Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2001-08-03
2002-07-30
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S021180
Reexamination Certificate
active
06426886
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to voltage converters that output multiple voltages, and more particularly to protecting a linear post-regulator used in such a converter against an overcurrent condition.
BACKGROUND OF THE INVENTION
As herein relevant, a voltage or power converter is a circuit or system that receives an input voltage (Vin) that is AC in an AC:DC voltage converter, or DC in a DC:DC voltage converter, and generates at least two voltages that are provided as rectified DC outputs (Vo, VoAUX). Typically the Vo voltage is sampled and fed back to the voltage feedback node (VFB) of a pulse width modulator (PWM) whose output can control magnitude of Vo. However the VoAUX voltage is neither sampled or used to control the PWM, nor otherwise voltage regulated. Instead, a linear regulator is used between the relevant converter output and the VoAUX node as a post-regulator.
Such converters may be implemented in a variety of topologies. By way of example,
FIG. 1A
depicts a prior art isolation-providing DC:DC voltage converter
10
. Converter
10
provides isolation in that the input ground is separate from the output ground. However for use with the present invention, it is not critical whether a converter does or does not provide isolation.
In the exemplary topology of
FIG. 1A
, converter
10
receives a source of input potential Vin on the system input side
20
, and converts Vin to a Vo potential and a VoAUX potential on the system output side
30
. Loads will be coupled between the Vo node and output-side ground, and between the VoAUX node and output-side ground. Other systems
10
could of course generate more than two output voltages, and if system
10
were an AC:DC converter, then Vin could represent a raw input AC voltage that has been rectified to yield Vin. As noted, while
FIG. 1A
shows an isolating converter having separate input-side ground and output-side ground, converter
10
is merely exemplary, and could in fact be non-isolating, with a common ground for the system input-side and output-side.
In the exemplary topology of
FIG. 1A
, transformer T
1
provide isolation between the input and output sides of system
10
, as does isolator unit
11
. Transformer T
1
typically comprises at least one primary winding W
1
and at least one secondary winding W
2
, shown here as being tapped, from which raw output voltages Vo and VinAUX will be provided. The input side of converter
10
includes a switch Q
1
coupleable in series between one end primary transformer winding W
1
and input-side ground (or other input-side reference potential). If additional primary side windings are present, each such winding could also have a switch, and be similarly coupleable. However, it is not required that converter
10
provide isolation, in which case I
1
could be omitted, and input side ground and output side ground would be a common ground.
Referring to
FIG. 1A
, in a fashion well known to those skilled in the relevant art, switch Q
1
opens and closes upon receipt of drive signal from a drive circuit
40
. In turn, circuit
40
outputs the drive signal in response to input signals from a pulse width modulator (PWM)
50
that operates preferably in response to a feedback sample (k·Vo) taken from output voltage Vo, e.g., via resistor string R
1
and R
2
. Voltage Vo is output from a rectifier circuit, here shown as a simple diode-capacitor, D
1
and C
1
. Commonly the k·Vo sample is coupled to a voltage feedback node (VFB) on PWM
50
. A source of Vbias (not shown) is coupled to provide operating potential for PWM
50
.
In operation, the k·Vo sample at the VFB node is compared within PWM
50
to a stable reference voltage (not shown). PWM
50
then generates an appropriate correction signal based upon the voltage difference between k·Vo and the reference potential. The correction signal is suitably coupled, e.g., via an isolator I
1
if required, to driver
40
to command switch Q
1
in a corrective fashion. For example, if PWM
50
determines that Vo is too low, the correction signal from the PWM can cause switch Q
1
to turn-on with increased pulse width, to increase duty cycle and thus magnitude of Vo. Or, if the PWM determines that Vo is too high, the PWM will cause drive circuit
40
to turn-on Q
1
with decreased pulse width, to decrease duty cycle and thus magnitude of Vo.
When switch Q
1
turns-on, Vin is impressed across input winding W
1
, and essentially Vin is sampled or chopped. The resultant chopped signal is inductively coupled via transformer T
1
to the secondary transformer winding W
2
. On the output side of system
10
, diode D
2
and capacitor C
2
filter the chopped AC to yield raw potential VinAUX, which is coupled as input to a post-linear regulator circuit
60
to yield VoAUX. Internal to regulator
60
is a feedback loop
70
that is used to limit the maximum current available from the VoAUX node.
System
10
in
FIG. 1A
is typical of many prior art converters in that the Vo voltage can be well regulated by feedback to the PWM, but there is no real regulation of the potential VoAUX, only a limit as to maximum current at the VoAUX node. For example, the PWM may control magnitude of Vo to within about ±2%, whereas VinAUX may vary ±5% to ±10% or so, as the magnitude of Vin andlor loads on either output node vary. Generally there is but one PWM in a converter system, and the VoAUX node simply is not voltage regulated using PWM feedback. A post-linear regulator can regulate VoAUX to within about ±2%. But protecting post-regulator
60
against thermal overload can be a challenge, especially if regulator
60
is implemented with discrete components, rather than as a single IC. For example, if the load resistance LOAD
AUX
becomes too low, or even a short circuit, regulator
60
must stand-off a voltage differential of (VinAUX−0) and a maximum value of load current I
AUX
. The pass device must dissipate the power equal to the product of the stand-off voltage and maximum current, and can readily be damaged. Some prior art topologies include current foldback to reduce magnitude of output current under short-circuit load conditions, but such topologies still do not use input-to-output feedback to voltage regulate the VoAUX node potential.
FIG. 1B
depicts an exemplary linear post-regulator
60
, a circuit that will be coupled in series between VinAUX and VoAUX to limit the maximum permissible load current (I
Aux
) available to LOAD
AUX
. Regulator
60
includes a pass element, here a bipolar transistor Q
pass
used as an emitter follower, coupled in series with a current sensor
70
, through which current I
AUX
passes.
Regulator
60
further includes a first amplifier
80
that compares a sensor
70
measure of I
AUX
with a reference voltage
90
representing a maximum threshold current. Regulator
60
also includes a second amplifier
100
that compares a measure of VoAUX potential to a reference potential
110
. A feedback loop
120
is provided such that the magnitude of the input or control signal to pass element Q
pass
is a function of the magnitude of sensed current I
AUX
. In the example shown in
FIG. 1B
, Q
1
is a bipolar transistor whose input signal is the base-emitter drive voltage established by amplifier
100
. If sensor
70
determines that I
AUX
is exceeding a threshold set by reference
90
, the effect of the feedback in the regulator is to decrease the Q
pass
base-emitter voltage, thus decreasing collector and emitter current, or I
AUX
. Diode Dr in feedback loop
120
protects amplifiers
80
and
100
from damage from each other's output signals.
An exemplary current sensor
70
is shown in FIG.
1
C. Sensor
70
can include a small impedance sense resistor Rs across which I
AUX
creates a voltage drop proportional to Rs·I
AUX
. This potential is sensed with a differential amplifier Ae whose output is coupled to amplifier
100
. If I
AUX
increases sufficiently, the output signal from the error amplifier Ae will exceed the threshold level set by reference
90
,
Dorsey & Whitney LLP
Nguyen Matthew
Switch Power, Inc.
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