Browse Prior Art Database

ISOLATED OUTPUT LOW-COST POWER SUPPLY

IP.com Disclosure Number: IPCOM000006163D
Original Publication Date: 1991-Jul-01
Included in the Prior Art Database: 2001-Dec-10
Document File: 2 page(s) / 79K

Publishing Venue

Motorola

Related People

Rafael Brody: AUTHOR

Abstract

In many applications it is needed to have an efficient regulated power supply with isolated output. The traditional approach is to use a pulse transformer to transfer energy from the primary to the secondary winding with feedback from the output of the power supply, implemented via an optocoupler or another transformer, to control the driving circuit of the primary to keep the output voltage in a predefined range.

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MOlOROLA INC. Technical Developments Volume 13 July 1991

ISOLATED OUTPUT LOW-COST POWER SUPPLY

by Rafael Brody

In many applications it is needed to have an efficient regulated power supply with isolated output.

  The traditional approach is to use a pulse transformer to transfer energy from the primary to the secondary winding with feedback from the output of the power supply, implemented via an optocoupler or another transformer, to control the driving circuit of the primary to keep the output voltage in a predefined range.

  A very inexpensive solution that uses the same pulse transformer to transfer the energy from the primary winding to the secondary as well as for transferring the control feedback from the secondary to the primary winding, is described in Figure 1.

The power supply uses a flyback transformer.

  Ao oscillator is connected to FFl. When the Q output of FFl is logic "l", transistor Ql is "ON" and the current through the primary of the transformer rises as a ramp. Ql will be turned "OFF" at the next clock to FFl or when the current through Ql is above a predefined value (which will be typically about four times the nominal average primary current) that is

detected by R, and Comparator 1.

The output voltage is regulated by Comparator 2 which turns "ON" Q2 when V, is above a predefined

value and "OFF" when V, is below another predefined value.

When Q2 is "OFF", the flyback current charges output capacitor Cl.

When Q2 is "ON", the flyback current pass...