ST AN2952 APPLICATION NOTE

AN2952

Application note

35 W electronic ballast for HID lamps

Introduction

Low-power metal halide lamps are becoming popular as lighting sources in indoor environments like shopping centers or malls, serving as alternatives to more traditional halogen lamps, thanks to their intrinsic higher efficiency, longer lifetime and optimal color rendering. Until now, electronic circuits supplying HID lamps made use of microcontrollers and complicated topologies. Generally four power switches are used in full-bridge topology, two of which work at high frequency (80 kHz-100 kHz) in order to regulate the lamp current, while the other two work at low frequency (150 Hz-400 Hz). This application note describes an innovative analog solution for a complete ballast circuitry optimized to feed a 35 W high intensity discharge (HID) metal halide lamp (MHL), without making use of any microcontroller to regulate and control the lighting output. This circuitry is used on demonstration board STEVAL-ILH002V1 shown in figure 1. The lamps used for this design are a 35 W metal halide lamp CDM-T (Philips) and an HCI-T (Osram).

Figure 1. STEVAL-ILH002V1

AM03330v1

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Contents	AN2952

Contents

1	Lamp descpription . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
2	HID technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
	2.1	HID ballast requirement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.2	Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
3	PFC section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
4	DC-DC fly-back converter design . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
5	DC-AC inverter and lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		20
6	Ignition stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		23
7	Re-start circuit and open load/short-circuit protection . . . . . . . . . . . .		27
Appendix A		Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	29
Appendix B		Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	31
Appendix C		L6562D transistor mode PFC controller . . . . . . . . . . . . . . . . . . . . .	35
Appendix D		L6571A high-voltage half-bridge driver with oscillator . . . . . . . . .	36
Appendix E		LM139 low-power quad voltage comparators . . . . . . . . . . . . . . . . .	37
Appendix F		PFC inductor 1.2 mH, 0.34 A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	38
Appendix G		Transformer ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	39
Appendix H		Inductor 680 µH, 0.3 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	40
Appendix I		Switch mode transformer 35 W, 70 kHz, 2.3 mH . . . . . . . . . . . . . . .	41
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .			42

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AN2952	List of tables

List of tables

Table 1. Osram HID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Table 2. Philips HID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Table 3. Converter specification data and fixed parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Table 4. STEVAL-ILH002V1 bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Table 5. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

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List of figures	AN2952

List of figures

Figure 1. STEVAL-ILH002V1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Lamp families . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 3. HID lamp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 4. Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. PFC schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. PFC envelope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 7. PFC detail during startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. DC-DC fly-back converter with the sum method control. . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 9. DC-DC converter with a linear regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 10. VCC driver regulation through MJD112T4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 11. STF5NK90Z in DC-DC converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Figure 12. DC-AC inverter stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 13. STGD3NB60SD in steady-state operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 14. STGD3NB60SD details during steady-state operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 15. Detail during turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 16. Detail during turn-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 17. Ignition circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 18. STGD3NB60SD operation during startup. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 19. Lamp startup evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Figure 20. Re-start circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Figure 21. Re-start control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 22. Protection control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 23. Board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Figure 24. Top board layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Figure 25. Bottom board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Figure 26. L6562D block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Figure 27. L6571A block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Figure 28. LM139 pin connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Figure 29. LM139 schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Figure 30. PFC inductor 1.2 mH, 0.34 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Figure 31. Transformer ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Figure 32. Inductor 680 µH, 0.3 A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Figure 33. Switch mode transformer 35 W, 70 kHz, 2.3 mH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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AN2952	Lamp descpription

1 Lamp descpription

In the last year high-intensity discharge (HID) lamps have become attractive lighting sources for their luminous efficacy and their long life. In Figure 2 the generic lamp families are

illustrated.

Figure 2. Lamp families

		,!-03
).#!.$%3#%.4		'!3 $)3#(!2'%
#ONVENTIONAL	(ALOGEN	,OWWPRESSURE	(IGH PRESSURE
		3ODIUM	3ODIUM
		FLUORESCENTELAMP	MERCURY VAPOR
			METALTHALIDE
			!-V

2 HID technology

Like fluorescent lamps, HID lamps require a ballast circuitry to properly supply the lamps themselves. Also the technology in high-intensity discharge lighting is in some ways similar to that of fluorescent lamps. An arc is established between two electrodes in a gas-filled tube which causes a metallic vapor to produce radiant energy. In fact when sufficient voltage is applied to the electrodes, an arc is formed between them. Electrons in the arc stream collide with atoms of vaporized metals, shifting the wavelength of this energy into the visible range, so light is produced without adding any phosphor coating in the inner side of the bulb. In addition, the length of the electrodes is only few inches and the gases in the tube are highly pressurized. The arc generates extremely high temperatures, causing the vaporization of metallic elements in the gas atmosphere and the release of massive amounts of visible radiant energy. There are three primary types of HID lamps: mercury vapor, sodium and metal halide. The names refer to the elements that are added to the gases in the arc stream which cause each type to have somewhat different color characteristics and overall lamp efficiency. Mercury vapor lighting is the oldest HID technology. The mercury vapor produces a bluish light that renders colors poorly. Therefore,

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HID technology	AN2952

most mercury vapor lamps have a phosphor coating that alters the color temperature and improves color rendering to same extent. Although they are not the most efficient, they are often used because of their longer lifetime with respect to the other types of HID lamps.

Concerning sodium lamps, high-pressure sodium sources were developed primarily for their energy efficiency. Mercury and sodium vapors in the ceramic arc tube produce a yellow/orange light with extremely high LPW performance and exceptionally long service life (up to 40,000 hours). High-pressure sodium lamps render colors poorly, which tends to limit their use to outdoor and industrial applications where high efficacy and long life are priorities. Metal halide lamps are among the most energy efficient sources of white light available today. These lamps feature special chemical compounds known as “halides” that produce light in most regions of the spectrum. They offer high efficacy, excellent color rendition, long service life and good lumen maintenance. Because of their numerous

advantages, metal halide lamps are used extensively in outdoor applications and in commercial interiors. An example of an HID lamp is illustrated in Figure 3.

Figure 3. HID lamp

Glass Tube

Spacer

Upper Support
Quartz Arc Tube
Tungsten Electrode
Return Lead
Connector Lead
	Mercury and metal halides are combined
Lower Support	under high pressure.
	Within arc stream, these atoms generate
	both ultraviolet radiation
	And visible light. The glass bulb filters the
	ultraviolet radiation
	without affecting the visible light

AM03332v1

As shown, it is composed of two coaxial bulbs. The inner tube that generates the arc is housed in a protective glass envelope that also functions to filter UV rays produced during the ionization of the gas in the inner tube. Unlike ballast for fluorescent tubes that generally make use of a relatively high switching frequency converter, the HID lamps are supplied with low-frequency square waves (150 Hz to 800 Hz) to avoid damage from acoustic resonance. The acoustic resonance may cause arc instability, fluctuation in light output, and in the worst case a cracking of tubes. In practice, the designer must ensure that the ripple current is well below 20% to prevent the acoustic resonance in a low-frequency square wave operation.

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AN2952	HID technology

2.1HID ballast requirement

Like any gaseous discharge light source, HID lamps have their own special electrical requirements that must be supplied by the ballast circuitry according to the power and the type of HID itself. HID lamps require a high ignition voltage (2 kV to 5 kV) and even higher (>20 kV) when the lamp is hot. In addition, an HID lamp requires a warm-up period to achieve its full light output. Even a momentary loss of power can cause the system to restrike and have to warm up again (a process that can take up to several minutes). The lamps used for this design are:

1.Osram power ball HCI-T35W/830WDL

2.Philips mastercolor CDM-T 35W/830

whose characteristics are given in Table 1 and Table 2 respectively.

Table 1.	Osram HID
	Parameter	Value	Unit
	Lamp wattage	37	W
	Lamp voltage	90	V
Ignition voltage min./max.		3.5 / 5	kV
	Lamp current	0.5	A
Nominal luminous flux		3400	Lm
	Luminous efficacy	92	Lm/W
	Color temperature	3000	K
	Color rendering index	84	Ra
	Base	G12
	Light center length	56	mm
	Length	100	mm
	Average lamp life	12000	h
	Burning position	Universal

Table 2.	Philips HID
	Parameter	Value	Unit
	Lamp wattage	38	W
	Lamp voltage	88	V
Ignition voltage min./max.		5	kV
	Lamp current	0.53	A
Nominal luminous flux		3300	Lm
	Luminous efficacy	93.3	Lm/W
	Color temperature	3000	K
	Color rendering index	81	Ra
	Base	G12

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HID technology					AN2952
	Table 2.	Philips HID (continued)
		Parameter		Value	Unit
		Light center length		56	mm
		Length		100	mm
		Average lamp life		6000 to 50% survival	h
		Burning position		Universal

2.2Block diagram

The ballast is composed of 4 main blocks:

●A boost converter with a function of power factor correction pre-regulator (PFC)

●A fly-back converter with a function of DC-DC converter

●The half-bridge inverter to drive the lamp with low-frequency square waves

●The ignition circuit to warm up the lamp

Figure 4 illustrates the block diagram of the 35 W HID ballast.

Figure 4. Block diagram

	$# $#		$# !#
	CONVERTER
	PLUS		INVERTER
	CURRENTRCONTROLLER			,AMP
			)'"4
0&STAGE	VOLTAGE CONTROLLER		HALF BRIDGE
&ILTER AND			)GNITION
RECTIFIERI			SINGLE
BRIDGE
			,OWWFREQUENCY
			)'"4
)NPUT	/PEN LOAD	2E STARTT
MAIN
	PROTECTION	CONTROL
				!-V

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AN2952	HID technology

The HID ballast block diagram includes:

●an EMI filter

●a bridge rectifier

●a boost converter for PFC

●a DC-DC converter to control the lamp current and power

●a DC-AC inverter for driving the lamp at low frequency

●a re-start control

●an open-load protection

●an ignition circuit (starter)

●various driver ICs and circuitry for controlling the different blocks

The complete electrical schematic diagram of the board as well as the board layout and the bill of material (BOM) are given in Appendix B: Bill of material.

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PFC section	AN2952

3 PFC section

The PFC pre-regulation circuit is a boost converter working in discontinuous mode. It makes use of a power bipolar transistor as the main switch and no dedicated IC is used to generate the proper PWM signal. The physical relation existing between the collector current (IC) and storage time (ts) of any power bipolar transistor is exploited to generate the PWM signal. In fact the overall switch-on time is given by the sum of “IBON time” plus the storage time.

Therefore keeping the “IBON time” constant, the duty-cycle changes according to the ts modulation. This natural duty-cycle variation generates an appropriate PWM signal to control the PFC stage. To drive the power bipolar transistor we use the same L6562D output used in the DC-DC fly-back converter with a fixed duty-cycle to regulate the lamp power. Figure 5 illustrates the electrical schematic related to the PFC stage.

Figure 5. PFC schematic

												"ASE 1
			$									2
												K
PINI			344( !								$	2
												6SENSING
						,
												K
2		4STORAGEGMODULATION				% M( 344( 2 ! $					344( !	#
	$		#								$	& 6
				N& 6			$
											6 7
	,,		2		1		344( 2 5	#
2						#					$
								& 6
		2								6R
	$				"5,		$				6 7
	,,							#	$	2
							344( 2 !
								& 6
			1			P& 6				7
									344( 2 5		2
			342								+
"ASE 1 342
								4
							1
		1
							+% !
2	#		2	0&INHIBITORHDURINGNSTART UP
K
	N& 6
							$
							344( !
								%&$
												!-V

For a better understanding of the proper design of the above described PFC circuit, the user may refer to the application note AN2349. The equations needed to calculate the inductance value and capacitance value are given as follows.

The PFC inductor is given by equation 1:

Equation 1:

P

t

A

+ t

B

2

1

V

− V

L =

OUT

M

=1.2mH

2

f

T

VOUT

IM

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AN2952	PFC section

where:

●P is the input power

●f is the operating frequency

●(tA+tB)/T is chosen as 0.7 in order to ensure that the circuit remains in the discontinuous mode leaving a deadtime of 0.3 T,

●VOUT is the PFC output voltage, imposed at 460 V

The output capacitor is given by equation 2:

Equation 2:

2 I

M

1

C

OUT

= I

M

−

= 21µF

π

2πf

V

ppripple

where:

●f*= 2 f = 100 Hz

●Vppriple is the peak-to-peak ripple voltage

For the actual demonstration board two capacitors of 47 µF / 300 V in series have been used since the output voltage at startup may exceed 550 V. Figure 6 shows the storage time

modulation in the PFC section.

Figure 6. PFC envelope

AM03335v1

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PFC section	AN2952

During the startup phase the PFC stage has to be inhibited since the output voltage V0 is as high as ~550 V, therefore the duty cycle increases with consequent excessive current on the PFC power switch. This effect could cause device failure. In fact, during the startup phase,

when the voltage on the secondary side is higher than 250 V (Zener diodes D20 and D22), a low current flows on the base of the Q14 transistor (see Figure 5) which starts conducting,

disabling the PFC circuit. After startup, the output voltage V0 on the secondary side of the transformer decreases, the bipolar Q14 switches off, and the PFC starts working. Figure 7 shows the waveform during the startup phase in which this effect is highlighted.

Figure 7. PFC detail during startup

Vce

Ic

Ib

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AN2952	DC-DC fly-back converter design

4 DC-DC fly-back converter design

The fly-back converter uses a central taped transformer. This allows us the use of two small capacitors (some hundreds of nF), to create the Vcc/2. In this way the lamp frequency can be set at about 200 Hz without using electrolytic capacitors that wouldn't permit precise control of the lamp current. In order to guarantee the stability and the regulation of the lamp current, some important conditions must be considered.

●The voltage on the output electrolytic capacitor that feeds the fly-back converter has to be very stable;

●The driver used in the DC-DC converter (L6562D) is generally used for PFC function. Unlike the PFC function here, the multiplier input (pin 5) is connected to a stabilized voltage that allows having a constant current envelope (necessary to proper feed the lamp) at the output of the fly-back;

●The two secondary sides of the fly-back that supply the lamp, through the half-bridge, provide a current with stable amplitude, where the high-frequency ripple is minimized by the two non-electrolytic capacitors C12 and C22. These two capacitors only have to filter the operating frequency of the fly-back (50 kHz-100 kHz), but they do not have to store high energy that would impose a fixed voltage on the lamp, thus reducing the lamp current control.

The power regulation uses the sum method that has the function to regulate the lamp power by two sensing signals on the two secondary sides of the DC-DC transformer T2. The current control is performed by reading the signal on the sensing resistor RI, while the voltage control is performed by reading the signal on the sensing resistor Rv. These two signals are then added in order to create the error signal to regulate the lamp power. In particular during startup this control assures a square wave voltage (about 200 Hz) of

+/- 280 V on the lamp, with additional voltage spikes of 2.5 kV-3 kV from the ignition circuit. After the ignition, the voltage spikes disappear and the square wave voltage suddenly decreases down to ~40% of the steady-state value and the current control takes over

regulating the power. Firstly the power delivered to the lamp is about 60% of its nominal value and after about one minute it reaches its steady-state value. Figure 8: DC-DC fly-back converter with the sum method control on page 14 shows the DC-DC fly-back converter

schematic where the sum method control used for the power control has been encircled.

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