The focus of this circuit was to take a design based on the LT3798 and make it Energy Star Compliant; reduce no load, standby power to less than 300mW. That was achieved by adding the components highlighted in the dotted line box in the schematic inside the primary side. This circuit is simply a switch that removes the power loss in R9 and R14. When the LT3798 starts switching, Q2 turns off, eliminating any current flow in its path. Standby power at no load stays within 300mW with an input of 85VAC to 150VAC. The converter will operate up to 265VAC, but the standby, no load power will increase another 100mW. This converter uses the LT3798 to regulate the 120VAC line voltage to an isolated 5V, 2.2A output. Output rectification is synchronous and handled by the LT8309.
Other important features of this circuit are
- Load/line regulation stays within +/-4%
- Output ripple at 85VAC is about 150mV, most of it line frequency ripple; a larger C3 can be used to reduce this ripple
- Output ripple above 100VAC stays within 50mV, including spikes
- Full load efficiency comes in at 80%
- Short circuit protection works fine at any input line
The LT3798 is a constant-voltage/constant-current isolated flyback controller that combines active power factor correction (PFC) with no opto-coupler required for output voltage feedback into a single-stage converter. A LT3798 based design can achieve a power factor of greater than 0.97 by actively modulating the input current, allowing compliance with most Harmonic Current Emission requirements. The LT3798 is well suited for a wide variety of off-line applications. The input range can be scaled up or down, depending mainly on the choice of external components. Efficiencies higher than 86% can be achieved with output power levels up to 100W. In addition, the LT3798 can easily be designed into high DC input applications.
The LT8309 is a secondary-side synchronous rectifier driver that replaces the output rectifier diode in a flyback topology. By replacing the diode with a N-channel MOSFET, applications are no longer restricted by the heat constraints of the output diode. The IC replicates the behavior of a diode by sensing the drain-to-source voltage to determine when the current becomes negative. The LT8309's low minimum on- and off-times help improve noise immunity. The fast propagation delay of 26ns allows applications to operate in discontinuous conduction mode (DCM) and critical conduction mode (CrCM). The gate driver features a 0.8Ω pull-down device for fast turn-offs. The 40V VCC rating allows the part to be driven from the output voltage or the rectified drain voltage of the MOSFET. A low quiescent current of 400μA maximizes efficiency at low output currents.