JP2003189598A - Switching power unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the malfunction of excessive current protection by detecting a voltage drop after a switching element is turned on without making an influence exerted upon characteristics of the switching element, in a switching power unit that performs the excessive current protection by the voltage drop due to a current flow to an on-resistor at the on-time of the switching element. <P>SOLUTION: The power unit comprises: an on-detection means that uses a voltage between main terminals of a first switching element; a current detection means that uses the on-resistor during an on-period; and a control means that turns off the first switching element when the on-detection is not obtained after a delay period that is determined by a control signal impressed to a control terminal of the first switching element. By using the voltage between the main terminals of the first switching element, the excessive current protection is surely performed during only the on-period, thus the sure excessive current protection can be achieved. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a switching power supply,
In particular, the present invention relates to a switching power supply having overcurrent protection utilizing a voltage drop caused by a current flowing through an ON resistance of a switching element.

[0002]

2. Description of the Related Art Heretofore, as a switching power supply device of this type, there has been one described in Japanese Patent No. 2888729, for example. FIG. 4 shows the conventional switching power supply device described in the above publication.

In FIG. 4, 1 is a first switching element, 3 is a diode, 4 is a coil, 5 is a smoothing capacitor, 50 is a synchronous switching element, 51 and 52 are resistors for voltage division, 53 is a drive circuit, and 54 is a resistor. 55 is a Zener diode, 56 is a comparator, and 57 is a PWM control circuit.

In the figure, a MOS-FET is used as the first switching element 1, and the drain-source voltage (Vds) when turned on is the drain current (Rds) flowing in the on-resistance (Rds) of the MOS-FET. It takes a value of Vds = Rds × Id in proportion to the value of Id).

Further, the first switching element 1 and the synchronous switching element 50 are simultaneously turned on by the drive circuit 53.
By turning off, when the first switching element 1 is turned on, the “input voltage −Vd
s ″ is divided by the voltage dividing resistors 51 and 52. The comparator 56 compares the reference voltage of the Zener diode 55 and the midpoint voltage of the voltage dividing resistors 51 and 52 to detect the overcurrent state.

At the rated load, when the first switching element 1 is on, the midpoint voltage of the resistors 51 and 52 is set higher than the reference voltage, and the output of the comparator 56 is Low. Next, due to overcurrent, the drain current (Id) when the first switching element 1 is on
The voltage drop due to the ON resistance (Rds) becomes larger than the rated load, and the midpoint voltage divided by the voltage dividing resistors 51 and 52 becomes lower than the reference voltage, so that the output of the comparator 56 changes from Low to High. . Comparator 56
The PWM control circuit 57 outputs the first
It is shown that the current control is performed by turning off the switching element 1 of 1.

In Japanese Patent No. 2888729, the operating point of the overcurrent protection changes depending on the temperature characteristic of the on-resistance of the MOS-FET, so that it is necessary to set it to 2 to 3 times the rated load. It is said that.

[0008]

However, in the above-described conventional configuration, the same characteristics are required in order for the first switching element 1 and the synchronous switching element 50 to be turned on at the same time, but in general, the first switching element is used. An element capable of allowing a large current to flow is used as the element, and the synchronous switching 50 is a signal element to sufficiently exhibit its performance. Since there is a difference in the time when the drive circuit 53 is turned on due to the difference in the element characteristics, the overcurrent may be detected in a transitional state of turning off or on. At this time, an overcurrent protection malfunction occurs, and the state in which the ON width of the first switching element 1 is narrow continues, and there is a problem that a stable output cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a switching power supply device which can obtain a stable output and at the same time improve an overcurrent protection operation.

[0010]

In order to solve the above-mentioned conventional problems, a switching power supply device according to the present invention includes an ON detecting means using a voltage between main terminals of a first switching element and an ON detecting means. The first switching element is turned off when the on detection cannot be obtained after a delay period determined from the current detection means using the ON resistance and the control signal applied to the control terminal of the first switching element. It consists of control means.

With this, it is possible to reliably detect the ON state by using the voltage between the main terminals of the first switching element,
When it is possible to operate the overcurrent protection only during the ON period with certainty, it is not affected by the characteristics of the element, and ON detection cannot be performed after the delay time from the control signal to the control terminal of the first switching element. By operating the control signal so as to turn off, an excessive current is prevented from flowing even in a state where it cannot be detected due to a load short circuit, a coil abnormality, or the like, and reliable overcurrent protection is achieved.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 is a first switching element connected to a DC power supply, an inductance connected between the first switching element and a load, and the inductance. A second switching element connected in parallel to a series circuit of the above, a smoothing capacitor connected in parallel with the load, the first switching element and the first switching element so that the voltage across the load becomes constant. In the switching power supply device using synchronous rectification having a control circuit for controlling the second switching element, the control circuit has an overcurrent protection function, and the overcurrent protection function is a voltage between main terminals of the first switching element. ON detection means using the above, a current detection means using ON resistance performed during the ON period, and a control signal applied to the control terminal of the first switching element. The switching power supply device is characterized in that it comprises a control means for turning off the first switching element when ON detection is not obtained after a predetermined delay period from the main terminal of the first switching element. It is possible to reliably detect ON by using the voltage between
It is possible to operate the overcurrent protection only during the ON period with certainty, it is not affected by the characteristics of the element, and ON detection cannot be performed after delaying the delay time from the control signal to the control terminal of the first switching element. In this case, by operating so as to turn off the control signal, an excessive current is prevented from flowing even in a state where ON detection cannot be performed due to a load short circuit or a coil abnormality, and reliable overcurrent protection is provided.

According to a second aspect of the invention, the switching power supply device according to the first aspect is characterized in that the current detecting means has a temperature characteristic for correcting the temperature characteristic of the ON resistance of the first switching element. As a result, it is possible to increase the accuracy of the operating value of overcurrent protection with respect to the temperature, and to obtain stable overcurrent protection.

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing a switching power supply device according to an embodiment of the present invention.

In FIG. 1, 1 is a first switching element, 2 is a second switching element, 3 is a diode, 4
Is an inductance, 5 is a smoothing capacitor, 6 is a load (resistor), 7 is a resistor connected to the first switching element 1, 8 is a resistor connected to the second switching element 2, and resistors 7 and 8 are provided. Is not always necessary. Reference numeral 9 is a DC power supply, 100 is a control circuit block, 101 is a synchronous rectification control block for performing synchronous rectification and overcurrent protection operation, and 102 is synchronous with a control signal for turning off the control terminal of the first switching element 1. Switch connected, 10
3 is a delay circuit, 104 is a drive circuit for the first switching element 1, 105 is an overcurrent threshold voltage, and 106 is a comparator for detecting ON of the first switching element 1 and detection of overcurrent. Comparing the input of the voltage dropped from the DC voltage 9 by the overcurrent threshold voltage 105 and the output voltage of the first switching element 1, the ON detection means and the current detection means are configured, and 107 is a first 1 is a switch connected when ON of the switching element 1 is detected, 108 is an OR circuit that takes the logical sum of ON detection by the delay circuit 103 and the comparator 106, 109 is a drive circuit of the second switching element 2, and 110 is It is a voltage control block including an error amplifier that detects the voltage across the load 6 and compares it with a reference voltage, an oscillation circuit, and PWM control.

The operation and action of the switching power supply device configured as described above will be described below.

First, when the first switching element 1 is in the ON state, the DC power supply 9 turns the first switching element 1 on.
The electric power is supplied to the inductance 4 via the. Next, when the first switching element 1 is turned off, a counter electromotive force is generated in the inductance 4 and the diode 3 is turned on,
A current flows from the diode 3 to the inductance 4.
After that, the second switching element 2 is turned on after a delay time from the turning off of the first switching element 1, so that the diode 3 is turned off. Next, when the second switching element 2 is turned off, the diode 3 is turned on again. Then, in the operation in which the first switching element 1 is turned on again after a delay time from the turning off of the second switching element 2, in the ON period of the first switching element 1 and the ON period of the second switching element 2. The voltage supplied to the load 6 is kept constant by controlling the voltage.

The overcurrent protection operates by detecting the voltage drop due to the current flowing through the on resistance during the on period of the first switching element 1.

FIG. 2 shows an example of operation waveforms at a rated load.

In the figure, when the first switching element 1 is turned on by the drive circuit 104, the comparator 1
06, the positive input is "DC power supply 9-threshold voltage 10
A voltage higher than 5 "is applied, and-" DC power supply 9-
Threshold voltage 105 "is input and output is from Low to H
change to high. The output of the comparator 106 is High
When it becomes h, the output of the OR circuit 108 changes from Low to H.
change to high. The synchronous rectification control block 101 is O
The switch 107 is provided while the output of the R circuit 108 is High.
Is connected to monitor whether the current flowing through the first switching element 1 is in an overcurrent state. When no overcurrent is recognized, the output of the drive circuit 104 is inverted by the voltage control block. In the process of turning off the first switching element 1, the + input of the comparator 106 becomes lower than the “DC power supply 9−threshold voltage 105”, and the output changes from High to Low.

When the switch 102 is connected at the same time when the output of the drive circuit 104 is inverted, the output of the OR circuit 108 changes from Low to High in synchronization with the High to Low timing of the comparator 106.

If switch 102 is always open,
After delaying the inversion of the output of the drive circuit 104 by the delay circuit 103, the output of the OR circuit 108 changes from Low to High.
Changes to.

FIG. 3 shows an example of operation waveforms in overcurrent.

In the figure, when the first switching element 1 is turned on by the drive circuit 104, the comparator 1
06, the positive input is "DC power supply 9-threshold voltage 10
A voltage higher than 5 "is applied, and-" DC power supply 9-
Threshold voltage 105 "is input and output is from Low to H
change to high. The output of the comparator 106 is High
When it becomes h, the output of the OR circuit 108 changes from Low to H.
change to high. The synchronous rectification control block 101 is O
The switch 107 is provided while the output of the R circuit 108 is High.
Is connected to monitor whether the current flowing through the first switching element 1 is in an overcurrent state. When an overcurrent is recognized, the output of the comparator 106 changes from High to Low, the synchronous rectification control block 101 changes the output of the drive circuit 104 to Low, and the first switching element 1 is turned off.

When the switch 102 is connected simultaneously with the inversion of the output of the drive circuit 104, the output of the OR circuit 108 changes from Low to High in synchronization with the timing of High to Low of the comparator 106.

If switch 102 is always open,
After delaying the inversion of the output of the drive circuit 104 by the delay circuit 103, the output of the OR circuit 108 changes from Low to High.
Changes to.

Here, when the load 6 is close to a short circuit, the first
Even if the switching element 1 of the
Sufficient voltage does not appear at the + input of 06, and ON detection cannot be performed. Although the threshold voltage 105 has two kinds of voltages suitable for ON detection and overcurrent protection, and can be improved by switching, the effect is not recognized depending on the characteristics of the inductance 4 (inductance, impedance saturation characteristics, etc.). . In this state, the comparator 106
Since a high level does not occur in the output of the switch 107, the switch 107 is connected by the output of the delay circuit 102 and the overcurrent is monitored.
Therefore, it is possible to avoid the problem that the overcurrent protection function does not operate because the is not connected and operates at the maximum duty to increase the stress applied to the component. The delay circuit 10
Although 3 is delayed from the signal from the synchronous rectification control block, other signals (for example,
From the voltage control block 110 to the synchronous rectification control block 11
A control signal to 0) may be used.

Further, the ON resistance of the MOS-FET is used for detecting the overcurrent in this embodiment. However, since the change in the temperature characteristic is large, an excessive current is generated especially when a condition close to a load short circuit occurs at a low temperature. Greater stress on the flow parts. Therefore, by giving the threshold voltage 105 or the comparator 106 a temperature characteristic for correcting the temperature characteristic of the MOS-FET, the influence can be reduced. Further, the accuracy of the operating point of the overcurrent is also improved.

[0030]

As described above, according to the first and second aspects of the present invention, it is possible to reliably detect the ON state of the first switching element, and prevent erroneous detection of overcurrent. Even if the ON state cannot be detected due to some abnormality, the ON pulse width is controlled with a delay time, so that stable overcurrent protection is performed.

[Brief description of drawings]

FIG. 1 is a block diagram of a switching power supply device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing operation waveforms at a rated load of the switching power supply device according to the first embodiment of the present invention.

FIG. 3 is a diagram showing operating waveforms of the switching power supply device according to the first embodiment of the present invention due to overcurrent.

FIG. 4 is a block diagram of a conventional switching power supply device.

[Explanation of symbols]

1 First switching element 2 Second switching element 3 diode 4 Inductance 5 Smoothing capacitor 6 load (resistance) 7 resistance 8 resistance 9 DC power supply 50 sync switch 51 division resistor 52 resistor for voltage division 53 Drive circuit 54 Resistance 55 Zener diode 56 comparator 57 PWM control circuit 100 control circuit block 101 Synchronous rectification control block 102 switch 103 delay circuit 104 drive circuit 105 threshold voltage 106 comparator 107 switch 108 OR circuit 109 drive circuit 110 voltage control block

Claims (2)

[Claims] 1. A first switching element connected to a DC power source, an inductance connected between the first switching element and a load, and a first switching element connected in parallel to a series circuit of the inductance and the first switching element. And a smoothing capacitor connected in parallel with the load, and a control circuit for controlling the first switching element and the second switching element so that the voltage across the load is constant. In the switching power supply device using synchronous rectification, the control circuit has an overcurrent protection function, and the overcurrent protection function is an on-detection unit using a voltage between main terminals of the first switching element and an on-period protection function. The ON detection is performed after a delay period determined from the current detection means using the ON resistance and the control signal applied to the control terminal of the first switching element. If you do not get the above, the first
2. A switching power supply device comprising a control means for turning off the switching element. 2. The switching power supply device according to claim 1, wherein the current detecting means has a temperature characteristic for correcting the temperature characteristic of the ON resistance of the first switching element.