JPH05344712A - Overload protection circuit for switching power supply - Google Patents

Abstract

PURPOSE:To remove the input voltage dependency of an overload detection signal by adding an input voltage correction signal detected by an ON-OFF type converter part to a load current signal detected by a current detection means and by forwarding an added signal to the overload protection terminal of a pulse-width control circuit. CONSTITUTION:In an ON-OFF type converter part 40, an input voltage is rectified by diodes D4 and D5 for the purpose of generating a negative voltage, smoothed by a capacitor C4 after high-speed wavenumber components are removed by a choke coil L, and converted by voltage divider resistances R1, R2 into a voltage corresponding to a load current signal outputted by a current detection circuit 30. Then, an input voltage correction signal obtained by the ON-OFF type converter part 40 is added to a load current signal outputted by the current detection circuit 30 and sent to an overload detection terminal OLP. Therefore, the input voltage dependency of an input current detection circuit is cancelled by the input voltage correction signal EC of the ON-OFF type converter using a bias winding so that an accurate overload protection can be performed even if the range of a supply voltage becomes wider.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overload protection circuit used in a stabilized DC power supply device and the like, and more particularly to improvement of operating characteristics when the range of input voltage fluctuates widely.

[0002]

2. Description of the Related Art The applicant of the present application has proposed an overload protection circuit for a switching power supply in Japanese Utility Model Publication No. 1-194866. FIG. 3 is a block diagram of the configuration of the conventional device disclosed in the prior application. In the figure, the DC-DC converter 10 is supplied with input power of an input current Iin at an input voltage Vin, and is supplied to the load side of a load current Iout at an output voltage Vout. When using a commercial AC power supply as the input power supply,
DC is converted by a rectifying and smoothing circuit. The PWM (pulse width) control circuit 20 stabilizes the output voltage by sending an on / off control signal to a switching element such as a transistor provided in the DC-DC converter 10. The current transformer CT is provided on the input side of the DC-DC converter 10. The rectifier circuit 32 converts a current signal into a voltage signal, and the averaging circuit 33 converts the input current Iin.
Is generated. The input voltage signal circuit 34 is a circuit that detects the input voltage Vin. Input voltage correction circuit 3
Reference numeral 5 is a circuit for correcting the input current signal with the input voltage signal and sending it to the PWM control circuit 20. For example, the correction is performed by adding both.

FIG. 4 is an explanatory diagram of the input voltage dependency of the overcurrent protection circuit, in which the horizontal axis represents the input voltage Vin and the vertical axis represents the load current detection signal. In the figure, the peak current Ip is the peak component of the switched input current signal. In the on-off converter, the PWM control circuit 20 controls the pulse width to be narrowed when the input voltage Vin becomes high, and at a constant switching frequency. When turning on and off, peak current Ip
Becomes smaller. The output power Po satisfies the following relationship. Po = Vin · Ip · D · η (1) Here, the output power Po is constant, D is the duty ratio, and η is the efficiency. The correction signal Ec is a signal whose absolute value increases in proportion to the increase of the input voltage Vin, and when it is added to the peak current, it becomes a corrected signal which is convex downward. This corrected signal is sent to the OLP (overload protection) detection terminal of the PWM control circuit 20, and when the detection level I _L is exceeded, overload protection is activated. Here, a negative voltage is used because of the logic.

[0004]

However, when the input voltage is widened to 90V to 240V as an AC voltage, the load current at the time of high input voltage at the time of overload detection becomes excessive, and the switching element on the primary side becomes excessive. There was a risk of destruction. Further, although the above-mentioned correction signal can deal with the fluctuation of the input voltage in a narrow range, the input voltage dependency of the overload detection signal cannot be sufficiently removed when the input voltage is widened. However, there is a problem that overload detection cannot be performed accurately.

The present invention solves such a problem, and an object of the present invention is to provide an overload protection circuit for a switching power supply which does not depend on the input voltage of an overload detection signal even when the input voltage is widened. And

[0006]

SUMMARY OF THE INVENTION The present invention which achieves such an object is to rectify and smooth AC power, apply it to the primary winding of a transformer, turn it on and off by a switching element, and apply it to the secondary winding of this transformer. An on-off type converter unit 10 for rectifying and smoothing the induced switching signal and outputting it.
And a control signal is sent to the switching element in a direction to reduce the error voltage by comparing the output voltage of the converter unit with a reference voltage, and the pulse width is controlled to be narrowed when the input voltage increases. Pulse width control circuit 2
0, a means 30 for detecting the peak component of the current flowing through the primary winding, and an on-on converter section 40 for rectifying and smoothing the switching signal induced in the bias winding provided on the primary side of the transformer. It has.

The load current signal detected by the current detecting means is added with the input voltage correction signal detected by the on-on converter section and sent to the overload protection terminal of the pulse width control circuit.

[0008]

The load current signal detected by the current detecting means is not linear with respect to the input voltage and cannot be used as it is as an accurate overload detection signal. Therefore, a signal for canceling the influence of the input voltage is obtained by the input voltage correction signal detected by the on-on converter unit. Then, by adding both, an accurate overload detection signal that does not depend on the input voltage can be sent to the overload protection terminal of the pulse width control circuit.

[0009]

The present invention will be described in detail below with reference to the drawings. FIG. 1 is a configuration block diagram showing an embodiment of the present invention. In the figure, the on / off converter unit 10 rectifies an alternating current AC by a diode bridge DB and smoothes it by a capacitor Co, and a DC power of an input voltage Ein is applied to a primary winding n1 of a transformer. The switching element Q such as FET turns on / off the current flowing through the primary winding n1. Then, since a switching signal is induced in the secondary winding n2 of this transformer, it is rectified and smoothed by the diode D1 and the capacitor C1 and output. Here, the on-off type converter is a converter in which the excitation energy is stored in the winding of the transformer when the switching element Q is turned on and is released when the switching element Q is turned off. The output voltage detection circuit 12 is an error amplifier that compares the output voltage Eo of the converter unit 10 with a reference voltage to obtain an error voltage. This error signal is passed through an insulation circuit 14 on the primary side and the secondary side of a power supply such as a photocoupler. And is sent to the feedback terminal F / B of the pulse width control circuit 20.

The pulse width control circuit 20 sends a control signal to the switching element Q in the direction of reducing the error signal, and controls the pulse width in the direction of narrowing when the input voltage increases. Further, when a signal recognized as an overload state is given to the overload detection terminal OLP, the switching operation is suppressed. The current detection circuit 30 detects the peak component of the current flowing through the primary winding n1, and uses the current transformer CT or the current-voltage conversion resistor to input the input current I, for example.
Generates a pulsed voltage signal proportional to in and holds the peak voltage.

The on-on converter section 40 rectifies and smoothes the switching signal induced in the bias winding n3 provided on the primary side of the transformer, and rectifies it with the diodes D4 and D5 to generate a negative voltage. , The high frequency component is removed by the choukk coil L and smoothed by the capacitor C4, and the current detection circuit 30 is formed by the voltage dividing resistors R1 and R2.
It is converted into a voltage that matches the load current signal output by the.
Here, the ON-ON type converter is a switching element Q.
Is turned on, the excitation energy is stored in the primary winding n1 of the transformer and at the same time the excitation energy is discharged from the secondary winding n2. The input voltage correction signal obtained by the on-on converter unit 40 is added to the load current signal output from the current detection circuit 30 and sent to the overload detection terminal OLP. The auxiliary power supply unit 50 supplies the operating voltage Vcc of the pulse width control circuit 20, and rectifies the switching signal induced in the bias winding n3 by the diode D2.
It is smoothed by the capacitor C2.

The operation of the thus constructed device will be described below. In the on-off converter 10, the input voltage Ein
And the output voltage Eo have the following relationship.

[Equation 1] Here, n ₁₂ is the turn ratio between the primary winding n1 and the secondary winding n2,
D is the duty ratio of the switching signal. In the on-on type converter, the following relationship is established between the input voltage Ein and the output voltage Eo.

[Equation 2] Here, n ₁₃ is the turn ratio between the primary winding n1 and the bias winding n3. It is assumed that the output voltage Eo is constant due to the output voltage stabilizing circuit. Then, the input voltage correction signal Ec output from the on-on converter 40 can be expressed by the following equation.

[Equation 3]

FIG. 2 is an explanatory diagram of the input voltage and the OLP terminal voltage. The pulse width control circuit 20 has an overload detection terminal OLP.
When the voltage applied to the voltage reaches a predetermined voltage, for example, -200 mV, it shuts down. The peak current Ip is the primary winding n
The following relationship is established with the duty ratio D at the peak of the pulse current flowing in No. 1. Vin · Ip · D · η = constant (1) ′ Then, as the input voltage Ein increases, the absolute value of the peak current Ip decreases, but there is a rising curve, but the relationship is the efficiency η of the power supply and the duty ratio. It has a nonlinearity that depends on D. On the other hand, the input voltage correction signal Ec has a downward-sloping curve as shown in Formula 3, so that a substantially constant OLP terminal voltage can be obtained by adding both.

[0014]

As described above, according to the present invention, the input voltage dependency of the input current detection circuit is canceled by the input voltage correction signal Ec of the on-on type converter using the bias winding. The effect is that accurate overload protection can be performed even if the voltage is widened. In addition, when the bias winding is used together with the auxiliary power supply unit as in the embodiment, the cost of the component is not increased, and the non-linearity can be compensated without using a relatively expensive component such as an operational amplifier. Contribute to lower costs.

[Brief description of drawings]

FIG. 1 is a configuration block diagram showing an embodiment of the present invention.

FIG. 2 is an explanatory diagram of an input voltage and an OLP terminal voltage.

FIG. 3 is a configuration block diagram of a conventional device.

FIG. 4 is an explanatory diagram of input voltage dependency of an overcurrent protection circuit.

[Explanation of reference numerals] 10 ON / OFF type converter section 20 Pulse width control circuit 30 Current detection means 40 ON / ON type converter section

Claims (1)

[Claims] 1. An on-off converter for rectifying and smoothing AC power, applying it to a primary winding of a transformer, turning it on and off by a switching element, and rectifying and smoothing a switching signal induced in a secondary winding of the transformer and outputting the switching signal. Part 10
A control signal is sent to the switching element in a direction to reduce the error voltage by comparing the output voltage of the converter unit with a reference voltage, and the pulse width is controlled to be narrowed when the input voltage increases. Pulse width control circuit 20
A means 30 for detecting the peak component of the current flowing through the primary winding, and an on-on type converter section 40 for rectifying and smoothing the switching signal induced in the bias winding provided on the primary side of the transformer. The input current correction signal detected by the on-on converter is added to the load current signal detected by the current detection means, and the sum is sent to the overload protection terminal of the pulse width control circuit. Load protection circuit.