VISHAY AN606 Technical data

Kandarp Pandya

INTRODUCTION

AN606

Vishay Siliconix

Current-Sensing Power MOSFETs

Vishay Siliconix current-sensing power MOSFETs offer a
simple means of incorporating a protection feature into an
electronic control circuit and avoiding catastrophic failures
resulting from overcurrent (overload) and/or short-circuit
conditions. The device package is a modified D2PAK with five
pins. The MOSFET termination retains the standard D

2

PAK
footprint for a three-pin device. The additional two pins provide
termination for a current-sense output and an internal Kelvin
connection to the source. For current sensing, the MOSFET
design employs a small number of the total number of
MOSFET cells in a known ratio. The latter define the
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
incorporation into the main control system.

DEVICE DESCRIPTION AND PRINCIPLE OF
OPERATION

D2PAK-5

D (Tab, 3)

between gate and drain-stub and between drain-stub and source,
respectively. See Application Note 826, Recommended Minimum

Pad Patterns With Outline Access for Vishay Siliconix
MOSFETs (http://www.vishay.com/doc?72286), for the

recommended PCB layout dimensional details of the pad pattern.
Modified-part library symbols for schematic symbol and PCB
layout are available on the “Protel” (PCB design software)
platform. For soft copy, please contact Vishay Siliconix in Santa
Clara, Calif., in the United States, by phoning 1-408-567-8927.

The Principle Behind the Current-Sensing Feature

The most efficient way to sense the drain-source current is to
use the ratio-metric measurement. In a power MOSFET, it is
possible to implement this method easily.

The cell density, a favored term within the power MOSFET
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,
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)

G

324

51

SENSE

SENSE

FIGURE 1. Package Information and Schematic Symbol

SGD

KELVIN

(2)

N-Channel MOSFET

S (5)

(4)

KELVIN

Package Information and Schematic Symbol, Figure 1, shows a
partial reproduction of a datasheet for a current-sensing
MOSFET, SUM50N03-13C. Gate, drain-stub/tab, and source
(pins 1, 2, and 3) are in the same position as in a standard D

2

PAK
(TO-263) MOSFET. However, pin-out modification is required to
incorporate current-sense (pin 2) and Kelvin-to-source (pin 4)

TABLE 1: Current Sense Characteristics

Current Sensing Ratio r ID = 1 A, V

Mirror Active Resistance r

Document Number: 71991
17-Dec-03

m(on)

Dividing the MOSFET cells in a known ratio creates two paths
that share the drain-source current. The path with the smaller
number of cells constitutes the sense current, which is much
smaller than the current conducting through the rest of the
cells. A very simple, low-power, external circuit can measure
this current. Multiplying this value with the cell ratio gives the
total drain-source current.

The classic Kelvin termination for the return of sense current
to the main source connection insures the measurement
accuracy. This terminal not only eliminates the ground loop,
but also minimizes the imbalance of internal structures with
two current paths.

The Current-Sensing Parameters, Table 1, and the
Current-Sense Die Characteristics and Schematic, Figure 2,
help to demonstrate the current-sensing operation and circuit
implementation.

= 10 V, R

GSS

VGS = 10 V, ID = 10 mA 3.5 W

= 1.1 W 420 520 620

SENSE

www.vishay.com

1

AN606

Vishay Siliconix

TYPICAL CHARACTERISTICS (25_C UNLESS NOTED) SENSE DIE

10

8

6

4

− On-Resistance (W)

m(on)

r

2

0

0.00 0.02 0.04 0.06 0.08 0.10

1200

1000

800

600

Ratio

400

On-Resistance vs. Sense Current

VGS = 4.5 V

I

(A)

SENSE

Current Ratio (I

vs. Gate-Source Voltage (Figure 1)

RS = 6.6 W

(MAIN)/IS

VGS = 10 V

)

RS = 4.7 W

RS = 2.2 W

RS = 1.1 W

RS = 0.5 W

On-Resistance vs. Gate-Source Voltage

10

8

6

4

− On-Resistance (W)

m(on)

r

2

0

0246810

− Gate-to-Source Voltage (V)

V

GS

G

SENSE S KELVIN

V

G

R

ID = 10 mA

S

200

0

048121620

VGS − Gate-to-Source Voltage (V)

FIGURE 2. Current-Sensing Die Characteristics and Schematic

Definition of Current-Sensing Parameters

The current-sense ratio, r, is the quotient of the number of cells
terminated on the sense terminal to the total number of cells on
the MOSFET die.

To derive the value of r using the above definition requires
detailed die design. However, the quotient of drain current to
the sense current provides the same value because these
current values are the sum of cell current in each path.

Mathematically:

r = I

D/ISENSE

ID is drain current

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2

I

is the current flowing out of the sense terminal and into

SENSE

the sense resistor, R

Mirror active resistance, r

SENSE

, is the resistance of parallel

m(on)

connected cells used in the sense chain when the device is on.
Being r

as in any other MOSFET, the value depends on

DS(on)

the gate drive, drain current, and junction temperature.
Accordingly, r

is defined at given values of VGS, I

m(on)

DRAIN

and TJ junction.

By definition, for the sense die, refer to Figure 2. Mirror active
resistance r
at 4.5 V and 10 V, corresponding drain-source current I
up to 0.1 A, and junction temperature TJ at 25
temperature coefficient of r

is specified at the gate-source voltages, V

m(on)

is the same as that of r

m(on)

SENSE

_

C. The

DS(on)

GS

Refer to the on-resistance vs. junction temperature curve in
Figure 3.

Document Number: 71991

17-Dec-03

,

.