Switching power supply having low loss characteristics

Details Switching power supply having low loss characteristics Switching power supply having low loss characteristics

2000-07-18

2001-06-12

Han, Jessica (Department: 2838)

Electric power conversion systems

Current conversion

Including d.c.-a.c.-d.c. converter

C363S098000

Reexamination Certificate

active

06246594

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of switching power supplies, and particularly to a switching power supply using the resonance phenomenon of a capacitor and a coil.
2. Description of the Related Art
Since power supplies having high efficiency compared to switching power supplies of the related art can be obtained, synchronous rectifier type power supplies have recently been receiving attention.
Reference numeral
501
in
FIG. 36
represents a synchronous rectifier type power supply of the related art. This power supply comprises a primary side bridge circuit
510
, a secondary side rectification and smoothing circuit
520
, a main transformer
530
, and a control circuit
540
.
The primary side bridge circuit
510
has four bridge transistors
511
a
,
511
b
,
512
a
and
512
b
(in this case, they are all n-channel MOSFETs).
The operation of the primary side bridge circuit
510
is divided into an A phase and a B phase, with bridge transistors that conduct during A phase operation being represented by reference numerals
511
a
and
512
a
, and bridge transistors that conduct during the B phase operation being represented by reference numerals
511
b
and
512
b.
A primary winding
531
and a secondary winding
532
(
532
a
,
532
b
) magnetically coupled to the primary winding
531
are provided inside the main transformer
530
.
Both ends of the primary winding
531
are connected to the output section of the primary side bridge circuit
510
, and the primary winding
531
and the four bridge transistors
511
a
,
511
b
,
512
a
and
512
b
are H-bridge connected.
Reference numeral
519
is a D.C. voltage source exemplified by a D.C. voltage obtained by rectifying and smoothing a commercial voltage, or a D.C. voltage output from a storage battery. The high voltage side of the D.C. voltage source
519
is connected to supply voltage line
517
, while a low voltage side is connected to a ground line
518
.
The primary side bridge circuit.
510
is connected to the supply voltage line
517
and the ground line
518
. When the A phase bridge transistors
511
a
and
512
a
are turned on with the B phase bridge transistors
511
b
and
512
b
turned off, A phase current I
A
is supplied from the D.C. voltage source
519
to the primary winding
531
.
On the other hand, when the B phase bridge transistors
511
b
and
512
b
are turned on with the A phase bridge transistors
511
a
and
512
a
turned off, B phase current I
B
. is supplied to the primary winding
531
. The A phase current I
A
and the B phase current I
B
are opposite in direction to each other.
The secondary winding
532
has a terminal at its electrical center and an A phase secondary winding
532
a
and a B phase secondary winding
532
b
use the terminal as their common terminal (center tap).
The secondary side rectification and smoothing circuit
520
comprises a choke coil
525
, an output capacitor
526
and two rectification transistors
523
a
and
523
b.
The center tap of the A phase secondary winding
532
a
and the B phase secondary winding
532
b
is connected to a ground terminal
528
, and the other terminals are connected to source terminals of respective rectification transistors
523
a
and
523
b.
The drain terminals of both of the rectification transistors
523
a
and
523
b
are commonly connected to one terminal of the choke coil
525
.
Reference numeral
527
represents the other end of the choke coil
525
, and is connected to an output terminal. The output capacitor
526
is connected across the output terminal
527
and the ground terminal
528
. Reference numeral
529
represents a load, which is also connected across output terminal
527
and the ground terminal.
The voltage on the output terminal
527
is isolated by a photocoupler
549
and input to the control circuit
540
.
The control circuit
540
comprises a reference voltage source
541
, a differential amplifier
542
, an oscillator
543
, a comparator
544
, and a drive circuit
545
. The differential amplifier
542
amplifies a difference between the voltage input from the photocoupler
549
and the output voltage of the reference voltage source
541
, and supplies its output to the comparator
544
.
The comparator
544
compares the voltage input from the differential amplifier
542
with the output waveform of the oscillator
543
, and outputs the comparison result to the drive circuit
545
.
The drive circuit
545
controls the time that the bridge transistors
511
a
,
512
a
,
511
b
and
512
b
are on so that a difference between the output voltage of the photocoupler
549
detected by the differential amplifier
542
and the output voltage of the reference voltage source
541
becomes small, based on the comparison result of the comparator
544
.
Accordingly, even when the output voltage of the output terminal
527
fluctuates due, for example, to load variations, the primary side bridge circuit
510
is controlled by operation of the control circuit
540
so as to absorb these fluctuations, and the output voltage of the output terminal
527
is kept at a constant voltage.
Operation of the power supply
501
will now be described.
FIG. 37
shows the situation when the power supply
501
is operating, with the A phase and B phase bridge transistors
511
a
,
512
a
,
511
b
and
512
b
tuned off and current flowing in the secondary side due to energy stored in the choke coil
525
.
Respective parasitic transistors
524
a
and
524
b
are formed inside the rectification transistors
523
a
and
523
b
. The parasitic diodes
524
a
and
524
b
are forward biased by electromotive force generated in the choke coil
525
, and respective currents I
551
and I
552
flow.
FIG. 41
is a timing chart showing the operation of the power supply
501
, and the above described state is represented as a waveform before time t
1
in the timing chart.
From this state, a positive voltage is applied to the gate terminals of the A phase bridge transistors
511
a
and
512
a
, and when they are turned on, the two ends of the primary winding
531
are connected the supply voltage line
517
and the ground line
518
. As a result, current represented by I
553
in
FIG. 38
flows.
The A phase secondary winding
532
a
is connected at a polarity to apply a positive voltage to a source terminal of the A phase rectification transistor
523
a
when the A phase bridge transistors
511
a
and
512
a
are on. In this state, a voltage of a polarity to apply a negative voltage to the source terminal of the B phase rectification transistor
523
b
is induced in the B phase secondary winding
532
b.
The drive circuit
545
applies a positive voltage to the gate terminal of the A phase rectification transistor
523
a
and the gate electrodes of the A phase bridge transistors
511
a
and
512
a.
In an n-channel MOSFET, when a voltage higher than the threshold voltage is applied to the gate terminal while the voltage on the source terminal is higher than the voltage on the drain terminal, current flows from the source terminal to the drain terminal in a direction that is the opposite of that for normal operation.
This operation is known as the third quadrant operation (in a p-channel MOSFET the condition where a voltage that is lower than the voltage on the drain terminal is applied to the source terminal and a voltage lower than the voltage on the drain terminal is applied to the gate terminal is called the third quadrant operation).
The solid line in
FIG. 42
is a graph showing the characteristic of an n-channel MOSFET, with the horizontal axis representing drain terminal voltage V
DS
with reference to the source terminal, and the vertical axis representing drain current I
D
when a flow direction from the drain terminal to the source terminal is a positive direction.
The range in the first quadrant of this graph is normal MOSFET operation, and the solid line characteristic in the range of the third quadrant is the third quadrant operation. While the drain voltage V
DS
is small, a

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