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A METHOD OF CONTINUOUS CURRENT SENSING IN SWITCHMODE CIRCUITS

IP.com Disclosure Number: IPCOM000005678D
Original Publication Date: 1987-Oct-01
Included in the Prior Art Database: 2001-Oct-25
Document File: 2 page(s) / 106K

Publishing Venue

Motorola

Related People

John E. Herman: AUTHOR [+2]

Abstract

In switchmode power supply and electronic motor drive design it is desirable to control load current. This is frequency done with current transformers or current sense resistors. This technique employs a combination of current sense methods to achieve continuous, instantaneous and average current sensing of the load.

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MOlYlROLA Technical Developments Volume 7 October 1987

A METHOD OF CONTINUOUS CURRENT SENSING IN SWITCHMODE CIRCUITS

by John E. Herman and Steve H. Noskowitz

   In switchmode power supply and electronic motor drive design it is desirable to control load current. This is frequency done with current transformers or current sense resistors. This technique employs a combination of current sense methods to achieve continuous, instantaneous and average current sensing of the load.

   Typically, a single current sense resistor R, is placed between the low side switch and ground of the switchmode circuitry. However, only a partial current wave form may be obtained which often gives rise to elaborate timing circuitry to compensate for the unknown current in the load. In the case of motor control cir- cuitry, other difficulties such as locked rotor further complicate the design as a result of inadequate current sensing. The DC Restored Dual Rail Current Sense Circuit described herein allows continuous current sensing as well as provides a voltage proportional to average current in the load.

   Figure 1 shows an inductive load, switch, catch diode and current sense resistor (R,). Figure 2 shows the current sense wave form that appears across R,. Current flows through the catch diode while switch Ql is open. The dotted lines represent the current in the load while switch Ql is open. The objective is to sense cur- rent in the load at all times. The DC Restored Dual Rail Current Sense Circuit of Figure 3 achieves continuous current sensing in the following manner.

   A current transformer is connected in series with the inductive load to obtain continuous sensing of the AC component of the load current. The transformer output or AC component of the current wave can be DC restored by direct use of the DC component of the partial wave form present across R,. The term "dual rail" has been used to indicate that current is sensed at two different points in the circuitry. The magnitude of the AC component of the current wave form and the partial current wave form may be matched (scale 1:l) by proper choice of R, and R,,,kP DC restoration may be achieved by use of a sample and hold circuit composed of a simple switch (Q2) connected between transformer terminal A and R, with a capacitor between terminal B of the transformer and ground. Q2 is turned on and off alo...