JP7266558B2 - switching power supply - Google Patents

Description

The present invention relates to a switching power supply device with a step-down chopper.

Conventionally, as shown in FIG. 8, there is a switching power supply device 10 that converts a commercial three-phase AC voltage into a predetermined DC voltage Vo and outputs the DC voltage Vo. The switching power supply device 10 performs full-wave rectification of a three-phase AC voltage supplied from a commercial three-phase AC power supply 12 with six rectifying elements, and outputs a DC voltage Vi on which a small ripple having a frequency six times the commercial frequency is superimposed. A rectifier circuit 14 is provided, and a noise filter 16 that constitutes a π-type filter with an inductor 16a and two capacitors 16b is connected after the rectifier circuit 14 . During normal operation, the voltage V16 at the output of the noise filter 16 is approximately equal to the DC voltage Vi.

A step-down chopper 18 that converts the DC voltage Vi that has passed through the noise filter 16 into an output voltage Vo that is lower than the DC voltage Vi is provided after the noise filter 16 . The step-down chopper 18 is a general non-isolated DC-DC converter, and is composed of a switching element 20 made of a MOSFET, a commutating element 22 made of a diode, a smoothing inductor 24, a smoothing capacitor 26 and an input capacitor 28. ing. Both ends of the smoothing capacitor 26 are output terminals of the switching power supply device 10, and a load 30 is connected.

Briefly explaining the operation of the step-down chopper 18, the switching element 20 receives a drive pulse Vg output from the switching control circuit 34 and turns on and off at a predetermined switching cycle Tsw. When the switching element 20 is turned on and off, an intermittent voltage is generated after the switching element 20, which is the input voltage Vi. A DC output voltage Vo is generated across both ends. The input capacitor 28 functions to prevent the AC component of the switching current I20 flowing through the switching element 20 from flowing out to the noise filter 16 and becoming input feedback noise.

An input line of the step-down chopper 18 is connected to an output line of the noise filter 16 via a switch 32 made of IGBT. The switch 32 receives a signal from the overvoltage detection means 36 and opens and closes the input line of the step-down chopper 18 instead of performing a switching operation like the switching element 20 . Step-down chopper 18 is only allowed to operate when switch 32 is closed.

A switching control circuit 34 controls the output voltage Vo of the step-down chopper 18 . The drive pulse Vg output by the switching control circuit 34 is a pulse voltage that repeats a high level and a low level at a predetermined switching cycle Tsw. By changing the output voltage Vo in the direction of approaching the target value Vr, the on-duty of the switching element 20 is changed to keep the output voltage Vo at the target value Vr.

When the overvoltage detection means 36 detects that the output voltage Vo of the step-down chopper 18 has risen due to the occurrence of some abnormality and has reached the overvoltage threshold value Vth (>Vr), the overvoltage detection means 36 outputs a signal to open the switch 32. to stop the operation of the step-down chopper 18 and protect the load 30 .

Generally speaking, the configuration of the switching power supply 10 can be said to be a combination of the configuration of the switching power supply disclosed in Patent Document 1 and the overvoltage protection function of the switching power supply disclosed in Patent Document 2.

JP 2007-26814 A JP-A-2006-33943

In the case of the conventional switching power supply device 10, if for some reason short-circuit failure occurs between both ends of the switching element 20, that is, between the drain and the source, even if the above-described overvoltage protection operation is performed, other circuit elements including the switch 32 will be damaged. There is a possibility that it will break down, and there is a problem in terms of safety.

FIG. 9 is a time chart showing the operation of the switching power supply device 10. When the switching element 20 is short-circuited, the output voltage Vo of the step-down chopper 18, the inductor current I16 of the noise filter 16, the output end of the noise filter 16 1 schematically illustrates each operating waveform of the voltage V16 of . During normal operation (before timing T1), the switch 32 is closed, each circuit element operates appropriately, and the output voltage Vo is held at the target value Vr.

When the switching element 20 has a short-circuit failure (timing T1), the switching element 20 is fixed in the ON state regardless of the duty ratio of the driving pulse Vg from the switching control circuit 34 . Therefore, the output voltage Vo sharply rises toward the DC voltage Vi, and the inductor current I16 also sharply rises at the same time. The voltage V16 does not change so much and is kept substantially at the DC voltage Vi. After that, when the output voltage Vo reaches the overvoltage threshold Vth (timing T2), the overvoltage detection means 36 performs control to switch the switch 32 to an open state, thereby stopping power supply to the step-down chopper 18 .

However, since the inductor current I16 has already reached a very large value at the timing T2, when the switch 32 is switched to the open state, a large back electromotive force is generated in the inductor 16a to keep the current I16 flowing. Excessive surge voltage (ringing) occurs in voltage V16. As a result, an excessive voltage exceeding the rated voltage of the switch 32 is applied across the switch 32, causing the switch 32 to malfunction.

If a short-circuit fault occurs between the collector and emitter of the switch 32, the power supply to the step-down chopper 18 is restarted and an excessive voltage is applied to the commutation element 22 and the smoothing capacitor 26, so that these circuit elements also fail. Probability is high. Similarly, the switching control circuit 34 and the overvoltage detection means 36 may also fail.

As a countermeasure to prevent the switch 32 from failing, for example, a method of inserting a protective fuse in series with the switch 32 is conceivable. difficult to protect. Also, it is conceivable to use a high withstand voltage switch that can withstand the surge voltage as the switch 32, but the quality is excessive during normal operation, and the cost increases. Also, a method of absorbing the surge voltage by connecting a Zener diode or a varistor in parallel across the capacitor 16b is conceivable, but if the energy to be absorbed is large, the Zener diode or the like will be severely destroyed and open. possible, and it is difficult to reliably protect the switch 32 .

Another possible method is to set the overvoltage threshold Vth to a value as low as possible (a value slightly higher than the target value Vr of the output voltage Vo), detect the overvoltage and open the switch 32 before the current I16 increases. However, even during normal operation, the output voltage Vo fluctuates within a certain range around the target value Vr (for example, when the current of the load 30 changes suddenly, the output voltage Vo rises or falls transiently). Therefore, if the overvoltage threshold value Vth is set to a value close to the target value Vr, the overvoltage detection means 34 may operate even during normal operation, and the step-down chopper 18 may be stopped. Therefore, there is a limit to how to lower the overvoltage threshold Vth.

A method of omitting the noise filter 16, which is a source of surge voltage (ringing), is also conceivable. However, in an actual usage environment, it is assumed that the wiring between the AC power supply 12 and the switching power supply 10 will be extremely long, and if the wiring inductance becomes too large to be ignored, the surge voltage will increase by the same mechanism as described above. Occur. Therefore, countermeasures must be taken on the assumption that the input line of the step-down chopper 18 has a certain amount of inductance.

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above background art, and when a switching element of a step-down chopper is short-circuited, it is possible to quickly stop power supply and reliably protect internal circuit elements and loads. An object of the present invention is to provide a switching power supply.

The present invention provides a step-down chopper having a switching element, a commutation element, a smoothing inductor, and a smoothing capacitor, a switching control circuit for generating a driving pulse for turning on and off the switching element at a predetermined switching cycle, and a step-down chopper in the front stage of the step-down chopper. provided with a switch, and a switching element monitoring circuit that opens and closes the switch according to the state of the switching element,
When the switching control circuit is outputting a drive pulse to make the on-duty of the switching element less than 100%, the switching element monitoring circuit detects that the ON state of the switching element is longer than the switching cycle. When it is detected that it has continued beyond the specified time, the switch is opened to stop the power supply to the step-down chopper,
When the switching control circuit is outputting a drive pulse for setting the on-duty of the switching element to 100%, the switching element monitoring circuit closes the switch regardless of the state of the switching element. It is a switching power supply that holds the voltage.

The switching element monitoring circuit includes a voltage detection circuit that detects a voltage across the switching element or a voltage corresponding thereto, and the change width of the voltage across the switching element exceeds the specified time and reaches a small value below a certain value. A configuration may be adopted in which it is determined that the ON state of the switching element has continued beyond the specified time when it is detected that it is held.

Alternatively, the switching element monitoring circuit includes a current detection circuit that detects a switching current flowing through the switching element, and upon detecting that the switching current continues to flow beyond the specified time, the switching element is turned on. It can be configured to determine that it has continued beyond the specified time. In this case, when the switching control circuit acquires the output signal of the current detection circuit and detects that the peak value of the switching current exceeds a threshold value, the switching element is forcibly operated during the remaining period of the switching cycle. It is preferable to change the driving pulse so as to turn it off.

A configuration in which a rectifier circuit that full-wave rectifies a commercial three-phase AC voltage and outputs a DC voltage with a ripple superimposed thereon is provided, and an input line of the step-down chopper is connected to the subsequent stage of the rectifier circuit via the switch. can do. Further, a configuration may be adopted in which a noise filter having an inductor is provided in the preceding stage of the switch.

According to the switching power supply device of the present invention, when the switching element of the step-down chopper is short-circuited, the power supply to the step-down chopper can be quickly stopped, and the internal circuit elements including the switch and the load can be reliably protected. be able to.

1 is a circuit diagram showing one form of a switching power supply device related to the present invention; FIG. 2 is a time chart showing the operation of the switching power supply device of FIG. 1 when a switching element has a short-circuit failure; FIG. 4 is a circuit diagram showing another form of a switching power supply device related to the present invention; 4 is a time chart showing the operation of the switching power supply device of FIG. 3 when a switching element has a short-circuit failure; 1 is a circuit diagram showing an embodiment of a switching power supply device of the present invention; FIG. 5 is a time chart showing the operation of the switching power supply device of this embodiment when a switching element has a short-circuit failure; A circuit diagram (a) showing a modified example of a step-down chopper and a switch, a circuit diagram (b) showing a modified example when the main power supply is a commercial single-phase AC power supply, and a circuit showing a modified example when the main power supply is a DC power supply It is a figure (c). 1 is a circuit diagram showing a conventional switching power supply; FIG. 4 is a time chart showing the operation of a conventional switching power supply when a switching element has a short-circuit failure.

One form of a switching power supply device related to the present invention will be described below with reference to FIGS. 1 and 2. FIG. Here, the same configurations as those of the above- described conventional switching power supply device 10 are denoted by the same reference numerals, and descriptions thereof are omitted.

Like the switching power supply 10, the switching power supply 38 of this form is a device that converts a commercial three-phase AC voltage into a predetermined DC voltage Vo and outputs it. The difference in configuration is that a switching control circuit 40 (including a current detection circuit 44) is provided instead of the switching control circuit 34, and a switching element monitoring circuit 42 (voltage detection circuit 46) is provided instead of the overvoltage detection means 36. ) is provided, and other configurations are the same. The following description will focus on the differences in configuration.

The switching control circuit 40, like the switching control circuit 34, is a circuit that controls the output voltage Vo of the step-down chopper 18, and generates a drive pulse Vg that repeats high level and low level at a predetermined switching cycle Tsw. . Then, by detecting the output voltage Vo and changing the duty ratio between high and low of the drive pulse Vg in the direction to bring the output voltage Vo closer to the target value Vr, the on-duty of the switching element 20 is changed, and the output voltage Vo is held at the target value Vr.

Also, the switching control circuit 40 generates the drive pulse Vg so that the maximum on-duty of the switching element 20 is limited to a value less than 100%. In other words, control is performed under the condition that a period in which the switching element 20 is turned off must be provided in one switching cycle Tsw.

Furthermore, the switching control circuit 40 has a current detection circuit 44 that detects the switching current I20 flowing through the switching element 20, and when it detects that the peak value of the switching current I20 exceeds a specified threshold, During the remaining period, the drive pulse Vg is changed so as to forcibly turn off the switching element 20 . This operation is a so-called pulse-by-pulse overcurrent protection operation.

The switching element monitoring circuit 42 has a voltage detection circuit 46 that detects the voltage V22 across the commutating element 22, and detects when the change width of the voltage V22 across the commutating element 22 exceeds a specified time Tth and is held at a value less than or equal to a certain value. When detected, it is determined that the ON state of the switching element 20 has continued beyond the specified time Tth, and the switch 32 is opened. The specified time Tth is longer than the switching period Tsw.

The specified time Tth can be set to an appropriate value by providing a CR timer circuit or a microcomputer counter inside the switching element monitoring circuit 42 . Here, it is set to approximately twice the switching period Tsw (Tth≈2×Tsw).

FIG. 2 is a time chart showing the operation of the switching power supply 38. When the switching element 20 is short-circuited, the output voltage Vo of the step-down chopper 18 and the voltage Vds22 across the switching element 20 (drain-source voltage) , the voltage V22 across the commutating element 22 (cathode-anode voltage), the inductor current I16 of the noise filter 16, and the voltage V16 at the output terminal of the noise filter 16. FIG.

During normal operation (before timing T1), the switch 32 is closed, each circuit element operates appropriately, and the output voltage Vo is held at the target value Vr. The voltage Vds20 across the switching element 20 has a pulse-like waveform that repeats high level (off) and low level (on), and the voltage V22 across the commutating element 22 has a pulse-like waveform that repeats low level and high level. , that is, a waveform in which high and low are reversed with respect to voltage Vds20.

When the switching element 20 has a short-circuit failure (timing T1), the switching element 20 is fixed in the ON state regardless of the duty ratio of the drive pulse Vg from the switching control circuit 40 . Therefore, the output voltage Vo rises toward the DC voltage Vi, and the inductor current I16 also increases at the same time. The voltage V16 does not change so much and is kept substantially at the DC voltage Vi. The voltage Vds20 across the switching element 20 is held at substantially zero volts, and the voltage V22 across the commutating element 22 is held at substantially the input voltage Vi.

After that, when the switching element monitoring circuit 42 detects that the change width of the voltage V22 across the commutating element 22 has exceeded the specified time Tth and is kept at a small value equal to or less than a certain value (timing T2), the switching element 20 It is determined that the change width of the voltage Vds20 across both ends has been maintained at a small value equal to or less than a certain value beyond the specified time Tth, and that the ON state of the switching element 20 has continued beyond the specified time Tth (≈2×Tsw).

Since the maximum on-duty of the switching element 20 during normal operation is limited to less than 100% by the drive pulse Vg, the ON state does not continue for the switching period Tsw or longer. In other words, it can be determined that both ends of the switching element 20 have a short-circuit failure if the ON state continues beyond the specified time Tth (≈2×Tsw). Therefore, the switching element monitoring circuit 42 performs control to switch the switch 32 to the open state at the timing T2, thereby quickly stopping the power supply to the step-down chopper 18 .

The time from timing T1 to T2 is Tth≈2×Tsw, which is considerably shorter than the time Tx shown in FIG. 9, so the inductor current I16 does not have a very large value at timing T2. Therefore, when the switch 32 is switched to the open state, the back electromotive force generated in the inductor 16a is small, and the surge voltage (ringing) generated in the voltage V16 is suppressed. As a result, the voltage applied across the switch 32 is suppressed to a low value, and the switch 32 is safely protected. Moreover, since the power supply to the step-down chopper 18 is also stopped, the other circuit elements of the step-down chopper 18 and the load 30 are also safely protected.

Considering only the protection of the internal circuit elements and the load 30, it is preferable to set the specified time Tth as short as possible (for example, Tth≈Tsw). The circuit 42 is likely to malfunction. For example, when external noise enters the switching power supply device 10, the switching element monitoring circuit 42 may malfunction and switch the switch 32 to the open state even during normal operation. Therefore, it is preferable to set the specified time Tth to, for example, 1.5 times or more the switching period Tsw.

Further, if the timing T2 for switching the switch 32 to the open state is set before the value of the current I16 reaches the current value at which the smoothing inductor 24 is completely magnetically saturated, higher safety can be ensured. Therefore, it is preferable to set the specified time Tth to, for example, 2.5 times or less of the switching period Tsw.

The circuit configuration of the switching power supply device 38 of this form is suitable for, for example, an active filter for power factor improvement of several kW class that steps down AC400V (3φ) to about DC370V. As shown in FIG. 1, the switching power supply device 10 has a configuration in which a rectifier circuit 14 performs full-wave rectification of a three-phase AC voltage to generate a DC voltage Vi. Even without it, the input voltage (DC voltage Vi) of the step-down chopper 18 can be maintained at a value higher than the output voltage Vo, realizing a high power factor equivalent to that of an active filter using a step-up chopper. becomes possible.

Further, when the configuration of the conventional switching power supply device 10 is applied to an active filter for improving the power factor of several kW class that steps down AC400V (3φ) to about DC370V, if the switching element 20 of the step-down chopper 18 short-circuits, When the current I16 reaches several hundred amperes at timing T2 in FIG. is added, and there is a high possibility that the switch 32 will short-circuit. When the switch 32 short-circuits, the power supply to the step-down chopper 18 is restarted, and an excessive voltage is applied to the smoothing capacitor 26 (for example, an aluminum electrolytic capacitor) to violently rupture, or a large current flows through the smoothing inductor 24. This is extremely dangerous because the commutation element 22 may be severely damaged, resulting in abnormal heat generation. On the other hand, if the configuration of the switching power supply device 38 is applied, the internal circuit elements including the switch 32 and the load 30 can be safely protected.

As described above, according to the switching power supply device 38, when the switching element 20 of the step-down chopper 18 is short-circuited, the power supply to the step-down chopper 18 can be quickly stopped, and the internal circuit including the switch 32 can be stopped. The element and the load 30 can be reliably protected.

Next, another form of the switching power supply device related to the present invention will be described with reference to FIGS. 3 and 4. FIG. Here, the same configurations as those of the switching power supply device 38 described above are denoted by the same reference numerals, and descriptions thereof are omitted.

Like the switching power supply 38, the switching power supply 48 of this form is a device that converts a commercial three-phase AC voltage into a predetermined DC voltage Vo and outputs it. The difference in configuration is that a switching element monitoring circuit 50 (including a current detection circuit 44) is provided instead of the switching element monitoring circuit 42, and other configurations are the same. The current detection circuit 44 of the switching element monitoring circuit 50 is also used by the current detection circuit 44 of the switching control circuit 40 . The following description will focus on the differences in configuration.

The switching element monitoring circuit 50 has a current detection circuit 44 that detects the switching current I20 flowing through the switching element 20, and when it detects that the switching current I20 continues to flow beyond the specified time Tth, the switching element 20 is turned on. has continued beyond the specified time Tth, and the switch 32 is opened. The specified time Tth is longer than the switching period Tsw, and is set in the same way as the specified time Tth of the switching element monitoring circuit 42 . Here, similarly to the above, it is set to approximately twice the switching period Tsw (Tth≈2×Tsw).

FIG. 4 is a time chart showing the operation of the switching power supply device 48. When the switching element 20 is short-circuited, the output voltage Vo of the step-down chopper 18, the switching current I20 flowing through the switching element 20, and the inductor of the noise filter 16 2 schematically illustrates operating waveforms of a current I16 and a voltage V16 at the output terminal of the noise filter 16. FIG.

During normal operation (before timing T1), the switch 32 is closed, each circuit element operates appropriately, and the output voltage Vo is held at the target value Vr. The switching current I20 has a pulse-like waveform that repeats high level (on) and low level (off).

When the switching element 20 has a short-circuit failure (timing T1), the switching element 20 is fixed in the ON state regardless of the duty ratio of the drive pulse Vg from the switching control circuit 40 . Therefore, the output voltage Vo rises toward the DC voltage Vi, and the inductor current I16 also increases at the same time. The voltage V16 does not change so much and is kept substantially at the DC voltage Vi. Switching current I20 increases as does inductor current I16.

After that, when the switching element monitoring circuit 50 detects that the switching current I20 continues to flow beyond the specified time Tth (timing T2), the ON state of the switching element 20 exceeds the specified time Tth (≈2×Tsw). determined to continue.

Since the maximum on-duty of the switching element 20 during normal operation is limited to less than 100% by the drive pulse Vg, the ON state does not continue for the switching period Tsw or longer. In other words, it can be determined that both ends of the switching element 20 have a short-circuit failure if the ON state continues beyond the specified time Tth (≈2×Tsw). Therefore, the switching element monitoring circuit 50 performs control to switch the switch 32 to the open state at the timing T2, thereby quickly stopping the power supply to the step-down chopper 18 .

In this manner, the switching power supply device 48 can quickly stop the power supply to the step-down chopper 18 when the switching element 20 of the step-down chopper 18 is short-circuited, similar to the switching power supply device 38 described above. Internal circuit elements including 32 and load 30 can be reliably protected. Further, since the current detection circuit 44 of the switching element monitoring circuit 50 is also used by the current detection circuit 44 of the switching control circuit 40, the increase in the number of parts and cost can be minimized. Of course, the current detection circuit of the switching element monitoring circuit 50 may be provided separately from the current detection circuit 44 .

Next, one embodiment of the switching power supply device of the present invention will be described with reference to FIGS. 5 and 6. FIG. Here, the same configurations as those of the switching power supply device 48 described above are denoted by the same reference numerals, and descriptions thereof are omitted.

The switching power supply device 52 of this embodiment, like the switching power supply device 48, is a device that converts a commercial three-phase AC voltage into a predetermined DC voltage Vo and outputs it. The difference in configuration is that a switching control circuit 54 (including the current detection circuit 44) is provided instead of the switching control circuit 40 (including the current detection circuit 44), and a switching element monitoring circuit 50 (the current detection circuit 44 is included). 44) is replaced with a switching element monitoring circuit 56 (including the current detection circuit 44), and other configurations are the same. The following description will focus on the differences in configuration.

The switching control circuit 54, like the switching control circuit 40, is a circuit that controls the output voltage Vo of the step-down chopper 18, and generates a driving pulse Vg that repeats high level and low level at a predetermined switching cycle Tsw. . Then, by detecting the output voltage Vo and changing the duty ratio between high and low of the drive pulse Vg in the direction to bring the output voltage Vo closer to the target value Vr, the on-duty of the switching element 20 is changed, and the output voltage Vo is held at the target value Vr. When the current detection circuit 44 detects the switching current I20 and detects that the peak value of the switching current I20 exceeds a specified threshold value, the switching element 20 is forcibly turned off during the remaining period of the switching period Tsw. , the driving pulse Vg is changed (pulse-by-pulse type overcurrent protection operation).

The difference from the switching control circuit 40 is that the drive pulse Vg is generated so that the maximum on-duty of the switching element 20 is not restricted. In other words, control is performed under the condition that the switching element 20 may be kept on during one switching period Tsw.

The switching element monitoring circuit 56 performs two types of operations according to the state of the drive pulse Vg. First, when the switching control circuit 54 is outputting the drive pulse Vg for making the on-duty of the switching element 20 less than 100%, the switching element monitoring circuit 56 detects that the switching current I20 is kept at a specified time Tth (≈2× Tsw), it is determined that the ON state of the switching element 20 has continued beyond the specified time Tth, and the switch 32 is opened. This is the same operation as the switching element monitoring circuit 50 described above.

On the other hand, when the switching control circuit 54 is outputting the drive pulse Vg to set the on-duty of the switching element 20 to 100%, the switching element monitoring circuit 56 monitors the state of the switching element 20 (change in switching current I20). method), the switch 32 is kept closed.

FIG. 6 is a time chart showing the operation of the switching power supply device 52. When the switching element 20 is short-circuited, the output voltage Vo of the step-down chopper 18, the switching current I20 flowing through the switching element 20, and the switching control circuit 54 are 4 schematically illustrates the operation waveforms of the driving pulse Vg to be output, the inductor current I16 of the noise filter 16, and the voltage V16 at the output end of the noise filter 16. FIG.

During normal operation (before timing T1), the switch 32 is in a closed state, each circuit element operates appropriately, and the output voltage Vo is held substantially at the target value Vr. The switching current I20 and the drive pulse Vg have pulse-like waveforms that repeat high level (on) and low level (off).

Note that, unlike the switching power supply device 48, the switching power supply device 52 may be in a situation where the on-duty of the switching element 20 becomes 100% under the control of the switching control circuit 54 during normal operation. For example, as shown in FIG. 6, when the current of the load 30 suddenly increases and the output voltage Vo becomes lower than the target value Vr. Therefore, if it takes a long time for the output voltage Vo to return to the target value Vr, there is a possibility that the ON state of the switching element 20 will continue beyond the specified time Tth.

However, the switching element monitoring circuit 56, when the driving pulse Vg for making the on-duty of the switching element 20 to 100% is output, regardless of the state of the switching element 20 (how the switching current I20 changes). , to keep the switch 32 closed. Therefore, during normal operation, the power supply to the step-down chopper 18 is not stopped even if the ON state of the switching element 20 continues beyond the specified time Tth.

When the switching element 20 has a short-circuit failure (timing T1), the switching element 20 is fixed in the ON state regardless of the duty ratio of the drive pulse Vg. Therefore, the output voltage Vo rises toward the DC voltage Vi, and the inductor current I16 also increases at the same time. The voltage V16 does not change so much and is kept substantially at the DC voltage Vi. Switching current I20 increases as does inductor current I16.

After that, when the switching element monitoring circuit 50 detects that the switching current I20 continues to flow beyond the specified time Tth (timing T2), the ON state of the switching element 20 exceeds the specified time Tth (≈2×Tsw). determined to continue.

Since the output voltage Vo is higher than the target value Vr between the timings T1 and T2, the switching control circuit 54 changes the drive pulse Vg so that the on-duty of the switching element 20 is reduced. Alternatively, since the crest value of the switching current I20 increases, the drive pulse Vg is changed so that the on-duty of the switching element 20 is reduced by the pulse-by-pulse type overcurrent protection operation. Then, if the switching element 20 is operating normally (if it is not out of order), there will always be a period during which the switching element 20 is turned off in one switching cycle Tsw.

That is, it can be determined that both ends of the switching element 20 have a short-circuit failure when it is detected that the ON state has continued beyond the specified time Tth (≈2×Tsw) at the timing T2. Therefore, the switching element monitoring circuit 56 performs control to switch the switch 32 to the open state, thereby quickly stopping the power supply to the step-down chopper 18 .

In this manner, the switching power supply 52 can quickly stop the power supply to the step-down chopper 18 when the switching element 20 of the step-down chopper 18 is short-circuited, similarly to the switching power supply 48 described above. Internal circuit elements including 32 and load 30 can be reliably protected. Also, during normal operation, there is no problem of erroneously stopping the power supply to the step-down chopper 18 .

It should be noted that the switching power supply device of the present invention is not limited to the above-described embodiment (switching power supply device 52) , and part of the techniques disclosed in the switching power supply devices 38 and 48 can be diverted. 1 , the voltage detection circuit 46 of the switching element monitoring circuit 42 is configured to detect the voltage V22 across the commutating element 22 as a substitute for the voltage Vds20 across the switching element 20. ing. This is because one end of the commutating element 22 is at the ground potential, so the voltage V22 across both ends can be easily detected. In this way, the voltage detection circuit can be configured to detect a voltage at another location that behaves similarly to the voltage Vds20 across the switching element when the switching element 20 is short-circuited. Of course, the configuration may be such that the both-end voltage Vds20 is directly detected.

In the case of the switching power supply device 48 shown in FIG. 3, the current detection circuit 44 of the switching element monitoring circuit 50 is configured to directly detect the switching current I20 flowing through the switching element 20. When the element 20 is short-circuited, the configuration can be changed to detect a current at another location that behaves similarly to the switching current I20. For example, instead of directly detecting the switching current I20 of the switching element 20, as a substitute characteristic, the current at the point where the anode of the commutating element 22 and one end of the capacitor 28 are connected may be detected.

Further, in the case of the switching power supply device 52 shown in FIG. 5, the switching element monitoring circuit 56 is configured to include the current detection circuit 44, but like the switching element monitoring circuit 42 in FIG. You can change the configuration.

In the switching power supplies 38, 48, 52 (FIGS. 2, 4, and 6), the prescribed time Tth is set to approximately twice the switching cycle Tsw, but the value of twice is merely an example. As described above, the specified time Tth is set not only to open the switch 32 at a safe timing, but also to prevent malfunction due to external noise (to clear the EMS test), and to set the smoothing inductor 24 of the step-down chopper 44. It is adjusted to an appropriate value for each device, taking into consideration the performance required for the device, such as the saturation characteristics of the device, and the convenience of design.

In switching power supply devices 38, 48 and 52, as shown in FIG. 7A, switch 32 is provided on the high side, but it may be provided at point A on the low side. Similarly, the switching element 20 and the smoothing inductor 24 may also be provided at points B and C on the low side. Also, in the switching power supply devices 38, 48, 52, the switching element 20 is a MOS type FET, but it may be changed to another transistor element or the like in view of the device specifications (input voltage, output voltage, power capacity). can be done. Similarly, although the commutating element 22 is a diode, it can be changed to a transistor element or the like for synchronous rectification. Similarly, although the switch 32 is an IGBT, it can be changed to a semiconductor switch such as a MOSFET, or a mechanical switch such as a relay.

The switching power supply devices 38, 48, and 52 are provided with a full-wave rectifier circuit (rectifier circuit 14) for the three-phase AC voltage at the input terminal because the original power supply is the three-phase AC power supply 12. FIG. However, if the original power supply is a single-phase AC power supply 58, a full-wave rectifier circuit (rectifier circuit 62) for single-phase AC voltage is provided at the input end like the switching power supply device 60 in FIG. good. In this case, it is preferable to further connect a large-capacity input smoothing capacitor 64 to the output end of the rectifier circuit 62 to smooth the rectified pulsating voltage and generate the DC voltage Vi. As a result, the period during which the input voltage Vi of the step-down chopper 18 is lower than the output voltage Vo can be eliminated, and the power factor improvement operation can be performed during the entire period. Further, when the original power supply is the DC power supply 66, the rectifier circuit can be omitted as in the switching power supply 68 of FIG. 7(c).

The switching power supply devices 38, 48, 52 are provided with the noise filter 16 before the switch 32, but the noise filter may be provided before the rectifier circuit, or if not necessary, the noise filter shown in FIGS. ), the noise filter may be omitted as in the switching power supply devices 60 and 68 of FIG.

The switching power supply devices 38, 48, and 52 have non-isolated inputs and outputs, but by further incorporating a converter other than the step-down chopper 18, an isolated switching power supply device can be configured. . For example, a configuration in which an insulated DC-DC converter is provided after the step-down chopper 18 and the output voltage of the DC-DC converter is supplied to the load 30 is conceivable. Alternatively, an insulated AC-DC converter may be provided in the preceding stage of the switch 32, and the step-down chopper 18 may be connected to the output line of the AC-DC converter via the switch 32. FIG. In either configuration, the intended effect of the present invention can be obtained.

10, 38, 48, 50, 52, 60, 68 switching power supply device 12 commercial three-phase AC power supply 14 rectifier circuit (full-wave rectifier circuit for three-phase AC voltage)
16 noise filter 16a inductor 16b capacitor 18 step-down chopper 20 switching element 22 commutation element 24 smoothing inductor 26 smoothing capacitor 28 input capacitor 30 load 32 switches 34, 40, 54 switching control circuits 42, 50, 56 switching element monitoring circuit 44 current detection Circuit 46 Voltage detection circuit 58 Single-phase AC power supply 62 Rectifier circuit (full-wave rectifier circuit for single-phase AC voltage)
64 Input smoothing capacitor 66 DC power supply
I16 Noise filter inductor current
I20 switching current
Tsw switching cycle
Tth regulation time
Vds20 Voltage across switching element
Vg drive pulse
V16 Voltage at the output end of noise filter 16
V22 Voltage across commutating element 22

Claims (6)

A step-down chopper having a switching element, a commutation element, a smoothing inductor and a smoothing capacitor, a switching control circuit for generating a driving pulse for turning on and off the switching element at a predetermined switching cycle, and a switch provided in the preceding stage of the step-down chopper. and a switching element monitoring circuit that opens and closes the switch according to the state of the switching element,
When the switching control circuit is outputting a drive pulse to make the on-duty of the switching element less than 100%, the switching element monitoring circuit detects that the ON state of the switching element is longer than the switching period. When it is detected that it has continued beyond the specified time, the switch is opened to stop the power supply to the step-down chopper,
When the switching control circuit is outputting a drive pulse for setting the on-duty of the switching element to 100%, the switching element monitoring circuit closes the switch regardless of the state of the switching element. A switching power supply device, characterized in that: The switching element monitoring circuit includes a voltage detection circuit that detects a voltage across the switching element or a voltage corresponding thereto,
3. When it is detected that the change width of the voltage across the switching element has been maintained at a small value below a certain value for more than the specified time, it is determined that the ON state of the switching element has continued beyond the specified time. 1. A switching power supply device according to claim 1. The switching element monitoring circuit includes a current detection circuit that detects a switching current flowing through the switching element,
2. The switching power supply device according to claim 1, wherein when it is detected that said switching current has continued to flow beyond said specified time, it is determined that said switching element has continued to be in the ON state beyond said specified time. The switching control circuit acquires the output signal of the current detection circuit, and when detecting that the peak value of the switching current exceeds a threshold, forcibly turns off the switching element for the remaining period of the switching cycle. 4. The switching power supply device according to claim 3, wherein said drive pulse is changed as follows. A rectifier circuit that full-wave rectifies a commercial three-phase AC voltage and outputs a DC voltage with a ripple superimposed thereon, wherein an input line of the step-down chopper is connected via the switch to a subsequent stage of the rectifier circuit. 5. The switching power supply device according to any one of 1 to 4. 6. The switching power supply device according to claim 1, wherein a noise filter having an inductor is provided in front of said switch.

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