JPH0576185A - Overcurrent detecting circuit - Google Patents

Abstract

PURPOSE:To enable a highly reliable overcurrent protection by adding minus voltage as bias to the voltage converted from a primary current, and compensating the difference of the detection level by the difference between an AC 100V system and an AC 200V system. CONSTITUTION:An overcurrent detection compensating circuit performs forward rectification from auxiliary winding N3, and clamps the output with a Zener diode D4 until it becomes a certain value, in respect of the minus voltage rectified with a diode D3. The minus voltage from the overcurrent detection compensating circuit is added as bias voltage to the voltage converted from the primary current 1 applied to the minus current limit detection terminal A of a control IC1. Accordingly, no matter how the input is, the overcurrent on load side can be detected with one threshold. And the difference of the detection level by the difference between an AC 100V system and an AC 200V system is compensated with the overcurrent detection compensating circuit. Hereby, highly reliable protection measures against overcurrents can be applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent detection circuit for detecting an overcurrent on the secondary load side of a power supply circuit on the primary side.

[0002]

2. Description of the Related Art A conventional overcurrent detection circuit for detecting an overcurrent on the secondary load side of a power supply on the primary side will be described. The input voltage, for example, AC100V is rectified, and the rectified primary side voltage is switched by the switching element. This primary side pulse voltage by switching is converted into a secondary side voltage via a voltage transformer. The secondary voltage is rectified to supply power to the load. 2 due to load short circuit
When detecting the overcurrent flowing to the secondary side on the primary side, the current flowing to the primary side is measured through the current transformer to detect the overcurrent on the secondary side, or the current flowing to the resistor provided in series is detected. The voltage drop due to the secondary current is measured to detect the overcurrent on the secondary side.

[0003]

However, in the conventional overcurrent detection for detecting an overcurrent on the secondary load side of the power source on the primary side, there is a problem when the input voltage is a single voltage such as AC100V. However, when the input voltage is wide, such as AC100V system and AC200V system, the current value flowing to the primary side varies depending on the input voltage even when the overcurrent value is constant. FIG. 3 shows the waveform of the primary side current. In the figure, the vertical axis represents the magnitude of the primary side current, and the horizontal axis represents the input voltage. As is clear from the figure, AC200V
The system has a smaller pulse width and a lower peak value than the AC100V system. Therefore, even if the overcurrent value on the secondary load side is constant depending on the input voltage, the overcurrent detection level varies, which is not preferable. When the input voltage is converted by the voltage transformer, the number of parts such as the voltage transformer and the voltage changeover switch increases, and problems such as cost and space arise.
If the input voltage is set to a single value, the number of product types will increase, product versatility will be lacking, and management will be enormous.

The present invention has been made in view of the above points, and an object of the present invention is that the input is AC100V.
The detection level of the overcurrent on the secondary load side by the input voltage in the circuit that detects the overcurrent flowing on the secondary load side of the switching regulator in the current continuous mode operation that is continuously used in a wide range from the system to 200V system. An object of the present invention is to provide an overcurrent detection circuit having no difference.

[0005]

In order to achieve the above object, an overcurrent detection circuit according to the present invention is provided with an AC100V system to an AC system.
Auxiliary winding that generates a voltage by starting the main circuit in an overcurrent detection circuit that detects overcurrent on the secondary load side of a switching regulator that operates continuously in current continuous mode operation with inputs up to 200 V 1 is applied to the negative current limit detection terminal of the control IC, which is provided with an overcurrent detection auxiliary circuit including a diode that performs forward rectification and a Zener diode that clamps the negative voltage rectified by this diode until it reaches a constant value. A negative voltage output from the overcurrent detection auxiliary circuit is added as a bias to the voltage obtained by converting the secondary current, and A
The feature is that the difference in the detection level due to the difference between the C100V system and the AC200V system is corrected.

[0006]

Function: The input from the AC100V system to the AC200V system is rectified by the rectifier. A voltage is generated on the secondary load side by starting the main circuit. When an overcurrent flows on the load side due to a short circuit or the like, the current flowing on the primary side also increases. 1 by resistance
The voltage drop of the secondary current is measured, and when it exceeds a certain value (threshold value), it is detected as an overcurrent on the secondary load side. In this case, the value of the voltage drop of the primary side current flowing through the resistor differs depending on the input voltage. Bias when the input voltage is high,
The value of the voltage drop of the primary-side current flowing through the resistor is corrected so that it is aligned when the input voltage is low. This correction circuit is an overcurrent detection correction circuit. The overcurrent detection / correction circuit performs forward rectification from the auxiliary winding that generates a voltage when the main circuit is activated, and clamps the output of the negative voltage rectified by the diode with a Zener diode until it reaches a constant value. The minus voltage from this overcurrent detection correction circuit is added as a bias to the voltage obtained by converting the primary side current applied to the minus current limit detection terminal of the control IC. Therefore,
The overcurrent on the load side can be detected with one threshold value regardless of the input.

[0007]

Embodiments of the present invention will be described below with reference to FIGS. FIG. 1 is a circuit configuration diagram of a power supply having an overcurrent detection circuit according to an embodiment of the present invention, and FIG. 2 shows a relationship between a detection terminal voltage at a negative current limit terminal of a control IC and an input voltage and a bias voltage. It is a figure. First, in FIG. 1, D1 is a rectifier, which rectifies a wide range of input voltages (AC100V system, 200V system). Reference numeral 1 is a control IC, which is the main switching element N
It controls the channel MOSFET Q1 and has a negative current limit detection terminal A. T1 is a transformer, which is a primary winding N1, a secondary winding N2, and a tertiary winding N
Have three. C1 to C5 are capacitors, D2, D3, D
5 is a diode, D4 is a Zener diode, R1 to R
6 is a resistance. However, the starting voltage circuit for starting the control IC 1 is omitted. The overcurrent detection correction circuit is D3.
It is composed of D4, C4, and R4 to R5.

Next, the operation will be described. Input AC10
0V-AC200V is rectified, the control IC1 controls the N-channel MOSFETQ1 which is a switching element, and this N-channel MOSFETQ1 switches the primary side voltage applied to the primary side. The resistance drop of the primary-side current I flowing through the resistor R1 inserted in series on the primary side is divided by the resistors R2 and R3. This divided voltage is the control IC
It is applied to the negative current limit detection terminal A of 1,
The primary current is monitored. This negative voltage threshold is 2
It is determined by the overcurrent value to be detected on the secondary load side.

In FIG. 2, when the input is an AC 100V system, the voltage applied to the negative current limit detection terminal A of the control IC 1 is V1, and V is up to the threshold value Vt.
There is a margin of t-V1. However, when the input is an AC200V system, it is necessary to add the bias voltage Vb to make it the same voltage as V1. If there is no bias Vb, the voltage applied to the negative current limit detection terminal A of the control IC1 becomes V2. The circuit that creates this bias is the overcurrent detection and correction circuit.

In FIG. 1, the tertiary winding N of the transformer T1
The voltage generated at 3 is flyback rectified by the diode D2 and used as the operating power supply of the control IC1. Since this voltage is flyback rectified, it is hardly affected by changes in the input voltage. Similarly, the voltage generated in the tertiary winding N3 of the transformer T1 is forward rectified by the diode D3 to generate a negative voltage. The Zener diode D4 applies a voltage obtained by subtracting the Zener voltage from the rectified negative voltage to the negative current limit detection terminal A. That is,
By performing forward rectification, a negative voltage that is almost proportional to the input voltage is created. In the case of an input AC100V system, the negative voltage created by forward rectification by the diode D3 is equal to the Zener voltage of the Zener diode D4, and the bias voltage is 0. Is. In the case of an input AC200V system, the difference between the negative voltage created by forward rectification by the diode D3 and the Zener voltage of the Zener diode D4 becomes the bias voltage. Therefore, with this bias, the overcurrent on the secondary load side can be detected at a constant level regardless of the input voltage.

[0011]

As described above, the overcurrent detection circuit of the present invention has a diode for performing forward rectification from an auxiliary winding that generates a voltage when the main circuit is activated, and a negative voltage rectified by this diode to a constant value. An overcurrent detection auxiliary circuit composed of a Zener diode that clamps until the current reaches a level is provided, and the overcurrent detection auxiliary circuit outputs a voltage obtained by converting the primary side current applied to the negative current limit detection terminal of the control IC. Since a negative voltage is applied as a bias and the difference in the detection level due to the difference between the AC100V system and the AC200V system is corrected, the input voltage of the AC10 can be adjusted by adding an overcurrent detection correction circuit consisting of a few parts.
It is possible to create an overcurrent detection circuit that can detect the overcurrent flowing in the secondary load side of the switching regulator in the current continuous mode operation that is continuously used in a wide range from 0 V system to 200 V system at the same level on the primary side, and the input voltage Power supplies used in many different countries can be provided with inexpensive and reliable overcurrent protection with the same specifications.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a power supply circuit including an overcurrent detection circuit according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between a detection terminal voltage at a negative current limit terminal of a control IC, an input voltage and a bias voltage.

FIG. 3 is a diagram showing a primary-side current waveform.

[Explanation of symbols]

 1 Control IC C1 to C5 Capacitor D1 Rectifier D2, D3, D5 Diode D4 Zener diode N1, N2, N3 Transformer winding Q1 N channel MOSFET R1 to R6 Resistor T1 Transformer

Claims (1)

[Claims] 1. An overcurrent detection circuit for detecting an overcurrent flowing on a secondary load side of a switching regulator of a current continuous mode operation which is continuously used with an input of AC100V system to AC200V system in a primary side, and starts a main circuit. The auxiliary current detection auxiliary circuit is composed of a diode that performs forward rectification from the auxiliary winding that generates a voltage by using a zener diode that clamps the negative voltage rectified by this diode until it reaches a certain value. To the voltage obtained by converting the primary side current applied to the limit detection terminal, a negative voltage output from the overcurrent detection auxiliary circuit was added as a bias to correct the difference in detection level due to the difference between the AC100V system and the AC200V system. An overcurrent detection circuit characterized in that.