t sensing techniques.
Philips Semiconductors
AN10322_1



Current Sensing Power MOSFETs
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Rev
Date
Description
01 20040909
Initial
version




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© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.00 09 September 2004
2 of 10
Philips Semiconductors
AN10322_1



Current Sensing Power MOSFETs
1. Introduction
Current sensing power MOSFETs provide an effective means of protecting automotive
electronic circuits from over current conditions. They offer an almost lossless method of
measuring the load current that eliminates the need for a current shunt resistor.

Philips has developed a range of current senseFET products to address the automotive
market as shown in Table 1. All are based on our low Rdson TrenchMOS
TM
PowerMOS
technology.

Table 1:
Philips SenseFET range

Device
Rdson max (m)
Sense Ratio
BUK7105-40AIE 5
500:1
BUK7905-40AI 5
500:1
BUK7C06-40AITE 6
560:1
BUK7108-40AIE 8
500:1
BUK7107-55AIE 7
500:1
BUK7C08-55AITE 8
500:1
BUK7109-75AIE 9
500:1
BUK7C10-75AITE 10
500:1



2. Principle of Operation
Current SenseFET technology is dependent on the close matching of transistor cells
within the powerMOS. TrenchMOS
TM
are made up of many thousands of transistor cells
in parallel. Elements within the device are identical, and the drain current is shared
equally between them. The more cells that are in parallel for a given chip area the lower
the on-state resistance of the MOSFET will be. This principle has been the key driving
force for automotive powerMOS for many years is well understood by both suppliers and
customers alike.

Furthermore, it is possible to isolate the source connections of several cells from those of
the majority, and bring them out onto a separate sense pin. The powerMOS can now be
thought of as two transistors in parallel with a common gate and drain but separate
sources (Figure 1). When the devices are turned on the load current will be shared in
ratio of their on-resistances.

The sense cells pass only a small fraction of the total load current in proportion to the
ratio of their areas. This ratio is typically 500:1.

The ratio of the current though the MainFET to the SenseFET is known as the sense
ratio, n. This ratio is defined for the condition where the source and sense terminals are
held at the same potential. An additional Kelvin connection to the source metallisation
enables accurate determination of the source potential.



© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.00 09 September 2004
3 of 10
Philips Semiconductors
AN10322_1



Current Sensing Power MOSFETs


Fig 1. SenseFET equivalent circuit


3. Virtual Earth current Sensing
The virtual earth technique gives the best performance in terms of accuracy and noise
immunity over the full temperature range of the device. This method is illustrated in
Figure 2.




Fig 2. Virtual Earth method
Fig 3. SenseFET response
+
-
OUT
LOAD
Rsense
Power Ground
Kelvin Source
Isense
Vbatt
Gate Drive
Vout
Isf
+
-
OUT
V-
V+
V-
V+
A/D
Microcontroller




© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.00 09 September 2004
4 of 10
Philips Semiconductors
AN10322_1



Current Sensing Power MOSFETs
Current feedback to the virtual earth of the op-amp ensures that the sense and Kelvin
terminals are held at the same potential so that the geometric ratio is measured.

n
R
I
Vsense
sense
D =

Equation 1
A typical response is shown in Figure 3. The sense signal closely mirrors the drain
current throughout the whole pulse duration.

Tolerances of
±
5% can be achieved with this circuit over temperature. The main
advantage of this method is that the sense signal is independent of temperature and
linearly proportional to load current. A negative supply is needed for the op-amp when
the MOSFET is used in the low side.


4. Sense Resistor current Sensing
The use of an external sense resistor in series with the sense pin offers a simpler
technique for monitoring the current through the device (Figure 4 & Figure 5).





Fig 4. Sense Resistor Circuit
Fig 5. Equivalent Model in the on state

+

-

OUT

LOAD

Rsense

R1

R2

Power Ground

Kelvin Source

Isense

Gate Drive

Vbatt

Vout
Isf

Vsense

Rmf(on)
Rsf(on)
Rsense
Rwire
Rd
Id
ISENSE
KELVIN
DRAIN
SOURCE
Isf
Vsense

In figure 4 above, an external op-amp circuit is used to amplify the sense signal.

In Figure 5, the resistance of the FET is separated in to active and passive components,
again with a common drain resistance. The active channels are modelled by R
mf(on)
for
the MainFET carrying the majority of the current and R
sf(on)
for the senseFET. The
passive contribution from the wire resistance is denoted R
wire
.





© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.00 09 September 2004
5 of 10
Philips Semiconductors
AN10322_1



Current Sensing Power MOSFETs
The circuit operates as a potential divider with the following design equations.
sf
sense
I
R
R
R
Vout
)
(
1
2
=

Equation 2
)
(
)
(
on
sf
sense
sense
on
mf
D
sense
R
R
R
R
I
V
+
=

Equation 3
The inclusion of R
sense
increases the resistance of the mirror leg and the sense ratio now
becomes






+ =
)
(
1
'
on
sf
sense
R
R
n
n

Equation 4

The maximum voltage seen on the Isense terminal occurs when the sense resistor is
infinite i.e. open circuit condition. Known as the compliance voltage this equals
(
)
d
on
mf
on
mf
on
DS
R
R
R
V
+
×
)
(
)
(
)
(
.
Therefore the mirror terminal only samples a proportion of the full drain-source voltage.
Fortunately for low voltage PowerMOS the contribution from the drain resistance is a
small proportion of the total Rds(on) and so the compliance ratio is high. This will
deteriorate in higher voltage devices.

As an example the BUK7905-40AIE has Rmf(on) = 3mOhm , Rsf(on) = 1.1Ohm and
nominal sense ratio of 500:1 . If R
sense
= 1 when 10A load current flows through the load
this will generate Vsense 14mV. In the low side a single rail amplifier can be used to
amplify this signal up to a more useful level.

The Kelvin connection to source is essential for accurate current sensing. Otherwise
voltage drops over the wire resistance that are caused by load current will add to the
sense voltage and introduce a source of error. In the past this was less of an issue as a
wire resistance of 2m

was only a small fraction of a 200m mainFET. But modern
PowerMOS products can have on-resistances as low as 1m
,
comparable with the
parasitic resistances. Referencing to the Kelvin pin eliminates the wire contribution.

The main disadvantage of the sense resistor technique is that the inclusion of Rsense
introduces a temperature dependence.

Imagine the case where Rsense 0 . The on-resistance of both the mainFET and
senseFET track together over temperature and the ratio of the two remains constant. In
this case the current sense ratio will also remain constant over temperature.
Conversely, if Rsense approaches infinity, the sense voltage now becomes
)
(
)
(
T
R
I
V
on
mf
D
sense
=
(Equation 3), and will follow Rmf(on) over temperature. The
mainFET on-resistance almost doubles between 25C and 175C thus eroding the
tolerance of the measurement. For values between the two a balance must be struck
between signal magnitude and accuracy. We normally recommend maintaining
)
(on
sf
sense
R
R
<<
and amplifying the Vsense signal accordingly.

A typical response is shown in Figure 6.


© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.00 09 September 2004
6 of 10
Philips Semiconductors
AN10322_1



Current Sensing Power MOSFETs




Fig 6. SenseFET response in resistor circuit

Looking at the trace above it appears that the senseFET reacts instantaneously to
changes in the drain current, and the sense signal contains false peaks at turn-on and
turn-off. These are due to a difference in current ratio between the linear and fully
enhanced operating regions and are circuit related.

This becomes clearer if we evaluate the response in terms of an effective sense ratio, n,
which is dependent on the ratio of
)
(on
sf
sense
R
R
(Equation 4). Lets assume that Rsense
= 1 .

At t0 the gate-source voltage is zero and no current flows. Once a gate voltage is applied
a channel forms and current begins to flow through the device, although the on-
resistance is still very high. At this point
and the geometric sense ratio,
n is measured. This will yield the maximum sense signal at time t1. As the gate is
overdriven the on-resistance of the senseFET falls and the
sense
on
sf
R
R
>>
)
(
)
(on
sf
sense
R
R
factor becomes
more significant (Figure 7). This continues until the final Vgs level and sense ratio are
reached (t2). During turn-off the process is reversed. Note that if Rsense = 0 no false
peaks are observed and the sense signal is akin to the virtual earth response (Figure 3).




© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.00 09 September 2004
7 of 10
Philips Semicondu