JPH0127425Y2 - - Google Patents

Description

[Detailed description of the invention] Industrial application field This invention is a flyback. The present invention relates to a converter-type switching power supply, and more specifically to its overcurrent control technology.

Conventional technology Flyback. A converter-type switching power supply switches DC input using a switching element connected to a winding of a transformer, and converts the induced voltage generated in another winding of the transformer into the other winding during the ON period of the switching element. The flyback voltage generated in the other winding of the transformer is rectified by the diode and smoothed by a smoothing capacitor during the OFF period of the switching element, and is smoothed by a smoothing capacitor to form a direct current. It has the advantage of being compact, highly efficient, and highly reliable, and has been widely used as a DC power source for computers and their terminal peripherals.

This flyback. Conventionally, when overcurrent protection is provided in a converter-type switching power supply, the current flowing when the switching element is turned on is detected on the primary side of the transformer using a current transformer or a resistor, or a resistor is used on the secondary side. It was detected using Furthermore, among the self-excited type, there are also known ones that perform overcurrent control by detecting changes in the switching frequency.

Disadvantages of the conventional technology However, among the conventional detection methods mentioned above, the resistance detection method uses the voltage drop in the detection resistor, so as the overcurrent detection value increases, the loss increases and the efficiency decreases. However, it has the disadvantage of increasing heat generation.

There are also methods that use a current transformer to detect the secondary current, but flyback. In the converter, the secondary current flows during the off period of the primary switching element. However, overcurrent protection must be performed during the on period of the switching element. Therefore, in the conventional method of detecting the secondary current using a current transformer, a peak hold circuit is used to hold the signal of the current flowing to the secondary side during the off period of the switching element until the switching element is next turned on. There were problems such as the need for overcurrent protection and a timing delay in the overcurrent protection operation. Moreover, when an overcurrent is detected and the on-width of the switching element is controlled to be reduced, the peak value of the current decreases, so in an overcurrent control method using a normal peak hold circuit, the control effect is becomes intermittent, and stable overcurrent control cannot be obtained. on the other hand,
If the off-period is extended so that the on-width does not widen beyond a certain level, the overcurrent control characteristic becomes a characteristic with an extended tail instead of a drooping characteristic.

Furthermore, since the primary side current detection method basically has a circuit configuration that controls the total power on the primary side and provides overcurrent protection, when applied to a multi-output type switching power supply, the secondary side output circuit Current cannot be detected individually. For this reason, in a multi-output type switching power supply that requires an overcurrent protection function for each of the multiple output circuits, it is necessary to configure the number of switching power supply circuits according to the number of outputs. As the circuit configuration becomes more complex and larger,
This will result in high costs. A similar drawback exists in a self-excited switching power supply that uses changes in the switching frequency to perform overcurrent control.

Purpose of the present invention Therefore, the present invention eliminates the above-mentioned conventional drawbacks,
A small and inexpensive flyback that can detect the current of each output circuit individually even when it is a multi-output type, has good drooping characteristics, and provides stable overcurrent protection. The purpose is to provide a converter type switching power supply.

Structure of the Present Invention In order to achieve the above object, the present invention switches DC input by a switching element connected to a winding of a transformer, and during the ON period of the switching element, switches the DC input to another winding of the transformer. The generated induced voltage is blocked by a diode connected to the other winding, and during the off period of the switching element, the flyback voltage generated in the other winding of the transformer is rectified by the diode. , in a switching power supply in which DC output is obtained by smoothing with a smoothing capacitor, a current transformer is inserted and connected in series in a circuit loop including the other winding of the transformer, the diode, and the smoothing capacitor; The present invention is characterized by comprising an overcurrent protection circuit that uses a voltage generated in the secondary winding of the current transformer as a detection signal when current stops flowing in the primary winding of the current transformer.

Embodiment FIG. 1 shows an electric circuit diagram of a switching power supply according to the present invention, in which a switching circuit 1 comprising switching elements such as transistors is connected in series with the primary winding N ₁ of a transformer T ₁ . Then, by the switching operation of the switching circuit 1, the DC input Vin applied through the primary winding _N1 of the transformer _T1 is switched, and the switching output is taken out to the secondary winding _N2 of the transformer _T1 . It's becoming like that. The switching circuit 1 may be a self-excited type that performs a switching operation by causing a switching element to perform blocking oscillation or the like, or a separately excited type that performs a switching operation by forcibly driving a switching element.

On the winding N ₂ side of the transformer T ₁ is a diode D ₁
An output circuit consisting of a capacitor _C1 and a capacitor C1 is connected. The diode D ₁ is connected so as to have a polarity opposite to the voltage induced in the winding N ₂ of the transformer T ₁ during the ON period of the switching circuit 1.

The above circuit configuration is a flyback. This is a general circuit configuration of a converter type switching power supply. In the present invention, in the above switching power supply, the secondary winding N ₂ of the transformer T ₁ and the diode
A current transformer CT is inserted and connected in series in a circuit loop including D ₁ and a smoothing capacitor C ₁ , and when current no longer flows in the primary winding Nct ₁ of the current transformer CT, the secondary winding Nct The feature is that it is equipped with an overcurrent protection circuit that performs overcurrent control using the voltage e ₁ generated at ₂ . In this embodiment, the current transformer CT connects both ends of the winding Nct ₁ to one end of the secondary winding N ₂ of the transformer T ₁ and the anode of the diode D ₁ , respectively, and connects the ends of the winding Nct ₂
One end of is connected to the input end (-) of comparator ₂ through a rectifying diode D2. R ₁ is the resistance.

A reference voltage source 3 and a sawtooth wave generating circuit 4 are connected in series to the input terminal (+) side of the comparator 2, and a comparison voltage E ₂ is applied from these to the input terminal (+).
and the detection voltage E ₁ input to the input terminal (-) side
People are starting to compare the two. The sawtooth wave generating circuit 4 generates a sawtooth voltage signal that is synchronized with the switching circuit 1. Reference numeral 5 denotes an insulating coupling circuit composed of an insulating transformer, a photocoupler, or the like.

Next, the operation will be explained with reference to the waveform diagram in FIG. When switching circuit 1 is on, transformer T ₁
As shown in FIG. 2d, a primary current _I1 having a triangular waveform flows through the primary side of the motor. Next, switching circuit 1
When is reversed from on to off, the primary current _I1 is cut off and at the same time the winding is turned off as shown in Figure 2a.
A flyback voltage Ef ₂ having a polarity opposite to the voltage when the switching circuit 1 is turned on is generated at N ₂ . Since this flyback voltage Ef ₂ is in the forward direction with respect to the diode D ₁ , the diode D ₁ becomes conductive and a triangular wave-shaped secondary current as shown in Fig. 2b is generated in the winding Nct ₁ of the current transformer CT. I ₂ flows, winding Nct ₂
A voltage as shown in FIG. 2c is induced on the side.

In the present invention, the detection voltage _e1 shown by the shaded area A in Figure 2c is rectified by the diode _D2 , and inputted to the comparator 2 to produce a comparison voltage as shown in Figure 2e.
_E2 , the region where the detected voltage _E1 is higher than the comparison voltage _E2 is defined as an overcurrent region, and overcurrent control is performed by turning off the switching circuit 1. That is, assuming that the level relationship _of the detection voltage _{E 1 to} the comparison voltage E ₂ is as shown in FIG. As a result, the switching circuit 1, which should normally be switched from on to off at time _t1 , turns off faster by time _Δt1 , as shown by the dotted line C in FIG. Shortened by ₁ .

Here, as shown in FIG. 3, as a general characteristic of a transformer, the voltage-time product EToff when off is equal to the voltage-time product ETon when it is on. Under these conditions, the on-time width of switching circuit 1
When Ton is shortened by ΔTon, the on-time voltage time product ETon is shortened by ΔTon mainly on the time axis, and the change width Δe ₁ of the detection voltage e ₁ becomes extremely small. On the other hand, the voltage-time product EToff when off is
Mainly on the voltage axis, there is a large change from voltage eo ₁ to voltage eo ₂ by Δeo. Since the voltages eo ₁ and eo ₂ correspond to the primary current I ₁ and the secondary current I ₂ , a large voltage change Δeo means that the changes in the primary current I ₁ and secondary current I ₂ are large. means. After all, when overcurrent control is performed using detection voltage e ₁ , secondary current I ₂ will change greatly in response to a small change in detection voltage e ₁ , so the overcurrent control characteristics will be
As shown in Figure 4, unlike the conventional hemline characteristic _L1 , there is a beautiful drooping characteristic _L2 that falls almost vertically.
becomes.

Furthermore, unlike the conventional method, a peak hold circuit or the like is not required, so there is no time delay element such as a capacitor, and an overcurrent protection circuit with quick response can be realized. Furthermore, compared to the case of resistance detection, the loss is much smaller and the efficiency is improved.

Furthermore, when configuring a multi-output type switching power supply, for example, as shown in Figure 5, a current transformer CT is inserted and connected to each output circuit,
By adopting a circuit configuration in which each detection voltage e ₁ is input to the input terminal (-) of the comparator 3 through OR connection of the diode D ₂ , the current of each output circuit can be detected individually. Therefore, when providing an overcurrent protection function to a multi-output type switching power supply, it is possible to simplify the circuit configuration, reduce the size and weight, and reduce costs.

FIG. 6 shows an electric circuit connection diagram in a more specific embodiment of the switching power supply according to the present invention. In the figure, the same reference numerals as in FIG. 1 indicate the same components. In this embodiment, the switching circuit 1 connects the winding N ₁ of the transformer T ₁
A transistor, which serves as a switching element, is connected in series with
Q ₁ is connected, and positive feedback is applied from the output side of the transistor _{Q 1} to the base input side using the winding N ₁ and winding N ₃ of the transformer T 1 to cause blocking oscillation operation in the transistor Q _1. A self-excited switching power supply is shown. Also, the secondary winding of transformer T ₁
Connect a resistor R ₂ and a capacitor C ₂ to one end of N ₂ ,
A sawtooth wave generating circuit 4 is formed by these. _D3 is a diode.

FIG. 7 shows another embodiment of the switching power supply according to the present invention. The feature of this embodiment is that a smoothing capacitor _C3 is added to the input terminal (-) of comparator 2, and the detected voltage _e1 is converted into DC and supplied to the input terminal (-) of comparator 2. That's true.

Effects of the present invention As stated above, the present invention provides flyback.
In a converter-type switching power supply, a current transformer is inserted and connected in series within a circuit loop that includes the transformer windings, a diode that rectifies the flyback voltage, and a smoothing capacitor, so that no current flows through the primary winding of the current transformer. The present invention is characterized by being equipped with an overcurrent protection circuit that uses the voltage generated in the secondary winding of the current transformer as a detection signal when the current transformer becomes overheated. A small and inexpensive flyback that can detect current individually, has good droop characteristics, and provides stable overcurrent protection. A converter type switching power supply can be provided.

[Brief explanation of the drawing]

Fig. 1 is an electric circuit connection diagram of a switching power supply according to the present invention, Fig. 2 a to e are waveform diagrams of each part thereof,
Fig. 3 is a diagram explaining overcurrent protection operation, Fig. 4 is a diagram showing overcurrent control characteristics, Fig. 5 is an electric circuit connection diagram in another embodiment of the switching power supply according to the present invention, and Fig. 6 is a diagram showing overcurrent control characteristics. Similarly, FIG. 7 is an electric circuit connection diagram in still another embodiment. 1...Switching circuit, 2...Comparator, 3...
...Reference voltage source, 4...Sawtooth wave generation circuit, T ₁ ...
...Transformer, CT...Current transformer, _D1 ...Diode.

Claims (1)

[Claims for Utility Model Registration] (1) Switching DC input by a switching element connected to a winding of a transformer, and reducing the induced voltage generated in other windings of the transformer during the ON period of the switching element. , by a diode connected to the other winding, and during the off period of the switching element, the flyback voltage generated in the other winding of the transformer is rectified by the diode, and by a smoothing capacitor. In a switching power supply that obtains a DC output by smoothing, a current transformer is inserted and connected in series in a circuit loop including the other winding of the transformer, the diode, and the smoothing capacitor,
A switching device comprising an overcurrent protection circuit that uses a voltage generated in the secondary winding of the current transformer as a detection signal when current stops flowing in the primary winding of the current transformer. power supply. (2) the transformer includes a plurality of other windings,
The current transformer is inserted for each of the plurality of other windings, and the logical sum of voltage signals generated in each secondary winding when current stops flowing in the primary winding of each current transformer is used as a detection signal. A switching power supply according to claim 1 of the utility model registration claim.