Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-12-19
2003-03-18
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S089000
Reexamination Certificate
active
06535400
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A
BACKGROUND OF THE INVENTION
The fields of power electronics, and power supplies in general, are concerned with the processing of electrical power using electronic devices. One class of power supply that is commonly used to provide power for electronic devices such as personal computers, laptop computers, personal communication devices, and personal digital assistants is referred to as a DC/DC switching converter power supply. In general, a DC/DC switching converter power supply contains a raw power input port that is typically coupled to a DC power source such as a battery and controller. The raw DC input power is processed according to one or more control signals provided by the controller and yields a conditioned output power signal. In particular, a DC/DC converter converts a DC input voltage to a conditioned DC output voltage that may have a larger or smaller voltage magnitude. One type of DC/DC converter is a forward converter illustrated in FIG.
1
. Typically, a DC/DC forward converter is used to provide a DC output voltage that has an output magnitude less than the input magnitude. In particular, as depicted in
FIG. 1
, a typical prior art forward converter
100
includes a raw DC voltage source
107
input between terminals
101
and
105
and coupled to the primary winding
103
of a power transformer
102
. A transformer reset circuit
104
is provided to demagnetize the power transformer during periods when no current is present in the primary coil. A first switch
106
, which is typically a power switching MOS transistor, is coupled between the primary winding of the power transformer
102
and the reference terminal of the voltage input
105
. The secondary coil
109
of the power transformer
102
is coupled to switching diodes
110
and
112
, inductor
114
, and output capacitor
116
, wherein zn output voltage is developed between terminals
117
and
119
. If the first electronic switching module is not a power MOSFET switch a protection diode
108
may be placed across the first electronic switching module to provide a discharge path for the inductance in series around the switching module
106
. A power MOSFET does not need a protection diode due to a parasitic diode that is inherently created due to the semiconductor structure of the MOSFET.
When an input pulse is applied to the primary winding
103
of the transformer
102
a voltage is induced in the secondary winding
109
of the power transformer
102
, the polarity of which is indicated by the respective dots shown on the windings in FIG.
1
. Accordingly, during a positive going pulse, switching diode
110
turns on and a circuit is formed that includes the secondary winding
109
of transformer
102
, inductor
114
, capacitor
116
and switching diode
110
. During the positive going pulse, inductor current I
L
120
flows into the inductor from the secondary winding. The inductor current
120
is equal to the integral of the voltage applied to the inductor divided by the inductance thereof. Accordingly, for a square wave pulse having a constant amplitude, the inductor current
120
will begin to increase in a substantially ramp like manner.
Similarly, when the first switching module
106
turns off the input voltage pulse, switching diode
110
turns off and the inductor current
120
begins to decrease as a linear function. Switching diode
112
will turn on when the voltage at node
121
has fallen below the threshold voltage of diode
112
. When conducting, switching diode
112
turns on to complete the circuit that includes switching diode
112
, inductor
114
and output capacitor
116
.
The output voltage provided between output terminals
117
and
119
is a function of the amplitude of the voltage input pulses provided by the input DC voltage source
107
, the turns ratio of power transformer
102
, the switching frequency of the first switching module
106
, and the duty cycle of the input pulses. For the forward converter illustrated in
FIG. 1
, the output voltage is less than or equal to the voltage across the secondary winding of the power transformer.
The switching diodes
112
and
110
each have a small non-zero resistance when forward biased, i.e., when the diodes are turned on and are conducting. When any current is flowing through the respective diode, the non-zero resistance results in a voltage drop being generated across the switching diode, resulting in a diode conduction loss equal to V*I, where V is the voltage drop and I is the current flowing through the diode. For a typical switching Schottky diode, this diode voltage drop may be as high as 0.4 volts. Because the switching diodes
110
and
112
are in each of the two circuit paths, the output voltage, which is less than the input voltage to begin with, is further reduced by the diode voltage drop. In some DC/DC converters, the diode conduction loss can contribute significantly to the overall power loss. In low output voltage applications the diode conduction loss can be particularly serious. Thus, as the supply voltages for next generation electronic equipment become lower, the forward conduction loss of the switching diodes becomes increasingly significant.
Many components used in current electronic products require 3.3 volts, and in some cases, most notably microprocessors, the voltage requirements have dropped below 2 volts. As this trend of lower supply voltages continues into the future, many electronic devices will be designed to operate at 1 volt or less. As an example of the problems associated with Schottky switching diodes in DC/DC power supplies, a power supply having an output voltage of 5 volts will have approximately 92% to 93% efficiency. However, as the output voltage drops the efficiency of the diode rectifiers drops as well. At 3.3 volts for example, the efficiency of the diode rectifiers is approximately 88%, at 2 volts the efficiency is approximately 83%, and at 1 volt the efficiency is less than 75%.
To ameliorate this condition, the switching diodes
110
and
112
used in
FIG. 1
are often replaced with other electronic switching modules that may include single or multiple MOSFETs, bi-polar transistors, or other semiconductor switches such as thyristors or SCR's. Typically, these electronic switches are referred to as synchronous rectifiers since they are switched on and off synchronously with the switching cycles of the first switching mode to rectify the pulsed DC voltages induced in the secondary coil
109
. Typically, synchronous rectifiers are large channel area power MOSFET switches that are able to clamp the various switching nodes to 0.1 volt or less thus reducing the forward conduction loss by a factor of 4 or more when compared to Schottky switching diodes. Synchronous rectifiers are typically driven using one of two methods. In the first method, a control circuit is used to drive the synchronous rectifiers. In this case, the trade off in using a switching diode or a MOSFET rectifier is whether the power needed to drive the MOSFET gate cancels the efficiency gained from a reduced forward voltage drop.
FIG. 2
depicts the second method for driving a pair of synchronous rectifiers in which the synchronous rectifiers are self-driven. A self-driven system does not suffer from the energy losses described above since the energy necessary to drive the gates of the two synchronous MOSFET rectifiers is returned to the inductor or transformer. In particular, a forward converter
200
uses MOSFET switches
210
and
214
to provide the necessary current paths to rectify the incoming power pulses. The efficiency gain of the synchronous rectifiers depends on the load current, the input battery voltage, the desired output voltage, the switching frequency of the first switching module, and the characteristics of the MOSFET switches. Typically in a DC/DC converter, a lower output voltage and higher load current will militate toward the use of synchronous rectification.
Synchronous rectification using MOSFET devices, however, i
Berhane Adolf Deneke
Brady W. James
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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