JP7734643B2 - Switching Power Supply - Google Patents

Description

The present invention relates to a switching power supply device equipped with a dummy circuit that consumes dummy power at the output terminal where the output voltage is generated.

Switching-type power conversion circuits find it difficult to operate properly when the output current drops below a certain level. For example, when the current output by a power conversion circuit approaches zero amperes, the voltage across the switching element cannot change quickly when it is turned on and off, preventing the desired zero-volt switching operation. Pulses may also be missing at intervals several times the switching period, resulting in large output ripples (a problem of unstable operation under light loads).

Furthermore, when the state of the externally connected load changes and the current output by the power conversion circuit suddenly drops to near zero amperes, the output voltage Vo of the power conversion circuit rises significantly, and because the output capacitor connected to the output terminal does not discharge, the output voltage Vo is maintained at a high value and it takes a long time to return to the original set value Vor [dynamic load fluctuation problem].

To avoid these problems, it has been customary to connect a dummy resistor (fixed resistor) to the output terminal where the output voltage Vo is generated. Here, if the current flowing through the load is the output current Io and the current flowing through the dummy resistor is the dummy current Id, then by connecting the dummy resistor, the current output by the power conversion circuit becomes Io + Id, and the dummy current Id continues to flow even when the output current Io becomes zero, making it less likely that the two problems mentioned above will occur.

In addition, Patent Document 1 discloses a multi-output switching power supply that stabilizes and controls a single reference output voltage, and controls the sum of the output current Io (load current) of the reference output and the dummy current Id (current of a current-variable dummy resistor circuit) to a constant value. The purpose of controlling the dummy current Id in this way is to reduce and stabilize fluctuations in the output voltages of outputs other than the reference output.

JP 2016-42765 A

There are switching power supplies that can externally adjust the output voltage Vo over a wide range, but if a dummy resistor (fixed resistor) is connected to the output terminal and the design is such that an appropriate amount of dummy current Id flows when the output voltage Vo is set to a low voltage, the dummy current Id will be larger than necessary when the output voltage Vo is set to a high voltage, resulting in large, unnecessary losses in the dummy resistor. Conversely, if the design is such that an appropriate amount of dummy current Id flows when the output voltage Vo is set to a high voltage, the dummy current Id will be insufficient when the output voltage Vo is set to a low voltage, making it impossible to avoid problems such as unstable operation under light loads and dynamic load fluctuations. This is also true for the multi-output switching power supply described in Patent Document 1.

The present invention was made in consideration of the above-mentioned background art, and aims to provide a switching power supply device that can externally adjust the output voltage over a wide range and that always flows an appropriate amount of dummy current even when the output voltage setting is changed.

The present invention is provided with a switching-type power conversion circuit that converts an input voltage into a predetermined output voltage Vo, outputs the output voltage Vo and an output current Io from a pair of output terminals, and supplies output power Po (=Vo·Io) to a load, and a dummy circuit that is connected between the pair of output terminals and into which a dummy current Id flows, and generates dummy power Pd (=Vo·Id) by the flow of the dummy current Id,
The power conversion circuit has a function of externally varying a set value Vor of the output voltage Vo,
the dummy circuit comprises a voltage-controlled current source unit that receives a DC control voltage Vs and generates the dummy current Id corresponding to the control voltage Vs, a calculation unit that performs digital calculation processing to determine the value of the control voltage Vs, and a control voltage generation unit that generates the control voltage Vs of the value determined by the calculation unit and outputs it to the voltage-controlled current source unit;
The calculation unit is a switching power supply device that is set with a predetermined constant, a power threshold Pth (>0), acquires information on the set value Vor and the output current Io, and determines the value of the control voltage Vs so as to satisfy the condition Vor·(Id+Io)≧Pth.
For example, the power conversion circuit controls its own switching operation so that the output voltage Vo approaches a set value Vor, and has a function of externally varying the set value Vor.

A dummy power lower limit value Pdmin (Pth > Pdmin > 0) is set in the calculation unit, and the calculation unit acquires information on the set value Vor and the output current Io and determines the value of the control voltage Vs so as to satisfy the conditions Vor·(Id + Io) ≧ Pth and Vor·Id ≧ Pdmin. In this case, the dummy power lower limit value Pdmin can be configured to be defined by a formula that includes the output current Io as a variable, so that it becomes smaller as the output current Io increases. Alternatively, the dummy power lower limit value Pdmin can be configured to be defined by a formula that does not include the output current Io as a variable.

Furthermore, the power conversion circuit has an overvoltage protection function that stops power conversion operation when the output voltage Vo exceeds an overvoltage threshold Vovp, and the calculation unit is set to a predetermined constant voltage threshold Vth (> 0), and the formula defining the dummy power lower limit Pdmin includes the set value Vor as a variable within the range where the set value Vor satisfies the condition Vovp - Vth < Vor < Vovp, the condition Pdmin < Pth is abolished, and the dummy power lower limit Pdmin increases as the set value Vor increases.

Furthermore, it is preferable that the control voltage generation unit is composed of a pulse width modulation unit that outputs a square wave voltage whose duty is set based on information about the value of the control voltage Vs determined by the calculation unit, and a low-pass filter that smooths the square wave voltage to generate the control voltage Vs.

The switching power supply of the present invention can externally adjust the output voltage Vo over a wide range, and the dummy current Id is always set to an appropriate magnitude even when the output voltage Vo setting is changed. Furthermore, the dummy current Id always changes to an appropriate value even when the output current Io flowing to the load changes. Therefore, it is possible to avoid problems such as unstable operation under light loads and dynamic load fluctuations in the power conversion circuit, while effectively suppressing unnecessary losses by reducing the dummy current Id.

1 is a circuit diagram showing a first embodiment of a switching power supply device according to the present invention; 2A is a block diagram showing the input and output of the calculation unit of FIG. 1, and FIG. 2B is a graph showing an outline of the operation of this calculation unit. 2A is a block diagram showing an example of a specific configuration of the control voltage generating unit of FIG. 1 , and FIG. 2B is a block diagram showing another example. 2A is a circuit diagram showing an example of a specific configuration of the voltage-controlled current source of FIG. 1, and FIG. 2B is a graph showing the relationship between input and output. 10A is a graph showing specific numerical examples of the change in dummy current and total current relative to the output current of the switching power supply device of the first embodiment, where FIG. 10B is a graph showing the change in output voltage set to 10V. 2A is a block diagram showing the input and output of a modified example of the calculation unit of FIG. 1 , and FIG. 2B is a graph showing an outline of the operation of this calculation unit. FIG. 4 is a circuit diagram showing a second embodiment of a switching power supply device according to the present invention. 8A is a block diagram showing the input and output of the calculation unit of FIG. 7, and FIG. 8B is a graph showing an outline of the operation of this calculation unit. 10A is a graph showing specific numerical examples of the change in dummy current and total current relative to the output current of the switching power supply device of the second embodiment, where FIG. 10A is a graph when the output voltage is set to 10 V, and FIG. 10B is a graph when the output voltage is set to 12 V.

<Switching power supply device 10 of the first embodiment>
A first embodiment of a switching power supply of the present invention will now be described with reference to Figures 1 to 5. As shown in Figure 1, a switching power supply 10 of this embodiment includes a switching-type power conversion circuit 12 that converts an input voltage Vi into a predetermined output voltage Vo. The power conversion circuit 12 has an input power supply 16 connected to a pair of input terminals 14 and a load 20 connected to a pair of output terminals 18, and outputs an output voltage Vo and an output current Io via the output terminals 18, supplying output power Po (=Vo·Io) to the load 20.

The power conversion circuit 12 also has the ability to externally vary the output voltage Vo. Specifically, a set value command S(Vor) that defines the set value Vor of the output voltage Vo is input from the outside, and switching operation is controlled so that the output voltage Vo approaches the set value Vor recognized from the set value command S(Vor). The method for inputting the set value command S(Vor) is not particularly limited; for example, it may be input as a digital signal from an external device, or a volume or the like may be connected to a dedicated variable terminal provided on the power conversion circuit 12, and the set value Vor may be manually adjusted.

A dummy circuit 22 is connected between the pair of output terminals 18, generating dummy power Pd (= Vo·Id) by the flow of a dummy current Id. The dummy circuit 22 is composed of a voltage-controlled current source unit 24 that receives a DC control voltage Vs and generates a dummy current Id corresponding to the control voltage Vs, a calculation unit 26 that performs digital calculation processing to determine the value of the control voltage Vs, and a control voltage generation unit 28 that generates the control voltage Vs of the value determined by the calculation unit 26 and outputs it to the voltage-controlled current source unit 24.

The calculation unit 26 is set with a predetermined constant power threshold Pth (>0) and a dummy power lower limit Pdmin (Pth > Pdmin > 0). In the case of this calculation unit 26, the dummy power lower limit Pdmin is set using a formula that includes the output current Io as a variable, so that the larger the output current Io, the smaller Pdmin becomes.

Then, as shown in FIG. 2(a), the calculation unit 26 acquires the set value information J(Vor) and the output current information J(Io), determines the value of the control voltage Vs so as to satisfy both conditions of the following equations [1] and [2], and outputs the control voltage command S(Vs), which is a digital signal.
Vor・(Id+Io)≧Pth [1]
Vor·Id≧Pdmin [2]

Note that the set value information J(Vor) is information about the set value Vor recognized from the set value command S(Vor), and the output current information J(Io) is information about the detected value of the output current Io. However, the set value information J(Vor) may use other information, i.e., information other than the set value Vor recognized from the set value command S(Vor). This other information will be explained in the <Other embodiments, variations, etc.> section below.

The control voltage generation unit 28 is a block that generates an analog control voltage Vs corresponding to the control voltage command S(Vs), which is a digital signal. As shown in Figure 3(a), it is composed of a pulse width modulation unit 28a that outputs a square wave voltage Vp (peak value V1) whose duty D is set based on the control voltage command S(Vs) output by the calculation unit 26, and a low-pass filter 28b that smooths the square wave voltage Vp to generate a control voltage Vs ≈ V1 · D.

The voltage-controlled current source unit 24 can use a constant current circuit that combines a transistor, an operational amplifier, and a resistor, as shown in Figure 4(a), for example. In this circuit, the dummy current Id increases as the control voltage Vs increases, as shown in Figure 4(b).

Next, we will explain in detail the meaning of the above equations [1] and [2] and the operation of the switching power supply 10 (the operation of the dummy circuit 22). Here, the current output by the power conversion circuit 12 (the sum of the output current Io and the dummy current Id) is referred to as the total current Iall, and the power output by the power conversion circuit 12 (the sum of the output power Po and the dummy power Pd) is referred to as the total power Pall.

The inventors noticed that when the power threshold Pth is set to the lower limit of the total power Pall that can avoid the problem of unstable operation under light loads in a switching-type power conversion circuit, the power threshold Pth remains almost constant even when the output voltage Vo is changed, and came up with the idea of adjusting the dummy power Pd so that the following equation [3] is satisfied.

Pall=Pd+Po=Vo・(Id+Io)≧Pth [3]
If the dummy power Pd is set to satisfy equation (3), the total power Pall can be maintained at a value equal to or greater than the power threshold Pth even if the output power Po becomes zero. Furthermore, if the output power Po increases from zero, the dummy power Pd can be reduced accordingly, reducing unnecessary losses. Furthermore, in many cases, satisfying equation (3) can avoid the problem of dynamic load fluctuations, so if Po increases to a value exceeding Pth, the dummy power Pd can be set to zero.

However, if the dummy power Pd is set to zero, problems other than the two problems mentioned above may occur, so it is preferable to set a dummy power lower limit value Pdmin (<Pth) as shown in equation [4] and generate a certain amount of dummy power Pd.
Pd＝Vo・Id≧Pdmin [4]

The power conversion circuit 12 operates so that the output voltage Vo is equal to the set value Vor, so Vo = Vor in effect. Therefore, by replacing Vo with Vor in equations [3] and [4], the above equations [1] and [2] can be obtained.

The dummy circuit 22 automatically adjusts the value of the dummy current Id based on equations [1] and [2], the set value information J(Vor), and the output current information J(Io). Figures 5(a) and (b) provide numerical examples to clearly illustrate the operation of the dummy circuit 22. Figure 5(a) is a graph showing the results when the power threshold Pth is 1W and the set value Vor is 5V (output voltage Vo = 5V). The horizontal axis represents the output current Io, and the vertical axis represents the current at each component. When the output current Io is zero, the dummy current Id is 0.2A (=1W/5V), which corresponds to the power threshold Pth. As the output current Io increases, the dummy current Id gradually decreases toward zero, gradually decreasing along the line corresponding to the dummy power lower limit Pdmin (the Pdmin/5V line).

Figure 5(b) is a graph showing the results when the power threshold Pth is 1W and the set value Vor is 10V (output voltage Vo = 10V). When the output current Io is zero, the dummy current Id is 0.1A (=1W/10V), which corresponds to the power threshold Pth. As the output current Io increases, the dummy current Id gradually decreases, approaching zero and gradually decreasing along the dummy power lower limit Pdmin line (Pdmin/10V).

In this way, when the output voltage Vo is high, the dummy current Id becomes relatively small, and when the output voltage Vo is low, the dummy current Id becomes relatively large, so that even if the output voltage Vo setting is changed, the dummy power Pd is kept almost constant.

As described above, the switching power supply 10 can externally vary the output voltage Vo over a wide range, and the dummy current Id is always set to an appropriate magnitude even when the output voltage Vo setting is changed. Furthermore, the dummy current Id always changes to an appropriate value even when the output current Io changes. Therefore, it is possible to avoid problems such as unstable operation under light loads and dynamic load fluctuations in the power conversion circuit 12, while effectively suppressing the occurrence of excessive dummy loss Pd by reducing the dummy current Id.

Furthermore, the operation of the calculation unit 26 of the dummy circuit 22 can be freely set by rewriting the program. For example, it can be set to an operating mode that "adjusts the dummy current" under normal circumstances, and then switched to a different operating mode when a specific situation arises. Therefore, by providing this dummy circuit 22, it is possible to perform dummy current control for a different purpose when a specific situation arises, enabling highly intelligent and advanced control.

<Modification of the first switching power supply device 10>
Modifications of the control voltage generating unit 28 and the calculating unit 26 of the first switching power supply device 10 will be described with reference to FIG. 3B and FIG.

The control voltage generator 28 (Figure 3(a)) described above has a unique pulse width modulation section 28a within a digital processor, combined with a low-pass filter 28b, which has the advantage of being able to generate the control voltage Vs with high resolution even when using an inexpensive, general-purpose digital processor. However, as shown in Figure 3(b), it may be replaced with a control voltage generator 28x consisting of a single D/A converter, thereby reducing the number of parts. However, when using the control voltage generator 28x, it should be noted that a relatively expensive, high-speed digital processor is required to increase the resolution of the control voltage Vs.

The calculation unit 26 (Figures 2(a) and 2(b)) is also configured with a dummy power lower limit Pdmin (Pth > Pdmin > 0), which is defined by a formula that includes the output current Io as a variable, so that the dummy power Pdmin decreases as the output current Io increases. However, the dummy power lower limit Pdmin (Pth > Pdmin > 0) may also be defined by a formula that does not include the output current Io as a variable. For example, in the calculation unit 26x of the modified example shown in Figures 6(a) and 6(b), the dummy power lower limit Pdmin is defined as a predetermined constant. In this case, although the amount of wasted dummy power Pd increases slightly when the output current Io is large, this has the advantage of reducing the processing load on the calculation unit 26x. Alternatively, if certain conditions are met, the dummy power lower limit Pdmin may be set to zero regardless of the magnitude of the output current Io.

<Switching power supply device 30 of second embodiment>
Next, a second embodiment of the switching power supply of the present invention will be described with reference to Figures 7 to 9. Here, the same components as those in the above embodiment are given the same reference numerals and the description thereof will be omitted.

As shown in FIG. 7, the switching power supply 30 of this embodiment differs from the switching power supply 10 in that the power conversion circuit 12 is replaced with a power conversion circuit 32, and the calculation unit 26 (calculation unit 26x) of the dummy circuit 22 is replaced with a calculation unit 34.

The power conversion circuit 32 operates in the same manner as the switching power supply 12 described above, and is further characterized by having an overvoltage protection function that stops power conversion operation when the output voltage Vo exceeds the overvoltage threshold Vovp. The calculation unit 34 basically operates in the same manner as the calculation unit 26x described above, and is further characterized by having a predetermined constant voltage threshold Vth (>0) set, and the formula defining the dummy power lower limit Pdmin including the set value Vor as a variable.

Here, we will explain the "malfunction of the overvoltage protection function" that is likely to occur in switching power supplies equipped with overvoltage protection functions.

When the set value Vor is close to the overvoltage threshold Vovp, the problem of dynamic load fluctuations described in the Background Art section becomes more pronounced. When the set value Vor is close to the overvoltage threshold Vovp, even a small increase in the output voltage Vo can cause the overvoltage threshold Vovp to be exceeded, which could activate the overvoltage protection function and shut down the power conversion circuit 12 despite normal power conversion operation. Therefore, when the set value Vor is close to the overvoltage threshold Vovp, preventing erroneous operation of the overvoltage protection function due to dynamic load fluctuations becomes a major issue.

The switching power supply unit 30 therefore has a unique calculation unit 34 that increases the dummy power Pd especially when the set value Vor is close to the overvoltage threshold Vovp, thereby avoiding the problem of the overvoltage protection function operating erroneously.

Explaining the calculation unit 34 in more detail, the formula of the calculation unit 34 defines the dummy power lower limit Pdmin as a predetermined constant (Pth > Pdmin > 0) as shown in Figure 8(b) when the set value Vor is in the range where the condition Vor <= Vovp - Vth is satisfied (when the set value Vor is sufficiently lower than the overvoltage threshold Vovp). This is similar to the calculation unit 26x described above, and the graph in Figure 8(b) is similar to the graph of the calculation unit 26x described above (Figure 6(b)).

Furthermore, the formula of the calculation unit 34 specifies that when the set value Vor is in the range where the condition Vovp - Vth < Vor < Vocp is satisfied (when the set value Vor is close to the overvoltage threshold Vovp), the condition Pdmin < Pth related to the dummy power lower limit Pdmin is abolished, and as shown in Figure 8 (c), the higher the set value Vor, the larger the dummy power lower limit Pdmin becomes. This is a unique feature added to the above-mentioned calculation unit 26x, and means that when the set value Vor is close to the overvoltage threshold Vovp, the dummy power Pd is made especially large.

Figures 9(a) and (b) are numerical examples to clearly illustrate the operation of the dummy circuit 34. Figure 9(a) is a graph showing the results when the power threshold Pth = 1W, the overvoltage threshold Vovp = 13V, the voltage threshold Vth = 2V, and the set value Vor = 10V (output voltage Vo = 10V). Here, the set value Vor = 10V is determined to be sufficiently lower than the overvoltage threshold Vovp = 13V, and the dummy circuit 26x operates in the same manner as described above. In other words, when the output current Io = zero, the dummy current Id is 0.1A (= 1W/10V), which corresponds to the power threshold Pth. As the output current Io increases, the dummy current Id gradually decreases and is maintained at a constant value along the line corresponding to the dummy power lower limit Pdmin (the Pdmin/10V line).

Figure 9 (b) is a graph showing the case where the power threshold Pth = 1W, the overvoltage threshold Vovp = 13V, the voltage threshold Vth = 2V, and the set value Vor = 12V (output voltage Vo = 12V). Here, the set value Vor = 12V is determined to be close to the overvoltage threshold Vovp = 13V, the dummy power lower limit Pdmin increases beyond the power threshold Pth, and the dummy current Id is maintained at a constant value along the line corresponding to the dummy power lower limit Pdmin (the Pdmin/12V line).

In this way, the dummy power Pd becomes particularly large when the set value Vor is close to the overvoltage threshold Vovp, so the increase in the output voltage Vo when a dynamic load fluctuation occurs can be made very small.Even if the overvoltage threshold Vovp is slightly exceeded, the time it takes to return to the original set value Vor is shortened.Therefore, by adjusting the responsiveness of the overvoltage protection, it is possible to avoid erroneous operation of the overvoltage protection function.

As described above, the switching power supply unit 30 can achieve the same effects as the switching power supply unit 10 described above, and can also easily and effectively prevent malfunction of the overvoltage protection function.

<Other embodiments, modifications, etc.>
The switching power supply of the present invention is not limited to the above embodiment. For example, in the case of the above-described calculation units 26, 26x, and 34, the set value information J(Vor) acquired is information on the set value Vor recognized from the set value command S(Vor). This has the advantage of enabling the dummy power Pd to be changed quickly even if there is a time delay between the issuance of a command to change the output voltage Vo and the actual change in the output voltage Vo. However, the set value information J(Vor) may be information other than information on the set value Vor recognized from the set value command S(Vor). For example, if the power conversion circuit 12 or 32 has an error amplifier circuit that amplifies the difference between the output voltage Vo and a reference voltage that is the target value of the output voltage Vo, information on the set value Vor recognized from the value of the reference voltage may be used as the set value information J(Vor).

The voltage-controlled current source unit is not limited to the configuration of the voltage-controlled current source unit 24 shown in Figure 4(a), as long as it can pass a dummy current Id corresponding to the control voltage Vs generated by the control voltage generation unit. The power conversion circuit may be a switching converter, and the inverter circuit type is not particularly limited, as long as the above-mentioned effects can be obtained. It also does not matter whether the input is AC or DC, or whether the input/output is isolated or non-isolated.

In addition, the specific numerical examples shown in Figures 5 and 9 are presented solely to make the operation easier to understand, and it goes without saying that the present invention is not limited to these numerical values.

10, 30 Switching power supply device 12, 32 Power conversion circuit 18 Output terminal 20 Load 22 Dummy circuit 24 Voltage controlled current source section 26, 26x, 34 Calculation section 28, 28x Control voltage generation section 28a Pulse width modulation section 28b Low pass filter
D Duty
Iall total current
Id dummy current
Io output current
J(Io) Output current information
J(Vor) setting value information
Pd Dummy power
Pdmin Dummy power lower limit
Po Output Power
Pth Power Threshold
S(Vor) Set point command
S(Vs) Control voltage command
Vo output voltage
Vor Output voltage setting
Vovp Overvoltage threshold
Vp Square wave voltage
Vs control voltage
Vth voltage threshold

Claims (6)

a switching-type power conversion circuit that converts an input voltage into a predetermined output voltage Vo, outputs the output voltage Vo and an output current Io from a pair of output terminals, and supplies output power Po (= Vo·Io) to a load; and a dummy circuit that is connected between the pair of output terminals and into which a dummy current Id flows, and generates dummy power Pd (= Vo·Id) when the dummy current Id flows,
the power conversion circuit controls its own switching operation so that the output voltage Vo approaches a set value Vor, and has a function of externally varying the set value Vor;
the dummy circuit comprises a voltage-controlled current source unit that receives a DC control voltage Vs and generates the dummy current Id corresponding to the control voltage Vs, a calculation unit that performs digital calculation processing to determine the value of the control voltage Vs, and a control voltage generation unit that generates the control voltage Vs of the value determined by the calculation unit and outputs it to the voltage-controlled current source unit;
a predetermined constant, a power threshold value Pth (>0), is set in the calculation unit; the calculation unit acquires information on the set value Vor and the output current Io, and determines the value of the control voltage Vs so as to satisfy the condition Vor·(Id+Io)≧Pth. The switching power supply device of claim 1, wherein a dummy power lower limit value Pdmin (Pth > Pdmin > 0) is set in the calculation unit, and the calculation unit acquires information on the set value Vor and the output current Io and determines the value of the control voltage Vs so as to satisfy the conditions Vor·(Id + Io) ≧ Pth and Vor·Id ≧ Pdmin. a switching-type power conversion circuit that converts an input voltage into a predetermined output voltage Vo, outputs the output voltage Vo and an output current Io from a pair of output terminals, and supplies output power Po (= Vo·Io) to a load; and a dummy circuit that is connected between the pair of output terminals and into which a dummy current Id flows, and generates dummy power Pd (= Vo·Id) when the dummy current Id flows,
The power conversion circuit has a function of externally varying a set value Vor of the output voltage Vo,
the dummy circuit comprises a voltage-controlled current source unit that receives a DC control voltage Vs and generates the dummy current Id corresponding to the control voltage Vs, a calculation unit that performs digital calculation processing to determine the value of the control voltage Vs, and a control voltage generation unit that generates the control voltage Vs of the value determined by the calculation unit and outputs it to the voltage-controlled current source unit;
a predetermined constant power threshold value Pth (>0) and a dummy power lower limit value Pdmin (Pth>Pdmin>0) are set in the calculation unit, and the dummy power lower limit value Pdmin is defined by a mathematical formula including the output current Io as a variable so that it becomes smaller as the output current Io is larger; and the calculation unit obtains information on the set value Vor and the output current Io, and determines the value of the control voltage Vs so as to satisfy the conditions Vor·(Id+Io)≧Pth and Vor·Id≧Pdmin . The switching power supply device of claim 2, wherein the dummy power lower limit value Pdmin is defined by a mathematical formula that does not include the output current Io as a variable. the power conversion circuit has an overvoltage protection function that stops a power conversion operation when the output voltage Vo exceeds an overvoltage threshold Vovp;
5. The switching power supply device according to claim 3, wherein a voltage threshold value Vth (>0) that is a predetermined constant is set in the calculation unit, and the formula that defines the dummy power lower limit value Pdmin includes the set value Vor as a variable within a range in which the set value Vor satisfies the condition Vovp-Vth<Vor<Vovp, the condition Pdmin<Pth is abolished, and the dummy power lower limit value Pdmin becomes larger as the set value Vor becomes higher. 5. The switching power supply device according to claim 1, wherein the control voltage generation unit comprises: a pulse width modulation unit that outputs a rectangular wave voltage whose duty is set based on information about the value of the control voltage Vs determined by the calculation unit; and a low -pass filter that smooths the rectangular wave voltage to generate the control voltage Vs.