Control circuit for changing frequency of switching power supply
Technical Field
The utility model relates to a constant voltage power supply control circuit especially indicates the external control circuit that can change switching power supply's inside clock frequency.
Background
It is important to provide clean and stable power for various products from a reasonably cost effective regulated power supply. The stabilized voltage power supply is used for stabilizing the voltage of an unstable power supply. The switching frequency of a switching power supply is determined by an internal oscillator circuit, usually having a fixed frequency.
When the switching frequency of the switching power supply is increased, the inductance of the filter inductor in the switching circuit and the capacity of the output capacitor are reduced. For a certain inductance value, a higher power switching frequency can reduce the ripple of the output voltage. For a certain output voltage ripple value, the higher switching frequency can reduce the numerical value and the volume of the inductor. Therefore, it is beneficial to miniaturize the switching power supply to increase the switching frequency of the switching power supply, and the switching power supply can be designed to have smaller volume and higher efficiency. However, with the increase in switching frequency, the loss of the switching power supply also increases; because the bandwidth of the power supply is increased when the switching frequency of the power supply is increased, the transient response speed of the power supply is increased.
As electronic products (such as mobile phones, MP3, MP4, etc.) powered by batteries become more popular, the demand for power sources with high efficiency and small volume is increasing. However, there is still a need to improve the switching power supply to minimize the loss of the switching power supply while ensuring high efficiency and small size.
A circuit conventionally used at present for generating a switching frequency of a switching power supply generally includes an oscillation circuit and a reference current generation circuit for supplying a reference current to the oscillation circuit. Fig. 1 shows a reference current generating circuit widely used in the prior art. It includes an operational amplifier 1 ', an NMOS source follower 2 ' and a constant value resistor 3 '. The non-inverting input terminal of the operational amplifier 1' is connected with a reference voltage VrefThe inverting input end of the NMOS source electrode tracker 2 ' is connected with the source electrode end of the NMOS source electrode tracker 2 ', and the output end of the NMOS source electrode tracker 2 ' is connected with the grid electrode end; the fixed resistor 3 'is connected between the source terminal and the ground terminal of the NMOS source follower 2'.
Since the voltage values of the non-inverting input terminal and the inverting input terminal of the operational amplifier 1 'are the same in this circuit diagram, the voltage applied to the constant value resistor 3' is also VrefFrom this, the reference current through the fixed resistor 3' is:
Iref=Vref/R;
wherein, R is the resistance value of the constant value resistor 3'.
A reference current I generated by the reference current generation circuitrefAn operating current is supplied to the oscillating circuit and a corresponding oscillating frequency, i.e. the switching frequency of the switching circuit, is generated. As can be seen from the above, the reference current I is usedrefIs fixed, the switching frequency of the switching circuit is also fixed.
In view of this, the utility model provides a simple and easy, can follow the control circuit of the internal frequency that the outside changed switching power supply to let the manufacturer can have corresponding more optimized design power, save the cost.
SUMMERY OF THE UTILITY MODEL
The utility model aims at providing a can change switching power supply frequency's control circuit, it is through increasing limited electronic components of external circuit and interconnecting link, realizes that the frequency to switching power supply is adjustable, can have corresponding optimization power supply design, reduce cost to the realization is guaranteeing under the prerequisite of high efficiency, little volume, makes this switching power supply's loss effectively reduce, prolongs its life.
In order to achieve the above object, the present invention provides a control circuit capable of changing the frequency of a switching power supply, which comprises: the circuit comprises an oscillating circuit for providing a clock frequency signal for a switching power supply and a reference current generating circuit for providing a reference current for the oscillating circuit;
the reference current generating circuit comprises an operational amplifier, an NMOS source electrode tracker and an external variable resistor; the positive phase input end of the operational amplifier is connected with a reference voltage, the negative phase input end of the operational amplifier is connected with the source end of the NMOS source electrode tracker, and the output end of the operational amplifier is connected with the grid end of the NMOS source electrode tracker; the external variable resistor is connected between the source terminal of the NMOS source follower and ground.
Furthermore, the reference current generating circuit also comprises an internal constant value resistor which is connected with the external variable resistor in parallel.
The oscillation circuit includes: an oscillation charging and discharging circuit and a threshold voltage selection circuit connected by a circuit.
Wherein, the oscillating charge-discharge circuit includes: the charging circuit is formed by connecting a current mirror charging current source and a charging control switch in series and then connecting the charging current source and the charging control switch in series with an oscillation capacitor, wherein the other end of the oscillation capacitor is grounded; the oscillating discharge circuit is formed by connecting a current mirror discharge current source and a discharge control switch in series and then connecting the current mirror discharge current source and the discharge control switch in parallel with an oscillating capacitor.
The threshold voltage selection circuit comprises a threshold voltage logic selector and a threshold voltage comparator; one input end of the threshold voltage comparator is connected with the output end of the threshold voltage logic selector; the other input end of the threshold voltage comparator is connected with the non-grounding end of the oscillation capacitor; the output end of the threshold voltage comparator is respectively connected with the charging control switch, the discharging control switch and the threshold voltage logic selector in a feedback mode.
Compared with the prior art, the utility model, its advantage lies in: by adding limited electronic components and connecting circuits of an external circuit, the power supply design is optimized in a targeted manner, the cost is reduced, and the power supply is widely applicable to high-efficiency and small-volume power supplies. Through the frequency of actual adjustment switching power supply, can realize guaranteeing under the prerequisite of high efficiency, small volume for this switching power supply's loss effectively reduces, prolongs its life.
Drawings
FIG. 1 is a circuit diagram of a prior art reference current generating circuit;
fig. 2 is a circuit diagram of an embodiment of the reference current generating circuit of the present invention;
fig. 3 is a circuit diagram of another embodiment of the reference current generating circuit of the present invention;
fig. 4 is a circuit diagram of the oscillating circuit of the present invention.
Detailed Description
The preferred embodiment of the present invention will be described in detail below with reference to fig. 2 to 4.
The utility model provides a can change switching power supply frequency's control circuit, it contains: an oscillation circuit and a reference current generation circuit that supplies a reference current to the oscillation circuit.
As shown in fig. 2, a circuit diagram of a reference current generating circuit 101 according to the present invention includes an operational amplifier 1, an NMOS source follower 2 and an external variable resistor 3. The normal phase input end of the operational amplifier is connected with a reference voltage VrefIts inverting input terminal is connected to the source terminal of the NMOS source tracker 2, and its output terminal is connected to the NMOS sourceThe gate terminal of the pole tracker 2 is connected; the external variable resistor 3 is connected between the source terminal and the ground terminal of the NMOS source follower 2.
Since the voltage values of the non-inverting input terminal and the inverting input terminal of the operational amplifier 1 are the same in this circuit diagram, the voltage applied to the external variable resistor 3 is also VrefFrom this, the reference current through the variable resistor 3 is:
Iref=Vref/Rext;
wherein R isextIs the present resistance value of the variable resistor 3.
Therefore, in the present invention, when the resistance R of the external variable resistor 3 is adjustedextA reference current I generatedrefThe value of (c) will change accordingly.
The preferred embodiment of the present invention further provides another reference current generating circuit 102, and the specific circuit diagram thereof is shown in fig. 3. The only difference between the reference current generating circuit 102 and the reference current generating circuit 101 is that the reference current generating circuit 102 further includes an internal constant resistor 4 connected in parallel to the external variable resistor 3. Therefore, the reference current generated by the circuit is as follows:
Iref=Vref/Rand are;
RAnd are=Rext//Rint;
Wherein R isintIs the resistance value of the internal constant value resistor 4; rAnd areThe resistance value is obtained by connecting the external variable resistor 3 and the internal constant value resistor 4 in parallel.
Therefore, in the present invention, when the resistance R of the external variable resistor 3 is adjustedextA reference current I generatedrefThe value of (c) will change accordingly. When the reference current generation circuit 12 is not connected to the external variable resistor 3, the resistance of the internal constant value resistor 4 determines the lowest reference current Iref. When the current resistance value of the external variable resistor 3 is increased by adjustment so as to be connected with the internal constant value resistor 4 in parallel, the total resistance value R after parallel connection is causedAnd areWill be compared with RintDecrease, then the reference current IrefAnd will increase accordingly.
As shown in fig. 4, which is a schematic diagram of the oscillating circuit of the present invention, the oscillating circuit provides a clock frequency signal for the switching power supply. Which comprises the following steps: an oscillation charging and discharging circuit and a threshold voltage selection circuit connected by a circuit.
Wherein, the oscillating charge-discharge circuit includes: the charging circuit is formed by connecting a current mirror charging current source 5 and a charging control switch 8 in series and then connecting the charging current source and the charging control switch in series with an oscillating capacitor 7, wherein the other end of the oscillating capacitor 7 is grounded; the oscillating discharge circuit is formed by connecting a current mirror discharge current source 6 and a discharge control switch 9 in series and then connecting the oscillating capacitor 7 in parallel.
The threshold voltage selection circuit includes a threshold voltage logic selector 10 (in this embodiment, a logic selection switch may be used to implement the threshold voltage logic selector) and a threshold voltage comparator 11; the threshold voltage logic selector 10 has 2 preset threshold voltage values VceilingAnd VbottomThe output end of the comparator is connected with one input end of the threshold voltage comparator 11; the other input end of the threshold voltage comparator 11 is connected to the non-grounded end of the oscillation capacitor 7, and the output end of the threshold voltage comparator 11 is respectively connected to the charge control switch 8, the discharge control switch 9 and the threshold voltage logic selector 10 in a feedback manner.
Reference current I generated by the aforementioned reference current generation circuitrefThe current I is generated by a current mirror charging current source 5 and a current mirror discharging current source 6 respectively by a current mirror (currentmirror) methodupAnd Idown. The current mirror described herein is a commonly used method of generating a current having a proportional relationship to a reference current, and therefore, IupAnd IdownWill be based on the reference current IrefIs changed. Wherein, the current IupIs used for providing oscillating capacitance7, and the current is used to discharge the oscillating capacitor 7.
When the charge control switch 8 is closed, the current mirror charge current source 5 is based on the reference current IrefGenerated current IupThe oscillating capacitor 7 is charged so that the voltage applied across the oscillating capacitor 7 rises. When the discharge control switch 9 is closed, the current source 6 is discharged by the current mirror according to the reference current IrefGenerated current IdownThe oscillating capacitor 7 is discharged, so that the voltage applied across the oscillating capacitor 7 decreases. I isdown/IupThe ratio between them determines the clock duty cycle of the switching power supply. The threshold voltage comparator 11 is used to determine the crossing value of the actual voltage on the oscillating capacitor 7 and the threshold voltage. The threshold voltage comparator 11 has two threshold voltages: vceilingAnd Vbottom. Threshold voltage logic selector 10 is used to select a threshold voltage for threshold voltage comparator 11.
When the charge control switch 8 is closed to charge the oscillation capacitor 7, the threshold voltage logic selector 10 selects VceilingAs a threshold value of the voltage comparator 11, the voltage across the oscillating capacitor 7 will increase when the voltage exceeds VceilingThen, the feedback output end of the voltage comparator 11 will control the charging control switch 8 to open and close the discharging control switch 9, thereby ending the charging process and starting the discharging process; at the same time, the feedback output of the voltage comparator 11 will also control the threshold voltage logic selector 10 to select VbottomAs a threshold of the voltage comparator 11 during discharge. During the discharge process, the voltage across the oscillating capacitor 7 will decrease until its voltage value is less than VbottomThe feedback output of the voltage comparator 11 will then control the discharge control switch 9 to open and the charge control switch 8 to close, thereby ending the discharge process and cycling the charge process again.
Therefore, the clock charging time of the switching power supply is as follows: t ison=Cosc×(Vceiling-Vbottom)/Iup(ii) a The clock discharge time of the switching power supply is as follows: t isoff=Cosc×(Vceiling-Vbottom)/Idown(ii) a Wherein, CoscIs the capacitance value of the oscillation capacitor 7. It follows that the clock period of the switching power supply is from TonAnd ToffAdded to each other, the switching frequency of the switching power supply is 1/Ton+Toff. Therefore, if the current I is adjusted accordinglyupAnd IdownThen the switching frequency of the switching power supply will also change.
As is apparent from the above description, since the external variable resistor 3 is provided in the reference current generating circuit, the reference current I can be changed by adjusting the resistance thereofrefThe value of (c). And because of the charging and discharging currents I generated by the current mirrorupAnd IdownIs related to a reference current IrefAnd finally, the clock charging and discharging time of the switching power supply is changed along with the proportional change, so that the switching frequency of the switching power supply is adjustable. Specifically, when increasing the resistance value of the external variable resistor 3, the reference current IrefWill increase, at the same time, the charge and discharge current IupAnd IdownWill also increase proportionally, thereby enabling the clock charging and discharging time T of the switching power supplyonAnd ToffThe switching frequency of the switching power supply is reduced and finally increased. On the contrary, when the resistance value of the external variable resistor 3 is reduced, the switching frequency of the switching power supply can be reduced in the same manner.
To sum up, the utility model provides a can follow outside control circuit who changes switching power supply frequency through increasing limited electronic components and connecting line, has corresponding optimization power supply design, reduce cost to extensively be applicable to high efficiency and small volume power. Through the frequency of actual adjustment switching power supply, can realize guaranteeing under the prerequisite of high efficiency, small volume for this switching power supply's loss effectively reduces, prolongs its life.
While the present invention has been described in detail with reference to the preferred embodiments thereof, it should be understood that the above description should not be taken as limiting the present invention. Numerous modifications and alterations to the present invention will become apparent to those skilled in the art upon reading the foregoing description. Accordingly, the scope of the invention should be limited only by the attached claims.