Abstrict
An electric heater for a motor vehicle uses heat generated by power
semiconductors as the heat source. The heat output by the power
semiconductors is used directly for heating. The power semiconductors
are regulated by circuit regulators to be able to adjust the heating
power continuously. In addition, switching devices are provided
which interrupt or shut down the respective branch circuits individually
in the event of short circuits in the power semiconductors.
Claims
1. An electric heater for a motor vehicle, using heat generated
by power semiconductors as the heat source, characterized in that
several branch circuits (Z1, Z2, Zn), each with one power semiconductor
(FET1, FET2, FETn) operated in high power loss operation, are connected
in parallel for generation of heat.
2. An electric heater for a motor vehicle, using heat generated
by power semiconductors as the heat source, characterized in that
several branch circuits (Z1, Z2, Zn), each with two series-connected
power semiconductors (FET11, FET12; FET21, FET22; FETn1, FETn2)
operated in high power loss operation, are connected in parallel
for generation of heat.
3. The electric heater according to claim 1 or 2, characterized
in that a switching device (FUSE1, FUSE2, FUSEn) which responds
to overload is connected in series to the power semiconductor (FET1,
FET2, FETn) in each branch circuit (Z1, Z2, Zn).
4. The electric heater according to claim 1 or 3, characterized
in that the power output by the power semiconductors (FET1, FET2,
FETn) can be regulated individually by a common predetermined setpoint
(Isoll) and by actual values (Ist1, Ist2, . . . , Istn) derived
from the power semiconductors (FET1, FET2, . . . , FETn).
5. The electric heater according to claim 2 or 3, characterized
in that the powers output by the respective first power semiconductors
(FET11, FET21, FETn1) of the branch circuits (Z1, Z2, Zn) can be
regulated individually by a common predetermined setpoint,(Isoll)
and by actual values (Ist11, Ist21, . . . , Istn1) derived at these
power semiconductors, and the powers output by the respective second
power semiconductors (FET12, FET22, . . . , FETn2) can be regulated
individually by a fixed predetermined control voltage (ust) and
by actual values (Ist12, Ist2, . . . , Istn2) derived at these power
semiconductors.
6. The electric heater according to one of claims 1 through 5,
characterized in that the power semiconductors (FET1, FET2, . .
. , FETn; FET11, FET12, FET21, FET22, . . . FETn1, FETn2) are operated
in the short circuit.
7. The electric heater according to one of claims 1 through 6,
characterized in that the branch circuits (Z1, Z2, . . . , Zn) supply
power at the output to a low-resistance series resistor as a load
impedance.
8. The electric heater according to one of claims 1 through 7,
characterized in that the switching devices (FUSE1, FUSE2, . . .
, FUSEn) are designed as a printed conductor part of the branch
circuits (Z1, Z2, . . . , Zn) which burn out in the event of a fault
at the elevated current occurring in the respective branch circuit
(Z1, Z2, . . . , Zn).
9. The electric heater according to one of claims 1 through 7,
characterized in that the switching devices (FUSE1, FUSE2, FUSEn)
are looped as shunts into branch circuits (Z1, Z2, Zn) which burn
out in the event of a fault at the elevated current occurring in
the respective branch circuit (Z1, Z2, . . . , Zn).
10. The electric heater according to one of claims 1 through 7,
characterized in that the connecting wires of the power semiconductors
(FET1, FET2, FETn; FET11, FET12, FET21, FET22, FETn1, FETn2) are
used as switching devices (FUSE1, FUSE2, FUSEn) which burn out in
the event of a fault at the elevated current occurring in the respective
branch circuit (Z1, Z2, Zn).
11. The electric heater according to one of claims 2, 3 and 5 through
7, characterized in that in the event of a short circuit in one
of the two power semiconductors (FET11 or FET12; FET21 or FET22;
FETn1 or FETn2) connected in series in a branch circuit (Z1, Z2,
Zn), an additional control signal can be derived from the defective
branch circuit (Z1, Z2, Zn) to reduce the power output by the respective
second power semiconductor (FET12 or FET11; FET22 or FET21; FETn2
or FETn1) or switch it to a disconnect status.
12. The electric heater according to one of claims 1 through 11,
characterized in that it is designed as a heater module, with the
power semiconductors (FET1, FET2, . . . , FETn; FET11, FET12; FET21,
FET22; FETn1, FETn2) mounted in thermal contact on a cooling body.
13. The electric heater according to claim 12, characterized in
that the power semiconductors (FET1, FET2, . . . , FETn; FET11,
FET12, FET21, FET22, . . . , FETn1, FETn2) and/or the cooling body
are monitored by temperature sensors to detect whether a predetermined
maximum temperature is exceeded, and in that the output signals
of the temperature sensor(s) reduce the power output of the respective
power semiconductors or all the power semiconductors or switch them
to a disconnect status.
14. The electric heater according to claim 13, characterized in
that the temperature sensors are integrated into the power semiconductors
(FET1, FET2, . . . , FETn; FET11, FET12, FET21, FET22, . . . , FETn1,
FETn2).
Description BACKGROUND INFORMATION
[0001] The present invention relates to an electric heater for
a motor vehicle, using the heat generated by power semiconductors
as the heat source.
[0002] Such a heater is known from German Patent No. 34 42 350
C2. With this known heater, the power semiconductor controls the
electric drive motor. The power semiconductor is connected to a
cooling body through which a liquid coolant flows, so the heat generated
is transferred to the liquid coolant by heat exchange. The liquid
coolant circulates in a closed line system having a pump and the
actual heater installation.
[0003] The efficiency of this known electric heater is not especially
great, because the heat generated by the power semiconductor must
be converted repeatedly. In addition, the heater installation has
a complicated design, depends on the engine current present and
thus cannot be regulated independently of the latter.
SUMMARY OF THE INVENTION
[0004] The object of the present invention is to create an electric
heater of the type defined in the preamble where the efficiency
is greatly increased with a simple design and independent regulation
of heating power is possible.
[0005] This object is achieved according to a first embodiment
of the present invention by connecting several branch circuits,
each with one power semiconductor operated in high power loss operation,
in parallel for generation of heat, or according to a second embodiment
by connecting several branch circuits, each with two series-connected
power semiconductors operated in high power loss operation, in parallel
for generation of heat.
[0006] In these embodiments, the current is converted directly
into heat by the power semiconductors, which greatly increases efficiency.
Another advantage of the new heater is that no additional control
module is needed for the heater. Installation of the heater in the
motor vehicle is also greatly simplified. In addition, the cabling
complexity and manufacturing costs of the new electric heater are
also reduced.
[0007] No separate fuse protection for the heater in the vehicle
electrical system is necessary. When starting operation of the heater,
the high starting current surge can be prevented by a regulated
smooth current rise. The new heating module can be cascaded in any
desired fashion to increase the heating power and can also be integrated
easily into a fan regulator.
[0008] To protect the power semiconductors, one embodiment provides
for a switching device that responds to overload to be connected
in series with the power semiconductor in each branch circuit. In
the event of a fault, the branch circuit affected can be shut down
with this switching device without having to lose heater function
as a whole. Heating power is reduced only by the ratio of defective
branch circuits to total branch circuits.
[0009] According to one embodiment, regulation of the heating power
is easily made possible by the fact that the power output by the
power semiconductors can be regulated individually by a common predetermined
setpoint and by actual values derived from the power semiconductors,
or by the fact that the powers output by the respective first power
semiconductors of the branch circuits can be regulated individually
by a common predetermined setpoint and by actual values derived
at these power semiconductors, and the powers output by the respective
second power semiconductors can be regulated individually by a fixed
predetermined control voltage and by actual values derived at these
power semiconductors.
[0010] If the branch circuits are to supply power at the output
to a low-resistance series resistor as a load impedance, then the
heat generated by the series resistor can contribute to an increase
in heating power. Each power semiconductor can supply power to an
individual series resistor. All the power semiconductors may also
supply power to a common series resistor, or groups of power semiconductors
may each be connected to a group-individual series resistor.
[0011] The switching devices for interrupting the branch circuits
can be implemented in various ways. Thus, according to one embodiment,
the switching devices may be designed as a printed conductor part
of the branch circuits which burn out in the event of a fault at
the elevated current occurring in the respective branch circuit.
The same effect can also be achieved by looping the switching devices
as shunts into the branch circuits, which burn out in the event
of a fault at the elevated current occurring in the respective branch
circuit, in which case the shunt can also be used to derive another
control signal. Finally, the branch circuit can also be interrupted
by using the connecting wires of the power semiconductors which
burn out in the event of a fault at the elevated current occurring
in the respective branch circuit.
[0012] A controlled reduction or interruption in the current in
a defective branch circuit occurs when measures are taken to ensure
that in the event of a short circuit in one of the two power semiconductors
connected in series in a branch circuit, an additional control signal
can be derived from the defective branch circuit to reduce the power
output by the respective second power semiconductor or switching
it to a disconnect status. The control signal picked off at the
shunt can be used as the control signal.
[0013] The structural design of the new electric heater can be
simplified by designing it as a heater module, with the power semiconductors
mounted in thermal contact on a cooling body, with the heat transfer
via the cooling body being improved.
[0014] Simple temperature monitoring can be achieved with the electric
heater by the fact that the power semiconductors and/or the cooling
body are monitored by temperature sensors to detect whether a predetermined
maximum temperature is exceeded, and by the fact that the output
signals of the temperature sensor(s) reduce the power output by
the respective power semiconductors or all the power semiconductors
or switch them to a disconnect status. If the power semiconductors
are monitored by individual temperature sensors, the expense of
this is reduced by integrating the temperature sensors into the
power semiconductors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention will now be explained in greater detail
on the basis of embodiments illustrated in the drawings, which show:
[0016] FIG. 1: a circuit diagram of an electric heater with n parallel
branch circuits, each containing one power semiconductor, and
[0017] FIG. 2: a circuit diagram of an electric heater with n parallel
branch circuits, each having two series-connected power semiconductors.
DETAILED DESCRIPTION
[0018] As FIG. 1 shows, branch circuits Z1, Z2, . . . Zn are connected
in parallel to one another at power supply voltage Ubatt, with each
branch circuit Z1, Z2, . . . , Zn having a power semiconductor FET1,
FET2, . . . , FETn. The connection to power supply voltage Ubatt
is by way of switching devices FUSE1, FUSE2, . . . , FUSEn, which
perform an individual interruption of branch circuit Z1, Z2, . .
. , Zn in the event of a fault, e.g., a short circuit of the power
semiconductor at which the multiple current occurs. A shunt which
is not shown in detail may be added to the connection of power semiconductors
FET1, FET2, . . . , FETn at the ground potential, where an individual
actual value Ist1, Ist2, . . . Istn can be derived for the branch
circuit Z1, Z2, . . . , Zn. In addition to the actual value picked
off at the shunt, a setpoint Isoll is supplied to the gate terminal
of power semiconductors FET1, FET2, FETn via a comparator or operational
amplifier that serves as a circuit regulator to permit continuous
regulation of the power in the respective power semiconductor. If
the actual value exceeds setpoint Isoll, then the circuit regulator
switches the power semiconductor into the disconnect status or reduces
the power output. In addition, switching device FUSE1, FUSE2, .
. . , FUSEn can completely interrupt branch circuit Z1, Z2, . .
. , Zn in the event of a short circuit of respective power semiconductor
FET1, FET2, . . . , FETn.
[0019] Printed conductor segments of branch circuit Z1, Z2, . .
. , Zn themselves can be used as switching devices FUSE1, FUSE2,
FUSEn. Depending on the design of branch circuits Z1, Z2, Zn and
the respective circuit regulator, the current may increase to a
level 25 to 50 times higher in the event of a short circuit, so
the printed conductor part burns out. The shunt can also be used
as a switching device if it burns out with this current rise and
interrupts branch circuit Z1, Z2, . . . , Zn. Even the connecting
wires of power semiconductors FET1, FET2, . . . , FETn can be dimensioned
to assume the function of switching devices FUSE1, FUSE2, . . .
, FUSEn. The electric heater of this type may be designed as a heater
module, mounted on a cooling body and integrated into a fan regulator;
furthermore, the heater module itself need no longer be fused with
respect to the vehicle's electrical system. However, it may be necessary
to fuse the feeder lines to the heater module.
[0020] In the embodiment according to FIG. 2, each branch circuit
Z1, Z2, . . . , Zn has two series-connected power semiconductors
FET11 and FET12, FET21 and FET22, . . . FETn1 and FETn2, each controlled
by its own circuit regulator. As in the embodiment according to
FIG. 1, a switching device FUSE1, FUSE2, FUSEn and a shunt can be
looped into branch circuits Z1, Z2, . . . , Zn. Switching devices
FUSE1, FUSE2, . . . , FUSEn in turn can be designed in the variants
described. Control signals characterizing actual value Ist11, Ist21,
. . . Istn1 of branch circuit Z1, Z2, . . . , Zn can be picked off
at the shunts of the branch circuits and sent to the circuit regulators
of the respective first power semiconductors FET11, FET21, . . .
, FETn1 to which can also be sent a setpoint Isoll to regulate the
power in branch circuit Z1, Z2, . . . , Zn. The second power semiconductors
FET12, FET22, . . . , FETn2 are controlled by separate circuit regulators
to which are sent a fixed predetermined control voltage ust and
an actual value Ist12, Ist22, . . . , Istn2, which is derived from
the voltage drop at the first upstream power semiconductor FET11,
FET21, FETn1. In the event of a short circuit or defect in a power
semiconductor such as FET11 with this design of the circuit regulators,
the respective second power semiconductor, such as FET12, in branch
circuit Z1 can be shut down or the power output by it can be reduced.
However, the functioning of the remaining system is not affected,
and the heating power is merely reduced by the ratio of defective
branch circuits to total branch circuits.
[0021] If both power semiconductors, e.g., FET21 and FET22, are
short-circuited, then the switching device, e.g., FUSE2 as in the
embodiment according to FIG. 1, goes into operation and interrupts
the branch circuit, e.g., Z2, at the high current rise occurring.
[0022] If only one common heat-dissipating, low-resistance series
resistor is used as the load impedance for all branch circuits Z1,
Z2, . . . , Zn to increase the heating power, then this resistor
is looped into the common line leading to battery voltage Ubatt.
This series resistor does not change the operation of the electric
heater, it merely limits the current rise to a lower level in the
event of a short circuit in a single power semiconductor (FIG. 1)
or both power semiconductors (FIG. 2), but this lower level is still
sufficient for a reliable response of switching device FUSE1, FUSE2,
. . . , FUSEn. The heat generated by the series resistor is also
used for heating, but it entails a power distribution which can
be utilized at a predetermined maximum heating power to expand the
temperature use range for the heater.
[0023] Each power semiconductor or each pair of power semiconductors
can also be connected to battery voltage Ubatt across an individual
series resistor. Groups of branch circuits may also supply a series
resistor. In any case, all the series resistors are involved in
the production of heat.
[0024] Temperature monitoring can easily be incorporated into the
new heater. Thus, a temperature sensor may be provided for each
power semiconductor and may also be integrated into the power semiconductor.
If a predetermined maximum temperature is exceeded at the power
semiconductor, the output signal of the temperature sensor then
controls the respective power semiconductor so that its power output
is reduced or it is completely shut down.
[0025] It is also possible to provide just one temperature sensor
for measuring the temperature of the cooling body, with all the
power semiconductors of the electric heater being in thermal contact
with it. If the temperature of the cooling body exceeds a predetermined
maximum temperature, then all the power semiconductors are controlled
with the output signal of the temperature sensor in such a way that
their power output is reduced or they are completely shut down.
Different values of the output signal of the temperature sensor
can be used for this purpose, with the output signal initially triggering
a power reduction at the first lower value and a complete shutdown
at the second higher value of the output signal.
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