Control circuit of MOSFET for synchronous rectification

Details Control circuit of MOSFET for synchronous rectification Control circuit of MOSFET for synchronous rectification

2003-06-27

2004-08-24

Lam, Tuan T. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Amplitude control

C327S531000, C327S532000, C363S089000

Reexamination Certificate

active

06781432

ABSTRACT:

BACKGROUND OF INVENTION
The present invention relates to a control circuit of a MOSFET used for rectifying or circulating an output current of a switching power supply or the like.
FIG. 16
depicts a conventional synchronous rectifying circuit in which a diode rectifying circuit is connected to a secondary side of a forward converter.
FIG. 17
is a timing chart illustrating an operation of the circuit shown in FIG.
16
. In
FIG. 16
, reference numeral
101
denotes a DC power supply,
102
denotes a MOSFET (n-channel depletion MOSFET),
103
denotes a transformer,
104
,
108
and
109
denote diodes,
105
denotes a control circuit of the MOSFET
102
,
106
denotes a smoothing reactor,
107
denotes a smoothing capacitor, and N
1
, N
2
and N
3
denote a primary winding, a secondary winding and a tertiary winding, respectively, (number of turns thereof are also taken as N
1
, N
2
and N
3
, respectively) of the transformer
103
. A load, not shown, is conned across the smoothing capacitor
107
.
In FIG.
16
and
FIG. 17
, the MOSFET
102
is subjected to on-off control by the control circuit
105
so that an output voltage becomes constant. When the MOSFET
102
is turned ON in a period (
1
) in
FIG. 17
, a DC power supply voltage V
in
is applied to the primary winding N
1
of the transformer
103
. In the secondary winding N
2
of the transformer
103
, a voltage of (N
2
/N
1
) times a primary winding voltage V
P1
is generated, which, while storing energy in the smoothing reactor
106
through the diode
108
, releases the energy to a load side. An exciting current I
m1
flows in exciting inductance (not shown) of the transformer
103
.
When the MOSFET
102
is turned OFF in a period (
2
) in
FIG. 17
, the exciting energy, being stored in the exciting inductance of the transformer
103
, is released from the tertiary winding N
3
of the transformer
103
to the DC power supply
101
through the diode
104
. In the secondary winding N
2
of the transformer
103
, a voltage of −(N
2
/N
1
) times to a primary winding voltage is generated, and the reverse voltage is applied to the diode
108
, which transfers a current I
D1
, having flowed in the diode
108
, into the diode
109
. At this time, the energy stored in the smoothing reactor
106
is released to the load side through the diode
109
.
In a period (
3
) in
FIG. 17
, when the exciting current I
m1
becomes zero, a reverse voltage V
in
is applied to the diode
104
to cut it off, which causes the primary winding voltage V
P1
of the transformer
103
to become zero. In the period (
3
), the energy stored in the smoothing reactor
106
is continuously released to the load side through the diode
109
.
Subsequent to this, in the period (
1
), the MOSFET
102
is turned ON again and a voltage of (N
2
/N
1
) times a primary winding voltage V
P1
is generated in the secondary winding N
2
of the transformer
103
, applying a reverse voltage to the diode
109
, which transfers a current I
D2
, having flowed in the diode
109
, into the diode
108
.
Thereafter, the period (
1
) to the period (
3
) are repeated, by which a waveform of a current I
L
flowing in the smoothing reactor
106
becomes a synthesized waveform of I
D1
and I
D2
.
FIG. 18
is a second conventional synchronous rectifying circuit in which MOSFETs (n-channel depletion MOSFETs) are used in the forward converter in
FIG. 16
instead of the diodes
108
and
109
.
FIG. 19
is a timing chart illustrating an operation of the circuit shown in FIG.
18
.
In
FIG. 18
, reference numerals
110
and
111
denote MOSFETs,
113
and
114
denote resistors each being connected between a gate of each MOSFET and each end of a secondary winding N
2
of the transformer
103
. In
FIG. 18
, components having the same functions as those in
FIG. 16
are denoted by the same reference numerals and signs with explanation thereof being omitted.
When an output voltage of a synchronous rectifying circuit using diodes as that in
FIG. 16
is a voltage as low as being on the order from 3.3V to 5V, a forward voltage drop of the diode (on the order of 0.5 to 1V) causes a proportion of a conduction loss to become very large.
In a MOSFET with a negative drain current, the drain current flows in a body diode of the MOSFET when no voltage is applied between the gate and the source. This causes a voltage drop on the order of 0.5V. The voltage drop, however, can be reduced by applying a positive voltage between the gate and the source which makes resistivity equivalent to that of the on-resistance to be exhibited. The prior art in
FIG. 18
is presented by noting this point.
The differences between the circuits of
FIGS. 18 and 16
are shown in FIG.
19
. In the period (
1
), a voltage V
Q3
, applied between a drain and a source of the MOSFET
111
, is applied to the MOSFET
110
as a gate signal for generating a negative drain current I
Q2
to reduce the conduction loss of the MOSFET
110
. In the period (
2
), a voltage V
Q2
, applied between a drain and a source of the MOSFET
110
, is applied to the MOSFET
111
as a gate signal for flowing a negative drain current I
Q3
to reduce the conduction loss of the MOSFET
111
. Hatched portions in I
Q2
and I
Q3
in
FIG. 19
represent periods in which the conduction losses are reduced.
With the prior art as shown in
FIG. 18
, during the period (
3
) shown in
FIG. 19
, a period appears during which no gate voltage is applied to the MOSFET
111
to reduce conduction loss. As a result, device efficiency is decreased and the cooling capacity against heat generation must be increased by enlarging a cooling device. Consequently, the entire device cannot readily be made compact and lightweight.
SUMMARY OF THE INVENTION
Accordingly, it is a subject of the present invention to provide a control circuit of a MOSFET for synchronous rectification in which a gate voltage is applied to the MOSFET in almost all of a period in which a current flows in a MOSFET, thereby reducing conduction loss and increasing device efficiency in a device that can be compact and light in weight.
In a preferred embodiment, a cathode of a first diode is connected to a drain of a MOSFET for synchronous rectification, a first current supplying unit is connected to an anode of the first diode, and a resistor is connected between the anode of the first diode and a source of the MOSFET to measure a voltage across the resistor. The voltage across the resistor varies depending on a voltage drop when a current flows in the MOSFET for synchronous rectification. Therefore the value of the voltage across the resistor is compared to a first reference voltage by a voltage comparing unit and the output is amplified. A gate voltage is applied between a gate and a source of the MOSFET for synchronous rectification by a gate driving unit.
Thus, by setting the current level taken as the reference to be small, it becomes possible to apply a gate voltage in almost all of a period in which a current flows in the MOSFET for synchronous rectification, which makes it possible to reduce a conduction loss more than in the prior art shown in FIG.
18
.
In a further embodiment, as the above-described first reference voltage, a forward voltage drop in a second diode to which a current is supplied from a second current supplying unit is used. This makes it possible to compensate temperature to forward voltage characteristics of the first diode to enhance a current detection accuracy.
Moreover, by making the first diode and the second diode have forward temperature characteristics the current of the forward direction to temperature-voltage characteristics approximately identical with each other, the current detection accuracy can be further enhanced.
Still further, when a difference between the voltage across the above-described resistor and the first reference voltage becomes equal to or less than a certain value, a gate voltage for the MOSFET is made so as not to be generated. Namely, when a negative current flowing in the MOSFET for synchronous rectification is reduced and the voltage ac

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