CN103051166A - Soft start circuit for switching power supply - Google Patents

Abstract

The invention discloses a soft start circuit for a switching power supply, and the soft start circuit comprises a benchmark current module, a current mirror module, a switch, a capacitor, a comparator, a benchmark voltage module, a first P-channel metal oxide semiconductor (PMOS) switch transistor and a second PMOS switch transistor. The switch drops the potential of the capacitor to the ground before the electrification, and the benchmark current module charges an upper polar plate of the capacitor through the current mirror module after the electrification. When the potential of the upper polar plate of the capacitor is smaller than the benchmark voltage, the comparator outputs low level, the first PMOS switch transistor is conducted, and the output end of the circuit is the potential of the upper polar plate of the capacitor; and when the potential of the upper polar plate of the capacitor is greater than the benchmark voltage, the comparator outputs the high level, the second PMOS switch transistor is conducted, and the output end of the circuit is the benchmark voltage. Due to the adoption of the soft start circuit, the current overshoot phenomenon can be eliminated, so that a chip is not damaged, the soft start effect is good, the circuit is simple in structure, and the area of the chip is saved.

Description

The soft starting circuit that is used for Switching Power Supply

Technical field

The present invention relates to field of switch power in the analog integrated circuit, particularly relate to a kind of soft starting circuit for Switching Power Supply.

Background technology

Along with popularizing rapidly of portable electric appts, switching power source chip is widely applied because it has the high characteristics of energy conversion efficiency.In the job applications of switching power source chip, because the phenomenon of current over pulse is arranged in power up, can cause chip to damage; So, must design soft starting circuit and eliminate this phenomenon.Existing various soft starting circuit has complex structure, works unreliable and the shortcoming such as chip occupying area is large.

Summary of the invention

The technical problem to be solved in the present invention provides a kind of soft starting circuit for Switching Power Supply, can eliminate the current over pulse phenomenon, and chip is not damaged.

For solving the problems of the technologies described above, soft starting circuit for Switching Power Supply of the present invention, comprise: a current mirror module that is formed by nmos pass transistor M1, M2, the source electrode of nmos pass transistor M1, M2 meets power vd D, and the grid of the drain and gate of nmos pass transistor M2 and nmos pass transistor M1 is connected with the reference current module; The drain electrode of nmos pass transistor M1 is connected with an end of switch S 1 and the top crown of capacitor C 1; The bottom crown ground connection of the other end of switch S 1 and capacitor C 1;

One comparator BJ, its inverting input is connected with the reference voltage module; Its normal phase input end is connected with the drain electrode of nmos pass transistor M1 and the source electrode of PMOS switching transistor M3; The output of this comparator BJ is connected with the input of inverter FX1;

The grid of PMOS switching transistor M3 is connected with the output of inverter FX1, and the drain electrode of PMOS switching transistor M3 is connected with the source electrode of PMOS switching transistor M4, and as the output of circuit;

The input of inverter FX2 is connected with the output of inverter FX1; The output of inverter FX2 is connected with the grid of PMOS switching transistor M4; The drain electrode of PMOS switching transistor M4 is connected with described reference voltage module.

Owing to adopt the soft starting circuit for Switching Power Supply of the present invention, utilize the mode to the charging of capacitor C 1 top crown, in power up, obtain a gradual reference voltage output; So that switching power source chip internal reference voltage rising in power up, thereby eliminate the current over pulse phenomenon, the protection chip is not damaged.

Compared with prior art, soft starting circuit soft start of the present invention is effective, and circuit structure is simple, and can effectively save the area of chip.

Description of drawings

The present invention is further detailed explanation below in conjunction with accompanying drawing and embodiment:

Accompanying drawing is the soft starting circuit one embodiment schematic diagram for Switching Power Supply.

Embodiment

In the job applications of switching power source chip, because the phenomenon of current over pulse is arranged in power up, therefore can cause chip to damage.The present invention can effectively address the above problem, so that switching power source chip internal reference voltage rising in power up, thereby eliminate the current over pulse phenomenon, the protection chip is not damaged.

Shown in accompanying drawing, in the present embodiment, described soft starting circuit for Switching Power Supply comprises: a current mirror module that is comprised of nmos pass transistor M1, M2, the reference current module, switch S 1, capacitor C 1, one comparator BJ, inverter FX1, FX2, PMOS switching transistor M3, M4, reference voltage module.

Switch S 1 is moved the current potential of capacitor C 1 to ground before circuit powers on, circuit powers on rear reference current module by the top crown charging of current mirror module to capacitor C 1.When the current potential of the top crown of capacitor C 1 during less than reference voltage, described comparator BJ output low level, PMOS switching transistor M3 conducting, output Vo is the current potential of the top crown of capacitor C 1; When the current potential of the top crown of capacitor C 1 during greater than reference voltage, described comparator BJ output high level, PMOS switching transistor M4 conducting, output Vo is reference voltage.

In the present embodiment, give the bias current size I of capacitor C 1 top crown charging and the time length that capacitance C has determined soft start.By the CV characteristic of electric capacity as can be known:

$C\frac{\mathrm{dV}}{\mathrm{dt}}=I---\left(1\right)$

Because bias current is steady state value, so the time of soft start can be expressed as:

$t=\frac{{\mathrm{CV}}_{\mathrm{ref}}}{I}---\left(2\right)$

Power supply electrifying comes control switch S1 by the short pulses of tens nanoseconds in a flash, so that S1 was carved at 0 o'clock in this time period of tens nanoseconds is closed, thereby moves capacitor C 1 top crown current potential to ground.After the short pulse, switch S 1 disconnects, the bias current that the reference current of 5 μ A (microampere) becomes one road 50nA after through the current mirror module that is comprised of nmos pass transistor M1 and M2 in proportion charges to capacitor C 1 top crown, and the current potential of capacitor C 1 top crown is lifted to V from 0 _RefIn this time period, described comparator BJ output low signal, PMOS switching transistor M3 is opened, and output Vo keeps idiostatic with capacitor C 1 top crown, is lifted to greater than V when the current potential of capacitor C 1 top crown is recharged _RefThe time, the high signal of comparator BJ upset output, this moment, PMOS switching transistor M4 was opened, and output Vo equals V _Ref, soft start-up process finishes.

Although the present invention utilizes specific embodiment to describe, the explanation of embodiment is not limit the scope of the invention.The one skilled in the art is by with reference to explanation of the present invention, in the situation that does not deviate from the spirit and scope of the present invention, carries out easily various modifications or can make up embodiment.In the situation that does not break away from the principle of the invention, many distortion and improvement that those skilled in the art makes also should be considered as protection scope of the present invention.

Claims (2)

1. a soft starting circuit that is used for Switching Power Supply is characterized in that, comprising:

One current mirror module that is formed by the first nmos pass transistor and the second nmos pass transistor, the source electrode of the first nmos pass transistor and the second nmos pass transistor connects power supply, and the grid of the drain and gate of the second nmos pass transistor and the first nmos pass transistor is connected with the reference current module; The drain electrode of the first nmos pass transistor is connected with an end of a switch and the top crown of an electric capacity; The bottom crown ground connection of the other end of described switch and described electric capacity;

One comparator, its inverting input is connected with the reference voltage module; Its normal phase input end is connected with the drain electrode of the first nmos pass transistor and the source electrode of a PMOS switching transistor; This comparator output terminal is connected with the input of the first inverter;

The grid of the one PMOS switching transistor is connected with the output of the first inverter, and the drain electrode of a PMOS switching transistor is connected with the source electrode of the 2nd PMOS switching transistor, and as the output of circuit;

The input of one second inverter is connected with the output of the first inverter; The output of this second inverter is connected with the grid of the 2nd PMOS switching transistor; The drain electrode of the 2nd PMOS switching transistor is connected with described reference voltage module.

2. soft starting circuit as claimed in claim 1 is characterized in that: when the current potential of the top crown of described electric capacity during less than reference voltage, circuit output end is the current potential of the top crown of this electric capacity; When the current potential of the top crown of described electric capacity during greater than reference voltage, circuit output end is reference voltage.

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