JPS631324A - Rush-current limiting circuit of switching source - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] [Technical field to which the invention pertains]

The present invention rectifies the AC input and converts the DC into one of the transformers.
In a so-called AC input type switching power supply circuit, which opens and closes switching via the secondary winding, thereby rectifying the voltage generated in the secondary winding of the transformer and supplying it to the load, rush current from the AC power supply is controlled. Regarding limiting circuits. Note that in the following figures, the same reference numerals indicate the same or corresponding parts.

[Prior art and its problems]

In this type of AC input type switching power supply circuit, immediately after the AC power is turned on, a large current (rush current) flows to charge the smoothing capacitor on the next side of the transformer. Conventionally, as a method of suppressing such rush current, a current limiting method is known in which a resistor or a thermistor is inserted in series with the smoothing capacitor. However, the inrush current prevention method using a resistor has the problem that although increasing the resistance value reduces the inrush current, the capacitor inflow current flows through this resistor even during steady operation, resulting in an increase in power loss. Furthermore, in the method using a thermistor, the inrush current can be limited by its large resistance value when it is cold (when the power is turned on), and the resistance value becomes small when it is hot (during steady operation), so that power loss can be reduced. However, even with this method, if the power is turned on and off repeatedly in a short period of time, due to the thermal time constant of the thermistor, it takes time for the resistance value that once decreased after the power is turned on to increase again after the power is turned off, resulting in an inrush current. The problem is that it is no longer possible to limit

[Purpose of the invention]

This invention solves the above-mentioned problems in an AC input type switching power supply circuit, and provides an inrush current limiting circuit that can reliably limit the inrush current from an AC power supply even when the AC power supply is repeatedly turned on and off in a short period of time. The purpose is to provide.

[Key points of the invention]

The key point of the present invention is that the DC output obtained by rectifying the AC input through the input side rectifier circuit (primary side rectifier circuit, etc.) is repeated through the first winding (primary winding, etc.) of the transformer. A switching power supply that is equipped with a switching means (switching transistor, etc.) that opens and closes, and that rectifies the voltage generated by the second winding (secondary winding, etc.) of the transformer (via a secondary side rectifier circuit, etc.) and supplies it to the load. In the circuit, a capacitor (
A series circuit of a primary side smoothing capacitor (such as a primary side smoothing capacitor) and a first diode (such as Dl) having a polarity that prevents charging of the capacitor by this DC output is connected, and a third diode of the transformer is connected in parallel with the capacitor. Winding wire (3
a second diode (D
2, etc.), the smoothing capacitor after AC input rectification is not directly charged by the rectified voltage of this AC input, but the so-called flyback current generated in the transformer when the switching means is turned off is The main advantage of this method is to prevent the temporary large current (rush current) that conventionally occurs to charge a smoothing capacitor when an AC power source is turned on.

[Embodiments of the invention]

The present invention will be explained in detail below based on FIGS. 1 and 2. Figures 1 (A) and (B) are configuration circuit diagrams of different embodiments of the present invention, and (1) and (2) in each figure (A) and (B) represent the forward method and flyback method, respectively, which will be described later. The circuit on the secondary side of the transformer for this method is shown. Also, Figure 2 is a waveform diagram to explain the operation of Figure 1.
Figures (1) and (2) are waveform diagrams for the forward method and flybank method, respectively. In FIG. 1(A), 1 is an AC input, and 2 is a primary side rectifier circuit for rectifying this AC human power 1, which is composed of a diode bridge in this example. 3 is a transformer, and 31.32 (3
2L322), 33 are the primary windings of the transformer 3,
These are the secondary winding and the tertiary winding. In addition, Fig. 2 (1), (2
1, (a), (11), (, c) are the currents of these windings 31 and 32.33, respectively, and are the primary currents II. Secondary current12. It is called flyback current IFB. Tr is a switching transistor as a switching means that switches the DC output of the primary rectifier circuit 2 through the primary winding 31 of the transformer 3, and 4A is a switching control circuit that controls the opening and closing of the transistor Tr. 6 (61, 62) are respectively forward method (f
il) and the secondary side rectifier circuit in the flyback method ((2) in the same figure), each of which is connected to the secondary winding 32 of the transformer 3.
The generated voltage of 1.322 is rectified and smoothed and supplied to the load as a DC cuff. Note that the forward method (1) means that when the switching transistor Tr is on (that is, the primary current 11 of the transformer 3 is flowing) as shown in FIG. 2 (1), the secondary winding 321 of the transformer 3 is This is a method in which a secondary current I2 is supplied to the load side, and the flyback method (2) is a method in which the switching transistor Tr is off (i.e., when the primary current In this method, a secondary current I2 is supplied from the secondary winding 322 of the transformer 3 to the load side in the energized state). Next, C1 is a primary side smoothing capacitor, and Di is a diode inserted in series with this capacitor C1. This diode D1 prevents a rush current from flowing into the capacitor C1 from the AC power source 1 side via the primary side rectifier circuit 2, and also prevents the DC output voltage of the primary side rectifier circuit 2 from being lower than the voltage across this capacitor C1. In this state, instead of the AC power 1, DC power is supplied from the capacitor CI to the switching transistor Tr. By the way, in this smoothing capacitor C1, as shown in FIG. 2, when the switching transistor Tr is on, the magnetic energy stored in the transformer 3 is transferred to the resistor R1 → tertiary winding 33 → diode D2 → capacitor C1 when the transistor Tr is off. It is charged by a so-called flyback current IFB as it is discharged in the path. Here, the diode D2 prevents the capacitor C1 from discharging toward the tertiary winding 33 when the transistor Tr is turned on. In the embodiment shown in FIG. 1(A), charging of the smoothing capacitor C1 is performed by switching the transistor Tr from a state where the charging voltage Vc is oV. is low. Therefore, since it takes time to release the flybank current IFB to the capacitor cl side, it is necessary to keep the transistor Tr in an off state until the release is completed. Therefore, the flyback current IFB is detected by the resistor R1 and the comparator 5, and its output is fed back to the switching control circuit 4A and the transistor T
I try to keep r in the off state. Due to the above operation, the charging voltage Vc of the smoothing capacitor CI gradually increases from immediately after the AC power is turned on, and the charging voltage Vc of the smoothing capacitor CI gradually increases.
The inflow current to I (fly bank current TFB) and the primary side + line 1 from this capacitor CI via diode D1.
When the current IC flowing to P is balanced, the voltage of capacitor c1 is kept approximately constant. Next, in the case of the embodiment shown in FIG. 1(B), instead of the flyback current detection circuit (resistor R1, comparator 5) shown in FIG. 1(A), the smoothing capacitor C1 is
A high resistance R2 for charging by the DC output of the primary side rectifier circuit 2, and a voltage Vc across the smoothing capacitor C1O.
The circuit has a circuit configuration in which a circuit (resistors R3 and R4) for detecting is added. Note that, along with this, the switching control circuit has also been replaced with a new circuit 4B. In the case of this embodiment, the method of charging the smoothing capacitor C1 is switched between the following two methods depending on whether the AC power is turned on or during normal operation. ■ Immediately after the power is turned on: While the smoothing capacitor C1 is being charged from the primary side line βP side through the high resistance R2, the charging voltage Vc becomes 0 ■
The capacitor charging voltage Vc is detected by the switching control port 94B via the resistors R3 and R4, and the switching of the transistor Tr is started gradually until the switching power supply reaches the minimum allowable charging voltage VA at which the switching power supply can supply the rated DC output to the load. be stopped. Note that the maximum inrush current 1i at this time is Ii = V imax / R2 (where V imax is the maximum value of the AC input voltage), so the inrush current can be limited by appropriately selecting the resistor R2. and this resistance R2 during steady operation
losses can also be limited. ■When the charging voltage Vc of the smoothing capacitor C1 is higher than the allowable minimum charging voltage VA: When the charging voltage Vc becomes higher than the voltage VA, the switching control circuit 4B detects this as described above and stops the switching of the transistor Tr. and start its switching operation. Thereafter, the smoothing capacitor C1 is charged by the fly bank current IFB generated when the switching transistor Tr is turned off, similarly to the embodiment shown in FIG. 1(A).

【Effect of the invention】

According to the present invention, the primary side DC output obtained by rectifying the AC input is switched open and closed via the primary winding of the transformer,
In a switching power supply circuit that supplies the rectified voltage on the secondary side of the transformer to the load, a capacitor is connected in series with the primary side smoothing capacitor to prevent inrush current to the smoothing capacitor due to the primary side outflow output. In addition to inserting a diode, the smoothing capacitor is charged by the flyback current through the tertiary winding of the transformer, so that rush current to the smoothing capacitor when the AC power is turned on is suppressed, and the power supply is Even if switching on and off is repeated in a short period of time, the effect of limiting inrush current can be obtained.

[Brief explanation of drawings]

FIGS. 1A and 1B are circuit diagrams showing the configurations of different embodiments of the present invention, and FIG. 2 is a waveform diagram for explaining the operation of FIG. 1. 1: AC input, 2: Primary side rectifier circuit, 3 Nidorance, 3
1: Primary winding, 32 (321, 322): Secondary winding, 33: Tertiary winding, 4A, 4B switching control circuit, 5: Comparator, 6 (61, 62): Secondary rectifier circuit, 7: DC output, C1: Primary side smoothing capacitor, Dl, D2: Diode, R1-R4: Resistor, IFB
: Flybank current. Moeiri Toyamaguchi Fu (1) Fuo 7-do Manbe (237:1 Ipattu 70,000 type

Claims (1)

[Claims] 1) Switching means for repeatedly opening and closing a DC output obtained by rectifying an AC input through an input side rectifier circuit through a first winding of a transformer; In a switching power supply circuit that rectifies the voltage generated by the winding and supplies it to the load, a capacitor is provided between the DC output terminals of the input side rectifier circuit, and a first polarity that prevents charging of the capacitor by the DC output is provided. connecting a series circuit with a diode, and a third winding of the transformer in parallel with the capacitor;
An inrush current limiting circuit for a switching power supply, characterized in that a series circuit is connected to a second diode having a polarity that charges the capacitor by the voltage generated in the winding when the switching means is off.