JP3145739B2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply.

[0002] Optimal overload protection is realized when a load that requires a relatively high peak current supply capability with respect to the average current of a motor or the like is connected.

[0003]

2. Description of the Related Art Switching type power supplies have been used as power supplies for various electronic devices. In particular, as a DC power supply, the switching method is widely adopted because of its advantages of small size, light weight, and high efficiency.

FIG. 5 shows a configuration of a conventional switching type DC power supply having an output stabilizing circuit.

In FIG. 5, reference numerals 1 and 2 denote input power supply terminals.
1 and the primary winding N1 of the transformer T1. One of the primary windings N1 is connected to the collector of the switching transistor Q1. T1 2
The secondary winding N2 is guided to a smoothing capacitor C2 via a rectifying diode Q3. The smoothed output from this filter circuit is supplied to a predetermined load via output terminals 4 and 5.

A voltage detection circuit 6 is connected in parallel to a load (not shown) connected to terminals 4 and 5 for detecting an output voltage to the load. The detected value is fed back to the control circuit including the PWM circuit 3 via the photocoupler Q4.

The photocoupler Q4 outputs the feedback detection voltage to the PWM circuit 3, and the PWM circuit 3 controls the switching duty ratio or the frequency of the driving pulse applied to the base of the switching transistor Q1 according to the detection value.

The output of the PWM circuit 3 drives the switching transistor Q1. As a protection function, a voltage proportional to the switching current of the switching transistor Q1 is obtained as a detection voltage by the current detection resistor R1. R2
R3 is a voltage dividing circuit that divides the detection voltage detected by R1 into an appropriate value. R4 and R5 are resistors for setting the threshold voltage of the detection reference value. Q2 is a comparator of the comparison circuit, which compares the detection voltage with the threshold voltage.

Next, the operation of overcurrent protection in the circuit of FIG. 5 will be described.

When a power supply is connected to the input power supply terminals 1 and 2, voltage application to the primary winding N1 of the transformer T1 is interrupted by the switching transistor Q1. The AC output generated by switching the switching transistor Q1 is insulated and transformed by the transformer T1. The isolated AC output is a diode Q3
Is rectified. The rectified output is supplied to the load from output terminals 6 and 7 after being smoothed by the smoothing capacitor C2.

When the load becomes heavy and the current of the switching transistor Q1 increases, the voltage generated at the detection resistor R1 also increases.
Outputs a signal to the PWM circuit 3. The PWM circuit 3 stops the pulse output by the signal from the comparator Q2. This operation is performed for each pulse.

[0012]

However, in such a conventional example, for a power supply having a small average current with respect to a peak current value such as a motor load, for example, the power is detected in accordance with the peak current value. To set the value,
When the overload state is continuous such as when the load is abnormal, there is a problem that the operation becomes continuous with the maximum current and the abnormal overheating occurs. Further, in order to prevent abnormal overheating, it is necessary to increase the heat capacity of the radiator plate or add an overheat protection circuit. Therefore, there are problems such as an increase in the size of the power supply and an increase in cost. .

It is an object of the present invention to provide a switching power supply which has solved the above problems.

[0014]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention relates to a switching power supply for intermittently connecting a power supply input to a switching element and converting an alternating current generated to a direct current to a load. Detection means for detecting, and protection means for restricting the operation of the switching element based on a detection signal from the detection means,
And a load current detection circuit. The load current detection circuit further includes an offset unit that offsets an input value of the detection unit based on an average value of the load current obtained by the load current detection circuit.

[0015]

According to the present invention, the protection means does not operate for the peak output up to the preset peak output level, but when the overload state continues, the offset means sets the average of the load current. The operation level of the protection means is reduced by giving an offset to the detection signal based on the value, thereby restricting the operation of the switching element.

[0016]

[Embodiment 1] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. However, in the following, the same parts as those of the above-described conventional example are denoted by the same reference numerals, and the detailed description thereof will be omitted.

In FIG. 1, the parts different from FIG. 5 are a current detection resistor R6 provided in series with a load, a detection circuit 7
And that a photocoupler Q5 is added.

The detection circuit 7 includes an operational amplifier, a transistor, and the like, and detects a voltage proportional to an output current generated in the current detection resistor R6. The output of the detection circuit 7 is input to an offset circuit 8 including resistors R2 and R3 via a photocoupler Q5.

Upon receiving the output from the detection circuit 7, the offset circuit 8 adds an arbitrary offset to the peak value output detected by R1 and outputs the result to the comparison circuit Q2.

Therefore, when there is no input from the current detection circuit 7, the input of the comparison circuit Q2 is at a level lower than the threshold value as shown in FIG.

When there is an input from the current detection circuit 7, the input of the comparison circuit Q2 is offset. And FIG.
When the input of the comparison circuit Q2 reaches the threshold value as shown in (B), a signal is output to the PWM circuit 3 to limit the output pulse width.

In this embodiment, the transistor section of the photocoupler Q5 is used as a variable resistance element. Although the present embodiment has been described by taking the flyback type as an example, the present invention can be applied to a forward type and a push-pull type switching power supply. Although a switching element uses a transistor, other elements such as an FET and a GTO can be used. Although the load current detection circuit (7) is used as the average value detection means, other means such as a temperature detection circuit may be used.

Embodiment 2 FIG. 3 shows a second embodiment of the present invention.

In the circuit of the first embodiment, a resistor is used for detecting a pulse current. In this embodiment, a current transformer T2 is used. Others are the same as the first embodiment.

The pulse current is transformed by a current transformer T2, converted into a voltage by a terminating resistor R7, and converted into a voltage by a diode Q6.
To obtain the detection voltage. This detection voltage is supplied to an offset circuit consisting of R2 and R3.

The subsequent operation is the same as in the first embodiment.

Third Embodiment FIG. 4 shows a third embodiment using the present invention.

In this embodiment, the detection circuit 7 of the circuit of the first embodiment is used.
And a photodetector Q5 with a delay circuit 9 added.

The delay circuit 9 can arbitrarily set the delay time of the output of the detection circuit 7, and sets an optimum detection delay time according to the load in the case of a load having a long rush current.

The subsequent operation is the same as in the first embodiment.

[0031]

As described above, according to the present invention, the output can be safely limited even when the overload state continues, and the heat sink conventionally had to be thermally designed with the continuous peak current. , Transformers, switching elements, etc., can be thermally designed with an average value, and downsizing and cost reduction can be realized.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing operation waveforms in the present invention.

FIG. 3 is a circuit diagram showing a second embodiment of the present invention.

FIG. 4 is a circuit diagram showing a third embodiment of the present invention.

FIG. 5 is a circuit diagram showing a conventional example.

[Explanation of symbols]

 1, 2 input power supply terminal 3 PWM circuit 4, 5 output terminal 6 voltage detection circuit 7 current detection circuit 8 delay circuit T1 transformer T2 current transformer Q1 switching transistor Q2 comparator Q3 diode Q4 photocoupler Q5 photocoupler C1, C2, C3 capacitors R1, R2 , R3, R4, R5, R6, R7 Resistor

Claims (1)

(57) [Claims] 1. A switching power supply for supplying a load by converting an alternating current generated by intermittently connecting a power supply input to a switching element to a direct current, to a load, a detecting means for detecting a peak current of the switching element, and a detection signal from the detecting means. A protection means for limiting the operation of the switching element based on the load current detection circuit, and an offset to an input value of the detection means based on an average value of the load current obtained by the load current detection circuit. A switching power supply comprising: