Design Article
Flyback converter simplifies isolated power design
Bruce Haug, Linear Technology Corp.
10/18/2012 7:16 AM EDT
Simple Flyback IC design
The LT8300 eliminates the need for an optocoupler, secondary-side reference voltage and extra third winding off the power transformer, all while maintaining isolation between the primary and secondary-side with only one part, the power transformer having to cross the isolation barrier. The LT8300 employs a primary-side sensing scheme which is capable of detecting the output voltage through the flyback primary-side switching node waveform. During the switch off-period, the output diode delivers the current to the output, and the output voltage is reflected to the primary-side of the flyback transformer. The magnitude of the switch node voltage is the summation of the input voltage and reflected output voltage, which the LT8300 is able to reconstruct. This output voltage feedback technique results in better than ±5 percent total regulation over the full line, load and temperature range. Figure 1 shows a flyback converter schematic using the LT8300.
Figure 1: LT8300 Flyback converter with primary side output voltage sensing
(Click on image to enlarge)
Primary-side output voltage sensing
Output voltage sensing for an isolated converter normally requires an optocoupler and secondary side reference voltage. An optocoupler transmits the output voltage feedback signal through the optical link while maintaining the isolation barrier. However, an optocoupler transfer ratio changes with temperature and aging, degrading its accuracy. Optocouplers also can be nonlinear from unit to unit which causes different gain/phase characteristics from circuit to circuit. A flyback design employing an extra transformer winding for voltage feedback can also be used to close the feedback loop instead of an optocoupler. However, this extra transformer winding increases the transformer’s size and cost.
The LT8300 eliminates the need for an optocoupler or extra transformer winding by sensing the output voltage on the primary-side of the transformer. The output voltage is accurately measured at the primary-side switching node waveform during the off time of the power transistor as shown in Figure 2, where N is the turns ratio of the transformer, VIN is the input voltage and VC is the maximum clamped voltage.
Figure 2: Typical switch node waveform
The LT8300 eliminates the need for an optocoupler, secondary-side reference voltage and extra third winding off the power transformer, all while maintaining isolation between the primary and secondary-side with only one part, the power transformer having to cross the isolation barrier. The LT8300 employs a primary-side sensing scheme which is capable of detecting the output voltage through the flyback primary-side switching node waveform. During the switch off-period, the output diode delivers the current to the output, and the output voltage is reflected to the primary-side of the flyback transformer. The magnitude of the switch node voltage is the summation of the input voltage and reflected output voltage, which the LT8300 is able to reconstruct. This output voltage feedback technique results in better than ±5 percent total regulation over the full line, load and temperature range. Figure 1 shows a flyback converter schematic using the LT8300.
Figure 1: LT8300 Flyback converter with primary side output voltage sensing
(Click on image to enlarge)
The LT8300 is available in a small 5-lead SOT-23 package and accepts an input voltage from 5V to 100V, which can be applied directly to the IC without the need for a series dropping resistor. It is able to reliably operate with a high input voltage due to the high voltage onboard LDO and the inherent extra spacing of pins 4 and 5 on the SOT-23 package. In addition, its onboard 260mA, 150V internal DMOS power switch allows it to deliver up to about 2W of output power.
Furthermore, the LT8300 runs in a low-ripple Burst Mode Operation at light load, which reduces the quiescent current to only 330µA, a feature that increases the battery run time during sleep mode. Other features include internal soft-start and undervoltage lockout. The transformer turns ratio and 1 external resistor are all that is needed to set the output voltage.
Furthermore, the LT8300 runs in a low-ripple Burst Mode Operation at light load, which reduces the quiescent current to only 330µA, a feature that increases the battery run time during sleep mode. Other features include internal soft-start and undervoltage lockout. The transformer turns ratio and 1 external resistor are all that is needed to set the output voltage.
Output voltage sensing for an isolated converter normally requires an optocoupler and secondary side reference voltage. An optocoupler transmits the output voltage feedback signal through the optical link while maintaining the isolation barrier. However, an optocoupler transfer ratio changes with temperature and aging, degrading its accuracy. Optocouplers also can be nonlinear from unit to unit which causes different gain/phase characteristics from circuit to circuit. A flyback design employing an extra transformer winding for voltage feedback can also be used to close the feedback loop instead of an optocoupler. However, this extra transformer winding increases the transformer’s size and cost.
The LT8300 eliminates the need for an optocoupler or extra transformer winding by sensing the output voltage on the primary-side of the transformer. The output voltage is accurately measured at the primary-side switching node waveform during the off time of the power transistor as shown in Figure 2, where N is the turns ratio of the transformer, VIN is the input voltage and VC is the maximum clamped voltage.
Figure 2: Typical switch node waveform
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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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