Download Fluorescent Lamp Applications

High Intensity Discharge (HID) Lamp Applications
The HID lamp produces light by sustaining an arc between two
electrodes in a gas-filled envelope. There are several different
types of HID lamps which are characterized by the type of gas
in the envelope. These include mercury vapor, metal halide,
sodium, and xenon.
The arc produced by a HID lamp directly provides the source of
illumination. In contrast, a fluorescent lamp uses a
phosphorous coating to shift UV light into the visible spectrum.
A high voltage is required to ignite a HID lamp, which varies
with the type and temperature of the lamp. Typical ignition
voltages can exceed 10KV when the lamp is hot.
The HID driver circuit must maintain constant power to the
lamp over a variety of operating conditions. At startup, the
voltage across the lamp will be low and the current flow will be
high. As temperature increases, current flow will decrease and
the voltage across the bulb can increase.
The HID driver circuit is essentially a switch-mode power supply (SMPS) that can regulate
voltage and current. Some HID driver circuits use an H-bridge circuit to supply an AC current
to the bulb. The AC frequency is typically less than 500 Hz to avoid resonance issues.
A HID driver can benefit from intelligent features to detect fault conditions. One common
problem with HID lamps is known as ‘cycling’, which occurs near the end of the bulb’s life. As
the lamp temperature increases, the required voltage increases to a point where the arc
cannot be maintained. The HID lamp then shuts down, the temperature decreases, and the
cycle starts again. An intelligent driver can detect this cycling condition and shut down the
lamp after a number of restart cycles.
Recommended Products for HID Lamp Control
The PIC16F785 is a 20-pin, 8-bit MCU with analog peripherals to implement a wide variety of
SMPS applications. These analog peripherals include two op-amps, two comparators, and two
PWM channels. There is also a voltage reference and 10-bit ADC. The combination of a MCU
with analog peripherals allows a configurable power supply solution.
The dsPIC DSC SMPS family of devices has a 16-bit MCU for fast math processing
performance. Furthermore, these devices have a high speed digital PWM peripheral and ADC
optimized for digital power applications. The dsPIC DSC allows the HID driver to be
implemented using a fully digital control loop, as shown in the Digital HID Reference
Design. The Digital HID Reference Design showcases a dsPIC implementing full digital control
of an HID ballast.