Electricity: electrical systems and devices – Safety and protection of systems and devices – Circuit interruption by thermal sensing
Reexamination Certificate
2000-02-22
2002-06-04
Huynh, Kim (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Circuit interruption by thermal sensing
C361S093800, C361S101000
Reexamination Certificate
active
06400545
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to a DC-DC converter, and more particularly to a fuseless DC-DC converter which can protect a circuit from overcurrent without involvement of a protective fuse.
2. Related Art
A 12-volt DC power source voltage is usually used as a DC source voltage in a vehicle. However, a load to be used in a vehicle is not limited to a load to be used at a 12 volts DC. For example, a load to be used at 42 volts DC is provided in a vehicle in Europe. In general, after a DC 42-volt source voltage has been lowered to a DC 12-volt source voltage to be used, the DC 12-volt source voltage is supplied to a load to be used at 12 volts DC. The DC 42-volt source voltage cannot be lowered, in unmodified form, to 12 volts DC. For this reason, there has been employed a DC-DC converter, wherein the DC voltage is converted into an AC voltage, the AC voltage is lowered to a desired voltage, and the thus-lowered AC voltage is converted into a desired DC voltage.
A known DC-DC converter has a circuit configuration such as that shown in
FIG. 6. A
DC power supply (i.e., a battery) is connected to a power MOSFET
11
and a power MOSFET
13
by way of a fuse
10
, and the source of the power MOSFET
11
is connected to the drain of a MOSFET
12
. Further, the source of the MOSFET
12
is connected to one end of a resistor R
10
. The remaining end of the resistor R
10
is grounded. The source of the power MOSFET
13
is connected to the drain of a MOSFET
14
, and the source of the MOSFET
14
is connected to one end of a resistor R
11
. The remaining end of the resistor R
11
is grounded. The power MOSFET
11
and the power MOSFET
13
constitute a higher-potential side of the DC-DC converter.
A primary coil
21
is connected to points between four terminals, one terminal belonging to each of the power MOSFET
11
, the MOSFET
12
, the power MOSFET
13
, and the MOSFET
14
; specifically, the primary coil
21
is connected across a junction G between the power MOSFET
11
and the MOSFET
12
and a junction H between the power MOSFET
13
and the MOSFET
14
. A secondary coil
22
is disposed so as to oppose the primary coil
21
. The turns ratio between the primary coil
21
and the secondary coil
22
is determined in accordance with a target voltage to which the source voltage is to be lowered. When an electric current flows through the primary coil
21
, a lower voltage determined by the turns ratio develops in the secondary coil
22
.
A driver circuit
15
is connected to the gate of the power MOSFET
11
, and the power MOSFET
11
is controlled so as to become active or inactive in response to a gate signal output from the driver circuit
15
. The driver circuit
15
is connected to a charge pump circuit
16
. The charge pump circuit
16
is made of; for example, a voltage-multiplication capacitor which is constituted by means of stacking capacitors into a plurality of layers, and boosts a source voltage of 12V supplied from the battery to a higher voltage (for example, 22V) and supplies the thus-boosted voltage to the driver circuit
15
.
A driver circuit
17
is connected to the gate of the power MOSFET
13
, and the power MOSFET
13
is controlled so as to become active or inactive in accordance with a gate signal output from the driver circuit
17
. The driver circuit
17
is connected to a charge pump circuit
18
. The charge pump circuit
18
is identical with the charge pump circuit
16
in terms of configuration and function.
A driver circuit
19
is connected to the gate of the MOSFET
12
, and the MOSFET
12
is controlled so as to become active and inactive in response to a gate signal output from the driver circuit
19
. Further, a driver circuit
20
is connected to the gate of the MOSFET
14
, and the MOSFET
14
is controlled so as to become active or inactive in response to a gate signal output from the driver circuit
20
.
In the DC-DC converter having the previously-described circuit configuration, in a case where the power MOSFET
11
, the MOSFET
12
, the power MOSFET
13
, and the MOSFET
14
are inactive and where the power MOSFET
11
and the MOSFET
14
are simultaneously turned on in response to the gate signals output from the driver circuits
15
and
20
, a DC current flows from the battery VB and through the primary coil
21
in the direction designated by arrow C, by way of the drain and source of the power MOSFET
11
. The DC current flows to the ground by way of the drain and source of the MOSFET
14
and the resistor R
11
. As a result of the power MOSFET
11
and the MOSFET
14
being turned on, a half-wave of an AC current (for example, a positive half-wave) is formed; specifically, a DC current whose voltage corresponds to a boosted voltage determined by the turns ratio (i.e., the remaining side of the half-wave; for example, a negative half-wave) arises in the secondary coil
22
.
After the power MOSFET
11
and the MOSFET
14
have been activated for a predetermined period of time, the driver circuit
15
deactivates the power MOSFET
11
, and the driver circuit
20
deactivates the MOSFET
14
. Simultaneously, the MOSFET
12
and the power MOSFET
13
are turned on in response to the corresponding gate signals output from the driver circuit
17
and the driver circuit
19
. When the MOSFET
12
and the power MOSFET
13
are turned on, a DC current flows from the battery VB and through the primary coil
21
in the direction designated by arrow D, by way of the source and drain of the power MOSFET
13
(i.e., in the direction opposite that in which the DC current flows when the power MOSFET
11
and the MOSFET
14
are turned on). The DC current flows to the ground by way of the drain and source of the MOSFET
12
and the resistor R
10
. As a result of the MOSFET
12
and the power MOSFET
13
being turned on, the DC current, which flows in the direction opposite that in which the DC current flows when the power MOSFET
11
and the MOSFET
14
are turned on, induces in the secondary coil
22
a DC current whose voltage corresponds to a lowered voltage determined by the turns ratio (i.e., the remaining half-wave; for example, a negative half-wave). The DC current is converted into an AC current by means of successive occurrence of two types of induced currents (i.e., two types of half-waves).
After the MOSFET
12
and the power MOSFET
13
have been activated for a predetermined period of time, the power MOSFET
11
and the MOSFET
14
are activated for a predetermined period of time. As mentioned above, the power MOSFETs
11
and
14
and the power MOSFETs
12
and
13
are turned on alternately, and a lowered AC current is output from the secondary coil
22
. The AC current arising in the secondary coil
22
in the manner mentioned previously is subjected to half-wave rectification (rectification of a positive half-wave) by a half-wave rectification circuit
23
. The thus-rectified current is smoothed by a smoothing circuit
24
, thereby producing a DC voltage which has been lowered by a predetermined level.
The resistor R
10
is for sensing an electric current. In the event that a short circuit or a like failure arising in the secondary circuit is detected as a result of monitoring a potential difference across the resistor R
10
, the driver circuit
19
is activated to interrupt the MOSFET
12
. Similarly, the resistor R
11
is for sensing an electric current. In the event that a short circuit or a like failure arising in the secondary circuit is found as a result of monitoring a potential difference across the resistor R
11
, the driver circuit
20
is activated to interrupt the MOSFET
14
.
In the event that a large current develops as a result of a short circuit or a like failure arising in the primary circuit, the fuse
10
is heated when the large current flows through the primary circuit. If an electric current of a predetermined value or higher flows through the primary circuit, the fuse
10
is melted, thereby interrupting the power supply to the primary circuit so as to protect the
Huynh Kim
Sughrue & Mion, PLLC
Yazaki -Corporation
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