Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – With compensation
Reexamination Certificate
1999-12-20
2001-09-04
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
With compensation
C327S109000, C327S110000
Reexamination Certificate
active
06285234
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of signal ground isolation and, more particularly, to signal ground isolation using magnetic coupling.
BACKGROUND OF THE INVENTION
In order to provide electrical isolation, e.g., for safety considerations, most switching DC-DC power converters employ conventional optocouplers.
FIG. 1
illustrates a prior art optocoupler circuit
100
. A steady DC voltage V
IN
received from the output of a main switching converter (not shown) is scaled by a resistor network
101
and
103
and is compared to a reference voltage V
REF
via a high gain amplifier
107
. Grounding point
102
comprises the power return path. The high gain amplifier
107
compares the scaled V
IN
and V
REF
and outputs an error signal voltage V
ERR
, representing the difference between V
IN
and V
REF
. The error signal voltage V
ERR
drives an LED
111
, causing LED
111
to emit light across isolation barrier
113
to a photo-transistor
119
. Grounding point
122
comprises the power return path for this side of the circuit. In a known manner, photo-transistor
119
converts the light emitted from LED
111
back to a current signal representing the difference between the scaled V
IN
and V
REF
, which is converted by resistor
117
back to a voltage representing the error signal voltage V
ERR
. Resister
105
converts the bias current input to or output from high gain amplifier
107
, thereby balancing any input bias-voltage imbalance.
The optocoupler circuit
100
of
FIG. 1
provides isolation across isolation barrier
113
; however, it operates with a restricted temperature range because the semi-conductor junction materials of photo-transistor
119
can only withstand temperatures between −20° centigrade and 95° centigrade, thereby limiting the dynamic range of the circuit. In addition, since the light emitted by light emitting diodes such as LED
111
is relatively weak in intensity, the isolation barrier
113
between LED
111
and photo-transistor
119
must be kept relatively small. Due to the close proximity of LED
111
with respect to photo-transistor
119
, capacitive coupling can occur between the two devices, thereby introducing AC coupling between the two devices and degrading the isolation that they provide.
It is also well known to utilize transformers to provide isolation between two electrical circuits so as to isolate a source of relatively high voltage that powers a device from low voltage devices and/or from a user of the isolated device. For example, isolation transformer are commonly used in medical equipment, such as temperature monitors, electro-cardiograms, oximeters, or invasive blood pressure monitors which include sensors which are in contact with the patient. U.S. Pat. No. 5,615,091, for example, incorporated fully herein by reference, is directed to an isolation transformer for medical equipment.
Non-isolated current sampling voltage summing circuits are also known.
FIG. 2
illustrates a prior art current sampling voltage summing circuit
200
. As shown in
FIG. 2
, a resistor-divider comprising resistors
201
and
203
scales a steady DC voltage V
IN
from a main switching converter and provides an input to a comparator, e.g. high gain amplifier
207
. Grounding points
202
comprise the power return path for this side of the circuit. High gain amplifier
207
compares this input with a reference voltage V
REF
input via resistor
205
, just as in FIG.
1
. The output V
ERR
of high gain amplifier
207
is an error voltage signal which is applied to the base of transistor
223
via resistor
221
. Transistor
223
acts as a voltage follower, since the error signal voltage V
ERR
will go across the base-emitter junction of transistor
223
and “sit” on top of the emitter. Thus, the emitter voltage of transistor
223
is the sum of the base-emitter junction voltage of the transistor
223
plus the error voltage V
ERR
, in volts.
A power transformer T
1
having a primary winding
227
and a secondary winding
229
is switchable between an energized and a de-energized state by switching transistor
225
. Grounding point
222
comprises the power return path for this side of the circuit. In a known manner, a non-isolated current sensing/sampling block
240
yields a current output kI
P
that is a scaled version of primary current I
P
at current input node
228
with the scaling factor k. The output current, kI
P
passes an emitter resistor
209
and produces a pulsating voltage V
SENSE
. This pulsating voltage V
SENSE
is added to the error voltage sitting at the emitter of transistor
223
, and the sum of these two voltages presents itself as a non-isolated feedback signal used in a pulse width modulator. By comparing the non-isolated feedback signal with another known reference voltage, a driving pulse with variable time duration (width) is provided for switching transistor
225
. However, due to the non-isolated nature of current sensing/sampling block
240
, grounding point
202
and grounding point
222
are in essence the same.
The magnetic coupling circuit
200
of
FIG. 2
has certain deficiencies. It does not provide isolation and it must “overcome” the base-emitter junction voltage of transistor
223
; thus it is unable to handle low-level signals. For example, if the primary current I
P
is relatively small and the sampled current output kI
P
multiplied by the ohmic value of the sensing resistor
209
is not large enough to overcome the base-emitter junction of transistor
223
, the circuit will not function because the circuit is, in effect, an open circuit. This will cause the control loop to be opened rendering it unable to control the converter output V
IN
. To properly function, the circuit must function at all times.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a signal isolator using magnetic coupling. In contrast to the prior art non-isolated voltage summing circuits, the present invention utilizes current summing and magnetic coupling. In addition to providing ground and signal isolation the circuit of the present invention also provides a current-summing node which is always in a high impedance state, thereby allowing ancillary control mechanisms to be easily implemented in the circuit.
REFERENCES:
patent: 4191929 (1980-03-01), Max et al.
patent: 4510476 (1985-04-01), Clatterbuck et al.
patent: 4525652 (1985-06-01), Sperzel et al.
patent: 4677536 (1987-06-01), Pepper
patent: 4698740 (1987-10-01), Rodgers et al.
patent: 4774419 (1988-09-01), D'Ariano
patent: 4853665 (1989-08-01), Olesak
patent: 5043598 (1991-08-01), Maeda et al.
patent: 5276357 (1994-01-01), Cripe
patent: 5539630 (1996-07-01), Pietkiewicz et al.
patent: 5615091 (1997-03-01), Palatnik
patent: 5917687 (1999-06-01), Fleckenstein
patent: 5939927 (1999-08-01), Myers
Synnestvedt & Lechner LLP
System Design Concepts, Inc.
Tran Toan
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