ST AN880 Application note

	AN880
®	APPLICATION NOTE

THE L6569: A NEW HIGH VOLTAGE IC DRIVER FOR ELECTRONIC LAMP BALLAST

by G. Calabrese and T. Castagnet

INTRODUCTION

Electronic lamp ballasts are now popular in both consumer and industrial lighting. They offer power saving, flicker free operation and reduced sizes. Improvements to the light control and cost reduction of the ballast will broaden their market acceptance.

Today designers focus on reducing the cost of the ballast, but also work to add features to the ballast like saving energy by dimming the light, or increasing the life time with better preheat and protections. Such requirements have contributed to the development of dedicated high voltage controllers like the L6569, which are able to drive the floating transistor of a symmetric half bridge inverter. This device is a simple, monolithic oscilla- tor-half bridge driver that allows quick design of the ballast.

HIGH VOLTAGE IC DRIVERS IN BALLAST APPLICATIONS

The voltage fed half bridge

Voltage fed series resonant half bridge inverters are currently used for Compact Fluorescent Lamp ballasts (CFL), for Halogen Lamp transformers, and for many European Tube Lamp (TL) ballasts. This simple converter is preferred for new designs, because it minimizes the off state voltage of the power transistors to the peak line voltage, and requires only one resonant choke. In addition this choke protects the half bridge against short circuits across lamp terminals. However overheating and overcurrent occur during open load operation. The inverter robustness must be improved, or some protections are required.

The half bridge inverter operates in Zero Voltage Switching (ZVS) resonant mode [1], to reduce the transistor switching losses and the electromagnetic interference generated by the output wiring and the lamp.

Fully integrated ballast controllers

By varying the switching frequency, the half bridge inverter is able to modulate the lamp power. However most current designs use a sin-

Figure 1: CFL series resonant half bridge inverter.

Figure 2: Current and voltage of the STD3NA50 MOSFETs when driven in ZVS with the L6569.

ID

VDS

GND

LVG

GND

RF

GND

2 μs/dv ; 50 V/dv ; 0.1 A/dv

gle frequency with a saturable pulse transformer (see fig. 1) to drive the transistors. This type of design has a higher component count, a higher tolerance on the switching frequency, and it cannot adjust the lamp power.

The only way to design a cost effective, compact and smart control of the lamp is to use a dedicated I.C. that is able to drive the upper transistor of an symmetric half bridge inverter. Such controllers require a high voltage capability for the floating transistor driver [2]. MOSFETs are preferred over Bipolar transistors as power switches because their gate driver requires a lower supply current and a smaller silicon size [3].

February 2003

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AN880 APPLICATION NOTE

THE L6569 AND ITS APPLICATIONS The L6569

The L6569 is able to directly control a symmetric half bridge inverter of a fluorescent lamp ballast, or a low voltage halogen lamp transformer.Two 270mA buffers drive the inverter MOSFETs in complementary fashion with a 1.25μs built-in dead time to prevent cross conduction. The buffer for the upper Mosfet is driven through a 600V level shifter realized in BCD off line technology. The oscillator, similar to a CMOS 555 timer, operates from 25 to 150 kHz with a +/-5% maximum tolerance. The internal 15V shunt regulator has a 9V Under Voltage Lock Out with an 1V hysteresis,

Figure 3: Block diagram of the L6569.

and the circuit requires only 150 μA at power up.

The L6569 integrates a high voltage Lateral DMOS transistor in place of the usual external diode [2] to charge the bootstrap capacitor for the upper buffer. Figure 5 shows DMOS operating as a synchronous rectifier.

The applications

The primary application for the L6569 is the Compact Fluorescent Lamp. With the oscillator, the supply and the Mosfet drivers it is the core of the application, and designers can customize the circuit to their requirements.

	VS	BOOT
UVLO	CHARGE
	PUMP
	LEVEL	HVG
	SHIFTER
		HIGH
RF		SIDE
		DRIVER
CF		OUT
	LOGIC CONTROL	LVG
	with DEAD TIME
		LOW
		SIDE
		DRIVER
	GND

Figure 4: Basic application diagram using the L6569 and two STD4NK50Z MOSFETs.

	180KΩ		100nF
	10µF		22Ω
	10KΩ	L6569	STD4NK50Z
			LAMP
	10µF
	AC LINE		22Ω
	1nF
			D02IN1385
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AN880 APPLICATION NOTE

Figure 5: Bootstrap capacitor charge.

15.6 V	ON
600V	120Ω
CHARGE PUMP CIRCUIT
LOGIC	ON
L6569

Figure 6: Basic diagram for 2x105 W lamp ballast in full bridge configuration.

				HV
		BOOT	100nF	100nF	BOOT
	VS					VS
VS
	RF
		HVG	47	47	HVG	RF
	CF	L6569			OUT	L6569
EXTERNAL		OUT				CF
OSCILLATOR
	GND	LVG	47	47	LVG	GND
				STB9NK50Z
D02IN1386

Typical industrial TL ballasts requires complex control with dimming or automation interface. Here the L6569 is a driver between the power and control blocks. To use it with an external oscillator, pin CF is used as an 0-12V logic input, and the L6569 becomes a high voltage buffer. Applications with power above 150W require a full bridge inverter. Figure 6 shows how two L6569 drive such a MOSFET bridge. If no external control is required, the first L6569 master can control the switching with its oscillator, and synchronizes the other driver as (slave).

The L6569 start up

Two versions of the L6569 are available with different start up characteristics. The L6569 drives the lower MOSFET ON at power-up until the supply voltage reaches the Under Voltage Lock Out. The bootstrap capacitor is precharged to 4.6V and both the lower and the upper MOSFETs will switch immediately with the oscillator. This is intended for inverters which use only one DC blocking capacitor connected to the power ground, as shown on figure 4 for CFL ballast.

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AN880 APPLICATION NOTE

The L6569A holds both MOSFETs OFF until the Under Voltage Lock Out is reached. This is intended for inverters using 2 decoupling capacitors in half bridge as shown on figure 12. The inverter is totally off, so that the voltage at the capacitors center node is not unbalanced by the leakage path during power on.

CONSIDERATIONS ON THE L6569 ENVIRONMENT

To illustrate the benefits of the L6569 in the CFL applications, a demonstration board was developed to supply Sylvania 18W DULUX lamp (ref: CF18DT/E). The following chapters summarize the application considerations applied in this design. The schematic, lay out and components list are shown in appendix A.

Symmetric half bridge operation

To supply a fluorescent lamp, the ballast has to achieve 3 functions: pre heat, ignition, and normal lamp operation. The serial resonance occurs between the choke and the capacitor in parallel with the lamp. The choice of these components determines the lamp ignition voltage and the nominal lamp current.

Since the inverter using the L6569 and MOSFETs can operate at a higher frequency than conventional solutions, the size of the passive components will be reduced. Such inverter can operate up to 150 kHz in ZVS mode, and the switching losses of the power transistors only limits the frequency. In new design this frequency should be set between 50 and 100 kHz. For instance with an 18W lamp, a frequency increase from 33 to 50 kHz will lead to a 40% reduction of the choke size.

To operate in Zero Voltage Switching (ZVS), the switching frequency is higher than the resonant frequency. All operation phases of the ballast are secure in this mode. When the bootstrap transistor is conducting, no pulse current will flow from pin BOOT to pin VS, as it might happen in Zero Current Switching. The bootstrap transistor remains in its Safe Operating Area, and its dissipation is negligible.

The MOSFET drive

The ZVS drive technique requires only a fast turn off capability as shown on figure 2, and the transistor buffers are designed with a stronger sink current. The two MOSFET buffers of the L6569 can sink a 400 mA peak current on capacitive load. Typically these buffers can drive any MOSFETs in TO220 package.

Figure 7 shows an example with the STP8NA50 that has an 0.85 Ω resistance RDS-ON.

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Figure 7: Current and voltage of the STP8NA50 MOSFET at turn off with the L6569. TGD = 245 ns ,Tc = 95 ns, E = 93 μJ @ Tj = 50°C, RG = 22 Ω.

TGD	Tc

ID

VGS

GND

GND

VD

GND

50 ns/dv ; 1 A/dv ; 5 V/dv ; 50V/dv

The built-in dead time circuit acts when a MOSFET turns off, delaying the turn on of the opposite transistor for 1.25 μs. The voltage VOUT between the 2 MOSFETs must switch within the minimum dead time (0.85 μs), as shown on figure 8, to avoid bridge cross conductions and transistors overheat.

Figure 8: STD3NA50 MOSFET turn off when driven by the L6569. TC + TGD < TD

TD

ID			VDS		GND
	TC		LVG		GND
		TGD	RF		GND

200 ns/dv ; 50 V/dv ; 0.1 A/dv

The MOSFET voltage selection

Since the ballast is connected to the ac mains, it must handle any spurious voltage spikes. When the front end RFI filter and the clamping device, such as a varistor, absorbes totally the spike energy, MOSFETs can have the same 600V minimum breakdown voltage BVDSS as the L6569.

Otherwise when the upper MOSFET is on, the residual default may be applied to the L6569. Although the pin OUT breakdown voltage is higher than 600V, it has a poor avalanche robustness. Therefore the lower MOSFET protects the driver by having a lower BVDSS. A MOSFET with a minimum BVDSS up to 500V will achieve safely this task.

AN880 APPLICATION NOTE

Figure 9. L6569 driver protection against voltage spikes.

BV OUT > 600V	H.V.+

ON

15V

V OUT

L6569											I
				OFF

The auxiliary supply of the converter

The circuit consumption is defined by the MOSFETs gate charge, the I.C. consumption, the oscillator, and the shunt regulator. Several circuits are possible.

In many applications a snubber is used to reduce the dissipation in the MOSFETs. When this snubber is used in conjunction with a start up resistor (RS in Figure 10), a non dissipative supply is achieved almost for free.

At start up the I.C. is consuming 150 mA, and therefore only a small supply resistor is required. During operation the capacitor provides the supply current. To avoid cross conduction, the capacitance is limited by the driver dead time TD . Hence the capacitive supply current IC is also limited.For a CFL ballast this circuit easily supplies the required operating current. Using a CF18DT lamp ( IL > 230 mA) the required capacitance is 470 pF on 230 Vac line. At 50 kHz the average capacitive current is 6 mA, as described in appendix B.

When the required driver current is higher than 10

mA, a secondary winding on the resonant choke is an easy supply alternative.

The ballast shutdown

The L6569 allows several ways (see figg. 11, 12 and 13) to shutdown the ballast [4]: by acting on the CF input oscillator pin to turn off the upper MOSFET or by acting on the VS supply pin with the Under Voltage Lock Out.

Acting on CF (Fig. 11) a limiting resistor RL has to be used, and it has to be: RL × CF > 1ms.

When the shutdown is realized acting on Vs pin, (see fig. 12) a limiting resistor Rs must be used to slow down the discharge of the supply filter Cs. The constant time of the discharge must be greater than 10 periods of the switching frequency:

RS ³	10
	Cs × fsw

Connecting the CF pin to ground GND stops the oscillator, and the lower MOSFET will remain ON. Therefore the bootstrap capacitor remains

Figure 10: Non dissipative auxiliary supply using the transistor snubber.

1mA WHEN STARTING
220kΩ	6 mA WHEN 50 kHz SWITCHING
Rs	C	470 pF
		310 V
	bootstrap
Cs	circuit
	L6569

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