AN606 Datasheet, PDF (1/4 Pages) Vishay Siliconix – Current-Sensing Power MOSFETs

AN606
Vishay Siliconix
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 D²PAK with five
pins. The MOSFET termination retains the standard D²PAK
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.
12345
(1)
G
(2)
SENSE
(4)
KELVIN
S (5)
G DS
SENSE
KELVIN
N-Channel MOSFET
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 D²PAK
(TO-263) MOSFET. However, pin-out modification is required to
incorporate current-sense (pin 2) and Kelvin-to-source (pin 4)
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.
Current Sensing Ratio
Mirror Active Resistance
TABLE 1: Current Sense Characteristics
r
r_m(on)
I_D = 1 A, V_GSS = 10 V, R_SENSE = 1.1 W
V_GS = 10 V, I_D = 10 mA
420
520
620
3.5
W
Document Number: 71991
17-Dec-03
www.vishay.com
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