Isolated discrete forward DC/DC design: Page 4 of 5

June 24, 2016 // By Bruce Haug
Isolated discrete forward DC/DC design
High density isolated DC/DC converters have significantly changed during the course of the past 25 years. The introduction of the full and half-brick form factors back then created a rush to use them in distributed power architectures in telecom, datacom, industrial and medical systems where a bus voltage is routed to every board within a system and each board had its own isolated DC/DC converter.

The LT8302 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 LT8302 employs a primary-side sensing scheme which is capable of sensing 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 LT8302 is able to reconstruct. This output voltage feedback technique results in better than ±5% total regulation over the full line, load, and temperature range.

Figure 3 shows a flyback converter schematic using the LT8302 with only 14 external components.

Fig. 3: LT8302 flyback converter with primary side output voltage sensing.

The LT8302 is available in an 8-lead thermally enhanced SO-8 package and accepts an input voltage from 2.8V to 42V. Its onboard 3.6A, 65V rugged internal DMOS power switch allows it to deliver up to about 18W of output power.

Furthermore, the LT8302 runs in a low-ripple Burst Mode Operation at light load which reduces the quiescent current to only 106µA, a feature that increases the battery run-time during sleep mode with the output voltage in regulation. 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.

 

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. However, this extra transformer winding increases the transformer’s size cost and does not provide very good output voltage regulation.

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