JPS5923190B2 - power circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION A switching type stabilized power supply circuit is configured as shown in FIG. 1, for example.

That is, in FIG. 1, a commercial AC voltage is supplied from a power plug 1 through a power switch 2 and a power filter 3 to a rectifier circuit 4, where it is rectified and smoothed into a DC voltage, and this DC voltage is connected to a primary coil Li of a transformer 5 and a switching circuit. For example, a PWM pulse from the PWM modulation circuit 16 is supplied to the transistor 6, which is turned on and off. Therefore, an AC voltage is taken out from the secondary coil L2 of the transformer 5, which is supplied to the rectifier circuit T, where it is rectified and smoothed into a desired DC voltage, and this DC voltage is taken out to the output terminal 8.

In order to stabilize the output voltage of this terminal 8, it is further configured as follows.

That is, the DC voltage at the terminal 8 is taken out by the variable resistor 11, supplied to the transistor 12, and compared with the reference voltage from the constant voltage diode 13. 1
5 to the modulation circuit 16 as its modulation input. The modulation circuit 16 also includes an oscillation circuit 1? Pulses for carrier waves are also supplied from . In this way, the pulse width of the PWM pulse from the modulation circuit 16 changes according to the DC voltage at the terminal 8, and the DC voltage at the terminal 8 is stabilized at a constant value.
At this time, the DC voltage from the rectifier circuit 4 is supplied through the transistor 21 to the control circuit of the transistor 6, that is, to the circuits 15 to 17 as its operating voltage.

In such a switching type power supply circuit, the input AC power supply side and the load side are electrically separated by the transformer 5 and the photocoupler 14.

Since the transistor 6 performs a switching operation, the efficiency is essentially high. However, in reality, the commercial AC voltage is 10
If it is 0, the DC voltage from the rectifier circuit 4 will be about 140, whereas the operating voltage of the circuits 15 to 17 will be about 6 to 15. Since the voltage must be lowered, efficiency decreases by this amount.

Further, the transistor 21 is required to have a high breakdown voltage, which lowers reliability and increases cost. Therefore, a power supply circuit as shown in FIG. 2 has also been considered.

That is, the AC voltage obtained in the tertiary coil L3 of the transformer 5 is supplied to the rectifier circuit 23 to convert it into a DC voltage, and this DC voltage is supplied to the circuits 15 to 17 as their operating voltages. Therefore, in this power supply circuit, compared to the power supply circuit shown in FIG. 1, there is no decrease in efficiency or reliability.

However, in this power supply circuit, when the switch 2 is turned on, the transistor 6 is off and the DC voltage from the rectifier circuit 23 is not obtained, so the circuits 15 to 17 do not work, and therefore the PWM pulse is not output from the modulation circuit 16. cannot be obtained, so transistor 6 continues to be off.

That is, this power supply circuit does not start even if switch 2 is turned on. For this reason, in general, the start circuit 24
is provided, and only for a few seconds after turning on switch 2,
The DC voltage from the rectifier circuit 4 is supplied to the circuits 15 to 17 as their operating voltage. However, this increases the cost due to the start circuit 24.
The present invention attempts to solve these problems.

Therefore, in the present invention, when the power switch 2 is turned on, the switching transistor 6 performs self-oscillation to obtain a DC voltage in the rectifier circuit 24, and this DC voltage starts the circuits 15 to 17. Thereafter, switching is performed by the transistor 6 to establish a steady operating state.

FIG. 3 shows an example of this, in which a coil L4 is provided in the transformer 5 for oscillation, a resonance capacitor 31 is connected in parallel to this, and one end of this parallel circuit is connected to one end of the coil L1 of the transformer 5. Connection tab The tap of coil L4 is connected to the base of transistor 6 through a series circuit of capacitor 30 and resistors 32 and 33.

Further, a series circuit of a resistor 35 and a capacitor 36 is connected to the output end of the rectifier circuit 24 as a time constant circuit, and the connection point between the resistor 35 and the capacitor 36 is connected to the transistor 41.
connected to the base of

This transistor 41 and a transistor 42 constitute a Schmitt circuit 43 for voltage level discrimination.The collector of the transistor 42 is connected to the base of a transistor 46 through a transistor 45, and the collector of the transistor 46 is connected to the resistors 32 and 33. connected to the connection point. In such a configuration, when switch 2 is turned on, since no output voltage is yet obtained from rectifier circuit 24, transistor 45 is off, and transistor 46 is also off.

Therefore, when the switch 2 is turned on, the output voltage of the rectifier circuit 4 is supplied to the base of the transistor 6 through the coil L4 → resistor 32 → resistor 33, and the transistor 6 is class A biased, and the capacitor 30 and Resistor 33
acts as a positive feedback path from coil L4 to the base of transistor 6, so transistor 6 and coils L, , L
4 constitutes an oscillation circuit and starts oscillation. Note that the oscillation frequency in this case is a frequency close to the switching frequency 3 of the transistor 6, for example, 20 kHz or more. When this oscillation occurs, an output voltage is obtained from the rectifier circuit 24, and this voltage charges the capacitor 36 through the resistor 35.
During this charging period, transistor 41 is off, so transistor 42 is on, transistor 45 is off, and transistor 46 is also off, so transistor 6 continues to oscillate. At this time, the output voltage of the rectifier circuit 24 is also supplied to the circuits 15 to 17 as their operating voltages. When the charging voltage of the capacitor 36 reaches a certain value, that is, after a predetermined period of time has passed after the switch 2 is turned on, the transistor 41 is turned on and the transistor 42 is turned off, so that the transistor 45 is turned on. Transistor 46 also turns on.

When the transistor 46 is turned on, the resistor 32,
The bias current and oscillation feedback signal that had been supplied to the base of the transistor 6 through 33 are cut off, and
At this time, the circuits 15 to 17 have already been supplied with the operating voltage from the rectifier circuit 24, so the PW from the modulation circuit 16 is
The transistor 6 starts switching due to the M pulse, and thereafter enters a steady operating state. In this way, in the present invention, when the switch 2 is turned on, the switching transistor 6 self-oscillates and enters a steady operation state, so there is no decrease in efficiency, the structure is simple, and there is no increase in cost. .

Furthermore, the time constants of the resistor 35 and capacitor 36 (including the time constant of the rectifier circuit 24) keep the transistor 6 in an oscillating state for a certain period of time after the switch 2 is turned on. The operating voltage at the time of starting can be reliably supplied, and therefore starting can be performed reliably.

Further, during steady operation, the transistor 46 is turned on and the transistor 6 is not oscillated by the coil L4.
Since the transistor 6 is turned on and off only by the PWM pulse from the modulation circuit 16, it is effective to stabilize the output voltage of the terminal 8 by changing the pulse width of the PWM pulse.
And it becomes certain. In the above example, the oscillation circuit constituted by the transistor 6 at the start is of the anti-coupling type, but it can also be of the blocking type, for example.

FIG. 4 shows an example of this. When switch 2 is turned on, transistor 6 is triggered through capacitor 38, self-oscillation begins at a frequency determined by the time constant of capacitor 31 and resistor 37, and after a predetermined period, transistor 6 is triggered through capacitor 38. When 46 is turned on, there is no feedback from the coil L4 to the base of the transistor 6, so switching is performed by the PWM pulse from the modulation circuit 16.

[Brief explanation of drawings]

1 and 2 are connection diagrams for explaining this invention,
3 and 4 are connection diagrams of an example of the present invention. 16 is a PWM modulation circuit, and 17 is an oscillation circuit.

Claims (1)

[Claims] 1. A first rectifier circuit that rectifies and smoothes an input AC voltage to extract a first DC voltage, a transformer having at least a primary coil and a secondary coil, and the primary coil at the output end of the first rectifier circuit. a switching element connected in series with the secondary coil; a second rectifier circuit connected to the secondary coil to rectify and smooth the output voltage of the secondary coil into a second DC voltage; a first detection circuit for detecting; a modulation circuit that uses the detection output to form a pulse that changes in response to a change in the second DC voltage and supplies it to the switching element as a control pulse; a positive feedback circuit that causes self-oscillation; a tertiary coil provided in the transformer; a third rectifier circuit that rectifies and smoothes the output voltage of the tertiary coil to supply an operating voltage to at least the modulation circuit; a second detection circuit that detects the rectified output of the rectifier circuit No. 3; and another switching element that controls positive feedback of the positive feedback circuit based on the detection output of the second detection circuit; is supplied, the switching element is caused to self-oscillate by the positive feedback of the positive feedback circuit, and an operating voltage is supplied from the third rectifier circuit to at least the modulation circuit, and after the input AC voltage is supplied, the switching element is caused to self-oscillate. When an output is obtained in the third rectifier circuit, the detection output of the second detection circuit controls the other switching element to cut off the positive feedback of the positive feedback circuit and stop the self-oscillation of the switching element. At the same time, the switching element is switched by the pulse from the modulation circuit to extract and supply the operating voltage of the modulation circuit from the third rectification circuit, and stabilized from the second rectification circuit. A power supply circuit designed to obtain a specified output voltage.