JPH02133067A - Switching power supply - Google Patents

Abstract

PURPOSE:To improve reliability at the time of abnormality, and to reduce material cost by dividing the output winding voltage of a transformer so as to be synchronized with ON currents of an inverter so that the overcurrent characteristics of the secondary side output of a power supply are not changed by input voltage and applying the output winding voltage to an overcurrent detector. CONSTITUTION:The ON voltage of a transformer winding 1b synchronized with the ON currents of an inverter 7 is divided by resistors 3, 4 and reverse bias voltage is applied to a comparator 9 for detecting overcurrents by a diode 11. Consequently, even when input voltage is fluctuated, the overcurrent characteristics of a secondary side output can be kept constant, and the maximum rated current values of diodes 10, 13 for rectifying the secondary side output can the inhibited at low values. Accordingly, material cost can be reduced while reliability at the time of abnormality can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a switching power supply having a flybank converter circuit configured such that the overcurrent protection characteristics of the power supply do not change depending on the input voltage.

2. Description of the Related Art In general, a switching power supply having a primary control type flyback converter circuit with a control circuit provided on the primary side of an output transformer has an overcurrent protection characteristic (DC output current - DC output voltage characteristic) of its output. Figure 2 Song Mi (input voltage A
C90V), j (input voltage AC IOOV),
k (input voltage AC IIOV), 1 (input voltage A
C 120 V), m (input voltage AC 132 V), which usually changes depending on the input voltage, and FIG. 2 shows a conventionally commonly used flyback converter circuit.

In Fig. 3, 17 is a converter transformer, 22 is an inverter (FET), 23 is a PWM control circuit, 24 is an overcurrent detection circuit (comparator), 29 is an output voltage detection circuit, 30 is a feedback circuit, and 18 is an overcurrent detection circuit. 19, 20 and 21 are resistors, 25 and 27 are diodes, and 26 and 2 are capacitors.

Problems to be Solved by the Invention As mentioned above, in a switching power supply having a conventional primary control type flyback converter circuit, the overcurrent protection characteristics of the output change depending on the input voltage. The diode 27 used for this must have a large maximum rated current value, which has the disadvantage of increasing costs.

Means for Solving the Problems The present invention solves the above-mentioned problems, and provides a method for ensuring that the output overcurrent detection point does not change depending on the input voltage of the power supply in a switching power supply of a primary control type flyback converter circuit. This is a switching power supply with special features.

In other words, in a switching power supply having a primary control type flyback converter circuit, the output winding voltage of the transformer is synchronized with the ON current of the inverter so that the overcurrent characteristics of the secondary output of the power supply do not change depending on the input voltage. This switching power supply is characterized in that the voltage is divided and applied to the overcurrent detection circuit.

By making the overcurrent detection point of the working output unchanged by the input voltage, the rectifier diode 13 of the output
It has the effect of reducing the maximum rated current value of , increasing reliability in the event of an abnormality and reducing material costs.

Example The present invention will be explained below with reference to an embodiment shown in FIG.

In Fig. 1, the ON voltage of the transformer winding 1b synchronized with the ON current of the inverter (FET) 7 is applied to the e terminal of the comparator (overcurrent detection circuit!) 9 for overcurrent detection.
The voltage is divided by the diode 11 and a reverse bias voltage is applied. l is converter 1 lance,
la, lc are transformer windings, 8 is a PWM control circuit, l5
16 is an output voltage detection circuit, 16 is a feedback circuit, 2 is an overcurrent detection resistor, 5 and 6 are resistors, 10 and 13 are diodes, and 12 and 14 are capacitors.

Note that another embodiment may be used in which a Zener diode is inserted in series with the resistor 3 in order to create this divided voltage. Various other modifications are possible, but in any case, the ON voltage of the output winding of the converter transformer is used to apply a reverse bias to the overcurrent detection circuit.

The voltage appearing in the winding 1b of the converter transformer when the inverter is ON is the voltage when the power input is high.
X) If the voltage when the power supply input is low is expressed as Vo(sxn), the following relationship is shown.

V+x(wax) > V+x(sin) ・---
−−・−−−−−−−− (1) Therefore, the divided voltage of the comparator (overcurrent detection circuit) and 9 human power is V' I
If expressed in terms of N, it becomes as follows, similar to the above equation.

V''+N(wax) >V'+N(min) 1・---------・------= (2) On the other hand, the peak value of the ON current flowing to the inverter is when the power input is high. If the current is expressed as I p (lIIax) and the current when the power supply input is low is expressed as I p (shin), then the flyback converter circuit has a relationship expressed by the following equation.

I p (IIIax) < I p (win)-
(3) Therefore, if the resistance value of the overcurrent detection resistor 2 is represented by R, Equation 3 becomes the following relational expression.

I p (wax) x R< I p (+++i
n) Therefore, even if the input voltage changes, it is possible to keep the overcurrent characteristic (regulation characteristic) of the secondary side output constant.

V′+N(max)+I p(lIIax)
x R#V +w (sin) →-1 p (win
)
a is input voltage AC90V, curve b is input voltage ACIOOO
Curve C is for an input voltage ACIIOV, curve d is for an input voltage of 120 VAC, and curve e is for an input voltage of 132 VAC.

Effects of the Invention As described above, in the switching power supply of the present invention, since the overcurrent characteristics (regulation characteristics) of the flyback converter circuit do not change depending on the input voltage of the power supply, the maximum rated current of the rectifying diode of the secondary output Since it is possible to keep the value low, it has remarkable effects such as lowering the material cost of switching power supplies and increasing reliability, and is of great industrial and practical value.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of an embodiment of the flybar solenoid converter circuit in the switching power supply of the present invention, Fig. 2 is a DC output current-DC output voltage characteristic diagram of the switching power supply, and Fig. 3
The figure is an explanatory diagram of a flybar sock converter circuit in a conventional switching power supply. 1: Converter transformer la, lb, lc: } Lance winding 7: Inverter 9: Comparator (overcurrent detection circuit)

Claims (1)

[Claims] In a switching power supply having a primary control type flyback converter circuit, the output winding voltage of the transformer is synchronized with the ON current of the inverter so that the overcurrent characteristics of the secondary side output of the power supply do not change depending on the input voltage. A switching power supply characterized by dividing the voltage into a voltage and applying it to an overcurrent detection circuit.