JPH07264866A - Ac-dc converter - Google Patents

Abstract

PURPOSE:To prevent breakdown of a switching element in a booster chopper circuit at starting time, in an AC-DC converter having a high power factor by providing the booster chopper circuit. CONSTITUTION:In an AC-DC converter comprising a rectifier 3, a booster chopper circuit 4 and a DC-DC converter circuit 5, a PWM circuit 6, frequency divider circuit 7 and a time constant circuit 8 are provided, to input the first pulse signal Spa output by the PWM circuit 6, to the first switching transistor Q1 of the DC-DC converter circuit 5 and the frequency divider circuit 7. In the frequency divider circuit 7, in accordance with a signal output by the time constant circuit 8, the frequency of the first pulse signal Spa is decreased, and the second pulse signal Spb thus generated is output to the second switching transistor Q2 of the booster chopper circuit 4. Accordingly, just after starting, a decreasing quantity of the pulse signal frequency of the frequency divider circuit is increased, and any high voltage due to saturation of a choke coil is not generated, to protect a switching element.

Description

Detailed Description of the Invention

[0001]

[Industrial field of application] This invention has realized a high power factor.
The present invention relates to soft start when the C-DC converter is started.

[0002]

2. Description of the Related Art A conventional AC-DC converter with improved power factor is an AC-DC converter including a rectifier for rectifying an AC input voltage and a DC-DC converter circuit for obtaining a stabilized DC output. Some have an active filter added by a chopper circuit. AC-D with an active filter that can obtain this high power factor
As a C converter, the present applicant has proposed an AC-DC converter having a circuit configuration as shown in FIG.
No. 25 application.

In the circuit configuration of FIG. 4, the first switching transistor Q1 of the DC-DC converter circuit 5 and the second switching transistor Q2 of the step-up chopper circuit 4 are
A drive signal v _P output by a pulse width modulation circuit (hereinafter referred to as a PWM circuit) as the same converter drive circuit
Are driven at the same time. for that reason,
In the DC-DC converter circuit 5 and the boost chopper circuit 4,
Since the control circuit, the multiplication circuit, and the PWM circuit that drives the switching transistor by the signal from the control circuit, which are conventionally configured independently of each other in both circuits, can be saved,
The circuit configuration is simple and the device can be downsized. Further, compared to an AC-DC converter having two control circuits and a signal frequency, the effect of significantly reducing noise generation due to EMI etc. was obtained.

[0004]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention Here, as shown in FIG.
Looking at the start-up operation of the C-DC converter, the output voltage is naturally zero at the start-up, and the low voltage continues for a certain period immediately after start-up. Therefore, the base of the first switching transistor Q1 of the DC-DC converter circuit 5 is The on-duty of the drive signal v _P applied to the maximum value becomes maximum during that period. As described above, in the circuit shown in FIG.
Since the first switching transistor Q1 of the DC-DC converter circuit 5 and the second switching transistor Q2 of the step-up chopper circuit 4 are simultaneously driven by the same drive signal, the second switching transistor Q2 of the step-up chopper circuit 4 also has an on-duty. The maximum period will continue.

In the AC-DC converter shown in FIG. 4, depending on how the maximum on-duty of the PWM circuit is set, the ON period of the second switching transistor Q2 is long, so that the current flowing through the choke coil L1 stores energy. The period becomes longer, and the choke coil L1 may be saturated due to DC superposition. If the choke coil L1 reaches the saturation state, the high voltage generated by the choke coil L1 becomes the second switching transistor Q2.
Will be destroyed. Therefore, the present invention is an AC-D that achieves a high power factor by adding a boost chopper circuit.
The purpose of the C converter is to reduce the on-duty of the switching element of the step-up chopper circuit at the time of startup to prevent the choke coil from becoming saturated, thereby preventing the switching element of the step-up chopper circuit from being destroyed at the time of startup. And

[0006]

The present invention is directed to a rectifier for rectifying an AC input voltage from an AC power supply line, a boost chopper circuit for boosting a rectified output voltage from the rectifier, and a high output voltage of the boost chopper circuit. In the AC-DC converter including a DC-DC converter circuit that receives the stabilized DC voltage to the load,
A converter drive circuit that detects an output voltage of the converter and supplies a first pulse signal having a pulse width corresponding to the output voltage to a switching element of the DC-DC converter circuit, and a pulse frequency of the first pulse signal. A frequency divider circuit for supplying the second pulse signal obtained by the step-down to the switching element of the step-up chopper circuit is provided, and the step-down amount of the pulse frequency of the frequency divider circuit is increased when the AC-DC converter is started. It is an AC-DC converter characterized in that.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a circuit of an embodiment of an AC-DC converter according to the present invention, which prevents the switching elements of the boost chopper circuit from being destroyed at the time of starting. The same components as those in FIG. 4 shown in FIG. 1 are designated by the same reference numerals. In FIG. 1, the AC-DC converter according to the present invention is configured as follows. Input terminals 1A and 1B are connected to the AC power supply line, and are connected to the AC input terminals of the rectifier 3, respectively. The (+) side output terminal of the rectifier 3 is connected to the anode of the diode D1 via the choke coil L1. The cathode of the diode D1 is connected to one end of the primary winding N1 of the converter transformer T, and the other end of the primary winding N1 is connected to the drain of the first switching transistor Q1.

The source of the first switching transistor Q1 is connected to the (-) side output terminal of the rectifier 3. The drain and source terminals of the second switching transistor Q2 are connected between the anode of the diode D1 and the (−) side output terminal of the rectifier 3, and between the cathode of the diode D1 and the (−) side output terminal of the rectifier 3. The output capacitor C1 is connected. One end of the secondary winding N2 of the converter transformer T is connected to the output terminal 2B, and the other end is connected to the anode of the diode D2. The cathode of the diode D2 is connected to the output terminal 2A via the choke coil L2. The cathode of the diode D2 is a flywheel diode D
Connect to the cathode of 3 and flywheel diode D3
Is connected to the output terminal 2B. Output terminal 2A,
A smoothing capacitor C2 is connected between 2B.

In the above circuit configuration, the choke coil L1, the diode D1, the second switching transistor Q2 and the output capacitor C1 form the boost chopper circuit 4, and the converter transformer T, the first switching transistor Q1, the diode D2 and the flywheel. The diode D3, the choke coil L2, and the smoothing capacitor C2 form the DC-DC converter circuit 5. Furthermore A
PWM circuit 6 as a circuit for driving the C-DC converter
, The feedback input terminal of the PWM circuit 6 is connected to the output terminal 2A, and the pulse output terminal is connected to the gate of the first switching transistor Q1 and the pulse input terminal of the frequency dividing circuit 7. Diode D1 of boost chopper circuit 4
A resistor R is provided between the cathode of the rectifier and the (-) side output terminal of the rectifier 3.
A time constant circuit 8 including a series circuit of the capacitor 3 and the capacitor C3 is provided. The resistor R3 and the capacitor C3 of the time constant circuit 8
Is connected to the control input terminal of the frequency dividing circuit 7, and the pulse output terminal of the frequency dividing circuit 7 is connected to the gate of the second switching transistor Q2.

A having the above-described circuit configuration is shown in FIG.
The operation of the C-DC converter at startup will be described below with reference to the waveform diagram of each signal shown in FIG. Immediately after the converter is started, the output voltage fed back to the PWM circuit 6 is low, so that the first pulse signal S _Pa having the maximum on-duty is transmitted to the first switching transistor Q1 of the DC-DC converter circuit 4. It outputs to the gate and the frequency dividing circuit 7. When the converter is started, the time constant circuit 8 starts charging the capacitor C3 by the boosted DC voltage of the boost chopper circuit 4, and inputs the charging voltage of the capacitor C3 to the frequency dividing circuit 7 as a control signal. Is done. Therefore, the frequency dividing circuit 7 decreases the pulse frequency of the first pulse signal S _Pa input to the pulse input terminal according to the magnitude of the signal S _T from the time constant circuit 8 input to the control input terminal. Will be changed.

Immediately after the converter is started, the charging voltage of the capacitor C3 is low, and the voltage signal S _T from the time constant circuit 8 to the frequency dividing circuit 7 is small. Therefore, the frequency dividing circuit 7 increases the step-down amount of the pulse frequency of the input pulse signal as shown in the pulse waveform on the left side of FIG. 2, and the second pulse whose on-duty is lower than that of the first pulse signal S _Pa. The signal S _Pb is _supplied to the second switching transistor Q of the boost chopper circuit 4.
It will be output to the gate of 2. As a result, the flow period of the current flowing to store energy in the choke coil L1 becomes relatively short, and the choke coil L1
1 is prevented from being saturated due to DC superposition.

After the start of the converter, the charging of the capacitor C3 of the time constant circuit 8 progresses as time passes, and the voltage signal S _T from the time constant circuit 8 to the frequency dividing circuit 7 increases. The frequency dividing circuit 7 gradually reduces the step-down amount of the pulse signal as shown in the center and the right side of FIG. 2 with respect to the increase of the signal of the control input terminal, and finally the first pulse signal S _Pa
And outputs a second pulse signal S _Pb having the same frequency as When the first pulse signal S _Pa and the second pulse signal S _Pb have the same frequency, the smoothing capacitor C2 of the DC-DC converter circuit 5 is being charged and the output voltage is also increasing. The on-duty of the first pulse signal S _Pa output from the PWM circuit 6 is lower than that immediately after the activation. Therefore, the on-duty of the second pulse signal S _Pb having the same frequency as that of the first pulse signal S _Pa is not enough to saturate the choke coil L1, and the second switching transistor Q2 does not operate as much as the choke coil L1. There is no risk of breakdown due to high voltage in saturation.

Here, in the circuit shown in FIG. 1, the time constant circuit 8 is provided between the cathode of the diode D1 and the (−) side output terminal of the rectifier 3, but the time constant circuit 8 on the rectifier 3 side of the choke coil L1 is provided. Between the terminal and the (−) side output terminal of the rectifier 3, or the second switching transistor Q2
It is also conceivable to provide it between the collector and the (−) side output terminal of the rectifier 3. Also, in the description of the operation, the first pulse signal S _Pa and the second pulse signal S _Pa from the start of the converter.
_The step-down amount of the pulse frequency of the frequency dividing circuit 7 is changed in two steps until _Pb becomes the same frequency. However, depending on the specifications of the converter, the step-down amount may be changed in only one step or in three or more steps. There may be cases.

The circuit shown in FIG. 3 shows another embodiment of the present invention in which the switching element of the boost chopper circuit is prevented from being destroyed during startup. 3, instead of the time constant circuit 8 of FIG. 1, a second switching transistor Q2
3 has the same circuit configuration as that of FIG. 1 except that the current detection unit 9 is provided in series with respect to the main current path. In the circuit of the embodiment of the present invention shown in FIG. 1, the amount of step-down of the pulse frequency of the frequency dividing circuit 7 is set large at the time of starting the converter, and the output signal S of the time constant circuit 8 is set after starting the converter. _The step-down amount of the pulse signal frequency of the frequency divider circuit 7 is gradually reduced according to _T , whereby saturation of the choke coil of the boost chopper circuit is prevented, and destruction of switching elements of the boost chopper circuit is prevented. ing. On the other hand, in the circuit shown in FIG. 3, the current flowing in the second switching transistor Q2 of the step-up chopper circuit 4 is detected by the current detecting unit 9, and the pulse signal of the frequency dividing circuit 7 is detected according to the magnitude of the detected voltage. It is intended to change the amount of frequency decrease.

When the converter is started up, the on-duty of the first pulse signal S _Pa output from the PWM circuit 6 becomes maximum, and the current detected when the current flowing through the second switching transistor Q2 becomes large according to the pulse width of the on-pulse. The signal to the frequency dividing circuit 7 of the unit 9 also becomes large, and accordingly, the frequency dividing circuit 7 increases the amount of step-down of the pulse signal frequency. As a result, the second pulse signal S _Pb whose on-duty is lower than that of the first pulse signal S _Pa is output from the frequency dividing circuit 7 to the base of the second switching transistor Q2, and the choke coil L1 is DC-superimposed. This prevents the saturation of the second switching transistor Q2 due to the high voltage generated by the saturation of the choke coil L1. The current detector 9
As the current detecting means, a resistor having a low resistance value, a current transformer, or the like can be considered.

[0016]

As described above, according to the present invention, at the time of starting the converter, the frequency dividing circuit which receives the signal from the time constant circuit or the current detecting unit at the control input terminal is controlled by the first pulse signal according to the signal. The second pulse signal obtained by greatly reducing the pulse frequency of is output. As a result, the on-duty of the second pulse signal at startup becomes lower than that of the first pulse signal, the saturation of the choke coil of the boost chopper circuit due to DC superposition is prevented, and the high duty that occurs when the choke coil is saturated. It is possible to prevent the switching element of the boost chopper circuit from being destroyed by the voltage.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an AC-DC converter of the present invention.

FIG. 2 is a waveform chart of each signal in the circuit shown in FIG.

FIG. 3 is a circuit diagram showing another embodiment of the AC-DC converter of the present invention.

FIG. 4 AC-proposed in Japanese Patent Application No. 4-61425
The circuit diagram of a DC converter.

[Explanation of symbols]

 1A, 1B input terminal 2A, 2B output terminal 3 rectifier 4 boost chopper circuit 5 DC-DC converter circuit 6 PWM circuit 7 frequency divider circuit 8 time constant circuit 9 current detector

Claims (5)

[Claims] 1. A rectifier for rectifying an AC input voltage from an AC power supply line, a boost chopper circuit for boosting a rectified output voltage from the rectifier, and a DC voltage stabilized by receiving a high output voltage of the boost chopper circuit. Supply DC to the load
In an AC-DC converter including a -DC converter circuit, an output voltage of the AC-DC converter is detected, and a first pulse signal having a pulse width corresponding to the output voltage is output to a switching element of the DC-DC converter circuit. And a frequency divider circuit for supplying a second pulse signal obtained by stepping down the pulse frequency of the first pulse signal to the switching element of the step-up chopper circuit. An AC-DC converter characterized by increasing a step-down amount of a pulse frequency of the frequency dividing circuit when the converter is started. 2. A rectifier for rectifying an AC input voltage from an AC power supply line, a boost chopper circuit for boosting a rectified output voltage from the rectifier, and a DC voltage stabilized by receiving a high output voltage of the boost chopper circuit. Supply DC to the load
In an AC-DC converter including a -DC converter circuit, an output voltage of the AC-DC converter is detected, and a first pulse signal having a pulse width corresponding to the output voltage is output to a switching element of the DC-DC converter circuit. And a frequency divider circuit for supplying a second pulse signal obtained by stepping down the pulse frequency of the first pulse signal to the switching element of the step-up chopper circuit. At the time of starting the converter, the step-down amount of the pulse frequency of the frequency dividing circuit is increased so that the AC-DC
After starting the converter, the amount of step-down of the pulse frequency is gradually reduced with the passage of time.
DC converter. 3. The step-up chopper circuit is provided with a time constant circuit, and the step-down amount of the pulse frequency of the frequency dividing circuit is changed according to a signal from the time constant circuit. The AC-DC converter according to item 1 and item 2. 4. The AC-DC converter according to claim 3, wherein the time constant circuit operates by receiving a high voltage output voltage of the boost chopper circuit. 5. A current detecting section is provided in series with a switching element of the boost chopper circuit, and a step down amount of a pulse frequency of the frequency dividing circuit is changed according to a signal from the current detecting section. The AC-DC converter described in claims 1 and 2.