JPH04304164A - Magnetic control switching power supply - Google Patents

Abstract

PURPOSE:To reduce power loss in a current detecting resistor while preventing erroneous function of an overcurrent protective circuit due to noise by connecting a current transformer, secondary winding thereof has higher number of turns than that of the primary winding, to the output side of a transformer. CONSTITUTION:A current transformer 24, secondary winding of which has higher number of turns than that of the primary winding, is connected in series with a line between the secondary winding of a transformer 3 and a smoothing circuit 5. Secondary current of the current transformer 24 is then subjected to half-wave rectification through a diode 25 and fed to an output current detecting resistor 26 in order to detect a half-wave voltage proportional to the peak value of output current Io. When the output current increases and the voltage at the inverted input terminal of a differential amplifier 9 reaches a reference voltage at the non-inverted input terminal to cause voltage drop at the output of the differential amplifier 9, a transistor 22 feeds a reset current to a saturable reactor 7 through a diode 23. Consequently, output voltage is restricted so that the output current does not increase thus realizing overcurrent protection.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit for a magnetically controlled switching power supply.

0002

2. Description of the Related Art A magnetically controlled DC-DC switching power supply device, as shown in the circuit diagram of FIG.
2) to connect the primary winding of the transformer (3) and the DC power supply (
The series circuit (4) is turned on and off, and the voltage generated in the secondary winding is thereby smoothed by the circuit (5) and power is supplied to the load (6). By the way, in this power supply device, the following circuit is provided in order to stabilize the output voltage Vout and protect against overcurrent. That is, as shown in Figure 2, a saturable reactor (7) is provided in series between the secondary winding of the transformer (3) and the smoothing circuit (5), and this is used as the reference voltage for setting the output voltage by the differential amplifier (8).
Vref1 and a voltage V proportional to the output voltage Vout
1 and a comparison difference voltage between the protection current setting reference voltage Vref2 and the voltage proportional to the output current I0 by the differential amplifier (9) to provide a constant voltage control function and an overcurrent protection function. things are being done. That is, in constant voltage control as shown in FIG. 2, the output voltage Vo
Voltage V1 obtained by dividing ut by resistors (10) (11)
is connected to the inverting input terminal of the differential amplifier (8), and the non-inverting input terminal is connected to the power supply Vcc, the resistor (12), and the resistor (
13) is connected to the reference voltage Vref1 created by step 13). The differential voltage generated when the output voltage rises above the set voltage causes the base current of the transistor (14) to increase and is passed through the diode (15) to the saturable reactor (7).
) by increasing the reset current of the saturable reactor (7) to increase the reactance of the saturable reactor (7) and keep the output voltage constant at the set value. For overcurrent protection, an output current detection resistor (16) is connected in series with the output circuit to detect the output current I0 as a voltage proportional to this, and this is divided by resistors (18) and (19). Differential amplifier (9
) to the non-inverting input terminal of the power supply Vcc and the resistor (20
) (21) is connected to the inverting input terminal of the differential amplifier (9). When the non-inverting input voltage V2 reaches the reference voltage Vref2 due to an increase or decrease in the output current I0, the base current of the transistor (22) is increased by the comparison difference voltage, and the base current of the saturable reactor (7) is increased through the diode (23). The output voltage Vout is controlled so that the output current I0 does not increase by increasing the reset current.

0003

[Problems to be Solved by the Invention] However, in the above-mentioned overcurrent protection circuit of the conventional device, a resistor (16) directly connected in series with the output circuit is used for detecting the output current I0, so a large power loss occurs. It is impossible to prevent this from occurring and reducing efficiency. Therefore, especially when used in a low voltage, large current load circuit, the operating efficiency of the entire system will be reduced. Of course, power loss can be reduced by reducing the resistance value of the current detection resistor (16), but this also reduces the level of the input signal voltage of the differential amplifier (9). For this reason, it is easy to malfunction due to noise and a stable overcurrent protection operation is inhibited, so there is a limit to how low the resistance of the output current detection resistor (16) can be made. Therefore, it is difficult for conventional circuits to meet the above-mentioned demand for improved operating efficiency.

0004

[Object of the Invention] An object of the present invention is to provide an overcurrent protection circuit for a magnetically controlled switching power supply device that can significantly reduce power loss in a current detection resistor while preventing malfunction of the overcurrent protection circuit due to noise. It is in.

[0005]

[Means of the present invention for solving the problems] FIG. 1 is a circuit diagram of one embodiment of the present invention (portions with the same reference numerals as those in FIG. 2 indicate equivalent parts), and the features of the present invention are as follows. At the point. That is, a current transformer (24) having a secondary winding with a larger number of turns W2 than the number of turns W1 of the primary winding is connected in series to a line between the secondary winding of the transformer (3) and the smoothing circuit (5). The secondary current is half-wave rectified by a diode (25) and then passed through the output current detection resistor (26), thereby detecting a half-wave voltage proportional to the peak value of the output current I0. After that, connect this to a diode (27) and a resistor (
28), a smoothing circuit consisting of a capacitor (29),
The DC voltage V2 is converted into a DC voltage V2 which changes in proportion to the peak value of the output current I0, and is applied to the inverting input terminal of the differential amplifier (9). Then, the output current I0 increases, and the voltage V2 at the inverting input terminal of the differential amplifier (9)
is the reference voltage Vref2 connected to the non-inverting input terminal.
When the output voltage of the differential amplifier (9) decreases, the transistor (22) causes a reset current to flow into the saturable reactor (7) via the diode (23), thereby increasing the output current I0. The output voltage Vout is suppressed to protect against overcurrent. In the figure, (30) is a resistor, which is caused by the opening of the secondary winding of the current transformer (24) during the period when the voltage of the secondary winding is not rectified by the diode (27), that is, the period when the diode (25) is off. , is used to prevent overvoltage from occurring, and has a large resistance value that does not affect the current transformation ratio.

[0006]

[Effect of the invention] As described above, in the present invention, the transformer (3)
A current transformer (24) having a secondary winding with a larger number of turns than the primary winding is connected to one line on the output side of the transformer. Therefore, the current flowing through the current detection resistor (26) connected to the secondary winding is 1/(W1/W) of the number of turns of the output current I0.
2) become. Therefore, to simplify the explanation, if we assume that there is no resistance (30) on the secondary side of the current transformer (21) and ignore the resistance of the diode (25), the resistance value of the current detection resistor (26) will be 1. When converted to the next side (W
1/W2) will be doubled. Therefore, when obtaining the same detection voltage, the value of the current detection resistor (26) can be made smaller than when the current detection resistor (26) is connected directly in series with the output circuit, so that the power loss that constantly occurs during operation can be significantly reduced. Therefore, even considering the increase in cost due to the use of a current transformer, it is economically advantageous, and the risk of malfunction due to noise can be reduced. In addition, since the voltage (2) of the differential amplifier (9) can be increased by increasing the current detection resistor (26) while reducing power loss compared to the conventional method, the reference voltage for setting the protection current also increases, and the protection current This has the advantage of increasing the degree of freedom in settings. In the above description, the case where the transformer (3) has one set of secondary outputs has been explained as an example, but it goes without saying that the present invention can also be applied to the case of multiple outputs of one or more sets. Furthermore, although the resistor (13) is used to obtain the reference voltage in the voltage control circuit as shown in FIG. 2, a constant voltage element such as a Zener diode may also be used.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional device.

[Explanation of symbols]

(1) Control unit (2) Switching element (3) Transformer (4) DC power supply (5) Smoothing circuit (6) Load (7) Saturable reactor (8) Differential amplifier for constant voltage control (9) Overcurrent Differential amplifier for protection (10) Voltage dividing resistor (11) Voltage dividing resistor (12) Resistor for obtaining reference voltage Vref1 (13)
Resistor for acquiring reference voltage Vref1 (14) Transistor (15) Diode (16) Resistor for output current detection (18) Voltage dividing resistor (19) Voltage dividing resistor (20) Resistor for acquiring reference voltage Vref2 (2
1) Resistor for obtaining reference voltage Vref2 (22)
Transistor (23) Diode (24) Current transformer (25) Diode (26) Output current detection resistor (27) Diode (28) Resistor (29) Capacitor (30) Resistor

Claims (1)

[Claims] 1. A DC power supply and a switching element are connected in series to the primary side of a transformer, and a voltage is obtained on the secondary side of the transformer by turning on and off the switching element, and this is smoothed by a smoothing circuit to produce an output voltage. At the same time, the reset current of the saturable reactor connected between the secondary side of the transformer and the smoothing circuit is controlled by the comparison difference voltage of the differential amplifier to which this output voltage and the reference voltage for setting the output voltage are connected. In a magnetically controlled switching power supply device that controls an output voltage using A differential amplifier is provided to compare a DC voltage proportional to the secondary winding current of the current transformer with a reference voltage for setting a protection current, and the output voltage is controlled by controlling the reset current of the saturable reactor using the comparison difference voltage. A magnetically controlled switching power supply device characterized in that overcurrent protection is provided by.