Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2003-05-05
2004-06-01
Riley, Shawn (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S056100
Reexamination Certificate
active
06744642
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates to a circuit for resetting a magnetic field of a transformer used in a power converter, and more particularly, to a reset circuitry and method for effectively transmitting the magnetizing energy stored in a transformer core of a power converter and extending a switch duty cycle of the power converter.
2. Description of the Prior Art
Three conventional methods for resetting magnetizing energy of a transformer mainly include a tertiary winding reset circuit, an RCD reset circuit, and a resonant reset circuit. Please refer to FIG.
1
and FIG.
2
.
FIG. 1
is a circuit diagram of a tertiary winding reset circuit according to the prior art.
FIG. 2
is a timing diagram of the circuit of FIG.
1
. The tertiary winding reset circuit as shown in
FIG. 1
includes a diode
17
connected in series with a tertiary winding of a transformer T. The transformer T has a magnetizing inductor
14
and a first switch
12
for controlling a duty cycle of the transformer T. In
FIG. 2
, a rectangular pulse
12
represents the signal voltage of the first switch
12
, V
S1
represents the voltage across the first switch
12
, and I
M
represents the current flowing through the magnetizing inductor
14
of the transformer T. When the first switch
12
is turned on, the transformer T charges the magnetizing inductor
14
until the first switch
12
is turned off. At this moment, the diode
17
connected in series with a tertiary winding of the transformer T is turned on and the magnetizing inductor
14
discharges through such loop until the magnetizing energy thereof is fully discharged. The magnetizing inductor
14
will not conduct current until the next ON period of the first switch
12
, as can be seen from the current waveform I
M
of FIG.
2
. If the turn ratio of the primary winding and tertiary winding of the transformer T is 1:1, the charging time and discharging time of the magnetizing inductor
14
will be equal with each other. In other words, the magnetizing energy stored in the magnetizing inductor
14
has to be fully discharged before the first switch
12
is turned on again, and thus the maximum duty cycle of the transformer T is limited within 0.5. Assuming that the duty cycle of the first switch
12
is D=⅓, then I
M
only needs one-half of the time necessary to release the magnetizing energy during the OFF period of the first switch
12
. When the magnetizing energy is fully discharged, the voltage drop across the primary winding of the transformer T is rated at zero, and the value of V
S1
decreases from 2V
IN
to V
IN
. According to the above-mentioned, it is known that the most serious drawback of the tertiary winding reset circuit is that the duty cycle of the first switch
12
is limited to a maximum value of 0.5.
Please refer to FIG.
3
and FIG.
4
.
FIG. 3
is a circuit diagram of an RCD reset circuit according to the prior art.
FIG. 4
is a timing diagram of the circuit of FIG.
3
. For the purpose of giving a clear explanation, like elements have the same reference numerals in the drawings. The RCD reset circuit includes a resistor
19
connected in parallel with a capacitor
18
, wherein both of them are then connected in series with a diode
17
, and finally connected to a primary winding of the transformer T. When the first switch
12
is turned on, the transformer T charges the magnetizing inductor
14
until the first switch
12
is turned off. At this time, the diode
17
of the RCD reset circuit is turned on, and the magnetizing inductor
14
resets the internal magnetic field of the transformer T via the RCD reset circuit until the next ON period of the first switch
12
. Because the parallel resistor
19
is a power-consuming element, when the RCD reset circuit resets the magnetizing energy of the transformer T, the resistor
19
will transform part of magnetizing energy into heat. It can be known from the foregoing that the most serious drawback of the RCD reset circuit is that the resistor
19
dissipates and transduces parts of the magnetizing energy into heat at the same time the inner magnetizing field of the transformer T is reset, and the dissipated energy cannot be retrieved. This reduces the efficiency of the transformer T.
Please refer to FIG.
5
and FIG.
6
.
FIG. 5
is a circuit diagram of a resonant reset circuit according to the prior art.
FIG. 6
is a timing diagram of the circuit of FIG.
5
. The resonant reset circuit includes a capacitor
18
connected in series with the resistor
19
, wherein both of them are then connected in parallel with a primary winding of the transformer T. When the first switch
12
is turned on, the transformer T charges the magnetizing inductor
14
until the first switch
12
is turned off. The magnetizing current discharges through a loop comprised of the magnetizing inductor
14
, the resistor
19
, and the capacitor
18
. This loop is referred to as a LC resonant loop. The operating efficiency of the transformer T is enhanced by resetting its internal magnetic via the LC resonant circuit, however the resonance caused by the LC resonant circuit will form a harmonic wave that causes an unexpected high voltage across the first switch
12
, as indicated by a voltage V
S1
shown in FIG.
6
. According to the above-mentioned, the most serious drawback of the resonant reset circuit is that the harmonic wave caused by resonance forms an enormous high voltage across the first switch
12
, so the transformer T requires a switch having a strong durability and resistivity against a high voltage as the first switch
12
.
SUMMARY OF INVENTION
It is therefore a primary objective of the claimed invention to provide a circuitry for resetting a magnetic field in a transformer of a power converter to solve the above-mentioned problems.
According to the claimed invention, there is provided a reset circuitry for resetting an internal magnetic field of a transformer of a power converter when a main switch of the power converter is turned off. The reset circuitry includes a first capacitor connected in series with a winding of the transformer, a rectifier connected in parallel with a series circuit comprising the first capacitor and the winding of the transformer, a second capacitor, an auxiliary switch connected in series with the second capacitor to form a series circuit to be connected in parallel with the rectifier, and a switch control circuit. The switch control circuit is configured to turn on the main switch and turn off the auxiliary switch instantaneously to release the magnetizing energy in a magnetizing inductor of the transformer, and then store the magnetizing energy stored in the magnetizing inductor and charge the first capacitor, and turn on the auxiliary switch and turn off the main switch to transmit the magnetizing energy stored in the magnetizing inductor and the energy stored in the first capacitor to the second capacitor after a short period of time, and charge the magnetizing inductor and the first capacitor by the second capacitor after the magnetizing energy stored in the magnetizing inductor is fully discharged, thereby reset an internal magnetic field of the transformer.
The claimed invention has a general form in which a power converter includes main switch, and a transformer having at least a primary winding and a secondary winding. The primary winding is connected in series with the main switch for receiving a DC voltage and induce an AC voltage on the secondary winding according to an on/off state of the main switch, a switch control circuit, and a reset circuitry for resetting a magnetic field in the transformer. In addition, the transformer has a magnetizing inductor. The switch control circuit can turn on the main switch and turn off the reset circuit in response to a pulse signal generated therefrom to store magnetizing energy in the magnetizing inductor. The switch control circuit can turn on the reset circuit and turn off the main switch in response to another pulse signal generated therefrom to tra
Ambit Microsystems Corp.
Riley Shawn
Winston Hsu
LandOfFree
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