Browse Prior Art Database

BI-DIRECTIONAL FIELD EFFECT TRANSISTOR (FET) SWITCH

IP.com Disclosure Number: IPCOM000009659D
Original Publication Date: 2000-Jan-01
Included in the Prior Art Database: 2002-Sep-09
Document File: 3 page(s) / 103K

Publishing Venue

Motorola

Related People

Mark J. Hockin: AUTHOR [+3]

Abstract

In some applications it is required to switch cur- rent flowing in different directions, for example in a battery backup application which will require charge to flow into the battery while input power is good and charge to flow out of the battery in the event of the input power failing. Known methods have used relays with the coil being driven by the control sig- nal and the battery current flowing in/out of the relay contacts.

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0 M MO-LA Technical Developments

BI-DIRECTIONAL FIELD EFFECT TRANSISTOR (FET) SWITCH

by Mark J. Hockin, Christopher Brice and Jason FL Ft. Jackson

  In some applications it is required to switch cur- rent flowing in different directions, for example in a battery backup application which will require charge to flow into the battery while input power is good and charge to flow out of the battery in the event of the input power failing. Known methods have used relays with the coil being driven by the control sig- nal and the battery current flowing in/out of the relay contacts.

  This proposal uses solid state devices to achieve this with a minimum of extra drive and control com- ponents.

  Specitically, this idea uses back p channel FETs with a diode biased drive topology to allow the use of a solid state device in this application. Its advan- tages are that it is small (surface mountable), shock resistant, (unlike the relay used in the known meth- ods) and as there are no moving parts, its reliability is greatly enhanced over the relay. With careful selection of the FETs and parallel operation (as shown in Figure 1,) the bidirectional FET switch will dissipate less energy than the relay.

  This idea also benefits from recent advances in low resistance FET technology.

  Figure 1 shows the circuit diagram for a two FET bidirectional switch.

  To prevent unwanted conduction occurring in the direction of the p channel FET body diode another FET is connected in series, "back to back" with its diode reversed to prevent this conduction path.

  Control of the bidirectional switch (Ql and 42) is via an n-channel FET QS. This FET is driven via R4 and R5 which reduce the magnitude of the con- trol signal to a level which is acceptable to Q5.

  To make the switch conduct, Q5 is switched on by taking the gate...