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Trends of resistor for current sensing |
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Resistance is decreased to measure
an increased current. The resistance may be less than 2m. Some additional consideration
is necessary for applications of such resistors.
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When the Resistance
is very low,
it becomes difficult to "sense the voltage precisely proportional to the
current that flows through a resistor", which is a fundamental function
of a resistor, when the switching frequency exceeds 100KHz.
(1)Minimize the inductance of resistor and PCB pattern
Parasitic inductance on a resistor normally ranges around several nH and it is
negligibly small for frequencies up to several 100KHz. But for an extremely low
value resistor, minute inductance affects relatively large impedance to cause
an error to the resistance. |
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Example of the error by inductance
An example is shown below. Parasitic inductance is assumed to be 1 nH in this
figure.
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Trimming disturbs the current flow
A resistor is trimmed to form a slit to adjust resistance. This may cause a non-uniformity
to the current flow, and may result in a heat spot. Local heating may aggravate
the linearity of resistance vs. current. |
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(2)Resistivity
should be uniform;
Not only a trimming line but voids at the junction with the resistor and the
electrode might cause non-uniformity of current density.
Tolerance of parasitic inductance might result in....
An error in the measurement, while error compensation may be possible for a fixed
value. Influence from inductance can be contained in some degree by the use of
filter. But scatter in parasitic inductance will also result in scatter of detected
current values. In the figures below, simulations of the scatter of detected
current values are shown.
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(3)Electric potential
across electrodes must be consistent;
The ratio of resistivity of electrode to that of resistive material is as small
as several dozens for ultra low resistance. When the electrode is not thick enough,
it cannot keep equi-potential regardless of the non-uniformity of current density
inside the resistor. This means depending on where in the electrode voltage is
extracted, resistance value varies. |
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Selection guide
for current sensing resistor:
From the above discussion, current sensing resistor for large current should
be:
(1) Minimum parasitic inductance and minimum tolerance
(2) Uniform structure of resistor and electrode
(3) Minimum potential difference across electrode
Uniform the current flow inside the resistor
Above conditions are achieved by completely getting rid of factors spoiling uniformity.
TLR, which is a metal plate type resistor, is the product developed under this
concept.
Actual structure of TLR
The structure of TLR is shown. It has a flat resistive metal plate with bulk
electrodes on it. Trimming is given not by slitting but by shaving the width
of the plate as to keep uniformity. Thus the TLR is suitable to sense the current
of DC/DC converters.
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Additional comments:
The following considerations are still more necessary:
(1)Fluctuation of detection in multi-phase DC/DC converter
(2)PCB pattern design including current and voltage pads
(3)Heat dissipation
(4)Filter to minimize sensing errors
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Current Detecting Resistor TLR |
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Application Notes
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