5 And 12V Ac Powered Switching Supply

This power supply utilizes an SGS-Thomson UC3842 integrated circuit in an off-line flyback regulator configuration, delivering +5 V at 4 A and ±12 V at 300 mA. This design allows for the use of a compact high-frequency (50 kHz) transformer capable of managing power levels typically handled by a 60 Hz transformer. The circuit features Q1, a 5-A 500-V MOSFET, alongside fast-recovery diodes. The transformer, T1, includes a primary winding with 45 turns of #26 wire. Each of the 12 V windings comprises 9 turns of #30 wire, wound bifilar. The 5 V winding consists of 4 turns of four bifilar #26 wires. The feedback control winding is implemented with two bifilar, parallel 10-turn #30 windings. The transformer core is a Ferroxcube EC35-3C8 with a 3/8" center leg.

The power supply circuit operates by converting high-voltage AC input into regulated DC output using the UC3842 controller. The UC3842 is a current-mode PWM controller that facilitates efficient power conversion. It regulates output voltage by adjusting the duty cycle of the MOSFET (Q1), which switches on and off to control energy transfer to the transformer (T1). The high-frequency operation at 50 kHz allows for a smaller transformer design, significantly reducing the size and weight compared to conventional 60 Hz transformers.

The transformer is designed with a 45-turn primary winding, which is critical for stepping down the voltage efficiently. The use of #26 wire for the primary winding ensures that it can handle the necessary current without excessive resistive losses. The bifilar winding technique used for the 12 V and 5 V secondary windings enhances coupling between the windings, improving efficiency and reducing electromagnetic interference.

Fast-recovery diodes in the rectification stage are employed to minimize switching losses, ensuring that the output voltage remains stable under varying load conditions. The feedback winding, consisting of two parallel 10-turn #30 bifilar windings, plays a crucial role in providing feedback to the UC3842, allowing for precise regulation of the output voltages.

The choice of the Ferroxcube EC35-3C8 core for the transformer is significant due to its high permeability and low core losses, which are essential for high-frequency applications. The dimensions of the core, particularly the 3/8" center leg, contribute to the magnetic performance, facilitating efficient energy transfer between the primary and secondary windings.

Overall, this power supply design exemplifies efficient power management in a compact form factor, suitable for applications requiring reliable +5 V and ±12 V outputs. This supply uses an SGS-Thomson UC3842 IC in an off-line flyback regulator, providing + 5 V at 4 A and ± 12 V at 300 mA. This enables a small high-frequency (50 kHz) transformer, to handle large amounts of power that are normally handled by a 60-Hz transformer. Q1 is a 5-A 500-V MOSFET, and the diodes are fast-recovery types. T1 has a 45-turn primary winding of #26 wire. The 12-V windings are each 9 turns of #30 wire, bifilar wound. The 5-V winding is 4 turns of four bifilar #26 wires. The control (feedback) winding is two bifilar, parallel 10-turn, #30 windings. The core is Ferroxcube EC35-3C8 with a 3/s" center leg.