Current-limited switch with fast transient response

Details Current-limited switch with fast transient response Current-limited switch with fast transient response

2001-08-21

2002-10-15

Nguyen, Matthew (Department: 2838)

Electricity: power supply or regulation systems

Self-regulating

Using a three or more terminal semiconductive device as the...

C323S315000, C323S273000

Reexamination Certificate

active

06465999

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to power MOSFET switches and in particular to a power MOSFET switch that has the capability of limiting the current that passes through the switch when the load becomes short-circuited.
BACKGROUND OF THE INVENTION
Power MOSFETs are widely used as switches in a variety of applications, including laptop computers, cellular phones and the like. Many of these products have internal circuit elements that are very sensitive to overcurrent conditions. If one element in the circuit becomes short-circuited, the resulting increase in current through the circuit may damage or destroy remaining elements in the circuit. For example, in a computer Universal Serial Bus (USB) application, there is a risk that if the user short-circuits the USB port the short-circuit will propagate back through the computer and damage other systems within the computer. It is therefore desirable to provide the MOSFET switch with a current-limiting capability that senses an overcurrent condition and closes the switch sufficiently that the current does not reach levels that will damage any of the internal components of the product.
Ideally, a MOSFET switch would have a very low on-resistance and would respond very quickly to an overcurrent condition by limiting the short-circuit current to a predetermined level. Such a switch would be highly efficient as a power supply and would protect upstream systems from short-circuit damage. The response time is particularly important because the longer the circuit is exposed to the overcurrent condition, the greater the likelihood of damage. The systems to be protected must inevitably be overdesigned to some extent to withstand the current pulse that occurs before the current-limiting circuitry is able to operate, and this leads to extra cost and weight. A fast response time in effect minimizes the amount of overdesign necessary.
In many current-detection circuits a “pilot” circuit is connected in parallel with the circuit to be monitored, and the current through the pilot circuit is detected. Such a prior art circuit is shown in FIG.
1
. The current through power MOSFET
10
(Iout) is mirrored by the current through pilot MOSFET
18
. A pilot resistor
26
is connected in the pilot circuit. The gate width of power MOSFET
10
is much larger than the gate width of pilot MOSFET
18
, the ratio of the gate widths being defined as “m” or as the scaling factor “SF” (m=SF). For example, if m=100, the impedance of MOSFET 18 is 100 times the impedance of MOSFET
10
, and the current through power MOSFET
10
should be 100 times the size of the current through pilot MOSFET
18
. Ideally, this ratio should remain the same regardless of the size of Iout, in which case the current through pilot MOSFET
18
accurately mirrors the current through power MOSFET
10
.
A reference current (Iref) is supplied through a reference resistor
30
, which is substantially equal to resistor
26
. A comparator
32
detects the difference between the voltage drops across pilot resistor
26
and reference resistor
30
, and when the voltage drops are equal comparator
32
delivers an output signal.
Iref
2
R
30
represents wasted energy (R
30
representing the size of resistor
30
), so it is desirable to increase the size of resistor
30
and reduce the size of Iref. For example, if R
30
is doubled, Iref can be reduced by one-half while obtaining the same voltage drop across resistor
30
. This requires, however, that the size of resistor
26
also be doubled, since R
26
≈R
30
. Increasing the size of resistor
26
(R
26
) increases the nonlinearity of the circuit, since the ratio of the currents through power MOSFET
10
and pilot MOSFET
18
becomes less constant as resistor
26
becomes larger. The current through the pilot MOSFET
18
thus becomes a less accurate “mirror” of the current through power MOSFET
10
.
The circuit shown in
FIG. 1
is discussed more fully in U.S. Pat. No. 5,867,014 to Wrathall et al., incorporated herein in its entirety.
This nonlinearity can be overcome by connecting a reference MOSFET
34
, equal in size to pilot MOSFET
18
, in parallel with resistor
30
and by driving the gate of reference MOSFET
34
in common with the gates of power MOSFET
10
and pilot MOSFET
18
, as shown in FIG.
2
. This arrangement provides an Iref that is equal to the current that would flow in the pilot circuit if resistor
26
were not present and proportional to the current through the power MOSFET
10
. Thus the ratio of the current through power MOSFET
10
to Iref is equal to the scaling factor (SF or m) and remains constant regardless of the size of the current through power MOSFET
10
. This allows large resistors to be used for pilot resistor
26
and reference resistor
30
without adversely affecting the linearity of the circuit. The circuit shown in
FIG. 2
is explained more fully in U.S. Pat. No. 4,820,968 to Wrathall et al., incorporated herein in its entirety.
Nonetheless, the limitations of transistor fabrication techniques limit the size of the scaling factor (the ratio of the gate widths of power MOSFET
10
and pilot MOSFET
18
), and therefore the size of Iref may still be larger than would be desirable to minimize energy losses. As is apparent from
FIG. 2
, Iref flows at all times, regardless of the state of power MOSFET
10
.
A solution to this problem is shown in
FIG. 3
, which represents the teaching of the above-referenced U.S. Pat. No. 5,867,014. Four reference MOSFETs
62
,
64
,
66
and
68
are connected in the reference circuitry. Each reference MOSFET is connected in parallel with a different reference resistor
70
,
72
,
74
and
76
. The circuit is similar to the circuit of
FIG. 2
except that four parallel MOSFET-resistor combinations similar to the parallel combination of MOSFET
34
-resistor
30
are connected in series. Each of MOSFETs
62
,
64
,
66
and
68
has electrical characteristics substantially similar to those of pilot MOSFET
54
. Thus, if the gate width of pilot MOSFET
54
is related to the gate width of power MOSFET
40
by the scaling factor SF=m, the gate width of each of MOSFETs
62
,
64
,
66
and
68
is also related to gate width of power MOSFET
40
by the factor m. Each of reference resistors
70
,
72
,
74
and
76
has an impedance equal to the impedance of pilot resistor
58
. The factor “n” represents the number of reference MOSFETs (i.e., in this case n=4).
It can be shown that, in the embodiment of FIG.
3
:
I
out=
I
ref·
m·n
Thus, for a given value of Iout, the size of Iref can be reduced by a factor of four in the circuit of
FIG. 3
as compared with the circuit of FIG.
2
.
The circuit of
FIG. 3
functions as a current detector but only when power MOSFET
40
is operating in its linear region.
A prior art circuit for limiting the load current in the event of a short-circuit is shown in FIG.
4
. The current through pilot MOSFET
82
is a predetermined percentage of the current through power MOSFET
80
. When there is no load current Iout, amplifier
88
biases MOSFET
90
off, and there is no current through the resistor Rset. When Iout increases as a result of a short in the load, the output of amplifier
88
controls MOSFET
90
so that MOSFET
90
gradually conducts more current. As MOSFET
90
begins to conduct, the current replica voltage SET increases and is delivered to the (+) input terminal of the current limit amplifier
86
. When the voltage SET exceeds an internal voltage Vref, the output of amplifier
86
reduces the current through power MOSFET
80
and MOSFET
82
. Because the feedback loop in this circuit contains two amplifiers, its response time to a short-circuit condition is rather slow. Moreover, the circuit does not limit Iout when the drain voltages of MOSFETS
80
and
82
(i.e., Vout) fall below Vref (about 1.2 V). When this point is reached, further decreases in Vout do not change the output of amplifier
86
. Since the gate voltages of MOSFETs
80
and
82
are therefore fixed, the drain to so

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