JPH061977B2 - Switching power supply start-up circuit - Google Patents

Description

The present invention relates to a starting circuit for a switching power supply.

[Conventional technology]

FIG. 3 is a circuit diagram showing an example of a conventional switching power supply, from an input power supply (hereinafter E) 1 to a switch (hereinafter SW).
2, a bias resistor (hereinafter R) 5, a Zener diode (hereinafter ZD) 6 and a bypass capacitor (hereinafter CB) 7 are connected in parallel to a power input terminal of a control circuit (hereinafter CTL) 8 and SW2 is turned on to turn on the CTL 8 To start.
The output of CTL8 is a switching transistor (hereinafter Q)
11 is driven to supply power to the primary winding (hereinafter P) of the voltage conversion transformer (hereinafter TR) 10 ', and the output of the secondary winding (hereinafter S) of the TR 10' is a rectifying diode (hereinafter D) 12
And smoothing with a smoothing capacitor (hereinafter CS) 13 to supply power to a load (hereinafter L) 14.

[Problems to be solved by the invention]

In the conventional switching power supply circuit described above, the resistance value of the bias resistor R5 is selected so as to supply the total current required for operating the CTL8, but generally, the current required for the CTL8 is on the order of several tens of mA, and the loss of R5. Is DC4
Since it reaches 1 to 2W for 8V input and 5 to 6W for 100V AC input, power conversion efficiency is greatly reduced especially for small output power sources, and R5 becomes a large power resistor, which causes heat generation and mounting space. Is a problem with.

Recently, the power consumption of the control IC has been reduced, and the power consumption has been reduced to 10 mA or more. However, the loss of the bias resistance at AC input is as large as about 2 W, and the problem has not been solved yet.

[Means for solving problems]

The start-up circuit of the switching power supply of the present invention connects an input power supply to a power supply input terminal of a drive circuit of a switching power supply circuit through a series circuit of a charging capacitor and a bias resistor, and connects a bypass diode to the bias resistor in parallel, The third winding of the voltage conversion transformer connected to the output side of the drive circuit is connected to the power input terminal of the drive circuit through a rectifying diode.

〔Example〕

 Next, the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a starting circuit for a switching power supply according to the present invention.

In FIG. 1, the same constituent elements as those in the conventional example are designated by the same reference numerals as those in FIG. The difference between this embodiment and the conventional example is that a closing capacitor (hereinafter C) is placed between SW2 and R5.
3 is inserted to form a series circuit, a diode (hereinafter DA4) is connected in parallel with R5, and a voltage conversion transformer (hereinafter TR
10) is a point that has a tertiary winding (hereinafter T) and is connected from this T to a power input terminal of the CTL 8 through a rectifying diode (hereinafter D9). That is, the feature of this embodiment is that the loss due to the bias resistor R5 is set to zero, R5 is driven through C3, and after starting, power is supplied from T of TR10 to CTL8.

Next, the operation of this embodiment will be described with reference to FIG.

FIG. 2 is a voltage waveform diagram showing the operation waveform of each part in FIG. 1, where the horizontal axis represents time t and the vertical axis represents voltage V.

First, when SW2 is turned on, an input voltage is applied to R5 as a differential waveform through C3 (the terminal voltage of R5 in FIG. 2 and the time constant is the capacitance of C3 × the resistance value of R5). The input terminal of CTL8 rises with the time constant of R5 and CB7, and ZD6
Clamped at the clamp voltage V _{Z of} and continues. CTL8
The converter operates to raise the output voltage and the output of T of TR10 rises, but this output voltage is set lower than the clamp voltage V _{Z in} order to protect ZD6. Therefore, in the initial state, as shown in FIG. The output voltage of D9 is as shown by the broken line, and the voltage of CTL8 is the clamp voltage V _Z.

When the charge of C3 progresses and the clamp current of ZD6 cannot be maintained, the voltage of CTL8 starts to decrease and becomes the stable voltage _{VA when} it becomes equal to the output voltage of D9, and the normal operation is performed.

Note that E1 may be a rectified AC. Also DA4
Is a bypass diode for accelerating the discharge of C3 when SW2 is off, and the charge charged in C3 is rapidly discharged until Q11 stops. Therefore, even if the interval between turning on after turning off the power is short, No startup failure will occur. Under a use condition in which it takes a long time until the power is turned on again so that the charged electric charge of C3 is naturally discharged after the power is turned off, even if DA4 is deleted, it does not affect the activation of the switching power supply.

〔The invention's effect〕

As described above, according to the present invention, power is supplied to the control circuit of the switching power supply through the capacitor and the resistor, and the third winding of the voltage conversion transformer is used as the power supply for the steady operation, so that the power of the bias resistor is reduced. Since power consumption can be suppressed only at start-up, the power loss at the time of steady operation of the switching power supply is reduced, and since the bias resistor only has to withstand the surge current when the input power is turned on, it can be made into a small power resistor. This has the effect of realizing a highly efficient switching power supply. In addition, since the bypass diode is connected in parallel to the bias resistor, there is an effect that the startup failure of the switching power supply does not occur even when the time until the power is turned on again after the power is turned off is short.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a switching power supply start-up circuit according to the present invention, FIG. 2 is a voltage waveform diagram showing operation waveforms of respective parts in FIG. 1, and FIG. 3 is an example of a conventional switching power supply. It is a circuit diagram shown. 1 ... Input power source (E), 2 ... Switch (SW), 3 ... Input capacitor (C), 4 ... Diode (DA), 5 ... Bias resistance (R), 6 ... Zener diode (ZD), 7
... bypass capacitor (CB), 8 ... control circuit (CT
L), 9, 12 ... Rectifying diode (D), 10, 10 '
... voltage conversion transformer (TR), 11 ... switching transistor (Q), 13 ... smoothing capacitor (CS), 14
... load (L).

Claims (1)

[Claims] 1. A power supply input terminal of a drive circuit of a switching power supply circuit is connected to an input power supply through a series circuit of a making capacitor and a bias resistor, and a bias diode is connected in parallel to the bias resistor to connect to the output side of the drive circuit. A starting circuit for a switching power supply, characterized in that it is configured by connecting a third winding of a connected voltage conversion transformer to a power supply input terminal of the drive circuit through a rectifying diode.