Design Article
Flyback converter simplifies isolated power design
Bruce Haug, Linear Technology Corp.
10/18/2012 7:16 AM EDT
Transformer selection & Design considerations
The transformer specification and design is probably the most critical part of successfully applying the LT8300. Since the voltage on secondary side of the transformer is inferred by the voltage sampled on the primary-side, the turns ratio must be tightly controlled to ensure a consistent output voltage.
Linear Technology has worked with leading magnetic component manufacturers to produce pre-designed flyback transformers for use with the LT8300. Table 1 shows an abbreviated list of recommended off-the-shelf transformers from Wurth Elektronik, Pulse Engineering and BH Electronics. A complete listing is shown in the LT8300 data sheet. These transformers typically withstand a 1,500VAC breakdown voltage for one minute from the primary to secondary. Higher breakdown voltages and custom transformers can also be utilized.
An LT8300 circuit can be modeled using any of the transformers listed in Table 1 by downloading a free copy of the LTspice software at www.linear.com/LTspice. This simulation produces realistic results to help further ease and confirm the design of such converters. The simulation circuit includes information on how the circuit starts up and its reaction to load steps for different input voltage. It is easy to make changes and see the impact to its circuit performance.
Conclusion
Power supply isolation can be utilized in medical systems to get off a noisy supply voltage and is also needed in distributed power architectures for safe operation. A LT8300 based circuit simplifies the design of an isolated flyback converter by eliminating the need for an optocoupler, secondary-side reference voltage and extra third winding off the power transformer. This device maintains primary to secondary isolation with only one part crossing the isolation barrier. Readily available off-the-shelf transformers prevent the need for a custom transformer. The LT8300 operates from a 5V to 100V input voltage range with up to 2 watts of output power making it well suited for a wide range of medical, industrial, telecom and datacom applications.
About the author:
Bruce Haug is Senior Product Marketing Engineer, Power Products, at Linear Technology Corp.
See related links:
Improving medical imaging with direct radiography
Smart devices are key for medical gear makers
Using clinical observation to facilitate medical device commercialization
An embedded solution for medical device interoperability
[Get a 10% discount on ARM TechCon 2012 conference passes by using promo code EDIT. Click here to learn about the show and register.]
The transformer specification and design is probably the most critical part of successfully applying the LT8300. Since the voltage on secondary side of the transformer is inferred by the voltage sampled on the primary-side, the turns ratio must be tightly controlled to ensure a consistent output voltage.
Linear Technology has worked with leading magnetic component manufacturers to produce pre-designed flyback transformers for use with the LT8300. Table 1 shows an abbreviated list of recommended off-the-shelf transformers from Wurth Elektronik, Pulse Engineering and BH Electronics. A complete listing is shown in the LT8300 data sheet. These transformers typically withstand a 1,500VAC breakdown voltage for one minute from the primary to secondary. Higher breakdown voltages and custom transformers can also be utilized.
Table 1: LT8300 off-the-shelf transformers
An LT8300 circuit can be modeled using any of the transformers listed in Table 1 by downloading a free copy of the LTspice software at www.linear.com/LTspice. This simulation produces realistic results to help further ease and confirm the design of such converters. The simulation circuit includes information on how the circuit starts up and its reaction to load steps for different input voltage. It is easy to make changes and see the impact to its circuit performance.
Conclusion
Power supply isolation can be utilized in medical systems to get off a noisy supply voltage and is also needed in distributed power architectures for safe operation. A LT8300 based circuit simplifies the design of an isolated flyback converter by eliminating the need for an optocoupler, secondary-side reference voltage and extra third winding off the power transformer. This device maintains primary to secondary isolation with only one part crossing the isolation barrier. Readily available off-the-shelf transformers prevent the need for a custom transformer. The LT8300 operates from a 5V to 100V input voltage range with up to 2 watts of output power making it well suited for a wide range of medical, industrial, telecom and datacom applications.
About the author:
Bruce Haug is Senior Product Marketing Engineer, Power Products, at Linear Technology Corp.
See related links:
Improving medical imaging with direct radiography
Smart devices are key for medical gear makers
Using clinical observation to facilitate medical device commercialization
An embedded solution for medical device interoperability
[Get a 10% discount on ARM TechCon 2012 conference passes by using promo code EDIT. Click here to learn about the show and register.]
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agk
10/19/2012 8:56 AM EDT
This is a clever design without the need for an opto coupler to sense the secondary side and to feed the primary side to regulate the secondary output.Many people will like to use this design even though the output load regulation is average.
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M2002
10/20/2012 1:51 AM EDT
Indead the load and line regulation is less than 1% across full load and line range (page 3).
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Etmax
10/22/2012 8:09 PM EDT
Not a very new technique, but much more practical when integrated into the IC. My only gripe with this method is the accuracy obtainable. I tend to do the sensing on the secondary and then use the opto as a critical switch. You get better regulation and the opto plays less of a role in accuracy. Stability is still an issue of course, but it becomes the same for each unit built making it something you can deal with.
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M2002
10/23/2012 1:43 AM EDT
What kind of accuracy are you looking for? What do you think of the load and line regulation in Figure 3? Thank you!
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studleylee
10/24/2012 12:59 PM EDT
@ETmax of course the ordinary crowd does it with an isolated device off the secondary, that's what makes this novel. that's why this way is cool and cheaper. You can always regulated down from the 5v rail.
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krh
10/22/2012 8:35 PM EDT
Bruce, a couple of questions:
- I didn't see any discussion in your article about efficiency (but maybe I just missed it).
- Does the design scale to higher output power..??
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M2002
10/23/2012 1:51 AM EDT
Krh, to answer your question:
1) 85% peak efficiency for 5V output and 87% peak efficiency for 12V output. A more detailed discussion can be found (http://cds.linear.com/docs/LT%20Journal/LTJournal-V22N3-06-df-LT8300-MinChen.pdf).
2) It does scale and you get better efficiency for higher output power. Check LTC's No-Opto Product Family.
Thank you!
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