Microchip HV809 Handleiding

Microchip Niet gecategoriseerd HV809

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Supertex inc.
Supertex inc.
www.supertex.com
Doc.# DSAN-AN-H36
A040313
AN-H36
Application Note
HV809 EL Lamp Driver for Battery Powered
and Off-line Equipment
by Scott Lynch, Senior Applications Engineer
Introduction
The Supertex HV809 is designed to drive large lamps at
high brightness. It can operate from a rectiîš¿ed/îš¿ltered 120V
AC power line or from any DC source in the range of 50 to
200V. For use in battery powered applications, an external
DC-DC converter is required.
This application note is divided into two sections, portable
applications and off-line applications. Section I describes the
operation of the Supertex’s HV809 EL lamp driver for a battery
operated (4 AA cells) application to drive a 12.5in2 EL lamp
to a brightness of 15ft-lm. Details are provided for designing
a high voltage output DC-DC converter. Applications can be
for PDA’s, GPS’s, hand held computers, and other portable
devices requiring high brightness EL backlighting. Section II
describes the HV809 operating from a 120V AC line to drive
a 100in2 EL lamp to a brightness of 20ft-lm. Applications can
be signage, courtesy lighting, and accent lighting.
Section I - Portable Applications
The basic circuit conîš¿guration is shown in Figure 1. There
are many different implementations in designing the DC-DC
converter. In this design, an inexpensive 555 timer IC was
used for the DC-DC converter. Details of the converter are
discussed in a later section.
Lamp Driver Circuit and Operation
The Supertex HV809 is capable of driving EL lamps of up to
350nF at 400Hz. Input supply can be any DC voltage source
from 50 to 200V. The HV809 supplies the EL lamp with an
AC square wave with a peak-to-peak voltage of two times
the input DC voltage.
The HV809 incorporates a lamp drive oscillator with frequency
controlled by a single resistor, REL-osc. The oscillator controls
the lamp driver output section, which consists of 4 transistors
arranged in a full bridge conîš¿guration as shown in Figure 2.
Figure 2: HV809 Lamp Driver
Figure 1: HV809 for Portable Applications
A1
A2
GND
REL
HVIN
VDD
OSC2
OSC1
HV809
DC-DC
Converter
CDD
REL
EL
Lamp
Batteries
HVOUT
VIN
Linear
Regulator
GND
HVIN
VDD
OSC2
OSC1
REL-Osc
Q
Q
Q
VA
Q
Logic
and
Osc
VB
2
AN-H36
Supertex inc.
www.supertex.com
Doc.# DSAN-AN-H36
A040313
The supply voltage can be supplied by a rectiîš¿ed/îš¿ltered AC
line or by an external high voltage power supply. Alternate
sets of output transistors are turned on by the drive oscil-
lator, providing a lamp drive waveform as shown in Figure
3. This design has excellent drive capability and provides
a symmetrical bipolar drive, resulting in a zero-bias signal.
Many lamp manufacturers recommend a zero-bias drive sig-
nal to avoid potential migration problems, thereby increasing
lamp life.
Figure 3: Lamp Drive Waveform
The design of the lamp driver section primarily consists of
selecting a lamp drive frequency and voltage. Lamp fre-
quency is controlled by REL-OSC. Typical values range from
510 to 5.1MΩ, with higher values yielding lower frequencies.
Lamp drive voltage is determined by the high voltage supply
(HVOUT).
Approximately a third of the power used by the lamp driver
is dissipated in the lamp resistance, and two thirds is dissi-
pated in the HV809’s bridge transistors during output transi-
tions. With high lamp drive frequencies, large lamps, or high
lamp voltages, power dissipation in the HV809 will rise. This
will be a limiting factor when using the HV809 in the SO-8
package, since power dissipation cannot exceed the pack-
age rating of 500mW. The TO-220 package is rated at 15
Watts.
Figures 4 and 5 show typical characteristics for a 12.5in2
lamp at two lamp drive frequencies. These graphs were de-
rived from a particular lamp and characteristics will vary with
other lamps.
Figure 4: Lamp Brightness for 12.5in2 Lamp
Figure 5: Input Power for 12.5in2 Lamp
40
30
20
10
0
50 100 150 200
Brightness (ft-lm)
-
Input DC voltage (V)
385Hz
179Hz
Brightness vs Voltage for 12.5 square inch lamp
1600
1200
800
400
0
25 50 100 150 200
385Hz
179Hz
Input Voltage (VDC)
Input Power (mW)
3
AN-H36
Supertex inc.
www.supertex.com
Doc.# DSAN-AN-H36
A040313
Figure 6: DC–DC Converter
HVOUT
THRSH
CNTRL
DIS
VCC
GND
TRIG OUT
RST
NC
VIN
CIN CHV
Batteries QSW
D
L
CT RD
RCRFB
ZFB
Battery–Powered DC-DC Converter
An inexpensive, regulated switchmode power supply can be
constructed using a 555 timer IC as shown in Figure 6. The
circuit is a basic yback boost converter using a 555 timer
to provide a PWM signal to control switch QSW. By varying
the duty cycle of the switch, output power can be controlled.
Normally, timing components RC, RD, and CT determine fre-
quency and duty cycle. In this circuit, feedback resistor RFB
and Zener ZFB add a positive bias to the timing circuit, with
bias voltage increasing with increasing output voltage. This
bias speeds up charging of timing capacitor CT but slows
down discharging, with the net result a decrease in duty
cycle as output voltage increases. This mechanism pro-
vides the negative feedback necessary for regulation. With
properly chosen components, this circuit regulates output
voltage while maintaining a reasonably constant switching
frequency.
Design of the converter consists of the following steps:
1. Establish requirements
2. Determine basic converter parameters of frequency,
duty cycle, and inductance (L)
3. Select switching transistor and rectiîš¿er (QSW
and D)
4. Select input and output capacitors (CIN and CHV)
5. Select timing components (RC, RD, and CT)
6. Select feedback components (RFB and ZFB)
Establish Requirements
When designing a DC-DC converter for the HV809, three
parameters are of primary importance: input voltage range
(VINmin/max ), output voltage (HVOUT
), and output power (P HV).
VIN is given, but HVOUT and PHV must be determined. If the
desired lamp frequency and voltage are known, the power
consumed by charging and discharging the lamp’s capaci-
tance can be estimated by the following equation:
PLAMP = 1 fLAMP • CLAMP • V2
LAMP (1)
2
where: fLAMP = lamp frequency
CLAMP = lamp capacitance
VLAMP = peak-to-peak lamp voltage
While this equation provides a general approximation of re-
quired power, it does not account for power loss due to lamp
and driver resistances. When establishing DC-DC converter
requirements, it is better to determine HVOUT and PHV em-
pirically. Construct an HV809 lamp driver circuit using the
intended lamp. Use a high voltage bench supply to power
the driver. Vary the input voltage and lamp frequency until
desired lamp brightness, color, and power consumption are
obtained. Measure the input voltage and current, and use
these numbers as the design requirements for the DC-DC
converter. If practical, make input current measurements
using several lamps and driver components to get a better
idea of maximum power requirements. Be sure to design
to a higher power level than is actually required to allow for
component tolerances and converter efîš¿ciency. Designing to
at least 125% of required power is usually adequate.
Determine Operating Frequency, Duty Cycle,
and Inductor
The next step is to establish the basic operating parameters
of the switching converter - frequency, duty cycle, and in-

Product specificaties

Merk: Microchip
Categorie: Niet gecategoriseerd
Model: HV809

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