Basic requirements for a sophisticated electronic ballast for HID lighting applications as prelimiary design considerations.

 

A. Basic considerations

Lamp current. As a solution to the acoustic resonance problem, caused by a high frequency lamp current, low frequency (50Hz - 500Hz) square wave lamp current is suggested. At high frequency operation the most problematic acoustic resonance range should be avoided.

At square wave operation the following advantages can be achieved:

  • Constant instantaneous lamp power, resulting in acoustic resonance free operation.
  • No temperature fluctuation, thereby eliminating flickering and less discharge tube wall loading.
  • Constant current density in the electrodes resulting in longer lamp life.
  • Constant temperature profile in the discharge tube and the constant (non-fluctuating) current density
  • EMI (according to the output) is radically decreased compared to the high frequency operation.
 

Ballast Curve. In order to avoid the extra power consumption, caused by the usual (nonideal) ballast curves, a nearly ideal ballast curve is recommended as it is illustrated in the overview.

According to an ideal ballast curve the following may be summarized:
  • Constant lamp current in the warm-up time.
  • Constant power, equal to the nominal lamp power in the required lamp voltage range. Therefore no extra energy consumption is required according to the line fluctuations and the lampe voltage increase caused by aging.
 
Efficency and temperature range. In industrial applications the ambient temperature range can be -40°C - 70°C. Assuming an average 30°C temperature rise in the ballast and according to experimental results approximately 94% efficiency, especially in the higher power range(250W - 400W), should be achieved.In the low power range (50W - 100W) the efficiency can be lower (92%) at the same ambient temperature. Automatic over temperature protection is recommended.
 
In the following some design recommendations are listed according to the high efficiency and reliability requirements: Application of highly reliable and stable film capacitors (polypropylene,etc.)are recommended. If it is possible application of electrolytic capacitors should be avoided. Selection of ferrite material having its minimum core loss density at high temperature(90°C - 100°C) is strongly recommended. Furthermore designing the magnetic elements(inductors and transformers) for low temperature rise(<20°C) even if it requires larger and somewhat more expensive ferrite cores is also recommended. Of course, further methods of designing efficient and reliable switchmode power supplies should be taken into consideration.

 

B. Recommendations for line (input) side

Input voltages range: wide input voltage range is not recommendedbecause it decreases the input unit(power factor preregulator) efficency and therefore overall efficiency.

Power Factor: 1.0 ( >98% )

THD: < 5% (< 10%)

Input conduction noise: filtered to an acceptable level

Inrush current: none or limited to an acceptable level

Input Transient: input transient voltages should be clamped to an appropriate level. Since the input transients depend strongly on the environment a careful selection of the clamping devices (MOV's) should be considered.

Over and under voltage protection: At an absolute maximum and minimum input voltage level determined by the circuit design an automatic switch off is recommended.

 

C. Recommendations for lamp (output) side

Short circuit protection: capability for continuous short circuit operation or(and) automatic switch off after a certain time.

No load condition: automatic switch off limited by the hot reignition time, reset by OFF/ON.

Cycling: the ballast should automatically switch off when the lamp voltage reaches a maximum level by aging.

Photo-switch: by applying a simple photoconductive cell(photoresistor) connected to the ballast an automatic night/day time switch for the ballas may be optionally realized.

Dimming: the lamp power can be continuously or by discrete step(s) dimmed to the half of the nominal lamp power (or lower for HPS lamps); a simple connection of several ballasts (50-100) realizing a lighting system controlled by a single low power dimming switch should be achieved(Fig.1).



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