CN109742737B - Overcurrent protection circuit based on fixed period switching power supply - Google Patents
Overcurrent protection circuit based on fixed period switching power supply Download PDFInfo
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- CN109742737B CN109742737B CN201910208309.2A CN201910208309A CN109742737B CN 109742737 B CN109742737 B CN 109742737B CN 201910208309 A CN201910208309 A CN 201910208309A CN 109742737 B CN109742737 B CN 109742737B
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02B—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO BUILDINGS, e.g. HOUSING, HOUSE APPLIANCES OR RELATED END-USER APPLICATIONS
- Y02B70/00—Technologies for an efficient end-user side electric power management and consumption
- Y02B70/10—Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes
Abstract
The invention provides an overcurrent protection circuit based on a fixed period switching power supply, wherein one end of an inductor L is connected with the anode of an input power supply Vin through a load, and the other end of the inductor L is connected with the drain electrode of a switching tube Q1; the oscillator is connected with the grid electrode of the switch tube Q1 through the RS trigger, the source electrode of the switch tube Q1 is grounded through a resistor Rcs, the constant current sampling unit is respectively connected with the source electrode of the switch tube Q1 and one input end of the AND gate G1, the minimum on-time timing unit is respectively connected with the grid electrode of the switch tube Q1 and the other input end of the AND gate G1, the output end of the AND gate G1 is connected with the R input end of the RS trigger, the input end of the on-time detection unit is respectively connected with the output end of the minimum on-time timing unit and the output end of the constant current sampling unit, the output end of the on-time detection unit is connected with the input end of the oscillator period adjusting unit, and the output end of the oscillator period adjusting unit is connected with the oscillator. The invention has the advantages of scientific design, strong practicability, simple structure and convenient use.
Description
Technical Field
The invention relates to an overcurrent protection circuit based on a fixed-period switching power supply.
Background
The fixed period switching power supply is widely applied in the power supply field due to the advantages of few devices, simple structure and the like of the system. As shown in fig. 1, in a conventional switching power supply structure, when a switching transistor Q1 is turned on, an output current is detected by a constant current sampling unit and a sampling resistor Rcs, and the switching state of the switching transistor Q1 is controlled together with an oscillator and an RS flip-flop, so that the output current is maintained at a desired current value.
Fig. 2 is a timing chart of the operation of a conventional switching power supply circuit, in which an oscillator is used to generate a square wave signal CLK with a fixed period, and the period T of the signal CLK is the switching period of the switching power supply circuit with the fixed period. When the CLK signal changes from low to high, the output signal DRV of the RS flip-flop changes from low to high, and the switching transistor Q1 changes from off to on. The switching tube on time is Ton, the switching tube off time is Toff, and t=ton+toff. In the on time Ton of the switching tube Q1, the inductor current increases, and the inductor current increases by the value: Δil+ = (Vin-Vout)/l×ton; during the off time Toff of the switching tube Q1, the inductor current decreases, and the inductor current decreases as follows: Δil- =vout/l×toff=vout/l× (T-Ton). When Δil+= Δil-, the average value of the inductor current is equal in each switching cycle, and the average value of the output current Iout is a constant value.
At the moment when the switch tube Q1 is changed from the off state to the on state, a voltage spike appears on the sampling resistor Rcs due to the influence of resonance of inductance and parasitic capacitance of the drain electrode of the power tube. In order to avoid the circuit operation state error caused by the voltage spike, the switching tube Q1 is generally allowed to be turned off after a fixed delay time passes after being turned on, and the fixed delay time is the minimum on-limit time ton_min of the switching tube, and is generated by a minimum on-time timing unit. When the switching tube Q1 is turned on from off, the minimum on-time timing unit starts timing, and ton_min is turned from low level to high level; after the minimum on-time timing unit finishes timing, ton_min changes from high level to low level.
When the switching tube Q1 is turned on, the current passing through the inductor L passes through both the switching tube Q1 and the sampling resistor Rcs, and the inductor current can be indirectly detected by detecting the voltage Vcs across the sampling resistor Rcs.
When the input voltage Vin is lower, the constant current sampling unit detects the inductance current by detecting the Vcs voltage in the on time of the switching tube Q1, when the average value of the inductance current is higher than the expected value, the output signal SHUT of the constant current sampling unit is changed from low level to high level, the input signal CLR of the RS trigger is changed from low level to high level, the output signal DRV of the RS trigger is changed from high level to low level, and the switching tube Q1 is changed from on state to off state.
As the input voltage Vin increases, the on-time Ton of the switching transistor Q1 decreases. When the voltage Vin increases to make the on-time Ton of the switching tube Q1 equal to ton_min, the on-time Ton of the switching tube Q1 is not reduced even if Vin continues to increase. The reason is that: when the average value of the inductance current is higher than the expected value, the output signal Shut of the constant current sampling unit is changed from low level to high level, but the output signal ton_min of the minimum on-time timing unit is not changed from low level to high level yet. After the output signal ton_min of the minimum on-time timing unit is changed from low level to high level, the input signal CLR of the RS flip-flop is changed from low level to high level and the switching transistor Q1 is turned off. In this case, the on time ton=ton_min of the switching transistor Q1, ton remains unchanged as Vin increases, and in one switching period, the inductor current increases by the value of: Δil+ = (Vin-Vout)/l×ton_min, the inductor current reduction value is: Δil- = Vout/l×toff=vout/L (T-ton_min), where the switching period T, the inductance value L, the minimum on-time ton_min, and the output voltage Vout are constant, as Vin increases, Δil+ increases, Δil-is constant, and thus Δil+ > - Δil-, i.e. in each switching period, the average value of the inductance current increases, so that the switching power supply output current Iout increases with Vin, and exceeds the constant current value set by the system. Fig. 3 is a graph comparing inductance current waveforms before and after Vin increases in a conventional switching power supply circuit.
In order to solve the above problems, an ideal technical solution is always sought.
Disclosure of Invention
The invention aims at overcoming the defects of the prior art, and provides an overcurrent protection circuit based on a fixed period switching power supply, which is scientific in design, strong in practicability, simple in structure and convenient to use.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows: an overcurrent protection circuit based on a fixed period switching power supply is provided, wherein the cathode of a diode is connected with the positive electrode of an input power supply Vin, the anode of the diode is connected with the drain electrode of a switching tube Q1, one end of an inductor L is connected with the positive electrode of the input power supply Vin through a load, the other end of the inductor L is connected with the drain electrode of the switching tube Q1, and the two ends of the load are output power supplies Vout; the output end of the oscillator is connected with the S input end of the RS trigger, the output end of the RS trigger is connected with the grid electrode of the switch tube Q1, the source electrode of the switch tube Q1 is grounded through a resistor Rcs, the input end of the constant current sampling unit is connected with the source electrode of the switch tube Q1, the output end of the constant current sampling unit is connected with one input end of the AND gate G1, the input end of the minimum conduction time timing unit is connected with the grid electrode of the switch tube Q1, the output end of the minimum conduction time timing unit is connected with the other input end of the AND gate G1, and the output end of the AND gate G1 is connected with the R input end of the RS trigger; the system comprises an oscillator period adjusting unit, a minimum on-time timing unit, an oscillator period adjusting unit, a constant current sampling unit, a switching-on time detecting unit and a switching-on time detecting unit, wherein the input end of the switching-on time detecting unit is respectively connected with the output end of the minimum on-time timing unit and the output end of the constant current sampling unit; the output end of the oscillator period adjusting unit is connected with the oscillator and is configured to adjust the period of the oscillator according to the sampling comparison output information of the on-time detecting unit.
Based on the above, the on-time detection unit includes a nor gate G2, a nor gate G3, and a nor gate G4, where one input end of the nor gate G2 is connected to the output end of the constant current sampling unit, the other input end of the nor gate G2 is connected to the output end of the nor gate G3, the output end of the nor gate G2 is connected to one input end of the nor gate G3, the input end of the nor gate G4 is connected to the output end of the minimum on-time timing unit, the output end of the nor gate G4 is connected to the other input end of the nor gate G3, and the output end of the nor gate G3 is connected to the oscillator period adjustment unit.
Based on the above, the oscillator period adjustment unit includes an inverter G5, a resistor R2, a resistor R3, a MOS transistor Q2, a MOS transistor Q3, a MOS transistor Q4, a MOS transistor Q5, a MOS transistor Q6, a MOS transistor Q7, a MOS transistor Q8, a capacitor C2, and a voltage regulator VR, the input terminal of the inverter G5 and the gate of the MOS transistor Q2 are respectively used as the input terminal of the oscillator period adjustment unit, the drain of the MOS transistor Q2 is connected to a power supply Vdd through the resistor R2, the source of the MOS transistor Q2 is respectively connected to the drain of the MOS transistor Q3 and one end of the capacitor C2, the output terminal of the inverter G5 is connected to the gate of the MOS transistor Q3, the source of the MOS transistor Q3 is grounded, one end of the capacitor C2 is connected to the gate of the MOS transistor Q4, the other end of the capacitor C2 is grounded, the source of the MOS transistor Q4 and the source of the MOS transistor Q5 are respectively connected to the gate of the MOS transistor Q6 and the drain of the MOS transistor Q6 through the resistor R3, the drain of the MOS transistor Q8 is grounded, and the drain of the MOS transistor Q8 is connected to the drain of the MOS transistor Q7 is grounded, and the drain of the MOS transistor Q8 is grounded.
Compared with the prior art, the overcurrent protection circuit has the advantages of scientific design, strong practicability, simple structure and convenient use by increasing the oscillation period of the oscillator so as to limit the output current of the switching power supply and protect the output current of the switching power supply from exceeding an expected value.
Drawings
Fig. 1 is a schematic diagram of a prior art switching power supply circuit of the present invention.
Fig. 2 is a schematic diagram of the operation timing of a prior art switching power supply circuit of the present invention.
Fig. 3 is a graph comparing inductor current waveforms before and after Vin increases in a prior art switching power supply circuit of the present invention.
Fig. 4 is a schematic diagram of the structure of the switching power supply circuit of the present invention.
Fig. 5 is a schematic circuit diagram of the on-time detecting unit according to the present invention.
Fig. 6 is a schematic circuit configuration diagram of the oscillator period adjustment unit of the present invention.
Fig. 7 is a schematic circuit configuration of an oscillator according to the prior art of the present invention.
Fig. 8 is a timing chart of the operation of the on-time detecting unit of the present invention.
Fig. 9 is a graph comparing inductor current waveforms before and after Vin increases in the switching power supply circuit of the present invention.
Detailed Description
The technical scheme of the invention is further described in detail through the following specific embodiments.
As shown in fig. 4, in the overcurrent protection circuit based on the fixed period switching power supply, the cathode of the diode is connected with the positive electrode of the input power supply Vin, the anode of the diode is connected with the drain electrode of the switching tube Q1, one end of the inductor L is connected with the positive electrode of the input power supply Vin through the load, the other end of the inductor L is connected with the drain electrode of the switching tube Q1, and the two ends of the load are output power supplies Vout; the output end of the oscillator is connected with the S input end of the RS trigger, the output end of the RS trigger is connected with the grid electrode of the switch tube Q1, the source electrode of the switch tube Q1 is grounded through a resistor Rcs, the input end of the constant current sampling unit is connected with the source electrode of the switch tube Q1, the output end of the constant current sampling unit is connected with one input end of the AND gate G1, the input end of the minimum conduction time timing unit is connected with the grid electrode of the switch tube Q1, the output end of the minimum conduction time timing unit is connected with the other input end of the AND gate G1, and the output end of the AND gate G1 is connected with the R input end of the RS trigger; the system comprises an oscillator period adjusting unit, a minimum on-time timing unit, an oscillator period adjusting unit, a constant current sampling unit, a switching-on time detecting unit and a switching-on time detecting unit, wherein the input end of the switching-on time detecting unit is respectively connected with the output end of the minimum on-time timing unit and the output end of the constant current sampling unit; the output end of the oscillator period adjusting unit is connected with the oscillator and is configured to adjust the period of the oscillator according to the sampling comparison output information of the on-time detecting unit.
The on-time detection unit compares the output signals of the minimum on-time timing unit and the constant current sampling unit. After the voltage of the input power Vin increases to make the on-time Ton of the switching transistor Q1 equal to ton_min, if Vin continues to increase, the output signal State of the on-time detecting unit changes from low level to high level. When the State signal is at a high level, the oscillator period adjustment unit adjusts the period of the oscillator so as to increase the period of the oscillator. The longer the State is high, the larger the period of the oscillator.
In this embodiment, as shown in fig. 5, the on-time detection unit includes a nor gate G2, a nor gate G3, and a nor gate G4, where one input end of the nor gate G2 is connected to the output end of the constant current sampling unit, the other input end of the nor gate G2 is connected to the output end of the nor gate G3, the output end of the nor gate G2 is connected to one input end of the nor gate G3, the input end of the nor gate G4 is connected to the output end of the minimum on-time timing unit, the output end of the nor gate G4 is connected to the other input end of the nor gate G3, and the output end of the nor gate G3 is connected to the oscillator period adjustment unit.
As shown in fig. 8, when Vin voltage is low, the on time Ton of the switching tube is greater than ton_min, and when the high level of the shutdown signal comes, ton_min is high, and the signal State is maintained at low level. When Vin voltage increases to make on time Ton of the switching transistor Q1 equal to ton_min, ton_min is low and the signal State becomes high if Vin continues to increase and the high level of the shutdown signal arrives. The higher Vin, the earlier the shift signal goes high, and the longer the State continues to be high in one switching cycle. The signal State is used to adjust the oscillator period.
As shown in fig. 6, the oscillator period adjustment unit includes an inverter G5, a resistor R2, a resistor R3, a MOS transistor Q2, a MOS transistor Q3, a MOS transistor Q4, a MOS transistor Q5, a MOS transistor Q6, a MOS transistor Q7, a MOS transistor Q8, a capacitor C2, and a voltage regulator VR, the input terminal of the inverter G5 and the gate of the MOS transistor Q2 are respectively used as the input terminal of the oscillator period adjustment unit, the drain of the MOS transistor Q2 is connected to a power supply Vdd through the resistor R2, the source of the MOS transistor Q2 is respectively connected to the drain of the MOS transistor Q3 and one end of the capacitor C2, the output terminal of the inverter G5 is connected to the gate of the MOS transistor Q3, the source of the MOS transistor Q3 is grounded, one end of the capacitor C2 is connected to the gate of the MOS transistor Q4, the other end of the capacitor C2 is grounded, the source of the MOS transistor Q4 and the source of the MOS transistor Q5 are respectively connected to the power supply through the resistor R3, the drain of the MOS transistor Q4 is respectively connected to the gate of the MOS transistor Q6 and the drain of the MOS transistor Q8, the drain of the MOS transistor Q5 is grounded, and the drain of the MOS transistor Q8 is connected to the drain of the MOS transistor Q7 is grounded. When the signal State is at a low level, the MOS transistor Q2 is turned off, the MOS transistor Q3 is turned on, the capacitor C2 is rapidly discharged to 0, and the voltage VC2 on the capacitor C2 is 0. When the signal State changes from low level to high level, the MOS transistor Q2 is turned on, the MOS transistor Q3 is turned off, the capacitor C2 is charged, and the voltage VC2 on the capacitor C2 gradually rises. The power supply Vdd, the resistor R3, the MOS transistor Q4, the MOS transistor Q5, the MOS transistor Q6, the MOS transistor Q7, the MOS transistor Q8 and the voltage stabilizing transistor VR form a voltage control current source, the voltage VC2 is connected to the input end of the voltage control current source, and the output of the voltage control current source is the current I2. When the voltage value of the voltage VC2 is 0, the output current I2 is 0; the larger the voltage value of the voltage VC2, the larger the output current I2.
In general, as shown in fig. 7, the period of the oscillator is determined by the charging current I1 and the capacitance C1. After the output current I2 of the oscillator period adjusting unit is connected to one end of the charging current I1 of the oscillator, the effective charging current of the oscillator becomes I1-I2. When the output current I2 of the oscillator period adjustment unit increases from 0, the effective charging current of the oscillator decreases and the period T of the oscillator increases. The rate at which the oscillator period T increases with Vin may be determined by the amplification of resistor R2, capacitor C2, and the voltage controlled current source.
As shown in fig. 9, when Vin voltage increases to make the on-time Ton of the switching tube Q1 equal to ton_min, vin continues to increase, and the on-time ton=ton_min of the switching tube. In the prior art, in one switching period, the inductance current increasing value is: Δil+ = (Vin-Vout)/l×ton_min, the inductor current reduction value is: Δil- =vout/l×toff=vout/l× (T-ton_min), where the switching period T, the inductance value L, the minimum on-time ton_min, and the output voltage Vout are constant. As Vin increases, Δil+ increases. In the present invention, the period T of the oscillator follows Vin and thereby ΔIL-increases. By designing the oscillator period adjusting unit, the rate of the oscillator period T increasing along with Vin is properly set, and the delta IL < + > and delta IL < - > in each switching period can be achieved, so that the output current Iout of the switching power supply does not increase along with the increase of Vin.
Finally, it should be noted that the above-mentioned embodiments are only for illustrating the technical scheme of the present invention and are not limiting; while the invention has been described in detail with reference to the preferred embodiments, those skilled in the art will appreciate that: modifications may be made to the specific embodiments of the present invention or equivalents may be substituted for part of the technical features thereof; without departing from the spirit of the invention, it is intended to cover the scope of the invention as claimed.
Claims (3)
1. An overcurrent protection circuit based on a fixed period switching power supply is provided, wherein the cathode of a diode is connected with the positive electrode of an input power supply Vin, the anode of the diode is connected with the drain electrode of a switching tube Q1, one end of an inductor L is connected with the positive electrode of the input power supply Vin through a load, the other end of the inductor L is connected with the drain electrode of the switching tube Q1, and the two ends of the load are output power supplies Vout; the output of oscillator connects the S input of RS trigger, the grid of switch tube Q1 is connected to the output of RS trigger, switch tube Q1' S source passes through resistance Rcs ground connection, the source of switch tube Q1 is connected to the input of constant current sampling unit, the output of constant current sampling unit connects an input of AND gate G1, the grid of switch tube Q1 is connected to the input of minimum on-time timing unit, the other input of AND gate G1 is connected to the output of minimum on-time timing unit, the R input of RS trigger is connected to the output of AND gate G1, its characterized in that: the system comprises an oscillator period adjusting unit, a minimum on-time timing unit, an oscillator period adjusting unit, a constant current sampling unit, a switching-on time detecting unit and a switching-on time detecting unit, wherein the input end of the switching-on time detecting unit is respectively connected with the output end of the minimum on-time timing unit and the output end of the constant current sampling unit; the output end of the oscillator period adjusting unit is connected with the oscillator and is configured to adjust the period of the oscillator according to the sampling comparison output information of the conduction time detecting unit; when the input voltage increases to reduce the on time of the switching tube Q1 to the minimum limit time, the oscillation period of the oscillator is increased, the output current of the switching power supply is limited, and the output current of the switching power supply is protected from exceeding an expected value.
2. The overcurrent protection circuit based on a fixed period switching power supply of claim 1, wherein: the on-time detection unit comprises a NOR gate G2, a NOR gate G3 and a NOR gate G4, wherein one input end of the NOR gate G2 is connected with the output end of the constant current sampling unit, the other input end of the NOR gate G2 is connected with the output end of the NOR gate G3, the output end of the NOR gate G2 is connected with one input end of the NOR gate G3, the input end of the NOR gate G4 is connected with the output end of the minimum on-time timing unit, the output end of the NOR gate G4 is connected with the other input end of the NOR gate G3, and the output end of the NOR gate G3 is connected with the oscillator period adjustment unit.
3. The overcurrent protection circuit based on a fixed period switching power supply of claim 1, wherein: the oscillator period adjusting unit comprises an NOT gate G5, a resistor R2, a resistor R3, an MOS tube Q2, an MOS tube Q3, an MOS tube Q4, an MOS tube Q5, an MOS tube Q6, an MOS tube Q7, an MOS tube Q8, a capacitor C2 and a voltage stabilizing tube VR, wherein the input end of the NOT gate G5 and the grid electrode of the MOS tube Q2 are respectively used as the input end of the oscillator period adjusting unit, the drain electrode of the MOS tube Q2 is connected with a power supply Vdd through the resistor R2, the source electrode of the MOS tube Q2 is respectively connected with the drain electrode of the MOS tube Q3 and one end of the capacitor C2, the output end of the NOT gate G5 is connected with the grid electrode of the MOS tube Q3, the source electrode of the MOS tube Q3 is grounded, one end of the capacitor C2 is connected with the grid electrode of the MOS tube Q4, the source electrode of the MOS tube Q4 and the source electrode of the MOS tube Q5 are respectively connected with a power supply Vdd through the resistor R3, the drain electrode of the MOS tube Q4 is respectively connected with the grid electrode of the MOS tube Q6 and the drain electrode of the MOS tube Q8, the drain electrode of the MOS tube Q8 is grounded, and the drain electrode of the MOS tube Q8 is connected with the MOS tube Q8 is grounded, and the drain electrode of the MOS tube Q8 is grounded.
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| CN201910208309.2A CN109742737B (en) | 2019-03-19 | 2019-03-19 | Overcurrent protection circuit based on fixed period switching power supply |
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