JPH0349476Y2 - - Google Patents

Description

[Detailed description of the invention] [Field of industrial application] The present invention relates to a FET (field effect transistor) self-excited switching circuit with a low input voltage. This relates to a FET self-excited switching circuit in which a winding is added to the FET and the induced voltage is superimposed on the output voltage of the feedback circuit and supplied to the gate of the field effect transistor, which is the main switching element.

[Prior Art] There is a single-stone self-excited switching power supply that uses blocking oscillation as a DC-DC converter. This type of power supply has the advantage of requiring a small number of parts, and is therefore widely used as a power supply with a relatively small output capacity. Conventional technology has exclusively used ordinary bipolar power transistors as the main switching elements, but in recent years, with the demand for higher speeds of, for example, 200kHz, there has been an increasing tendency to use field-effect transistors (FETs). It's coming. Bipolar transistors have the effect of accumulating fractional carriers, which causes a time delay in switching speed, and in order to improve the switching speed, it is necessary to apply various measures to the base circuit to extract accumulated charge, making the circuit quite complex. There was a problem that the number of parts increased as the number of parts increased.
Another problem was that the transistor could not be used to its maximum performance due to the current amplification factor _hFE .

On the other hand, field effect transistors have no charge accumulation effect, and the drain current changes immediately in response to changes in the signal voltage applied to the gate.Therefore, they are not only advantageous for speeding up, but also have a very simple gate circuit. Because it's over.

Now, as a conventional technology, this type of converter using a power MOS FET as the main switching element is
It is constructed as shown in FIG. Its operating principle can be basically considered to be the same as the conventionally known blocking oscillation operation. Feedback winding N _G1 of transformer T
The field effect transistor _Q1 is turned on and off using the feedback voltage induced in the field effect transistor Q1 to convert direct current to alternating current, and the voltage is converted between the primary winding _N1 and the secondary winding _N2 ,
The output is rectified and smoothed by diodes D ₁ and D ₂ , a choke coil L, and a smoothing capacitor C.

[Problems that the invention aims to solve] However, field effect transistors have a lower threshold voltage V _TH than conventional bipolar transistors.
Since the voltage is quite high (for example, about 3 to 4 V), when the input voltage to the switching circuit is low and used in a state close to the threshold voltage, there is a problem that the drive is insufficient and large losses occur. For this reason, power supplies that use field-effect transistors as main switching elements have an input voltage of 5 to 5
It was considered unsuitable for power supplies below 6V. This is because in order to sufficiently drive this type of transistor, a gate-source voltage of about 8 to 10 V is required.

Now, in the prior art, it may be thought that simply increasing the number of turns in the feedback winding N _G1 would increase the induced voltage and solve the problem of insufficient drive, but this is not the case at all. Generally, when the shape of the transformer T is small, an on-time limiting circuit 2 as shown in FIG. 5 is provided to regulate the on-time and avoid saturation of the core. This on-time limiting circuit 2 includes a time constant circuit consisting of a resistor _R4 and a capacitor _C2 connected across the feedback winding N _G1 ,
and a transistor _Q2 connected between the gate and source of the transistor Q1 and driven by the terminal voltage of _the capacitor _C2 .

It is true that increasing the number of turns of the feedback winding N _G1 increases the induced voltage, but at the same time the base-emitter voltage V _BE of the transistor Q ₂ also increases, and although it varies depending on the type of transistor, according to the standard usually,
It will be destroyed if it is not kept below 5V. Therefore, the voltage induced in the feedback winding N _G1 is the same as that of the transistor _Q2.
It was necessary to set the V _BE within a range that did not exceed the standard, and for this reason, conventional technology could not avoid a shortage of drives.

In order to solve this problem, it is conceivable to install a separate power supply and use it to drive the field effect transistor, but installing the separate power supply requires space and requires a large number of parts.
Such a configuration cannot be adopted because it would be disadvantageous in terms of cost.

The purpose of the present invention is to eliminate these drawbacks of the conventional technology, and to be able to sufficiently drive a field effect transistor even with a low input voltage, to sufficiently reduce loss, and to take advantage of the advantages of field effect transistors. An object of the present invention is to provide a FET self-excited switching circuit which can achieve higher frequency and smaller size of the circuit.

[Means for Solving the Problems] The present invention, which can achieve the above objects, connects a field effect transistor, which is a main switching element, in series with the primary winding of a transformer, and connects the field effect transistor, which is a main switching element, to the gate of the transformer. In a self-excited switching circuit, the feedback winding formed in the transformer is connected to a feedback circuit including a feedback winding, and the self-excited oscillation is performed using the feedback voltage, and the output is taken out from the secondary winding of the transformer. This is a FET self-excited switching circuit in which the voltage induced in the other winding is connected via a diode so as to be applied to the gate of the field effect transistor in addition to the feedback voltage.

In the present invention, the winding already installed in the transformer may be used as is, or a separate winding may be added, and the induced voltage is used to drive the field effect transistor at a sufficiently high voltage. It is configured as follows.

[Function] With this configuration, even if the input voltage to the switching circuit is low, about 5 to 6 V or less, the gate of the field effect transistor will have no voltage induced in the windings other than the feedback winding. The applied voltage and the output voltage of the feedback circuit are applied additively (superimposed), and the field effect transistor can be driven with a sufficiently high voltage, such as 8 to 10 V, that does not cause insufficient drive. Therefore, there is no shortage of drives, and losses can be kept very low.

[Example] Hereinafter, the present invention will be explained in more detail based on the drawings. FIG. 1 is a circuit diagram showing an embodiment of the FET self-excited switching circuit according to the present invention.
The basic idea is the 5th one mentioned above as conventional technology.
It is almost the same as the case shown in the figure. A feedback circuit ₁ uses a field effect transistor Q1 as a main switching element, connects the field effect transistor _Q1 in series with the primary winding _N1 of the transformer T, and includes the feedback winding _NG1 of the transformer T at its gate. is connected and the combined feedback voltage is used to generate self-oscillation. Feedback circuit 1 consists of feedback winding N _G1 , resistor R ₁ , and capacitor.
Consists of C ₁ .

In this embodiment as well, in order to avoid saturation of the core of the transformer T, an on-time limiting circuit 2 for the field effect transistor _Q1 is provided. This on time limit circuit 2
As in the case of the prior art, there is a time constant circuit consisting of a resistor _R4 and a capacitor _C2 connected across the feedback winding N _G1 , and the gate-source circuit of the transistor _Q1 . It consists of a transistor _Q2 driven by the terminal voltage of a capacitor _C2 .

A rectifying and smoothing circuit including diodes D ₁ and D ₂ , a choke coil L, and a smoothing capacitor C is connected to the output side of the secondary winding N ₂ of the transformer T. Further, the output thereof is subjected to constant voltage control by a constant voltage stabilizing circuit 3.

Here, the present invention is significantly different from the prior art in that a winding N _G2 is wound around the transformer T, and its output is supplied to the gate of the field effect transistor Q ₁ via the diode D ₃ and the resistor R ₂ . The points are connected like this. These circuits are connected so that the induced voltage in the winding N _G2 is added to the feedback voltage. Here, the voltage induced in the feedback winding _NG1 is set within a range that does not exceed the _VBE specification of the transistor _Q2 . And a winding different from the feedback winding N _G1
The voltage induced in N _G2 is a voltage that can minimize the loss of the field effect transistor due to overdrive, and the voltage applied to the field effect transistor by both windings is equal to the threshold voltage V _TH (usually 3~4V) is approximately 2.5 times. Therefore, as a result, the voltage applied by the feedback winding N _G1 is about 5V, and the voltage induced in another winding N _G2 is added, and the voltage applied to the gate of transistor _Q1 is about 8 to 10V. It is appropriate to

Note that in FIG. 1, a diode _D4 is inserted in series with the resistor _R3 , but this is for preventing backflow. Without this, when the input voltage is low (e.g. 5-6V) and the gate drive voltage of transistor _Q1 is high (e.g. 8-10V), the current in the opposite direction will flow through resistor _R3 . .

The operation of the circuit configured as described above is as follows. First, when the power is turned on, a voltage is applied to the gate through the starting resistor _R3 , which turns the field effect transistor _Q1 into a conductive state and transforms the transformer T.
A drain current flows through the primary winding _N1 , and a voltage is induced in each of the other windings.

The feedback voltage generated in the feedback winding _NG1 is positively fed back to the gate of the transistor _Q1 via the resistor _R1 and the capacitor _C1 . As a result, the transistor _Q1 becomes conductive, causing the excitation current of the transformer T to increase approximately linearly. Eventually, when the terminal voltage of the feedback winding N _G1 begins to decrease, the drain current of the transistor Q ₁ begins to decrease, and then the transistor Q ₁ suddenly enters the cut-off state. Capacitor _C1 is charged by the gate current and discharges its charge at cut-off, which is repeated. As a result, the gate-source region is biased in the forward direction, and the drain current flows again to return to the initial state.

By repeating the above operations, the field effect transistor _Q1 automatically repeats on and off. In this way, at startup immediately after the power is turned on, even if the input voltage to the switching circuit is low, the low voltage is used as it is to temporarily turn on and off the transistor _Q1 to operate the power supply.
Therefore, since the threshold voltage of the field effect transistor _Q1 is high, the drive is insufficient immediately after the power is turned on, and the loss becomes somewhat large.

However, once self-oscillation occurs in this manner, a voltage is induced in the winding N _G2 as well. This voltage is connected to diode D ₃ and resistor R ₂
The voltage is applied to the gate via the feedback circuit 1, but the voltage is connected so that the positive half wave is in phase with the output voltage from the feedback circuit 1, and acts additively, so the high voltage. Since the gate of transistor _Q1 is driven by this high voltage, even when the input voltage to the switching circuit is as low as 5 to 6 V, a high voltage of about 8 to 10 V is applied to the gate. As a result, the field effect transistor will never run out of drive, and the increase in loss can be suppressed.

The situation is shown in Figure 2. In the figure, the solid line indicates the gate-source voltage according to the embodiment shown in Figure 1 above.
It shows V _GS and drain-source voltage V _DS . Incidentally, the case of the prior art as shown in FIG. 5 is shown by a broken line. This waveform diagram also suggests that the circuit according to the present invention has less loss.

Although this configuration is extremely easy to use from a circuit perspective, the present invention can be modified in various other ways. In Figure 3, a winding N _G2 is further wound around the transformer primary winding _N1 , and its output is connected to a diode _D3.
and is applied to the gate of transistor _Q1 via resistor _R2 . Further, in FIG. 4, the secondary winding _N2 of the transformer is used as is, and the voltage induced therein is applied to the gate of the transistor _Q1 via the diode _D3 and the resistor _R2 . In any case, the polarity of the winding is chosen such that the voltage induced therein is additively superimposed on the feedback voltage and can be applied as a high voltage to the gate of the transistor, connected via diode _D3 . It turns out. In these cases, since almost no current flows, the resistor R ₂ can be omitted.
In order to make the drawings easier to read, illustration of the on-time limiting circuit is omitted. The structure and operation of these embodiments are basically the same as the circuit of the embodiment shown in FIG. 1, so corresponding parts are denoted by the same reference numerals and description thereof will be omitted.

[Effects of the invention] Since the present invention is a FET self-excited switching circuit configured as described above, it is possible to achieve
In some cases, even if the input voltage to the switching circuit is close to the threshold voltage of the field effect transistor, it is possible to use an extremely simple configuration in which the already installed winding is used as is and a diode is added. Even in cases where the field effect transistor is extremely low, the drive power of the field effect transistor will not be insufficient.Therefore, the reduction in loss can be prevented and reliable and stable switching operation can be performed. By taking advantage of the advantages, it is possible to achieve high frequency and miniaturization of circuits, and it can produce extremely excellent practical effects.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing one embodiment of the FET self-excited switching circuit according to the present invention, Fig. 2 is an explanatory diagram of its operating waveforms, and Figs. 3 and 4 respectively show other embodiments of the present invention. FIG. 5 is a circuit diagram showing a conventional technique. 1... Feedback circuit, 2... On-time limiting circuit, 3... Constant voltage stabilization circuit, Q ₁ ... Field effect transistor,
N ₁ ...Primary winding, N _G1 ...Feedback winding, N _G2 ...Winding, D ₃
…diode.

Claims (1)

[Scope of utility model registration request] A field effect transistor, which is a main switching element, is connected in series with the primary winding of the transformer, a feedback circuit including a feedback winding of the transformer is connected to the gate thereof, and the feedback voltage is used to cause self-oscillation,
In a self-excited switching circuit that extracts output from the secondary winding of a transformer, a voltage induced in a winding other than the feedback winding formed in the transformer is added to the feedback voltage through a diode. A FET self-excited switching circuit, characterized in that the FET self-excited switching circuit is connected to apply voltage to the gate of the field effect transistor.