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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 VOUT WITHOUT OPTOCOUPLER OR SIGNAL TRANSFORMER • Easy Design • Few Components VIN VOUT LT35XX NO Feedback Signal Transformer • High Reliability • VOUT Accuracy 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 www.linear.com • Ready-made demo board / circuit / application • Easy design with standard components • Adjustable to different input and output voltages 1 Special issue: Industrial Solution Kit / December 2011 Isolated Monolithic Flyback Simplifies Design & Eliminates Optocoupler – LT3573 • VIN 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 • VOUT 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 MSOP16 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 • VIN 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 • VOUT Set with Two External Resistors • Off-the-Shelf Transformers • Programmable Soft-Start • Programmable Undervoltage Lockout • Output Voltage Temperature Compensation • Adjustable Current Limit More info: www.linear.com/3574 2 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. HIGHLIGHT MAGAZINE Isolated Monolithic Flyback Regulator Simplifies Design & Eliminates Optocoupler – LT3575 • VIN 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 • VOUT Set with Two External Resistors • Off-the-Shelf Transformers • Programmable Soft-Start • Programmable Undervoltage Lockout designs. The high level of integration and the use of boundarymode 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. • 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 More info: www.linear.com/3575 100V Isolated Flyback DC/DC Controller Simplifies Design & Eliminates Optocoupler – LT3748 • VIN 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 • VOUT 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. 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 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. Winding build of 750311675 as example of an advanced winding build. 0,1794mm Secondary 1 Red: Green: Primary 1 Violet: Auxilliary Blue: Secondary 2 Orange: Primary 2 Chart of Design Kits Order Code LTC IC Demo Board Power (W) 750 105 750 103 750 106 750 107 750 108 DC1636A DC1228A DC1643A DC1557A DC1694A LT3574 LT3573 LT3575 LT3748 LT3748 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 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 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 Linear Technology (UK) Ltd. 3 The Listons, Liston Road Marlow, Buckinghamshire SL7 1PD Tel.: +44 (0)1628 477066 Fax: +44 (0)1628 478153 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 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 4www.linear.com , 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.