George Qian, Michael Wu, Analog Devices
Design Note 1047
Introduction
Isolated flyback converters are used in automotive, industrial, medical and telecom applications where the power supply must be reliable, easy-to-use, high voltage and isolated, and must provide excellent regulation over load, line and temperature. LT8304-1 is an isolated no-opto flyback converter specially optimized for high output voltage applications – providing outputs up to 1000 V.
Traditionally, the regulation feedback loop requires a bulky high voltage divider to directly sense the high output voltage, along with opto-couplers to convey feedback information back through the isolation barrier. The bulky resistor solution results because a 1206 resistor can handle 200 V maximum. So to sense 1000 V, at least six 1206 resistors are required, plus a small bottom resistor.
1000 V/15 mA Output, from a 4 V–28 V Input
An LT8304-1 flyback converter design features a low component count. Figure 1 shows a complete 4 V–28 V input to 1000 V output solution capable of supporting 15 mA loads. The output current capability increases with input voltage, reaching 13 mA when the input voltage is greater than 24 V. The LT8304-1’s ability to sense the output voltage through the primary-side waveform eliminates the need for a bulky high voltage divider, and no-opto coupler is required.
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| Figure 1. | A Complete 1000 V/15 mA Isolated Flyback Converter from a 4 V–28 V Input. | |
The guidelines for calculating voltage and current stress on the components surrounding the LT8304-1 are detailed in the LT8304-1 data sheet. Notably, this 1000 V solution uses a transformer with three split-output windings on the secondary side. The primary side to secondary side turns ratio is 1:10:10:10, instead of a single-secondary-winding 1:30 transformer. The 1:10:10:10 transformer enables the output voltage stress to be split among three high voltage output diodes and three high voltage output capacitors. Individual component voltage ratings need only be one-third of the total voltage, facilitating more options for output diode and output capacitor selection.
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| Figure 2. | Efficiency of Figure 1 at Various Input Voltages | |
Figure 2 shows efficiency at various input voltages. This flyback converter achieves 90.5% peak efficiency. Even with no opto-coupler, load regulation at different input voltages remains tight, typically 2% to 3%, as shown in Figure 3.
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| Figure 3. | Load Regulation of Figure 1 at Various Input Voltages. |
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800 V/10 mA Output, from 4 V–18 V Input
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| Figure 4. | A Complete 800 V/10 mA Isolated Flyback Converter from a 4 V–18 V Input. | |
Figure 4 shows a complete 4 V–18 V input to 800 V output solution capable of providing up to 10 mA output current. This flyback converter achieves 88.2% peak efficiency when the input is 18 V and the load current is 10 mA. Figure 5 shows the efficiency curve at different input voltages; Figure 6 shows the excellent load regulation. This solution also features a low component count.
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| Figure 5. | Efficiency of Figure 4 at Various Input Voltages. | |
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| Figure 6. | Load Regulation of Figure 4 at Various Input Voltages. |
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Conclusion
The LT8304-1 is an easy-to-use monolithic micropower isolated flyback converter optimized for high output voltage applications. By sampling the isolated output voltage directly from the primary-side flyback waveform, complete solutions maintain tight regulation without either output voltage divider or opto-isolator. The output voltage is simply programmed with two external resistors and a third optional temperature compensation resistor. Boundary mode operation enables a small magnetic solution with excellent load regulation. A 2 A, 150 V DMOS power switch is integrated, along with all the high voltage circuitry and control logic, in a thermally enhanced 8-lead SO package. The LT8304-1 operates an input voltage range of 3 V to 100 V, and delivers up to 24 W of isolated output power.
