A wide-input range 12V output ac-dc power supply reference design kit (RDK-734) is now available from Power Integrations. It contains the core components to build an 8.4W open frame power supply for appliances using PI’s energy-efficient, accurate primary-side regulation LinkSwitch™-3 CV/CC switcher IC.
The highly-integrated LinkSwitch-3 ICs dramatically simplify low power CV/CC designs by eliminating optocouplers and all secondary control circuitry.
Other key features of the design include:
- Efficiency >83% at full load
- Meetst ErP regulation with >70% efficiency at 500mW output
- Maximum power output <15W to simplify UL qualification
Design Key Points
This reference design kit targets to meet the standard basic voltage DoE Level VI and COC5 Tier 2 regulatory requirements. Efficiency was optimized with the transformer design, active devices, primary clamp and filter components, and bias voltage. For the transformer design, maximizing the wire gauge of the primary and secondary windings to fit the bobbin width helps decrease copper loss with the increased conducted effective area.
To minimize power losses, the primary clamp or RCD snubber circuit were selected such that the peak MOSFET drain voltage is maintained to <680V while still meeting efficiency and thermal performance of these components. Also, since LinkSwitch-3 uses primary side regulation (PSR), the resulting peak MOSFET drain voltage will affect the feedback voltage sensing and should settle at 1% within 1.2µs from the turn-off of the primary MOSFET for best output regulation. The use of slow recovery type (typical trr >1µs) reduces the feedback voltage ringing and further improve regulation.
An external bias supply through the bias winding of the transformer also increases efficiency, especially at light load, and lowers the no-load input power consumption by disabling the internal high-voltage supply for LNK6407D.
Proper selection of the secondary side components (output diode, output filter capacitor, and preload) likewise helps meet efficiency specifications. A low forward voltage and low leakage current Schottky-barrier type diode was used with a voltage rating enough to have around 20% margin from actual diode voltage stress to the peak inverse voltage (PIV) rating of the diode. It is also necessary to take into consideration the diode’s self-heating to meet thermal and efficiency margins.
For the output filter capacitor, a very-low ESR type with a high ripple current rating can be used to meet output voltage ripple specification and improves efficiency. And since the preload is a fixed loss to the power supply, it should be sized such that the output voltage at no-load is within the target regulation.
The design also aims to have a good margin for conducted EMI performance and simplify the input filter stage by eliminating the need for a common mode choke or Y-class capacitor. The transformer uses E-Shield technique to minimize the noise coupling by having a cancellation shield as the first layer of the transformer filling the bobbin width followed by the primary winding. To decrease the loss incurred by parasitic capacitance between the shield and the primary winding, the 4 layers of tape were added.
Succeeding windings are for the bias, feedback and the secondary. The secondary was wound in counter-clockwise direction with respect to the other windings with the physical start connected to the GND and terminating the winding to the anode of the output diode. To further decrease noise coupling, the core was connected to B- (negative side of the input bulk capacitor).