JP2014075657A - Power supply control device - Google Patents

Abstract

In a power supply control device having a blown fuse, a technique for optimizing selection of a blown fuse is provided.
A power supply control device is provided in a power supply path for supplying power from a power supply to a load, and has a fuse having a predetermined fusing characteristic indicating a relationship between current and fusing time, and is provided in the power supply path from the power supply. When a switch circuit for switching on / off the supply of the load current Ir to the load and the fuse is used in an allowable use area due to a fusing characteristic, the average value Pfav of the fuse power loss is the rated current of the fuse. A control circuit that controls on / off switching of the switch circuit so as to be substantially equal to a predetermined power loss Po when used at a predetermined load current Io of Ifs or less.
[Selection] Figure 3

Description

  The present invention relates to a power supply control device, and in particular, in a power supply control device that is provided between a power source and a load and controls power supply to the load, a load current considering the use of a fuse provided in the power supply control device Related to control technology.

  Conventionally, a high-power semiconductor switch element such as a power MOSFET is provided in a power supply path connecting a power source and a load, and the current supply to the load is controlled by turning on and off the semiconductor switch element. There is provided a power supply control device that protects a power supply path to an overcurrent from overcurrent. In such a power supply control device, when an overcurrent flows, a device that controls the potential of the control terminal of the semiconductor switch element by a control circuit to turn off the semiconductor switch element to cut off the power supply is known. In addition, such a power supply control device is known in which a fuse (blown fuse) is provided for further protection (see Document 1).

JP 2011-77698 A

  By the way, the fuse is selected in consideration of the case where the load current increases due to the temperature characteristics of the load, the variation of the load, the rise of the power supply voltage, and the like. At that time, the fuse is usually selected so that the rated current of the fuse is not exceeded even when the load current increases. For example, even if the normal load current is 12 A, if there are fuses with rated currents of 15 A and 20 A, a fuse with a rated current of 20 A is often selected in anticipation of fluctuations in the load current. In this case, the selection of the fuse is not always appropriate, and there is a possibility that the fuse becomes over-specific and the fuse becomes larger than necessary, thereby increasing the wire diameter.

  The present invention provides a technique capable of optimizing the selection of a fuse and reducing the diameter of an electric wire in a power supply control device having a fuse.

  The power supply control device disclosed in this specification is a power supply control device that is connected to a power supply path that supplies power from a power supply to a load, and that controls power supply from the power supply to the load. A fuse having a predetermined fusing characteristic indicating a relationship between current and fusing time, a switch circuit that is provided in the power supply path and switches on / off of supply of load current from the power source to the load; When the fuse is used in an allowable use area due to the fusing characteristics, the average power loss of the fuse is approximately equal to the predetermined power loss when the fuse is used at a predetermined load current equal to or lower than the rated current of the fuse. And a control circuit for controlling on / off switching of the switch circuit so as to be equal.

  According to this configuration, even when the load current exceeds the rated current of the fuse, the switching of the switch circuit is controlled so that the average power loss is substantially equal to the predetermined power loss. As a result, even if the load current exceeds the rated current of the fuse, the fuse can be safely used in its allowable use range. Therefore, for example, in selecting a fuse when the predetermined load current is 12 A, a fuse with a rated current of 15 A can be selected instead of a fuse with a rated current of 20 A. That is, the selection of the fuse can be made appropriate. As a result, it is possible to avoid the use of a fuse with a rated current higher than necessary, and the fuse to be used can be reduced in size. That is, in the power supply control device having a fuse, the selection of the fuse can be optimized, and thereby the wire diameter can be reduced.

In the power supply control device, the control circuit sets and sets a duty ratio for performing on / off control of the switch circuit so that an average value of power loss of the fuse is substantially equal to the predetermined power loss. The switch circuit may be PWM controlled according to the duty ratio.
According to this configuration, by setting the duty ratio for controlling on / off of the switch circuit, the average value of the power loss of the fuse can be made almost equal to the predetermined power loss easily and preferably.

The power supply control device may further include a current detection unit that detects the load current and generates a current detection signal, and the control circuit monitors the load current based on the current detection signal, and A current ratio of the predetermined load current to current may be calculated, and the square of the current ratio may be set as the duty ratio.
According to this configuration, since the power loss of the fuse is proportional to the square of the load current, the average of the power loss of the fuse is preferably set by setting the square of the current ratio of the predetermined load current to the load current as the duty ratio. The value can be approximately equal to the predetermined power loss.

The power supply control device may further include a voltage detection unit that is connected to the power supply path, detects a power supply voltage of the power supply, and generates a voltage detection signal. The control circuit is based on the voltage detection signal. The power supply voltage may be monitored, a voltage ratio of a predetermined power supply voltage corresponding to the predetermined load current to the power supply voltage may be calculated, and the square of the voltage ratio may be set as the duty ratio.
The fuse power loss is proportional to the square of the fuse voltage drop. Therefore, when the voltage drop of the fuse is proportional to the power supply voltage, the average of the power loss of the fuse is preferably set by setting the square of the voltage ratio of the predetermined power supply voltage corresponding to the predetermined load current to the power supply voltage as the duty ratio. The value can be approximately equal to the predetermined power loss.

  Further, in the power supply control device, a current detection unit that detects the load current, a temperature detection unit that detects an environmental temperature, and a temperature rise of the fuse from the environmental temperature is determined by the load current flowing through the fuse. A fuse temperature calculation circuit for calculating the calculation fuse temperature by calculating the difference between the heat generation of the fuse and the heat dissipation of the fuse, and adding the rising temperature of the fuse to the environmental temperature, and the control The circuit estimates the temperature of the fuse from the arithmetic fuse temperature, and controls switching of the switch circuit after the fuse temperature reaches a first threshold temperature, and the fuse temperature is set to the first temperature. By maintaining at one threshold temperature, or by maintaining between the first threshold temperature and a second threshold temperature below the first threshold temperature, The average value of the power loss of the serial fuse, the fuse may be controlled to be substantially equal to the predetermined power loss when used in rated current below a predetermined load current of the fuse.

  According to this configuration, the selection of the fuse can be made appropriate, and even if the load current or the power supply voltage fluctuates, the fuse temperature is estimated, so the selected fuse is blown unnecessarily. Instead, it can be used in the state of the first threshold temperature that is set or in the state of the temperature within a predetermined temperature range between the first threshold temperature and the second threshold temperature lower than the first threshold temperature.

In the power supply control device, the control circuit turns off the switch circuit at the first threshold temperature and turns on the switch circuit at the second threshold temperature, thereby setting the fuse temperature to the first threshold temperature. A value between the temperature and the second threshold temperature may be maintained.
According to this configuration, the fuse temperature can be maintained at a temperature within a predetermined temperature range between the first threshold temperature and the second threshold temperature.

At that time, the control circuit may determine the length of the next on-period according to the rate of increase in the fuse temperature during the on-period of the switch circuit.
According to this configuration, the fuse temperature can be controlled so as not to exceed the threshold temperature in accordance with the rate of increase in the fuse temperature, that is, in accordance with the magnitude of the load current.

  According to the power supply control device of the present invention, selection of a fuse can be optimized in the power supply control device having a fuse.

1 is a schematic block diagram of a power supply control device according to a first embodiment of the present invention. A graph that schematically shows the fusing characteristics of a fuse FIG. 3 is a time chart schematically showing a time transition of each signal according to the first embodiment. FIG. Schematic block diagram of a power supply control apparatus according to Embodiment 2 of the present invention Time chart which shows roughly time transition of each signal concerning Embodiment 2

<Embodiment 1>
Embodiment 1 of the present invention will be described with reference to FIGS. 1 to 3.
1. Circuit Configuration As shown in FIG. 1, the power supply control device 10 is connected between a power source Ba and a load 50 to an electric wire (an example of a power feeding path) 51 that supplies power from the power source Ba to the load 50. The power supply from Ba to the load 50 is controlled.

  The power supply control device 10 includes a control circuit 20, a fuse 21, a voltage detection circuit 22, a current detection circuit 23, a switch circuit 30, and a power supply line (an example of a power feeding path) 40.

  In addition, in Embodiment 1, the power supply control apparatus 10 shows the example arrange | positioned in the engine room of a motor vehicle, for example. Further, the power source Ba is a battery, and an example is shown in which, for example, a light as the load 50 is driven and controlled by the power supply control device 10 via an electric wire 51.

  The fuse 21 is provided in the power supply line 40 and has a predetermined fusing characteristic indicating the relationship between the current and the fusing time. Here, the rated current Ifs of the fuse 21 is set to 15 A (ampere), for example, and its fusing characteristics are indicated by a fusing characteristic line in FIG. Normally, the rated current Ifs of the fuse 21 guarantees that the fuse 21 will not be blown regardless of the time during which the current I flows within the allowable use range divided by the blow characteristic line of the fuse 21, as shown in FIG. Current value to be set. In addition, each axis | shaft of FIG. 2 is shown by logarithm.

  The switch circuit 30 is provided in series with the power supply line 40, that is, provided in the middle of the power supply line 40, and turns on / off the supply of the load current Ir from the power supply Ba to the load 50 from the control circuit 20. Switching is performed according to the control signal Sgc. Here, the switch circuit 30 is configured as a semiconductor switch, and includes a main switch 31 that supplies power to the load 50 and a sense transistor (an example of a current detection unit) 32 for detecting the load current Ir. The main switch 31 and the sense transistor 32 are configured by, for example, an N-channel FET (field effect transistor) as shown in FIG.

  A voltage detection circuit (an example of a voltage detection unit) 22 is connected in parallel to the power supply line 40, detects the power supply voltage Vb of the power supply Ba, converts the power supply voltage Vb from analog to digital, and detects a voltage that is a digital signal. A signal SV is generated. The voltage detection signal SV is supplied to the control circuit 20. The voltage detection circuit 22 is configured by, for example, a voltage follower or a voltage dividing resistor. In the present embodiment, the voltage detection circuit 22 may be omitted.

  A current detection circuit (an example of a current detection unit) 23 is connected to the sense transistor 32, detects the load current Ir, converts the load current Ir from analog to digital, and generates a current detection signal SI that is a digital signal.

  The control circuit 20 is configured by a CPU, for example. The control circuit 20 generates a control signal Sgc for controlling on / off switching of the switch circuit 30 based on the current detection signal SI or the voltage detection signal SV, and controls the switch circuit 30 by the control signal Sgc. To do. Specifically, the control circuit 20 supplies a control signal Sgc to the gates of the main switch 31 and the sense transistor 32 to control on / off switching of the main switch 31 and the sense transistor 32.

2. Operation of Power Supply Control Device Next, a characteristic operation of the power supply control device 10 according to the first embodiment will be described with reference to FIG. For convenience of explanation, it is assumed that the resistance value of the load 50 is 1 ohm (Ω), and the resistance value of the fuse 21 is negligibly smaller than the resistance value of the load 50. Therefore, as shown in FIG. 3, the numerical value of the detection voltage Vd of the power supply voltage Vb is equal to the numerical value of the detection current Id of the load current Ir.

  As a characteristic operation, when the fuse 21 is used in the use allowable region of FIG. 2, the control circuit 20 has an average value of power loss of the fuse 21 (hereinafter, referred to as “average power loss”) Pfav On / off switching of the switch circuit 30 is controlled so as to be approximately equal to a predetermined power loss Po when 21 is used at a predetermined load current Io equal to or less than the rated current Ifs.

  At that time, the control circuit 20 sets the duty ratio Du for controlling the on / off of the switch circuit 30 so that the average power loss Pfav is substantially equal to the predetermined power loss Po, and the switch circuit is set according to the set duty ratio Du. 30 is PWM controlled.

  At that time, the control circuit 20 monitors the load current Ir based on the current detection signal SI, calculates the current ratio of the predetermined load current Io to the load current Ir (predetermined load current Io / load current Ir), The square of the calculated current ratio is set as the duty ratio Du.

  By setting the duty ratio Du of PWM control, that is, the duty ratio Du of the control signal Sgc in this way, the fuse 21 can be used even if the load current Ir exceeds the rated current Ifs of the fuse 21 as shown in FIG. It can be used safely in the allowable range.

  Hereinafter, a specific example will be described with reference to FIG. Here, the rated current Ifs of the fuse 21 is “15 A”, and the predetermined load current Io equal to or lower than the rated current Ifs is “12 A”. Also, the rated current of the load is 12A, and since it operates normally at 12A, it is not necessary to supply more current. The duty ratio Du of the switch circuit 30 is set so that the average power loss Pfav is substantially equal to the predetermined power loss Po when the fuse 21 is used with the load current Ir of “12A”. At that time, the duty ratio Du is set to the square of the current ratio of the predetermined load current Io to the load current Ir. Here, as the power loss of the fuse 21, transient power loss such as when the power supply voltage Vb is turned on is excluded.

  As shown in FIG. 3, the control circuit 20 sets the duty ratio Du to “100%” when the load current Ir, that is, the detected current value Id is “12A” which is the predetermined load current Io. The power loss Pf of the fuse 21 when the detected current value Id (load current Ir) is 12 A is set to a predetermined power loss Po. As shown in FIG. 3, the predetermined power loss Po is smaller than the power loss Pfs when the fuse 21 is used at the rated current Ifs. Note that the detected current value Id shown in FIG. 3 is the maximum value of the current detection signal SI, that is, the maximum value of the detected current.

For example, when the detected current value Id of the load current Ir is “14A”, that is, in the period up to time t1 in FIG. 3, the square of the current ratio of the predetermined load current Io to the load current Ir is (12/14 ) becomes a ² ≒ 0.734. Therefore, the control circuit 20 sets the duty ratio Du of the gate control signal Sgc to about “73%”. At this time, the load current Ir is a pulse current with a duty ratio Du of 73%, as shown in FIG.

Here, if the resistance value of the fuse 21 is Rf, the average power loss value Pfav of the fuse 21 is Pfav = Id ² × Du × Rf = 196 × 0.73 × Rf≈144Rf.
On the other hand, the predetermined power loss Po is Po = (predetermined load current Io) ² × Rf = 12 ² × Rf = 144Rf, and the average power loss Pfav is almost equal to the predetermined power loss Po.

Similarly, for example, when the detected current value Id of the load current Ir is “16 A” exceeding the rated current, that is, in the period from time t1 to time t2 in FIG. 3, the predetermined load current with respect to the load current Ir The square of the current ratio of Io is (12/16) ² = 0.56. Therefore, by setting the duty ratio Du of the gate control signal Sgc to “56%”, the average power loss Pfav becomes substantially equal to the predetermined power loss Po.

  In this way, the average power loss Pfav of the fuse 21 is made approximately equal to the predetermined power loss Po when the fuse 21 is used at a predetermined load current Io that is equal to or less than the rated current Ifs. As a result, even when the load current Ir exceeds the rated current Ifs of the fuse 21 in the allowable use range, the fuse 21 can be safely used in the allowable use range.

3. Effect of Embodiment 1 Even when the load current Ir exceeds the rated current Ifs of the fuse 21, the duty ratio Du of the PWM control, that is, the control signal is set so that the average power loss Pfav is substantially equal to the predetermined power loss Po. The duty ratio Du of Sgc is set. As a result, even if the load current Ir exceeds the rated current Ifs of the fuse 21, the fuse 21 can be safely used in the allowable use region of FIG. Therefore, for example, in selecting the fuse 21 when the predetermined load current Io is 12 A, a fuse with a rated current of 15 A can be selected instead of a fuse with a rated current of 20 A. That is, the selection of the fuse 21 can be optimized and the wire diameter can be reduced. As a result, it is possible to avoid the use of the fuse 21 having a rated current more than necessary, and the fuse 21 to be used can be downsized.

<Embodiment 2>
Next, with reference to FIGS. 4 and 5, the power supply control device 10 </ b> A of the second embodiment will be described. Note that differences from the power supply control device 10 according to the first embodiment will be mainly described, and redundant descriptions will be omitted.

  As illustrated in FIG. 4, the power supply control device 10 </ b> A includes an environmental temperature sensor (an example of a temperature detection unit) 24 and a fuse temperature calculation circuit 25 instead of the voltage detection circuit 22.

  For example, the environmental temperature sensor 24 is provided in the vicinity of the fuse 21 and detects the environmental temperature Ta in the vicinity of the fuse 21. Information on the detected ambient temperature Ta is provided to the fuse temperature calculation circuit 25.

  As shown below, the fuse temperature calculation circuit 25 increases the temperature rise ΔTf from the ambient temperature Ta of the fuse 21 based on the difference between the heat generation of the fuse 21 due to the load current Ir flowing through the fuse 21 and the heat dissipation of the fuse 21. The calculated fuse temperature Tf is calculated by adding the rising temperature ΔTf of the fuse 21 to the environmental temperature Ta, and the actual fuse temperature TF is estimated from the calculated fuse temperature Tf. Then, the fuse temperature calculation circuit 25 provides the control circuit 20 with information on the calculation fuse temperature Tf.

Here, for example, the fuse temperature calculation circuit 25 samples the load current Ir every predetermined time Δt, and substitutes the value of each load current Ir into the following equation (1) to calculate the fuse rising temperature ΔTf.
ΔTf (n) = ΔTf (n−1) × exp (−Δt / τf) + Rthf
× Rf (n−1) × Ir (n−1) ² × (1-exp (−Δt / τf)) (1)
Here, Ir (n): detection load current value (A) for detection n (an integer of 1 or more)
ΔTf (n): Fuse rise temperature at detection n times (° C)
Rf (n) = Rf (0) × (1 + κf × (Tf−To))
: Fuse resistance at detection n times (Ω)
Rf (0): Fuse resistance at temperature To (Ω)
Rthf: Thermal resistance of fuse (° C / W)
τf: fuse heat dissipation time constant (s)
κf: Fuse resistance temperature coefficient (/ ° C)
In Equation (1), the first term that does not include the current Ir indicates heat dissipation of the fuse 21, and the second term that includes the load current Ir indicates heat generation of the fuse 21 due to the load current Ir. That is, when the switch circuit 30 is turned off and the load current Ir is cut off and there is no load current Ir, the calculation fuse temperature Tf is determined by the heat radiation of the fuse 21.

  The control circuit 20 estimates the fuse temperature TF from the calculated fuse temperature Tf. Further, after the fuse temperature TF reaches the threshold temperature (an example of the first threshold temperature) Tth, the control circuit 20 controls the switching of the switch circuit 30 on and off by the same method as in the first embodiment. Thus, the fuse temperature TF is maintained at the threshold temperature Tth. As a result, the average power loss Pfav of the fuse 21 is controlled to be substantially equal to the predetermined power loss Po.

  Note that the threshold temperature Tth is determined in advance through experiments or the like as the upper limit temperature of the fuse 21 in the case of controlling switching of the switch circuit 30 between on and off. At that time, the threshold temperature Tth is determined with a predetermined margin with respect to the fusing temperature TM of the fuse 21, as shown in FIG.

  At that time, the control circuit 20 may determine the length of the next ON period Kon according to the rate of increase of the fuse temperature TF during the ON period Kon of the switch circuit 30. For example, the length of the next on period Kon may be shortened as the rate of increase in the fuse temperature TF increases.

  For example, in FIG. 5, it is assumed that the supply of the load current Ir is started at time t0 and the fuse temperature TF reaches the threshold temperature Tth at time t3. Assuming that the increase rate of the fuse temperature TF from the time t0 to the time t3 is “α”, the length of the ON period Kon from the time t4 is shortened as the increase rate α increases.

  This is due to the following reason. This is because the increase rate α usually depends on the load current Ir, and the decrease in the fuse temperature TF in the off period Koff of the load current Ir does not depend on the load current Ir. That is, a large increase rate α corresponds to a large load current Ir. Therefore, in order to suppress the rise in the fuse temperature TF, it is necessary to reduce the load current Ir and reduce the average power loss P of the fuse 21.

Note that the ON / OFF switching period of the switch circuit 30 after the fuse temperature TF reaches the threshold temperature Tth may be determined as appropriate according to the set amount of the temperature hysteresis shown in FIG.
Further, the control circuit 20 is not limited to maintaining the fuse temperature TF at the threshold temperature Tth after the fuse temperature TF reaches the threshold temperature (first threshold temperature) Tth. The control circuit 20 turns off the switch circuit 30 at the threshold temperature Tth and turns on the switch circuit 30 at the second threshold temperature lower than the threshold temperature Tth, thereby changing the fuse temperature TF between the threshold temperature Tth and the second threshold temperature. The value may be maintained. In this case, the fuse temperature TF can be maintained at a temperature within a predetermined temperature range between the threshold temperature Tth and the second threshold temperature.

4). The effect of Embodiment 2,
As described above, in the second embodiment, on / off switching of the switch circuit 30 is controlled while estimating the fuse temperature TF so that the fuse temperature TF does not exceed the predetermined threshold temperature Tth. As a result, the average power loss Pfav of the fuse 21 is controlled to be substantially equal to the predetermined power loss Po. Therefore, the selection of the fuse 21 can be made appropriate, and even if the load current Ir or the power supply voltage Vb fluctuates, the load 50 can be supplied with electric power without unnecessarily fusing the selected fuse 21. Can be supplied.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.

  (1) In the first embodiment, the control circuit 20 monitors the load current Ir based on the current detection signal SI, calculates the current ratio of the predetermined load current Io to the load current Ir, and squares the current ratio. Although an example in which is set as the duty ratio Du is shown, the present invention is not limited to this. The control circuit 20 monitors the power supply voltage Vb based on the voltage detection signal SV, and a voltage ratio of the predetermined power supply voltage Vo (12V in FIG. 2) corresponding to the predetermined load current Io to the power supply voltage Vb (predetermined power supply voltage Vo / The power supply voltage Vb) may be calculated, and the square of the voltage ratio may be set as the duty ratio of the control signal (PWM signal) Sgc.

  The power loss Pf of the fuse 21 is proportional to the square of the voltage drop of the fuse. Therefore, when the voltage drop of the fuse 21 is proportional to the power supply voltage Vb, for example, when the load resistance value is constant, the square of the voltage ratio of the predetermined power supply voltage Vo corresponding to the predetermined load current Io to the power supply voltage Vb is duty cycle By setting the ratio, the average value Pfav of the power loss of the fuse can be preferably made substantially equal to the predetermined power loss Po.

  (2) In the above embodiment, the duty ratio Du is set when controlling the on / off switching of the switch circuit 30 so that the average power loss value Pfav of the fuse 21 is substantially equal to the predetermined power loss Po. Although the example in which the switch circuit 30 is PWM-controlled by the set duty ratio Du is shown, the present invention is not necessarily limited thereto. For example, without setting a predetermined duty ratio Du, an average power loss value Pfav for a predetermined time is calculated, and an arbitrary on-time and an arbitrary on time so that the calculated average power loss value Pfav is substantially equal to the predetermined power loss Po. The switch circuit 30 may be turned on / off according to the off time.

  (3) In the said embodiment, although the example which comprises each circuit of the power supply control apparatus 10 and 10A as an individual circuit was shown, it is not restricted to this. For example, except for the environmental temperature sensor 24 and the switch circuit 30, the power supply control devices 10 and 10A may be configured by an ASIC (application-specific integrated circuit).

  DESCRIPTION OF SYMBOLS 10 ... Electric power supply control apparatus, 20 ... Control circuit, 21 ... Fuse, 22 ... Voltage detection circuit, 23 ... Current detection circuit (current detection part), 24 ... Environmental temperature sensor, 25 ... Fuse temperature calculation circuit, 30 ... Switch circuit , 31 ... main switch (switch circuit), 32 ... sense transistor (current detection unit), 40 ... power supply line (feeding path), 51 ... electric wire (feeding path), Ir ... load current, Ta ... environmental temperature, TF ... fuse Temperature, Tf: operational fuse temperature, ΔTf: rising temperature of the fuse,

Claims (7)

A power supply control device that is connected to a power supply path that supplies power from a power source to a load and controls power supply from the power source to the load,
A fuse provided in the power supply path and having a predetermined fusing characteristic indicating a relationship between current and fusing time;
A switch circuit that is provided in the power supply path and switches on / off of supply of load current from the power source to the load;
When the fuse is used in an allowable use area due to the fusing characteristics, the average power loss of the fuse is approximately equal to the predetermined power loss when the fuse is used at a predetermined load current equal to or lower than the rated current of the fuse. A control circuit for controlling on / off switching of the switch circuit so as to be equal;
A power supply control device comprising:   The control circuit sets a duty ratio for on / off control of the switch circuit so that an average value of power loss of the fuse is substantially equal to the predetermined power loss, and the switch is set according to the set duty ratio. The power supply control device according to claim 1, wherein the circuit performs PWM control. A current detection unit that detects the load current and generates a current detection signal;
The control circuit monitors the load current based on the current detection signal,
The power supply control device according to claim 2, wherein a current ratio of the predetermined load current to the load current is calculated, and a square of the current ratio is set as the duty ratio. A voltage detection unit that is connected to the power supply path, detects a power supply voltage of the power supply, and generates a voltage detection signal;
The control circuit monitors the power supply voltage based on the voltage detection signal, calculates a voltage ratio of a predetermined power supply voltage corresponding to the predetermined load current to the power supply voltage, and calculates the square of the voltage ratio as the duty The power supply control device according to claim 2, wherein the power supply control device is set as a ratio. A current detection circuit for detecting the load current;
A temperature detector for detecting the environmental temperature;
The temperature rise from the environmental temperature of the fuse is calculated based on the difference between the heat generation of the fuse due to the load current flowing through the fuse and the heat dissipation of the fuse, and the temperature rise of the fuse is calculated as the environmental temperature. A fuse temperature calculation circuit for adding and calculating a calculation fuse temperature,
The control circuit estimates the temperature of the fuse from the arithmetic fuse temperature,
After the temperature of the fuse reaches the first threshold temperature, the switching of the switch circuit is controlled to maintain the fuse temperature at the first threshold temperature, or the first threshold temperature And the second threshold temperature lower than the first threshold temperature, the average value of the power loss of the fuse is determined as the predetermined power when the fuse is used at a predetermined load current lower than the rated current of the fuse. The power supply control device according to claim 1, wherein the power supply control device is controlled so as to be substantially equal to the loss.   The control circuit turns off the switch circuit at the first threshold temperature and turns on the switch circuit at the second threshold temperature, thereby setting the fuse temperature between the first threshold temperature and the second threshold temperature. The power supply control device according to claim 5, wherein the power supply control device is maintained at a value of.   The power supply control device according to claim 5 or 6, wherein the control circuit determines a length of a next on-period according to a rate of increase in the fuse temperature during the on-period of the switch circuit.

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