AN606

Vishay Siliconix

Document Number: 71991

17-Dec-03

www.vishay.com

Current-Sensing Power MOSFETs

Kandarp Pandya

INTRODUCTION

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 D

PAK with five

pins. The MOSFET termination retains the standard D

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

KELVIN

D (Tab, 3)

S (5)

N-Channel MOSFET

PAK-5

SENSE

(1)

(2)

(4)

KELVIN

FIGURE 1. Package Information and Schematic Symbol

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

PAK

(TO-263) MOSFET. However, pin-out modification is required to

incorporate current-sense (pin 2) and Kelvin-to-source (pin 4)

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.

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.

TABLE 1: Current Sense Characteristics

Current Sensing Ratio

= 1 A, V

GSS

= 10 V, R

SENSE

= 1.1 W

420

520

620

Mirror Active Resistance

m(on)

= 10 V, I

= 10 mA

3.5

AN606

Vishay Siliconix

www.vishay.com

Document Number: 71991

17-Dec-03

TYPICAL CHARACTERISTICS (25

_C UNLESS NOTED)

SENSE DIE

0.00

0.02

0.04

0.06

0.08

0.10

On-Resistance vs. Sense Current

SENSE

(A)

200

400

600

800

1000

1200

Current Ratio (I

(MAIN)/IS

)

vs. Gate-Source Voltage (Figure 1)

Ratio

On-Resistance vs. Gate-Source Voltage

= 10 V

= 4.5 V

- Gate-to-Source Voltage (V)

= 10 mA

= 6.6 W

= 4.7 W

= 1.1 W

= 0.5 W

SENSE

KELVIN

= 2.2 W

FIGURE 2. Current-Sensing Die Characteristics and Schematic

(
on)

-
On-Resistance (

)

(
on)

-
On-Resistance (

)

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

SENSE

is drain current

SENSE

is the current flowing out of the sense terminal and into

the sense resistor, R

SENSE

Mirror active resistance, r

m(on)

, is the resistance of parallel

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

Being r

DS(on)

as in any other MOSFET, the value depends on

the gate drive, drain current, and junction temperature.

Accordingly, r

m(on)

is defined at given values of V

, I

DRAIN

and T

junction.

By definition, for the sense die, refer to Figure 2. Mirror active

resistance r

m(on)

is specified at the gate-source voltages, V

at 4.5 V and 10 V, corresponding drain-source current I

SENSE

up to 0.1 A, and junction temperature T

at 25

C. The

temperature coefficient of r

m(on)

is the same as that of r

DS(on)

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

Figure 3.

AN606

Vishay Siliconix

Document Number: 71991

17-Dec-03

www.vishay.com

FIGURE 3. Normalized r

m(on)

for the Sense Die

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

-50 -25

100 125 150 175

On-Resistance vs. Junction Temperature

= 10 V

= 25 A

- Junction Temperature (_C)

(
on)

-
On-Resistance (

)

Normalized

DESIGN EQUATIONS

The following three equations enable circuit design and

analysis.

SENSE

= x I

= I

SENSE

x [r

m(on)

+ R

SENSE

] or

= I

x r

DS(on)

/(r

m(on)

+ R

SENSE

)

SENSE

= I

SENSE

x R

SENSE

= V

x R

SENSE

/(r

m(on)

+ R

SENSE

)

Where I

SENSE

Current flowing out of sense terminal

Current-sensing ratio

Drain-source

current

Drain-source voltage

m(on)

Mirror active resistance

SENSE

External current-sense resistor

Application Aspects and Design Examples

The current-sense ratio r, even though fixed by design, is

dependent on manufacturing process variations.

Furthermore, mirror active resistance r

m(on)

depends on circuit

parameters V

and I

and junction temperature T

. As a

result, a practical design can realize an accuracy of 15% --

20% for current sensing. Accordingly, the current-sensing

MOSFET is most suitable for supervisory functions such as

overcurrent and/or short-circuit protection.

Three keys to a successful design are to:

1. have an adequate margin between the normal

operating-current value and the trip-current value;

2. use a minimum value of the I

SENSE

signal at the maximum

value of I

; and

3. use a fast comparator with hysterisis to control and protect

the MOSFET.

Typical schematic configurations for implementing the current

sense are shown in Figure 4 and Figure 5.

The Virtual Earth Sensing Scheme, Figure 4, is suitable for

applications aiming at higher noise immunity and speed. This

approach also improves measurement accuracy by

eliminating the sense resistor. However, a dual power supply

and inverted (negative) output signal are the price designers

pay for deriving these benefits.

The Resistor Sensing Scheme shown in Figure 5 is a quite

simple and economical approach. The accuracy of current

measurement is affected by the introduction of an external

sense resistor R

. However, the latter aids in lowering the

temperature sensitivity of the current-sense signal.

Sense

Kelvin

Load

Stray

Resistance

-
+

Sense

Kelvin

Load

-
+

-V

FIGURE 4. Virtual Earth Sensing Scheme

FIGURE 5. Resistor Sensing Scheme

AN606

Vishay Siliconix

www.vishay.com

Document Number: 71991

17-Dec-03

TABLE 2: Current Product Range

Part #

Channel Type

(VDC)

(W)

(A)

(W)

Package

Si6862DQ

0.026/4.5 V

6.6

1.8

TSSOP-8*

Si4730EY

0.015/10 V

11.7

3.6

SO-8*

SUM50N03-13LC

0.013/10 V

PAK-5

SUM60N08-07C

0.007/10 V

300

PAK-5

Recommended minimum pads for current-sensing MOSFETs in TSSOP-8 and SOIC-8 packages see application note AN826

(

http://www.vishay.com/doc?72286

CONCLUSION

Vishay Siliconix current-sensing power MOSFETs enable

implementation of a simple solution for incorporating

supervisory protection features such as overcurrent and/or

short circuit. This approach offers the freedom and flexibility of

control-circuit design, though the accuracy of measurement is

not suitable for current-control applications. Virtually any

power MOSFET from the Vishay Siliconix product range can

be supplied with a current-sensing feature.

Электронный компонент: SI6862DQ