A Study of the High Intensity Discharge Lamp -

Electronic Ballast Interface

Mark W. Fellows HID Lamp Development

Philips Lighting Company Bath, New York, USA

mark.fellows@philips.com

Abstract—New electronic ballasts for High Intensity Discharge lamps have been introduced recently. The purpose of this paper is to identify requirements of the ballast interface from the perspective of the lamp.

Acceptable electronic ballasts for HID lamps will result in lamp performance with equivalent or better then ANSI compliant magnetic ballasts. This study addresses proper starting, run-up, and steady state operation of HID lamps on electronic ballasts. Resonant and pulse ignition, run-up current and voltage requirements, acoustic resonance effects, and electrode instabilities will be covered. The paper concludes with a summary of ballast requirements necessary to provide long lamp life.

1. Introduction Electronic ballasts for High Intensity Discharge

(HID) lamps have been introduced recently. While electronic ballast design for HID lamps is still in its infancy, the potential advantages of electronic ballasts are very great. As many of the specifications and standards for these ballasts are being developed it is critical that all of the lamp ballast interactions are clearly understood and considered. A lack of knowledge of lamp characteristics by many designers has resulted in some ballast-lamp systems with low performance.

In a general sense, this paper addresses the lamps needs and lays out the consequences if the ballasts do not meet those requirements. The purpose is to summarize the basic requirements and set up ground rules for “correct designs”. This paper will identify requirements of the ballast interface from the perspective of the lamp. This study addresses proper starting, run-up, and steady state operation of HID lamps on electronic ballasts. Included is a proposal for defining acceptable resonant ignition for HID lamps. Acoustic resonance effects, including definitions of the major types of resonance, are presented along with straightforward useful equations for identifying their locations in the frequency domain.

2. Principle of Hid lamp operation Energy in HID lamps is coupled in via the electrons

in the discharge. These electrons are accelerated by the external electric field and transfer their energy to the atoms via elastic and in-elastic collisions3. Elastic collisions lead to heating of the gas, while in-elastic collisions give rise to excitation and ionization. Electron collisions also limit the electron flow and thus determine lamp current. The resultant operating pressure in these high-pressure lamps in the steady state is on the order of 10 – 100 bars. Combinations of several gases are found within the discharge, all of which have their unique roles to play.

A starting gas such as argon or xenon is present in the cold lamp. The initial discharge in this starting gas result in the lamp heating up to the point where the other components of the lamp fill will evaporate. The main gas called a buffer gas, is primary composed of mercury vapor and plays an important role in the electrical and thermal properties of the lamp during steady state operation. Mercury is chosen as the buffer gas as it is can provide high electric field strength and low heat losses.

In most HID lamps additional elements are responsible for the radiation properties. The elements most commonly used are Sodium, Scandium, Thallium, Dysprosium, Cesium, and Indium as they are efficient radiators in the visible part of the spectrum. It is essential that the vapor pressure of these elements be high enough to be effective in the discharge. With pure metals, the required temperatures to obtain satisfactory vapor pressures are often far too high for practical use. To overcome this problem, these metals can be added in the form of metal halides, where the halides are iodides. Vapor pressures of most metal halides are much higher than that of the basic element. At high temperatures – in the center of the discharge – the metal halide molecules dissociate, allowing the metal atoms to join in the discharge. Even though the pressure of these added metals is much lower than that of the mercury buffer gas,

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they are responsible for the major part of the excitation and ionization, as their excitation and ionization levels are much lower than those of mercury are. As a result, lamp operation and emitted spectrum will be sensitive to changes in vapor pressure and spatial distribution of the metal halides.

3. Lamp Functions

In order for the lamp to perform within the system, the ballast portion must address various lamp functions. These phases of lamp operation are starting phase (including breakdown); heating of the discharge tube (including electrode electron emission and run-up); and steady state operation. In turn each phase has primary and secondary functions. Primary functions (parameters) are directly adjustable while secondary effects are the result of actions by the primary function. For the lamp a few primary parameters are current, frequency of operation (time), electrode geometry, fill pressure, chemical fill (salts), and discharge tube volume. Examples of secondary functions are lamp power and voltage, color (CCT), color rendering index (CRI), lumen output, cold spot temperature, etc. Although the ballast by itself cannot influence electrode geometry, fill pressure, chemical fill (salts) and discharge tube volume, it can modify the behavior of those primary lamp characteristics by its ability to transform and adjust the secondary effects.

The starting phase includes breakdown, electrode heating, and discharge tube run-up, with focus on electrode heating and run-up. Breakdown is dependent on fill pressure of the gas, choice of gas, electrode construction and geometry of the discharge tube. Electrode heating is taken as the time from breakdown until the electrodes reach thermionic emission, while the run-up phase is the time from thermionic emission of the electrodes until the lamp reaches its nominal operating temperature. Lamp maintenance can be varied and effected if the time of the electrode-heating phase is decreased, resulting in less electrode sputtering1. Increasing the lamp current during run-up will decrease the time (duration) of the lamp electrode-heating phase, with the possibility of improved maintenance. Electronic ballasts have the capability to have increased run-up current while at the same time keeping the run-up current crest factor low 2. It is not so much the absolute amplitude of the peak current as it is the ratio of the peak current to its RMS value during the time of run-up which may lead to sputtering and loss of tungsten from the electrode.

During the steady state phase the lamp will be operated at its nominal design temperature. The electrode temperatures and arc termination (to the electrode) should be stable, well defined, and attached to the electrode tip.

Lamp chemistry should be stable, resulting in color stability, good lumen output, and consistent photometry properties. If the discharge arc is distorted phenomena such as acoustic resonance, helical or electrode instability may be occurring.

4. Acoustic resonance Acoustic waves in a discharge tube can be generated

by periodic power, which will periodically heat the gas in the discharge tube, resulting in pressure oscillations (superimposed on an average pressure)10. These waves reflect against the discharge tube walls and at certain frequencies standing pressure waves, called acoustic resonance (AR), occur. Acoustic resonance may cause strong arc instabilities or even extinction of the arc. They can change the lamp properties. As a result of the acoustic resonance the discharge path may increase in length, lamp voltage may rise, the arc may oscillate, all resulting in arc flicker.

To calculate the frequencies where acoustic resonance may occur, basic wave equations8 are used to define a cylindrical volume (representing the elongated portion of the arc tube) with internal length L and internal radius R. Periodic pressure oscillations behave according to the Morse wave equation9:

In cylindrical coordinates:

Acoustic resonance boundary conditions require a sound velocity of zero perpendicular to the walls. Assuming that a single effective temperature can describe the gas behavior, the pressure p is given by:

p = gas pressure (Pa) c = sound velocity (m/s) t = time (s)

p = gas pressure (Pa) z, r = cylindrical coordinates (m) φ = cylindrical coordinate (rad)

ωz = longitudinal angular velocity (r/s) ωr = radial angular velocity (r/s) ω2 = ωz

2 + ωr2

z, r = cylindrical coordinates (m) φ = cylindrical coordinates (rad) Jl = Bessel function of order l c = sound velocity (m/s)

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In a cylinder with length L and radius R, ωz and ωr

are given by:

The αlm is the mth zero of the first derivative of the Bessel function of order l (Jl). Longitudinal modes in the z-direction and radial/azimuthal modes are identified. The index n is for longitudinal modes, while l and m refer to azimuthal and radial modes. A mode is represented as l.m.n. Thus 0.0.2 refers to the second longitudinal mode. According to the results of zeros of the first derivative of the Bessel function, the first radial mode is 0.2.0, and the first azimuthal mode is 1.1.0.

Combining the resonance equations provides the following expression for the resonance frequency ωlmn:

The sound velocity C depends on temperature. For an ideal gas:

In actual use it is necessary to assume different sound velocities for longitudinal and radial direction because of differences in temperature profiles for the two directions. The inner dimension of the discharge tube and the velocity of sound in the gas only determine the acoustic resonance frequencies. If the electrode dimensions are small compared to the inner tube diameter, the inner tube length can be used for calculating the basic resonance frequencies because the pressure waves are reflected against the ends of the tube.

Generation of acoustic resonance at the theoretical predicted frequencies only occurs if the driving force (high frequency power) of the acoustic wave is

sufficiently strong. Certain resonances are more likely to be present and may be stronger than others, depending on discharge tube orientation and asymmetry.

5. Helical instability In HID discharge tubes when the self magnetic force

that is generated by the discharges current exceeds the wall-stabilizing forces, there may be a change in the arc from a cylindrical to a helical form5. The helical form will continually rotate about the discharge tube. Increasing “arc current or constriction of the arc increases the self-magnetic force and thus favors the onset of the helical instability”5. According to Zollweg and Ragaller6 the magnetic forces, which favor the helical arc configuration, are normally balanced by stabilizing forces favoring the cyclindical form.

Helical instability has recently arisen as an explanation for some of the instabilities seen when discharges operated on high frequency ballasts apparently “wander” around a fixed point. This is a slow rather periodic movement of the arc when operated in the vertical orientation. It appears to be most visible when the lamp is being dimmed at high frequencies. These arcs do not appear to be showing acoustic resonance, shear instability or electrode instabilities.

6. Electrode instability In a high pressure mercury vapor discharge at steady-

state and 60 Hz the discharge terminates on the coil in the cathode phase and on the rod tip in the anode phase [Fig. 2]. During the first part of the run-up of a Na/Sc metal halide lamp the wall temperature is not sufficient to volatize the NaI and ScI3 and the discharge should behave like a mercury discharge with increasing mercury pressure4. The above description describes the normal electrode and arc behavior during run-up at 60 Hz. For lamps operated on high frequency ballasts this is often the exception. The arc is often located on the coil rather then the tip, for both anode and cathode cycles. After some time the arc may move to the tip.

cr = radial sound velocity cz = longitudinal sound velocityl, m, n = mode indexes

cp/cv = specific heat ratio Rgas = gas constant (JK-1mol-1) Taverage = average gas temperature (K) Maverage = average molar weight (kg/mole)

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7. Ballasting of the lamp

Many of the lamp ballast compatibility issues2 arise from ballast designers attempting to build HID ballasts just as they would fluorescent electronic ballasts. Before starting a ballast design, the following information should be gathered [Table 1]. This includes characteristics such as primary operating frequency, ignition form, waveform ripple component, in rush current (peak and rms) upon lamp ignition, ballast efficiency, lamp voltage and current waveshapes and design features for lamp end-of-life (EOL) protection—rectification and lamp-out conditions

Ballast Information / Design features

Input operating voltage range In rush current

Ballast topology Ballast efficiency

Lamp types Lamp end-of-life (EOL) protection

Lamp operating frequency Lamp-out conditions

Ignition form (pulse, resonance) Safety Circuits

Timing functions Table 1 Ballast Information / Design features

There are many varieties of ballasts. The following definitions will be used for this paper [Table 2].

Definitions of Electronic Ballasts

Direct Current (DC) ballast

Electronically controlled power source that operates a lamp with direct current

Low Frequency (LF) Square Wave ballast

Electronically controlled power source for a lamp which has an square wave output waveform between 50 and 400 cycles per second

High Frequency (HF) ballast

Output frequency is above 19 kilohertz and below 100 kilohertz, typically with sine wave output

Very High Frequency (VHF) ballast

Above 100 kilohertz, typically with sine wave output. Although an upper frequency limit for operation of existing HID lamps has yet to be defined, the practical limit appears to be in the 500kHz to 1000kHz range. Above this range special lamp designs, or electrodeless lamps may be needed

Table 2 Definitions of electronic ballasts

Ballasts for HID lamps should be designed to meet appropriate national HID recommendations and standards such as ANSI, UL, CSA, FCC, IEEE, IESNA, CISPA and IEC 11-20 [ Table 3].

National Recommendations and Standards

(Source - ANSI, UL, CSA, FCC, IEEE, IESNA CISPA, IEC)

Safety Total Harmonic Distortion

EMI/RFI Requirements Ambient Temperature Range Allowed For Ballast Operation

Surge Protection Maximum Ballast Surface Temperature (at test point if specified)

Power Factor Mean Time between Ballast Failures

Table 3 National Recommendations and Standards

In addition, it is helpful to have a detailed overview of how the ballast is resistant to acoustic resonance phenomena. Resonance varies with frequency as a function of lamp wattage, arc tube geometry, internal fill pressure, arc tube salts composition, and lamp burning time. Low frequency square wave ballasts operate below resonance, thus providing good lamp stability [Fig. 2], as long as the high frequency ripple riding on the low frequency square wave doesn’t trigger acoustic resonance. Very high Frequency (VHF) can also provide satisfactory operation, with the proper lamp and frequency combination. Figure 2 Regions of Lamp Stability

8. Ignition techniques Ignition techniques for HID lamps typically consist

of pulse or resonant starting. Pulse ignition characteristics have been used for

years and are well defined by the various regulatory agencies. The pertinent parameters follow [Table 4].

Pulse Ignition Characteristics

Pulse waveform (shape)

Pulse repetition rate and polarity

Pulse height Pulse rise and fall times

Pulse width Note any excessive pulse ringing

Table 4 Pulse Ignition Characteristics

Acoustic Resonances

Stable operation

Transition High Frequency Transition Very High FrequencZone Zone

Low FrequencySine or Square Wave

Below Resonance

Lam

p In

stab

ility

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The pulse characteristics (height, width, etc.,) should meet the appropriate ANSI or latest ANSI proposed lamp specifications. The minimum pulse height requirements, for North America, for metal halide lamps are greater than 3000 volts but less than socket/base requirements (typically 4000 volts). Pulse width should be greater than 1µs wide at 2700V for low wattage HID lamps (39 to150 watts) and greater than 1.3µs for medium wattage lamps (175 to 400 watts). Pulse position is preferred to be near the peak of the open circuit voltage (OCV) waveform, on the rising portion of a sine type waveform, or near the initial plateau of a square wave OCV waveform. For metal halide lamps, pulse polarity should agree with OCV polarity. A minimum of 240 pulses per second for low wattage and 120 pulses per second for lamp powers above 150 W is recommended. The above recommendations are found in either issued or proposed ANSI and / or IES lamp specifications.

In addition to these measurements, the rise and fall times associated with the ignition pulse as well as the leakage current associated with the pulse, are of interest. The faster the transition of the pulse, the higher these currents may be. Leakage currents will be found lead-to-lead, as well as lead-to-ground. If the pulse is very narrow, much of its energy may be coupled to ground21. A “best practice” design will have rise and fall times and leakage current to be, at a minimum, comparable to that of ANSI compliant magnetic control ballasts.

Resonant ignition is coming into its own for HID

lamps. There are a number of commercial ballasts that today use resonant ignition, but not all are lamp friendly. Defining the peak ignition voltage for resonance starting is not as simple as it sounds. Many new lamps will ignite with considerable lower peak voltages. However as the lamp ages and the electrodes deteriorate, additional voltage is necessary to ensure lamp ignition under the conditions of cold temperature, operation in the dark, reduced salts, and variation in internal dimensions. If an “UV enhancer” is used within the lamp (found in many lamps from various manufacturers) to ensure consistent and reliable starting, a voltage peak of at least 2800 volts is needed to ensure operation of the enhancer under all conditions. This would suggest that the resonant peak should be allowed to ramp to at least that level. This is near the 2700 volts where existing ANSI lamp specifications require the pulse width be measured. If the resonant voltage peak were set at 3000 volts, we would have a safety margin and also be at the existing lamp minimum ANSI peak voltage. Thus compatibility between resonant and pulse ignition. A lower voltage may be satisfactory for low wattage lamps with shorter arc lengths and krypton 85 gas fill. Our testing shows unsatisfactory ignition below 2400 volts, over life.

The improvement in lamp starting (startability) with resonance appears to be both a function of the increased

“individual pulse cycle width” and the increased “aggregate pulse width” due to the huge quantity of high frequency oscillations during the resonance burst. It is recommended that 3000 volts be the minimum peak resonant voltage, for all of the reasons mentioned above.

ANSI values (recommendations) will insure satisfactory ignition for both high frequency and low frequency lamp operation. Certainly other values may also provide excellent ignition.

In our experience lamps driven by low frequency square waves have good ignition and stable operation when the open circuit voltage is in the 250 – 300 volt range. With Resonant swept ignition the open circuit voltage before resonance begins is not an issue. As the voltage sweeps to the point of glow to arc transition it is not necessary for that sweep to start from a point of fixed value, as long as the ballast is capable of supplying a compliance voltage sufficient to maintain the arc once ignition occurs. 9. Ballast criteria for successful lamp operation

If the ballast design is to be considered successful, the following criteria should be met [Table 5].

Successful Ballast criteria All appropriate test lamps should operate within lamp data sheet limits Photometry data for the test lamps operated with the test ballasts should be comparable to data for the same lamps operated with ANSI reactors For ±10% nominal input voltage, all lamps with lamp voltages within the allowed ANSI lamp voltage range should have a lamp maximum power output deviation of ±3% Examine voltage and current waveforms for reignition voltage, crest factors, power factor, etc Reignition voltage for the electronic ballasts should be comparable to or less than that of control ballasts At room temperature (in the dark), all lamps should attain full-arc within the appropriate ANSI or proposed ANSI time limit All lamps should start within 2 minutes under cold box conditions (lamps at –30°F) at –10% nominal ballast input voltage Full run-up should be realized within 2 minutes for low wattage lamps (a longer time period may be needed for high wattage lamps)—final lamp voltage should be within 10% of lamp voltage measured at 10 minutes Hot restrike times of lamps with test ballasts should be comparable to, or less than, hot restrike times of lamps operated with control ballasts. Hot restrike within 10 minutes is considered acceptable Neither acoustic resonance nor arc instabilities under the full range of lamp operating positions may be present

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It is preferable that no frequency component of current, voltage or power falls within known acoustic resonance frequencies There should be no change in OCV or ignition waveform characteristics from 0 to 100 hours

Table 5 Successful ballast criteria:

At rated lamp life the following lifetime criteria should be reviewed to ensure a successful ballast design [Table 6]

Successful lifetime test criteria Lamps operating on test ballasts show photometry and electrical maintenance data at least as good as control ballasts Examine lamp end-of-life (EOL) effects on ballasts. Test ballasts need to survive all EOL conditions Examine lamp X-rays for life effects, electrode wear, arc-tube deterioration, etc. Lamps operated on test ballasts should show comparable or reduced life effects to those operated on control ballasts.

Table6 Successful life test criteria

10. Conclusions

Acceptable electronic ballasts for HID lamps will result in lamp performance with equivalent or better then ANSI compliant magnetic ballasts. This study has addressed proper starting, run-up, and steady state operation of HID lamps on electronic ballasts. Resonant and pulse ignition has been reviewed. Run-up current and voltage requirements have been discussed. Acoustic resonance effects, helical instabilities, and electrode instabilities were covered. The paper concludes with a table of ballast criteria for successful lamp operation requirements necessary to provide long lamp life, and a table of lifetime test criteria for review at rated lamp life..

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12. IESNA LM-47-1995. 1995. IESNA Approved Method for Life Testing of High Intensity Discharge (HID) Lamps. J of the IESNA.

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21. Jackson A. 1999. Parasitic Capacitance in Pulse Start HID Lighting Systems, J of the IESNA. 28 (No. 1) pp. 174-181.

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