Constant current flyback power supply having forward converter mode-configured auxiliary secondary windings producing constant voltage output

A constant current power supply contains a multimode transformer that supplies constant current power over a two or four wire link transporting telecommunication signals to a remote telecommunication unit. It also supplies a reduced magnitude, constant supply voltage for powering the circuitry of the constant current power supply itself. A primary transformer winding receives a pulse width-modulated voltage from a switched mode voltage generator, which is powered by a reduced magnitude, constant voltage derived from a first auxiliary, forward converter mode, secondary winding of the multimode transformer, through a first diode and a first inductor. The multimode transformer also includes a flyback mode secondary winding coupled to the two wire link and operative to deliver a prescribed constant current to the remote load circuit. A current detector circuit coupled in circuit with the two wire link monitors current supplied to the remote load from the flyback mode secondary winding, and controls the operation of the switched mode voltage generator, to maintain constant current flow to the remote load. Constant voltage power for the current detector is derived through a second diode and a second inductor from a second auxiliary, forward converter mode, secondary winding of the multimode transformer.

FIELD OF THE INVENTION 
The present invention relates in general to communication systems and 
networks and is particularly directed to a new and improved constant 
current power supply apparatus, that includes a multimode transformer 
winding arrangement having a primary winding coupled to receive a 
modulated voltage, a flyback mode secondary winding, which is configured 
to supply constant current power from a central office over a 
telecommunication two or four wire pair transporting telecommunication 
signals to a remote telecommunication unit, and a pair of auxiliary 
forward converter mode secondary windings, which are configured to supply 
a reduced magnitude constant supply voltage for powering the circuitry of 
the power supply itself. 
BACKGROUND OF THE INVENTION 
In an effort to ensure that the operation of a remote telecommunication 
unit being serviced from a central office is effectively independent of a 
locally available power source, it is common practice to power the remote 
unit from the central office using the same pair of wires that transport 
telecommunication signals between the central office and the remote unit. 
Where the power parameters for the remote site equipment require delivery 
of a constant current (typically on the order of 40-100 mA), then, in 
order for the voltage output by the upstream (central office) power supply 
to realize the desired constant current at the remote unit, it is 
necessary that the power supply voltage be adjustable (typically between 
40 and 100 V), so as to accommodate variations in the physical separation 
or distance between the central office and the remote site. 
It is customary practice to employ a (pulse width modulator-based) switch 
mode power supply operating off the 48V supply rail provided for the 
central office equipment. Because the components of the internal circuitry 
of the switch mode power supply require a voltage considerably less than 
the available 48 volt supply rail (e.g., on the order of 12V), lossy 
components, such as a resistor and Zener diode circuit, are typically 
employed to provide the requisite regulated reduced power, which is very 
inefficient. 
An alternative, and more efficient, scheme such as that described in the 
Chavannes U.S. Pat. No. 5,534,768, is to employ an auxiliary winding of 
the main (flyback) transformer that is used to couple the switched (pulse 
width modulated) voltage to the telecommunication two wire pair serving 
the remote unit. For a given set of operational and separation parameters, 
the turns ratio of the auxiliary winding of the flyback transformer is 
defined so as to effectively tap off a prescribed fraction of the 
relatively large voltage required to supply constant current power to the 
remote unit. While such a scheme is satisfactory for one set of 
parameters, it will not maintain a constant current at the remote unit, if 
the distance between the remote unit and the central office is changed. 
More particularly, in accordance with the operation of a flyback 
mode-configured transformer, during the first part of the switching cycle, 
the main power switch of the voltage modulator (usually a MOSFET) is 
conductive. As a consequence, energy is stored in the transformer core, 
and no energy is transferred to the secondary winding. During the second 
part of the switching cycle, the main switch is open and conduction 
through the transformer's primary winding is terminated. The resulting 
collapsing magnetic field induces current flow in the secondary windings 
of the transformer. The secondary winding voltage increases to a level 
sufficient to cause current flow, such that the flux in the transformer 
core is the same immediately after the opening of the switch as prior to 
its opening. The actual voltages reached by the secondary windings are 
controlled by voltage regulation circuitry and not by the turns ratio of 
the primary winding to the secondary; the voltage reached by the various 
secondary windings are locked to one another in accordance with their 
respective turns ratios. 
As a result, if the main secondary winding is required to produce only half 
as much voltage as in another application, then the auxiliary windings 
will produce a similarly reduced voltage. As an example, if the distance 
between the remote unit and the central office is decreased, the magnitude 
of the switched voltage required to provide the same constant current to 
the remote equipment will also decrease. However, decreasing the magnitude 
of the switched power supply voltage will also reduce the magnitude of the 
voltage produced by the auxiliary winding of the flyback mode transformer 
for powering the internal circuitry of the switched voltage supply. To 
remedy this problem, it is standard practice to employ an additional 
switched mode power supply operating in a constant voltage configuration, 
with the output winding of its associated transformer used exclusively for 
providing its own supply voltage and that of the main pulse width 
modulated voltage generator. A major shortcoming of this approach is that 
it significantly increases circuit complexity and cost. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, these problems are effectively 
solved by a constant current power supply apparatus containing a multimode 
transformer winding arrangement, which is configured to supply constant 
current power over a two or four wire link transporting telecommunication 
signals to a remote telecommunication unit, such as a repeater, and to 
supply a reduced magnitude, constant supply voltage for powering the 
circuitry of the constant current power supply itself. 
For this purpose, the multimode transformer winding arrangement has a 
primary winding that is coupled to receive a pulse width-modulated voltage 
generated by a switched mode voltage generator. The switched mode voltage 
generator is powered by a relatively reduced magnitude, constant voltage 
derived from a first auxiliary, forward converter mode, secondary winding 
of the multimode transformer. This first auxiliary secondary winding is 
coupled through a first diode and a first inductor to the switched mode 
voltage generator. 
The multimode transformer also includes a flyback mode secondary winding, 
which is coupled to the two or four wire link and is operative to deliver 
a prescribed constant current to the remote load circuit. A current 
detector circuit is coupled in circuit with the two wire link to the load 
and is operative to monitor the current being supplied to the load by way 
of the flyback mode secondary winding. The current detector circuit 
controls the operation of the switched mode voltage generator, so as to 
maintain a constant current flow to the remote load. Power for the current 
detector is derived through a second diode and a second inductor from a 
second auxiliary, forward converter mode, secondary winding of the 
multimode transformer, which also produces a relatively reduced magnitude, 
constant voltage. 
Upon start-up, the switched mode voltage generator is initially powered by 
a relatively inefficient linear voltage regulator circuit, which produces 
the relatively reduced magnitude, constant voltage (e.g., 12V) required by 
the pulse width modulator. Once the switched mode voltage generator 
becomes operative, so that a constant voltage is generated by the first 
auxiliary, forward converter mode-configured secondary winding, this 
constant voltage is applied as a back bias control input to the linear 
voltage regulator, thereby turning off the linear regulator, and powering 
the switch mode regulator. 
In operation, during the first portion of the switching cycle, as flux is 
stored in the main transformer's core for subsequent conversion into 
current in the flyback mode, secondary winding, it is also coupled 
directly into each of the first and second auxiliary secondary windings. 
The actual voltage produced by these auxiliary secondary windings is 
defined in accordance with the turns ratio coupled to the primary winding. 
The output voltage produced by each of the auxiliary secondary windings 
will remain at a prescribed constant value (e.g., 12V), irrespective of 
the voltage produced by the transformer's flyback secondary winding, in 
accordance with the value established by the current detector circuit to 
maintain a constant current flow via the two-wire path to the remote load. 
Voltage pulses produced by the secondary auxiliary windings will be 
transformer ratio-coupled to the primary voltage, and can therefore be 
controlled to the correct amplitude by properly setting the turns ratios 
of the transformer's windings. The widths of these pulses will be 
controlled by the amount of constant current power delivered from the 
secondary flyback winding over the telecommunication link to the remote 
load. The pulse trains produced by a respective secondary auxiliary 
winding will be a series of pulses having a prescribed amplitude and width 
having an average value controlled by load conditions. Capacitors 
downstream of rectifier diodes are large relative to the loading of the 
auxiliary secondary windings, so that the auxiliary secondary windings 
operate in a peak hold mode and are not sensitive to pulse width.

DETAILED DESCRIPTION 
The multimode transformer-based, constant current power supply apparatus 
according to the present invention is diagrammatically illustrated in the 
single FIGURE, as comprising a switched mode voltage generator (pulse 
width modulator (PWM)) 10, which is operative to produce a pulse width 
modulated (PWM) voltage output signal on an output line 11. Output line 11 
is coupled through an oscillation-inhibiting resistor 12 to a controlled 
power switching device 20 (such as a power MOSFET), the current flow 
(source-drain) path of which is coupled in circuit between a (-48V) power 
supply reference bias line 30 and a first terminal 41 of a primary winding 
40 of a main power transformer 50. A second terminal 42 of primary winding 
40 is coupled to a reference (ground) terminal 31. 
PWM 10 may comprise a commercially available fixed frequency current mode 
controller, having a prescribed controllable duty factor range. In the 
illustrated embodiment, PWM 10 provides a modulated (variable pulse width) 
output voltage over the pulse width modulation output line 11 through the 
oscillation-inhibiting resistor 12 to the gate terminals 21 of power 
MOSFET 20. Coupled between output line 11 and line 30 is a Zener diode 13, 
which is operative to prevent excessive transient voltages from being 
applied to the gate terminal 21 of the MOSFET 20. 
With its drain-source path being coupled in circuit with the primary drive 
winding 40 of the main power transformer 50, MOSFET 20 is operative to 
controllably modulate or `chop` the current through the main power 
transformer 50, and thereby controllably modulate the voltage across 
respective ones of a plurality of secondary windings, which include a 
first, flyback mode, secondary winding 140, and second and third auxiliary 
forward converter mode secondary windings 150 and 160, respectively. The 
modulated voltages across these secondary windings are rectified and 
filtered to provide respectively different magnitude output voltages, as 
will be described. 
MOSFET 20 has its source terminal 22 coupled through a sense resistor, 
shown as a series-connected resistor pair 24-25, to bias reference line 
30. Source terminal 22 is also coupled through a primary current sense 
filter comprised of a resistor 34 and capacitors 35 and 36 over a line 37 
to a current sense input port IS of PWM 10. This filter circuit is 
operative to provide current limit level control and to filter out voltage 
overshoot associated with the inductance of the sense resistor pair 24-25. 
The current sense input port IS of the PWM 10 serves as its the primary 
pulse width control input; namely, the width or duty cycle of the output 
pulse signal generated by the PWM 10 on its modulation output line 11 is 
dependent upon the voltage coupled to its current sense input port IS, 
based upon the primary winding current sensed via sense resistor pair 
24-25. The operating frequency of the pulse width modulation output signal 
generated by the PWM 10 is defined by an RC time constant circuit 
comprised of a resistor 46 and a capacitor 47, coupled in circuit between 
reference voltage line 30 and a voltage reference (VREF) port of the PWM 
10. The junction 48 of this RC time constant circuit is coupled to a 
frequency control input port RT/CT of PWM 10. A capacitor 49 provides a 
low AC impedance bypass path to the reference line 30 for the VREF port of 
the PWM 10. 
PWM 10 has a compensation output port COMP coupled via line 52 to a ground 
fault protection circuit 54, which is operative, as necessary to limit 
current flow in paths other than the intended constant current path to the 
remote unit. These other paths might be unintentionally provided by a 
person's body and such current should be limited. The ground fault 
protection circuit 54 may comprise an opto-isolator coupled in circuit 
between the COMP port of the PWM 10 and reference line 30. The operation 
of the ground fault protection circuit 54 is responsive to the control 
output of a ground fault detection circuit (not shown). The compensation 
output port COMP of the PWM 10 is also coupled through coupling/biasing 
resistors 56 and 57 and capacitor 58 to output terminal 92 of an 
opto-isolator circuit 90. The output terminal 92 of opto-isolator circuit 
90 serves as a pulse width modulation control node, and is coupled through 
parallel-connected resistor 95 and capacitor 96 to reference bias line 30. 
The junction between resistor 57 and coupling capacitor 58 is coupled to a 
voltage feedback port VFB of PWM 10. Voltage feedback port VFB provides a 
second control input to the PWM 10 (in addition to current sense input 
port IS) for controlling the modulated output waveform at its output port 
OUT. 
PWM unit 10 is powered by a prescribed voltage (12V) applied from a 
constant voltage supply node 80 to its supply voltage input port (VCC), 
and a ground return line 30 to its ground terminal GND. This ground return 
is also capacitively coupled via capacitor 61 to its reference terminal 
(REF). Opto-isolator 90 has input terminals 96 and 97 coupled over 
respective lines 106 and 107 to output terminals 116 and 117 of a current 
detector circuit 100, of conventional construction, installed in a 
two-wire path (telecommunications link) 110 to a remote unit. Current 
detector 100 is powered by its own associated constant (12V) power supply 
derived from the secondary side of the main power transformer 50, as will 
be described, and is operative to generate an output representative of the 
current flow through the two-wire path 110. 
As a start-up supply for the PWM 10, a conventional, relatively inefficient 
linear regulator circuit, shown at 120, is operative to apply a 12V output 
to the constant voltage supply node 80. Once a constant (12V) voltage is 
generated by the first forward converter-configured auxiliary secondary 
winding 150 of the main power transformer 50 and applied to node 80, that 
voltage is effective to back-bias a diode 121 coupled in circuit with the 
emitter of a transistor 122, so as to turn off the linear voltage 
regulator 120. A Zener diode 123 regulates the base voltage applied to 
transistor 122 to a prescribed value sufficient to meet the power up 
requirements of the VCC port of the PWM 10. 
As pointed out above, the modulated voltages across the secondary windings 
of the main power transformer 50 are rectified and filtered to provide 
respectively different magnitude output voltages. In particular, flyback 
winding 140 operates in a conventional flyback mode to deliver a 
controlled constant current (as rectified by an output diode 142) over the 
two-wire path 110 to the remote unit being powered. Each of the secondary 
windings 150 and 160 is configured to supply a reduced magnitude constant 
(12V) supply voltage for powering the circuitry of the power supply 
itself. For this purpose, auxiliary secondary winding 150 is coupled 
through a first diode 152 and a first inductor 154 to the constant voltage 
supply node 80. Also, auxiliary secondary winding 160 is coupled through a 
second diode 162 and a second inductor 164 to the current detector circuit 
100. 
It will be recognized that a standard flyback mode secondary configuration 
does not employ an inductor, since the transformer normally provides its 
circuit function. However, since the auxiliary secondary windings 150 and 
160 operate in forward converter mode, the transformer itself does not 
provide this function, so that inclusion of these auxiliary inductors is 
preferred. Although failure to include these inductors will result in a 
substantial reduction in the power efficiency of the auxiliary secondary 
windings, when compared to the main flyback secondary winding 140, because 
the auxiliary secondary windings normally deliver very little power, 
failure to provide these additional inductors will have a much smaller 
effect on the overall efficiency of the power supply. Thus, insertion of 
these inductors, while preferred, is a matter of choice. 
During the first portion of the switching cycle output of the PWM 10, as 
flux is stored in the main transformer's core for subsequent conversion 
into current in the flyback mode secondary winding 140, it is also coupled 
directly into each of the auxiliary, forward converter mode secondary 
windings 150 and 160. The actual voltage produced by these auxiliary 
secondary windings is defined in accordance with the turns ratio coupled 
to the primary winding 40. The output voltage produced by each of the 
auxiliary secondary windings will remain at a prescribed constant value 
(e.g., 12V), irrespective of the voltage produced by the transformer's 
flyback secondary winding 110, in accordance with the value established by 
the current detector circuit 100 and output over output lines 116 and 117 
to maintain a constant current flow via the two-wire path 110 to the 
remote load. 
The voltage pulses produced by the secondary windings will be transformer 
ratio-coupled to the primary voltage, and can therefore be controlled to 
the correct amplitude by properly setting the turns ratios of the 
transformer windings. The widths of these pulses will be controlled by the 
amount of power required to be delivered by the secondary flyback winding. 
Thus, the pulse trains produced by a respective secondary auxiliary 
winding will be a series of pulses of desired amplitude and of a width 
having an average value controlled by load conditions. It is necessary 
that the value of capacitance downstream of rectification diodes be large 
relative to the loading of the auxiliary windings. As a result, the 
auxiliary secondary windings 150 and 160 operate in a peak hold mode and 
are not sensitive to pulse width. 
As will be appreciated from the foregoing description, the above discussed 
problems of conventional flyback mode transformer configured, constant 
current power supply configurations are effectively remedied in accordance 
with the present invention, by means of a multimode transformer winding 
arrangement, which is not only configured to supply constant current power 
over a two wire link transporting telecommunication signals to a remote 
telecommunication unit, but is augmented to include auxiliary forward 
converter mode secondary windings, which are configured to supply a 
reduced magnitude constant supply voltage for powering the circuitry of 
the power supply itself, including the switched voltage generator on the 
primary side of the transformer, and a current detector on the secondary 
side of the transformer. 
While I have shown and described an embodiment in accordance with the 
present invention, it is to be understood that the same is not limited 
thereto but is susceptible to numerous changes and modifications as known 
to a person skilled in the art, and I therefore do not wish to be limited 
to the details shown and described herein but intend to cover all such 
changes and modifications as are obvious to one of ordinary skill in the 
art.