AN14

Application Note 14
Issue 2 March 1996
Transistor Considerations for LCD Backlighting
High Efficiency DC to AC Conversion
Neil Chadderton
Introduction
LCD Backlighting has generated
widespread interest from many diverse
disciplines within the engineering
industry. This has no doubt been fueled
by the trend to portability and
particularly to the enormous growth of
the computing market. Products such as
notebook, laptop, and palmtop personal
computers, portable televisions,
viewcams, point of sale terminals,
a u t o m o t i v e d a s h boa r ds , a vi oni c s
displays, metering and instrumentation
usually employ an LCD screen, and as
such require a means of backlighting. To
date the most prevalent method has
been to use a small cold cathode
fluorescent (CCFL) tube that is usually
integrated with a reflector/diffuser into
the display unit. The CCFL power
consumption can account for a
significant portion (up to 50%) of the
total system requirement. Therefore to
achieve marketable advantages in
battery life and re-charge frequency,
much attention must be applied to the
CCFL power supply, so as to attain the
highest possible conversion efficiency.
This problem has been the focus of
many electronic component vendors:
much research and design effort being
invested in order to offer system
designers the most attractive
components/solutions in terms of
efficiency, cost, weight, and size. Many
of t he analog IC companies have
published application specific reports,
and characterised or developed
specifically, integrated circuits for the
application.
This note acknowledges this work, and
will draw upon such sources and
reproduce these vendors’ circuits where
appropriate (a list of references is
included in Appendix A) but it is focused
primarily on the transistor requirements
– their mode of operation within the
backlighting circuit, important
parameters, and their impact on the
system efficiency.
CCFL Lamp Characteristics
An understanding of the requirements
for the backlighting power supply
should begin with a description of the
load involved. The fluorescent tube
presents a serious challenge to the
circuit designer. Around 1kV is required
to strike the tube (initiate conduction), at
which event the tube’s gaseous contents
ionise and it begins to conduct at a lower
sustaining voltage – thus a negative
AN14-1
Application Note 14
Issue 2 March 1996
resistance characteristic is evident.
Other power supply constraints include
an intolerance of DC current, a
sensitivity to waveform crest factor, and
RFI criteria.
The curve tracer plots shown in Figures
1 and 2 show the negative resistance
region for two typical CCFL units: – the
first for a 150mm linear, 10mm diameter
backlight tube for a laptop display, and
the other a “U” tube as produced for a
car dashboard display. Referring to
figure 1, the high striking voltage can be
seen at 560V and the negative resistance
excursion to 240V is self evident.
Similarly, these values for Figure 2 are
1240V and 900V. Note should also be
made of the slope impedance in the
conducting state. The power supply
must accommodate this, and in some
cases provision made to regulate the
lamp current to ensure a long tube life.
For drive waveforms at low frequencies,
a fluorescent tube has time to react to
the changing waveform potential, and
effectively re-strikes on each reversal of
the waveform polarity, (perceived as
flicker on line frequency units). At high
drive waveform frequencies, this effect
is not apparent, and the lamp can be
approximated to a resistive load. Usual
operating frequencies range from 25 to
120kHz, this being dictated by
consideration
of
inaudibility
requirements, converter inductor size,
and at the extreme, parasitic and
HV-lead-to-ground coupling capacitance.
in Figure 3. This is also referred to as the
Royer Converter, after G.H. Royer who
proposed the topology in 1954 as a
power converter. (Note: Strictly
speaking the backlighting converter
uses a modified version of the Royer
converter – the original used a
saturating transformer to define the
operating frequency, and therefore
produced a squarewave drive
Prior to the tube striking, or when no
t u b e i s c o n n e c t e d , t he o p e r at i n g
frequency is set by the resonant parallel
circuit comprising the primary
capacitance C1, and the transformer’s
primary winding W2+W3. Once the tube
has struck, the ballast capacitor C2 plus
distributed tube and parasitic
capacitances are reflected back through
the transformer, and the operating
frequency is lowered.
C2
W1
L1
T1
W2
W3
L1
C1
+V
R1
C3
R2
The secondary load can become
dominant in circuits with a high
transformer turns ratio, Eg. those
designed to operate from very low DC
input voltages.
W4
Q1
Q2
0V
Figure 3.
Generalised Royer Converter.
Figure 1.
CCFL Characteristics - 150mm linear;
100V/div horizontal, 200µA/div vertical.
Basic Operation Of Converter
The drive requirements dictated by the
CCFL tube’s behaviour and preferred
operating conditions can be achieved by
the resonant push-pull converter shown
Application Note 14
Issue 2 March 1996
Figure 2.
CCFL Characteristics - “U” tube;
200V/div horizontal, 1mA/div vertical.
AN14-2
waveform). The circuit looks simple but
this is very deceptive: many
components interact, and while the
circuit is capable of operation with
widely varying component values,
(useful
during
development)
optimisation is required for each design
to achieve the highest possible
efficiencies.
Transistors Q1 and Q2 are alternatively
saturated by the base drive provided by
the feedback winding W4. The base
current is defined by resistors R1 and R2.
Supply inductor L1 and primary
capacitance C1 force the circuit to run
sinusoidally thereby minimising
harmonic generation and RFI, and
providing the preferred drive waveform
to the load. Voltage step-up is achieved
by the W1:(W2 + W3) turns ratio. C2 is
the secondary winding ballast capacitor,
and effectively sets the tube current.
Each transistor’s collector is subject to a
voltage= 2 x π/2 x VS, (or just π x VS)
where VS is the DC input voltage to the
converter. (The π/2 factor being due to
the relationship between average and
peak values for a sinewave, and the x2
multiplier being due to the 2:1
autotransformer action of the
transformer’s centre-tapped primary).
This primary voltage is stepped up by
the transformer turns ratio Ns:Np, to a
high enough level to reliably strike the
tube under all conditions:- starting
voltage is dependent on display
housing, location of ground planes, tube
age, and ambient temperature.
The basic converter shown in Figure 3 is
a valid and useful circuit that has been
utilised for many systems and indeed
offered as a sub-system by several
manufacturers.
AN14-3
Application Note 14
Issue 2 March 1996
resistance characteristic is evident.
Other power supply constraints include
an intolerance of DC current, a
sensitivity to waveform crest factor, and
RFI criteria.
The curve tracer plots shown in Figures
1 and 2 show the negative resistance
region for two typical CCFL units: – the
first for a 150mm linear, 10mm diameter
backlight tube for a laptop display, and
the other a “U” tube as produced for a
car dashboard display. Referring to
figure 1, the high striking voltage can be
seen at 560V and the negative resistance
excursion to 240V is self evident.
Similarly, these values for Figure 2 are
1240V and 900V. Note should also be
made of the slope impedance in the
conducting state. The power supply
must accommodate this, and in some
cases provision made to regulate the
lamp current to ensure a long tube life.
For drive waveforms at low frequencies,
a fluorescent tube has time to react to
the changing waveform potential, and
effectively re-strikes on each reversal of
the waveform polarity, (perceived as
flicker on line frequency units). At high
drive waveform frequencies, this effect
is not apparent, and the lamp can be
approximated to a resistive load. Usual
operating frequencies range from 25 to
120kHz, this being dictated by
consideration
of
inaudibility
requirements, converter inductor size,
and at the extreme, parasitic and
HV-lead-to-ground coupling capacitance.
in Figure 3. This is also referred to as the
Royer Converter, after G.H. Royer who
proposed the topology in 1954 as a
power converter. (Note: Strictly
speaking the backlighting converter
uses a modified version of the Royer
converter – the original used a
saturating transformer to define the
operating frequency, and therefore
produced a squarewave drive
Prior to the tube striking, or when no
t u b e i s c o n n e c t e d , t he o p e r at i n g
frequency is set by the resonant parallel
circuit comprising the primary
capacitance C1, and the transformer’s
primary winding W2+W3. Once the tube
has struck, the ballast capacitor C2 plus
distributed tube and parasitic
capacitances are reflected back through
the transformer, and the operating
frequency is lowered.
C2
W1
L1
T1
W2
W3
L1
C1
+V
R1
C3
R2
The secondary load can become
dominant in circuits with a high
transformer turns ratio, Eg. those
designed to operate from very low DC
input voltages.
W4
Q1
Q2
0V
Figure 3.
Generalised Royer Converter.
Figure 1.
CCFL Characteristics - 150mm linear;
100V/div horizontal, 200µA/div vertical.
Basic Operation Of Converter
The drive requirements dictated by the
CCFL tube’s behaviour and preferred
operating conditions can be achieved by
the resonant push-pull converter shown
Application Note 14
Issue 2 March 1996
Figure 2.
CCFL Characteristics - “U” tube;
200V/div horizontal, 1mA/div vertical.
AN14-2
waveform). The circuit looks simple but
this is very deceptive: many
components interact, and while the
circuit is capable of operation with
widely varying component values,
(useful
during
development)
optimisation is required for each design
to achieve the highest possible
efficiencies.
Transistors Q1 and Q2 are alternatively
saturated by the base drive provided by
the feedback winding W4. The base
current is defined by resistors R1 and R2.
Supply inductor L1 and primary
capacitance C1 force the circuit to run
sinusoidally thereby minimising
harmonic generation and RFI, and
providing the preferred drive waveform
to the load. Voltage step-up is achieved
by the W1:(W2 + W3) turns ratio. C2 is
the secondary winding ballast capacitor,
and effectively sets the tube current.
Each transistor’s collector is subject to a
voltage= 2 x π/2 x VS, (or just π x VS)
where VS is the DC input voltage to the
converter. (The π/2 factor being due to
the relationship between average and
peak values for a sinewave, and the x2
multiplier being due to the 2:1
autotransformer action of the
transformer’s centre-tapped primary).
This primary voltage is stepped up by
the transformer turns ratio Ns:Np, to a
high enough level to reliably strike the
tube under all conditions:- starting
voltage is dependent on display
housing, location of ground planes, tube
age, and ambient temperature.
The basic converter shown in Figure 3 is
a valid and useful circuit that has been
utilised for many systems and indeed
offered as a sub-system by several
manufacturers.
AN14-3
Application Note 14
Issue 2 March 1996
Backlight Converters Within
Control Loops
C2
W1
L1
T1
W2
PWM
W3
L1
+V
C1
C3
D1
Application Note 14
Issue 2 March 1996
R1
R2
W4
Q2
Q1
0V
Figure 4a.
Royer Converter With PWM Control - High Side Current Fed Version.
Variations on the basic topology are
possible, perhaps the most important
being to include the converter within a
control loop. This can be used to
regulate the tube current:- this
maximises tube lifetime, ensures a
constant light output as the battery pack
voltage decreases, and enables
adjustment of tube brightness. The
usual form of the circuit is to employ a
Buck or step-down converter (directly
from the battery pack to increase
efficiency) feeding the centre tap of the
transformer, or the emitter current of the
transistors, depending on the
controller’s technology and capability.
Figures 4a and 4b show these
arrangements in conceptual form. The
controller can monitor the tube current
directly in the secondary, or in some
recent systems, by the primary current.
This latter method allows the tube to be
fully floating thus minimising HV losses.
Figure 5 shows a circuit published by
linear IC manufacturer LINEAR
TECHNOLOGY CORP. that exhibits a
significant efficiency improvement over
previous designs; primarily due to the
choice of the ZETEX FZT849. It is based
on the Buck converter current fed Royer
scheme of Figure 4b, and monitors the
lamp’s current directly by averaging the
positive half cycles of lamp current, and
applying this signal to the controller’s
feedback pin. The electrical conversion
efficiency using this form of circuit can
be very high, the stated value for Figure
5 being 88%. Higher efficiencies up to
92% are possible by using larger
transformers to reduce copper and core
losses.
C2
5mA MAX
CCFT
15pF
W1
W2
3KV
L1
T1
2
+4.5 to 20V
W3
9
W1
Vin
C1
1K
+V
R1
C3
10uF
R2
W4
Q1
FZT
849
0V
L1
D1
W2
0V
Figure 4b.
Royer Converter With PWM Control - Low Side (or tail) Current Fed Version.
4
COILTRONICS
CTX110092-1
33nF
W3
5
FZT
849
1
1N5818
CTX300-4
300uH
Vin
NC
Vsw
E1
LT1172
Vfb
E2
Gnd Vc
PWM
AN14-4
3
Q2
Connect to lowest
voltage available
(Vmin=3V)
7
W4
2.2uF
Gnd
Figure 5.
Linear Technology LCD Backlight Converter.
AN14-5
1/2
BAV99
10K
1uF
50K
560
1/2
BAV99
Application Note 14
Issue 2 March 1996
Backlight Converters Within
Control Loops
C2
W1
L1
T1
W2
PWM
W3
L1
+V
C1
C3
D1
Application Note 14
Issue 2 March 1996
R1
R2
W4
Q2
Q1
0V
Figure 4a.
Royer Converter With PWM Control - High Side Current Fed Version.
Variations on the basic topology are
possible, perhaps the most important
being to include the converter within a
control loop. This can be used to
regulate the tube current:- this
maximises tube lifetime, ensures a
constant light output as the battery pack
voltage decreases, and enables
adjustment of tube brightness. The
usual form of the circuit is to employ a
Buck or step-down converter (directly
from the battery pack to increase
efficiency) feeding the centre tap of the
transformer, or the emitter current of the
transistors, depending on the
controller’s technology and capability.
Figures 4a and 4b show these
arrangements in conceptual form. The
controller can monitor the tube current
directly in the secondary, or in some
recent systems, by the primary current.
This latter method allows the tube to be
fully floating thus minimising HV losses.
Figure 5 shows a circuit published by
linear IC manufacturer LINEAR
TECHNOLOGY CORP. that exhibits a
significant efficiency improvement over
previous designs; primarily due to the
choice of the ZETEX FZT849. It is based
on the Buck converter current fed Royer
scheme of Figure 4b, and monitors the
lamp’s current directly by averaging the
positive half cycles of lamp current, and
applying this signal to the controller’s
feedback pin. The electrical conversion
efficiency using this form of circuit can
be very high, the stated value for Figure
5 being 88%. Higher efficiencies up to
92% are possible by using larger
transformers to reduce copper and core
losses.
C2
5mA MAX
CCFT
15pF
W1
W2
3KV
L1
T1
2
+4.5 to 20V
W3
9
W1
Vin
C1
1K
+V
R1
C3
10uF
R2
W4
Q1
FZT
849
0V
L1
D1
W2
0V
Figure 4b.
Royer Converter With PWM Control - Low Side (or tail) Current Fed Version.
4
COILTRONICS
CTX110092-1
33nF
W3
5
FZT
849
1
1N5818
CTX300-4
300uH
Vin
NC
Vsw
E1
LT1172
Vfb
E2
Gnd Vc
PWM
AN14-4
3
Q2
Connect to lowest
voltage available
(Vmin=3V)
7
W4
2.2uF
Gnd
Figure 5.
Linear Technology LCD Backlight Converter.
AN14-5
1/2
BAV99
10K
1uF
50K
560
1/2
BAV99
Figure 6 shows Linear Technology’s
latest design using the LT1182 and the
Zetex ZDT1048 dual transistor. The
LT1182 provides a low component count
circuit and contains all control functions
f o r t h e R o y e r c o n ve r t e r , a nd t h e
control/switch for the LCD contrast
converter within one package. Primary
Royer converter current is sensed by the
IC, so that the CCFL tube can be operated
in a “floating” mode thereby decreasing
losses in the secondary circuit. The
FZT849 transistors, or the ZDT1048 dual
package are preferred options for this
converter circuit.
Detailed reports on these circuits can be
found via the references listed.
Application Note 14
Issue 2 March 1996
Application Note 14
Issue 2 March 1996
Figure 7 shows an oscillograph of the
transistor’s operating conditions in such
a circuit. The Collector-Emitter voltage
peaks at 28V (less than π x VS due to the
lamp load); the Emitter current is almost
constant at 0.5A (with a ripple
component dependent on the Buck
inductor); and the base voltage appears
as a clipped (due the transistor’s VBE)
version of the primary waveform.
unnecessary on-resistance losses. The
primary breakdown voltage BVCBO, of a
planar bipolar transistor depends on the
epitaxial layer - specifically its thickness
and resistivity. The breakdown voltage
of most interest to the designer is
usually that attained across the
Collector-Emitter (C-E) terminals. This
value can vary between the primary
breakdown BVCBO and a much lower
voltage dependent on the state of the
base terminal bias.
Requisite Transistor
Characteristics
The relatively low operating frequency as
required by the backlighting Royer
Converter (to minimise HV parasitic
capacitance losses), and the ease of
transformer drive, makes this circuit
particularly suitable for bipolar transistor
Figure 7.
Royer Converter Operating Waveforms:
VCE 10V/div; IE 0.5A/div; VBE 2V/div
respectively, 2µs/div horizontal .
implementation. This isn’t to exclude
M O S F E T b a s e d d e s i g n s ( so me IC
vendors have specified MOS as this suits
t heir t echno logy) but in terms of
equivalent on-resistance and silicon
efficiency, the low voltage bipolar device
has no equal. For example, the ZETEX
ZTX849 E- L ine ( TO-92 compatible)
transistor exhibits a RCE(sat) of 36mΩ. This
can only be matched by a much larger
(and expensive) MOSFET die, only
available in TO-220, D-Pak, and similar
larger packages.
The important transistor characteristics
are voltage rating, VCE(sat), and hFE, and
are detailed below.
Figure 6.
Linear Technology Floating Tube LCD Backlight Converter.
AN14-6
The voltage rating required deserves
some thought with respect to the
standard transistor breakdown
parameters, as it is possible to
over-specify a device on grounds of
voltage rating, and thereby incur a
reduction in efficiency due to
[The breakdown mechanism is caused
by the avalanche multiplication effect,
whereby free electrons can be imparted
with sufficient energy by the reverse
bias electric field such that any collisions
can lead to ionisation of the lattice
atoms. The free electrons thus
generated are then accelerated by the
field and produce further ionisation. This
multiplication of free carriers increases
the reverse current dramatically, and so
the junction effectively clamps the
applied voltage. The base terminal can
obviously influence the junction current
– ther eby modulating t he voltage
required for a breakdown condition.]
Figure 8 shows how the breakdown
characteristic is seen to vary for different
circuit conditions. The BVCEO rating (or
when the base is open circuit) allows the
Collector-Base (C-B) leakage current ICBO
to be effectively amplified by the
transistor’s β thus significantly
increasing the leakage component to
ICEO. Shorting the Base to the Emitter
(BVCES) provides a parallel path for the
C-B leakage, and so the voltage required
for breakdown is higher than the open
base condition. BVCER denotes the case
between the open and shorted base
options:- R indicating an external
base-emitter resistance, the value of
AN14-7
Figure 6 shows Linear Technology’s
latest design using the LT1182 and the
Zetex ZDT1048 dual transistor. The
LT1182 provides a low component count
circuit and contains all control functions
f o r t h e R o y e r c o n ve r t e r , a nd t h e
control/switch for the LCD contrast
converter within one package. Primary
Royer converter current is sensed by the
IC, so that the CCFL tube can be operated
in a “floating” mode thereby decreasing
losses in the secondary circuit. The
FZT849 transistors, or the ZDT1048 dual
package are preferred options for this
converter circuit.
Detailed reports on these circuits can be
found via the references listed.
Application Note 14
Issue 2 March 1996
Application Note 14
Issue 2 March 1996
Figure 7 shows an oscillograph of the
transistor’s operating conditions in such
a circuit. The Collector-Emitter voltage
peaks at 28V (less than π x VS due to the
lamp load); the Emitter current is almost
constant at 0.5A (with a ripple
component dependent on the Buck
inductor); and the base voltage appears
as a clipped (due the transistor’s VBE)
version of the primary waveform.
unnecessary on-resistance losses. The
primary breakdown voltage BVCBO, of a
planar bipolar transistor depends on the
epitaxial layer - specifically its thickness
and resistivity. The breakdown voltage
of most interest to the designer is
usually that attained across the
Collector-Emitter (C-E) terminals. This
value can vary between the primary
breakdown BVCBO and a much lower
voltage dependent on the state of the
base terminal bias.
Requisite Transistor
Characteristics
The relatively low operating frequency as
required by the backlighting Royer
Converter (to minimise HV parasitic
capacitance losses), and the ease of
transformer drive, makes this circuit
particularly suitable for bipolar transistor
Figure 7.
Royer Converter Operating Waveforms:
VCE 10V/div; IE 0.5A/div; VBE 2V/div
respectively, 2µs/div horizontal .
implementation. This isn’t to exclude
M O S F E T b a s e d d e s i g n s ( so me IC
vendors have specified MOS as this suits
t heir t echno logy) but in terms of
equivalent on-resistance and silicon
efficiency, the low voltage bipolar device
has no equal. For example, the ZETEX
ZTX849 E- L ine ( TO-92 compatible)
transistor exhibits a RCE(sat) of 36mΩ. This
can only be matched by a much larger
(and expensive) MOSFET die, only
available in TO-220, D-Pak, and similar
larger packages.
The important transistor characteristics
are voltage rating, VCE(sat), and hFE, and
are detailed below.
Figure 6.
Linear Technology Floating Tube LCD Backlight Converter.
AN14-6
The voltage rating required deserves
some thought with respect to the
standard transistor breakdown
parameters, as it is possible to
over-specify a device on grounds of
voltage rating, and thereby incur a
reduction in efficiency due to
[The breakdown mechanism is caused
by the avalanche multiplication effect,
whereby free electrons can be imparted
with sufficient energy by the reverse
bias electric field such that any collisions
can lead to ionisation of the lattice
atoms. The free electrons thus
generated are then accelerated by the
field and produce further ionisation. This
multiplication of free carriers increases
the reverse current dramatically, and so
the junction effectively clamps the
applied voltage. The base terminal can
obviously influence the junction current
– ther eby modulating t he voltage
required for a breakdown condition.]
Figure 8 shows how the breakdown
characteristic is seen to vary for different
circuit conditions. The BVCEO rating (or
when the base is open circuit) allows the
Collector-Base (C-B) leakage current ICBO
to be effectively amplified by the
transistor’s β thus significantly
increasing the leakage component to
ICEO. Shorting the Base to the Emitter
(BVCES) provides a parallel path for the
C-B leakage, and so the voltage required
for breakdown is higher than the open
base condition. BVCER denotes the case
between the open and shorted base
options:- R indicating an external
base-emitter resistance, the value of
AN14-7
Application Note 14
Issue 2 March 1996
[Note: The voltage applied by the
feedback winding must not exceed the
BVEBO of the transistor. This is specified
at 5V usually, against an actual of 7.5 to
8.5V].
which is typically 100 to 10kΩ. BVCEV or
BVC E X is a special case where the
base-emitter is reverse biased; this can
provide a better path for the C-B leakage,
and so this rating yields a voltage close
to, or coincident with the BVCBO value.
Constant IB Curves
(Normal Operation)
BVCEO
BVCER
BVCES
BVCEX
0
BVCBO
VCE - Collector Emitter Voltage
Figure 8.
Voltage Breakdown Modes of Bipolar
Transistor.
Figure 9.
Breakdown modes of the ZTX849 Bipolar
Transistor.
events of course being in perfect
synchronism. An expanded view of the
C-E and B-E waveforms is shown in
Figure 10.
Figure 9 shows a curve tracer view of the
relevant breakdown modes of the
ZTX849 transistor, including a curve
showing the device in the “on” state.
Curves 1 and 2 are virtually coincident
and show BVCBO and BVCES respectively.
Curve 3 shows the BVCEV case with an
applied base bias (VEB) of -1V. Curve 4
shows BVCEO at approximately 36V.
Curve 5 is a BVCE curve, showing how the
breakdown condition is affected by a
positive base bias of 0.5V.
The BVCEV rating has particular relevance
to the Royer Converter, as can be
surmised from Figure 7. Examination of
this will show that the transistor only
experiences the high C-E voltage when
t h e b a s e v o l t a g e h a s b e en t a k e n
negative by the feedback winding, these
Application Note 14
Issue 2 March 1996
The VCE(sat) and hFE parameters have a
direct bearing on the circuit’s electrical
conversion efficiency. This is especially
true of low voltage battery powered
systems, due to the high current levels
involved. Selection of standard LF
amplifier transistors provides far from
ideal results; these parts are for general
purpose linear and non-critical switching
use only. The high VCE(sat) inherent to these
parts, and low current gain could reduce
circuit efficiency to less than 50%. For
example, the stated VCE(sat) maximum
measured at 500mA, for the FZT849
SOT223 transistor, and a LF device
sometimes quoted as a suitable Royer
Converter transistor are 50mV and 0.5V
respectively. Eg.
VCE(sat)
@IC
IB
FZT849
50mV
0.5A
20mA
BCP56
0.5V
0.5A
50mA
For the above reasons, transistors
designed and optimised for high current
switching applications offer the most
cost-effective and efficient solutions.
The table presented in Appendix C lists
several ZETEX transistors that are
e m i n e n t l y s u i t a b l e f o r t h e R oy e r
converter. All of these parts offer
outstanding VCE(sat) and high current
performance for their size, and many are
so-called “Super-β” transistors; thereby
helping to simplify and improve drive
current requirements. Figure 11 shows
the VCE(sat) exhibited by the ZTX1048A for
a range of forced gain values. This
device is one of the ZTX1050 series of
transistors that employ a scaled up
variant of the highly efficient Matrix
geometry, developed for the ZETEX
“SuperSOT” series. This enables a
VCE(sat) p e r f o r m a n c e s i m i l a r t o t h e
ZTX850 series at the low to moderate
currents relevant to this application,
though utilising a smaller die, and
therefore providing a cost and possibly
a space saving advantage.
300mV
250mV
Figure 10.
Royer Converter: VCE and VBE Waveforms
5V/ div and 2V/ div respectively.
AN14-8
To address the VCE(sat) issue, large power
transistors are occasionally specified.
Unfortunately their capacitance, and
characteristic low base transport factor
(a feature of Epitaxial Base devices) can
lead to problems with cross-conduction
losses due to long storage and switching
times. The current gain is also
important, as the losses in the base bias
can be significant to the overall figure;
judicious selection of the bias resistor to
e n s u r e a m i n i m u m VCE(sat) w h i l e
preventing base overdrive needs to
consider supply variation, maximum
l a m p c u r r e n t , a n d t r a n s i s t o r hFE
minimum value and range.
200mV
150mV
100mV
50mV
0
1mA
10mA
100mA
1A
10A
Figure 11.
VCE(sat) v IC for the ZTX1048A Bipolar
Transistor: Forced gains of 10,20,50,100.
AN14-9
Application Note 14
Issue 2 March 1996
[Note: The voltage applied by the
feedback winding must not exceed the
BVEBO of the transistor. This is specified
at 5V usually, against an actual of 7.5 to
8.5V].
which is typically 100 to 10kΩ. BVCEV or
BVC E X is a special case where the
base-emitter is reverse biased; this can
provide a better path for the C-B leakage,
and so this rating yields a voltage close
to, or coincident with the BVCBO value.
Constant IB Curves
(Normal Operation)
BVCEO
BVCER
BVCES
BVCEX
0
BVCBO
VCE - Collector Emitter Voltage
Figure 8.
Voltage Breakdown Modes of Bipolar
Transistor.
Figure 9.
Breakdown modes of the ZTX849 Bipolar
Transistor.
events of course being in perfect
synchronism. An expanded view of the
C-E and B-E waveforms is shown in
Figure 10.
Figure 9 shows a curve tracer view of the
relevant breakdown modes of the
ZTX849 transistor, including a curve
showing the device in the “on” state.
Curves 1 and 2 are virtually coincident
and show BVCBO and BVCES respectively.
Curve 3 shows the BVCEV case with an
applied base bias (VEB) of -1V. Curve 4
shows BVCEO at approximately 36V.
Curve 5 is a BVCE curve, showing how the
breakdown condition is affected by a
positive base bias of 0.5V.
The BVCEV rating has particular relevance
to the Royer Converter, as can be
surmised from Figure 7. Examination of
this will show that the transistor only
experiences the high C-E voltage when
t h e b a s e v o l t a g e h a s b e en t a k e n
negative by the feedback winding, these
Application Note 14
Issue 2 March 1996
The VCE(sat) and hFE parameters have a
direct bearing on the circuit’s electrical
conversion efficiency. This is especially
true of low voltage battery powered
systems, due to the high current levels
involved. Selection of standard LF
amplifier transistors provides far from
ideal results; these parts are for general
purpose linear and non-critical switching
use only. The high VCE(sat) inherent to these
parts, and low current gain could reduce
circuit efficiency to less than 50%. For
example, the stated VCE(sat) maximum
measured at 500mA, for the FZT849
SOT223 transistor, and a LF device
sometimes quoted as a suitable Royer
Converter transistor are 50mV and 0.5V
respectively. Eg.
VCE(sat)
@IC
IB
FZT849
50mV
0.5A
20mA
BCP56
0.5V
0.5A
50mA
For the above reasons, transistors
designed and optimised for high current
switching applications offer the most
cost-effective and efficient solutions.
The table presented in Appendix C lists
several ZETEX transistors that are
e m i n e n t l y s u i t a b l e f o r t h e R oy e r
converter. All of these parts offer
outstanding VCE(sat) and high current
performance for their size, and many are
so-called “Super-β” transistors; thereby
helping to simplify and improve drive
current requirements. Figure 11 shows
the VCE(sat) exhibited by the ZTX1048A for
a range of forced gain values. This
device is one of the ZTX1050 series of
transistors that employ a scaled up
variant of the highly efficient Matrix
geometry, developed for the ZETEX
“SuperSOT” series. This enables a
VCE(sat) p e r f o r m a n c e s i m i l a r t o t h e
ZTX850 series at the low to moderate
currents relevant to this application,
though utilising a smaller die, and
therefore providing a cost and possibly
a space saving advantage.
300mV
250mV
Figure 10.
Royer Converter: VCE and VBE Waveforms
5V/ div and 2V/ div respectively.
AN14-8
To address the VCE(sat) issue, large power
transistors are occasionally specified.
Unfortunately their capacitance, and
characteristic low base transport factor
(a feature of Epitaxial Base devices) can
lead to problems with cross-conduction
losses due to long storage and switching
times. The current gain is also
important, as the losses in the base bias
can be significant to the overall figure;
judicious selection of the bias resistor to
e n s u r e a m i n i m u m VCE(sat) w h i l e
preventing base overdrive needs to
consider supply variation, maximum
l a m p c u r r e n t , a n d t r a n s i s t o r hFE
minimum value and range.
200mV
150mV
100mV
50mV
0
1mA
10mA
100mA
1A
10A
Figure 11.
VCE(sat) v IC for the ZTX1048A Bipolar
Transistor: Forced gains of 10,20,50,100.
AN14-9
Application Note 14
Issue 2 March 1996
Package Options
Conclusions
ZETEX can offer a range of packages to
allow complete circuit size and layout
optimisation. Figure 12 illustrates these,
f r om th e TO92 compatible E- Line
through-hole package, to surface mount
options SOT23, SOT223, and SM-8.
The advanced transistor geometries,
and optimised processing employed by
ZETEX leads to a range of transistors
that are ideally suited to the LCD
b ac k l i ght in g inv e r t e r a pplication.
Attention has been applied to specifying
a range of devices relevant to, and
exhibiting a superior performance
within the Royer inverter topology.
References
“Transistors as On-Off Switches in
Saturable Core Circuits”
Bright, Pittman and Royer.
Westinghouse Electric Corp.,
Electrical Manufacturing Dec 1954.
“Techniques for 92% Efficient LCD
Illumination”
Applications Note 55 August 1993
Jim Williams
Linear Technology Corp.,
Figure 12.
Package Options.
The SM-8 is a dual island, eight leaded
package that possesses the same body
dimensions as the industry standard
SOT223. These attributes allow it to
r e p l a c e t h e t w o R o y e r C on ve r t e r
transistors with a single package two
chip device, yielding a significant cost
and space saving.
“A Fourth Generation of LCD Backlight
Technology - Component and
Measurement Improvements Refine
Performance” Application Note 65
October 1995
Jim Williams
Linear Technology Corp.
“Switching and Linear Power Supply,
Power Converter Design”
A. Pressman
Hayden Press.
For example, the ’1048A transistor is
available as an uncommitted dual within
the SM8 package as the ZDT1048.
AN14-10
Application Note 14
Issue 2 March 1996
Appendix A
Appendix B
LT1070, 1170 Series Switching
Regulators
LT1182, 1183 CCFL/LCD Contrast Dual
Switching Regulator
Linear Technology Corporation,
1630 McCarthy Blvd.,
Milpitas, CA 95035-7487
TEL: (408) 432 1900
CCFL Inverter Transformer and
Inductor Manufacturers
Linear Technology (UK) Ltd.,
TEL:(01276) 677676
Linear Technology KK
Tokyo, 102 JAPAN
TEL: 81-3-3237-7891
Coiltronics Inc.,
TEL: (407) 241-7876
(Transformers and inductors)
Represented by METL in the UK
TEL: 01844-278781
Sumida Electric Co., Ltd.
Tokyo 125 JAPAN
TEL: 03-3607-5111
(Inductors)
Represented by ACAL Electronics Ltd.,in
the UK
TEL: 0344-727272
Sumida Electric (USA) Co., Ltd
TEL: (708) 956-0666
(Transformers and Inductors)
Coilcraft
TEL: (708) 639-6400
(Inductors)
Coilcraft (UK)
TEL: 0181-301-3553
Newport Components Ltd.,
TEL: 01908-615232
(Inductors)
Pico Electronics Inc.,
NY 10552
TEL: (914) 699-5514
(Inductors)
Represented by Ginsbury Electronics
Ltd., in the UK
TEL: 01634-290040
AN14-11
Application Note 14
Issue 2 March 1996
Package Options
Conclusions
ZETEX can offer a range of packages to
allow complete circuit size and layout
optimisation. Figure 12 illustrates these,
f r om th e TO92 compatible E- Line
through-hole package, to surface mount
options SOT23, SOT223, and SM-8.
The advanced transistor geometries,
and optimised processing employed by
ZETEX leads to a range of transistors
that are ideally suited to the LCD
b ac k l i ght in g inv e r t e r a pplication.
Attention has been applied to specifying
a range of devices relevant to, and
exhibiting a superior performance
within the Royer inverter topology.
References
“Transistors as On-Off Switches in
Saturable Core Circuits”
Bright, Pittman and Royer.
Westinghouse Electric Corp.,
Electrical Manufacturing Dec 1954.
“Techniques for 92% Efficient LCD
Illumination”
Applications Note 55 August 1993
Jim Williams
Linear Technology Corp.,
Figure 12.
Package Options.
The SM-8 is a dual island, eight leaded
package that possesses the same body
dimensions as the industry standard
SOT223. These attributes allow it to
r e p l a c e t h e t w o R o y e r C on ve r t e r
transistors with a single package two
chip device, yielding a significant cost
and space saving.
“A Fourth Generation of LCD Backlight
Technology - Component and
Measurement Improvements Refine
Performance” Application Note 65
October 1995
Jim Williams
Linear Technology Corp.
“Switching and Linear Power Supply,
Power Converter Design”
A. Pressman
Hayden Press.
For example, the ’1048A transistor is
available as an uncommitted dual within
the SM8 package as the ZDT1048.
AN14-10
Application Note 14
Issue 2 March 1996
Appendix A
Appendix B
LT1070, 1170 Series Switching
Regulators
LT1182, 1183 CCFL/LCD Contrast Dual
Switching Regulator
Linear Technology Corporation,
1630 McCarthy Blvd.,
Milpitas, CA 95035-7487
TEL: (408) 432 1900
CCFL Inverter Transformer and
Inductor Manufacturers
Linear Technology (UK) Ltd.,
TEL:(01276) 677676
Linear Technology KK
Tokyo, 102 JAPAN
TEL: 81-3-3237-7891
Coiltronics Inc.,
TEL: (407) 241-7876
(Transformers and inductors)
Represented by METL in the UK
TEL: 01844-278781
Sumida Electric Co., Ltd.
Tokyo 125 JAPAN
TEL: 03-3607-5111
(Inductors)
Represented by ACAL Electronics Ltd.,in
the UK
TEL: 0344-727272
Sumida Electric (USA) Co., Ltd
TEL: (708) 956-0666
(Transformers and Inductors)
Coilcraft
TEL: (708) 639-6400
(Inductors)
Coilcraft (UK)
TEL: 0181-301-3553
Newport Components Ltd.,
TEL: 01908-615232
(Inductors)
Pico Electronics Inc.,
NY 10552
TEL: (914) 699-5514
(Inductors)
Represented by Ginsbury Electronics
Ltd., in the UK
TEL: 01634-290040
AN14-11
Application Note 14
Issue 2 March 1996
Appendix C
ZETEX Royer Converter Transistors
Device
BVCEV BVCES /
* BVCBO
BVEBO
IC
(DC)
hFE
@
IC / Vce
A/V
VCE(sat)
@
V
IC / IB
A/A
Package Surface
Mount
Option
V
V
V
A
ZTX849
_
80
6
5
100 - 300
1/1
25mV typ
50mV Max
0.5 / 0.02
E-Line
FZT849
(SOT223)
ZTX869
_
60
6
5
300 min
1/1
20mV typ
50mV Max
0.5 / 0.01
E-Line
FZT869
(SOT223)
ZTX689B
_
50(typ)
5
3
450 min
1/2
60mV typ
0.5 / 0.005 E-Line
FZT689B
(SOT223)
FMMT619
(SuperSOT)
_
50
5
2
200 min
1/2
55mV typ 0.5 / 0.01
125mV typ 1.0 / 0.01
200mV Max
SOT23
-
ZTX1048A
50
50
5
4
300 1200
1/2
24mV typ
45mV Max
0.5 / 0.02
E-Line
ZDT1048
(SM-8)
ZTX1049A
80
80
5
4
300 1200
1/2
35mV typ
60mV Max
0.5 / 0.02
E-Line
ZDT1049
(SM-8)
* If specified. For those devices that don’t include a BVCEV test, the actual value will
be close to the BVCES/BVCBO figure - please refer to text.
AN14-12
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