Bi-directional MOS current sense circuit

A current sensing circuit comprises a power device adapted to conduct a bidirectional current between first and second terminals thereof, first and second sensing devices operatively coupled to the power device, a sense amplifier providing first and second voltages to the first and second sensing devices, and a gate drive device providing activating signals to the power switching device and the first and second sensing devices. The first and second sensing devices each has an active area that is substantially identical and significantly smaller than a corresponding active area of the power switching device. The sense amplifier measures the voltage of the first sensing device and maintains the voltage on the second sensing device at the same level as the first sensing device by injecting an additional current into the second sensing device. The sense amplifier further provides an output signal proportional to the bidirectional current. The first and second sensing devices have k times higher resistance than a corresponding resistance of the power device when in an active state.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to voltage regulator circuits, and more particularly to circuits for measuring the bidirectional current through a switching device of a switched mode voltage regulator circuit.

2. Description of Related Art

Switched mode voltage regulators (also known as switched mode power converters) are known in the art to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode voltage regulator provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. A buck converter is one particular type of switched mode voltage regulator that includes two power switches that are typically provided by MOSFET transistors. The power switches are referred to individually as the high side switch and the low side switch, corresponding to their placement within the buck converter as referenced to the voltage source and ground, respectively. A filter capacitor coupled in parallel with the load reduces ripple of the output current. A pulse width modulation (PWM) control circuit is used to control the gating of the power switches in an alternating manner to control the flow of current in the output inductor. The PWM control circuit uses feedback signals reflecting the output voltage and/or current level to adjust the duty cycle applied to the power switches in response to changing load conditions.

It is known to measure the output current level by sensing the current passing through a resistor coupled to the load. The voltage across the sense resistor is detected using a sense amplifier to produce a signal corresponding to the output current. This type of current sense circuit has the drawback of reducing the efficiency of the voltage regulator by the voltage drop across the sense resistor. Alternatively, it is known to use one of the power switches as a sense resistor and detect the voltage drop across the internal resistance between drain and source of the MOS device (RDSON). This alternative approach overcomes the efficiency reduction caused by a sense resistor. Nevertheless, since the current through the power device is bidirectional, it is often difficult or impractical to measure the bi-directional current.

FIG. 1shows an exemplary circuit10to measure the current IPthrough an MOS power device12having an active area A. A second MOS device14having an active area A/k is used to split the load current. Gate driver16provides the pulse modulated signal to activate the power device12and the second device14. An operational amplifier20has a non-inverting terminal coupled to the source of the power device12and an inverting terminal coupled to the source of the second device14. The operational amplifier20includes a feedback resistor18coupled between the inverting terminal and output terminal. The operational amplifier20maintains the source voltage of the second device14at the same level as the power device12, such that the current through the second device14is IP/k. The output terminal of the operational amplifier20provides sense voltage Vsensethat is proportional to the load current IP. Both directions of current IPcan be measured with the circuit, but it should be appreciated that the sense voltage VSensewill be negative with respect to the source terminal of the power device12for positive load currents IP. This requires an auxiliary negative power supply for the operational amplifier20, which is in many cases unavailable or costly.

Other known current sense circuits are capable of measuring a bi-directional current through a high side shunt resistor without the need for auxiliary power supplies. But, these known circuits are not suited to measure the current of a MOS power device that is continuously turned on and off. Still other known current sense circuits can measure the current through a power switch, while also suffering from limited linear operating range. These circuits are also undesirable because they require sensing devices that are scaled much larger than necessary to avoid measuring errors.

Thus, it would be advantageous to provide a bidirectional current sensing circuit for a power device that has wide linear operating range, minimal matching requirements, and fast response.

SUMMARY OF THE INVENTION

The present invention overcomes the deficiencies of the prior art by providing a current sensing circuit that measures bidirectional current through a power switching device without the need for an auxiliary negative power source.

More particularly, the current sensing circuit comprises a power device adapted to conduct a bidirectional current between first and second terminals thereof, first and second sensing devices operatively coupled to the power device, a sense amplifier providing first and second voltages to the first and second sensing devices, and a gate drive device providing activating signals to the power switching device and the first and second sensing devices. The first and second sensing devices each has an active area that is substantially identical and significantly smaller than a corresponding active area of the power switching device. The sense amplifier measures the voltage of the first sensing device and maintains the voltage on the second sensing device at the same level as the first sensing device by injecting an additional current into the second sensing device. The sense amplifier further provides an output signal proportional to the bidirectional current. The first and second sensing devices have k times higher resistance than a corresponding resistance of the power device when in an active state.

In an embodiment of the invention, the sense amplifier comprises an operational amplifier having a first input terminal coupled to the first sensing device and a second input terminal coupled to the second sensing device, a feedback transistor coupled between the first input terminal and an output of the operational amplifier, and first and second resistors coupled to the first and second input terminals, respectively. The first and second resistors may be provided by first and second matched CMOS transistors. In another embodiment of the invention, the sense amplifier comprises plural CMOS transistors.

A more complete understanding of the bidirectional current sensing circuit for a power device will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a bidirectional current sensing circuit for a power device that has wide linear operating range, minimal matching requirements, and fast response. In the detailed description that follows, like element numerals are used to describe like elements illustrated in one or more figures.

FIG. 2depicts a current sensing circuit40in accordance with an embodiment of the invention. The current sensing circuit40is divided into four parts, including: (1) the power device42having an active area A through which current is to be measured; (2) a pair of MOS sensing devices44,46of the same type as the power device42, but with each having a much smaller active area A/k; (3) a sense amplifier including operational amplifier48, MOS device52, and loading resistors54,56; and (4) a gate drive device58. The gate drive device58applies a gate voltage to the gate terminals of power device42and sensing devices44,46in accordance with a determined duty cycle to control their on/off states. The sensing devices44,46have their drain terminals coupled respectively to the drain and source of the power device42. The power device42is assumed to operate in the triode region, i.e., the device characteristic can be approximated by a low resistor with value RQPwhen in the on state. The sensing devices44,46are also operated in the triode region and therefore can be assumed to have k times higher resistance (RQ1, RQ2) than power device42when turned on. In the embodiment ofFIG. 2, the power device42corresponds to the low side switch of a switched mode power converter.

The operational amplifier48has a non-inverting terminal coupled to a first voltage node (Vp) and an inverting terminal coupled to a second voltage node (Vn). The first voltage node Vpis coupled to the source terminal of sensing device44and to the drain terminal of power device42through resistor56. The second voltage node Vnis coupled to the source terminal of sensing device46and to the drain terminal of power device42through resistor54. MOS device52provides a feedback path for operational amplifier48, with the operational amplifier output driving the gate terminal of the MOS device52and the drain terminal of MOS device52coupled to the second voltage node Vn. A first current source I1is defined between supply voltage VDD and first voltage node Vp, and a second current source I2is defined between supply voltage VDD and source terminal of MOS device52.

In operation, the operational amplifier48maintains the first voltage node Vpat the same level as the second voltage node Vnby injecting current Ininto the node Vn. The second node voltage is determined as follows:

Vn=R2·RQ⁢⁢2R2+RQ⁢⁢2·(Ioffset+Iout)
in which R2is the resistance of resistor54and RQ2is the drain-source resistance of sensing device46. If the drain-source resistance of the power device42(RQP) is much less than the drain-source resistance of sensing device44(RQ1), then the positive node voltage is determined as follows:

Accordingly, the feedback loop of operational amplifier48through MOS device52maintains Vnequal to Vp, and with resistors54,56equal and the drain-source resistances of sensing devices44,46equal, the foregoing two equations will be equal and can be simplified to:
Iout·RQ2≈IP·RQP
and with
RQ2=k·RQP
the equation further simplifies to:

Iout≈Ipk
In other words, the current Ioutis proportional to the current IPthrough the power device42. This equation is valid for positive and negative currents of Ipas long as the current Inremains positive. In the case in which Ioffsetis chosen to be larger than the maximum absolute value of Ip/k, the voltages Vpand Vnwill also remain positive. It should be appreciated that this simplifies the design of the operational amplifier48and eliminates the need for a negative auxiliary supply for the operational amplifier. When the power device42is off, Ioutwill be equal to zero since R1equals R2and the feedback loop maintains Vpequal to Vn.

In an embodiment of the invention, the gate drive device58applies a gate voltage simultaneously to the gate terminals of power device42and sensing devices44,46. Alternatively, the gate drive device58may apply the gate voltage to the sensing devices44,46after a certain amount of delay following application of the gate voltage to the power device42. This delay period would ensure that the power device42is on before activating the sensing devices44,46, and thereby serve to avoid any initial voltage spikes in the measuring current.

FIG. 3depicts an alternative current sensing circuit60that provides bi-directional current sensing within a CMOS process. As in the preceding embodiment, the circuit includes power device62having an active area A through which current is to be measured, and a pair of MOS sensing devices64,66of the same type as the power device62, but with each having a much smaller active area A/k. The resistors54(R2),56(R1) are replaced by CMOS transistors70,68operated in the triode region. The operational amplifier48is replaced by CMOS transistors74,72forming a simple amplifier circuit, with transistor78providing a feedback loop. Gate drive device76applies a gate voltage to the gate terminals of power device62and sensing devices64,66in the same manner as described above. In the embodiment ofFIG. 3, the power device62corresponds to the low side switch of a switched mode power converter.

As in the preceding embodiment, a first voltage node (Vp) is coupled to the source terminal of sensing device64and to the drain terminal of power device42through the drain-source resistance of transistor68. A second voltage node Vnis coupled to the source terminal of sensing device66and to the drain terminal of power device62through the drain-source resistance of transistor70. CMOS transistors74,72have respective current sources providing a bias current to source terminals thereof and to the gate of feedback transistor78. Current source I1provides offset current to the first voltage node Vp, and current source I2provides offset current to the drain terminal of MOS device78, which is in turn connected to the second voltage node Vn. The operation of the current sensing circuit60is generally the same as the embodiment of theFIG. 2.

From the equations derived above, it should be appreciated that R1needs to only match R2, and that RQ1needs to match RQ2and RQP. Therefore, transistors68,70do not have to be the same type of devices MOS as sensing devices64,66or power device62. For example, transistors68,70may be low voltage devices (e.g., sustaining only 5 volts), and MOS sensing devices64,66and power device62may be devices that sustain higher voltage (e.g., 20 volts). Since power device62may in some applications be formed of an array of transistors connected in parallel, it would be advantageous to use two of the transistors of the array to form MOS sensing devices64,66in order to achieve optimal matching. Since the active area of the MOS sensing devices64,66is k times smaller (e.g., k equal to 100,000), the impact on the resistivity of the power device62would be minimal. It may also be advantageous to replace CMOS transistors72,74with bipolar devices to minimize the offset voltage of the amplifier. This would further improve the measuring accuracy of the overall circuit.

FIG. 4depicts an alternative current sensing circuit80that provides bi-directional current sensing within a CMOS process. In the embodiment ofFIG. 4, the power device82corresponds to the high side switch of a switched mode power converter, with the current sensing circuit80providing a floating ground. As in the preceding embodiment, power device82has an active area A and MOS sensing devices84,86each have a much smaller active area A/k. CMOS transistors90,88operate in the triode region to provide the resistors R1, R2. CMOS transistors94,92provide the amplifier circuit, with transistor96providing a feedback loop. Gate drive device98applies a gate voltage to the gate terminals of power device92and sensing devices94,96in the same manner as described above.

Unlike the preceding embodiments, the orientation of the MOS sensing devices84,86is reversed such that their source terminals are coupled to the drain and source of power device82, respectively. Likewise, the orientations of CMOS transistors90,88, and94,92are reversed in contrast to the preceding embodiment. Accordingly, a first voltage node (Vp) is coupled to the drain terminal of sensing device86and to the source terminal of power device82through the drain-source resistance of transistor88, and a second voltage node (V,nis coupled to the drain terminal of sensing device84and to the drain terminal of power device82through the drain-source resistance of transistor90. The current sources I1, I2, IB1, IB2are each referenced to ground. Otherwise, the circuit operates substantially as in the preceding embodiments. It should be appreciated that exemplary power device82is illustrated in this and the preceding embodiments as being an NMOS power device, although it should be appreciated that the circuit could be readily adapted by persons having ordinary skill in the art for use with a PMOS power device.

Having thus described a preferred embodiment of a circuit for measuring the bi-directional current through a switching device of a switched mode voltage regulator circuit, it should be apparent to those skilled in the art that certain advantages of the system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is solely defined by the following claims.