JP3429417B2 - Forward type DC-DC converter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power factor improvement of a forward type DC-DC converter which receives a commercial power source as an input.

[0002]

[Prior art]

(2) In the conventional forward type DC-DC converter, a technique for improving the power factor without using a smoothing capacitor for the rectifier output is widely known, and FIG. 1 is an example of the circuit. Further, FIG. 2 shows the operation waveforms of each part in this circuit.

In the figure, the output of the full-wave rectification circuit 2 that receives the commercial power supply 1 as an input is the full-wave rectification voltage without using a smoothing capacitor in series with the primary winding 5 of the transformer 3 and the main switching element 4. It is applied to a switching circuit composed of a circuit.

When the main switching element 4 is turned on and a voltage is applied to the primary winding 5, the transformer 3 starts winding the dots shown in the figure, and the secondary winding 6 wound on the same pole supplies power to the output. Is configured to supply.

On the secondary side of the transformer, from the beginning of the winding of the secondary winding 6, a rectifying diode 7 which is in the forward direction when the main switching element 4 is ON, a smoothing circuit composed of an inductance 9 and a capacitor 10, and a load 15 are provided. Supply power to. A flywheel diode 8 is connected so that the energy of the inductance 9 flows to the load 15 when the main switching element 4 is off.

Further, as the control circuit, the error amplifier 1
1 is a predetermined reference voltage 1 for the voltage of the capacitor 10.
It is connected so as to compare and detect 2 and increase the error voltage to supply it to the pulse generator 13.

Further, the pulse generator 13 sends a PWM-controlled pulse having a constant frequency to the drive circuit 14 to drive the main switching element 4.

In such a configuration, the operation waveform of each part is as shown in FIG. That is, when the main switching element 4 is driven by the PWM pulse as shown in FIG. 2C, (3) the current of the inductance 9 becomes a sawtooth wave as shown in FIG. 2A.

The inductance current of the sawtooth wave is in a completely discontinuous state. Moreover, the peak value of the inductance current is low in the low voltage part of the half-cycle of the input voltage (full-wave rectified voltage), so the discontinuity period is long, and the peak value of the inductance current is high in the high voltage part. Since it becomes higher, the discontinuity period becomes shorter.

Therefore, the input current of the converter has a similar shape to the waveform shown in FIG. 2 (d) in which the average value of the inductance current is connected by the envelope. Therefore, the input current is distorted,
The current waveform has many harmonic components.

Further, in a period in which the input full-wave rectified voltage is lower than the output average value voltage, the input current does not flow, so that the input power factor deteriorates.

Further, when the direct current (b) obtained by averaging the inductance current is supplied to the load 15, the peak current increases due to the discontinuous current, which causes a decrease in efficiency and an increase in noise.

[0013]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems. That is, according to the present invention, the on-pulse is applied to the main switching element immediately when the inductance current becomes zero. In this way, frequency control is performed in which the on-width is constant and the off-width is variable during the period when the input full-wave rectified voltage is low and the period when it is high.

As a result, the discontinuous period of the inductance current is eliminated, and as a result, a forward type DC-DC converter with a high power factor and low noise can be realized without an input electrolytic capacitor.

[0015] (4)

The present invention is characterized by taking the following means in order to solve the above problems.

That is, according to the present invention, a full-wave rectifier circuit having a commercial power supply as an input, a primary winding of a transformer and a main switching circuit connected in series to a rectified output of the full-wave rectifier circuit, and this transformer A rectifying / smoothing circuit composed of a rectifying diode and a flywheel diode connected to the secondary winding in a direction to supply electric power to the load, a smoothing inductance and a capacitor, and a control circuit for driving the main switching element. Composed. The control circuit performs duty control by detecting the output voltage and performing frequency control in which the ON width is constant and the OFF width is variable. In this way, the forward type D that supplies a stable output voltage to the load
It is a C-DC converter. That is, the forward type DC-DC converter is characterized in that the control circuit detects the current of the inductance and controls to start supplying the ON pulse when the current becomes zero.

As a means for detecting the current of the inductance, it is preferable to use a resistor provided on the negative side of the smoothing capacitor and the anode side of the flywheel diode.

More preferably, the control circuit is characterized in that a limiter for producing a minimum OFF width is provided for the drive pulse of one cycle for controlling the main switching element.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 3 shows a forward type DC-DC according to the present invention.
FIG. 4 is a circuit diagram of an embodiment of a converter, and FIG. 4 shows operation waveforms of respective parts of this circuit. Further, in FIG. 3, the same parts as those described in FIG. 1 are designated by the same reference numerals. Further, a description that overlaps with the related art of FIG. 1 will be omitted. (5)

In the present invention, the parts added to the conventional circuit of FIG. 1 are as follows. That is, the inductance current detecting resistor 16 detects the inductance current, the comparator 17 compares the preset reference value, and inputs the reference value to the pulse generator 13 '. The circuit for detecting the output voltage and inputting it from the error amplifier 11 to the pulse generator 13 'is the same as the conventional circuit.

That is, the comparator 17 is so constructed as to detect that the inductance current has become zero and send a signal for turning on the switch to the pulse generator 13 '.

In this way, the pulse generator 13 'is
From the outputs of the error amplifier 11 and the comparator 17, the frequency-modulated pulse whose ON width is constant and whose OFF width is controlled is controlled to the minimum OF.
It is sent to the drive circuit 14 through the F width limiting circuit 18 to drive the main switching element 4. By the minimum OFF width limiting circuit, the ON pulse does not continue even during the period when the full-wave rectified voltage is low.

The operation of the above circuit configuration will be described below with reference to the operation waveforms of FIG.

First, in one cycle of the full-wave rectified waveform, the drive pulse is controlled so that the ON period is constant as shown in FIG. 4 (c).

Since the inductance current linearly rises during the ON period of the main switching element and linearly falls during the OFF period, it becomes a sawtooth wave as shown in FIG. 4 (a). Therefore, the peak value is low in the part where the full-wave rectified voltage is low, so the period when the inductance current reaches zero is short, and the peak value of the inductance current is high in the part where the full-wave rectified voltage is high, so the period when the inductance current reaches zero. Becomes longer. (6)

When the inductance current reaches zero, an ON pulse of the drive signal is generated and the switching element 4 is turned on. Therefore, the switch OFF period is set to the period until the inductance current reaches zero.

Therefore, OF is low in the portion where the full-wave rectified voltage is low.
The F period is short (and therefore the frequency is high), and the OFF period is long (and therefore the frequency is low) in the portion where the full-wave rectified voltage is high.
Become. As a result, the switching frequency is shown in FIG.
The waveform becomes as shown in (c), and frequency control is performed.

By such control, the inductance current disappears in the discontinuous period as shown in FIG. 4 (a), so the envelope connecting the average values of the inductance current has a sinusoidal waveform as shown in FIG. 4 (d). Become. As a result, the input power supply can have a waveform with few harmonic components.

Further, since such frequency control is performed, the switching frequency rises during a period when the full-wave rectified voltage is lower than the average value of the output voltage, that is, within the non-conduction angle, and a large exciting current of the transformer 3 can flow. Power factor can be improved.

DC which is obtained by averaging the inductance current
When the output current (FIG. 4 (b)) is supplied to the load, the inductance current is zero-cross controlled, so the ratio of the peak current to the average current is reduced, and the efficiency and noise can be reduced.

Further, in the circuit of the present invention, the OFF period theoretically disappears during the period when the full-wave rectified voltage is low,
There is a risk that the ON pulse will continue and the pulse transformer will be saturated. Therefore, in order to solve this problem, the minimum OFF width limiting circuit 18 is provided and the OFF period is forcibly provided every (7) cycles.

[0032]

As described above, the input rectifier circuit output capacitorless forward type DC-DC converter of the present invention solves the drawbacks of the conventional system by the control method. That is, by detecting the inductance current and generating the ON pulse by detecting the zero-cross point of this current, it is possible to perform frequency control that controls the OFF width with a constant ON width. You can As a result, it is possible to realize a forward type DC-DC converter aiming at power factor improvement, efficiency improvement, and noise reduction by adding a few circuits to the conventional technique.

[Brief description of drawings]

FIG. 1 is a basic circuit diagram of a conventional forward type DC-DC converter.

FIG. 2 is a waveform of each part of a conventional forward type DC-DC converter.

FIG. 3 is a basic circuit diagram of a forward type DC-DC converter of the present invention.

FIG. 4 is a waveform of each part of the forward type DC-DC converter of the present invention.

[Explanation of symbols]

1 Commercial power supply 2 full wave rectifier 3 transformers 4 Main switching element 7,8 diode 9 Inductance 10 Output smoothing capacitor (8) 11 Error amplifier 12 Reference voltage 13,13 'pulse generator 14 Drive circuit 15 load 16 Inductance current detection resistor 17 Comparator 18 Minimum OFF width limit circuit

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Claims (3)

(57) [Claims] 1. A full-wave rectifier circuit using a commercial power supply as an input,
Connected in series to the full-wave rectified output of the full-wave rectifier circuit,
A switching circuit including a primary winding of a transformer and a main switching element, a rectifying diode and a flywheel diode connected to a secondary winding of the transformer in a direction for supplying power to a load, and smoothing. It comprises a rectifying / smoothing circuit composed of an inductance and a capacitor for control, and a control circuit for driving the main switching element. The control circuit detects the output voltage and performs frequency control with a constant on-width, thereby achieving a duty cycle. In a forward type DC-DC converter that performs a tee control and supplies a stable output voltage to a load, the control circuit detects the current of the inductance and when the current becomes zero. Then, a forward type DC-DC converter characterized by starting to supply an ON pulse. 2. The forward type DC-DC according to claim 1.
In the converter, the means for detecting the current of the inductance is a resistor provided on the negative side of the smoothing capacitor and the anode side of the flywheel diode, and is a forward type DC-DC converter. 3. The forward type DC-DC converter according to claim 1 or 2, wherein the control circuit has a minimum OFF width for each drive pulse of one cycle for controlling the main switching element. A forward type DC-DC converter characterized by: