A Universal Programmed Start Dimming Ballast Platform

Wei Xiong Product Development

Universal Lighting Technologies Madison, AL 35756. wxiong@unvlt.com

Ana V. Stankovic Department of Electrical and Computer Engineering

Cleveland State University Cleveland, OH 44115

a.stankovic@csuohio.edu

Abstract -- In this paper, a novel universal programmed start dimming ballast platform for fluorescent lamps is proposed. The proposed platform consists of an independent PWM type filament heating control circuit for filaments preheat and filament heating in dimming condition. Mathematical analysis of the proposed method is discussed in the paper. Experimental results of T8 32watt and T5 28watt lamps are presented to validate the proposed method.

Index Terms--Programmed Start Ballast, Dimming, Fluorescent Lamps, Preheat control, PWM

I. INTRODUCTION

Fluorescent lamps are very popular and widespread on the consumer market because of the energy they save [1-5]. Unlike incandescent lamps, fluorescent lamps always require a ballast to regulate the power flow through the lamp. Generally, there are two major types of fluorescent lamp ballasts made for low pressure, hot cathode discharge lamps. First type of ballasts is instant start ballast, which has a very short period of starting time. Instant start ballasts employee a very high voltage to start the lamp. The high voltage produces a glow discharge current that degrades the integrity of the cathodes during the brief period before a lamp strikes. The drawback of instant start ballasts is a premature lamp failure since the high starting voltage burns through cathodes quickly. The second type of ballasts is programmed start ballast which provides filament heating before lamp ignition so that the glow current is minimized and so is the voltage needed to strike the lamp. By using the programmed start ballast, a lamp filament damage is minimized during starting and lamp life is drastically improved. Therefore the programmed start ballasts are desirable in order to improve the lamp life and reduce the overall cost. To make programmed start electronic ballast economically more attractive, the universal dimming control is required. Dimming ballast technology has been extensively analyzed over the last decade [6-15]. All existing technologies are designed for a specific lamp load because the filament heating is related to the main lamp drive circuit. Filament heating

circuit can not be flexibly adjusted according to different lamp loads, i.e. different lamp currents. In the commercial ballast development it’s desirable to consolidate as many types of lamp loads as possible into one ballast design so that the development and storage cost are minimized.

In this paper a new universal programmed start platform for continuous dimming is proposed. The proposed platform has the following advantages as compared to the traditional programmed start dimming topologies:

1. Zero voltage applied to the lamp before lamp ignition so that zero glow-current is achieved and lamp life is dramatically improved.

2. A completely independent filament heating method employed to preheat the filament prior to lamp ignition in order to provide optimum filament heating during dimming.

3. Instead of sinusoidal-wave filament heating voltage, square-wave filament heating voltage is proposed. Therefore, the peak to peak filament heating voltage is minimized in order to avoid arcing across the filament and to achieve maximum filament heating power.

4. Independent filament heating control method which shuts down filament heating whenever it is not needed in order to achieve higher ballast efficacy.

5. A filament type sensing circuit and PWM filament voltage control is proposed to optimize the filament heating in dimming condition for different type of lamps.

6. The proposed electronic ballast is universal since it can be used for multiple types of lamp loads.

II. INDEPENDENT FILAMENT HEATING CONTROL

Filament heating is the most important part when designing a programmed start dimming ballast. This includes both filament preheat before ignition and filament heating when dimming the lamp.

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Programmed start ballast requires proper filament preheat before lamp ignition to ensure a long lamp life. An important parameter in predicting lamp life is the ratio of the hot electrode resistance (RH) to the cold electrode resistance (RC). The optimal ratio of RH/RC is higher than 4.25 [10].

During dimming, an optimum supplemental electrode heating as a function of discharge lamp current must be maintained in order to keep lamp life within a reasonable range. In general, during dimming, the filaments must be provided with additional heating to keep the filament temperature at approximately 700-900°C. For example, ANSI_IEC C78.81 has a requirement for providing proper filament heating for T8-32Watt fluorescent lamp during dimming. The requirement is listed below:

Maximum heating voltage (V): EVmax = 5.3 Minimum heating voltage (V):

EVmin = 5.0 – 20.0*ID 0.060 ≤ ID < 0.100(A) EVmin = 8.5 – 54.5*ID 0.100 ≤ ID < 0.155(A)

EVmin = 0 0.155 ≤ ID (A) ID is the lamp discharge current. EV is the filament voltage. Nowadays in the lighting market there are 3 primary

fluorescent lamp series for commercial lighting application: T5 28watt series (35watt, 21watt, and 14watt), T8 32watt series (28watt, 25watt, and 17watt), and T5 54wattt series. Each series of lamps has the same filament. T5 28watt series generally has 10ohm cold resistance, T8 32watt series generally has 3 ohm cold resistance and T5 54watt series has 2 ohm cold resistance. T5 28watt series and T8 32watt lamp series have similar lamp current and power rating so it’s possible and desirable to design a ballast that can drive both lamp series in order to reduce the commercial development and storage cost.

For example, the preheat requirement for T5-28w and T8-32w lamps are the same (RH/RC > 4.25), whereas the filament heating requirement are different when dimming each lamp series. The proposed independent filament drive method can effectively solve the filament drive problem for different lamp series when consolidating the ballast design.

Fig.1 shows the proposed independent filament drive topology.

In Fig.1 transistors, Q1, Q2, the blocking capacitor, C_dc_block and the primary of the filament drive transformer, T_preheat form a square wave voltage generator. The resonant frequency, of the blocking capacitor, C_dc_block, and the primary inductance of the filament drive transformer, L_preheat, is much smaller than the operating frequency of transistors Q1 and Q2 driven by IC_1. The condition is given in equation (1).

1_____2

1 ICfpreheatLblockdcC

fres <<⋅⋅⋅

=π

(1)

L_preheat is the primary inductance of the filament drive transformer T_preheat. f_IC_1 is the IC_1 driving frequency. If condition (1) is satisfied the peak voltage of the primary winding of T_preheat is equal to V_RAIL/2. The voltage across C_dc_block is equal to V_RAIL/2 too. The voltage across the secondary winding of T_preheat is V_RAIL/2N (N is the turn’s ratio between the primary and secondary winding). When transistors, Q1 and Q2, are continuously driven by IC_1 at the frequency, f_IC_1, the voltage waveform across the primary winding of T_preheat and lamp filament is shown in Fig.2. The current through the magnetizing inductance of T_preheat, Imag, and current though the filament, I_filament_p is shown in Fig.3.

During the preheat mode, the micro-controller unit (MCU) continuously enables the integrated circuit, IC_1, for 1-1.5seconds to preheat the lamp filaments before ignition and to ensure optimal RH/RC. Meanwhile the MCU measures the

V_filament

t

t

V_T_Preheat

V_rail / 2

-V_rail / 2

V_rail / 2N

-V_rail / 2N

Fig.2 Voltage waveforms across the primary winding of T_preheat and lamp

filament Self_oscillatingHalf_bridgedriver

V_RAIL

DIMMING CONTROLINPUT

FILAMENT TYPE SENSE

PWM FILAMENT DR OUT

Micro-Controller

R_f ilament_a

INVERTER CONTROL

C_dc_block

Rf _senseMCU

T_p

rehe

at_A

1

2

Q2

0

LAMP CURRENT SENSING

T_p

rehe

at_B

1

2

IC_1

0

T_preheat

1

2

R_f ilament_b

Q1

Enable

Fig.1. Block diagram of the independent filament drive method

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voltage across Rf_sense, which represents the primary current through T_preheat, to determine what kind of lamp is connected to the filament drive. Since the hot resistance of T8-32watt series lamp is around 10ohm and hot resistance of T5-28watt series lamp is around 40ohm the primary current of T_preheat will be different. The primary current through the sensing resistor Rf_sense can be obtained from (2):

hotfilament

railrailsense RN

V

preheatLf

VI

_2

__

2_32 ⋅⋅

+

⋅⋅

= (2)

The voltage across the sensing resistor Rf_sense can be

obtained from (3):

⋅⋅

+

⋅⋅

⋅=

hotfilament

railrailsensefsense RN

V

preheatLf

VRV

_2

___ 2_32

(3)

The MCU stores the lamp type after the preheat mode and uses this information to determine the filament heating voltage during the dimming mode. The MCU will take the dimming command from the dimming interface to control the lamp current and filament heating. The PWM filament heating is proposed in this paper to control the filament voltage according to the lamp current level in the dimming mode. The PWM output of the MCU is connected to the enable pin of the integrated circuit, IC_1 (Fig.1). When the PWM output is high, IC_1 is enabled and the square-wave voltage is generated; when the PWM output is low, IC_1 is disabled and there is no voltage across the filament. Typical voltage waveforms across the primary winding of the T_preheat and the filament are shown in Fig.4.

The RMS voltage across the filament can be easily obtained from Fig.4 as:

N

VDDV rail

f⋅

⋅=

2)( _ (4)

From equation (4), it follows that the filament voltage is adjusted by the MCU through the PWM duty ratio D. If a dimming command is received by MCU through the dimming interface, the MCU can optimize the filament heating according to the target dimming lamp current level for the specific lamp type which information was obtained during the preheat mode. This flexible filament heating control method makes it possible to consolidate the ballast design for T5-28watt and T8-32watt lamp series.

III. CIRCUIT TOPOLOGY FOR UNIVERSAL DIMMING

BALLAST

Class D series resonant inverter with the proposed universal programmed start dimming method is shown in Fig. 5.

The integrated circuit, IC_2 shown in Fig.5 is a half-bridge driver. Q3 and Q4 are the main power switches, i.e. MOSFETs, of the half-bridge. C_dc is a DC blocking capacitor that prevents DC current from flowing through the lamp. L_resonant and C_resonant are the main resonant components that offer a high starting voltage and maintain a certain level of lamp current. A conventional programmed start ballast has to minimize the voltage across the lamp during the preheat mode in order to avoid a damaging glow current that has a negative impact on the lamp life. Since the filament heating circuit is typically integrated into the lamp drive tank, in order to minimize the lamp voltage during the preheat mode, the resonant frequency of the resonant tank has to be much smaller than the frequency in the preheat mode.. It further affects the value,

I_mag

t

t

V_T_Preheat

V_rail / 2

-V_rail / 2

V_rail / 8fL_preheat

- V_rail / 8fL_preheat

I_filament_p

t

V_rail / 2N2Rhot

-V_rail / 2N2Rhot

Fig. 3. Voltage across the primary winding, the magnetizing current and current through the primary winding of T_preheat

V_filament

t

t

V_T_Preheat

V_rail / 2

-V_rail / 2

V_rail / 2N

-V_rail / 2N

1/ f_PWM

D/ f_PWM

Fig.4 PWM filament voltage control

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size and cost of the resonant tank (L_resoant and C_resonant). This problem has been eliminated by using the proposed universal programmed start dimming ballast shown in Fig.5. Since the MCU disables the integrated circuit (IC_2) during the preheat mode, there will be zero voltage across the lamp and therefore no glow current will be flowing through the lamp.

The resonant tank optimization method is summarized as follows:

1. Given the lamp operating frequency: f_full_bright 2. Minimize the resonant inductor current, I_L_resonant

(f_full_bright) Lamp current and resonant inductor current can be

obtained by using the sinusoidal approximation:

π

2_ ⋅= railrmseq VV (5)

Where, Veq_rms is the RMS value of the fundamental voltage at the input of the resonant tank.

resonantbrightfulllampbrightfull

brightfulllampresonantbrightfull

rmseq

brightfullresonantL

CRj

RLj

V

I

⋅⋅⋅+

+⋅⋅

=

___

___

_

__

1

...)(

ωω

ω

(6)

resonantbrightfulllampbrightfullbrightfullresonantL

brightfullbrightfulllamp

CRjI

I

⋅⋅⋅+

⋅

=

_____

___

1

1)(

....)(

ωω

ω

(7)

)

1

arg(

...)(

___

___

_

___

resonantbrightfulllampbrightfull

brightfulllampresonantbrightfull

rmseq

brightfullphaseresonantL

CRj

RLj

V

I

⋅⋅⋅+

+⋅⋅

=

ωω

ω

(8)

resonantfulllampfull

fulllampresonantfull

rmseq

fullresonantL

CRj

RLj

V

I

⋅⋅⋅+

+⋅⋅

=

dim__dim_

dim__dim_

_

dim__

1

...)(

ωω

ω

(9)

resonantfulllampbrightfullfullresonantL

fullfulllamp

CRjI

I

⋅⋅⋅+

⋅

=

dim___dim__

dim_dim__

1

1)(

...)(

ω

ω

ω

(10)

Self_oscillatingHalf_bridgedriver

C_dc_block

V_rail

IC_1

0 0

0

Q4

L_resonant1 2

D1

IC_2

Q2

MCU

Rf _sense

DIMMING CONTROL INPUT

Lamp

LAMP CURRENT SENSING

C1

V_RAIL

R_f ilament_b

INVERTER CONTROLD2

R_I_sense

Q1

IC_2 Enable

Half_BridgeDriver

PWM FILAMENT DR OUT

T_preheat

1

2

C_dc

T_preheat_A

1

2

R1

T_preheat_B

1

2

C_resonant

Enable

Q3

FILAMENT TYPE SENSE

Micro-Controller

R_f ilament_a

Fig.5 Universal Programmed Start Dimming Ballast

MCU enables the filament voltage control and disables the lamp control by setting: PWM OUT Duty=100%

IC2_enable=0

MCU counts for time T_preheat to preheat the lamp filament and senses the lamp type

When t=T_preheat, MCU enables the lamp voltage control by setting: IC2_enable=1

MCU sets the operating frequency through the Inverter_Control by using the negative current PI feedback

algorithm and the actual lamp current

MCU adjusts the PWM output according to the dimming command and the lamp type to control the filament voltage

MCU keeps receiving dimming command through the dimming control interface

MCU sets the starting frequency to IC_2 through the Inverter_Control to start the lamp

MCU receives dimming command from dimming interface

MCU waits for T_start to ensure the reliable lamp starting

Fig.6.The MCU control flow chart

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The optimization process can be summarized as follows: Given:

ettbrightfulllampbrightfullbrightfulllamp II arg______ )( =ω (11)

0)( ___ <brightfullphaseresonantLI ω (12)

ettfulllampfullfulllamp II argdim___dim_dim__ )( =ω (13)

KHzfull 80dim_ <ω (14)

Minimize:

)( __ brightfullresonantLI ω (15)

Equation (11) ensures that the lamp current is right on the design target at the given operating frequency for full bright. Equation (12) ensures that the tank is inductive when operating at full bright so that Q3 and Q4 can be soft-switched in full bright condition. Equation (14) ensures that the operating frequency for full dimming is not too high so that the parasitic capacitance of the output lead wire and lamp to ground will not have a big impact on the lamp current control. Equation (13) is to ensure that the lamp full dimming current is right on the design target at the optimized full dimming operating frequency. In the steady state, the MCU senses the lamp current, receives the dimming command and automatically adjusts the operating frequency. The MCU control flow chart is shown in Fig. 6.

IV. EXPERIMENTAL RESULTS

The proposed universal programmed start continuous dimming ballast has been built in the laboratory. Experimental results for T5-28watt and T8-32watt lamp series is shown below: Vrail = 500V

f_full_bright = 35KHz f_full_dim < 80KHz I_full_bright = 180mA I_full_dim = 10mA RH/RC > 4.25

Vf - filament voltage Vf_peak_to_peak =20V, (Vf_peak_to_peak < 30V) Cdc_block =10nF

T8 32w lamp filament heating when dimming must follow ANSI_IEC C78.81 T5 28w lamp filament heating when dimming must follow the target given below:

Vf = 5.3V 0.01A < Ilamp <0.05A 3.5V<Vf <5.3V 0.05A< Ilamp <0.10A

1.5V<Vf <3.5V 0.10A< Ilamp <0.15A Vf =0V Ilamp >0.15A

By applying the proposed optimization program (equation 11-15), resonant tank component values are obtained:

Lresonant=5.7mH Cresonant=2.5nF

Cdc=100nF (chose Cdc>20*Cresonant) By applying (4) turn’s ratio of Tpreheat is obtained: N=25 By applying,

3___21 _ brightfull

res

fpreheatLblockdcC

f <⋅⋅⋅

=π

(16)

L_preheat is obtained: L_preheat=15mH IR2104 half-bridge drive is selected for IC_2. STL6569 self-oscillating half-bridge drive IC is selected for IC_1. Rh/Rc are calculated as:

c

pin

f

c

hR

I

V

RR

= (17)

Cold resistance Rc for T8 lamp filament is 3.8ohm Cold resistance Rc for T5 lamp filament is 9.0ohm. From Fig.7 and Fig.8 Rh/Rc are obtained as:

Rh/Rc for T5: 8.921/0.231/9.0=4.29 >4.25 Rh/Rc for T8: 8.773/0.405/3.8=5.70 >4.25

Fig. 7 represents the filament preheat voltage (channel 1), enlarged filament voltage (channel 1), the filament current (channel 3) and the lamp current (channel 2) for T5 28 watt lamp.

Fig. 8 represents the filament preheat voltage (channel 1), enlarged filament voltage (channel 1), the filament current (channel 3) and the lamp current (channel 2) for T8 32 watt lamp.

Fig.9 shows the T5 28watt filament voltage when the lamp current is I lamp=0.18A

Fig.10 shows the T8 32watt filament voltage when the lamp current I lamp=0.18A

From Fig.9 and Fig.10, it is observed that the filament voltage is around 0, which improves the ballast efficiency for the full bright.

Fig.7 Preheat filament voltage (channel 1) for T5 28watt lamp

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Fig.11 shows the T5 28watt filament voltage when the

lamp current I lamp=0.115A Fig.12 shows the T8 32watt filament voltage when the

lamp current I lamp=0.115A From Fig.11 and Fig.12, it follows that the filament

voltage for T5 is 2.425V and filament voltage for T8 is 2.0V, which meets the filament drive requirement for dimming condition.

Fig.13 shows the T5 28watt filament voltage when the

lamp current I lamp=47mA Fig.14 shows the T8 32watt filament voltage when the

lamp current I lamp=47mA From Fig.13 and Fig.14 it is observed that the filament

Fig.8 Preheat filament voltage (channel 1) for T8 32watt lamp

Fig.9 T5 28watt filament voltage when I lamp=0.18A

Fig.10 T8 32watt filament voltage when I lamp=0.18A

Fig.11 T5 28watt filament voltage when I lamp=0.115A

Fig.12 T8 32watt filament voltage when I lamp=0.115A

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voltage for T5 is 5.329V and filament voltage for T8 is 5.0V, which meets the filament drive requirement for dimming condition

Fig.15 shows the T5 28watt filament voltage when the lamp current, I lamp=10mA.

Fig.16 shows the T8 32watt filament voltage when the lamp current, I lamp=10mA.

From Fig.15 and Fig.16 , it follows that the filament voltage for T5 is 5.343V and filament voltage for T8 is around 4.872V, which meets the filament drive requirement for dimming condition.

A T5 28watt lamp has been working in 47mA dimming

condition continuously for 1.5years without showing filament damage symptoms, which proves that the PWM filament driving method can be successfully used in controlling lamp filament voltage in dimming condition.

V. CONCLUSION

A universal programmed start dimming ballast platform for fluorescent lamps is proposed. The proposed platform has an independent PWM type filament heating control circuit for filament preheat and filament heating in dimming condition. The proposed platform makes it possible to consolidate the ballast design for T528att and T832watt series lamp loads. In addition, the proposed platform also simplifies the resonant tank design. The tank optimization program has been discussed in the paper. The laboratory prototype has been built and tested on T8 32watt and T5 28watt lamps. It has been verified that lamps did not show filament damage symptoms after operating continuously for one and a half year.

REFERENCES [1] W. Xiong, A.V. Stankovic and L. Nerone," Modeling and Design of L-Complementary Self-Oscillating Class D Inverter with Output Voltage Clamping”, Conference Proceedings of IEEE IAS 2011 Annual Meeting, Orlando, Florida, pp 1-8.

Fig.14 T8 32watt filament voltage when I lamp=47mA

Fig.15 T5 28watt filament voltage when I lamp=10mA

Fig.16 T8 32watt filament voltage when I lamp=10mA

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[2] Wei Xiong, Ana V. Stankovic and Louis Nerone," Modeling and Design of L-Complementary Self-Oscillating Class D Inverter with Output Voltage Clamping during Starting",Conference Proceedings of IEEE ECCE 2011, Phoenix, Arizona, pp.1132-1136. [3] W. Xiong, A.V. Stankovic and L. Nerone, " New Model of L-Complementary Self-Oscillating Class D Inverter with Output Voltage Clamping", Proceedings of IEEE Applied Power Electronics Conference 2011, Forth Worth, Texas, March [4] E.E. Hammer, “high frequency Characteristics of fluorescent lamps up to 500KHz,” Journal of the Illum. Engin. Soc., pp. 56-61 [5] E.E. Hammer, and T.K. McGowan, “Characteristics of various F40 fluorescent systems at 60Hz and high frequency”, IEEE Trans. Ind. Applicant., vol. 21, n. 1, pp. 11-16, 1985. [6] C.S. Moo, H.L. Cheng and H.C. Yen, “Designing Dimmable electronic ballast with frequency control”, in Proc, IEEE applied power electronics Conf., Vol.2, pp 727-733, March 1999 [7] S.Y.R. Hui, Y.K.E. Ho and H. Chung, “An Electronic Ballast with dimming range, High PF, and Low EMI”, IEEE Trans. On power electronics, Vol. 16, No. 4, pp.465-472, July 2001 [8] W.H. Ki, J.Y.Shi, E. Yau, P.K.T. Mok and J.K.O. Sin, “phase-controlled dimmable electronics ballast for fluorescent lamps”, in Proc. IEEE Power Elector. Spec. Conf., Vol.2, pp. 1121-1125, 1999 [9] F.J. Azcondo, S. Bracho and C. Branas, “Pulsewidth Modulation control of electronic ballast for dimming control of fluorescent lamps”, in Proc. IEEE Int. Symp. Industry Electronics, Vol.2, pp.537-542, July 1997 [10] C. Contenti, “digitally addressable DALI Dimming ballast”, in Proc. IEEE Applied Power Electronics Conf., Vol2, pp 936-942, 2002 [11] H. Y. Wang, A.V. Stankovic, D. Kachmarik and L. Nerone,"A Novel Discrete Dimming Ballast for Linear Fluorescent Lamps", IEEE Transactions on Power Electronics, Vol. 24, No. 6, June 2009 [12] Chen et al. “dimming ballast and method”, US patent 7,420,336 B2 issued Jul.24, 2007 [13] A.V. Stankovic, L. Nerone and P. Kulkarni,"Modified Synchronous Buck Converter for Dimmable HID Electronic Ballast",IEEE Transactions on Industrial Electronics,Vol.59,No.4,April, 2012. [14] A. V. Stankovic, D. Uppala, D. Kachmarik, M.C Cosby Jr. and L. Nerone," Design, Analysis and Optimization of a Universal Power Factor Correction Circuit for Linear Fluorescent Lamps", Journal of the Illuminating Engineering Society, vol. 33, No.1, Winter 2004.pp.43-54. [15] Yunfen Ji, et al., “compatibility Testing of fluorescent lamp and ballast systems”, IEEE transactions on industry applications, IEEE service center, Piscataway, NJ, vol. 35, No.6, Dec. 1999

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