CN101594048B - PWM buck convertor with overcurrent protection function - Google Patents
PWM buck convertor with overcurrent protection function Download PDFInfo
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- CN101594048B CN101594048B CN2009101060776A CN200910106077A CN101594048B CN 101594048 B CN101594048 B CN 101594048B CN 2009101060776 A CN2009101060776 A CN 2009101060776A CN 200910106077 A CN200910106077 A CN 200910106077A CN 101594048 B CN101594048 B CN 101594048B
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Abstract
The invention relates to a PWM buck convertor with overcurrent protection function, which comprises a control circuit, a first switching tube connected between an input power source and an inductor, and a current-limit circuit, wherein the current-limit circuit is used for sampling current signals on the first switching tube and outputting overcurrent signals; the control circuit judges whether the overcurrent signals are greater than a preset current maximum to control the connection and cut off of the first switching tube; and the PWM buck convertor is provided with the current-limit circuit, so that a power tube is cut off when the current is increased to the preset current maximum to protect a chip and ensure the normal work of a switch power supply. Furthermore, a capacitor in an RC circuit outputs sampling voltage so as to avoid the influence of power supply voltage vibration on the current-limit circuit in the direct sampling process; when the overcurrent happens, the cut-off time of the power tube is fixed so as to avoid the repeat starting of the power tube in short time and reduce power consumption; and two shifts of cut-off time are automatically selected according to the output voltage of the buck convertor so as to ensure the stability of the output voltage.
Description
Technical field
The present invention relates to PWM type decompression transformation, more particularly, relate to a kind of PWM type buck converter with overcurrent protection function.
Background technology
Along with in the world wide to the continuous pursuit of energy-conserving and environment-protective, switching power circuit more and more obtains paying attention to as one of microelectric technique important developing direction.Especially in fields such as automotive electronics, Industry Control, LED illuminations, the adjusting and voltage-reduction switch power circuit is widely adopted with its high conversion efficiency, and plays an important role.The adjusting and voltage-reduction switch power circuit can be converted to the constant low power supply of direct current to dc high voltage or constant current is an electric.Except performance will satisfy the requirement of powered products, the safeguard measure of Switching Power Supply self was also extremely important.Overcurrent protection is exactly one of important safeguard measure.When chip is exported situations such as taking over heavy duty or generation output short-circuit, the reduction voltage circuit power switch pipe is opened for a long time, inductance in this process in the topology and the electric current on the power switch pipe can increase so that certain slope is linear.Big electric current flows through power tube and inductance for a long time and can produce a large amount of heats and can not distribute, and finally causes burning of inductance or power switch, causes damage.
Summary of the invention
The technical problem to be solved in the present invention is; flowing through power tube and inductance for a long time at big electric current in the PWM type buck converter of prior art can produce a large amount of heats and can not distribute; finally cause burning of inductance or power switch; defective such as cause damage provides a kind of PWM type buck converter with overcurrent protection function.
The technical solution adopted for the present invention to solve the technical problems is: construct a kind of PWM type buck converter with overcurrent protection function, comprise control circuit, be connected first switching tube between input power supply and the inductance, and current limit circuit, current signal and output overcurrent signal on its described first switching tube that is used to sample, whether control circuit judges described over-current signal greater than the predetermined current maximum, with conducting or the shutoff of controlling described first switching tube;
Described current limit circuit comprises:
Current sampling circuit, the current signal on described first switching tube that is used to sample, and output correspondent voltage signal;
The RC circuit is used to receive the corresponding voltage signal of described current sampling circuit output, and exports corresponding sampled voltage;
First reference voltage source is used to be provided with the maximum current that flows through described first switching tube, and exports first reference voltage;
First comparator is used for the more described sampled voltage and first reference voltage, with the output overcurrent signal to control circuit.
In the PWM type buck converter with overcurrent protection function of the present invention, described current limit circuit also comprises:
Second reference voltage source is used to export second reference voltage;
Second comparator is used for comparison second reference voltage and system feedback voltage, sluggishness whether occurs to judge described first comparator.
In the PWM type buck converter with overcurrent protection function of the present invention; described current sampling circuit comprises first resistance and the second switch pipe between described input power supply and inductance in sequential series; wherein; the connected node of described first resistance and second switch pipe is exported corresponding voltage signal; the resistance of first resistance 〉=600K Ω also≤1M Ω, the breadth length ratio of second switch pipe≤40/1.4.
In the PWM type buck converter with overcurrent protection function of the present invention, the RC circuit comprises the electric capacity and second resistance that is connected in series, and wherein, an end of second resistance is as the input of described RC circuit, and the bottom crown of electric capacity is exported described sampled voltage.
In the PWM type buck converter with overcurrent protection function of the present invention, be parallel with the 3rd switching tube at the two ends of described electric capacity, the voltage of the described electric capacity that is used to reset.
In the PWM type buck converter with overcurrent protection function of the present invention; first reference voltage source comprises the 4th switching tube and a reference source between input power supply and ground in sequential series; wherein; the connected node of described the 4th switching tube and a reference source is exported first reference voltage, the 4th control end of switching tube ground connection.
In the PWM type buck converter with overcurrent protection function of the present invention, be connected with an inverter at the output of described first comparator, be used for the output signal of anti-phase described first comparator, with the output overcurrent signal to described control circuit.
In the PWM type buck converter with overcurrent protection function of the present invention, described first comparator is monolateral controlled hysteresis comparator, and described current limit circuit also comprises:
Level displacement circuit is used for the output signal of described second comparator of level shift;
NAND gate is used to receive the output signal and the over-current signal of described level displacement circuit, to control the lag function of described first comparator.
In the PWM type buck converter with overcurrent protection function of the present invention, described first switching tube, second switch pipe and the 3rd control end of switching tube all are coupled to described control circuit.
In the PWM type buck converter with overcurrent protection function of the present invention, described first switching tube, second switch pipe and the 3rd switching tube are the PMOS power switch pipe.
Implement the PWM type buck converter with overcurrent protection function of the present invention; has following beneficial effect: by in PWM type buck converter, increasing current limit circuit; make switch-off power pipe when electric current rises to the predetermined current maximum; the protection chip, the operate as normal of assurance Switching Power Supply.Further, by electric capacity output sampled voltage in the RC circuit, when having avoided Direct Sampling, the influence that supply voltage shake convection current restricting circuits is brought; The power tube turn-off time is fixed behind the overcurrent, has avoided power tube to open repeatedly at short notice, has reduced power consumption; Size according to the buck converter output voltage is selected two grades of turn-off times automatically, guarantees the stability of output voltage.
Description of drawings
The invention will be further described below in conjunction with drawings and Examples, in the accompanying drawing:
Fig. 1 is the circuit block diagram that the present invention has the PWM type buck converter of overcurrent protection function;
Fig. 2 is the circuit theory diagrams of an embodiment of current limit circuit shown in Figure 1;
Fig. 3 is the circuit theory diagrams of first reference voltage source shown in Figure 2;
Fig. 4 a is the oscillogram of electric capacity bottom crown voltage Vc shown in Figure 2;
Fig. 4 b is the oscillogram of electric capacity bottom crown voltage Vc shown in Figure 2.
Embodiment
As shown in Figure 1, in the PWM type buck converter with overcurrent protection function of the present invention, mainly comprise control circuit, be connected first switching tube 101 between input power supply Vin and the inductance L X112, and related peripheral circuit, in this related peripheral circuit, has fly-wheel diode 111, filter capacitor 113, load resistance 114, and first the feedback divider resistance 115 and second the feedback divider resistance 116, wherein first the feedback divider resistance 115 and second the feedback divider resistance 116 connected node output system feedback voltage; The invention of the technical program is; at PWM type buck converter current limit circuit is set; current signal and output overcurrent signal on its first switching tube 101 that is used to sample; control circuit judges that whether described over-current signal is greater than the predetermined current maximum; with conducting or the shutoff of controlling first switching tube 101; promptly when electric current rises to the predetermined current maximum, turn-off first switching tube 101, this electric current is limited within the predetermined current maximum protects chip, guarantee the operate as normal of Switching Power Supply.
This current limit circuit comprises current sampling circuit 100 and comparison circuit as shown in Figure 1, wherein, sample circuit is used to read the current signal on first switching tube 101 and sampled signal is exported to comparison circuit, by comparison circuit sampled signal and predetermined current maximum are compared, the output overcurrent signal is to control circuit, thereby control circuit judges whether to occur overcurrent according to this over-current signal, then turn-offs first switching tube 101 if overcurrent occurs.
As illustrated in fig. 1 and 2, current sampling circuit comprises first resistance 104 (sampling resistor) and the second switch pipe 102 (sampling switch pipe) between input power supply Vin and inductance L X112 in sequential series, wherein, one end of first resistance 104 is connected with input power supply Vin, the other end is connected with first end of second switch pipe 102, second end of second switch 102 is coupled to inductance L X112, thereby form relation in parallel with first switch 101, the connected node of first resistance and second switch pipe is as the output of this sample circuit 100, with the electric current correspondent voltage signal on the output and first switching tube 101, in this embodiment, consider the power consumption and the chip area of device simultaneously, the resistance of first resistance 104 being set at [600K Ω, 1M Ω], the breadth length ratio of second switch pipe 102 is set at≤and 200/7.
As shown in Figure 2, comparison circuit comprises RC circuit, first reference voltage source 301, first comparator 201, second reference voltage source 302 and second comparator 202, wherein, the RC circuit is used for the correspondent voltage signal of received current sample circuit 100 outputs, and exports the positive input terminal of corresponding sampled voltage to first comparator 201; In a preferred embodiment, the RC circuit comprises the electric capacity 106 and second resistance 105 that is connected in series, wherein, one end of second resistance 105 is coupled to the output of current sampling circuit 100 as the input of RC circuit, and the other end is connected with an end of electric capacity 106, the other end of electric capacity 106 is connected with input power supply Vin, second resistance 105 is used to determine electric capacity 106 to discharge and recharge time constant, and the bottom crown of electric capacity 106 output sampled voltage is with as judging whether first switching tube 101 overcurrent occurs.First reference voltage source 301 is used to be provided with the maximum current that flows through first switching tube 101, and exports first reference voltage, it is as the floating voltage source, variation with the input power supply changes, and makes the maximum constraints electric current of circuit keep definite value, does not change with input voltage.First comparator 201 compares the sampled voltage and first reference voltage that its positive input terminal and negative input end receive, and with the output overcurrent signal to control circuit, overcurrent whether occurs by the control circuit circuit judges, and control the conducting or the shutoff of first switching tube 101.Second reference voltage source 302 is used to export second reference voltage, second comparator 202 is used for comparison second reference voltage and system feedback voltage (the connected node output system feedback voltage of the first feedback divider resistance 115 and the second feedback divider resistance 116), length or the weak point of turn-off time behind the judgement overcurrent, provide by system's a reference source, to determine further whether first comparator 201 sluggishness occurs.
As shown in Figure 2, in a preferred embodiment, first comparator adopts monolateral controlled hysteresis comparator, and be parallel with the 3rd switching tube 103 at two ends at electric capacity 106, its first end is connected with input power supply Vin, second end is connected with the positive input terminal of first comparator 201, to be used for the voltage of reset capacitance 106.Output at first comparator 201 is connected with an inverter 304, is used for the output signal of anti-phase first comparator 201, to export the over-current signal that finally sends to control circuit.At the output termination level displacement circuit 303 of second comparator 202, the output signal of its second comparator 202 is converted to the high pressure logical signal; The output signal of output termination NAND gate 305, the second comparators 202 of level displacement circuit 303 after level shift with the common input nand gate 305 of over-current signal, the lag function of its output control first comparator 201.Whether first comparator 201 jumps to low level by high level and does not have sluggishness, and exist lag function to depend on the external control logic level by low transition during to high level.
Among the embodiment as shown in Figure 3, first reference voltage source 301 comprises the 4th switching tube 401 and a reference source 402 between input power supply Vin and ground in sequential series, wherein, the connected node of the 4th switching tube 401 and a reference source 402 is exported first reference voltage, the control end ground connection of the 4th switching tube 401.A reference source 402 keeps constant current, is provided by internal reference.
In above embodiment, the control end of first switching tube 101, second switch pipe 102 and the 3rd switching tube 103 all is coupled to control circuit, and first switching tube 101, second switch pipe 102, the 3rd switching tube 103 and the 4th switching tube 401 are the PMOS power switch pipe.
Its operation principle is: the breadth length ratio of establishing first switching tube 101 is
During conducting on it electric current be I
1, conducting resistance is R
OnThe breadth length ratio of second switch pipe 102 is
During conducting on it electric current be I
2The two unit are grid oxygen electric capacity all is C
Ox, threshold voltage is V
Thp, mobility is μ
pInput voltage is V
IN, transistor drain terminal (with the inductance joint) current potential is V
LX, second switch pipe 102 source ends (S point) current potential is Vs, first resistance, 104 sizes are Rs.Because first switching tube 101 and second switch pipe 102 all are operated in linear zone, then have
The S point voltage is:
V
S＝V
IN-I
2R
S (3)
Bring (1), (2) into (3) and abbreviation, and order
Obtain
Among the present invention, first resistance 104 is got higher value, guarantees
Then obtaining the S point voltage is
V
S≈V
IN-I
1R
on (5)
At first reference voltage source 301, the reference voltage of its output is Vref1.Input voltage vin is not a fixed value in the practical application of PWM type reduction voltage circuit, must guarantee that under different input voltages the maximum that limits electric current does not change.Therefore adopted floating reference voltage source the 4th switch controlled end 401 ground connection as Fig. 3 in the present invention, be operated in linear zone during current constant, its conducting resistance is approximate only relevant with input supply voltage Vin.If the conducting resistance of the 4th switching tube 401 is R '
On, the reference current that flows through the 4th switching tube 401 is Iref, the breadth length ratio of the 4th switching tube 401 is (W/L) 401, according to the formula of conducting resistance:
Be carved with during overcurrent:
V
IN-I
ref·R′
on＝V
IN-O
max·R
on＝Vref1(7)
Promptly
I wherein
MaxAllow the maximum current that flows through on first switching tube of setting for needs 101.Utilize formula (8) that (W/L) 401 is set, obtain unsteady Vref1, make lowest high-current value and Vin on first switching tube irrelevant with input power supply Vin.
The current-limiting circuit principle analysis:
If the value of electric capacity 106 is C, the value of first resistance 104 is R1, and the value of second resistance 105 is R2.
When the circuit operate as normal when overcurrent (not), over-current signal ILMT is a low level, and the signal Dctrl of switch controlled end is a low level, first switching tube 101 and 102 conductings of second switch pipe, the 3rd switching tube 103 turn-offs, and electric capacity 106 begins discharge, and its bottom crown voltage Vc begins with (R from Vin
1C) descend for time constant.In Vc decline process, 201 couples of Vc of first comparator and reference voltage V ref1 compare, and detect whether overcurrent of first switching tube:
A. if in whole switching tube turn on process Vc＞Vref1 is arranged all the time, first comparator 201 is exported high level, and over-current signal ILMT is a low level, and expression does not have overcurrent to take place.When the switching tube normal turn-off arrived constantly, first switching tube 101, second switch pipe 102 turn-offed simultaneously, and the 3rd switching tube 103 is opened, and the voltage on the electric capacity 106 returns zero at once, makes Vc=Vin and keeps whole Toff process.When arriving to following one-period, first switching tube 101, second switch pipe 102 are opened, and the 3rd switching tube 103 turn-offs, and detects the electric current on first switching tube 101 again.Carry out this process repeatedly, the cycle is identical with the pwm signal cycle.C point voltage waveform is seen Fig. 4 a.
B. if in Vc decline process, Vc=Vref1 occurs, first comparator, 201 output low levels, over-current signal ILMT saltus step is a high level, then thinks overcurrent takes place this moment.Signal ILMT makes the Dctrl signal for high, turn-offs first switching tube 101 at once, prevents that overcurrent condition from taking place.Because first switching tube 101 and second switch pipe 102 turn-offs, S point voltage Vs=Vin, and be electric capacity 106 chargings by the RC circuit.Vc is begun with [(R by Vref1
1+ R
2) C] for time constant rises, reach the sluggish overturn point Vref1+ Δ V of first comparator 201 up to Vc.Over-current signal ILMT keeps high level in this process, and first switching tube 101, second switch pipe 102, the 3rd switching tube 103 keep turn-offing.This process duration depends on the amount of hysteresis of first comparator 201.If second reference voltage source, 302 output voltages are Vref2, system feedback voltage is Vfb, then when output voltage is not too low, when being Vfb＞Vref2, second comparator, 202 output high level, make first comparator 201 have amount of hysteresis Δ V, then 101 turn-off times of first switching tube are long fixed value behind the overcurrent; When output voltage was low, promptly during Vfb＜Vref2, second comparator, 202 output low levels made first comparator 201 not possess amount of hysteresis, and then 101 turn-off times of first switching tube are the time of delay of circuit self behind the overcurrent.C point voltage waveform is seen Fig. 4 b.
Reach the sluggish overturn point Vref1+ Δ V of first comparator 201 as Vc, first comparator, 201 output switching activities are high level, and over-current signal ILMT becomes low level, and be non-over-current state this moment, transfer to control circuit and control the switch of first switching tube 101, beginning current detecting next time.
The present invention describes by several specific embodiments, it will be appreciated by those skilled in the art that, without departing from the present invention, can also carry out various conversion and be equal to alternative the present invention.In addition, at particular condition or concrete condition, can make various modifications to the present invention, and not depart from the scope of the present invention.Therefore, the present invention is not limited to disclosed specific embodiment, and should comprise the whole execution modes that fall in the claim scope of the present invention.
Claims (10)
1. PWM type buck converter with overcurrent protection function, comprise control circuit, and be connected first switching tube between input power supply and the inductance, it is characterized in that, also comprise current limit circuit, current signal and output overcurrent signal on its described first switching tube that is used to sample, whether control circuit judges described over-current signal greater than the predetermined current maximum, with conducting or the shutoff of controlling described first switching tube;
Described current limit circuit comprises:
Current sampling circuit, the current signal on described first switching tube that is used to sample, and output correspondent voltage signal;
The RC circuit is used to receive the corresponding voltage signal of described current sampling circuit output, and exports corresponding sampled voltage;
First reference voltage source is used to be provided with the maximum current that flows through described first switching tube, and exports first reference voltage;
First comparator is used for the more described sampled voltage and first reference voltage, with the output overcurrent signal to control circuit.
2. the PWM type buck converter with overcurrent protection function according to claim 1 is characterized in that described current limit circuit also comprises:
Second reference voltage source is used to export second reference voltage;
Second comparator is used for comparison second reference voltage and system feedback voltage, sluggishness whether occurs to judge described first comparator.
3. the PWM type buck converter with overcurrent protection function according to claim 1; it is characterized in that; described current sampling circuit comprises first resistance and the second switch pipe between described input power supply and inductance in sequential series; wherein; the connected node of described first resistance and second switch pipe is exported corresponding voltage signal; the resistance of first resistance 〉=600K Ω also≤1M Ω, the breadth length ratio of second switch pipe≤40/1.4.
4. the PWM type buck converter with overcurrent protection function according to claim 1; it is characterized in that the RC circuit comprises the electric capacity and second resistance that is connected in series, wherein; one end of second resistance is as the input of described RC circuit, and the bottom crown of electric capacity is exported described sampled voltage.
5. the PWM type buck converter with overcurrent protection function according to claim 4 is characterized in that, is parallel with the 3rd switching tube at the two ends of described electric capacity, the voltage of the described electric capacity that is used to reset.
6. the PWM type buck converter with overcurrent protection function according to claim 1; it is characterized in that; first reference voltage source comprises the 4th switching tube and a reference source between input power supply and ground in sequential series; wherein; the connected node of described the 4th switching tube and a reference source is exported first reference voltage, the 4th control end of switching tube ground connection.
7. the PWM type buck converter with overcurrent protection function according to claim 2; it is characterized in that; output at described first comparator is connected with an inverter, is used for the output signal of anti-phase described first comparator, arrives described control circuit with the output overcurrent signal.
8. the PWM type buck converter with overcurrent protection function according to claim 7 is characterized in that described first comparator is monolateral controlled hysteresis comparator, and described described current limit circuit with overcurrent protection function also comprises:
Level displacement circuit is used for the output signal of described second comparator of level shift;
NAND gate is used to receive the output signal and the over-current signal of described level displacement circuit, to control the lag function of described first comparator.
9. the PWM type buck converter with overcurrent protection function according to claim 3 is characterized in that the control end of described first switching tube and second switch pipe all is coupled to described control circuit; Described first switching tube and second switch pipe are the PMOS power switch pipe.
10. the PWM type buck converter with overcurrent protection function according to claim 5 it is characterized in that described the 3rd control end of switching tube is coupled to described control circuit, and described the 3rd switching tube is the PMOS power switch pipe.
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| CN2009101060776A CN101594048B (en) | 2009-03-19 | 2009-03-19 | PWM buck convertor with overcurrent protection function |
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