Four-Output Flyback Regulator Demonstration Board
The DC659 demo board from Analog Devices is a four-output flyback regulator based on the LTC3806 control IC. Input voltages between 25 and 60 Volts are stepped down to 12V, 5V, 3.3V and 2.5V. Total load power is 22W.
As a result of the use of synchronous rectification, efficiency and cross regulation are significantly improved versus a conventional flyback regulator. This demo circuit is suitable for powered Ethernet or 42V automotive applications.
The DC659 steps a high input voltage down to multiple low voltage outputs with high efficiency. The demo circuit features the LTC3806 synchronous flyback controller. The synchronous flyback topology replaces the normal output diode of the flyback converter with a MOSFET.
(click on schematic to enlarge)
The MOSFET has a lower forward voltage drop than the diode it replaces, which improves efficiency particularly at low and very low output voltages. The second benefit achieved by synchronous rectification is that unlike a diode, the MOSFET synchronous switch will allow current to flow in both directions.
Therefore, during the ON time of the secondary side switches, all of the outputs are able to share power. This improves the cross regulation of the outputs so that output voltage regulation is maintained even when one output is lightly loaded and the other outputs are heavily loaded. Normally with diode rectification, the lightly loaded output rises, as its output capacitor is peak charged with voltage spikes caused by transformer leakage inductance.
This effect can still be seen on the diode derived (nonsynchronous) 12V output on the demo circuit. This output will rise in the event that its load is relatively small compared to the other output loads. This output has a correspondingly loose output accuracy specification.