CN100487995C

CN100487995C - Short-circuit protective circuit

Info

Publication number: CN100487995C
Application number: CNB011380950A
Authority: CN
Inventors: 甘旭; 王满堂
Original assignee: ZTE Corp
Current assignee: ZTE Corp
Priority date: 2001-12-31
Filing date: 2001-12-31
Publication date: 2009-05-13
Anticipated expiration: 2021-12-31
Also published as: CN1430314A

Abstract

A short-circuit protecting circuit is composed of a time delay circuit for holding the sampled output current level of power unit for PWM switching power supply for a certain time and slowly lowering it to become a continuous signal, a short-circuit current threshold setting-up circuit for generating threshold level, an over-current detecting circuit for comparing the output of time delay circuit with the said threshold level and making judgement, and a lagging circuit for forming window comparactor, elongating short-circuit protecting signal, feeding the short-circuit protecting signal back to threshold setting up circuit, and improving the anti-interference power of over-current detecting circuit.

Description

A kind of short-circuit protection circuit

Technical field

The present invention relates to field of switch power, be specifically related to current source PWM (PWM) Switching Power Supply.

Background technology

The application of Switching Power Supply at present more and more widely in the defencive function of Switching Power Supply, generally all needs short-circuit protection circuit, damages power supply and load to prevent that Switching Power Supply operating current when the short circuit is excessive.And the quality of short-circuit protection circuit performance directly affects the reliability of whole power supply.At present, the short-circuit protection circuit in the Switching Power Supply mostly adopts the current peak restricting circuits of pulse width modulating chip inside to detect the operating current on the former limit of transformer, limits the function that its peak value reaches short-circuit protection.Because this scheme just limits the current peak of major loop, frequent always protection and recovery under short-circuit condition, the operating current of major loop is still very big, exists Switching Power Supply short circuit output residual voltage height, and loss is big, the problem that reliability is low.

U.S. Pat 5; 400; adopted a kind of predeterminable short circuit prelibation to recover the short-circuit protection circuit of number of times among 206 " the Short circuit and overload protection circuitwhich allows a predetermined number of reconnection attempts "; use one counter can be set limit the short circuit prelibation and recover number of times and the circuit of time; when protection is moved above after number necessarily is set; protective device is locked; be in guard mode always; this scheme can solve the big problem of Switching Power Supply short circuit loss; but after short circuit is removed, can not recover automatically, and need manually to reset.

Summary of the invention

Technical problem to be solved by this invention is the shortcoming that overcomes prior art, proposes a kind of short-circuit protection circuit, and the operating current and the loss of Switching Power Supply during with the reduction short-circuit protection improve the reliability of power supply, and can resume work automatically after short circuit is removed.

Technical scheme of the present invention is as follows: comprise delay circuit, short circuit current threshold setting circuit, short circuit overcurrent testing circuit and hysteresis circuit; Wherein:

Delay circuit is used for current sampling level maintenance a period of time (this time is by the applied environment decision of power supply) and slow decline with the main loop of power circuit output of PWM Switching Power Supply, becomes a continuous signal;

Short circuit current threshold setting circuit is used to produce threshold level;

The short circuit overcurrent testing circuit is used for the current sampling level of comparison delay circuit output and threshold level and the judgement that short circuit current threshold setting circuit is produced; When the current sampling level surpasses the preset threshold level, produce the short-circuit protection signal; When the current sampling level drop to be lower than threshold level after, cancel the short-circuit protection signal;

The hysteresis circuit forms window formula comparison circuit, prolongs and sends the short-circuit protection signal recovers operate as normal to next time time; And with the short-circuit protection signal feedback of output to short circuit current threshold setting circuit, make short circuit current threshold setting circuit reduce threshold level, and strengthen the antijamming capability of short circuit overcurrent testing circuit.

The short-circuit protection signal can be used to control the power supply of pulse width modulating chip, perhaps drags down the pressure feedback port of pulse width modulating chip, the work of shutdown switch power supply.Behind the short-circuit protection, the current sampling level that delay circuit is exported is in constantly slowly in the decline process, when drop to be lower than threshold level after, over-current detection circuit is cancelled the short-circuit protection signal, Switching Power Supply attempts resuming work; If Switching Power Supply also is in short-circuit condition; protective circuit of the present invention repeats the short-circuit protection process again, changes guard mode over to, if the short circuit of Switching Power Supply is removed; then this protective circuit stops the output short circuit protection signal, recovers the normal operating conditions of Switching Power Supply.

Adopt device provided by the present invention; the current peak restricting circuits of the dependence pulse width modulating chip inside of partly mentioning with background technology is carried out the scheme of short-circuit protection and is compared; because the introducing of time-delay and hysteresis function; the operating current of Switching Power Supply and loss when having reduced short-circuit protection have effectively improved the reliability of supply unit.

Description of drawings

Fig. 1 is a theory diagram of the present invention.

Fig. 2 is the physical circuit figure that realizes apparatus of the present invention.

Embodiment

Below in conjunction with Fig. 1,2 the invention will be further described:

The parallel branch of being made up of first capacitor C 1 among Fig. 2 and resistance R 2 is connected across between the negative electrode of diode VD1 and the ground and has constituted delay circuit 101; The parallel branch that second capacitor C 2 and resistance R 4 formed has constituted threshold setting circuit 102 with being connected across after resistance R 5 is connected between VREF power end and the ground; Hysteresis circuit 104 can be realized by resistance R 3; But short circuit overcurrent testing circuit 103 comparator D1 realize;

The VCC port is for comparator D1 power supply and provide and draw level, and D1 is open-collector comparator, connects the VCC port by pull-up resistor R1, if when using non-open-collector comparator or amplifier, pull-up resistor R1 can remove;

VREF is a stable DC voltage, is used for to 102 power supplies of current threshold initialization circuit;

The anode of diode VD1 in the delay circuit 101 is connected on the main loop of power circuit current sampling resistor or current transformer of Switching Power Supply 100; it among Fig. 2 the current sampling signal that current sampling resistor R6 from the main loop of power circuit of PWM Switching Power Supply obtains this loop; this signal is through behind the delay circuit 101; even the current sampling signal on the R6 disappears; because the integral action of C1 in the delay circuit 101; the current sampling level also can keep a period of time and slowly descend; the current sampling level is sent into over-current detection circuit 103; the comparator D1 place of threshold level in over-current detection circuit 103 that produces with current threshold initialization circuit 102 compares judgement; when the current sampling level on the C1 surpasses the preset threshold level; D1 produces the short-circuit protection signal; output low level; because the effect of hysteresis circuit R3; form window formula comparison circuit, prolonged short-circuit protection to the time of recovering operate as normal once more.Short-circuit protection signal CTL is low level when guard mode; can be used for driving the power supply of triode or metal-oxide-semiconductor control PWM chip; also can be used for dragging down the pressure feedback port (as the COM end) of pulse width modulating chip; the work of shutdown switch power supply; discharge into the recovery threshold value of R3, R4, R5 generation up to C1; comparator just overturns, and is output as high-impedance state, recovers the operate as normal of pulse-width modulation circuit.

Wherein, C1 in the delay circuit 101 and the value of R2 should make delay time longer greatly like this, and the loss of power during to the reduction short-circuit protection has positive role;

Hysteresis circuit R3 influences the reduction degree of the threshold voltage that threshold setting circuit 102 produced, and resistance is more little, and the hysteresis effect is strong more, and the reduction degree of threshold voltage is big more, and short-circuit protection is also long more to the time of recovering.

Disclosed herein have time-delay and the short-circuit protection control circuit of hysteresis function can be widely used in all kinds of switch power supply systems, during particularly to power source short-circuit protection loss little, the occasion that reliability is high.

Claims

1, a kind of short-circuit protection circuit comprises delay circuit (101), short circuit current threshold setting circuit (102), short circuit overcurrent testing circuit (103) and hysteresis circuit (104);

Described delay circuit (101) is used for current sampling level maintenance a period of time and slowly decline with the main loop of power circuit output of PWM Switching Power Supply, becomes a continuous signal;

Described short circuit current threshold setting circuit (102) is used to produce threshold level;

Described short circuit overcurrent testing circuit (103) is used for the current sampling level of comparison delay circuit (101) output and threshold level and the judgement that short circuit current threshold setting circuit (102) is produced; When the current sampling level surpasses the preset threshold level, produce the short-circuit protection signal; When the current sampling level drop to be lower than threshold level after, cancel the short-circuit protection signal;

Described hysteresis circuit (104) forms window formula comparison circuit, prolongs and sends the short-circuit protection signal recovers operate as normal to next time time; And with the short-circuit protection signal feedback of output to short circuit current threshold setting circuit; make short circuit current threshold setting circuit reduce threshold level; and the antijamming capability of enhancing short circuit overcurrent testing circuit; described hysteresis circuit (104) is realized by resistance; one end of this resistance links to each other with the output of described short circuit current threshold setting circuit (102) and the input of described short circuit overcurrent testing circuit (103), and the other end links to each other with the output of described short circuit overcurrent testing circuit (103).