Current Sensing Power
Current sensing power MOSFETs provide a highly
effective way of measuring load current in power
conditioning circuits. Conceptually simple in nature, these
devices split load current into power and sense
components, and thereby allow signal level resistors to be
used for sampling. Since this technique results in higher
efficiency and lower costs than competing alternatives,
understanding how to use SENSEFET product is an
important design issue.
Getting accustomed to these devices is relatively, but not
completely, straightforward. They are conceptually simple,
but have their own unique set of characteristics and subtle
properties. The following discussion examines both, and
starts with a description of how SENSEFET devices work.
Principle of Operation
Their operation is based on the matched devices
principle that is so commonly used in integrated circuits.
Like integrated circuit transistors, the on-resistance of
individual source cells in a power MOSFET tends to be
well matched. Therefore, if several out of several thousand
cells are connected to a separate sense pin, a ratio between
sense section on-resistance and power section
on-resistance is developed. Then, when the SENSEFET
device is turned on, current flow splits inversely with
respect to the two resistances, and a ratio between sense
current and source current is established.
The separate source connection is called a mirror.
Typically SENSEFET product is designed such that the
ratio between mirror cells and source cells is on the order
of 1:1000 Schematically, this looks like two parallel FETs
with common gate and drain connections, but separate
source leads. An illustration of this configuration appears
in Figure 1. The relative size of the two devices determines
how current is split between source and mirror terminals.
The ratio of source current to mirror current is specified by
n, the “Current Mirror Ratio”. This ratio is defined for
conditions where both source and mirror terminals are held
at the same potential. Since n is on the order of 1000:1, load
current is approximately equal to source current, and the
current mirror ratio also describes the ratio of load current
to sense current.
Figure 1. SENSEFET Equivalent Circuit
When a signal level resistor is connected between mirror
and source terminals, a known fraction of load current is
sampled without the insertion loss that is associated with
power sense resistors. For this reason, the technique of
measuring load current with SENSEFET devices is called
“lossless current sensing”. As long as the sense resistor is
less than 10% of the mirror section’s on-resistance
RDM(on), the current that is sampled is approximately load
current divided by the current mirror ratio or ILOAD/n. In
practice, the amount of sense voltage that is developed with
such low values of sense resistance is usually not sufficient
to drive current limiting circuits. Therefore, larger values
of RSENSE are normally used. These larger values appreciably
affect the total resistance in the mirror leg, and therefore,
alter the current mirror ratio. How to model this behavior
and calculate sensing parameters is discussed as follows.
Calculating Sense Resistance
With the aid of the model that is shown in Figure 2,
calculating sense voltage and sense resistance is very
straightforward. In this model, RDS(on) is separated into
bulk and active components. Bulk drain resistance is
common to the entire device, and is represented by Rb.
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© Semiconductor Components Industries, LLC, 2002
March, 2017 − Rev. 6
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