ST AN2835 APPLICATION NOTE

AN2835

Application note

70 W HID lamp ballast

based on the L6569, L6385E and L6562A

Introduction

This application note describes the electronic lamp ballast for 70 W high intensity discharge (HID) metal halide lamps (MHL) used for general indoor applications. The ballast is composed of a boost converter and an inverter. The inverter is realized by a full bridge driver with a power control circuit.

The booster converter for power factor correction (PFC) is controlled by the L6562A controller (U1). The inverter is a full bridge topology driven by two pairs of half bridge buck converters, L6385E (U3) and L6569 (U4), with the constant power control circuit L6562A (U2).

In this note the dual-buck converter is introduced. One works in high frequency and the other works in complementarity with necessary dead time at a lower frequency.

Figure 1. The demonstration board

May 2010 Doc ID 15073 Rev 1 1/21

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Contents AN2835

Contents

1 Safety instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 The selected solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 The dual-buck converter topology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 The power control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Ignition circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Description of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1 Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2 The PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3 Electrical schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4.1 Test with HID lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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AN2835 List of figures

List of figures

Figure 1. The demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. The fundamental diagram for the HID lamp ballast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 3. The dual-buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 4. The timing chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 5. Input power, output voltage and input peak current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 6. Indirect constant power control circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 7. Average current sense circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 8. Ignition circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 9. Electrical characteristics of LIC01 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 10. Demonstration board top-side view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 11. Demonstration board bottom-side view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 12. Schematic diagram of demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 13. Lamp current at warm-up state. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 14. Load with 30 W during warm-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 15. Load with 50 W during warm-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 16. Load with 100 W in steady-state. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 17. Load with 140 W in steady-state. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 18. Steady-state at 110 Vac input. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 19. Steady-state at 220 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 20. The timer circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 21. Lamp current during start up with HID lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 22. The lamp current in warm-up state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 23. The lamp current in steady-state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Doc ID 15073 Rev 1 3/21

Safety instructions AN2835

1 Safety instructions

Warning: The demonstration board must be used in a suitable

laboratory by qualified personnel who are familiar with the installation, use, and maintenance of electrical systems.

Intended use

The demonstration board is designed for demonstration purposes only, and must not be used for domestic installations or for industrial installations. All technical data, including the information concerning the power supply and working conditions, should only be taken from the documentation included in the pack and must be strictly observed.

Installation

The installation instructions for the demonstration board must be taken from the present document and strictly observed. The components must be protected against excessive strain, and in particular, no components are to be bent, or isolating distances altered during transportation, handling or use. The demonstration board contains electrostatically sensitive components that are prone to damage through improper use. Electrical components must not be mechanically damaged or destroyed (to avoid potential risk and personal injury).

Electrical connection

Applicable national accident prevention rules must be followed when working on the mains power supply. The electrical installation must be carried out in accordance with the appropriate requirements (e.g. cross-sectional areas of conductors, fusing, and PE connections).

Board operation

A system architecture which supplies power to the demonstration board must be equipped with additional control and protective devices in accordance with the applicable safety requirements (e.g. compliance with technical equipment and accident prevention rules).

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AN2835 The selected solution

2 The selected solution

2.1 The dual-buck converter topology

The fundamental application circuit in Figure 2 is composed of a PFC stage and a power inversion stage. As the boost converter for power factor correction (PFC) is commonly used, only the power inversion stage is introduced in this application note.

Figure 2. The fundamental diagram for the HID lamp ballast

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The full bridge inverter consists of two half bridge buck converters. This is shown in

Figure 3. Both converters have the same L2 load, C2 and lamp. One of the buck converters

(S2 and S4) works in high frequency (several tens of kHz) and the second buck converter (S3 and S5) works in complementarity with necessary dead time at a lower frequency (a few hundred Hertz). This kind of full bridge stage is also called dual-buck converter.

Figure 3. The dual-buck converter

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The timing diagram in Figure 4 indicates the relationship of a dual-buck converter and lamp current.

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The selected solution AN2835

Figure 4. The timing chart

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Description of the four operating states

● State 1 in Figure 3a (from t0 to t1, see Figure 4):

– S3 and S4 in off-state, S2 and S5 are turned on, C1 discharges through S2, L2,

Lamp and S5. Certain energy stores to L2 and C2.

● State 2 in Figure 3b (from t1 to t2, see Figure 4):

– S3 and S4 remain in off-state. S2 is working in high frequency in off-state, and S5

is working in low frequency and remains in on-state. The energy of L2 and C2 keeps on releasing through Lamp, S5 and the reversed body diode of S4. As S2 is working at a higher frequency, state 1 and state 2 is repetitive until the S5 is turned-off at t3.

● State 3 in Figure 3c (from t3 to t4, see Figure 4):

– S2 and S5 in off-state, S3 and S4 are turned on, C1 discharges through S3, Lamp,

L2 and S4. Certain energy stores to L2 and C2.

● State 4 in Figure 3d (from t4 to t5, see Figure 4):

– S2 and S5 remain in off-state. S4 is working in high frequency in off-state, and S3

is working in low frequency and remains in on-state. The energy of L2 and C2 keeps on releasing through the reversed body diode of S2, then S3 and lamp. S4 is working at a high frequency from t3 to t6, therefore state 3 and state 4 is repetitive until S3 is turned-off at t6. One full operating cycle is completed. Starting from t6, the behavior of t0 is repeated again. From the above analysis, we realize the lamp current flow to this dual-buck converter, and loads to the same L2 inductor, C2 output capacitor, and HID lamp. The lamp current at state 3 and state 4 is in the opposite direction.

2.2 The power control circuit

There are two main functions of the power control circuit. One is constant current control during warm-up and the other is constant power control during steady-state operation.

● Constant current control

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Normally, the lamp current is higher during the warm-up stage than when working at steady-state. But a warm-up current that is too high may cause the electrode to decay and shorten the operating life of the lamp. If warm-up current is too low, the time to steady-state is postponed. Therefore providing a value with 20% higher than the rate of warm-up current during warm-up time is respected. The constant current control is

AN2835 The selected solution

realized by controlling the peak inductor current of the dual-buck converter. Assume the input voltage of the buck converter is V input peak current is I

, as the buck converter is working at a critical discontinuous

in_pk

, the output voltage is Vbo, the duty cycle is D,

mode, and the average input current is:

Equation 1

-- -

D⋅=

inpk

And the duty cycle is:

Equation 2

---------=

The input power becomes:

Equation 3

inVbiIin

⋅=

Thus the relationship between the input power (P voltage (V

) is:

), input peak current (I

) and output

in_pk

Equation 4

If the lamp is operating with a constant current source, once input peak current (I selected, we observe the input power (P

) is proportional to the output voltage (Vbo).

in_pk

Despite the power losses, the output power is also proportional to the output voltage.

Figure 5. Input power, output voltage and input peak current

) is

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In Figure 5, there are three different I

curves, this helps to choose the proper input peak

in_pk

current according to the different types of lamp. After warming-up, the lamp voltage increases slowly to the minimum value of the rated power, the duty cycle increases accordingly. And then the input peak current decreases. In order to power up the lamp in steady-state, the circuit changes from constant current control function to constant power control.

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