JPH01278258A - Switching power source - Google Patents

Abstract

PURPOSE:To prevent a resistor from generating heat by connecting a thyristor in parallel with a rush current preventing resistor, and turning ON it only when an oscillator is oscillating and a smoothing capacitor is an oscillation starting voltage or more. CONSTITUTION:In a switching power source, a smoothing capacitor C1 is connected after a rectifier 1, and an oscillator 6 in which the primary winding L1 of a power transformer T1 is connected in series with a switching transistor(Tr) Q1 is connected thereto. A rush current preventing circuit 5 in which a resistor R1 is connected in parallel with a thyristor S1 is provided at a power source line 4. Further, a detector 9 obtains a DC voltage by a Zener diode D3 when an AC input is presented, which is divided and applied to the base of a Tr Q2. Thus, even if the oscillator 6 is oscillated by the momentary breakage of a commercial power source, the detector 9 does not detect the presence of an AC input. Accordingly, the Tr Q2 is turned OFF, and even if a power source is turned again ON, it can prevent a rush current.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an improvement in an inrush current prevention circuit for a switching power supply.

[Prior art and its problems]

A conventional switching power supply using a commercial power supply has a smoothing capacitor C1 having a large capacitance connected after a rectifier circuit 1 for rectifying AC input, as shown in the circuit diagram of FIG. When commercial power is first turned on to input terminal 2.3, or -
In order to prevent the inrush current that transiently flows into the capacitor C1 when the commercial power is turned on again after being temporarily cut off, the AC input power line 4 is equipped with a resistor R1 and a thyristor S1 to prevent inrush current. A rush current prevention circuit 5 connected in parallel is provided.

The thyristor S1 of this circuit 5 is connected to the thyristor S1 of the power transformer T1.
The oscillation state of the oscillation circuit 6 formed by connecting the main current path of the next winding ILI and the switching transistor Q1 in series is detected by the winding L2, and when the detection output is present, it is triggered and turned on, so the AC input is The zero oscillation state flowing through the thyristor S1 is such that the DC power obtained by the capacitor CI is interrupted by the transistor Q1.

Note that the resistor R2, the resistor R3, and the diode DI are circuit elements for obtaining a trigger voltage to be applied to the gate of the thyristor S1 from the voltage generated in the winding L2 in the power transformer T1.

Therefore, when commercial power is turned on for the first time, or even if it is temporarily cut off, if it is turned on after the voltage of capacitor C1 has decreased and oscillation has stopped, thyristor S1 is "off" and AC input flows through the rectifier circuit 1 through the resistor R1, so no large rush current flows through the capacitor C1.

However, it is not possible to prevent the inrush current that occurs when recovering from a commercial power supply cutoff for a very short time during which oscillation does not stop.

In other words, even if the AC input is lost for a short time, a voltage is generated by the accumulated charge in the capacitor C1, and it is generally designed so that the oscillation state continues even if this voltage drops below normal. be. Therefore, capacitor C1
This is because if the commercial power supply is turned on with the voltage considerably lower than normal, an inrush current may occur even in the oscillation state.

[Problem to be solved by the invention]

An object of the present invention is to provide a switching power supply that can prevent rush current even when the commercial power supply is cut off for a short time during which the oscillation of the oscillation circuit does not stop.

[Means to solve the problem]

The switching power supply of the present invention includes at least an AC input rectifier circuit, a smoothing capacitor connected to the output side of the rectifier circuit,
An oscillation circuit in which the primary winding of a power transformer and the main current path of a switch element are connected in series and the smoothed DC output is intermittent by the switch element, and the oscillation state is detected by coupling with the primary winding of the oscillation circuit. It consists of a winding and a detection circuit that detects AC input, and a resistor and a thyristor are connected in parallel on the input side of the rectifier circuit, and the thyristor is activated only when detection signals from both the winding and the detection circuit are present. It is characterized by having an inrush current prevention circuit that turns "on."

Still another switching power supply of the present invention includes at least an AC input rectifier circuit, a smoothing capacitor connected to the output side of the rectifier circuit, a primary winding of a power transformer, and a main current path of a switching element connected in series to obtain smoothed DC current. An oscillation circuit whose output is intermittent by a switch element, a winding that is coupled to the primary winding of the oscillation circuit to detect its oscillation state, and a detection system that detects that the voltage of the smoothing capacitor is higher than the voltage that starts oscillation. The circuit consists of a resistor and a thyristor connected in parallel between the rectifier circuit and the smoothing capacitor, and the inrush current prevention circuit turns on the thyristor only when a detection signal from both the winding and the detection circuit is present. It is characterized by having a circuit.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a switching power supply according to the present invention will be described below with reference to a circuit diagram shown in FIG. Note that the same parts as in FIG. 3 are given the same reference numerals.

Also in FIG. 1, a smoothing capacitor C1 is connected after the rectifier circuit 1 that rectifies the AC input.

The smoothing capacitor CI has the primary t of the power transformer T1.
An oscillation circuit 6 is connected in series between the line Ll and the collector-emitter, which is the main current path of the switching transistor Q1, and its oscillation state is controlled by a signal from the terminal 7 applied to the base of the transistor Q1. be done. The control method may be passive or automatic (any known method may be used, such as the secondary winding of the power transformer TI,
The rectifier circuit and the like connected to the secondary winding when obtaining DC output are not essential parts of the present invention and are therefore not shown.

An AC input power line 4 connected to the input terminal 3 is provided with a rush current prevention circuit 5 in which a resistor R1 and a thyristor S1 are connected in parallel. The gate of the iris resistor S1 is connected to one end of the winding L2 of the power transformer T1 through a resistor R3 and a diode D1, and the other end of the winding L2 is connected to the power supply line 4 through a resistor R2. Connect to the power supply line 4 through the collector and emitter of Q2.

The resistor R2, the resistor R3, and the diode D1 are circuit elements for obtaining the l.rega voltage to be applied to the gate of the thyristor Sl from the voltage generated in the winding L2 coupled to the primary winding L1 in the power transformer T1. .

A series circuit of a diode D2, a resistor R4, and a resistor R5 is connected between the AC input power supply lines 8 and 4, and the resistor R4
A Zener diode D3, a capacitor C2, and a series circuit consisting of a resistor R6 and a resistor R7 are connected in parallel between the connection point of the resistor R5 and the power supply line 4, respectively. A connection point between resistor R6 and resistor R7 is connected to the base of transistor Q2.

The part of the circuit surrounded by this dotted line, which is connected between the power supply lines 8.4 of the AC input, obtains a DC voltage much lower than the AC input set by the Zener diode D3 when the AC input is present, and when this DC input voltage to resistor R6 and resistor R
A detection circuit 9 detects an AC input divided by 7 and applied to the base of the transistor Q2. Transistor Q2 is "on" because the low voltage obtained by the detection circuit 9 is applied to its base, regardless of whether the AC input is high or low.

The switching power supply of the present invention configured as described above operates as follows.

When an AC input is first applied, the detection circuit 9 generates a signal indicating the presence of the AC input, ie a DC voltage, turning transistor Q2 "on".

However, since the oscillation circuit 6 has not yet started oscillation,
Winding L2 coupled to primary winding L1 in power transformer T1 does not detect its oscillation state.

Thyristor S1 is then "off" since no trigger voltage is applied to its gate, so the AC input flows through resistor R1 to rectifier circuit l and no inrush current occurs in capacitor C1.

Then, when the voltage of the capacitor C1 rises to the voltage at which the oscillation circuit 6 starts oscillating and enters the oscillation state, the winding L
2 detects the oscillation state of the primary winding L1 and generates a voltage as a detection signal. This voltage is applied as a trigger voltage to the gate of thyristor S1, so that thyristor S1 turns "on" and the AC input flows through thyristor S1.

Next, even if the commercial power supply is cut off momentarily and the oscillation circuit 6 is still in an oscillating state, the detection circuit 9 will not detect the presence of AC input (so the transistor Q2 will be turned off. Therefore, the winding Even if there is voltage on L2, thyristor S1
is "off" and the AC input passes through resistor R1. During this time, regardless of whether the voltage of the capacitor C1 is at a voltage that allows the oscillation circuit 1 to continue oscillating or is at a lower voltage,
When the AC input is removed and the commercial power is turned on again, the AC input flows through the resistor R1, so no inrush current occurs. Note that since the capacitance value of the capacitor C1 is large, it goes without saying that the detection circuit 9 detects the interruption of the commercial power supply much faster than the voltage drop.

FIG. 2 is a circuit diagram showing another embodiment of the switching power supply of the present invention. The same parts as in FIG. 1 are given the same reference numerals.

Also in Fig. 2, a smoothing capacitor C1 is connected after the rectifier circuit 1 that rectifies the AC input, and the primary winding L1 of the power transformer T1 and the main current path of the switching transistor Q1 are connected in series to the smoothing capacitor C1. The oscillation circuit 6 formed by the connection is connected. The oscillation state may be controlled by any known method, either passive or automatic.

The secondary winding of the power transformer TI and the rectifier circuit connected to the secondary winding when obtaining DC output are not essential parts of the present invention, and therefore are not shown in the figure as in the case of Fig. 1. It is.

An inrush current prevention circuit 1o, in which a resistance R1 for inrush current prevention and a thyristor s2 are connected in parallel, is provided in the hot side power supply line 11 between the output side of the rectifier circuit 1 and the smoothing capacitor 01, and is connected to the input of the rectifier circuit 1. The side is connected to the input terminal 2.3 via a power supply line 8.4.

The gate of thyristor S2 is connected to diode D5 and resistor R1.
3 to one end of the winding L2 of the power transformer T1, and to the power supply line 11 via a resistor R14. The other end of the winding L2 is connected to the power supply line 11 via the collector and emitter of the transistor Q4. Resistor R13,
Resistor R14 and diode D5 are circuit elements for obtaining a trigger voltage to be applied to the gate of thyristor S2 from the voltage generated in winding L2 coupled to primary winding L1 in power transformer T1.

A series circuit of a resistor R8 and a resistor R9 is connected between the DC hot side power line 11 and the cold side power line 12, and the connection point between the resistors R8 and R9 and the power line 12 are connected.
A series circuit of a Zener diode D4 and a resistor RIO is connected between them. Furthermore, the base of the transistor Q3 is connected to the connection point between the Zener diode D4 and the resistor RIO, and its collector is connected to the transistor Q4 through the resistor R12.
The emitter is connected to the power supply line 12. A resistor R11 is connected between the base and emitter of the transistor Q4.

Hot side power line 11 and cold side power line 1
The circuit part connected between 2 and surrounded by this dotted line is connected to the capacitor C at a value higher than the voltage at which the oscillation circuit 6 starts oscillating.
When a voltage of 1 is present, a base voltage is generated to turn on transistor Q4, and capacitor C
This is a detection circuit 13 that detects that the voltage of I is higher than the voltage at which the oscillation circuit 6 starts oscillating.

The switching power supply of the present invention configured as described above operates as follows.

Until the voltage on capacitor CI reaches the voltage at which oscillation begins when commercial power is first applied, circuit portion 13 does not produce a detection signal, transistor Q4 is "off", and winding L2 also has oscillation. does not generate a voltage to be detected. Therefore, since thyristor S2 is "off", the DC input from rectifier circuit 1 flows through resistor R1.

When the voltage of capacitor C1 reaches the voltage that starts oscillation,
The detection circuit 13 generates a detection signal to switch the transistor Q4 to "
When the thyristor S2 is turned on, a voltage is also generated in the winding L2, and a trigger voltage is applied to the gate of the thyristor S2, turning it on. The DC input then flows through thyristor S2.

Next, even if the commercial power supply is cut off momentarily and the oscillation circuit 6 is still in an oscillating state, the voltage of the capacitor C1 is normally set higher than the voltage that stops oscillation, and exceeds the voltage that starts oscillation. When the voltage drops, transistor Q4 turns "off", the trigger voltage is cut off, and thyristor S2 turns "off". And since the DC input flows through resistor R1,
Even if commercial power is turned on again, no inrush current will occur.

The detected value of the voltage of the capacitor C1 by the detection circuit 13 is
It may be set to any value higher than the voltage at which the oscillation circuit 6 starts oscillation. As described above, the voltage for stopping oscillation is set lower than the voltage for starting oscillation, so detecting the vicinity of the voltage for stopping oscillation cannot achieve the purpose of preventing inrush current.

In each of the embodiments, the oscillation circuit is of a single-stone type, but a two-stone type such as a half-bridge type may be used, and there is no need to limit the circuit configuration.

〔effect〕

As described above, in the switching power supply of the present invention, the thyristor connected in parallel with the inrush current prevention resistor is in an oscillating state, and the smoothing capacitor on the output side of the AC input or rectifier circuit starts oscillating. It has an inrush current prevention circuit that turns on only when the voltage is above that level.

If the commercial power supply is cut off momentarily, disappearance of AC input or drop in voltage of the smoothing capacitor will be detected, so even if the oscillation state is '1akt-b', the input flowing to the smoothing capacitor will be used to prevent inrush current. Since the current flows through the resistance, no inrush current occurs.

Note that when detecting AC input, even if the voltage of the smoothing capacitor continues to drop, if AC input is present and the oscillation state continues, the thyristor is "on".

When detecting the voltage of a smoothing capacitor, the thyristor turns ``off'' when the voltage drops beyond a certain set value above the voltage that starts oscillation, and when the oscillation state m'tE occurs at that low voltage, an inrush current occurs. The prevention resistor may generate heat. This is convenient when detecting AC input because there is no such fear.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply according to the present invention, FIG. 2 is a circuit diagram showing another embodiment of the present invention, and FIG. 3 is a circuit diagram showing a conventional switching power supply. 1: Rectifier circuit 2.3: Input terminal C1: Smoothing capacitor Q1 switching transistor Ll:
Primary winding L1 winding

Claims (2)

[Claims] (1) Oscillation in which the AC input rectifier circuit, the smoothing capacitor connected to the output side of the rectifier circuit, the primary winding of the power transformer, and the main current path of the switch element are connected in series, and the smoothed DC output is intermittent by the switch element. A circuit, a winding that is coupled to the primary winding of the oscillation circuit to detect its oscillation state, a detection circuit that detects AC input, and a resistor and a thyristor connected in parallel to the input side of the rectifier circuit, and the winding The thyristor is turned “on” only when the detection signals of both the detection circuit and the detection circuit are present.
A switching power supply characterized by having an inrush current prevention circuit. (2) Oscillation in which the AC input rectifier circuit, the smoothing capacitor connected to the output side of the rectifier circuit, the primary winding of the power transformer, and the main current path of the switch element are connected in series, and the smoothed DC output is intermittent by the switch element. A circuit, a winding that is coupled to the primary winding of the oscillation circuit to detect its oscillation state, a detection circuit that detects that the voltage of the smoothing capacitor is higher than the voltage that starts oscillation, and a rectifier circuit and a smoothing capacitor. A switching power supply comprising a resistor and a thyristor connected in parallel between them, and an inrush current prevention circuit that turns on the thyristor only when detection signals from both the winding and the detection circuit are present.