Methods and apparatus for current sensing
View Patent ↗Methods and apparatus for current sensing according to various aspects of the present invention operate in conjunction with a current sensor adapted to sense the current provided to a load via a transistor. In one embodiment, a power supply includes the current sensor, and supplies the current through a series combination of an inductor and the transistor. The current sensor is adapted to generate the inductor current signal according to a sensed current in the inductor, and may comprise a series combination of a first resistor and a second resistor. The resistive series combination is adapted to be connected in parallel to the series combination of the inductor and the transistor.
1. A power supply for supplying a current through series combination of an inductor and a transistor, comprising:
a control system adapted to control the current according to an inductor current signal;
a current sensor connected to the control system, wherein the current sensor:
is adapted to generate an inductor current signal according to a sensed current in the inductor; and
comprises a series combination of a first resistor and a second resistor, wherein the series combination is adapted to be connected in parallel to the series combination of the inductor and the transistor; and
a differential amplifier comprising a first input adapted to be connected to a node between the inductor and the transistor and a second input connected to a node between the first resistor and the second resistor.
2. A power supply according to claim 1 , wherein a ratio of a resistance of the first resistor to a resistance of the second resistor substantially matches a ratio of a lead inductance of the transistor to an inductance of the inductor.
3. A power supply according to claim 1 , wherein the current sensor further comprises a third resistor comprising a first terminal adapted to be connected to a terminal of the transistor and a second terminal connected to the control system, wherein a resistance of them third resistor is selected according to a selected range of inductor current signal values to represent a selected range of sensed currents.
4. A power supply according to claim 1 , wherein the current sensor further comprises a third resistor, wherein:
the first input of the amplifier is adapted to be connected to the node between the inductor and the transistor via the third resistor; and
a resistance of the third resistor is selected according to a scaled range of output signals corresponding to a selected range of sensed current.
5. A power supply according to claim 1 , further comprising a second transistor having a first terminal connected to an output of the amplifier, wherein the second transistor is adapted to generate the inductor current signal according to a signal from the amplifier.
6. A power supply according to claim 1 , wherein the control system comprises a PWM controller.
7. A power supply according to claim 1 , wherein a voltage Vp at a node between the first resistor and the second resistor is configured to be:
V
P
=
R
L
1
R
L
1
+
R
L
2
×
V
out
where R L1 is a resistance of the first resistor, R L2 is a resistance of the second resistor, and V out is an output voltage of the series combination of the inductor and the transistor.
8. A power supply according to claim 1 , wherein a voltage V N at a node between the inductor and the transistor is configured to be:
V
N
=
L
lead
L
lead
+
L
buck
×
V
out
-
R
FET
×
I
out
where L lead is a lead inductance of the transistor, L buck is an inductance of the inductor, R FET is a resistance of the transistor, I out is the current, and V out is an output voltage of the series combination of the inductor and the transistor.
9. A power supply according to claim 1 , wherein the inductor current signal is proportional to the sensed current independent of an absolute value of the inductor and an absolute value of a lead inductance of tile transistor.
10. A power supply according to claim 1 , wherein the inductor current signal is proportional to the sensed current independent of a regulated output voltage of the power supply.
11. A current sensor for providing an inductor current signal corresponding to a sensed current in a series combination of an inductor and a synchronous MOSFET during an ON state of the MOSFET, comprising:
a first resistor comprising a first terminal connected to a source terminal of the MOSFET;
a second resistor having a first terminal connected to a second terminal of the first resistor and a second terminal connected to the inductor; and
a differential amplifier comprising:
a first input connected to a node between the first resistor and the second resistor; and
a second input adapted to be connected to a drain terminal of the MOSFET,
wherein the inductor current signal is proportional to the sensed current independent of an absolute value of the inductor and an absolute value of a lead inductance of the MOSFET.
12. A current sensor according to claim 11 , wherein a ratio of a resistance of the first resistor to a resistance of the second resistor substantially matches a ratio of a lead inductance associated with the MOSFET to an inductance of the inductor.
13. A current sensor according to claim 11 , wherein the current sensor further comprises a third resistor comprising a first terminal adapted to be connected to the drain terminal of the MOSFET, wherein a resistance of the third resistor is selected according to a selected range of inductor current signal values to represent a selected range of sensed currents.
14. A current sensor according to claim 11 , wherein the current sensor further comprises a third resistor, wherein:
the first input of the amplifier is adapted to be connected to the drain terminal of the MOSFET via the third resistor; and
a resistance of the third resistor is selected according to a scaled range of output signals corresponding to a selected range of sensed current.
15. A current sensor according to claim 14 , further comprising a transistor having a control terminal connected to an output of the amplifier, wherein the transistor is adapted to generate the inductor current signal according to a signal from the amplifier.
16. A current sensor according to claim 11 , wherein a voltage V p at the node between the first resistor and the second resistor is configured to be:
V
P
=
R
L
1
R
L
1
+
R
L
2
×
V
out
where R L1 is a resistance of the first resistor, R L2 is a resistance of the second resistor, and V out is an output voltage of the series combination of the inductor and the MOSFET.
17. A current sensor according to claim 11 , wherein a voltage V N at the drain terminal of the MOSFET is configured to be:
V
N
=
L
lead
L
lead
+
L
buck
×
V
out
-
R
FET
×
I
out
where L lead is a lead inductance of the MOSFET, L buck is an inductance of the inductor, R FET is a resistance of the MOSFET, I out is the sensed current, and V out is an output voltage of the series combination of the inductor and the MOSFET.
18. A method of generating all inductor current signal corresponding to a current in a series combination of an inductor and a transistor, comprising:
connecting a series combination of a first resistor and a second resistor in parallel with the series combination of the inductor and the transistor;
selecting resistance values for the first resistor and the second resistor according to a lead inductance associated with the transistor and an inductance of the inductor, wherein a ratio of a resistance of the first resistor to a resistance of the second resistor substantially matches a ratio of the lead inductance of the transistor to the inductance of the inductor;
receiving a first signal at an amplifier from a node between the first resistor and the second resistor;
receiving a second signal at the amplifier from a node between the inductor and the transistor; and
generating the inductor current signal according to a difference between the first signal and the second signal.