ST EVAL6520-1421 User Manual

EVAL6520-1421

14 W / 21 W T5 miniature ballast driven by L6520 and STT13005D bipolar transistors

Features

■Drives either T5-14W-HE or T5-21W-HE lamps

■Standard form factor (19 mm x 120 mm)

■Compliance with IEC61347-2-3, IEC61000-2-3 and EN55022 Class-C

Description

The EVAL6520-1421 is a demonstration board able to drive either a 14 W or 21 W linear T5 fluorescent lamp with the L6520 low voltage ballast controller.

The half bridge consists of NPN high voltage power transistors driven by a suitable pulse transformer.

Data brief

EVAL6520-1421

November 2011	Doc ID 018537 Rev 2	1/16
For further information contact your local STMicroelectronics sales office.		www.st.com

Contents	EVAL6520-1421

Contents

1	Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 3
2	Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 4
3	Board performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
4	Application specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		9
5	Bill of material and board schematics . . . . . . . . . . . . . . . . . . . . . . . . . .		10
	5.1	Board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12

Appendix A Magnetic components data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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EVAL6520-1421	Introduction

1 Introduction

The L6520 low voltage ballast controller is intended to drive extremely compact applications based on either MOSFETs or bipolar transistors.

The EVAL6520-1421 is capable of driving either a T5-14W-HE or T5-21W-HE lamp, with the same miniatured (16 mm wide) ballast.

The selection of both the resonant components and the bipolar transistors, together with the design of the suitable pulse transformer, and IC power supply is also described.

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Board description	EVAL6520-1421

2 Board description

The board is supplied by any AC voltage in the European mains range and does not need any power factor correction having an input power of less than 25 W.

The half bridge voltage is obtained by filtering the rectified input voltage. This allows the use of a cheaper bulk capacitor and bipolar transistors.

The selection of a target condition is required by the range of the input voltage together with the necessity to drive two different kinds of lamps. In particular, the best driving condition and the best efficiency is obtained at 240 Vac with a 14 W lamp connected.

An EMI filter is placed at the board’s input to meet IEC61000 standards.

The lamp's cathodes are current preheated to make the ballast choke more compact thanks to the absence of auxiliary windings.

The resonant network design starts from the selection of the resonant capacitor (C10) that corresponds to the desired ballast efficiency. The inductance (L1) can be obtained by the following equation:

Equation 1

1

Lamp =

R Lamp

V HB

1

+ j ωC RES

j ωL RES

+

1

R Lamp

1

+ j ωC RES

R

Lamp

VHB is the effective voltage obtained across the half bridge along one mains cycle. The lower voltage is obtained at 50 Hz and can be approximately computed as:

Equation 2

V

2

V

+

2

V

2

−

1.54 105 P

=

2

− V

2

− V

f

C

LAMP

HB

IN

F

IN

F

OUT

MAINS

where:

●VF is the forward voltage drop of the rectifier bridge (1 V typ.)

●VIN is the RMS value of the input voltage

●COUT is the value of the bulk capacitor in µF (In this case C3 = 4.7 µF has been selected)

A resonant capacitor equal to 3.9 nF has been selected and resonant inductor equal to 3 mH has been calculated.

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EVAL6520-1421	Board description

It is now possible to estimate the ignition frequency, which must be higher than 46 kHz (minimum programmable value), and the minimum preheating frequency that guarantees a preheating voltage higher than 130 Vrms, as required by the lamp specifications.

Equation 3

VPH−IGN

A preheating frequency equal to 70 kHz (R5) between the FPRE pin and GND.

		VPFC		2			1
=			π				jωC
							RES
		1			+ jωLRES
			jωCRES

has been selected by connecting a 2.49 kΩ resistor

The half bridge is based on two STT13005D power bipolar transistors (Q1 and Q2). Both the high side and low side transistor are driven by a pulse transformer (T2).

To design this pulse transformer, the following parameters must be taken into account:

●Maximum available spacing on the PCB: this determines the core dimension.

●Maximum magnetizing current (Imag,rms) on the primary side of the transformer: this current causes core losses to not be transferred as a useful signal on the secondary side of the transformer. To minimize it, a higher primary inductance should be adopted. Typical inductances are between 6 mH and 40 mH, depending on the core dimension and the core permeability.

●Primary to secondary transfer ratio (n): the output voltage of a step-down transformer is lower than the input voltage, whereas the output current is higher than the input current. This helps to obtain higher DC currents with lower IC power dissipation. The minimum Vbe(sat) must be guaranteed in any condition as well as the minimum IB that guarantees the saturation condition of the BJT.

The Imag,rms(MAX) is selected lower than 10 mA when Vcc is equal to 13 V and a typical storage time of 1.2 µs is considered, therefore the primary inductance must be:

Equation 4

				1	−T	−T	−200ns
	Ton,max =				dt	sto			9.42us - Tsto
Lpri ≥ Vcc			3 Vcc	2 frun				= 1.732 Vcc		= 6.18mH
					I				I
	Imag,rms
					mag,rms				mag,rms
	3

The Vbe of the bipolar transistor can be calculated as follows (see Figure 2):

Equation 5

Vbe = Vpri n −Ib Rb =	Vcc −	Ib	(Rds,on_h +Rpri +Rds,on_l)	n −Ib Rb
		n

The Rds,on_h and the Rds,on_l are the ON resistances of the L6520 drivers and can be considered equal to 10 Ω each.

Ib is equal to Ic/hfe, and can be considered equal to Ic/6. Ib times Rb can be set between 0.7 V and 1 V, during run mode: in this design Rb (R6 and R8) can be selected between 10 Ω and 13 Ω.

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