Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2001-05-30
2002-06-18
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S127000
Reexamination Certificate
active
06407934
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a switching power converter for use in various electronic apparatuses, and more particularly to a synchronous rectifying circuit in the switching power converter.
Technologies regarding synchronous rectifying circuits in conventional switching power converters for use in various electronic apparatuses have been disclosed, for example, in the Japanese Laid-open Patent Application, Publication No. Hei 5-137326.
FIGS. 8 and 9
are circuit diagrams of switching power converters disclosed in the Japanese Laid-open Patent Application, Publication No. Hei 5-137326.
A conventional switching power converter shown in
FIG. 8
is configured so that an AC input Vin is rectified by a rectifying diode bridge
601
and a DC high voltage (100 V for example) is generated at a smoothing capacitor
602
. Energy is stored in and released from the excited inductance of a transformer
604
by a power MOSFET
603
on/off-controlled by a control circuit (not shown). A rectifying diode
620
is connected to the secondary winding of the transformer
604
, and the current in the secondary winding of the transformer
604
flows through the diode
620
to charge a smoothing capacitor
606
. In addition, the current is smoothed by a smoothing reactor
608
and a smoothing capacitor
607
, and a DC voltage is output as a DC output Vout. Since the diode
620
is used in the conventional switching power converter configured as described above, the proportion of a loss owing to the diode
620
in the total loss of the apparatus becomes large when obtaining a DC output of a low voltage (3 V for example), thereby raising a problem.
A conventional switching power converter shown in
FIG. 9
has been proposed to solve the problem encountered in the switching power supply circuit shown in FIG.
8
. In the switching power converter shown in
FIG. 9
, instead of the diode
620
shown in
FIG. 8
, a power MOSFET
705
is connected. In the switching power converter shown in
FIG. 9
, an N-channel MOSFET is used as the power MOSFET
705
. The on/off control of the power MOSFET
705
is carried out by using a voltage generating in the auxiliary secondary winding
704
C of a transformer
704
. The conventional switching power converter shown in
FIG. 9
has a lower conduction loss in comparison with the apparatus comprising the rectifying circuit including the diode shown in
FIG. 8
, and therefore has high power efficiency in the whole apparatus.
FIG. 10
is a waveform diagram of the synchronous rectifying circuit of the switching power converter shown in
FIG. 9. A
part (a) of
FIG. 10
shows the waveform of a primary current flowing through the power transistor
603
serving as a main switch, a part (b) of
FIG. 10
shows the waveform of a voltage generating at the auxiliary secondary winding
704
C of the transformer
704
, and a part (c) of
FIG. 10
shows the waveform of a secondary current flowing through the power MOSFET
705
. Synchronous rectifying means that a switching device, such as the power MOSFET
705
, is used as a rectifying switch as described above.
A problem arising in the above-mentioned switching power converter is to control the timing of the on/off control of the synchronous rectifying switch highly accurately. For example, if the turn-on timing of the synchronous rectifying switch in the switching power converter shown in
FIG. 9
is too early, a large turn-on loss occurs because the voltage of the synchronous rectifying switch is not lowered sufficiently. Conversely, if the turn-on timing of the synchronous rectifying switch is too late, a conduction loss at a body diode inside the synchronous rectifying switch increases. On the other hand, if the turn-off timing of the synchronous rectifying switch is too early, the conduction loss at the above-mentioned body diode increases. Conversely, if the turn-off timing of the synchronous rectifying switch is too late, a period occurs during which the synchronous rectifying switch and the main switch turn on simultaneously. As a result, a large loss owing to a short-circuit current occurs.
In the conventional switching power converter shown in
FIG. 9
, the turn-off of the synchronous rectifying switch
705
is carried out by the voltage reversion of the auxiliary secondary winding
704
C. This voltage reversion takes place when the main switch
603
turns on. Hence, a period occurs during which the main switch
603
and the synchronous rectifying switch
705
turn on simultaneously, although the period is instantaneous. As a result, a large loss owing to a short-circuit current occurs in the conventional switching power converter.
In addition, as a conventional switching power converter of a transformer-insulation type, wherein an AC voltage generated at a secondary winding is synchronously rectified and a power is supplied to a load, apparatuses disclosed in U.S. Pat. No. 5,383,106 and U.S. Pat. No. 5,430,633 are available. Both the switching power converters are flyback converters wherein a series circuit comprising a capacitor and a switch is connected to the primary winding of a transformer. In these apparatuses, when magnetic energy stored in the transformer is released from the secondary winding, an inductance and a capacitor connected equivalently in series with the winding of the transformer cause resonance, and the current flowing through the secondary winding has a resonance waveform. U.S. Pat. No. 5,430,633 discloses a circuit wherein rectifying means connected to the secondary winding is a synchronous rectifier.
FIG. 11
is a circuit diagram of a switching power converter with a synchronous rectifier disclosed in U.S. Pat. No. 5,430,633.
FIG. 11
simply shows only the main configuration portion of the switching power converter with the synchronous rectifier disclosed in U.S. Pat. No. 5,430,633.
As shown in
FIG. 11
, in the switching power converter of U.S. Pat. No. 5,430,633, a capacitor
125
and two switches
110
and
120
are connected to the primary winding
132
of a transformer
130
. The two switches
110
and
120
comprise transistors
111
and
121
and the body diodes
112
and
122
thereof, respectively. A coil
142
and a capacitor
144
are connected in series with the secondary winding
134
of the transformer
130
so as to produce resonance. Furthermore, a synchronous rectifier
440
having a synchronous rectifying transistor
441
and a body diode
442
is connected to the secondary winding
134
of the transformer
130
. This synchronous rectifying transistor
441
is configured so as to be controlled depending on the change of the voltage of the tertiary winding
136
of the transformer
130
. Still further, the output Vout of this switching power converter is fed back to the switches
110
and
120
via control means
160
.
A problem arising in the switching power converter configured as described above is to control the timing of the on/off control of the synchronous rectifying transistor
441
highly accurately. In the conventional switching power converter shown in
FIG. 11
, the on/off control of the synchronous rectifying transistor
441
is based on the change of the voltage of the tertiary winding
136
. After the first switch
110
(hereafter referred to as the first switch) on the primary side of the transformer
130
turns off, when the voltage of the tertiary winding
136
of the transformer
130
becomes higher than the threshold value of the gate voltage of the synchronous rectifying transistor
441
, the synchronous rectifying transistor
441
turns on. Therefore, the turn-on of the synchronous rectifying transistor
441
may become earlier than the turn-on (the start of conduction of the body diode
442
) of the synchronous rectifier
440
or the turn-on (the start of conduction of the body diode
122
) of the second switch
120
(hereafter referred to as the second switch) on the primary side of the transformer
130
. In this case, a turn-on loss occurs because the voltage of the synchronous rectifying transistor
441
is not lowered su
Hamaguchi Toshio
Ishii Takuya
Murakami Takaharu
Pearne & Gordon LLP
Sterrett Jeffrey
LandOfFree
DC/DC switching power supply with optimally timed... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with DC/DC switching power supply with optimally timed..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and DC/DC switching power supply with optimally timed... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2913023