JPH0318270A - Power-supply circuit - Google Patents

Abstract

PURPOSE:To effectively avoid the breakdown of a field-effect transistor by detecting the frequency of driving signal to stop-control the working of the field-effect transistor, in a power-supply circuit. CONSTITUTION:Oscillating output signal SOSC is waveform-shaped and directed to an integrating circuit 32. Then, via the integrating circuit 32, the terminal voltage VIN of a DC level changing according to the frequency of the oscillating output signal SOSC can be obtained, and the lowering of the frequency of the oscillating output signal SOSC is detected. The detecting result of a voltage detecting circuit 40 is applied to a control circuit 50 and a stop controlling circuit 52 one after another, and when the frequency of the oscillating output signal SOSC is lowered, then the working of a field-effect transistor 8 is stop- controlled, and the breakdown of the field-effect transistor 8 can be avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Field of Industrial Application The present invention relates to a power supply circuit, and is particularly suitable for application to a power supply circuit configured to drive a switching transformer using field effect transistors.

B. Summary of the Invention According to the present invention, in a power supply circuit, by detecting the frequency of a drive signal and controlling the operation of the field effect transistor to stop, it is possible to effectively avoid destruction of the field effect transistor.

C. Prior Art Conventionally, some power supply circuits have used field effect transistors as switching elements instead of bipolar transistors.

That is, as shown in FIG. 5, in the power supply circuit 1,
A predetermined DC power is applied from a power source 3 to an oscillation circuit 4, a drive control circuit 5, and a drive circuit 6 via a switch 2.

The drive control circuit 5 receives an oscillation output signal of a predetermined frequency output from the oscillation circuit 4, varies the duty ratio of the oscillation output signal according to the output voltage of the switching transformer 7, and then applies an electric field via the drive circuit 6. effect type transistor 8
Output to.

The drain of the field-effect transistor 8 is connected to the power supply 3 via the primary winding of the switching transformer 7, so that the field-effect transistor 8 is repeatedly turned on and off in response to the drive signal SIIR output from the drive circuit 6. .

In this manner, the field effect transistor 8 drives the switching transformer 7, thereby obtaining a desired output voltage.

D Problems to be Solved by the Invention In such a power supply circuit, in order to prevent the field effect transistor 8 from being destroyed by excessive current,
A protection circuit is provided.

However, as shown in Figure 6, when the power supply rises or falls, this protection circuit may not be able to function sufficiently, and excessive current may flow (Figure 6 (A)) or excessive voltage may flow. In some cases, the field effect transistor 8 was destroyed due to the applied voltage (FIG. 6(B)).

The present invention has been made in consideration of the above points, and aims to propose a power supply circuit that can effectively avoid destruction of field effect transistors.

E Means for Solving the Problem In order to solve this problem, the present invention provides the switching transformer 7, the drive signal generation circuits 4, 5, 6 that generate the drive signal SDR of the switching transformer 7, and the drive signal SDR. A field effect transistor 8 that drives the switching transformer 7 based on the signal SDII and a drive signal SDR.
frequency detection circuits 12, 18, 32 that detect the frequency of , and control circuits 40, 50, 52 that control the operation of the field effect transistor 8 to stop based on the detection result vIN of the frequency detection circuits 12, 18, 32. and be prepared.

If the frequency of the F action drive signal SDR is detected and the operation of the field effect transistor 8 is controlled to stop based on the detection result (2), if the frequency of the drive signal SDR is low, the operation of the field effect transistor 8 is stopped. It is possible to control the stoppage and effectively avoid destruction of the field effect transistor 8.

G example An embodiment of the present invention will be described in detail below with reference to the drawings.

In FIG. 1, in which parts corresponding to those in FIG.

As shown in FIGS. 2 and 3, the differentiator circuit 12 is composed of a series connection circuit of a capacitor 14 and a coil 16, and outputs an oscillation output signal S. , , (Figure 3 (A)) differential waveform signal S
++v (FIG. 3(B)) is applied to the waveform shaping circuit 18.

? The shaping circuit 18 is connected to the power supply line V via the coil 20.
It has a grounded-type transistor 22 whose collector is connected to CC, and a differential waveform signal SDV is applied to the transistor 22 via a base resistor 24.

As a result, the oscillation output signal S
Collector output signal S whose signal level falls at the timing when 03, rises. IjTl ((Fig. 3(C)) can be obtained.

Further, the waveform shaping circuit 18 connects the collector to the power supply line Vc via the coil 26 and the voltage regulator diode 2, respectively.
It has a common emitter type transistor 30 connected to C and ground, and a collector output signal S is supplied to the transistor 30. It is designed to give UTI.

Therefore, via transistor 30, an oscillating output signal S is generated. 3
After the signal level rises at the timing ξ when c rises, the signal level falls after a certain period determined by the constant voltage diode 28 has elapsed. tit■(
Is it possible to obtain Figure 3 (D))? Wear.

On the other hand, the integrating circuit 32 outputs a collector output signal S to a parallel connection circuit of a capacitor 34 and a coil 36 via a diode 38. uT. It is designed to give

Therefore, the terminal voltage V IN of the coil 36 (Fig. 3 (E))
In , the collector output signal S. During the period when the signal level of utt rises, the collector voltage of the transistor 30 is applied via the diode 38, and the voltage increases rapidly, whereas the collector output signal S. U
During the period when the signal level of T■ falls, capacitor 3
The voltage gradually drops at a rate determined by the time constants of the coils 4 and 36.

This causes an oscillation output signal S. When the frequency of sc becomes lower (Fig. 3 (F)), the period in which the signal level falls in the collector output signal soutz (Fig. 3 (G)) becomes longer than the period in which the signal level rises. The DC level of the terminal voltage VIN (Fig. 3 (H)) decreases, and the oscillation output signal S. It is possible to obtain a terminal voltage vIN whose DC level changes depending on the frequency of sc.

On the other hand, the voltage detection circuit 40 has a collector connected to the power supply line VCC through a coil 42, and a vine-grounded transistor 46 whose base is grounded through a coil 44, and has a constant voltage. A terminal voltage V0 is applied to the transistor 46 via a diode 48.

Therefore, when the terminal voltage (2) drops below the voltage determined by the constant voltage diode 48, the transistor 46 is turned off, and the oscillation output signal (s) is turned off.
It is designed to detect a decrease in the osc frequency.

On the other hand, when the transistor 46 is switched to the OFF state, the control circuit 50 outputs a control signal to the stop control circuit 52, thereby controlling the drive circuit 6 to stop the driving operation of the field effect transistor 8.

Incidentally, in this embodiment, the control circuit 50 and the stop control circuit 52 are composed of a transistor 54, and when the transistor 46 is turned off, the transistor 54 is turned on.

On the other hand, the drive circuit 6 is composed of transistors 56 and 58 connected in a Toten ball manner.
By connecting the collector of transistor 54 to the collectors of transistors 6 and 58 and lowering the potential, the operation of field effect transistor 8 is controlled to stop.

In practice, the field effect transistor 8 has the characteristic that it can be easily driven even when the power supply voltage is low, and while it starts operating immediately in a transient state when the power supply rises, it It maintains operation for a long time in certain conditions.

On the other hand, in the oscillation circuit 4, the oscillation frequency generally increases gradually to a predetermined value when the power supply is turned on, whereas the oscillation frequency gradually decreases from the predetermined value when the power supply falls.

Therefore, in the field effect transistor 8, the oscillation output signal S is generated when the power supply rises and falls. , C operated despite the low frequency, it was found that excessive current would flow or excessive voltage would be applied, leading to destruction.

Furthermore, it has been found that the conventional protection circuit cannot protect the field effect transistor 8, although it may function satisfactorily during power up and down times.

Therefore, in this embodiment, the oscillation output signal s osc
A decrease in the frequency of the field effect transistor 8 is detected and the operation of the field effect transistor 8 is controlled to be stopped, thereby effectively preventing destruction of the field effect transistor 8.

Thus, in this embodiment, the oscillation circuit 4, the drive control circuit 5, and the drive circuit 6 serve as a drive signal generating circuit for producing the drive signal SDR of the switching transformer 7, whereas the differentiating circuit 12, the waveform shaping circuit Circuit 18 and integration circuit 3
2 is a frequency detection circuit that detects the frequency of the drive signal SDR.

Furthermore, the voltage detection circuit 40, the control circuit 50, and the stop control circuit 52 stop the operation of the field effect transistor 8 based on the detection result of the frequency detection circuit. A control circuit is configured to control the system.

In the above structure, the oscillation output signal S output from the oscillation circuit 4. .

Furthermore, the oscillation output signal S. 3c is converted into a differential waveform signal SDV by the differentiating circuit 12, then waveform-shaped by the waveform shaping circuit 18, and output to the integrating circuit 32.

As a result, an oscillation output signal S is generated via the integrating circuit 32. s
Terminal voltage VI whose DC level changes according to the frequency of c
N can be obtained, and the terminal voltage VIN is applied to the voltage detection circuit 40 to detect a decrease in the frequency of the oscillation output signal S0, .

The detection result of the voltage detection circuit 40 is sequentially given to the control circuit 50 and the stop control circuit 52, thereby generating the oscillation output signal S. , C decreases, the operation of the field effect transistor 8 is controlled to stop, and destruction of the field effect transistor 8 can be avoided.

According to the above structure, the oscillation output signal S. By controlling the operation of the field effect transistor 8 to stop according to the frequency of the sc, it is possible to control the operation of the field effect transistor 8 to stop in a state where the frequency of the drive signal SDV is decreased,
In this way, destruction of the field effect transistor 8 can be effectively avoided.

In the above embodiment, by lowering the collector voltages of the transistors 56 and 58 connected to the field effect transistor 8, the field effect transistor 8
Although the case has been described in which the operation of the controller is controlled to stop, the present invention is not limited to this, and may be configured as shown in FIG.

That is, the transistor 4 is used instead of the vine grounded type transistor 62 whose collector is connected to the power supply line VCC via the coil 60.
Connect to 6.

Further, the collector of the transistor 62 is connected via a base resistor 66 to a transistor 64 whose collector is connected to the power supply line Vcc.

? The transistor 64 is connected to the ground via a capacitor 68 and to the emitter of the transistor 56, thereby generating an oscillation output signal S. , C decreases, the transistor 62 is turned on, and in response, the transistor 64 is turned off.

Therefore, the oscillation output signal S. When the frequency of sc decreases, the power supply voltage is no longer applied to the transistors 56 and 58, so that the operation of the field effect transistor 8 can be controlled to stop and destruction of the field effect transistor 8 can be avoided.

Furthermore, in the above embodiment, the oscillation output signals S0, .
Although the case of detecting the frequency of the field effect transistor has been described, the present invention is not limited to this, and the operation of the field effect transistor may be stopped and controlled by directly detecting the frequency of the drive signal S.

H Effects of the Invention As described above, according to the present invention, by detecting the frequency of the drive signal and controlling the operation of the field effect transistor to stop, the operation of the field effect transistor can be controlled even when the frequency of the drive signal is reduced. operation can be avoided, and thus destruction of the field effect transistor can be effectively avoided.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a power supply circuit according to an embodiment of the present invention, Fig. 2 is a connection diagram showing its detailed structure, Fig. 3 is a signal waveform diagram for explaining its operation, and Fig. 4 is a diagram showing other circuits. FIG. 5 is a block diagram showing a conventional power supply circuit, and FIG. 6 is a signal waveform diagram for explaining problems. 1, 10...Power supply circuit, 4...Oscillation circuit, 5...Drive control circuit, 6...Drive circuit, 7...... Switching transformer, 8...
...Field-effect transistor.

Claims (1)

[Claims] A switching transformer; a drive signal generation circuit that generates a drive signal for the switching transformer; a field effect transistor that drives the switching transformer based on the drive signal; A power supply circuit comprising: a frequency detection circuit that detects a frequency; and a control circuit that controls the operation of the field effect transistor to stop based on a detection result of the frequency detection circuit.