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Combined current sense for high- and low-side power transistors

IP.com Disclosure Number: IPCOM000018993D
Published in the IP.com Journal: Volume 3 Issue 9 (2003-09-25)
Included in the Prior Art Database: 2003-Sep-25
Document File: 2 page(s) / 230K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

Up to now two separated gain stages are used simultaneously for current sensing, one with a high-side the other with a low-side power transistor. The drawback of this technology is the essential well matching of the both gain stages. It requires an expensive and complex trimming of this two stages. Also the offsets of the gain stages are usually not exactly identical what introduces output signal errors. This invention shows a method to unite the two gain stages in one circuit with a half bridge architecture. The core of the invention is a simple operational transconductance (OTA) amplifier which is used for a synchronous DC-to-DC converter (figure 1). The current IL is the sum of IHS (current at high-side power transistor) and ILS (current at low-side power transistor). If no current is flowing through the power transistors the voltages Va and Vb are respective:

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© SIEMENS AG 2003 file: ifx_2003J51656.doc page: 1

Combined current sense for high- and low-side power transistors

Idea: Guiseppe Bernacchia, IT-Padova; Maurizio Galvano, IT-Padova; Giorgio Chiozzi, IT-Padova

Up to now two separated gain stages are used simultaneously for current sensing, one with a high- side the other with a low-side power transistor. The drawback of this technology is the essential well matching of the both gain stages. It requires an expensive and complex trimming of this two stages. Also the offsets of the gain stages are usually not exactly identical what introduces output signal errors.

This invention shows a method to unite the two gain stages in one circuit with a half bridge architecture. The core of the invention is a simple operational transconductance (OTA) amplifier which is used for a synchronous DC-to-DC converter (figure 1). The current IL is the sum of IHS (current at high-side power transistor) and ILS (current at low-side power transistor). If no current is flowing through the power transistors the voltages Va and Vb are respective:

      b [g61][g61]VVa V (On condition that the both RS have the same value.)

During the ON- and OFF-time (TON and TOFF, shown at the diagram of figure 1) the current IL is the sum of the currents IHS and ILS. The voltage Va has the value Va=Vb-0,5*ILRS.

During the ON-time (TON) the current IHS is flowing through the high-side sense resistor RS and excites the Voltage VHS=IHS*RS . The low-side power is switched off and the current ILS=0. The voltage Va falls down by the value of Va=0,5VHS.

During the OFF-time (TOFF) ILS is flowing t...