Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2002-01-25
2003-06-24
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S017000, C363S132000
Reexamination Certificate
active
06583999
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
DC to DC converters of the resonant type often require the use of parallel-connected power switches to provide adequate current levels, and they require large input and output filters. In addition, parallel operation of switching devices has the problem of current sharing, and large filters are disadvantageous from a size and weight standpoint. A typical prior art converter is shown in
FIG. 1
, and discussed later.
The output voltage of any DC-DC converter can be regulated by varying either: a) the switching frequency of the converter with respect to the resonant frequency at a constant pulse width, b) the pulse width of the converter at a constant switching frequency in the so-called Pulse Width Modulation (PWM) method, or c) the voltage of a voltage-fed pre-regulator in order to stabilize the output voltage of the power supply.
Each of these methods has its disadvantages:
a) Varying the switching frequency of the converter with respect to the resonance frequency does not allow operation in Zero Voltage Switch (ZVS) and Zero Current Switch (ZCS) mode over the entire range of the frequency operation, thus decreasing efficiency and increasing noise and electromagnetic interference (EMI).
b) The PWM method suffers from turn-on and turn-off losses, and requires use of lossy snubbers across the switches. The turn-on and turn-off losses increase as the operating frequency of the converter is increased.
c) The topology that employs a voltage-fed pre-regulator receives an input voltage and produces a constant output voltage signal across output nodes “A” and “B” (FIG.
1
), even if the input voltage to the power supply exhibits substantial voltage transients. This is why a configuration whereby feedback from the output that varies the input voltage and output load, in order to regulate the output voltage, is a preferable circuit configuration. However, this topology includes two separate, series connected stages: a boost converter and a series resonance converter. As a result, the overall efficiency decreases, since the overall efficiency is a multiplication of two efficiencies, that of the boost converter and that of the series half-bridge converter.
Various attempts have been made to address the disadvantages of existing converters (power supplies) by providing a constant frequency, multiphase, full-resonant mode DC to DC converter, suitable for use in low voltage, high current DC power distribution systems. For example, U.S. Pat. No. 4,533,986 to Jones discloses a compact electrical power supply, shown in FIG.
1
. Jones's converter includes a serial sequence of a DC to DC boost converter
4
, followed by a capacitive energy storage bank
6
, and a half-bridge series resonant converter
8
. Half-bridge converter
8
includes a transformer
10
, an output rectifier and filter
12
, a load
14
, and a feedback module
15
. Boost converter
4
increases the voltage available from a DC source (typically while presenting an inductive load thereto). Boost converter
4
includes an inductor L
boost
connected to one input terminal of converter
4
, and a solid state switch Q
boost
that is serially connected with L
boost
between the input terminals of converter
4
. A series switching diode D
boost
is connected between L
boost
and capacitive energy storage bank
6
, which includes three capacitors C
S
connected in parallel to the output of the boost converter.
Conduction of Q
boost
causes current to flow in inductor L
boost
and electrical energy to be stored in the inductor's magnetic field and in switching diode D
boost
. When Q
boost
is not conducting, D
boost
is forward biased in the direction to allow the energy stored in inductor L
boost
to be transferred in the form of an induced current to the boost converter output.
The capacitive energy in energy storage bank
6
is transferred to half-bridge series resonant converter
8
. Half-bridge series resonant converter
8
is constructed from two solid state switches Q
1
and Q
2
that are serially connected to each other across capacitive energy storage bank
6
, and an output transformer
10
having it's primary winding T
1
connected to the common output of switches Q
1
, Q
2
. Half-bridge series resonant converter
8
supplies the energy received from capacitive energy storage bank
6
to load
14
through output rectifier and filter
12
. Half-bridge series resonant converter
8
is operated by controlling the gates of Q
1
and Q
2
, through feedback module
15
, causing alternate conduction of either Q
1
or Q
2
through primary winding T
1
to a common terminal A of a series resonant circuit comprising two resonant elements L
r
and C
r
. This produces a pulse input to primary winding T
1
of transformer
10
, and causes energy to be supplied via a secondary winding T
2
of transformer
10
, and via output rectifier and filter
12
to load
14
. The boost circuit parameters are selected to produce a continuous current flow in the L
boost
inductor under normal load conditions.
The switching rates of solid state switches Q
1
and Q
2
always exceed the limits of human audibility, and are consistent with tolerable switching losses in the Q
boost
solid state switch, and with a minimum size for inductor L
boost
, while achieving a significant voltage boost for increased energy storage in capacitive energy storage bank
6
.
Regulating means in the form of a line regulator
30
are provided in boost converter
4
, responsive to the voltage stored in C
S
, for adjusting the duty cycle of Q
boost
to insure the average stored voltage remains constant. Regulation of the output voltage occurs by adjusting the output voltage of the boost converter, or by adjusting the switching rate of Q
1
and Q
2
in relation to the resonant frequency of the series resonant elements L
r
and C
r
.
The disadvantages of the DC-DC converter disclosed by Jones, and of all substantially similar converters or power supplies include: a) boost converter
4
affects the entire input voltage, causing insufficient efficiency, especially on low input voltage conditions; b) series half-bridge resonant converter
8
has high RMS currents at both input and output, causing I
2
R losses and requiring the usage of large filter capacitors, in both input and output; c) resonant converter
8
has large switching losses because of the apparent absence of Zero Voltage and Zero Current switching, thus reducing the overall efficiency; and, d) the apparent absence of Zero Voltage and Zero Current switching results in a great deal of noise and EMI, requiring the addition of large filter elements in order to meet accepted standards.
There is thus a widely recognized need for, and it would be highly advantageous to have, a constant-frequency, multiphase, full-resonant mode, DC to DC converter (power supply), suitable for use in low voltage, high current DC power distribution systems, that does not suffer from the disadvantages of prior art systems listed above.
SUMMARY OF THE INVENTION
The present invention is of a novel, high efficiency DC-DC power supply, and of a method to operate it to increase its efficiency. Specifically, the present invention is of a low voltage, high current, half-bridge, series-resonant, multiphase, DC-DC power supply. The present invention discloses a series-resonant switching DC-DC power supply that includes a combination of a boost type pre-regulator (hereafter simply “pre-regulator”) and a “base-sub” half-bridge series-resonant converter unit (hereafter “base-sub converter unit”). The base-sub converter unit includes two (a “base” and a “sub”) serial half-bridge converters, connected in parallel to the pre-regulator, and having outputs that are combined to produce the required output power. This unique topology differentiates the base-sub converter unit of the present invention from prior art series-resonant converters. The power supply has inherently a Zero Voltage and a Zero Current switching feature. The power supply of the present invention provides improved ability for high current and
Gabdoulin Valery
Spindler Leonid
Appletec Ltd.
Friedman Mark M.
Nguyen Matthew
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