driven by a dual frequency inverter. It uses a smaller value of ignition capacitor (C34) than capacitor used in conventional ignition circuit. With this configuration, it can generate a higher pulse power, which improves ignition success rate especially for hot lamp strike. A single digital signal controller, the dsPIC33FJ06GS202, controls both the DC/DC converter and the DC/AC inverter. The dsPIC DSC is responsible for detecting the voltage lamp and lamp current through its ADC pins (AN0 and AN1). The controller adjusts the PWM duty cycle of the DC/DC converter to control the lamp current. The DSC also monitors several fault signals. The open circuit protection and short circuit protection need rapid response in which the internal comparators are selected to implement these two protections. At the same time, the digital signal controller measures the battery voltage through another ADC pin (AN2). If the battery voltage is outside the normal operation range, the ballast will stop working. In addition, Timer2 of the DSC is used to control the operation frequency of the full-bridge inverter, and the inverter drive signal is produced through the I/O port, RB14.


The digital HID ballast presents a handful of advantages compared to the traditional analog one. Other than being low cost, digital HID ballast also offers robustness to design, high performance operation, effective protection and convenient implementation of sophisticated control algorithms. Traditional inductive ballast also has many problems such as large bulk capacitors, low Power Factor (PF) and difficulty reigniting. While in using digital electronic ballast, Instant start-up, small size, high PF, and high efficiency can be achieved.

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