JPH07106057B2 - Switching power supply circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit, and more particularly to a switching power supply circuit having a capacitor for smoothing an input power supply and a precharge means for suppressing a charging current when the input power supply of the capacitor is applied. Regarding the power circuit.

[Conventional technology]

An example of a conventional switching power supply circuit is shown in FIG. The input side of the switching unit 5 is connected to the input power supply 1 via the input smoothing capacitor 3 and the precharge unit 2 to receive power. The switching unit 5 operates under the control of the output control circuit 51 that operates by the power supply from the auxiliary power supply 4,
The output of the received power is converted by the transformer 54 and the rectifying / smoothing circuit 55 and supplied to the load 7.

In order to suppress the disturbance of the input power supply line due to the inrush charging current to the input smoothing capacitor 3 at the start of power supply from the input power supply 1 and prevent the influence on other devices, the precharge resistor 25 of the precharge unit 2 Limits the charging current. The completion time of charging is detected by the timer resistor 21 and the timer capacitor 22, and the precharge switch FET 24 to which the bias resistor 23 is connected is driven to short-circuit the precharge resistor 25. Enable the function of.

[Problems to be Solved by the Invention]

In the conventional switching power supply circuit described above, since the auxiliary power supply 4 having the timer resistor 41 and the constant voltage diode 42 for driving the output control circuit 51 of the switching unit 5 is directly connected in parallel with the input smoothing capacitor 3, Even when the terminal voltage of the capacitor 3 is smaller than the clamp voltage of the constant voltage diode 42, a bias is applied to the output control circuit 51, and the switching unit 5 is started in a state where the input voltage is insufficient, which causes a start error or the main switch 52. May cause destruction of.

In order to prevent this, normally, the output of the output control circuit 51 itself is delayed by inserting the timer capacitor 43 in parallel with the constant voltage diode 42 to stabilize the auxiliary power supply 4 and delaying the rise of the voltage applied to the output control circuit 51. At the time when the voltage of the constant voltage diode 42 rises to the input detection level of the input voltage, the voltage of the input smoothing capacitor 3 reaches the operating range of the switching unit 5, and the power consumption increases when the output control circuit 51 starts up. It is configured such that it is self-excited and compensated through the auxiliary power supply rectifying diode 53 by its own operation.

Therefore, the operating condition of this circuit requires that the two time constants have the following relationship.

R25 × C3 << R41 × C43 ……… (1) However, R25: resistance value of precharge resistor 25 C3: essential value of input smoothing capacitor 3 R41: resistance value of timer resistor 41 C43: capacitance of timer capacitor 43 In a high-power power supply, the capacity of the input smoothing capacitor 3 increases, so it is difficult to keep equation (1), and
If the capacity of the timer capacitor 43 is increased and the rise of the voltage is delayed too much, the auxiliary power supply cannot be secured by self-excitation after the startup, and there is a drawback that it will not be started.

An object of the present invention is to provide a switching power supply circuit which can be stably activated regardless of the capacity of a capacitor for smoothing an input power supply and which prevents a failure of an internal circuit.

[Means for Solving the Problems]

A switching power supply circuit of the present invention comprises a capacitor for smoothing power supply from an input power supply, a precharge means for suppressing a charging current of the capacitor for a predetermined time when the input power supply is applied, and an input power supply via the capacitor. Switching means for performing output conversion control of the supplied power to the load, an auxiliary power supply for supplying driving power to the switching means, and the auxiliary power supply in conjunction with the charging current suppressing operation of the precharge means. And a start control means for suppressing the start and stop of the power supply.

〔Example〕

 The present invention will now be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. In the switching power supply circuit of the present embodiment, the input side of the switching unit 5 has an input smoothing capacitor 3, a timer resistor 21,
It is connected to the input power source 1 via the timer capacitor 22, the bias resistor 23, the precharge switch FET 24, and the precharge unit 2 having the precharge resistor 25, and the output side of the switching unit 5 is connected to the load 7. The auxiliary power source 4 is connected in parallel with the input smoothing capacitor 3, and its timer resistor 41, constant voltage diode 42 and timer capacitor are connected.
The connection point with 43 is connected to the auxiliary power supply rectifier diode 53, the output control circuit 51, and the collector of the detection transistor 61 of the activation control unit 6 via the transformer 54 of the switching unit 5. The base of the detection transistor 61 is a precharge switch of the precharge unit 2 via a bias resistor 62.
It is connected to the drain of the FET 24, and the emitter of the detection transistor 61 is connected to the input negative electrode together with the source of the precharge switch FET24.

The functions and configurations of the precharge unit 2, the input smoothing capacitor 3, the auxiliary power supply 4, and the switching unit 5 are the same as those in the conventional art, and detailed description of their operations will be omitted.

Now, when the input power supply 1 is turned on, the timer capacitor 22 of the precharge unit 2 is charged from "0", so that the voltage of the gate of the precharge switch FET24 connected thereto reaches the threshold voltage. Precharge switch FET24
Is off, and the input smoothing capacitor 3 is charged through the precharge resistor 25. On the other hand, the detection transistor 61 of the activation control unit 6 is turned on during a period (precharge time) during which the charging current flows through the precharge resistor 25 and the voltage drop thereof is the V _BE voltage or more (precharge time). , The timer capacitor 43 is not charged and its terminal voltage remains '0'.

The charging voltage of the timer capacitor 22 is the precharge switch
When the gate threshold voltage of FET24 is reached, the precharge switch FET24 turns on, and at this point the detection transistor 6
Since the base-emitter of 1 is short-circuited, the detection transistor 61 is turned off, the auxiliary power supply 4 starts charging the timer capacitor 43 through the timer resistor 41, and the output control circuit 51
The switching unit 5 is activated at the time when the input detection voltage is exceeded.

As described above, the charging time constant of the timer capacitor 43 can be set independently of the precharge time of the precharge unit 2, and the charging voltage of the input smoothing capacitor 3 is sufficient at the time when the precharge switch FET24 is turned on. Since it has reached the upper limit, the switching power supply circuit is stably activated, and the main switch 52 is not destroyed.

〔The invention's effect〕

As described above, according to the present invention, after the operation of suppressing the charging current of the capacitor for smoothing the input power supply by the precharge means is finished, the power supply from the auxiliary power supply to the switching means is started to control the output conversion of the power supplied from the input power supply. So do
It is possible to stably start up regardless of the capacity of the capacitor for smoothing the input power source, it is possible to reduce the timer capacitor of the auxiliary power source, and it is possible to prevent the failure of the internal circuit.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing an example of a conventional switching power supply circuit. 1 ... Input power supply, 2 ... Precharge unit, 3 ... Input smoothing capacitor, 4 ... Auxiliary power supply, 5 ... Switching unit, 6 ... Startup control unit, 7 ... Load, 21 ... Timer resistor , 22 …… Timer capacitor, 23 …… Bias resistor,
24 …… precharge switch FET, 25 …… precharge resistor, 41 …… timer resistor, 42 …… constant voltage diode, 43 …… timer capacitor, 51 …… output control circuit, 52
...... Main switch, 53 …… Auxiliary power rectifier diode, 54…
… Transformer, 55 …… Rectification smoothing circuit, 61 …… Detection transistor, 62 …… Bias resistor.

Claims (1)

[Claims] 1. A capacitor for smoothing power supply from an input power supply, precharging means for suppressing a charging current of the capacitor for a predetermined time when the input power supply is applied, and power supplied from the input power supply via the capacitor. Switching means for performing output conversion control to supply the load to the load, an auxiliary power supply for supplying driving power to the switching means, and a supply of the auxiliary power supply interlocked with the charging current suppressing operation of the precharge means. A switching power supply circuit comprising a start control means for controlling start and stop.