JPH0746068Y2 - Pulse width modulation power supply frequency synchronization circuit - Google Patents

Description

[Detailed Description of the Invention] (Industrial field of application) The present invention relates to the frequency of a switching power supply for a constant voltage power supply,
The present invention relates to a circuit that automatically synchronizes with the frequency of another circuit.

(Prior art and its problems) In a commercially available pulse width modulation (PWM) type constant voltage power supply controller IC, it is not possible to synchronize with an external signal, for example, a device using a cathode ray tube. In addition, when a constant voltage power supply using this IC is used, the deflection yoke transformer of the cathode ray tube is affected by the leakage magnetic flux of the transformer of the constant voltage power source, and there is a drawback that the stripe noise is noticeable on the screen of the cathode ray tube. . To prevent this, the distance between the deflection yoke transformer and the power transformer should be increased, but it was difficult to remove noise in a small device that cannot keep the distance.

As described above, it has been desired that the noise generated on the screen of the cathode ray tube when a commercially available controller IC for a constant voltage power supply is used can be removed by a simple means.

(Means for Solving Problems) In order to eliminate these drawbacks, the present invention uses a reference voltage as a power source for a resistor (R _T ) and a capacitor (C _T ) externally connected to the control side and the output side, respectively. Controller for pulse width modulation type constant voltage power supply that supplies constant current of the same magnitude from constant current power supply to perform pulse width modulation by built-in oscillator (OSC) that determines the oscillation cycle ( 5) Means (4) for converting the oscillation pulse of the oscillator (OSC) into a square wave signal, means (2) for converting an external synchronization signal into a square wave signal, and these two square wave signals Means (3) for obtaining the exclusive OR of
A capacitor (C ₁ ) that is charged and discharged through a resistor (R ₂ ) with a time constant longer than the oscillation cycle according to the output level H (high) or L (low), the resistor (R _T ) and the capacitor. A current control resistor (R ₁ ) connected between (C ₁ ) to control the current of the constant current power supply of the oscillator (OSC) is added to synchronize the oscillation cycle of the oscillator of the controller with a synchronization signal from the outside. This will be described below with reference to the drawings.

(Embodiment) FIG. 1 shows a frequency synchronizing circuit of a pulse width modulation type power supply according to an embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an external synchronizing input terminal for inputting a horizontal synchronizing signal of a television,
2, 4 and 7 are flip-flop circuits (F / F), 3 is an exclusive OR circuit, 5 is a pulse width modulation type switching regulator controller IC, and 6 is a transformer which constitutes a part of the power supply circuit. A constant voltage power supply circuit is configured in combination with. 8
Is a DC input terminal and 9 is a DC output terminal. Oscillator OSC as internal are shown in FIG. 2 of the IC 5, the flip-flop F / F, the reference power supply VC, is configured by a comparator COMP, the oscillator OSC resistor R _T, to determine the frequency by the constant of the capacitor C _T FIG. 3 shows a concrete circuit sequence of the above.

In FIG. 3, a constant current I of the same magnitude as the constant current I flowing through the resistor R _T externally connected to the control side flows through the capacitor C _T externally connected to the output side of the oscillator OSC,
Since C _T is charged with a constant current, a sawtooth waveform having an oscillation period T as shown in FIG. 4 is obtained and oscillates.

Let V _EB (Q ₂ ) and V _EB (Q ₃ ) be the emitter-base voltages of the transistors Q ₂ and Q 3 in FIG. ₃ , respectively.
Assuming that the voltage change ΔV of the terminal voltage V _CT of the capacitor C _T is as shown in FIG. 4, the current I of the oscillator circuit shown in FIG. 3 can be expressed by the following equation.

ΔV = V _H −V _L (2) Further, the oscillation period T can be expressed by the following equation.

As is clear from FIG. 3 and the above equations (1) to (3), the current control resistor R ₁ is connected in parallel with the resistor R _T that determines the oscillation period T connected to the collector of the transistor Q ₁ of the oscillator circuit OSC. Since the current I flowing through the capacitor C _T also changes when the current I is changed by connecting to the capacitor, it is possible to change the oscillation period T from the above equation (3).

Now, returning to FIG. 1, when an external synchronizing signal (Fig. 5a) is input to the external synchronizing input terminal 1, it is converted into a square wave (Fig. 5b) by the flip-flop 2 (F / F). It Meanwhile I
Oscillation cycle pulse from the oscillator output terminal OSCOUT of C5 (5th
In Fig. C), the waveform is converted into a square wave (Fig. 5d) by the flip-flop 4 (F / F).

In this case, for an IC in which the oscillation cycle pulse of IC5 (Fig. 5c) is output at twice the frequency of the synchronization signal from the outside, flip-flop 7 (F / F) is added to flip-flop 4 (F / F). To reduce the frequency to 1/4. The output waveform is shown in FIG.

The outputs of the flip-flops 2 and 7 (FIGS. 5b and e) are ORed by the exclusive OR circuit 3, the output of which is connected to the capacitor C ₁ through the resistor R ₂ and the current control resistor.
It is connected to the R _T terminal of the switching controller 5 through R ₁ . The capacitor C ₁ is charged at the H level and discharged at the L level according to the output state of the exclusive OR circuit 3.

FIG. 5f shows the output waveform of the exclusive OR circuit 3. The charging / discharging voltage of the capacitor C ₁ is as shown in FIG. That is, the oscillation cycle T of the oscillator OSC of the switching controller 5 is
External from the synchronizing signal is larger compared to the period of (FIG. 5 a), it decreases the charging voltage of the capacitor C _1, R _T terminal current control resistor from R ₁ (FIG. 1) of the switching controller 5
Since the current flows in the equation (3), the oscillation period T of the oscillator OSC becomes smaller as the current I in the equation (3) increases.

On the contrary, if the oscillation cycle T of the oscillator OSC is smaller than the cycle of the synchronizing signal from the outside, the charging voltage of the capacitor C ₁ rises and the current from the _RT terminal of the switching controller 5 to the resistor R ₁ stops flowing. , The oscillation period T increases as the current I in the equation (3) decreases.

As described above, by controlling the current I of the _RT terminal of the switching controller 5, the oscillation cycle T of the oscillator OSC can be synchronized with the external synchronization signal. This method is the fourth
Since it does not affect ΔV, V _H , and V _L in the figure, the pulse width modulation control function of the switching controller 5 (IC)
It can be synchronized externally with 100% utilization.

As a concrete example, when the horizontal sync signal (15.75KHz) of the TV is connected to the external sync input terminal 1 in Fig. 1, a switching power supply synchronized with this frequency can be created and displayed on the CRT screen. The noise that is heard is inconspicuous because it is synchronized. In addition to this, the frequency of the switching power supply is kept stable by the horizontal synchronizing signal, so the problem that this frequency drifts and enters the receiver band to become noise is also solved.

(Effects of the Invention) As described above, the present invention is designed to synchronize the oscillation frequency of the controller IC of the pulse width modulation type switching power supply with the synchronization signal input from its external terminal.
To constant circuit composed of the R _T resistor and a capacitor C _T for controlling the oscillation frequency of the R _T resistor in parallel with the current control resistor
R ₁ is connected in parallel and the oscillation frequency is adjusted by changing the current flowing from the R _T terminal according to the period of the external periodic signal.

Therefore, the oscillation frequency of the oscillator of the switching controller is synchronized by the synchronizing signal input from the outside with a simple circuit configuration. Therefore, when the circuit of the present invention is used for the cathode ray tube display, no noise is displayed on the display screen, and convenient.

[Brief description of drawings]

FIG. 1 is a frequency synchronization circuit diagram according to an embodiment of the present invention,
2 is a circuit configuration diagram of a controller IC for a switching regulator, FIG. 3 is an oscillator circuit of FIG. 2, FIG. 4 is its C _T terminal waveform diagram, and FIG. 5 is a diagram for explaining the operation of FIG. It is a waveform diagram of each part. 1; Input terminal for external synchronization, 2, 4 and 7; Flip-flop (F /
F), 3; exclusive OR circuit, 5; controller IC for switching regulator, 6; transformer, 8 and 9; DC input terminal and output terminal, Q ₁ , Q ₂ , Q ₃ ; transistor, _RT ; resistance, C _T ;
Capacitor, R ₁ ; current control resistor.

Claims (1)

[Scope of utility model registration request] 1. A constant current of the same magnitude is supplied from a constant current power source using a reference voltage as a power source to a resistor (R _T ) and a capacitor (C _T ) externally connected to a control side and an output side, respectively. Built-in oscillator (OSC) that determines the oscillation cycle
A controller (5) for a pulse width modulation type constant voltage power source that performs pulse width modulation to perform a constant voltage operation by means (4) for converting an oscillation pulse of the oscillator (OSC) into a square wave signal; Means (2) for converting the synchronizing signal from the above into a square wave signal, means (3) for taking the exclusive OR of these two square wave signals, and the output level H (high)
Alternatively, a capacitor (C ₁ ) which is charged and discharged through a resistor (R ₂ ) at a L (low) level with a time constant longer than the oscillation cycle is connected between the resistor (R _T ) and the capacitor (C ₁ ). A current control resistor (R ₁ ) for controlling the current of the constant current power source of the oscillator (OSC) is added to enable the oscillation cycle of the oscillator of the controller to be synchronized with an external synchronization signal. Pulse width modulation type power supply frequency synchronization circuit.