OFF_LINE_24_V_100_W_DC_SUPPLY_

Published:2009/7/24 3:48:00 Author:Jessie | From:SeekIC

Today's electronic systems often rely on distributed power to support a myriad of control functions. This contrasts with historical approaches, which provided a regulated dc voltage to each of the subsystems with an off-line converter, often an expensive off-the-shelf box with less-than-optimal reliability. Shown here is a discrete approach that simplifies the task of distributed power design, saving time and money during system design. The LT1105 current-mode PWM control IC makes a simple, low-cost, yet highly reliable distributed power supply with the added advantage of a customizable footprint. The 24-V output is regulated to better than ±1 percent over a line range of 90 to 260 Vac and an 8:1 dynamic load range. A maximum output current of 4.2 A is available for a total of 100 W. All components, including the transformer, are specified in this design. The trans-former meets international safety standards UL 1950 and IEC950. This transformer and others that provide 36 or 48 V at 100 W are available off the shelf. The LT1105 uses unique design techniques, eliminating the optocoupler feedback normally found in off-line supplies, yet enabling the regulator to provide tight line and load regulation. A totem-pole output drives the gate of an external high-voltage FET (Q1), and switch current is monitored by sense resistor R22. Short-circuit protection is provided by burp mode operation, whereby the LT1105 will continuously shut down and restart during the fault condition.



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