JPS614416A - Overcurrent detector for switching power source - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an overcurrent detection circuit for protecting a switching power supply from total overcurrent.

[Conventional technology]

A switching power supply as shown in FIG. 2 has conventionally been used as a DC-DC conversion power supply. That is, it includes a switching transistor 1 and a switching transformer 2 in which one end of a primary coil is connected to the collector of the transistor 1. Then, when a DC power supply 3 is connected between the other end of the primary coil of the switching transformer 2 and the emitter of the transistor 1, and a pulse signal is applied from the drive circuit 4 to the base of the transistor 1, the DC power from the DC power supply 3 is The DC current flowing through the primary coil of the switching transformer 2 in response to the input voltage is switched by the transistor 1 to generate a 7-cycle current, which induces a 7-cycle voltage in the secondary coil of the switching transformer 2. be done. This signal voltage is rectified and smoothed by a rectifier/smoothing circuit 5, and then output at an output terminal 6.
A desired DC voltage is extracted from the

On the other hand, the DC output voltage obtained at the output terminal 6 is monitored by an output voltage detection circuit 7, and if a fluctuation thereof is detected, an output is generated in the output voltage detection circuit 7 and applied to the control circuit 8. 8 stabilizes the DC output voltage by changing the frequency and data of the pulse signal applied from the drive circuit 4 to the base of the transistor 1.

Furthermore, the switching power supply shown in FIG. 2 is equipped with a protection circuit in order to prevent excessive current from flowing through the switching transformer 2 and the switching transistor 1 due to output terminal short-circuiting, etc. A secondary overcurrent detection circuit 9 is provided.

By the way, regarding this overcurrent detection, in low power switching power supplies, since the current flowing is generally small, a resistor is inserted in the path of the current flowing through the components to be protected, and this is used to detect the overcurrent. High-power switching power supplies have a large amount of current flowing through them, so not only do they require expensive high-power resistors, but inserting a resistor into the current path reduces the conversion efficiency of the entire power supply, which is a feature of switching power supplies. This has the disadvantage of impairing high efficiency. In addition, since the control circuit 8 is generally connected to the secondary side circuit of the switching transformer 2, if the object to be protected against overcurrent is the primary side of the switching transformer 2, the current value detected on the primary side However, this analogy reduces the insulation between the primary and secondary, which is required by safety standards around the world. It also ended up not meeting the standards regarding the insulation between the parts.

Therefore, as shown in FIG. 2, a current transformer 10 with high insulation between the primary and secondary and low insertion loss has been used in the overcurrent detection circuit 9. In FIG. 2, the primary coil of the current transformer 10 is inserted between the collector of the switching transistor 1 and the primary coil of the switching transformer 2, and the secondary coil of the current transformer 10 is connected to a diode D1. The rectifier circuit consisting of the capacitor C1 is connected, and the output voltage of this rectifier circuit is applied to the error amplifier circuit 11. If an overcurrent is detected, the output of the error amplifier circuit 11 is given to the control circuit 8, and thereby the switching Transistor 1 can be controlled so that the current flowing through transistor 1 and the primary coil of switching transformer 20 is reduced.

[Problem that the invention seeks to solve]

However, the conventional overcurrent detection circuit 9 shown in FIG. 2 also has the following problems. That is, even if the peak value of the pulse current flowing through the primary coil of the current transformer 10 is constant, if the data of the pulse current () changes due to a change in the input DC voltage of the switching power supply, the overcurrent detection circuit 9 The output voltage of the rectifier circuit fluctuates, and therefore the operating point of the overcurrent detection circuit 9 deviates from the set value.

Now, the primary coil of the current transformer 10 is shown in Figure 6 (
When the ie pulse current ■ shown in a) is flowing, the current flow time It in each period of this pulse current I is,
Assuming that the non-circulation time is t2, the duty d is calculated by the following equation (1).

Generally, tl is 2~10μ5ec5 t2 is 10~30μ
sec, and when such a no-E pulse current ■ flows through the primary coil of the current transformer 10,
As shown in FIG. 6(b), both ends of the secondary coil have a waveform corresponding to the Norse current ■ and a p-p value corresponding to the peak value of the pulse current ■. A positive voltage V is induced. This pulse voltage V is the P pulse current■
A positive voltage component v1 corresponding to when the current is flowing, a negative voltage component v2 corresponding to the time when the current is not flowing, and the total voltage V1 . V2 has the relationship expressed by the following equation (2).

Therefore, (], ), (From equation 2i, the relationship between voltage v1 and data d is shown in FIG. 4.

By the way, in the switching power supply shown in FIG. 2, control is generally such that when the input DC voltage changes by 10%, the duty d of the pulse current (2) changes by about 10%. Therefore, the date d is 0,3
Assuming that the voltages Vl and V2 when the input DC voltage increases by 1.0% are Vl' and V2', respectively, then V4 + V22 v1' + v2' (the peak value of the pulse current ■ is constant) Therefore, ■, ′/v1-0
．． 957, resulting in a fluctuation of approximately 4%.

Therefore, when the rectifier circuit consisting of the diode D1 and the capacitor C1 of the overcurrent detection circuit 9 detects the voltage v1 that fluctuates depending on the change in the data d as shown in FIG. This will result in an error.

Therefore, in the above-mentioned conventional switching power supply, it was necessary to increase the withstand strength of components that should be protected, such as switching transformers and switching transistors, to take into account the possibility that a slight overcurrent would flow. This was a contributing factor to the increase in the price of switching power supplies.

[Failure to solve the problem]

Therefore, the present inventor made various considerations as to what means should be used to accurately detect fluctuations in the pulse current flowing through the part to be protected without being affected by changes in the duty d of the pulse current. 1- As a result, when the peak value of the pulse current flowing through the primary coil of the current transformer is constant, the p-p value of the pulse voltage induced in the secondary coil of the current transformer is data d
The present invention was conceived by paying attention to the fact that it remains constant regardless of changes in .

That is, the present invention is capable of generating currents in opposite directions that are induced in the secondary coil of the current transformer in response to current flow and non-current flow in each period of the pulse current flowing through the primary coil of the current transformer. A means for rectifying and adding the first and second DC voltages obtained thereby is connected to the secondary coil of the current transformer, and a control circuit is operated based on the voltage obtained from the rectifying and adding means. This is what I did.

[For production]

By providing means for rectifying the currents in opposite directions induced in the secondary coil of the current transformer and adding the first and second DC voltages obtained thereby, the secondary coil of the current transformer can be rectified. The pp value of the pulse voltage induced in the side coil will be detected.

〔Example〕

FIG. 1 shows a switching power supply device equipped with a filtered current detection circuit according to the present invention, which has the same configuration as the device shown in FIG. 2 except for the rectifier circuit included in the overcurrent detection circuit. have. Therefore, in FIG. 1, the same reference numerals are given to the parts corresponding to those in FIG. The rectifier circuit is composed of two diodes D1 and D2 and two capacitors C7 and C2. That is, the anode of diode D1 and the cathode of diode D2 are connected, and the connection point A
A capacitor C2 is connected between the current transformer 10 and one end of the secondary coil of the current transformer 10. Also, diode D1
One end of the capacitor C1 is connected to the cathode of the current transformer 100, and the other end of the capacitor C1 and the anode of the diode D2 are connected to the other end of the secondary coil of the current transformer 100.

The rectified output is taken from both ends of capacitor C1.

In the above configuration, during the non-circulating time t2 in which the negative voltage v2 shown in FIG. 6(b) is induced in the secondary coil of the uninterrupted current transformer 100, the diode D2
A charging current flows from the capacitor C2 to the capacitor C2, and the capacitor C2 is charged to the voltage v2.

Next, during the current flow time t1 when a positive voltage v1 is induced, a charging current flows from the diode D1 to the capacitor C1, and the capacitor C1 has a voltage v due to this charging current.
1 and the voltage (2) charged in the capacitor C2 to a voltage v1+v2. This voltage v1 +
V2 is taken out from both ends of the capacitor C1 and applied to the error amplification circuit 11, and the output of this error amplification circuit 11 is applied to the control circuit 8.

- [Effects of the Invention] As explained above, according to the present invention, the p-
The p-value can be checked with a very simple configuration. The p=p value of this pulse voltage corresponds to the peak value of the pulse current ■ flowing through the primary coil of the current transformer 10, and is not affected by changes in the duty d of the pulse current ■, so the switching power supply Even if the data d changes due to fluctuations in the DC input voltage, the point of the current set as the overcurrent does not shift, so that the overcurrent of the preset value can be detected accurately.

This eliminates the risk of current exceeding the set value flowing through the parts to be protected, which not only reduces the withstand strength of those parts, but also simplifies the design of switching power supplies and reduces the time required for adjustment and inspection. This has the advantage that switching power supplies can be manufactured at low cost.

Furthermore, according to the present invention, not only the pulse current flowing in one direction as shown in FIG. This has the advantage that even if an overcurrent occurs, it can be detected.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a switching power supply device equipped with an overcurrent detection circuit according to the present invention, FIG. 2 is a block diagram of a switching power supply device equipped with a conventional overcurrent detection circuit, and FIG.
Figure (a) is a waveform diagram of the pulse current flowing through the primary coil of the current transformer, Figure 6 (b) is a waveform diagram of the pulse voltage induced in the secondary coil of the current transformer, and Figure 4 is a waveform diagram of the pulse voltage flowing through the primary coil of the current transformer. This is a graph showing the relationship between the duty of the current transformer and the positive voltage induced in the secondary coil of the current transformer. In the picture: 13″・Chia’tora″″″′″″・2″’″
4 is a drive circuit, 8 is a control circuit, 9 is an overcurrent detection circuit, 10 is a current lance, and 11 is an error amplification circuit.

Claims (1)

[Claims] A pulse voltage is generated based on a pulse current obtained by switching a DC current flowing in response to a DC input voltage using a switching circuit, and the pulse voltage is converted into a DC voltage and can be extracted. An overcurrent detection circuit for a switching power supply device, comprising a current transformer whose primary side is connected in a path of overcurrent that may occur during switching, and one of the current transformers.
The pulse current is passed through the secondary side of the current transformer, the pulse current induced on the secondary side of the current transformer is rectified to obtain a DC voltage, and the switching circuit is controlled based on this DC voltage. The current detection circuit detects currents in mutually opposite directions induced in the secondary side of the current transformer corresponding to when the current flows and when the pulse current does not flow in each period of the pulse current flowing through the primary side of the current transformer. A means for rectifying and adding the first and second DC voltages obtained thereby is connected to the secondary side of the current transformer, and controlling the switching circuit based on the voltage obtained from the rectifying and adding means. An overcurrent detection circuit for a switching power supply device, characterized in that: