JP3598896B2 - Power supply - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a power supply device, and more particularly, to a power supply device including a snubber circuit that suppresses a surge voltage and the like.
[0002]
[Prior art]
FIG. 7 shows a configuration example of a conventional power supply device. As shown in the figure, the power supply device 50 includes a transformer 12 having a primary winding 12A, a secondary winding 12B, and a control winding 12C, and the voltage of an input power supply 14 to the primary winding 12A of the transformer 12. A switching element 18 composed of a transistor for performing switching of application / non-application (switching), a control circuit 52 connected to a control winding 12C of the transformer 12 for controlling a switching operation of the switching element 18, and a secondary of the transformer 12 The power supply includes a diode 24A for rectifying and smoothing the output from the winding 12B and a rectifying / smoothing circuit 24 having a capacitor 24B. This type of power supply device is generally called a ringing choke converter.
[0003]
In addition, there is a power supply device having a delay circuit in the control circuit 52 in FIG. 7 and performing an operation of delaying the turn-on timing of the switching element 18 by the delay circuit. This type of power supply device is generally called a quasi-resonant power supply. I have.
[0004]
In such a power supply device as a ringing choke converter or a quasi-resonant power supply, a snubber capacitor 20 as a snubber circuit is often connected in parallel with the switching element 18 as shown in FIG. This snubber capacitor 20 is widely used to suppress an increase in the oscillation frequency and intermittent oscillation under a light load, to reduce the rising slope of the voltage applied to the switching element 18, to suppress the switching loss and the peak value of the surge voltage, and the like. The ringing choke converter is called a snubber capacitor, and the quasi-resonant power supply is called a resonance capacitor.
[0005]
Also, a technique of connecting a resistor of several ohms to several tens of ohms in series with the snubber capacitor 20 for suppressing the peak value of the voltage of the switching element 18 and the vibration is widely known.
[0006]
In the power supply device as described above, as means for detecting the output current level of the power supply device inside the power supply device, as shown in FIG._C, Or a load current I output from the rectifying / smoothing circuit 24 and flowing to a load (not shown)._OThere is known a technique in which a current detection circuit 56 for detecting a current is applied (for example, see JP-A-7-59346).
[0007]
8A and 8B show specific examples of the current detection circuit 54, and FIGS. 8C and 8D show specific examples of the current detection circuit 56.
[0008]
That is, FIG. 8A shows the case where the resistor 60 is applied as the current detection circuit 54, and FIG. 8B shows the case where the current detection circuit 54 is connected to the current transformer 62 and the secondary winding of the current transformer 62. This is a circuit to which a circuit composed of a diode 64 and a resistor 66 is applied. 8C illustrates the current detection circuit 56 to which the resistor 68 is applied as in FIG. 8A, and FIG. 8D illustrates the current detection circuit 56 as in FIG. 8B. And a circuit formed by a current transformer 70 and a diode 72 and a resistor 74 connected to the secondary winding of the current transformer 70.
[0009]
The current detection circuit (hereinafter, referred to as a “current detection method using a resistor”) illustrated in FIGS. 8A and 8C detects a current flowing through the resistor, and outputs an output of the power supply device based on the detected current. The current level is detected, and the current detection circuit shown in FIGS. 8B and 8D (hereinafter, referred to as a “current detection method using a current transformer”) detects the current flowing in the current transformer. , Detecting the output current level of the power supply device based on the detected current.
[0010]
In the power supply device 50 configured as described above, the control circuit 52 controls the overcurrent protection based on the current level detected by the current detection circuit 54 or the current detection circuit 56, and controls the frequency at light load. Various controls such as reduction are performed.
[0011]
However, the power supply device configured as described above has the following problems depending on the current detection method.
[0012]
In the current detection method using a resistor, all of the current flowing through the primary winding 12A or the secondary winding 12B of the transformer 12 flows through the current detection resistor, so that the power loss is large. Further, in the current detection method using the current transformer, the cost is significantly increased as compared with the current detection method using the resistor, and if the current transformer is an ideal transformer, no power loss occurs, but it is not an ideal transformer. Therefore, power loss occurs.
[0013]
As techniques that can be applied to solve such problems, there are techniques described in JP-A-6-245498, JP-A-7-274493, and JP-A-8-47252.
[0014]
Japanese Patent Application Laid-Open No. 6-245498 discloses a technique in which a no-load winding is provided separately from a load winding for supplying a load current to a transformer, and an output voltage obtained by the load winding and a terminal voltage of the no-load winding. And the output current of the load winding is determined based on the difference voltage.
[0015]
Further, the technology described in Japanese Patent Application Laid-Open No. 7-274493 discloses that a voltage generated in a current detection resistor connected in series with a switching element is superimposed on a previously generated voltage, and the superimposed voltage and a reference voltage are compared with a comparator. Thus, the overcurrent can be detected even when the resistance value of the current detection resistor is low, and accordingly, the power consumption of the detection resistor is reduced. As a result, the loss can be reduced as a result.
[0016]
Further, the technique described in Japanese Patent Application Laid-Open No. 8-47252 is to apply a FET (field effect transistor) having a sense terminal as a switching element to detect a current flowing through the sense terminal. Since the current is a small current at a fixed ratio to the current, the loss can be reduced as compared with the related art.
[0017]
[Problems to be solved by the invention]
However, the technique described in Japanese Patent Application Laid-Open No. 6-245498 has a problem that the cost is increased and the circuit configuration is complicated because it is necessary to provide a no-load winding as described above.
[0018]
Further, according to the technique described in Japanese Patent Application Laid-Open No. 7-274493, when the threshold voltage of the comparator varies, the detection level varies greatly as compared with a conventional circuit, and current detection with high accuracy can be performed. There was a problem that it could not.
[0019]
Furthermore, the technique described in Japanese Patent Application Laid-Open No. 8-47252 has a problem in that it is necessary to use a special component such as an FET with a sense terminal, so that the flexibility of circuit design is low and the cost of a power supply device is high. there were.
[0020]
SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a power supply device capable of detecting an output current level with a simple circuit configuration, with low loss, high accuracy, and low cost. Aim.
[0021]
[Means for Solving the Problems]
To achieve the above object, a power supply device according to claim 1 includes a transformer having a primary winding and a secondary winding, and an input connected to the primary winding of the transformer connected in series to the primary winding. A switching element for switching, a snubber circuit connected in parallel to the switching element, a detecting means for detecting a current flowing through the snubber circuit, and a detection result obtained by the detecting means.At least one of control for overcurrent protection and control of a switching frequency of the switching element.And control means for performing the above control.
[0022]
According to the power supply device of the first aspect, the input to the primary winding is switched by the switching element connected in series to the primary winding of the transformer having the primary winding and the secondary winding, and the snubber circuit is connected in parallel. Is done. Note that the switching element includes a bipolar transistor, a MOS transistor, and the like. In addition, the snubber circuit suppresses the rise of the oscillation frequency and the intermittent oscillation at the time of light load as described above, reduces the slope of the rise of the voltage applied to the switching element, suppresses the switching loss and the peak value of the surge voltage, and the like. Widely used for this purpose, such as those composed of only a capacitor, those composed of a series circuit of a capacitor and a resistor, those composed of semiconductor elements having non-linear characteristics, etc. There is. The snubber circuit may be called a surge absorber circuit.
[0023]
Further, in this power supply device, the current flowing through the snubber circuit is detected by the detection means, and based on the detection result,At least one of control for overcurrent protection and control of the switching frequency of the switching elementIs controlled byDone.
[0024]
That is, when the switching element is turned off, the peak value of the current flowing into the snubber circuit is substantially equal to the peak value of the current flowing in the primary winding of the transformer at that time.
[0025]
Therefore, at the time of rated output of the power supply device, since the peak value of the current flowing through the primary winding is large, a current having substantially the same peak value flows into the snubber circuit. On the other hand, when the load is light, the peak value of the current flowing through the primary winding is small, and the peak value of the current flowing into the snubber circuit is also small.
[0026]
That is, by detecting the peak value of the current flowing into the snubber circuit, the control means can determine the output current level of the power supply device. Since the effective value of the current flowing into the snubber circuit is usually sufficiently smaller than the current flowing through the primary winding, a simple circuit configuration can be obtained by detecting the current flowing into the snubber circuit from the current flowing through the primary winding. As it is, the power loss in the detection means can be greatly improved.
[0027]
As described above, according to the power supply device of the first aspect, since the current flowing through the snubber circuit is detected by the detecting means, the output current level can be detected with a simple circuit configuration, with low loss, high accuracy, and low power. Cost can be done.
According to the power supply device of the first aspect, at least one of the control for the overcurrent protection and the control of the switching frequency of the switching element is performed based on the detection result by the detection unit. At least one of the control for current protection and the control of the switching frequency of the switching element can be performed with a simple circuit configuration, with low loss, high accuracy, and low cost.
[0028]
Further, in the power supply device according to claim 2, in the invention according to claim 1, when the snubber circuit is constituted by a capacitor, the detection means is a resistor connected in series to the capacitor, When the circuit is composed of a series circuit of a capacitor and a resistor, it is a voltage dividing resistor connected in parallel to the resistor of the snubber circuit.
[0029]
Therefore, according to the power supply device of the second aspect, the same effect as the invention of the first aspect can be obtained, and the detecting means is constituted only by the resistor. The circuit configuration can be simplified as compared with the case of using the device, and the cost of the device can be reduced.
[0030]
Further, in the power supply device according to claim 3, in the invention according to claim 2, when the snubber circuit is configured by a capacitor, a capacitor is connected in parallel with a resistor connected in series to the capacitor, and When the circuit is constituted by a series circuit of a capacitor and a resistor, a capacitor is connected in parallel with the voltage dividing resistor.
[0031]
That is, the smaller the capacitance of the capacitor of the snubber circuit is, the shorter the period of the current flowing into the capacitor is. Therefore, when the capacitance of the capacitor is relatively small, the current flowing through the resistor connected in series with the capacitor or the voltage dividing resistor is used. It is difficult to detect the peak value directly from the data. Therefore, in the invention according to the third embodiment, the level of the current can be detected as an integrated value by connecting a capacitor connected in series to the capacitor or a capacitor in parallel with the voltage dividing resistor. ing.
[0032]
Thus, according to the power supply device of the third aspect, the same effect as the invention of the second aspect can be obtained, and when the snubber circuit is constituted by a capacitor, it is connected in series to the capacitor. When the snubber circuit is composed of a series circuit of a capacitor and a resistor, the capacitor is connected in parallel with the voltage dividing resistor. As a result, the output current level can be reliably detected.
[0033]
The power supply device according to the present invention further includes a drive circuit that generates a drive signal for causing the switching element to perform a switching operation, as in the invention described in claim 4, and performs control for the overcurrent protection. The drive signal may be stopped.
In particular, the invention according to claim 4 further comprises a comparator that outputs a latch signal when the level of the voltage generated in the detection means exceeds a predetermined reference voltage, as in the invention according to claim 5. The control for overcurrent protection may be control for stopping the drive signal by the latch signal.
In the power supply device according to the present invention, the switching frequency may be controlled when an output current level determined by a current detected by the detection unit is smaller than a predetermined value. Control may be performed so that the oscillation frequency of the switching element does not increase.
In particular, according to a sixth aspect of the present invention, as in the seventh aspect of the invention, a minimum off-time determining circuit for determining a minimum off-time of the switching element, and a drive signal for causing the switching element to perform a switching operation. A drive circuit that generates the output of the drive circuit by the minimum off-time determination circuit when the output current level is smaller than a predetermined value. Control that does not turn on until it passes can be performed.
[0036]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of a power supply device of the present invention will be described in detail with reference to the drawings.
[0037]
[First Embodiment]
First, a configuration of a power supply device 10 according to the present embodiment will be described with reference to FIG. As shown in FIG. 1, a power supply device 10 according to the present embodiment includes a transformer 12 having a primary winding 12A, a secondary winding 12B, and a control winding 12C. One terminal of the line 12 </ b> A is connected to one terminal of the input power supply 14.
[0038]
Further, the power supply device 10 includes a switching element 18 composed of a transistor for switching (switching) application / non-application of a predetermined voltage to the primary winding 12A by the input power supply 14, and the collector of the switching element 18 is a primary winding. The emitter is connected to the other terminal of the input power supply 14.
[0039]
On the other hand, a control circuit 16 for controlling the switching operation of the switching element 18 and the like is connected to both terminals of the control winding 12C of the transformer 12, and one output terminal of the control circuit 16 is connected to the switching element 18. The other output terminal is connected to the base, and the other output terminal is connected to the emitter of the switching element 18.
[0040]
Further, a snubber capacitor 20 and a current detection circuit 22 connected in series are connected in parallel between the collector and the emitter of the switching element 18.
[0041]
The current detection circuit 22 according to the present embodiment includes a resistor 30, as shown in FIG. 2A, and both terminals of the resistor 30 are connected to the control circuit 16. In this embodiment, the resistor 30 has a relatively small resistance value of 1 ohm or less. This is to minimize the power loss due to the resistor 30.
[0042]
Further, as shown in FIG. 1, an anode of a diode 24A is connected to one terminal of a secondary winding 12B of the transformer 12, and a cathode of the diode 24A is connected to one terminal of a capacitor 24B. The other terminal of the capacitor 24B is connected to the other terminal of the secondary winding 12B.
[0043]
That is, the rectifying / smoothing circuit 24 for the output from the secondary winding 12B is configured by the diode 24A and the capacitor 24B. Here, both terminals of the capacitor 24B are connected to a load (not shown), and are configured as external output terminals of the power supply device 10.
[0044]
The transformer 12 is the transformer of the present invention, the control circuit 16 is the control means of the present invention, the switching element 18 is the switching element of the present invention, the snubber capacitor 20 is the snubber circuit of the present invention, and the current detecting circuit 22 is the present invention. Each corresponds to detection means.
[0045]
Next, the operation of the power supply device 10 according to the first embodiment will be described with reference to FIGS. 3 and 4 show the current (hereinafter, referred to as the primary current) I flowing through the primary winding 12A of the transformer 12 at the time of rated output and at the time of light load, respectively.₁And a current flowing through the switching element 18 (hereinafter, referred to as a switching element current) I._CAnd the current flowing through the snubber capacitor 20 (hereinafter, referred to as snubber current) I_SAnd a voltage between the collector and the emitter of the switching element 18 (hereinafter, referred to as a switching element voltage) V_CEFIG. 7 is a waveform diagram showing a change in level over time.
[0046]
As is clear from FIG. 1, the primary current I₁, Switching element current I_CAnd snubber current I_SHas the relationship shown by the following equation (1).
[0047]
I₁= I_C+ I_S                                                                        (1)
A period t1 in FIG. 3 is a period during which the state of the switching element 18 shifts from ON to OFF, and the switching element current I flowing through the switching element 18 during the period t1._CDecreases sharply.
[0048]
Accordingly, the primary current I just before the switching element 18 is turned off₁As if the peak value of the snubber current I was maintained._SFlows into the primary current I₁Is kept substantially constant.
[0049]
Thereafter, in period t2, snubber current I flowing into snubber capacitor 20_SIs the switching element voltage V_CEIs V_in+ N × V_oUntil the primary current I₁Flows so as to keep it approximately constant. Where V_inIs the voltage between the terminals of the input power supply 14, and n is the value obtained by dividing the number of turns of the primary winding 12A in the transformer 12 by the number of turns of the secondary winding 12B._oIs a voltage between terminals of the capacitor 24B, that is, a load voltage.
[0050]
Then, in the period t3, the switching element voltage V_CEIs V_in+ N × V_oIs reached, the snubber current I_SVibrates at a predetermined frequency determined by the leakage inductance and the parasitic capacitance of the transformer 12 and other circuit patterns.
[0051]
As described above, when the switching element 18 is turned off, the snubber current I_SIs the primary current I at that time.₁Is almost equal to the peak value of
[0052]
Therefore, at the time of rated output, the primary current I₁Is large, the snubber current I having substantially the same peak value is_SFlows into the snubber capacitor 20. On the other hand, when the load is light, on the other hand, as shown in FIG.₁Is small, and the snubber current I flowing into the snubber capacitor 20 is small._SBecomes smaller similarly.
[0053]
That is, the snubber current I_SThe control circuit 16 can determine the output current level of the power supply device 10 by detecting the peak value of. And the snubber current I_SIs usually the primary current I₁Is smaller than the primary current I₁More snubber current I_S, The loss in the current detection circuit 22 can be significantly improved with a simple configuration.
[0054]
As described in detail above, in the power supply device 10 according to the present embodiment, the current flowing through the snubber circuit is detected by the current detection circuit, so that the output current level can be detected with a simple circuit configuration, with low loss and high power. It can be performed accurately and at low cost.
[0055]
Further, in the power supply device 10 according to the present embodiment, the current detection circuit is configured only by the resistor, so that the circuit configuration is simplified as compared with the case where the current detection circuit is configured using a current transformer or the like. And the cost of the apparatus can be reduced.
[0056]
In this embodiment, the case where the resistor 30 is applied as shown in FIG. 2A as the current detection circuit 22 as the detection means of the present invention has been described, but the present invention is not limited to this. Instead, for example, a mode in which the mode illustrated in FIG.
[0057]
The current detection circuit 22 'shown in the figure is a circuit in which a capacitor 32 is connected in parallel to the current detection resistor 30 in the form shown in FIG. 2A, and in this configuration, the current level is detected as an integral value. can do.
[0058]
On the other hand, in the case where a circuit in which a snubber resistor 34 (generally a resistance of about several tens of ohms) is connected in series to the snubber capacitor 20 in addition to the snubber capacitor 20 is applied as a snubber circuit, FIG. As shown in the figure, a voltage dividing resistor 36A and a resistor 36B are connected between both terminals of the snubber resistor 34 as the current detecting circuit 22 '', and the current level is determined based on the voltage level divided by the resistors 36A and 36B. It is also possible to adopt a form for detection. Therefore, in this case, a circuit constituted by the series connection of the snubber capacitor 20 and the snubber resistor 34 corresponds to the snubber circuit of the present invention, and a circuit constituted by the series connection of the resistors 36A and 36B corresponds to the voltage dividing resistor of the present invention. , Respectively.
[0059]
[Second embodiment]
In the second embodiment, the current detection circuit shown in FIG. 2A, that is, the resistor 30 is applied, and the snubber current I_SAn embodiment in which overcurrent protection is performed on the basis of the peak value will be described.
[0060]
First, the configuration of the power supply device 10 according to the second embodiment will be described with reference to FIG. The same reference numerals as in FIG. 1 denote the same parts in FIG. 5, and a description thereof will be omitted.
[0061]
As shown in the figure, the control circuit 16 in the power supply device 10 according to the second embodiment is configured as an IC, and has a predetermined level between the power supply terminal 40A and the ground terminal 40B provided as external terminals. Can be driven by the application of the above voltage.
[0062]
Therefore, in the power supply device 10 according to the second embodiment, each terminal of the control winding 12C of the transformer 12 is connected to the power supply terminal of the control circuit 16 via the rectifying and smoothing circuit 38 including the diode 38A and the capacitor 38B. By connecting to the 40A and the ground terminal 40B, the control circuit 16 is driven by supplying a predetermined level of power supply voltage to the control circuit 16 only during the operation of the power supply device 10.
[0063]
The control circuit 16 includes a comparator 44 having an inverting input terminal connected to a ground terminal 40B via a reference power supply 42 having a predetermined reference voltage. The comparator 44 has a non-inverting input terminal connected to a current detection circuit. It is connected to one terminal of a resistor 30 provided as 22.
[0064]
On the other hand, the output terminal of the comparator 44 is connected to a drive circuit 48 that generates a drive signal for causing the switching element 18 to perform a switching operation. The output terminal of the drive circuit 48 that outputs the drive signal is switched via a resistor 28. It is connected to the base of element 18.
[0065]
In the power supply device 10, when the output current of the power supply device 10 increases, the primary current I₁, Snubber current I_SIn both cases, the peak value increases.
[0066]
At this time, the voltage generated in the current detecting resistor 30 also increases, and when the voltage level exceeds the reference voltage of the reference power supply 42 in the control circuit 16, a latch signal is output from the comparator 44 to the drive circuit 48. Then, the drive signal output from the control circuit 16 to the base of the switching element 18 is stopped by the latch signal, and overcurrent protection is performed.
[0067]
As described above in detail, in the power supply device 10 according to the second embodiment, control for overcurrent protection is performed based on the detection result of the current detection circuit. Can be performed with a simple circuit configuration, with low loss, high accuracy, and low cost.
[0068]
[Third embodiment]
In the third embodiment, the current detection circuit shown in FIG. 2B, that is, a current detection circuit 22 'in which a capacitor 32 is connected in parallel in addition to the resistor 30, is applied and detected by the current detection circuit 22'. Snubber current I_SA case in which the switching frequency of the switching element 18 is controlled based on the following will be described.
[0069]
First, the configuration of a power supply device 10 'according to the third embodiment will be described with reference to FIG. The same reference numerals as in FIG. 5 denote the same parts in FIG. 6 as in FIG. 5, and a description thereof will be omitted.
[0070]
As shown in the drawing, a control circuit 16 'in a power supply device 10' according to the third embodiment is configured as an IC similarly to the control circuit 16 according to the second embodiment, and is provided as an external terminal. It can be driven by applying a voltage of a predetermined level between both the power supply terminal 40A and the ground terminal 40B.
[0071]
Therefore, also in the power supply device 10 'according to the third embodiment, each of the terminals of the control winding 12C of the transformer 12 is connected to the power supply terminal 40A and the ground terminal 40B of the control circuit 16' via the rectifying and smoothing circuit 38. Thus, a power supply voltage of a predetermined level is supplied to the control circuit 16 'only during the operation of the power supply device 10' to drive the control circuit 16 '.
[0072]
Further, the control circuit 16 ′ according to the third embodiment differs from the control circuit 16 according to the second embodiment in that the one connected to the inverting input terminal and the one to the non-inverting input terminal of the comparator 44 are reversed. And that the output terminal of the comparator 44 is connected to a drive circuit 48 via a minimum off-time determining circuit 46 for determining the minimum off-time of the switching element 18.
[0073]
In the power supply device 10 ′, when the output current of the power supply device 10 ′ increases, the primary current I₁, Snubber current I_SIn both cases, the peak value increases.
[0074]
At this time, the integrated voltage obtained by the action of the capacitor 32 in the current detection circuit 22 'also increases, and the level of the integrated voltage is compared with the reference voltage of the reference power supply 42 by the comparator 44 in the control circuit 16'. Is smaller than the reference voltage, that is, when the output current level is smaller than the predetermined value, the output of the comparator 44 becomes high level.
[0075]
When the output of the comparator 44 is at a high level, the output of the drive circuit 48 is not turned on by the minimum off-time determining circuit 46 until a predetermined minimum off-time has passed.
[0076]
That is, when the output current level is smaller than the predetermined value, control can be performed so that the oscillation frequency of the switching element 18 does not increase, and energy consumption can be reduced.
[0077]
As described above in detail, in the power supply device 10 ′ according to the third embodiment, the switching frequency (oscillation frequency) of the switching element is controlled based on the detection result of the current detection circuit, so that the energy consumption is Can be reduced with a simple circuit configuration, with low loss, high accuracy, and low cost.
[0078]
In each of the above embodiments, a case where a bipolar transistor is applied as the switching element of the present invention has been described. However, the present invention is not limited to this. For example, a MOS transistor may be applied. It goes without saying that you can do it.
[0079]
Further, in the second and third embodiments, the case where the control circuit 16 (16 ′) is configured by an IC has been described. However, the present invention is not limited to this, and the control circuit 16 (16 ′) is not limited thereto. Each part in parentheses) can be constructed by assembling various electronic components.
[0080]
Further, in the second embodiment, the present invention employs the snubber current I_SIn the third embodiment, the snubber current I is applied to the case where the overcurrent protection is performed based on the peak value of_SAlthough the description has been given of the case where the present invention is applied to a mode in which the switching frequency of the switching element 18 is controlled based on the above, the present invention is not limited to this. Can all be applied.
[0081]
【The invention's effect】
According to the power supply device of the first aspect, since the current flowing through the snubber circuit is detected by the detection means, the output current level can be detected with a simple circuit configuration, with low loss, high accuracy, and low cost. Can be obtained.
According to the power supply device of the first aspect, at least one of the control for the overcurrent protection and the control of the switching frequency of the switching element is performed based on the detection result by the detection unit. At least one of the control for current protection and the control of the switching frequency of the switching element can be performed with a simple circuit configuration, with low loss, high accuracy, and low cost.
[0082]
Further, according to the power supply device of the second aspect, the same effect as that of the first aspect of the invention can be obtained, and the detection means is constituted only by the resistor. As compared with the case of using a circuit, the circuit configuration can be simplified, and the cost of the apparatus can be reduced.
[0083]
According to the power supply device of the third aspect, the same effect as the invention of the second aspect can be obtained, and when the snubber circuit is formed of a capacitor, the snubber circuit is connected in series to the capacitor. If a capacitor is connected in parallel with the resistor and the snubber circuit is composed of a series circuit of the capacitor and the resistor, the capacitor is connected in parallel with the voltage dividing resistor, so the current level is detected as an integral value. As a result, the output current level can be reliably detected.
[Brief description of the drawings]
FIG. 1 is a circuit diagram illustrating a configuration of a power supply device according to a first embodiment.
FIGS. 2A, 2B, and 2C are circuit diagrams each showing a specific configuration example of a current detection circuit of the power supply device according to the present invention.
FIG. 3 is a diagram illustrating a primary current I flowing through a primary winding of a transformer at a rated output of the power supply device according to the first embodiment.₁And the switching element current I flowing through the switching element_CAnd the snubber current I flowing through the snubber capacitor_SAnd the switching element voltage V between the collector and the emitter of the switching element._CEFIG. 7 is a waveform diagram showing a change in level over time.
FIG. 4 is a diagram illustrating a primary current I flowing through a primary winding of a transformer when the power supply device according to the first embodiment is lightly loaded.₁And the switching element current I flowing through the switching element_CAnd the snubber current I flowing through the snubber capacitor_SAnd the switching element voltage V between the collector and the emitter of the switching element._CEFIG. 7 is a waveform diagram showing a change in level over time.
FIG. 5 is a circuit diagram illustrating a configuration of a power supply device according to a second embodiment.
FIG. 6 is a circuit diagram illustrating a configuration of a power supply device according to a third embodiment.
FIG. 7 is a circuit diagram showing a configuration example of a conventional power supply device.
8A and 8B are specific examples of the configuration of a current detection circuit 54 in the power supply device of FIG. 7, and FIGS. 8C and 8D are specific examples of a current detection circuit 56 of the power supply device of FIG. Is a circuit diagram showing a typical configuration example.
[Explanation of symbols]
10, 10 'power supply
12 transformer
12A primary winding
12B Secondary winding
12C control winding
14 Input power
16, 16 'control circuit (control means)
18 Switching element
20 Snubber capacitor (snubber circuit)
22, 22 'current detection circuit (detection means)
24 Rectifier smoothing circuit
34 snubber resistor (snubber circuit)
36A, 36B resistance (voltage division resistance)

Claims (7)

A transformer having a primary winding and a secondary winding,
A switching element that is connected in series to the primary winding of the transformer and switches an input to the primary winding;
A snubber circuit connected in parallel to the switching element,
Detecting means for detecting a current flowing through the snubber circuit;
Control means for controlling at least one of control for overcurrent protection and control of a switching frequency of the switching element based on a detection result by the detection means;
Power supply device. The detecting means is a resistor connected in series with the capacitor when the snubber circuit is formed of a capacitor, and a snubber circuit when the snubber circuit is formed of a series circuit of a capacitor and a resistor. 2. The power supply device according to claim 1, wherein said power supply device is a voltage dividing resistor connected in parallel to said resistor. When the snubber circuit is composed of a capacitor, a capacitor is connected in parallel with a resistor connected in series to the capacitor.When the snubber circuit is composed of a series circuit of a capacitor and a resistor, the capacitor is connected. 3. The power supply device according to claim 2, wherein a capacitor is connected in parallel with the pressure resistor. Further comprising a drive circuit for generating a drive signal for performing a switching operation of the switching element,
  The control for the overcurrent protection is a control for stopping the drive signal.
  The power supply device according to any one of claims 1 to 3, wherein: A comparator that outputs a latch signal when a level of a voltage generated in the detection unit exceeds a predetermined reference voltage,
  The control for the overcurrent protection is control to stop the drive signal by the latch signal.
  The power supply device according to claim 4, wherein: The control of the switching frequency is controlled such that the oscillation frequency of the switching element does not increase when the output current level determined by the current detected by the detection unit is smaller than a predetermined value.
  The power supply device according to any one of claims 1 to 3, wherein: A minimum off-time determination circuit that determines a minimum off-time of the switching element, and a drive circuit that generates a drive signal for causing the switching element to perform a switching operation, further comprising:
  The control of the switching frequency is such that when the output current level is smaller than a predetermined value, the output of the drive circuit is not turned on by the minimum off-time determining circuit until a predetermined minimum off-time has passed.
  The power supply device according to claim 6, wherein:

Images