Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Reexamination Certificate
2003-05-21
2004-10-05
Zarneke, David A. (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
C324S1540PB, C323S316000
Reexamination Certificate
active
06801030
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a current sense apparatus and method, and more particularly, to a temperature independent current sense apparatus and method.
BACKGROUND OF THE INVENTION
Multi-phase DC-to-DC converter has been widely used in power supplier circuits. A multi-phase buck converter typically employs a pair of MOSFETs connected in series for each phase as an output stage connected between a high voltage and a low voltage to produce a phase output. To obtain stable and balanced output, the output voltage and phase currents of a converter are sensed and fed back to the control circuit of the converter to produce the suitable control signals to manipulate the MOSFETs of the output stage. To feed back the current of each phase, a current sense apparatus is used to detect the current flowing through the phase, for example a scheme provided by U.S. Pat. No. 6,246,220 issued to Isham et al. producing the current sense signal by use of a current feedback resistor to feed back to the control circuit. Since the control of each phase is achieved by referring to the phase current detected by a current sense apparatus, the accuracy of the current sense apparatus will directly affect the phase balance and performance of the converter. However, the introduced resistors will affect the phase current, and unfortunately, the factors of electronic devices are temperature dependent, especially the resistances or transistors made of semiconductor. The increasing working temperature not only produces signal error but also brings the phase at higher temperature further sharing more currents, and thus leads to be burnt out.
Various conventional current sense apparatus used in synchronous switching mode buck converters are shown in FIG.
1
. In
FIG. 1A
, a sense resistor
76
is introduced to be connected in series between the input voltage VIN and high side MOSFET
72
, and the produced voltage drop further produces a current sense signal by an operational amplifier
25
. In
FIG. 1B
, the sense resistor
76
is connected in series between a ground and the low side MOSFET
74
, and the operational amplifier
25
detects the voltage drop across the resistor
76
to produce the current sense signal. Both of them introduce the additional sense resistor
76
, and thus increase the cost and reduce the system efficiency. In
FIG. 1C
, the operational amplifier
25
directly detects the voltage drop across the conductive high side MOSFET
72
to produce the current sense signal. In
FIG. 1D
, the operational amplifier
25
directly detects the voltage drop across the conductive low side MOSFET
74
to produce the current sense signal. Both of them utilize the internal resistance of the MOSFET
72
or
74
as the sense resistor, and thus need not more cost for the sense resistor. However, the internal resistance of MOSFET varies with temperature, and the varied rate is about 5000 ppm, it is therefore not accurate of the measured current sense signal. In
FIG. 1E
, the parasitic resistor
78
of the output inductor
23
is used as the sense resistor, and it can be treated as connected in series between the inductor
23
and converter output
70
. The operational amplifier
25
detects the voltage drop across the parasitic resistor
78
to produce the current sense signal, while the resistance of the parasitic resistor
78
is too small and hard to control. In
FIG. 1F
, the sense resistor
76
is connected in series between the inductor
23
and converter output
70
, and the operational amplifier
25
detects the voltage drop across the resistor
76
to produce the current sense signal. This method introduces an additional resistor, and hence higher cost and poor system efficiency.
FIG. 6
shows a converter employing a conventional current sense apparatus as that in
FIG. 1D
, and only one phase is shown for simplicity. The operational amplifier
25
detects the voltage drop across the conductive low side MOSFET
74
and the produced current sense signal is connected to a sampling/holding circuit
50
that is also connected to the non-inverting input
302
of the error amplifier
30
. Additionally, a voltage follower
32
connected with an original reference voltage REF produces a reference voltage to the node between resistor
34
and capacitor
36
. The other terminal of the resistor
34
is connected to the non-inverting input
302
of the error amplifier
30
. The inverting input
301
of the error amplifier
30
is connected with the output voltage VOUT, and a feedback signal
303
and the output of the sampling/holding circuit
50
are connected to the control logic
40
together to manipulate the output stage circuit, i.e., MOSFETs
72
and
74
. Due to the current sense signal relating to the internal resistance of the MOSFET
74
, which is temperature dependent, the current sense signal will change with temperature and result in error. Moreover, the converter output varies when load
60
changes, as shown in FIG.
7
.
FIG. 7A
shows the waveforms of the converter output at low temperature, of which the upper one shows the transient performance of the variation A lout of the converter output current lout resulted from load variation, and the lower one shows the ripple performance of the converter output voltage VOUT induced by this transient effect.
FIG. 7B
shows the waveforms of the converter output at high temperature. For the same load variation, the droop VDROOP of the converter output voltage VOUT is smaller at high temperature than that at low temperature. In other words, the performance of a converter is much affected by temperature.
FIG. 8
shows a curve of the internal resistance of MOSFET to temperature variation. When temperature rises, the internal resistance of MOSFET also becomes larger, and therefore all operations incorporating the utilization of the internal resistance of MOSFET are affected by temperature.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a current sense apparatus and method for temperature independent current sense.
Another object of the present invention is to provide a multi-phase switching mode DC-to-DC converter and method thereof incorporating a temperature independent current sense apparatus and method for balance control between each phase of the converter.
A further object of the present invention is to provide a multi-phase switching mode DC-to-DC converter and method thereof, whose control to voltage droop resulted from load variation is temperature independent by incorporating a temperature independent current sense apparatus and method.
The invented current sense apparatus and method uses a common drain DMOSFET and a MOSFET serving as an output stage connected between a high voltage and a low voltage, and connects the current and voltage sense outputs from the DMOSFET to a servo amplifier. When a phase output current is flowing through the DMOSFET, a mirror current mirrored from the phase output current and a sense voltage are produced, and the servo amplifier is connected with the mirror current and sense voltage to produce a current sense signal. Due to the mirror current from the DMOSFET proportional to the phase output current, the current sense thus obtained is temperature independent. Application of the current sense apparatus to a multi-phase switching mode DC-to-DC converter will make the performance of the converter temperature independent.
REFERENCES:
patent: 5021730 (1991-06-01), Smith
patent: 6246220 (2001-06-01), Isham et al.
patent: 6414470 (2002-07-01), Liu et al.
Hsieh Shwu-Liang
Liu Jing-Meng
Tai Liang-Pin
Wang Hung-I
Nguyen Tung X.
Richtek Technology Corp.
Rosenberg , Klein & Lee
Zarneke David A.
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