AN-35

Integrated switching power supply technology, offering small size, low weight and universal AC input voltage operation, has finally evolved to cost-effectively replace linear transformerbased power supplies for low power applications. LinkSwitch reduces the cost of switching battery chargers and AC adapters to the level of linear transformer power supplies. LinkSwitch also easily meets standby and no-load energy consumption guidelines specified by worldwide egulatory programs such as the USA's Presidential 1 W Standby Executive Order and the European Commission's 2005 requirement for 300 mW no-load consumption.

The feature set of LinkSwitch offers the following advantages over other solutions:
• Lowest cost and component count for a constant voltage, constant current (CV/CC) solution
• Extremely simple circuit only 14 components required for a production-worthy design
• Primary based CV/CC solution eliminates 10 to 20 components for low system cost
• Up to 75% lighter power supply reduces shipping costs
• Fully integrated auto-restart for short circuit and open loop fault protection
• 42 kHz operation simplifies EMI filter design
• 3 W output with EE13 core for low cost and small size

LinkSwitch is designed to produce an approximate CV/CC output characteristic as shown in Figure 2. In charger applications, a discharged battery operates on the CC portion of the curve until almost fully charged and then naturally transitions to the CV portion of the curve. Below an output voltage of approximately 2 V (consistent with a failed battery pack), the supply enters auto-restart, reducing the average output current to approximately 8% of nominal.

In an AC adapter, normal operation occurs only on the CV portion of the curve, the CC portion providing overload protection and auto-restart short circuit protection. LinkSwitch is a fixed frequency PWM controlled device, designed to operate with flyback converters in discontinuous mode. In the CV portion of the curve, the device operates using voltage mode control and changes to a current limit mode during the CC portion of the curve. Total system CV accuracy is typically ±10% at the peak power point, including all device tolerances and line input voltage variations. With transformer primary inductance variations within ±10%, the total system CC accuracy is typically ±20% (LNK501) or ±25% (LNK500) compared to nominal values.

During CV operation, the reflected output voltage (VOR) controls the duty cycle. LinkSwitch is placed in the high side rail, as shown in Figure 1, such that VOR can be sensed directly, requiring no additional subtraction of the input voltage component.