Patent Publication Number: US-2022216785-A1

Title: Control circuit and switching converter

Description:
RELATED APPLICATIONS 
     This application claims the benefit of Chinese Patent Application No. 202110014397.X, filed on Jan. 6, 2021, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of power electronics, and more particularly to control circuits and switching converters. 
     BACKGROUND 
     A switched-mode power supply (SMPS), or a “switching” power supply, can include a power stage circuit and a control circuit. When there is an input voltage, the control circuit can consider internal parameters and external load changes, and may regulate the on/off times of the switch system in the power stage circuit. Switching power supplies have a wide variety of applications in modern electronics. For example, switching power supplies can be used to drive light-emitting diode (LED) loads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an example control circuit for the switching converter under the constant on-time control based on a ripple signal. 
         FIG. 2  is a waveform diagram of the example ripple signal of  FIG. 1 . 
         FIG. 3  is a schematic block diagram of an example switching converter, in accordance with embodiments of the present invention. 
         FIG. 4  is a schematic block diagram of an example ripple signal generation circuit, in accordance with embodiments of the present invention. 
         FIG. 5  is a schematic block diagram of an example control circuit, in accordance with the embodiments of the present invention. 
         FIG. 6  is a waveform diagram of example operation of the switching converter, in accordance with embodiments of the present invention. 
         FIG. 7  is a waveform diagram of example operation of the switching converter when the load changes, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference may now be made in detail to particular embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention may be described in conjunction with the preferred embodiments, it may be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it may be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, processes, components, structures, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention. 
     A switching converter may utilize voltage mode and current mode, in order to control the state of the power stage circuit to generate a stable output voltage. In order to address sub-slope oscillation of the output of the switching converter, a constant on-time control based on the ripple voltage can be used to control the switching converter. This control method can actively improve the stability of the system. 
     Referring now to  FIG. 1 , shown is a schematic block diagram of an example control circuit for the switching converter under constant on-time control based on a ripple signal. In this example, ripple signal Vrip can be superimposed on feedback voltage V FB  representing an output voltage of the switching converter to generate input voltage Vb. Referring now to  FIG. 2 , shown is a waveform diagram of the example ripple signal. As shown, assuming that a peak-to-peak value of ripple signal Vrip is Vp, ripple signal Vrip can vary between −½ Vp and ½ Vp, and the average value of ripple signal Vrip may be zero. Due to the existence of the ripple signal, there can be a difference of a DC bias voltage between feedback voltage V FB  and reference voltage V REF . Referring back to  FIG. 1 , in order to eliminate the DC offset voltage caused by injection of the ripple signal, correction signal Vcorr can be superimposed on reference voltage V REF  to generate input voltage Vc. In this example, correction signal Vcorr can be generated according to an error between feedback voltage V FB  and reference voltage V REF . When the switching converter operates in a steady state, the amplitude of correction signal Vcorr can be ½ Vp. The driving circuit can control the switching state of the power switch in the power stage circuit according to input voltages Vb and Vc. 
     However, on the one hand, this control method may be equivalent to having two voltage loops. The inner voltage loop can respond to feedback voltage V FB , in order to quickly respond to load dynamic “jumps.” The outer voltage loop may respond to correction signal Vcorr, to eliminate steady-state errors. The regulation speed of the inner loop can be relatively fast and the regulation speed of the outer loop relatively slow, which can affect dynamic performance. On the other hand, the correction signal may need to be designed according to the peak-to-peak value of the ripple signal, and may have a preset variation range. This can lead to the output voltage being out of adjustment due to limitations of the variation range of the correction signal under light load conditions, thereby reducing system accuracy. 
     Referring now to  FIG. 3 , shown is a schematic block diagram of an example switching converter, in accordance with embodiments of the present invention. The switching converter can include power stage circuit  21  (e.g., a buck topology) and control circuit  22 . In this example, power stage circuit  21  can include power switch S 1 , power switch S 2 , inductor L, and output capacitor Co. For example, a first terminal of power switch S 1  can connect to input voltage Vin, a second terminal of power switch S 1  can connect to a first terminal of inductor L, and a second terminal of inductor L can connect to an output terminal of the switching converter. A first terminal of power switch S 2  can connect to a common terminal of the second terminal of power switch S 1  and the first terminal of inductor L, and a second terminal of power switch S 2  can connect to a reference ground of the switching converter. Output capacitor Co can connect between the output terminal of the switching converter and the reference ground, and in parallel with load R L  for receiving output voltage Vo. In this example, power switch S 1  is the main power switch, and power switch S 2  is a rectifier switch. It should be understood that power switches S 1  and S 2  can be any type of field-effect transistors, such as metal-oxide-semiconductor field-effect transistors (MOSFET), other types of field-effect transistors and/or any other suitable types of transistors. 
     Control circuit  22  may adopt a closed-loop control mode to generate switching control signal pulse-width modulation (PWM) according to output voltage Vo, in order to control the switching states of power switches S 1  and S 2  to provide energy to load R L . In this example, control circuit  22  can include ripple signal generation circuit  221 , superimposing circuit  222 , and switching control signal generation circuit  223 . For example, ripple signal generation circuit  221  can generate ripple signal Vrip having the same frequency and phase as inductor current IL flowing through inductor L, and the variation range of ripple signal Vrip can be between zero and a preset value. That is, the peak-to-peak value of ripple signal Vrip can be the preset value (e.g., a value greater than zero). Further, superimposing circuit  222  can superimpose ripple signal Vrip on feedback voltage V FB  to generate loop control signal V FB1 . Switching control signal generation circuit  223  can generate switching control signal PWM according to loop control signal V FB1  and reference signal V REF . 
     In one example, ripple signal generation circuit  221  can generate a triangular wave signal with the same frequency and phase as inductor current IL flowing through inductor L according to input voltage Vin and duty ratio D of the switching converter, and may generate ripple signal Vrip according to the triangular wave signal and the valley value of the triangular wave signal. It should be understood that ripple signal generation circuit  221  in this particular example generates the triangular wave signal with the same frequency and phase as inductor current IL flowing through inductor L according to the known variables of the switching converter, but other circuit structures realizing this functionality can also be utilized in certain embodiments. 
     As compared with other approaches, the control circuit in particular embodiments can generate loop control signal V FB1  by superimposing ripple signal Vrip on feedback voltage V FB , and can control the operation state of the power stage circuit according to loop control signal V FB1  and reference signal V REF . Since the variation range of ripple signal Vrip can be between zero and the preset value, the valley value of ripple signal Vrip may not change with the duty cycle of the switching converter, such that there may be no DC bias voltage between feedback voltage V FB  and reference voltage V REF  under the valley control method. Therefore, the control circuit may not need to correct the reference signal, so the correction circuit can be omitted. Also, there may be only one voltage loop used in the control circuit, which can quickly respond to a dynamic jump of the load, and may also ensure that the output voltage accuracy of the switching converter is increased in different applications. 
     Referring now to  FIG. 4 , shown is a schematic block diagram of an example ripple signal generation circuit, in accordance with embodiments of the present invention. In this particular example, ripple signal generation circuit  221  can include triangular wave generation circuit  41 , valley generation circuit  42 , and difference circuit  43 . For example, triangular wave generation circuit  41  can generate triangular wave signal Vtri with the same frequency and phase as inductor current IL, according to input voltage Vin and duty ratio D of the switching converter, in order to control the switching states of the power stage circuit. In this way, the resonance problem that may be caused by output voltage phase lag can be addressed since the equivalent series resistance of the output capacitor is too small. Further, valley generation circuit  42  can sample the valley value of triangular wave signal Vtri according to the switching state of the power switch, in order to generate valley signal Vva. Difference circuit  43  can perform a difference operation on triangular wave signal Vtri and valley signal Vva, in order to generate a ripple signal Vrip, where ripple signal Vrip has a variation range between zero and a preset value. 
     For example, triangle wave generation circuit  41  can include current source I, switch K 1 , and switch K 2 , which can connect in series between supply voltage VCC and the reference ground of the switching converter. Triangle wave generation circuit  41  can also include capacitor C 1  and resistor R 1 . In this example, resistor R 1  can connect in series with switch K 2  to form a series structure, and capacitor C 1  can connect in parallel with the series structure. Current source I can be controlled by input voltage Vin, in order to generate a predetermined current. Switch K 1  can be controlled by switching control signal PWM, and switch K 2  can be controlled by current zero-crossing signal NCL that represents inductor current IL reaches zero. Further, switch K 2  can be controlled by an inverted signal of current zero-crossing signal NCL. When power switch S 1  of the switching converter is turned on, switching control signal PWM can be active, thus switch K 1  may be turned on, and current source I can begin to charge capacitor C 1 . When switching control signal PWM is inactive, switch K 1  can be turned off. When current zero-crossing signal NCL is inactive (e.g., inductor current IL is not zero), switch K 2  can be turned on, and thus capacitor C 1  may be discharged through resistor R 1 . 
     When the switching converter operates in a steady state, the charge and discharge of capacitor C 1  can reach a balanced state, and a stable triangular wave signal Vtri may be generated at non-grounded terminal of capacitor C 1 . Further, the rising duration and amplitude of triangular wave signal Vtri may be proportional to the duty cycle of the switching converter, and the phase and amplitude change of triangular wave signal Vtri can be consistent with those of the inductor current. In addition, when the switching converter operates in the discontinuous current mode, and current zero-crossing signal NCL is active, switch K 2  can be turned off, and triangular wave signal Vtri may not be discharged through resistor R 1  and remains stable. In this example, triangle wave generation circuit  41  can also include diode D connected in parallel at both ends of current source I, in order to provide a freewheeling loop for current source I when switches K 1  and K 2  are off. 
     In one example, valley generation circuit  42  can include switch K 3 , switch K 4 , switch K 5 , and capacitor C 2 . A first terminal of switch K 3  can be an input terminal of valley generation circuit  42  for receiving triangular wave signal Vtri, a second terminal of switch K 3  can connect to a first terminal of capacitor C 2 , and a second terminal of capacitor C 2  can be grounded. A first terminal of switch K 4  can connect to the first terminal of capacitor C 2 . A first terminal of switch K 5  can be coupled to the first terminal of switch K 3  through resistor R 2 , and a second terminal of switch K 5  can connect to a second terminal of switch K 4 . In this example, switch K 4  can be controlled by switching control signal PWM, switch K 3  may be controlled by switching control signal PWMB that is an inverted signal of switching control signal PWM, and switch K 5  can be controlled by current zero-crossing signal NCL. 
     Valley generation circuit  42  may also include a filter circuit connected to the common node of switches K 4  and K 5 , and which can filter the voltage at the common node of switches K 4  and K 5 , in order to generate valley signal Vva. In this example, the filter circuit can include resistor R 3  and capacitor C 3  connected in series between the common node of switches K 4  and K 5  and the reference ground, and valley signal Vva may be generated at the common node of resistor R 3  and capacitor C 3 . In this example, valley generation circuit  42  can also include buffer A 0  connected between the output terminal of triangular wave generating circuit  41  and the input terminal of valley generation circuit  42  to avoid the influence of valley generation circuit  42  on triangular wave signal Vtri, in order to optimize the circuit performance. 
     When the switching converter operates in the current continuous mode, since inductor current IL may not reach zero, current zero-crossing signal NCL can remain inactive, and switch K 5  may remain in the off state. When switching control signal PWMB is active, power switch S 2  in the switching converter can be turned on, switch K 3  may be turned on, and capacitor C 2  may receive the voltage across capacitor C 1 . That is, the voltage across capacitor C 2  can be equal to triangular wave signal Vtri. When switching control signal PWM is active, power switch S 1  in the switching converter may be turned on, power switch S 2  can be turned off, and switch K 4  may be turned on. At this time, the voltage across capacitor C 2  can be the valley value of triangular wave signal Vtri, which may be transmitted to the filter circuit to generate valley signal Vva. 
     When the switching converter operates in the current discontinuous mode, and when current zero-crossing signal NCL is inactive, the operation process of valley generation circuit  42  can be the same as that in the current continuous mode. When switching control signals PWM and PWMB are both inactive, power switches S 1  and S 2  may both be turned off, and current zero-crossing signal NCL can be active. Thus, switch K 5  may be turned on, and triangular wave signal Vtri can directly pass through the filter circuit, in order to generate valley signal Vva. 
     Referring now to  FIG. 5 , shown is a schematic block diagram of an example control circuit, in accordance with the embodiments of the present invention. In this example, superimposing circuit  222  can superimpose ripple signal Vrip on feedback signal V FB  to generate loop control signal V FB1 . Switching control signal generation circuit  223  may generate switching control signals for power switches S 1  and S 2  according to loop control signal V FB1  and reference signal V REF . In particular embodiments, switching control signal generation circuit  223  may utilize ripple control under a valley control mode to control the power stage circuit of the switching converter, and also utilize constant on-time control, in order to simplify the circuit structure. 
     For example, power switch S 1  can be controlled to be turned on when the valley value of loop control signal V FB1  reaches reference signal V REF ; that is, when feedback signal V FB  is equal to reference signal V REF . In one example, switching control signal generation circuit  223  can include a constant on-time control circuit for generating a reference signal according to the average value of triangular wave signal Vtri to adjust the on-time of power switch S 1 . For example, in the constant on-time control circuit, a ramp signal can be generated, and when the ramp signal rises from zero to the reference signal, power switch S 1  may be turned off. 
     In this example, switching control signal generation circuit  223  can include comparator  50  and driving circuit  51 . The inverting input terminal of comparator  50  may receive loop control signal V FB1 , the non-inverting input terminal may receive reference signal V REF , and the output terminal may generate a comparison signal by comparing loop control signal V FB1  with reference signal V REF . Driving circuit  51  can connect to the output terminal of comparator  50 , and may generate switching control signals PWM and PWMB according to the comparison signal. In this example, switching control signals PWM and PWMB can control power switches S 1  and S 2 , respectively. 
     Referring now to  FIG. 6 , shown is a waveform diagram of example operation of the switching converter, in accordance with embodiments of the present invention. In (a) of  FIG. 6 , shown is a waveform diagram of example operation of the switching converter under the current continuous mode. In (b) of  FIG. 6 , shown is a waveform diagram of example operation of the switching converter under the current discontinuous mode. In this example, the switching converter may utilize the ripple control under the valley control mode. As shown in (a) of  FIG. 6 , triangular wave signal Vtri and the inductor current of the switching converter may have the same frequency and phase. The maximum and minimum values of voltage Vc 2  across capacitor C 2  can be the same as the maximum and minimum values of triangular wave signal Vtri, such that valley signal Vva may be obtained according to voltage Vc 2 . Also, valley signal Vva and the valley value of triangular wave signal Vtri can remain the same. 
     Triangular wave signal Vtri and valley signal Vva can be subjected to the difference operation to obtain ripple signal Vrip. Assuming that the peak-to-peak value of ripple signal Vrip is Vp, and ripple signal Vrip varies between zero and Vp, feedback signal V FB  and ripple signal Vrip can be superimposed to generate loop control signal V FB1 , and thus loop control signal V FB1  may vary between V FB  and V FB +Vp. At time t 0 , when the valley value of loop control signal V FB1  is equal to reference signal V REF , that is, feedback signal V FB  and reference signal V REF  are equal, the control circuit can control power switch S 1  to turn on, and the inductor current may begin to rise. At time t 1 , power switch S 1  can be turned off, power switch S 2  may be turned on, and the inductor current can begin to decrease. At time t 2 , power switch S 1  may be turned on again, and the switching converter can operate in a steady state in cycles. 
     As shown in (b) of  FIG. 6 , in the current discontinuous mode, the generation principles of triangular wave signal Vtri, voltage Vc 2  across capacitor C 2 , valley signal Vva and ripple signal Vrip, are the same as those in (a) of  FIG. 6 . At time t 0 , when the valley value of loop control signal V FB1  is equal to reference signal V REF , that is, feedback signal V FB  (shown by the dotted line in the figure) is equal to reference signal V REF , the control circuit can control power switch S 1  to turn on, and the inductor current may begin to rise. At time t 1 , power switch S 1  can be turned off, power switch S 2  may be turned on, and the inductor current can begin to decrease. At time t 2 , power switches S 1  and S 2  may both be turned off, and the inductor current can be zero, and the valley value may be retained. At time t 3 , power switch S 1  can be turned on again, and the switching converter may operate in a steady state in cycles. 
     Referring now to  FIG. 7 , shown is a waveform diagram of example operation of the switching converter when the load changes, in accordance with embodiments of the present invention. In this example, the buck converter is taken as an example for illustration. During time period t 0 -t 1 , the switching converter may operate in a steady state. Power switch S 1  can be controlled to be turned on when the valley value of loop control signal V FB1  is equal to reference signal V REF . Then, inductor current IL can begin to rise, and power switch S 1  may be controlled to be turned off after a predetermined time. Then, inductor current IL may decrease, such that a stable output voltage Vo can be generated during the steady state. 
     At time t 1 , the load jumps from a heavy-load to a no-load. Since inductor current IL cannot suddenly change, the energy is excessive, so output voltage Vo rises, such that loop control signal V FB1  may remain greater than reference signal V REF . At time t 2 , the load jumps from a no-load to a heavy-load. Since there may only be one voltage loop in the control circuit in this example, it can quickly respond to the sudden change of the load in response to loop control signal V FB1 . Thus, output voltage Vo may drop only a small voltage drop ΔV in a relatively short period of time, and the switching converter can return back to be in a steady state after a relatively short period of time. Therefore, the switching converter in particular embodiments has a relatively good dynamic response. 
     In particular embodiments, by superimposing a ripple signal with a variation range between zero and a preset value on the feedback voltage, a loop control signal may be generated, and the power stage circuit can be controlled according to the loop control signal and the reference signal. Further, the control circuit may not need to correct the reference signal, such that a correction circuit can be omitted. In addition, there may only be one voltage inner loop, which can quickly respond to the dynamic change of the load, and can ensure that the output voltage accuracy of the switching converter can be increased in different applications. 
     The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with modifications as are suited to particular use(s) contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.