Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-08-11
2002-09-10
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C323S251000
Reexamination Certificate
active
06449175
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to power converters having magnetic amplifier (magamp) post regulators and, more particularly, to circuitry used to reduce the power and efficiency loss in the control transistor of a set-mode magamp post regulator.
BACKGROUND OF THE INVENTION
The magamp post regulator is a popular power supply topology for regulating the outputs of a power converter in many applications. Modern electronic devices often require several voltage outputs; and need a low cost, energy efficient and well regulated way of providing these outputs. Magamps are typically used to provide an efficient and reliable way of providing precise voltage regulation of independent outputs of a multiple output power converter. A magamp post regulator provides improved regulation of power converter output voltage using a small control current.
The basis function of a magamp is to block a positive incoming voltage for a certain time (t
block
) before allowing it to pass through an output filter. The duty cycle reduction occurs because the magamp delays the leading edge of the voltage waveform. The magamp acts to reduce the duty cycle to the rest of the circuit from the duty cycle of the incoming voltage so as to maintain the required average output voltage.
Conventional magamp post regulator circuits use a reset control to control the magamp using a control transistor operated in a linear mode.
FIG. 1
illustrates a prior art example of a conventional reset-controlled magamp circuit
10
.
FIG. 2
illustrates the hysteresis characteristic of the core element of the magamp of the circuit of FIG.
1
. The conventional magamp circuit includes a magamp
16
, a diode
12
, a reset transistor
20
and an error amplifier (error amp)
18
. In
FIG. 1
, when a power switch
34
is turned on, a secondary voltage V
sec
is developed across a transformer
14
secondary winding. Magamp
16
is forced into saturation due to the action of the voltage V
sec
forced upon it. The B-H hysteresis curve in
FIG. 2
shows the saturation point, B
saturation
at the top of the path. Since the magamp
16
is “in saturation”, forward biased and highly conductive, current flows through the magamp to a forward output rectifier diode
22
after which it is filtered by an L-C circuit, comprised of inductor
24
and capacitor
26
. The output voltage is coupled to a load, not shown, and is also divided by a voltage divider formed by series resistors
36
and
38
to generate a Voltage sense signal at node
35
. At the end of the switch “on” time, the magamp
16
remains forward biased and in saturation.
When the main power switch
34
turns off and the transformer
14
voltage reverses polarity to −V
sec
the current through the magamp
16
is caused to ramp down. As a result, a vertical rectifier diode
30
must pick up the output current, causing the voltage at node
15
to drop. In this off state the magamp voltage V
m
is not allowed to reach zero. Instead a reset control circuitry supplies a voltage that reversely biases the magamp
16
, such that the magnetic flux density is reset to a point below remanence (below the point B
remanence
of the left side of dark shaded area in FIG.
2
.). Then the main switch is turned on and the transformer
14
secondary voltage becomes +V
sec
. Since Magamp
16
is well below the saturation point and not conductive, it acts as an open circuit and blocks the secondary voltage. Vertical diode
30
continues to provide a path for the output current so the voltage at node
15
remains at zero. The magamp voltage Vm then equals +V
sec
. In time, the voltage across magamp
16
causes it to reach saturation and become conductive. The current through the magamp rises to the output current level and remaining at this level till the end of the on time.
The flux excursion on the B-H curve of
FIG. 2
depends on how much volt-time is applied across the magamp
16
during resetting. The amount of volt-seconds is controlled by the output of error amp
18
. The blocking time equation is given by
t
block
=
Δ
B
·
turns
·
A
core
V
;
where A
core
is the core area, &Dgr;B is the change in flux density, turns is the number of turns for the core, and V is the voltage. It can be seen from this equation that the loop in
FIG. 2
corresponding to &Dgr;B
2
gives a longer blocking time that the loop of &Dgr;B
1
. The cores required for this prior art method of reset control exhibit a relatively square B-H curve. To lower the output voltage and increase the blocking time, the loop followed is the lightly shaded part of the B-H curve as compared to the dark part. The control circuit forces the B-H loop larger by pushing the vertical, descending part of the locus. Thus, the minimum blocking voltage-time is the locus where it just touches the vertical axis. To maximize the difference between maximum and minimum volt-time blocking, the B-H loop of the core material must have a small difference between B
saturation
and B
remanence
, where it intercepts the vertical axis.
Compared to square loop amorphous core magamps, ferrite magamps are lower cost, better for high frequencies and can run at higher temperature. However, a drawback associated with this conventional reset control approach is that lower cost non-square ferrite cores perform poorly under reset control because the power dissipation at high flux excursion is too large, especially for operation at high frequency.
A prior art example of a conventional circuit for magamp post regulator control without using reset control but instead using a “set” mode with a control circuit in a linear mode, is shown in FIG.
3
. This set control enables the use of lower cost ferrite cores for the magamp core, however, operation in linear mode leads to unacceptable losses in the circuit. The corresponding B-H hysteresis characteristic of the core member of the magamp of the circuit is shown in FIG.
4
. For the magamp post regulator
40
in
FIG. 3
, an error amp
48
feeds a control transistor
50
which is operated in linear mode. When the transformer
44
secondary voltage V
sec
turns negative in response to power switch
64
, a diode
42
and a control transistor
50
“catch” the current through magamp
46
. Depending on the voltage output from error amp
48
, the current through the loop of diode
42
, control transistor
50
and magamp
46
is decreased, and the corresponding change in &Dgr;H and &Dgr;B is achieved (as shown in
FIG. 4
, the current is related to H by the equation H*L
core
=turns * I.) During the next positive cycle, the magamp
46
will block the secondary voltage V
sec
The blocking time, T
block
, according to the equation described above,
t
block
=
Δ
B
·
turns
·
A
core
V
,
is proportional to &Dgr;B (turns, A
core
and V are constant for the equation). As the curve in
FIG. 4
illustrates, set control mode operates only at one quadrant of the B-H curve while the reset control, as shown in
FIG. 2
, can operate at all four quadrants. In this “set” mode circuit, the control circuit tries to prevent the core from resetting, i.e. tries to make a smaller loop. Since there is no requirement for the core to be square, non-square less costly ferrites can be used.
FIG. 5
shows another prior art version of set control for a magamp post regulator. For this magamp post regulator circuit
70
, in addition to the magamp
76
power winding, there is an extra magamp control winding
77
. A driver diode
72
and a control transistor
80
control the magamp control winding
77
, with the control elements isolated from the transformer
74
secondary power winding. The current through the diode
72
and control transistor
80
can be reduced depending on the turns ratio of the control winding and power winding.
FIG. 6
shows a corresponding set of timing curves for the magamp set control circuit of FIG.
5
. The top curve
1
, is the secondary voltage and V
p
is the transformer
74
primary voltage, curve
2
is
Cuadra Jason
Melgarejo Manolo Mariano M.
Astec International Limited
Berhane Adolf Deneke
Coudert Brothers LLP
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