JPH0121700B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a variable output voltage switching power supply that can vary the output voltage over a wide range.

[Prior art and problems]

A switching power supply as a DC stabilized power supply generally compares the output voltage with a reference voltage, uses a pulse width control circuit to generate a pulse with a width corresponding to this error voltage, and uses this pulse to perform switching.
By controlling the transistor on and off,
This stabilizes the output voltage.

In such a switching power supply, when trying to vary the output voltage over a wide range, if the output voltage is set to a low value, the pulse width control circuit generates a pulse with an extremely narrow ON width to turn the switching transistor on and off. Conversely, when the output voltage is set high, the switching transistor must be controlled by a pulse with a wide on-width near the maximum allowable on-width determined by the switching circuit system.

When setting a low output voltage, if the on-width of the control pulse is narrower than the storage time of the switching transistor, current flows through the transistor only during the storage time, making control extremely difficult. Therefore, the lower limit of the output voltage is It is determined by the storage time of the transistor.

In addition, since the minimum and maximum values of the on-width of the control pulse are limited, as the variable range of the output voltage becomes wider, the stability accuracy of the output voltage against load fluctuations and input fluctuations becomes rougher. The lower the output voltage is set, the worse the output accuracy (rate of fluctuation) becomes.

[Purpose and structure of the invention]

The present invention has been made in view of the above-mentioned drawbacks, and it is an object of the present invention to provide a variable output voltage switching power supply that is capable of varying the output voltage over a wide range and has a stable output voltage with high precision.

The purpose of this is to control the pulse width according to the error voltage between the output voltage and the set voltage value of the variable reference voltage in a switching power supply that uses pulse width control to control switching elements on and off to obtain a DC stabilized output voltage. a first pulse width control circuit; a second pulse width control circuit that controls the pulse width to be proportional to the variable voltage value of the variable reference voltage at a constant frequency lower than the frequency of the first pulse width control circuit;
A logic circuit that performs a logical operation on the output pulse of the second pulse width control circuit and the output pulse of the first pulse width control circuit, and controls the switching element to be turned on or off by the output pulse of the logic circuit. This is achieved by a variable output voltage switching power supply characterized by the following characteristics.

[Embodiments of the invention]

Hereinafter, the present invention will be explained based on examples.

FIG. 1 is a circuit diagram of an embodiment of the present invention.
Vin is AC input power supply, REC1 and REC2 are rectifier circuits, T is main transformer, Tr is switching transistor, L is inductance, C is capacitor, Vout is DC output voltage, OP1 and OP2 are operational amplifiers, E is variable reference voltage , R1 and R2 are resistances,
PWM1 indicates a first pulse width control circuit, PWM2 indicates a second pulse width control circuit, and AND indicates a logic circuit.

FIG. 2 is a waveform diagram of control pulses as an explanatory diagram of the operation of the embodiment.

In FIG. 1, the first pulse width control circuit
PWM1, like the conventional one, generates control pulses for stabilizing the output voltage in response to output load fluctuations and input voltage fluctuations.

In other words, a rectifier circuit REC1 converts AC input power supply Vin to DC, and a switching transistor Tr.
2 of the main transformer T by the switching operation of the switching circuit composed of the main transformer T and the main transformer T.
The rectangular wave AC voltage induced in the next winding is rectified by the rectifier circuit REC2, smoothed by the inductance L and the capacitor C, and the DC output voltage is
Output as Vout.

This DC output voltage Vout is divided and detected by resistors R1 and R2, compared with a variable reference voltage E in an operational amplifier OP1, error amplified, and input to the first pulse width control circuit PWM1.

The first pulse width control circuit PWM1 generates a constant frequency pulse whose on width is controlled according to this error amplified signal, and controls the switching transistor.
A rectangular wave pulse as shown in FIG. 2A is sent out as a drive pulse for controlling the Tr on and off.

On the other hand, the second pulse width control circuit PWM2 sends out control pulses according to the variable setting value of the output voltage, and the setting value of the variable reference voltage E is the minimum value necessary for stabilizing the control circuit system. It is input to the second pulse width control circuit PWM2 via the operational amplifier OP2 with a limited gain.

In the second pulse width control circuit PWM2,
This is a frequency lower than the frequency of the first pulse width control circuit PWM1, and can be any constant frequency depending on the variable width of the output voltage (setting the frequency lower as the variable width becomes wider improves control accuracy). Then, a pulse whose on width is controlled to be proportional to the set value of the variable reference voltage E is generated and output.

The waveform B in Figure 2 is the output pulse of the second pulse width control circuit PWM2 when the variable reference voltage E is set low, and the waveform C in Figure 2 is the output pulse of the variable reference voltage E.
The output pulse waveforms are shown when the variable reference voltage E is set high.
As the setting of (output voltage) becomes higher, the duty ratio becomes larger.

Next, the output pulses of the first pulse width control circuit PWM1 and the second pulse width control circuit PWM2 are as follows.
A logical AND operation is performed in the logic circuit AND, and this output pulse causes the switching transistor Tr to
is driven and controlled.

When the variable reference voltage E is set low, the pulse of FIG. 2D is obtained as the AND of the rectangular wave pulses of FIG. 2B and FIG. 2A as the output of the logic circuit AND.

This control pulse is passed through the second pulse width control circuit.
During the period of the frequency determined by PWM2, the number of on-pulses is small and the off-period is long, and this drives the switching transistor Tr.
By supplying less electrical energy to the next winding than conventionally, a lower output voltage than conventionally can be obtained.

In addition, the on-width of the pulse at this time detects a sufficiently low output voltage with respect to the variable reference voltage E that is set low, so it does not have an extremely narrow width as in the past, but due to output load fluctuations and input It is possible to sufficiently control the on-width even against fluctuations.

When the variable reference voltage E as the output voltage is set high, the pulse in Figure 2 E is the logical product of the rectangular wave pulses in Figure 2 C and Figure 2 A, and
This control pulse is obtained as the output of AND, and this control pulse has a large number of on-width pulses in one cycle by the second pulse width control circuit PWM2, so a high output voltage can be obtained, and it fluctuates in the same way as in the case of a low setting. It is possible to sufficiently control the on-width of the current, and to perform highly accurate stabilization control.

In this way, in the circuit of the present invention, as the output voltage setting becomes higher, the second pulse width control circuit PWM2
The first pulse width control circuit PWM1 controls to increase the number of pulses in one cycle by
The on-width of the pulse is controlled in response to output load fluctuations and input fluctuations.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to easily set and control a low output voltage without making the on-width of the pulse extremely narrow as in the past, and it is also possible to vary the output voltage over a wide range. A variable output voltage switching power supply with sufficiently high output accuracy at a set voltage can be realized, and is extremely useful in practice.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a waveform diagram of control pulses as an explanatory diagram of the operation of the embodiment of the present invention. In the figure, REC1 and REC2 are rectifier circuits, T is the main transformer, Tr is the switching transistor, and OP
1, OP2 is an operational amplifier, E is a variable reference voltage,
PWM1 is a first pulse width control circuit, PWM2 is a second pulse width control circuit, and AND is a logic circuit.

Claims (1)

[Claims] 1. In a switching power supply that obtains a DC stabilized output voltage by controlling switching elements on and off using pulse width control, a first switch that controls the pulse width according to the error voltage between the output voltage and the set voltage value of the variable reference voltage is used. a pulse width control circuit; a second pulse width control circuit that controls the pulse width to be proportional to the variable voltage value of the variable reference voltage at a constant frequency lower than the frequency of the first pulse width control circuit; and a logic circuit that performs a logical operation on the output pulse of the pulse width control circuit and the output pulse of the first pulse width control circuit, and the switching element is controlled to be turned on or off by the output pulse of the logic circuit. A variable output voltage switching power supply.