HIGHLIGHT MAGAZINE
Special issue:
Industrial Solution Kit
/ December 2011
Easy Isolated Flyback DC/DC Converter Design Solution
Evaluation Kits
To offer easy isolated flyback design solutions for the LT3748 & LT357x family of ICs and WE-FB flyback transformers,
Linear Technology and Würth Elektronik have developed evaluation kits
with five different power output configurations:
1.
2.
3.
4.
5.
Kit
750105:
Kit
750103:
Kit
750106:
Kit
750107:
Kit
750108:
2.5 Watt LT3574 including Flyback Transformers for 3.3V, 5V and 12V
7 Watt LT3573 including Flyback Transformer for 5V
10 Watt LT3575 including Flyback Transformers for 3.3V, 5V and 12V
12 Watt LT3748 including Flyback Transformers for 3.3V, 5V and 12V
30 Watt LT3748 including Flyback Transformers for 12V, 15V and 24V
ACCURATE V
OUT
WITHOUT OPTOCOUPLER OR SIGNAL TRANSFORMER
V
IN
V
OUT
• Easy Design
• Few Components
LT35XX
• High Reliability
• V
OUT
Accuracy
NO Feedback
Signal Transformer
The output voltage of all these Linear Technology parts is
easily set with two external resistors and the transformer turns
ratio. Over 60 additional off-the-shelf transformers from Würth
Elektronik are available for many applications, with other
output voltages stocked at Farnell.
The
key advantages
of the evaluation kits are:
NO Feedback
Optocoupler
• Ready-made demo board / circuit / application
• Easy design with standard components
• Adjustable to different input and output voltages
www.linear.com
1
Special issue:
Industrial Solution Kit
/ December 2011
Isolated Monolithic Flyback Simplifies Design & Eliminates Optocoupler
–
LT3573
• V
IN
Range from 3V to 40V
• Onboard 1.25A, 60V Integrated NPN Power Switch
• No Transformer, Third Winding or Opto-Isolator Required
for Feedback
• Current Mode Control
• Boundary Mode Operation
• V
OUT
Set with Two External Resistors
• Programmable Soft-Start
• Programmable Undervoltage Lockout
• Output Voltage Temperature Compensation
• Adjustable Current Limit
The LT3573 is an isolated monolithic flyback switching
regulator with an integrated 1.25A NPN power switch. This
device significantly simplifies the design of a flyback converter
since it does not require a third winding or optoisolator and can
sense the output voltage directly from the primary side flyback
waveform. The LT3573 operates over an input voltage range of
3V to 40V at output power levels up to 7 watts, and can be used
in a wide variety of industrial, medical, datacom and automotive
applications requiring isolated power.
The LT3573 operates in boundary mode, which is a variable
frequency current mode switching scheme, resulting in a total
regulation band of ±5% over a wide input voltage range and
output load current in a typical application. Boundary mode
operation also permits the use of a smaller transformer when
compared to equivalent continuous conduction mode designs.
The output voltage is easily set by two external resistors.
The LT3573 has additional features that include an onboard
LDO, programmable soft start, undervoltage lockout, adjustable
current limit and output voltage temperature compensation.
The LT3573 is available in a small thermally enhanced MSOP-
16 package and is offered in an extended and industrial operating
junction temperature range at -40°C to 125°C.
More info: www.linear.com/3573
Low Power Isolated Monolithic Flyback Switching Regulator Simplifies Design &
Eliminates Optocoupler
–
LT3574
• V
IN
Range from 3V to 40V
• Onboard 0.65A, 60V Integrated NPN Power Switch
• No Transformer, Third Winding or Opto-Isolator Required for Feedback
• Current Mode Control
• Boundary Mode Operation
• V
OUT
Set with Two External Resistors
• Off-the-Shelf Transformers
• Programmable Soft-Start
• Programmable Undervoltage Lockout
• Output Voltage Temperature Compensation
• Adjustable Current Limit
The LT3574 is an isolated monolithic flyback switching regulator
that significantly simplifies the design of a low power isolated DC/
DC converter. No opto-isolator, third winding or signal transformer
is required for feedback since the output voltage is sensed from
the primary-side flyback signal. The LT3574 operates over a 3V
to 40V input voltage range, has a 0.65A NPN power switch and
can deliver up to 3 watts of output power, making it well suited
for a wide variety of industrial, medical, telecom, datacom and
automotive applications.
The LT3574 operates in boundary mode, which is a variable
frequency current mode control switching scheme, resulting in
±5% regulation over the full line, load and temperature range.
Boundary mode also permits the use of a smaller transformer
compared to equivalent continuous conduction mode designs.
The output voltage is easily set by two external resistors and
the transformers turns ratio. Several off-the-shelf transformers
identified in the data sheet can be used for numerous applications.
The high level of integration and use of boundary mode operation
results in a simple, clean application solution to the traditionally
tough problem of isolated power delivery.
Additional features include an onboard low dropout regulator
for IC power, programmable soft-start, undervoltage lockout,
adjustable current limit and output voltage temperature
compensation.
The LT3574 is available in a small MSOP-16 package and
is offered in an extended and industrial operating junction
temperature range at -40°C to 125°C.
More info: www.linear.com/3574
2
HIGHLIGHT MAGAZINE
Isolated Monolithic Flyback Regulator Simplifies Design & Eliminates Optocoupler
–
LT3575
• V
IN
Range from 3V to 40V
• Onboard 2.5A, 60V Integrated Power Switch
• No Transformer, Third Winding or Opto-Isolator Required for Feedback
• Current-Mode Control
• Boundary-Mode Operation
• V
OUT
Set with Two External Resistors
• Off-the-Shelf Transformers
• Programmable Soft-Start
• Programmable Undervoltage Lockout
• Output Voltage Temperature Compensation
• Adjustable Current Limit
The LT3575 is an isolated monolithic flyback switching regulator
that significantly simplifies the design of an isolated DC/DC
converter. No opto-isolator, third winding or signal transformer is
required for feedback since the output voltage is sensed from the
primary-side flyback signal. The LT3575 operates over a 3V to
40V input voltage range, has a 2.5A power switch and delivers up
to 14 Watts of output power.
The LT3575 operates in boundary mode, which is a variable
frequency current-mode control switching scheme, typically
resulting in ±5% regulation over the full line, load and temperature
range. Boundary mode also permits the use of a smaller
transformer compared to equivalent continuous conduction-mode
designs. The high level of integration and the use of boundary-
mode operation results in a simple, clean application solution to
the traditionally tough problem of isolated power delivery.
Additional features include an onboard low dropout regulator
for IC power, programmable soft-start, undervoltage lockout,
adjustable current limit and output voltage temperature
compensation.
The LT3575 is available in a small thermally enhanced TSSOP-16
package and is offered in an extended and industrial operating
junction temperature range from -40°C to 125°C.
More info: www.linear.com/3575
100V Isolated Flyback DC/DC Controller Simplifies Design & Eliminates Optocoupler
–
LT3748
• V
IN
Range from 5V to 100V
• No Opto-Isolator, Transformer or Third Winding Required for Feedback
• Drives External N-channel Power MOSFET
• Current Mode Control
• Boundary Mode Operation
• V
OUT
Set with Two External Resistors
• Off-the-Shelf Transformers
• Programmable Soft-Start
• Programmable Undervoltage Lockout
• Output Voltage Temperature Compensation
The LT3748 is a high input voltage isolated flyback DC/DC
controller that significantly simplifies the design of an isolated DC/
DC converter. No opto-isolator, third winding or signal transformer
is required for feedback since the output voltage is sensed from
the primary-side flyback signal. The LT3748 operates over a 5V to
100V input voltage range and drives an external N-channel power
MOSFET, making it well suited for a wide variety of industrial,
medical, telecom, datacom and automotive applications.
The LT3748 operates in boundary mode, which is a variable
frequency current mode control switching scheme, resulting in
±5% typical regulation over the full line, load and temperature
range. Boundary mode also permits the use of a smaller
transformer compared to equivalent continuous conduction
mode designs. The output voltage is easily set by two external
resistors and the transformer turns ratio. Several off-the-shelf
transformers identified in the data sheet can be used for
numerous applications. The high level of integration and the
use of boundary mode operation results in a simple, clean
application solution to the traditionally tough problem of isolated
power delivery.
Additional features include an onboard low dropout regulator
for IC power, programmable soft-start, undervoltage lockout,
adjustable current limit and output voltage temperature
compensation.
The LT3748 is available in a small MSOP-16 package with
4 pins removed for additional high voltage pin spacing.
More info: www.linear.com/3748
www.linear.com
3
HIGHLIGHT MAGAZINE
Transformers for
LT3573/3574/3575
and
LT3748
Linear Technology launched a family of ICs for isolated flyback
converters which allow a very simple. In close cooperation with
Würth Elektronik, there are more than 60 standard transformers
developed which comply with the requirements of these ICs.
By selecting the correct transformers and simply changing
two resistors on the board, users can select the output voltage
required.
All transformers are optimized for a very low leakage inductance
in order to maximize efficiency of the circuit. High leakage
inductances cause high voltage spikes on the switch, which
therefore need to be reduced by a snubber network. The losses
generated in the snubber are related to the height of the voltage
spike. This means lower leakage inductance causes a lower
voltage peak and therefore lower losses.
The low leakage inductance is achieved by full flat layers and
split windings. Wire is selected to fill the layers. When needed
multiple strands are used to fully fill the layer.
As an example Figure 1 shows the winding build of transformer
750311675. You can clearly see that primary and secondary
are constructed from multiple strands (bifilar resp. quadfilar).
In addition, they are split in parallel.
So the winding order is:
Secondary 1 => Primary 1 => Auxiliary => Secondary 2 => Primary 2
Secondary 1 and 2 resp. Primary 1 and 2 are connected in
parallel.
With this structure, the typical leakage inductance of the
750311675 transformer is 130nH inductance at 100kHz.
As shown, Würth Elektronik offers a wide range of standard
transformers which are suited for the LT357x and LT3748 IC
families. These are optimized for a very low leakage inductance.
Würth Elektronik provides LTspice
®
models for all standard
transformers.
The demo boards feature simple design and small solution
size. Würth Elektronik offers a wide variety of demo kits, each
of which includes a demo board. This board can be adjusted to
input and output requirements by changing only two resistors
and the transformer, thus offering a quick and easy solution for
an isolated switch mode supply.
Figure 1
Winding build of
750311675 as example
of an advanced
winding build.
Red:
Green:
Violet:
Blue:
Orange:
Secondary 1
Primary 1
Auxilliary
Secondary 2
Primary 2
0,1794mm
Chart of Design Kits
Order Code
LTC IC
750 105
750 103
750 106
750 107
750 108
LT3574
LT3573
LT3575
LT3748
LT3748
Demo Board Power (W)
DC1636A
DC1228A
DC1643A
DC1557A
DC1694A
2.5
7
10
12
30
Transformer assembled onboard
Order Code
Output Voltage
750 370 041
5V / 0.35A
750 370 047
5V / 1A
750 311 675
5V / 1.4A
750 311 439
12V / 1A
750 311 607
12V / 2.5A
Transformer 2**
Order Code Output Voltage
750 311 304
3.3V / 0.7A
–
–
750 310 564
12V / 0.5A
750 310 688
3.3V / 4A
750 311 599
15V / 2A
Transformer 3**
Order Code Output Voltage
750 310 799
12V / 0.15A
–
–
750 310 559
3.3V / 1.5A
750 311 689
5V / 3A
750 311 608
24V / 1.3A
Linear Technology AB
Linear Technology AB
P.O. Box 35 (Teknobulevardi 3-5) Electrum 204, Isafjordsgatan 22
Fin-01531 Vantaa / FINLAND
S-164 40 Kista / SWEDEN
Tel.: +358 (0)46 712 2171
Tel.: +46-(0)8-623 1600
Fax: +358 (0)46 712 2175
Fax: +46-(0)8-623 1650
Linear Technology (UK) Ltd.
3 The Listons, Liston Road
Marlow, Buckinghamshire SL7 1PD
Tel.: +44 (0)1628 477066
Fax: +44 (0)1628 478153
Würth Elektronik eiSos
GmbH & Co. KG
Max-Eyth-Straße 1
D-74638 Waldenburg
Tel.: +49 (0)7942/945-0
Fax: +49 (0)7942/945-5000
www.we-online.com
Linear Technology Highlight Magazine
Special Industrial Solution Kit issue 12/11 - English
version
Layout/Design: J+G Werbeges. mbH - www.jgwerbung.de • © Linear Technology Corporation 2011 • Printed in Germany
4
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, LTC, LT, LTM, LTspice, Linear Technology and the Linear logo are
registered trademarks of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
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