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Nondissipative Overcurrent Protection for a High Frequency Power Supply

IP.com Disclosure Number: IPCOM000081499D
Original Publication Date: 1974-Jun-01
Included in the Prior Art Database: 2005-Feb-28
Document File: 3 page(s) / 32K

Publishing Venue

IBM

Related People

Nunnery, WB: AUTHOR

Abstract

Illustrated is a small size, light weight, high-efficiency power supply. Small size and light weight are met by utilizing a high frequency in the supply, however high efficiency involves design of an improved sort. Most conventional power supplies utilize a direct current sensing operation to limit overcurrent due to overload or shorts. Typically, a resistor is inserted in the main stream of the power supply circuit and a voltage is developed across it. The voltage is amplified and compared against a threshold reference and if the peak current allowed is exceeded, a shutdown signal is generated for the power supply and a switch is turned off.

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Nondissipative Overcurrent Protection for a High Frequency Power Supply

Illustrated is a small size, light weight, high-efficiency power supply. Small size and light weight are met by utilizing a high frequency in the supply, however high efficiency involves design of an improved sort. Most conventional power supplies utilize a direct current sensing operation to limit overcurrent due to overload or shorts. Typically, a resistor is inserted in the main stream of the power supply circuit and a voltage is developed across it. The voltage is amplified and compared against a threshold reference and if the peak current allowed is exceeded, a shutdown signal is generated for the power supply and a switch is turned off.

The disadvantages with the normal scheme are that the resistor must be inserted in the current path of the power supply, in order to convert the current to voltage and power will, therefore, be dissipated. This loss of power results in a loss of efficiency. By making the resistor small, the power loss can be minimized, but the sense signal will be small and will be more difficult to amplify, especially in the DC mode and with the inherent noise problems that exist in most environments. Small and precise values of the sensing resistor are fairly scarce and such resistors are expensive relative to others.

The figure illustrates a nondissipative indirect sensing overcurrent protection circuit for a power supply. A method is shown for simulating the current ramp that occurs in the primary of transformer T1 during the time that a power supply switching transistor Q1 is on, without actually sampling the current through the primary of transformer T1 (designated as current Il). The circuit operates as follows: when Q3 is turned off, Q1 will be turned on and Q4 will be turned off. In this condition, two events occur: 1) the current in the primary of transformer 1 rises at a linear rate r as defined by the equation r=V(1) dt divided by L(P), where L(P) is the inductance of the primary coil in transformer 1, 2) Q...