AN2747 Application note
250 W HID metal halide electronic ballast
Introduction
This application note describes the two stages of electronic ballast for a 250 W HID metal halide lamp. The ballast is composed of a boost converter (power factor controller PFC) working in fixed OFF time and an inverter composed of a full bridge that drives the lamp at low frequency square wave. This design was realized thanks to the very large STMicroelectronics product portfolio. The components include a PFC driver, half-bridge drivers, a microcontroller, an auxiliary power supply, a voltage reference, logic parts, an amplifier, comparators, power devices as Power MOSFETs, IGBTs, and fast diodes. In the present note special attention has been given to the full bridge stage. The tests have been conducted using a 250 W metal halide lamp HQI - T (OSRAM) and some design criteria are given with test results. In the full bridge section all lamp phases have been analyzed. Figure 1. STEVAL-ILH001V1

May 2008

Rev 1

1/44
www.st.com

Contents

AN2747

Contents
1 Lamp description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Lamp phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.1.1 1.1.2 1.1.3 Ignition phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Warm-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Burn phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2

General circuit description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 2.2 Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Lamp power management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3

Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1 3.2 Electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4 5

L6385 high-voltage high and low side driver . . . . . . . . . . . . . . . . . . . . 20 ST7LITE39 microcontroller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
5.1 Application pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

6 7 8 9 10 11 12

Auxiliary power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 TS272 high-performance CMOS dual operational amplifiers . . . . . . . 28 LM119 high-speed dual comparators . . . . . . . . . . . . . . . . . . . . . . . . . . 29 74AC00 QUAD2-input NAND gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Lamp data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 PFC section design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Full bridge design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
12.0.1 Inductor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

12.1 12.2
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Filter capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Igniter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

AN2747

Contents

12.3 12.4

Power MOSFET selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 IGBT selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

13

Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
13.1 13.2 13.3 13.4 13.5 13.6 Init phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Lamp ignition phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Warm-up phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Burn phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 PFC run phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Thermal measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

14 15 16

Firmware flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

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List of figures

AN2747

List of figures
Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Figure 30. Figure 31. Figure 32. Figure 33. Figure 34. Figure 35. Figure 36. Figure 37. Figure 38. Figure 39. Figure 40. STEVAL-ILH001V1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Lamp families . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Luminous efficiency of various lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 HID lamps operation frequency: high pressure sodium lamps - acoustic resonance duty frequency map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 HID lamps operation frequency: metal halide lamps - acoustic resonance duty frequency map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Instantaneous lamp power with sinusoidal and square waveforms . . . . . . . . . . . . . . . . . . . 7 HID warm-up phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 250 W HID lamp ballast - block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Inductor current during charge phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Inductor current during discharge phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 PFC electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Full bridge electrical schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Control board electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Power board layout (not to scale) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Control board layout (not to scale) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 L6385 block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 ST7FLIT3xB general block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 ST7FLITE39 pinout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 MU reference voltage circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Pin usage and reset circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Vbus measurement circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Lamp voltage sensing circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Lamp voltage measurement circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Constant current control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Rsense circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Current regulation circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Auxiliary power supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 LM119 pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 74AC00 pinout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Eye sensitivity versus frequency. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Igniter circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Full bridge init phase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Lamp ignition voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Lamp current and voltage during warm-up phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Warm-up phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 Steady state phase: lamp current, voltage and lamp power . . . . . . . . . . . . . . . . . . . . . . . . 39 Steady state phase: lamp current, voltage and lamp power . . . . . . . . . . . . . . . . . . . . . . . . 39 Ballast efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Firmware flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

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AN2747

Lamp description

1

Lamp description
New products and devices are being launched in lighting applications and new solutions for these applications are required. High intensity discharge (HID) lamps have become attractive lighting sources for their luminous efficacy and their long life. Figure 2 illustrates the families of electric lighting for which HID lamps are intended. Figure 2. Lamp families
LAMPS

INCANDESCENT

GAS-DISCHARGE

HB LEDs

CONVENTIONAL

HALOGEN

MERCURY

SODIUM

LOW PRESSURE

HIGH PRESSURE

CFL/TL HID

In this document we will refer to high pressure sodium and mercury lamps as HID lamps. Figure 3 shows the luminous efficiency for each lamp type. Figure 3. Luminous efficiency of various lamps

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Lamp description

AN2747

Referring to Figure 3 we can see that HID lamps have higher luminous efficacy, but some negative aspects must be considered. When comparing HID and fluorescent lamps, two important issues should be considered: a greater starting voltage and the presence of acoustic resonance. To solve the issue of higher HID ignition voltage, a sort of starting aid, called igniters, are used to ignite the lamp. The problem of acoustic resonance is more complex. Acoustic resonance is characteristic of HID lamps operating at frequencies greater than 1 kHz and appears when lamp power fluctuation exceeds a threshold value. The current/voltage lamp waveforms and the operating frequency cannot be chosen freely, but they are dependent on lamp type, condition and temperature. A wrong choice of frequency and/or waveform can have a very negative effect on lamp performance and/or lifetime and sometimes the discharge tube may be mechanically damaged.

Figure 4.

Figure 5. HID lamps operation frequency: high pressure sodium lamps acoustic resonance duty frequency map
~1MHz Frequency

HID lamps operation frequency: metal halide lamps - acoustic resonance duty frequency map

~1kHz

~1kHz

~1MHz Frequency

Stable

Unstable

Stable

Unstable

As shown in Figure 4 and 5 for different lamps the free bands are completely different.

For HID lamps the solution proposed to avoid acoustic resonance uses the same principle. The designer must avoid a combination of power fluctuation and operating frequency . Another parameter that must be taken into account is flickering. For this reason, the frequency used in the application is higher than 100 Hz and below 1 kHz.

For sinusoidal waveforms the instant lamp power is variable and it has twice the frequency of the voltage and current frequency. In Figure 6 a diagram of instantaneous power for different waveforms is shown.

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AN2747 Figure 6.

Lamp description Instantaneous lamp power with sinusoidal and square waveforms
Instantaneous Power

Current Voltage

Instantaneous Power Voltage

Current

For ideal square waveforms, the instant power delivered to the lamp is constant and not modulated by any frequency. This driving plus the low frequency driving allows obtaining a system that is immune to the problem of acoustic resonance. For this reason HID lamp applications using square wave current techniques are the best solution.

1.1
1.1.1

Lamp phases
Ignition phase
The ignition voltage for HID lamps is higher with respect to the mains voltage. The peak voltage value to initiate the discharge is very high. Typically this is about 3 kV ÷ 5 kV. The voltage level at which an HID lamp ignites is called "ignition voltage". This voltage level is referred to the lamp when it is cold but this value increases with lamp temperature. To ensure immediate HID lamp restart, called "hot restrike", very high re-ignition peaks are necessary (about 20 kV). Standard luminaries are not designed for hot restrike voltages. For this reason in order to obtain hot lamp re-ignition, several minutes must elapse for the lamp to cool. The restart time depends on lamp temperature.

1.1.2

Warm-up
When the discharge has been initiated by the starting gas and the lamp still does not burn properly, a warm-up time is required. During the warm-up time the gas temperature increases, increasing the light output. The lamp voltage starts approximately from a quarter of Vlamp and increases up to Vlamp.

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Lamp description

AN2747

During this phase the maximum current must be limited and maintained at a value higher than 30% of the nominal value. The time to reach 80% of full light/power is called "warm-up time". HID lamps have a warm-up time of approximately 2-5 minutes. In Figure 7 typical behaviors of lamp voltage and current during warm-up is shown. Figure 7. HID warm-up phase

1.1.3

Burn phase
After the warm-up phase when the lamp reaches nominal power, the lamp current must be regulated taking into account the lamp voltage to ensure fixed rated power.

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AN2747

General circuit description

2
2.1

General circuit description
Block diagram
The block diagram of the ballast is shown in Figure 8. The complete circuit is composed of two stages:

The boost converter which regulates the output voltage and performs the power factor correction. The inverter stage composed of a full bridge that converts the DC current coming from the PFC stage into an AC current for the lamp.

The operation mode of the full bridge realizes a synchronous buck converter. The full bridge moreover drives the igniter block to generate the high-voltage pulses. Figure 8. 250 W HID lamp ballast - block diagram
STTH8R06DIRG STP20NM50FD
IGNITER L6385 L6385

Main

Filter + Bri dge STF12NM50N

L6562

Lamp

STGF10NB60SD ST7FLITE39 74AC00 LM119 TS272I LE50AB

VIPER12

To generate a square wave current in the lamp, the circuit is driven in the following way: a) When low side device L2 is switched ON, the high side Power MOSFET H1 operates with a high-frequency pulse width modulation (PWM). The duty cycle D is established by a constant-current control circuit. Inductor current during charge phase
DC Bus

Figure 9.

ON H1

I

H2 L
Lamp

ON L2

L1

In this condition the current increases linearly and the voltage across the inductor L is:
9/44

General circuit description Equation 1
VL = Vd c Vl a m p

AN2747

where

VL= lamp voltage Vdc = DC bus voltage Vlamp = lamp voltage b) When the high side device H1 is switched OFF, the current flows in the low side devices. The freewheeling diode integrated in the low side device IGBT L1 operates at high frequency with duty cycle 1-D (see Figure 10).

Figure 10. Inductor current during discharge phase
DC Bus

OFF H1

I

H2 L
Lamp

ON L2

L1

The voltage across the inductor is: Equation 2
VL = Vl a m p

The current through L decreases linearly. This circuit is actually a synchronous rectifier buck converter. To minimize the current ripple through the lamp, the circuit operates in continuous conduction mode (CCM) with fixed frequency. The circuit operates in mode A and B complementarily in low frequency supplying the lamp with square wave alternate current.

2.2

Lamp power management
The lamp power is obtained by multiplying the lamp voltage signal and the lamp current. The lamp voltage is sensed directly across the lamp while the lamp current is obtained using the relation given below. In continuous mode the lamp current is coincident with average buck coil current. Starting from peak inductor current the average value is: Equation 3
I I l a m p = I A V = I p e a k ---2

10/44

AN2747 where

General circuit description

Ilamp = lamp current IAV = average current Ipeak = inductor peak current I = inductor current ripple

The ripple current in a buck converter working in continuous mode is expressed as: Equation 4
Vb u s I = ----------- ( 1 ) fL

where

Vbus= DC bus voltage L= inductance value f= switching frequency = duty cycle

For the buck in continuous mode the duty cycle relation is: Equation 5
Vl a m = -------------p Vb u s

Using the relation (Equation 4) in the equation (5) we obtain: Equation 6
1 ---I = --------------------------------- V l a m p ( V b u s V l a m p ) 2 f L Vb u s 2

Assuming Vbus and f constant it is possible to write: Equation 7
1 K = --------------------------------2 f L Vb u s

The relation (Equation 3) can be written as: Equation 8
Il a m p = Ip e a k K Vl a m p ( Vb u s Vl a m p )

This relation is valid because the average current is equal to the lamp current. This formula is implemented in the microcontroller in order to calculate the lamp current.

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Board description

AN2747

3

Board description
The detailed electrical schematics are given in the following figures.

3.1

Electrical schematic
Figure 11. PFC electrical schematic

12/44

AN2747 Figure 12. Full bridge electrical schematic

Board description

13/44

Board description Figure 13. Control board electrical schematic

AN2747

14/44

AN2747 Figure 14. Power board layout (not to scale)

Board description

Figure 15. Control board layout (not to scale)

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Board description

AN2747

3.2

Bill of material
Table 1.
Name C1 C2 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 CX1, CX2 D2, D3, D4, D13 D5A D6 D8 D9 D10 D11 D13 F1 L1 L2 L3 M1,M2 NTC1 R1 R2A R2B R3

PFC bill of material
Value 1 F 10 nF 1 F 330 pF 560 pF 470 nF 220 F 330 pF 22 nF 47 F 100 nF 10 F 330 nF 200 mA 3 A, 600 V STTH8R06DIRG 8 A 1000 V Zener diode STTH1L06 STTH1L06 1N4148 FUSE 2 x 6.8 mH 550 H 4 A 2.2 mH STF12NM50N S237 330 680 k 10 k 1% 1/4 W 1% 1/4 W 1% 1/4 W STMicroelectronics EPCOS B57237S0259M 250 V 8 A 15 V 1/2 W 1 A, 600 V 1 A, 600 V STMicroelectronics STMicroelectronics Rated 400 V 50 V 50 V 50 V 50 V 630 V 450 V 50 V 50 V 35 V 50 V 50 V 275 V 100 V Type polyester capacitor ceramic ceramic ceramic ceramic EPCOS B32523Q8474K EPCOS electrolytic capacitor B43504-B5227-M7 ceramic ceramic Electrolytic ceramic Electrolytic EPCOS X2 capacitor B32922A2334K Signal diode General purpose rectifier STMicroelectronics Bridge rectifiers

4 A 250 V

EPCOS Common mode chokes B82725-J2402-N20 Magnetica part number 1555.0009

16/44

AN2747 Table 1.
Name R5 R6 R7A,R7B,R7C,R7D R8 R9 R10A R10B R11 R12 R13 TR1 U1 U2

Board description PFC bill of material (continued)
Value 6.8 k 6.8 0.68 1.5 k 12 k 470 k 560 k 6.8 k 12 k 6.8 BC557 L6562 VIPer12AS-E Rated 1% 1/4 W 1% 1/4 W 1% 1/4 W 1% 1/4 W 1% 1/4 W 1% 1/4 W 1% 1/4 W 1% 1/4W 1% 1/4 W 1% 1/4 W TO92 STMicroelectronics STMicroelectronics Type

Table 2.

Full bridge bill of material
Name Value 100 pF 10 F 3.3 F 33 pF 68 pF 100 nF 680 nF 220 pF 68 pF 200 mA STTH1L06 800 H STP20NM50FD STGF10NB60SD 470 m 820 k 100 1% 2 W 1% 1/4 W 1% 1/4 W 3.5 A Rated 630 V 25 V 50 V 50 V 50 V 400 V 630 V 6 KV 50 V 100 V EPCOS polyester capacitor B32529-D5335-K000 ceramic ceramic EPCOS B32652A6104K EPCOS B32523Q8684K Murata DECB33J221KC4B ceramic Signal diode STMicroelectronics Magnetica part number 1775.0001, VOGT: SL0607112101 STMicroelectronics STMicroelectronics Type

C100,C101 C102, C103 C104, C106 C105, C107 C108,C109 C110 C111 C112 C113,C114,C115 D100,D101,D102,D103 D104,D105 L100 Q100,Q102 Q101, Q104 R102 R103 R104,R106,R108, R110

17/44

Board description Table 2. Full bridge bill of material (continued)
Name R105, R109 R111 R112, R115 R113 R114, R116 R117, R118 T100 U100 U101, U102 Z1,Z2 Value 22 820 k 10 k 15 k 47 k 1 M High voltage Transformer SPARK GAP L6385 Zener diode 16 V 1/2 W Rated 1% 1/4 W 1% 1/4 W 1% 1/4 W 3W 1% 1/4 W 1% 1/4 W N1/N2=55/3 235 V Type

AN2747

VOGT part number SL0607111102 EPCOS B88069X4290T502 STMicroelectronics

Table 3.

Control board bill of material
Name C2 C3 C4 POS 100 nF 10 F 100 nF 0.1 F 0.47 F 2.2 F 22 pF 1 nF 1 F BAT42 Green LED Red LED 47 k 100 k 5.6 k 4.7 k 1 k 10 10 k 470 15V Rated Type SMD 1206 through hole SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206 STMicroelectronics SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206 SMD 1206

C5,C8,C9 C6 C7 C11 C17, C18 C19,C20 D1,D2 LD2 LD3 R2 R5,R6 R7 R9 R10,R15,R20 R11,R12 R3,R4,R8,R14,R17, R18,R19,R21,R22, R26 R23,R27

18/44

AN2747 Table 3. Control board bill of material (continued)
Name R24 R25 U1,U2 U3 U4 U5 U6 V1 74AC00 LM119 TS272 STF7LITE39B LE50-CD 100 k 5 V 100 mA POS Rated

Board description

Type

STMicroelectronics STMicroelectronics STMicroelectronics STMicroelectronics STMicroelectronics

19/44

L6385 high-voltage high and low side driver

AN2747

4

L6385 high-voltage high and low side driver
Figure 16. L6385 block diagram

The L6385 is a high-voltage device, manufactured with BCD "OFF-LINE" technology. It has a driver structure that enables to drive independently referenced N-Channel Power MOSFETs or IGBTs. The upper (floating) section is enabled to work with voltage rail up to 600 V. The logic inputs are CMOS/TTL compatible for easy interfacing with controlling devices. The main characteristic of this driver are:

high-voltage rail up to 600 V dV/dt immunity 50 V/nsec in full temperature range driver current capability: 400 mA source, 650 mA sink switching times 50/30 nsec rise/fall with 1 nF load CMOS/TTL Schmitt trigger inputs with hysteresis and pull-down undervoltage lockout on lower and upper driving sections internal bootstrap diode outputs in phase with inputs

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ST7LITE39 microcontroller

5

ST7LITE39 microcontroller
The ST7LITE3 is a member of the ST7 microcontroller family of products. All ST7s are based on a common 8-bit core, able to run up to 8 MHz clock frequency. Enhanced instructions like an 8x8-bit unsigned multiplication and indirect addressing allow good efficiency and a compact application code. Figure 17. ST7FLIT3xB general block diagram

The MCU main characteristics are:

internal RC oscillator with 1% precision at 8 MHz CPU frequency seven input channels 10-bit resolution A/D converter with 3.5 s of conversion time two 8-bit lite timers with prescaler two12-bit auto-reload timers with 4 independent PWM outputs and programmable dead time generation 8 Kbytes flash program memory 384 bytes RAM 256 bytes data EEPROM SPI and SCI communication interfaces

Figure 18 shows the ST7FLITE39 pinout.

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ST7LITE39 microcontroller Figure 18. ST7FLITE39 pinout

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5.1

Application pins

Pin 1: GND Pin 2: VDD: main supply voltage. Power is supplied using an STMicroelectronics LE50 which is able to supply 5 V with 1% tolerance. Figure 19 shows the adopted circuit.

Figure 19. MU reference voltage circuit

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ST7LITE39 microcontroller

Pin 3: reset pin

Figure 20. Pin usage and reset circuit

Pin 4: ADC channel 0 analog input, to measure the main DC bus voltage (VBUS). A resistor partition is used to obtain, starting from a +400 V of bus a maximum of 5 V, compatible with the MCU voltage input, see Figure 23.

Figure 21. Vbus measurement circuit

Pin 5: ADC analog input 1: not used Pin 6: ADC analog input 2: This pin is used to measure the lamp voltage. In Figure 23 there are two nets. The first one is a partitioning circuit composed of resistors R118R114, the other one composed of R117-R116. The voltage across the capacitors C114 and C115 is used as input of U4a to obtain a signal compatible with the MCU input.

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ST7LITE39 microcontroller Figure 22. Lamp voltage sensing circuit

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Figure 23. Lamp voltage measurement circuit

Pin 7: PB3 digital floating input with interrupt. This pin is used for maximum current protection. Pin 8: PB4 digital floating input. It is used for MCU Vref calibration. Pin 9: push-pull output, used to drive two status leds. A green LED indicates the normal status. A red LED indicates a fault condition (for example overcurrent protection). Pin 10: SCI RXD, used for external communication, power line modem or PC Pin 11: SCI TXD, used for external communication, power line modem or PC Pin 12-13: PA6-PA5 not used Pin 14: PA4 output PWM3. It is used to generate a reference voltage for the constant current control.

The current signal is obtained through a sense resistor R102 (ILAMP signal Figure 25). This voltage across R102 is amplified by U4b STMicroelectronics amplifier TS272 and it is compared using the comparator LM119 with the reference voltage coming from the MCU. When the Ilamp signal exceeds the defined threshold, the comparator output falls, giving the reset signal to the drivers.

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AN2747 Figure 24. Constant current control

ST7LITE39 microcontroller

Figure 25. Rsense circuit

Pin 15-16: PA3-PA2 output PWM1 and PWM0. These signals are connected to two flipflops realized using U1 and U2. These flip-flops are used to obtain the drive signal for the high side switches, see Figure 25. The set signal is obtained by PWM rising edge, directly from the signal PWM1 and PWM0. This signal is generated at 40 kHz fixed frequency. The reset signal is obtained by the output comparator U3. In this way it is possible to generate a PWM signal for drivers with fixed frequency and controlled duty cycle. Since the system works in continuous conduction mode to avoid instability in the current control circuit, the maximum duty cycle is limited at 50%.

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ST7LITE39 microcontroller Figure 26. Current regulation circuit

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Pin 17-18: PA1-PA0 push-pull outputs. These generate the signals for the low side driver. They are directly connected to the L6385 low_side_input pins. Pin 19-20: OSC2-OSC1 External quartz input: not used.

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Auxiliary power supply

6

Auxiliary power supply
The proposed power supply can be successfully applied in applications requiring 15 V for the power switch gate driver. This circuit assures good performance in terms of size and performance at very low cost. It is based on the VIPer12A-E in nonisolated buck configuration. Figure 27 shows the schematic. Figure 27. Auxiliary power supply

The VIPer12A-E is a low-cost smart power device with an integrated PWM controller that is suitable for such applications.

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TS272 high-performance CMOS dual operational amplifiers

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7

TS272 high-performance CMOS dual operational amplifiers
The TS272 devices are low cost, dual operational amplifiers designed to operate with single or dual supplies. These operational amplifiers use the ST silicon gate CMOS process allowing an excellent consumption-speed ratio. These series are ideally suited for lowconsumption applications. Figure 28. Pinout

The main characteristics of this device are:

output voltage can swing to ground excellent phase margin on capacitive loads gain bandwidth product: 3.5 MHz stable and low offset voltage three input offset voltage selections

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LM119 high-speed dual comparators

8

LM119 high-speed dual comparators
These products are precision high-speed dual comparators designed to operate over a wide range of supply voltages down to a single 5 V logic supply and ground and have low input currents and high gains. Although designed primarily for applications requiring operation from digital logic supplies, the comparators are fully specified for power supplies up to 15 V. Figure 29. LM119 pinout

The main characteristics of this device are:

two independent comparators supply voltage: +5 V to 15 V typically 80 ns response time at 15 V minimum fan-out of 2 on each side maximum input current of 1 A over operating temperature range inputs and outputs can be isolated from system ground high common-mode slew rate

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74AC00 QUAD2-input NAND gates

A N2747

9

74AC00 QUAD2-input NAND gates
The 74AC00 is an advanced high-speed CMOS QUAD 2-INPUT NAND gate fabricated with sub-micron silicon gate and double-layer metal wiring C2MOS technology. The internal circuit is composed of 3 stages including buffer output, which enables high noise immunity and stable output. All inputs and outputs are equipped with protection circuits against static discharge, giving them 2 KV ESD immunity and transient excess voltage. Figure 30 shows the device pinout. Figure 30. 74AC00 pinout

The main characteristics are:

high speed: tPD = 4 ns (Typ.) at VCC = 5 V low-power dissipation: ICC = 2 mA (max) at TA = 25 C high noise immunity: VNIH = VNIL = 28 % VCC (min.) 50 W transmission line driving capability symmetrical output impedance: |IOH| = IOL = 24 mA (min.) balanced propagation delays: tPLH at tPHL operating voltage range: VCC (OPR) = 2 V to 6 V pin and function compatible with 74 series 00 improved latch-up immunity

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Lamp data

10

Lamp data
The lamp data are given below and each is valid for the corresponding operating phase.

Ignition phase: The ignition voltage in case of a cold lamp is about 4-5 kV. The ignition voltage increases with lamp temperature. The ignition voltage in case of a hot re-strike can reach 25 kV. The circuit is not designed to supply this very high voltage pulse. Warm-up phase: During this phase a high warm-up current must be supplied (30% higher than nominal current) to prevent lamp turnoff. The lamp voltage increases gradually starting from a quarter of nominal lamp voltage up to the nominal value. The warm-up time is about 2 minutes. For a 250 W metal halide lamp a current of 3.2 amps is applied. Burn phase: The lamp is designed to be driven with a low frequency square wave AC current to avoid acoustic resonance of the electric arc. Acoustic resonance occurs approximately in the frequency domain: 1 kHz - 1 MHz. Some frequency ranges free of acoustic resonance exist. The commutating frequency of the full bridge should be limited to the domain: 50 Hz - 10 kHz to avoid any risk of acoustic resonance. In this application the commutating frequency of 160 Hz has been chosen. This frequency has been chosen in order to avoid a flickering effect. For frequencies higher than 130 Hz eye sensitivity is practically zero. In Figure 31 a diagram of eye sensitivity is shown.

Figure 31. Eye sensitivity versus frequency

The nominal lamp voltage is approximately 100 V and the nominal lamp power is 250 W. The differential resistance of the lamp is small and negative. To obtain a stable operating point, impedance in series with the lamp is needed.

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PFC section design criteria

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11

PFC section design criteria
For PFC design criteria see AN 1875.

12

Full bridge design criteria
The design of full bridge section involves the inductor (buck) component and device selection.

12.0.1

Inductor design
The design specs are: 1. 2. 3. 4. 1. 2. 3. 4. lamp current lamp ripple current lamp voltage lamp ripple voltage Fix the switching frequency fs and consequently the period Ts also is fixed. This frequency for power application is in the range 20 kHz ÷ 100 kHz
out lam Fix the duty cycle estimation as = --------------- = -------------p -

The design steps are summarized below:

V Vi n p u t

V Vb u s

Fix the current ripple in the inductor L Calculate the inductance value using the relation:

Equation 9
Vl a m ( 1 ) Ts -L = -------------p-------------------------------I

For the inductance calculation:

fs = 40 kHz, Ts = 25 s Vlamp =100 V Vbus = 400 V = 0.25 I = 2 A

Using these parameters the inductance value is 0.93 mH. For this project an inductance of 0.8 mH is used.

12.1

Filter capacitor
To calculate the capacitor value that is in parallel to the lamp, the max current lamp ripple must be considered. The lamp current ripple must be limited in order to avoid acoustic resonance. In this application the current ripple was fixed at Ilamp 5% of nominal current

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AN2747 Equation 10
V l a m p = R l a m p I l a m p

Full bridge design criteria

where Rlamp can be obtained by the relation (in the hypothesis of linear value): Equation 11
V lamp R l a m p = ----------------Pl a m p
2

Substituting this relation in the previous relation it is possible to obtain Vlamp.The relation to calculate the output capacitor is: Equation 12
C = -------------------I-------------8 f s V l a m p

I= inductor current ripple = 2 A Ilamp =2.5 A Ilamp = 5% Ilamp = 125 mA = 250 mA Rlamp = 40 Vlamp = 10V C= 625 nF. A capacitor of 680 nF 400 V was selected.

In this case we have:

12.2

Igniter
For the design of the igniter the maximum ignition voltage must be considered. This ignition voltage is obtained by charging a capacitor (100 nF) using a resistor. When the voltage exceeds the gas spark gap threshold (235 V) capacitor is discharged in the primary winding of the high voltage transformer. This voltage is transferred at the secondary winding and a high voltage pulse is obtained, see Figure 32. Figure 32. Igniter circuit

V > 235V

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Full bridge design criteria

AN2747

12.3

Power MOSFET selection
For the selection of the Power MOSFET, the following rules must be considered:

VDSS > Vout ID > IT(pk) Vbus = 400 V I T(peak) = 4 A

VDSS greater than 20% of Vbus that is 480 V

The STP20NM50FD, FDmeshTM Power MOSFET was selected because it satisfies the following specifications:

VDSS: 500 V RDS(on): < 0.25 ID: 20 A

This device was selected considering the losses in these devices which are are composed of three parts: switch-on, conduction and switch-off losses. The first loss is caused by the voltage across the MOSFET at switch-on, the second one depends on RDS(on) and RMS current, and the last one is caused by the current at switchoff. The general formulas for conduction and switch-off losses are given below: Equation 13
Pc o n d = I
2 RMS

R D S( o n )

Equation 14
Pt u r n o f f = Vb u s Ir m s tc r o s s f

12.4

IGBT selection
Considering the current level and the working frequency (160 Hz) the best choice for the low side device is the IGBT device. In parallel at this power switch the freewheeling diode works at high frequency in continuous conduction mode. Since the diode is stressed with high di/dt, Trr, Qrr, and Irms became key parameters especially when power loss must be minimized. For both reasons the STGF10NB60SD is the right choice for this application. Using high voltage technology based on a patented strip layout, STMicroelectronics has designed an advanced family of IGBTs, the PowerMESHTM IGBTs, with outstanding performances. The suffix "S" identifies a family optimized with minimum on-voltage drop for low frequency applications (<1 kHz).

VCes: 600 V Vcesat(max): < 1.8 V ID: 10 A Trr(typ): 50 ns Qrr(typ): 70 nC

Moreover in the same package a fast diode is integrated with the following characteristics:

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AN2747

Full bridge design criteria The power losses can be divided in two quantities: IGBT losses and diode losses. The IGBT power losses are mainly due to the conduction phase and depend on Vcesat. Equation 15
PI G B T c o n d = Vc e IF ( a v )

In the diode the conduction losses (considering a duty cycle of 50%) can be estimated using the following formula: Equation 16
PD c o n d = Vt o IF ( a v ) + r d I
2 F(rms)

Where:

PDcond= diode conduction losses IF(av)= average forward current IF(rms)= RMS forward current

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Experimental results

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13

Experimental results
These results have been obtained at a rated input voltage of 230 V. Ambient temperature: 23 C. In the full bridge section the following phases have been analyzed: 1. 2. 3. 4. init phase ignition phase warm-up phase steady state phase

13.1

Init phase
During this phase the DC bus voltage is sensed and both low side devices are switched ON. In this way both bootstrap capacitors are charged. The init phase is shown below in Figure 33. Figure 33. Full bridge init phase

C1 = low side IGBT 1 gate voltage C2 = low side IGBT 2 gate voltage C3 = inductor current C4 = lamp voltage

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AN2747

Experimental results

13.2

Lamp ignition phase
The high voltage transformer generates a proper ignition voltage to ignite the lamp. The voltage across the lamp is shown below in Figure 34. Figure 34. Lamp ignition voltage

13.3

Warm-up phase
During this phase the lamp current is limited and the lamp voltage increases starting from a quarter of nominal voltage until the rated voltage. During this phase the lamp power increases and at 80% of nominal lamp power the control power is activated, regulating the lamp power.

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Experimental results Figure 35. Lamp current and voltage during warm-up phase

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C2 = lamp current C3 = inductor current C4 = lamp voltage

Figure 36. Warm-up phase

13.4

Burn phase
During this phase the lamp is supplied with low-frequency square wave current. The lamp current and the lamp power are maintained constant and some waveforms are shown in Figure 37 and 38.

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AN2747

Experimental results Figure 37. Steady state phase: lamp current, voltage and lamp power

Figure 38. Steady state phase: lamp current, voltage and lamp power

13.5

PFC run phase
During the run phase the input power the power factor and the input current THD have been measured. The results are given in Table 4.

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Firmware flowchart

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Table 4.

Power factor and input current THD measurements
Vin (rms) V 115 230 Power factor 0.994 0.940 THD% 10 27

Below Figure 39 shows a diagram of total ballast efficiency versus input voltage. The system efficiency is obtained as a ratio of lamp power to input power. Figure 39. Ballast efficiency

13.6

Thermal measurement
In the output stage a thermal measurement on the power device has been performed. On the devices a heat sink having thermal resistance Rth = 3.23 C/W has been mounted. The temperature was measured on the top of the packages of the power devices by means of a thermocouple. A T of 55 C above ambient temperature has been measured.

14

Firmware flowchart
A simplified firmware flowchart is given in Figure 40.

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AN2747 Figure 40. Firmware flowchart

Firmware flowchart

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References

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15

References
1. 2. 3. 4. 5. 6. 7. 8. 9. "Design of Fixed-Off-Time controlled PFC Preregulators with the L6562" (AN1792) "EVAL6562-375W, 375 W Fot-Controlled PFC pre-regulator with the L6562 (AN1895) "L6562-250W High performance TM PFC" "L6562 Transition-Mode PFC Controller" (datasheet) "STF12NM50 N-channel 500 V 0.29 11A MDMesh Power MOSFET" (datasheet) "STTH8R06DT Turbo 2 ultrafast high voltage rectifier" (datasheet) "L6385 HIGH-VOLTAGE HIGH AND LOW SIDE DRIVER" (datasheet) "STF20NM50 N-channel 500 V 0.29 11A MDMesh Power MOSFET" (datasheet) "STGP10NB60S N-channel 10 A 600 V PowerMesh IGBT" (datasheet)

10. "VIPer12 Low Power OFF-Line SMPS Primary Switcher" (datasheet) 11. "ST7FLITE39 8-bit MCU with single voltage Flash, data EEPROM, ADC, timers, SPI, LINSCI" (datasheet) 12. "74AC00 QUAD 2-INPUT NAND GATE" (datasheet) 13. "LM119 High speed dual comparators" (datasheet) 14. "TS272I HIGH PERFORMANCE CMOS DUAL OPERATIONAL AMPLIFIERS" (datasheet) 15. "LE50AB Very low drop voltage regulators with inhibit" (datasheet)

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Revision history

16

Revision history
Table 5.
Date 06-May-2008

Document revision history
Revision 1 Initial release Changes

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