U.S. patent application number 09/777079 was filed with the patent office on 2001-08-09 for voltage step down type dc-dc converter having a coupled inductor.
Invention is credited to Nakagawa, Shin.
Application Number | 20010011885 09/777079 |
Document ID | / |
Family ID | 27481115 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010011885 |
Kind Code |
A1 |
Nakagawa, Shin |
August 9, 2001 |
Voltage step down type DC-DC converter having a coupled
inductor
Abstract
In the DC-DC converter according to the present invention, an
intermediate tap is provided in the free-wheel inductor; the
free-wheeling current goes out through the tap. The switch at the
main switch side is driven by an N-channel FET (NPN transistor) and
a pulse transformer, or the switch constitutes a P-channel FET (PNP
transistor), so that the driving circuit at the rectifying side can
be made simple. Further, the DC-DC converter includes loss-less
snubber circuits or a soft switching circuit having a sub switch in
order to absorb and recover the surge energy from a leaked
inductance, which is generated by the addition of the intermediate
tap. Moreover, the driving circuit for the soft switching circuit
is also simplified.
Inventors: |
Nakagawa, Shin; (Kiyose,
JP) |
Correspondence
Address: |
BRUCE LONDA
NORRIS, MCLAUGHLIN & MARCUS, P.A.
220 EAST 42ND STREET, 30TH FLOOR
NEW YORK
NY
10017
US
|
Family ID: |
27481115 |
Appl. No.: |
09/777079 |
Filed: |
February 5, 2001 |
Current U.S.
Class: |
323/224 |
Current CPC
Class: |
H02M 1/342 20210501;
H02M 3/158 20130101; Y02B 70/10 20130101; H02M 1/34 20130101 |
Class at
Publication: |
323/224 |
International
Class: |
G05F 001/613 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2000 |
JP |
2000-69443 |
Apr 10, 2000 |
JP |
2000-145736 |
Claims
What is claimed is:
1. A DC-DC converter comprising, a DC power supply, a main switch,
a freewheel inductor and a free-wheel switch, wherein an
intermediate tap is provided in said free-wheel inductor so that a
free-wheeling current goes out through said intermediate tap, and
wherein an EFT is used for said free-wheel switch.
2. A DC-DC converter according to claim 1, wherein a pulse
transformer and an N-channel FET or a pulse transformer and an NPN
transistor are used for the main switch.
3. A DC-DC converter according to claim 1, wherein a P-channel FET
or a PNP transistor is used as the main switch.
4. A DC-DC converter according to 1, wherein a capacitor for
resonating is connected to said main switch or to said free-wheel
switch so as to conduct a soft switching operation.
5. A DC-DC converter according to claim 1, said converter further
comprises a sub switch to constitute an active clamp or a sub
switch for zero voltage transition to conduct a soft switching
operation.
6. A DC-DC converter according to claim 1, said DC-DC converter
further comprises one or more loss-less snubber circuit.
7. A DC-DC converter comprising a DC power supply, a main switch, a
transformer, a rectifying switch, and a sub switch for conducting a
zero voltage transition operation, wherein an FET is used for said
free-wheel switch.
8. A DC-DC converter according to claim 1, further comprising one
or more a loss-less snubber circuit.
9. A DC-DC converter comprising two or more converters mentioned in
any one of claims 1 to 8 are connected together in a parallel
manner, wherein switching operation of the thus connected
converters is driven in a multiple phased manner.
Description
BACKGROUND OF THE INVENTION
[0001] 1) Field of the Invention
[0002] The present invention relates to a voltage step down type
DC-DC converter having a coupled inductor, which is preferably used
for a case required that a voltage should be largely lowered, for
instance, used for a power supply apparatus where a voltage of 1.6V
is obtained from a power source with a voltage of 20V.
[0003] Such a voltage step down-type DC-DC buck converter is
generally used for the MPU (Microprocessor Unit) in the recently
developed personal computers; the MPU is driven with a low voltage,
for instance 1.6V, however it requires a large current.
Particularly, for lap-top type personal computers, it is required
to make the efficiency higher while keeping the above condition in
order to make the operating time of the battery long and to reduce
heat generation.
[0004] 2) Related Art
[0005] In the field of the power supply apparatus for personal
computers where it is required to largely drop the voltage down,
normal type buck converters have mainly been used. FIG. 1 is a
circuit diagram showing a basic construction of the conventional
buck converter. However, according to the conventional buck
converters, when the voltage is greatly lowered, the duty ratio of
a main switch becomes very small. Therefore, it becomes difficult
to control the converter and the switching loss of the main switch
becomes great. In order to solve this problem, it has begun common
to use a voltage step down type converter where the voltage is
decreased in two steps; an intermediate voltage is generated in a
first step and a finally demanded low voltage is obtained in a
second step. However, such a type of converter has problems in that
the circuit thereof becomes complicated and also it is difficult to
make the apparatus compact.
[0006] The present invention has for its purpose to provide a DC-DC
converter, where the duty ratio of a main switch is made broader to
make it easy to be controlled and a high efficiency can be realized
by decreasing the switching loss of the main switch. According to
the DC-DC converter of the invention, it is possible to largely
lower the voltage, while, the complicity caused in the conventional
buck converter where the voltage is decreased in two steps is
withdrawn, the size of the apparatus becomes compact, and the cost
for manufacturing can be saved.
SUMMARY OF THE INVENTION
[0007] In order to realize the purpose, the DC-DC converter
according to the present invention has a characteristic in that the
DC-DC converter comprises a DC power supply, a main switch, a
free-wheel inductor and a free-wheel switch, wherein an
intermediate tap is provided in the free-wheel inductor, thereby a
free-wheel current goes out through the intermediate tap. According
to this construction, the duty ratio of the main switch becomes
broader in comparison to the conventional buck converter that does
not have a tap in the free-wheel inductor.
[0008] The DC-DC converter according to the present invention has
an aspect in that an N-channel FET is used for the free-wheel
switch, while a P-channel FET or a PNP transistor is used for the
main switch. According to this aspect, the driving circuit for both
the switches can be made simple.
[0009] The DC-DC converter according to the present invention has
another aspect in that an N-channel FET or an NPN transistor is
used for the main switch being combined with a pulse transformer.
According to this aspect, the driving circuit for the main switch
can also be made simple.
[0010] The DC-DC converter according to the present invention has
still another aspect in that the DC-DC converter further comprises
one or more loss-less snubber circuits, where a surge energy
generated from a leakage inductance, due to the existence of the
intermediate tap, is reused, so that an energy recovery is
realized. The DC-DC converter may further comprise a sub switch for
conducting a soft switching operation. According to the present
invention, the driving circuits for driving the main switch and the
free-wheel switch, the active snubber circuit and the sub switch
for the soft-switching operation can be made simple.
[0011] The DC-DC converter according to the present invention has
still another aspect in that in case a sufficiently high input
voltage can be obtained, both the driving circuit for the main
switch (high side) and the driving circuit for the free-wheel
switch (low side) may be connected together in series, so that the
power for driving can be saved.
[0012] Furthermore, the DC-DC converter according to the present
invention has still another aspect in that the DC-DC converter has
a construction where a sub switch for ZVT (zero voltage transition)
operation is added to the construction of a flyback converter so
that each switching element there conducts a soft switching
operation. Furthermore, one or more loss-less snubber circuits is
added to the construction to realize a more preferable
operation.
[0013] The DC-DC converter system according to the present
invention has still another aspect in that two or more DC-DC
converters having the characteristics mentioned above are connected
together in parallel so that so-called interleave construction is
realized, driving signals are driven in a multi-phase manner.
According to this interleave construction, it is possible to reduce
noise and/or ripples in the output of the converter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a circuit diagram depicting a conventional buck
converter;
[0015] FIG. 2 is a circuit diagram showing a basic construction of
the DC-DC converter according to the present invention;
[0016] FIG. 3 is a circuit diagram illustrating a first embodiment
according to the present invention;
[0017] FIG. 4 is a circuit diagram representing a second embodiment
according to the present invention;
[0018] FIG. 5 is a circuit diagram showing a third embodiment
according to the present invention;
[0019] FIGS. 6(a) to 6(f) show operational waveforms of the DC-DC
converter according to the present invention; and
[0020] FIGS. 7(a) to 7(f) illustrate operational waveforms of the
DC-DC converter according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] FIG. 2 is a circuit diagram showing a first embodiment of a
DC-DC converter according to the present invention. In FIG. 2, the
numerical reference 101 represents a DC power supply. Three drive
signals 102, 102, 102, which are supplied from a control circuit
(not shown), are given to a main switch 103, a free-wheel switch
107 and a sub switch 104, respectively, to control the operation of
the switching elements, respectively. It should be noted that the
operating circuits for stabilizing a voltage of the DC-DC converter
is omitted here. In the first embodiment, a P-channel FET is used
for the main switch 103, while an N-channel FET is for the
free-wheel switch 107; these switches 103 and 107 become ON
alternatively. The numerical reference 106 is a free-wheel
inductor, which has a tap 109; the reference 108 is a capacitor for
making the output voltage smooth.
[0022] The sub switch 104 is provided for conducting a soft
switching operation and the numerical reference 105 denotes a
capacitor for resonating. It should be noted that an N channel FET
is used for the sub switch 104. Diodes and capacitors, provided in
parallel with the source and drain of each FET (103, 104, 107), are
body diodes and capacitance between electrodes, respectively. It
may be possible to provide another diode and capacitor outside of
each switching element 103, 104, 107 in the same direction to that
of the diodes 103a, 104a, 107a and in parallel with the switching
element.
[0023] The basic construction of the DC-DC converter according to
the present invention, where the sub switch 104 and the capacitor
105 for resonance are omitted from the circuit shown in FIG. 2,
will be explained first. According to the basic construction, when
the main switch 103 is turn ON, a magnetizing current coming
through the main switch 103 energizes the inductor 106. Then the
main switch 103 turns OFF and the free-wheel switch 107 turns ON,
so that a free-wheel current goes out through the intermediate tap
109 of the inductor 106 toward the smoothing capacitor 108. Since a
coupled inductor is used, in other words, since the intermediate
tap 109 is provided in the inductor 106, the inductance, during
when the free-wheeling switch 107 is ON, becomes smaller than the
inductance when the main switch is ON. Therefore, during when the
free-wheel switch 107 is ON, much more free-wheeling current goes
through in a short time period. As a result, the duty ratio of the
main switch 103 can be made wider. It should be noted that the
position of the intermediate tap 109 is not limited to the center
of the inductor 106 but an arbitrary point can be selected.
Further, it may be possible to arrange such that the free-wheel
electric current goes through another coil provided in the inductor
106.
[0024] In addition to the above-mentioned basic construction, a
capacitor for resonating is connected to the main switch 103 in a
parallel manner, which resonates to the leakage inductance
generated by the existence of the coupled inductor 106. According
to this construction, by arranging the ON-OFF timing of the
switching elements such that the free-wheel switch 107 ON, the main
switch 103 ON, the freewheel switch 107 OFF and the main switch 103
OFF, in this order, the main switch 103 operates as a zero voltage
switch (ZVS) and the free-wheel switch 107 operates as a zero
current switch (ZCS). FIGS. 6(a) to 6(f) show voltage waveforms
when the main switch 103 turns OFF and current waveforms when the
free-wheel switch 107 turns ON. As shown in FIGS. 6(a) to 6(f),
these waveforms become a half waveform of a sign waveform. When a
PFM control is conducted fixing the off time of the main switch
103, the DC-DC converter according to the above-mentioned
construction operates in a preferred manner.
[0025] FIG. 6(a) shows a voltage waveform of the main switch 103,
FIG. 6(b) a current waveform of the main switch 103, FIG. 6(c) a
driving voltage waveform of the main switch 103, FIG. 6(d) a
voltage waveform of the free-wheel switch 107, FIG. 6(e) a current
waveform of the free-wheel switch 107, and FIG. 6(f) shows a
driving voltage waveform of the free-wheel switch 107,
respectively. According to the construction shown in FIG. 2, the
capacitor for resonating is connected to the plus side of the DC
power supply 101. However even if the capacitor is connected to the
minus side, it still operates in a parallel manner with the main
switch 103.
[0026] In the basic construction of the present invention mentioned
above, when the capacitor for resonating is provided at the
free-wheel switch 107 side in parallel to the switch 107 and the
switching timing of the main switch 103 and the free-wheel switch
107 is arranged as mentioned above, the main switch 103 conducts
the zero current switching operation and the free-wheel switch 107
conducts the zero voltage switching operation. In this case, when a
PFM control is conducted fixing the ON time of the main switch 103,
the DC-DC converter operates in a preferred manner.
[0027] In case that, an intermediate tap 109 is provided in the
inductor 106, when the wheel switch 107 becomes ON, the voltage at
the left side of the inductor 106 becomes minus. Therefore, if an N
channel FET or an NPN transistor is used for the main switch 103, a
minus power source is required in addition to a plus power source,
which is basically required. In the case that an N-channel FET or
an NPN transistor is used for the main switch, the driving circuit
becomes complicated as mentioned above, however, by adding a pulse
transformer to the N-channel FET or the NPN type transistor, a
minus power supply is not required and it is possible to make the
construction of the driving circuit for the main switch 103
simple.
[0028] On the other hand, if a P-channel FET is used (or a PNP
transistor may be used) for the main switch 103, it is possible to
make the driving circuit more simple. However, P-channel FETs
generally have a large ON resistance, so that a loss of the main
switch 103 becomes large due to the large ON resistance. According
to the present invention, the root mean square current of the main
switch 103 becomes so small due to the coupled construction of the
inductor 106 that the loss of the main switch 103 caused by the ON
resistance of the P-channel FET is not increased. Therefore, even
if a P-channel FET or a PNP transistor is used for the main switch
103 in order to make the driving circuit thereof simple, a
loss-less power supply apparatus can be realized.
[0029] In addition to this, since the voltage applied to the
free-wheel switch 107 becomes small by the existence of the tap 109
of the inductor 106, a switching element having a low ON resistance
can be selected for the free-wheel switch 107, so that the loss
caused by the ON resistance of the free-wheel switch 107 can also
be reduced.
[0030] In the converter where the intermediate tap 106 is provided
in the free-wheel inductor 106, a leakage inductance is caused from
the tap, so that a surge energy is generated at a moment when the
main switch 103 turns OFF. According to the invention, the thus
generated surge energy is reused by providing a loss-less snubber
circuit(s), which will be explained below, or by arranging such
that the switching elements conduct a soft switching operation. As
shown in FIG. 2, the sub switch 104 and the capacitor for
resonating 105 constitutes so-called active clamp construction,
which is for conducting the soft switching operation. According to
this construction, a ZVS operation by soft switching operation can
be realized by making the main switch 103 and the sub switch 104
alternatively ON with a little simultaneous overlapped OFF time. As
a result, it is possible to realize the DC-DC converter having a
high efficiency and to make the noise low. It should be noted that
if an N-channel FET is used for the free-wheel switch 104, the
driving circuit thereof becomes simple.
[0031] If the sub switch 104 for soft switching operation and the
capacitor 105 for resonating are provided, the ON-OFF timing of the
switching elements in the apparatus may not be in the best
condition. However, this problem can be easily solved by adjusting
the timing to make the free-wheel switch 107 ON or OFF.
[0032] In case that the input voltage is sufficiently high so that
a voltage which is required to drive the switching elements can be
obtained even if the input voltage is divided, it may be possible
to connect the driving circuits for the main switch and the
free-wheel switch together in a series manner. According to this
arrangement, the power for driving the elements can be reduced.
[0033] By using a coupled inductor 106, i.e. an inductor 106 with a
tap 109, a step is caused between the current at the main switch
103 side and the free-wheeling current. Due to the current step, an
output ripple current is increased. It is possible to reduce the
ripple current by providing a smoothing capacitor having a small
equivalent series register. However, in case that it is required to
make the ripple current caused by a switching operation extremely
small, or in case that it is required to make the size of the
smoothing capacitor extremely small, it may be possible to connect
two or more DC-DC converters according to the present invention in
parallel and drive the switching operation in these converters in a
multiple phase so that the ripple current can be cancelled. In this
case, one or more servo controlling system can be provided. The
method for conducting this arrangement is obvious for the skilled
in the art, so the explanation therefor is omitted here.
[0034] The detail of the first embodiment of the present invention
is illustrated in FIG. 3. The input power supply 301 is divided by
the resisters 303 and 304 into almost one to two (1/2). The divided
power is supplied to a center point of a high side driving circuit
A and a low side driving circuit B, respectively, and supplied to a
control circuit 302 via an operational amplifier 305; the high side
driving circuit A and the low side driving circuit B are connected
together in series as shown in FIG. 3. In such a manner, in case
that a required voltage can be obtained even if the input voltage
is divided, it is possible to reduce the driving power by dividing
the input voltage. It should be noted a capacitor 306 is provided
in order to make the voltage stable even if a pulse current is
given. An emitter follower type amplifier can be used for the
operational amplifier 305. It should be noted that the function for
making the control circuit 302 stable is not explained here.
[0035] The output O1 of the control circuit 302 drives an N channel
FET 319, i.e. a switching element at the free-wheel side, via the
low side driver B being constituted of transistors 307, 309, 310
and a resister 308. The output O2 of the control circuit 302 drives
an N-channel FET 318, i.e. a main switch 318, via the high side
driver A being constituted of transistors 311, 314, 315 and
registers 312 and 313, and further via a pulse transformer 316 and
a capacitor 317. The outputs O1 and O2 operate at almost
simultaneous timing; since the pulse transformer 316 is, however,
connected in the direction shown in FIG. 3, the switching elements
318 and 319 operate almost alternatively.
[0036] In the embodiment shown in FIG. 3, the free-wheel inductor
320 constitutes a transformer 320, which basically operates with
coils a and b. However, in the first embodiment, the transformer
320 further comprises a coil c in addition to the coils a and b.
The coil c serves to operate the below-mentioned second loss-less
snubber circuits, i.e. an active snubber circuit. Loss-less snubber
circuits will be explained below, which absorbs and reuses a surge
energy generated by the leakage inductance which is caused
depending on the degree of coupling of the coils a and b.
[0037] The surge energy generated at the moment when the main
switch 318 turns OFF, is stored in a capacitor 323 through a diode
322 . Then, during when the main switch 318 is ON, the energy is
recovered to the output 330 through an inductor 324. This is a
first loss-less snubber circuit. It may be possible to replace the
inductor 324 by a resistance.
[0038] The surge energy generated at a moment when the main switch
318 turns OFF is also stored in a capacitor 326 via diodes 325.
Then at the timing when the main switch 318 turns ON, a recovery
switch 327 is driven by the coil c of the transformer 320 and then
an inductor 328 is magnetized thereby. Then, the recovery switch
327 is turn OFF; the surge energy is recovered to the input power
supply through a diode 329 at this timing. It may be possible to
obtain the drive signal for the recovery switch 327 from another
coil added to the pulse transformer 316. This is the second
loss-less snubber circuit. It should be noted that in case that a
resister or a Zener diode is provided in parallel to the capacitor
326 to exchange the surge energy to a thermal energy thereby, the
recovery switch 327, the inductor 328 and the diode 329 may be
omitted.
[0039] The surge energy generated at a moment when the main switch
318 is turn OFF is further stored in a capacitor 331 through a
diode 330. Then, at a timing when the main switch 318 is turn OFF,
the surge energy is recovered to the input power supply through the
inductor 332 and the diode 333. This is the third loss-less snubber
circuit.
[0040] The DC-DC converter according to the invention may have only
one or two of the above mentioned snubber circuits, or all of them.
All of the snubber circuits shown in FIG. 3 can be called loss-less
snubber circuits.
[0041] FIG. 4 shows a second embodiment according to the invention,
where a soft-switching technique, so-called ZVT (Zero Voltage
Transition) is applied to the DC-DC converter of the present
invention. It should be noted that the same reference numbers are
used for the same components and the explanation therefor is
omitted here.
[0042] The control circuit 302 comprises an additional output
terminal O3; the output from the terminal O3 drives a sub switch
for soft switching operation, i.e. an FET 426, through a driving
circuit 425. The drain of the transistor 426 is connected to the
input power supply 301 via a diode 427 and the coil c of the
transformer 320. The connection of the drain of the sub switch 426
and the input power supply 301 using the coil c is so designed that
no high side driver or no pulse transformer is necessary in the
driving circuit of the sub switch 426. It should be noted that the
coil c for the ZVT may be additionally provided from the connecting
point of the main switch 318, and the sub switch 426 may be
arranged at the high side. In this case, the sub switch 426 may be
altered by the combination of a pulse transformer and an N-channel
FET or a P-channel FET.
[0043] The operational timing of the switches 319, 426 and 318 is
as follows: the free-wheeling switch 319 is ON, the sub switch 426
is ON, the free-wheel switch 319 is OFF, the main switch 318 is ON,
the sub switch 426 is OFF, the main switch 318 is OFF, in this
order. It may be possible to add another diode, for instance, a
Shottky barrier diode and/or another capacity so as to be parallel
to the body diode of each switching element 318, 319, 426.
According to the invention, the leakage inductance existing in each
coil of the transformer 320 is effectively used. However, it may be
possible to add an external inductor. According to the ZVT
mentioned above, the turn-on loss of the main switch 318 is
effectively reduced, and the loss-less snubber circuit constituting
the diode 322, the capacitor 323, the inductance 324 effectively
reduces the turn-off loss of the main switch 318. Therefore, all of
the switching elements 318, 319 and 426 conduct in a soft switching
operation due to the geometrical effect of the ZVT and the
loss-less snubber circuits, so that the DC-DC converter works with
a high efficiency and a high frequency.
[0044] In the second embodiment shown in FIG. 4, the power for ZVT
is obtained form the power supply 301, however, it may be possible
to arrange such that the power is obtained from the electric charge
stored in the capacitor 323 that is provided for storing the surge
energy generated when the main switch 318 turns OFF. The electric
charge is supplied to the sub switch 426 via the coil c of the
transformer 320, and the diode 427. In this case, the inductor 324,
which constitutes the first snubber circuit, may be omitted. It may
be possible to exchange the position of the diode 322 and the
capacitor 323 so as to make the driving operation easier.
[0045] FIGS. 7(a) to 7(f) show waveforms as an example of the Zero
Voltage Transition. FIGS. 7(a) and 7(b) are the waveforms showing
the operation of the main switch 318; FIG. 7(c) and 7(d)
illustrating the operation of the free-wheel switch 319; and FIG.
7(e) and 7(f) depicting the operation of the ZVT switch 426. It
should be noted that voltage and current are illustrated in a
crossing manner in FIGS. 7(a) to 7(e) in order to clearly show the
moment of voltage.times.current, caused by the soft switching
operation. In FIG. 7(a) showing the operation of the main switch
318, the current waveform has a narrower duty ratio, while in FIG.
7(c) illustrating the operation of the free-wheel switch 319,
voltage waveform has a narrower duty ratio. Concerning the
waveforms of the ZVT switch 426 in FIG. 7(e), the current waveform
shows a trianglular shape. FIGS. 7(b), 7(d) and 7(e) show waveforms
of the driving signals for the switches 318, 319 and 426,
respectively.
[0046] FIG. 5 is a circuit diagram showing the third embodiment
according to the present invention. If a tap is provided in the
inductor of a buck converter, it shows an intermediate operation of
a buck converter and a flyback converter. However, if a complete
construction of a flyback converter is applied and an intermediate
tap is provided in the coil of the transformer 320 provided on the
main switch side as shown in FIG. 5, it becomes possible to use an
N-channel FET for all switching elements 318, 319 and 426. In
addition, the driving circuit for these switching elements becomes
very simple, where a pulse transformer or a high side driver is not
required. It should be noted that in the circuit shown in FIG. 5
the sources of the switches 318 and 426 may be connected to an
upper side of the capacitor 321.
[0047] In the third embodiment shown in FIG. 5, the ON/OFF timing
of each switch 318, 319, 426 is the same as that of the second
embodiment shown in FIG. 4. It should be noted that in the third
embodiment shown in FIG. 5 the driving circuits for the switches
are included in the control circuit 302. In the third embodiment, a
flyback converter having the main switch 318 is simultaneously
rectified with the rectifying swith 319, and the sub switch 426,
the diode 427 and the coil c' are further added in order to conduct
the ZVT operation, and furthermore, the loss-less snubber circuit
constituting of the diode 322, the capacitor 323 and the inductor
324 is combined. According to such a construction, all of the
switching elements 318, 319 and 426 conduct a soft switching
operation. It should be noted that the positions of the diode 322,
the inductor 324 constituting of the loss-less snubber circuit are
arranged to be opposite to those in the second embodiment, in order
to have a preferred operation.
[0048] According to the DC-DC converter of the present invention,
an intermediate tap is provided in the free-wheel inductor.
Therefore, the duty ratio of the main switch can be made so wide
that it becomes easy to control the converter and the switching
loss can be reduced to make the efficiency higher. Further, by
using a P-channel FET (or PNP transistor) as the main switch or by
using a pulse transformer and an N-channel FET (or NPN transistor),
a simple driving circuit can be realized. Furthermore, by using a
loss-less snubber circuit and/or the resonance of the soft
switching operation, it can be realized to make the noise lower and
the efficiency higher. Therefore, the buck converter according to
the present invention is suitably used for a power supply apparatus
of for example a laptop type computer, which has a MPU with a low
operating voltage and is required to be compact in size.
* * * * *