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
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.
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