Vishay AN606 Data Sheet

AN606 Vishay Siliconix

Current-Sensing Power MOSFETs

Kandarp Pandya

INTRODUCTION

Vishay Siliconix current-sensing power MOSFETs offer a between gate and drain-stub and between drain-stub and source, simple means of incorporating a protection feature into an respectively. See Application Note 826, Recommended Minimum electronic control circuit and avoiding catastrophic failures Pad Patterns With Outline Access for Vishay Siliconix resulting from overcurrent (overload) and/or short-circuit MOSFETs (http://www.vishay.com/doc?72286), for the conditions. The device package is a modified D2PAK with five recommended PCB layout dimensional details of the pad pattern. pins. The MOSFET termination retains the standard D2PAK Modified-part library symbols for schematic symbol and PCB footprint for a three-pin device. The additional two pins provide layout are available on the “Protel” (PCB design software) termination for a current-sense output and an internal Kelvin platform. For soft copy, please contact Vishay Siliconix in Santa connection to the source. For current sensing, the MOSFET Clara, Calif., in the United States, by phoning 1-408-567-8927.

design employs a small number of the total number of MOSFET cells in a known ratio. The latter define the

The Principle Behind the Current-Sensing Feature

current-sense parameters. A typical control interface uses a simple circuit with an op-amp or a comparator. This approach

offers the freedom of control-level setting and facilitates its The most efficient way to sense the drain-source current is to incorporation into the main control system. use the ratio-metric measurement. In a power MOSFET, it is possible to implement this method easily.

DEVICE DESCRIPTION AND PRINCIPLE OF The cell density, a favored term within the power MOSFET

OPERATION industry, conveys that the power MOSFET structure consists of many cells connected in parallel. In principle, these cells constitute a resistive path for drain-source current. Electrically,

D (Tab, 3)

D2PAK-5 these cells are parallel connected resistors, r DS(on)s. Each cell

- being identical in structure and electrical characteristics - shares the current equally when the device is on. This property enables design of a MOSFET with a current-sensing feature.

(1) (4) Dividing the MOSFET cells in a known ratio creates two paths

KELVIN

that share the drain-source current. The path with the smaller number of cells constitutes the sense current, which is much

SENSE smaller than the current conducting through the rest of the cells. A very simple, low-power, external circuit can measure

S (5) this current. Multiplying this value with the cell ratio gives the

total drain-source current.

SG N-Channel MOSFET

SENSE KELVIN

The classic Kelvin termination for the return of sense current to the main source connection insures the measurement

FIGURE 1. Package Information and Schematic Symbol

accuracy. This terminal not only eliminates the ground loop, but also minimizes the imbalance of internal structures with Package Information and Schematic Symbol, Figure 1, shows a two current paths.

partial reproduction of a datasheet for a current-sensing

MOSFET, SUM50N03-13C. Gate, drain-stub/tab, and source The Current-Sensing Parameters, Table 1, and the (pins 1, 2, and 3) are in the same position as in a standard D2PAK Current-Sense Die Characteristics and Schematic, Figure 2, (TO-263) MOSFET. However, pin-out modification is required to help to demonstrate the current-sensing operation and circuit incorporate current-sense (pin 2) and Kelvin-to-source (pin 4) implementation.

TABLE 1: Current Sense Characteristics

Current Sensing Ratio ID = 1 A, VGSS = 10 V, RSENSE = 1.1 Mirror Active Resistance rm(on) VGS = 10 V, ID = 10 m A 3.5

Document Number: 71991 www.vishay.com 17-Dec-03