# Patent

## Title
Constant voltage constant current control circuits and methods with improved load regulation

## Abstract

The present invention discloses CVCC circuits and methods with improved load regulation for an SMPS. In one embodiment, the CVCC can include: a voltage feedback circuit to generate an output voltage feedback signal; a current feedback circuit to generate an output current feedback signal; a control signal generating circuit that receives the output voltage feedback signal and the output current feedback signal, and generates a constant voltage/constant current control signal; a first enable signal generating circuit that compares a first reference voltage and the constant voltage/constant current control signal to generate a first enable signal; and a PWM controller that generates a PWM control signal based on the constant voltage/constant current control signal to control a main switch of the flyback SMPS.

## Background
RELATED APPLICATIONS
This application claims the benefit of Chinese Patent Application No. 201210082610.1, filed on Mar. 26, 2012, which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
The present invention relates to the field of switch mode power supply (SMPS), and more specifically to constant voltage constant current control (CVCC) circuits and methods with improved load regulation.
BACKGROUND
Switch mode power supply (SMPS) may be characterized as small volume, light weight, high power conversion efficiency, etc., and SMPS has been widely used in industrial automation, instruments and meters, medical facilities, liquid crystal display (LCD), communication apparatus, audio-visual products, digital products, and other fields. SMPS is a power supply that utilizes modern power electronics to control a ratio of a switch's on time and off time, so as to remain the output of the power supply substantially steady. Usually, electronic devices, such as MOSFETs controlled by pulse-width modulation (PWM) are used in SMPS circuits.
SUMMARY
In one embodiment, a constant voltage constant current (CVCC) circuit for a switch mode power supply (SMPS), can include: (i) a voltage feedback circuit configured to generate an output voltage feedback signal; (ii) a current feedback circuit configured to generate an output current feedback signal; (iii) a control signal generating circuit configured to receive the output voltage feedback signal and the output current feedback signal, and to generate a constant voltage/constant current control signal; (iv) a first enable signal generating circuit configured to compare a first reference voltage and the constant voltage/constant current control signal to generate a first enable signal; and (v) a pulse-width modulation (PWM) controller configured to generate a PWM control signal based on the constant voltage/constant current control signal to control a main switch of the flyback SMPS, where the PWM control signal is configured to turn on the main switch when the first enable signal is inactive, and wherein the main switch remains off when the first enable signal is active.
In one embodiment, a method can include: (i) obtaining an output current feedback signal and an output voltage feedback signal; (ii) generating a constant voltage/constant current control signal based on the output current feedback signal and the output voltage feedback signal; (iii) generating a first enable signal based on the constant voltage/constant current control signal and a first reference voltage; (iv) controlling a main switch of the flyback SMPS based on the first enable signal, where the main switch is turned off by the first enable signal when the first enable signal is active; (v) generating, in a first operation mode, a PWM control signal based on the constant current control signal to control the main switch to maintain substantially constant output current; and (vi) generating, in a second operation mode, the PWM control signal based on the constant voltage control signal, where the PWM control signal is used to control the main switch when the first enable signal is inactive.
Embodiments of the present invention can advantageously provide several advantages over conventional approaches. For example, particular embodiments can provide CVCC control circuits and methods with improved load regulation, and that have a simplified circuit structure, relatively small volume, and relatively low product cost. In addition, when under light load or even a no-load state, the main switch of the SMPS can be turned off to avoid power loss. Other advantages of the present invention may become readily apparent from the detailed description of preferred embodiments below.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an example flyback SMPS.
FIG. 2 is a block diagram of a first example power supply controlled by a CVCC control circuit in accordance with embodiments of the present invention.
FIG. 3 is a block diagram of a second example power supply controlled by a CVCC control circuit in accordance with embodiments of the present invention.
FIG. 4 is a schematic diagram of an example specific implementation of the CVCC control circuit shown in FIG. 3.
FIG. 5 is an operation waveform diagram of the CVCC control circuit shown in FIG. 4 with a light load.
FIG. 6 is an operation waveform diagram of the CVCC control circuit shown in FIG. 4 with an even lighter load.
FIG. 7 is a schematic diagram of another example specific implementation of the CVCC control circuit shown in FIG. 3.
FIG. 8 is an operation waveform diagram of the CVCC control circuit shown in FIG. 7.
FIG. 9 is another example specific implementation of the CVCC control circuit shown in FIG. 3.
FIG. 10 is a flow diagram of an example CVCC method with improved load regulation, 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 fourth 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.
In a common switch mode power supply (SMPS) arrangement, a flyback converter can achieve substantially constant voltage or constant current output by applying a secondary-side control circuit to control a main switch of the power circuit. With reference to FIG. 1, shown is a schematic diagram of an example flyback SMPS. The flyback SMPS can control main switch QM by secondary-side feedback control circuit 102 to realize substantially constant voltage or constant current output. For example, an output voltage signal and an output current signal of the flyback SMPS can be detected by voltage sampling resistors, current sampling resistors, and opto-coupler 101. Secondary-side feedback control circuit 102 can be used to process the detected voltage signal and current signal, and to generate a corresponding control signal to control main switch QM. In this way, the flyback SMPS can generate substantially constant voltage output Vout or constant current output Iout.
The constant voltage constant current (CVCC) circuit can include secondary-side feedback control circuit 102, and opto-coupler 101, used to detect the feedback signal. However, the use of opto-coupler 101 may be difficult to integrate with other circuit portions, and may thus increase overall circuit volume and product cost. Also, in many practical applications, when the load of the main circuit is reduced to a certain extent (e.g., becomes a “light” load), the primary-side circuit may still transfer energy to the output terminal, resulting in power loss.
In particular embodiments, constant voltage constant current (CVCC) control circuits and methods with improved load regulation, can include: a voltage feedback module/circuit, a current feedback module/circuit, a control signal generating module/circuit, a first enable signal generating module/circuit, and a pulse-width modulation (PWM) controller. The control signal generating module can control the flyback SMPS operating in a constant voltage or constant current state, and generate a constant voltage/constant current control signal. The PWM controller can generate a PWM control signal based on the constant voltage/constant current control signal to control the main switch. The CVCC control circuit can achieve substantially constant current control and constant voltage control through select control, so the circuit structure can be relatively simple. Also, when the example CVCC control circuit is operating under a light load or even a no-load state, the main switch can be turned off by the enable signal to stop energy from being transferred from the input side to avoid associated power loss.
In one embodiment, a CVCC circuit for a SMPS, can include: (i) a voltage feedback circuit configured to generate an output voltage feedback signal; (ii) a current feedback circuit configured to generate an output current feedback signal; (iii) a control signal generating circuit configured to receive the output voltage feedback signal and the output current feedback signal, and to generate a constant voltage/constant current control signal; (iv) a first enable signal generating circuit configured to compare a first reference voltage and the constant voltage/constant current control signal to generate a first enable signal; and (v) a PWM controller configured to generate a PWM control signal based on the constant voltage/constant current control signal to control a main switch of the flyback SMPS, where the PWM control signal is configured to turn on the main switch when the first enable signal is inactive, and wherein the main switch remains off when the first enable signal is active.
Particular embodiments can provide a CVCC control circuit with improved load regulation for a flyback SMPS. Referring now to FIG. 2, an example CVCC control circuit can include voltage feedback module 201, current feedback module 202, control signal generating module 203, first enable signal generating module 204, and PWM controller 205. As used herein, a “module” can include a circuit or circuit portion, such as a circuit integrated on an integrated circuit (IC), or a separate IC or other device, such as may be integrated on a printed-circuit board (PCB).
The CVCC control circuit can generate a corresponding control signal to control main switch QM of the power supply by processing output voltage feedback signal VFB and output current feedback signal IFB. The operation mode of the flyback SMPS can be controlled to achieve substantially constant voltage output Vout and/or substantially constant current output Iour of the SMPS. For example, the flyback converter can include primary winding Np, secondary winding Ns, and auxiliary winding NT. In this example, secondary winding Ns can be coupled to the load.
Voltage feedback module 201 can generate output voltage feedback signal VFB, and current feedback module 202 can generate output current feedback signal IFB. Control signal generating module 203 can receive output voltage feedback signal VFB and output current feedback signal IFB, and may generate constant voltage/constant current control signal Vcomp. First enable signal generating module 204 can compare reference voltage Vref1 and constant voltage/constant current control signal Vcomp to generate enable signal EN1.
PWM controller 205 can generate a PWM control signal based on constant voltage/constant current control signal Vcomp to control main switch QM of the flyback SMPS. For example, when enable signal EN1 is inactive (e.g., a logic low level), the PWM control signal can be used to turn on or otherwise control main switch QM. However, when enable signal EN1 is active (e.g., a logic high level), main switch QM may be off. In other examples, EN1 may be active to remain main switch QM on, and may be inactive to allow the PWM control signal to effectively control main switch QM. The particular enable single control state and main switch on, off, or other control states, can depend on the type of main switch QM (e.g., PMOS or NMOS transistor), as well as the logic gates utilized in the control circuitry. Particular embodiments are amenable to any such type of enable-based control or enable/disable function of main switch QM.
In particular embodiments, the example CVCC control circuit can control the output of the SMPS without an opto-coupler. As a result, the circuit structure can be much simpler and easier to be implemented, and the circuit volume can be relatively small as compared to conventional approaches. Also, when the operation is under a light load or even a no-load state, main switch QM can be turned off to prevent associated power loss.
Referring now to FIG. 3, shown is a specific implementation of control signal generating module 203 of CVCC control circuit. For example, control signal generating module 203 can include current controller 301 that can calculate a difference between output current feedback signal IFB and reference current Iref1, so as to generate error signal Verr. Voltage controller 302 can compare output voltage feedback signal VFB against reference voltage Vref2, so as to generate control signal Vctrl1. Also, select controller 303 can generate constant voltage/constant current control signal Vcomp to control the flyback SMPS to operate in a constant voltage mode or a constant current mode based on control signal Vctrl1 and error signal Verr.
The following will describe example operation procedure with reference to various portions of the example circuit of FIG. 3. The flyback SMPS with substantially steady output can be controlled by the CVCC control circuit of particular embodiments, and can include signal input portion A, transformer B, load portion C, CVCC control portion D, and signal collection portion E. Signal input portion A, transformer B, load portion C, and CVCC control portion D as shown in FIG. 3 may have a similar circuit structure as the flyback SMPS discussed above.
Signal collection portion E can collect the output signals of load portion C. Specifically, auxiliary winding NT coupled to secondary winding NS of the flyback converter can obtain output voltage information. Also, secondary-side output voltage VS obtained through dividing resistors R11 and R22 can be converted by voltage feedback module 201 to obtain output voltage feedback signal VFB. In addition, primary-side current IS induced through current sense resistor RS can be converted by current feedback module 202 to obtain output current feedback signal IFB.
Output current feedback signal IFB and output voltage feedback signal VFB can be input to current controller 301 and voltage controller 302, respectively. Current controller 301 can calculate a difference between output current feedback signal IFB and reference current Iref1, to generate output error signal Verr. Also, voltage controller 302 can compare output voltage feedback signal VFB against reference voltage Vref2, to generate control signal Vctrl1.
Select controller 303 can select an operation mode of the flyback SMPS based on control signal Vctrl1 and error signal Verr, and may generate constant voltage/constant current control signal Vcomp. When control signal Vctrl1 is at high level, the flyback SMPS may operate in a first operation mode (e.g., a constant current operation mode). When control signal Vctrl1 is a pulse type of signal with a certain duty cycle, the flyback SMPS may operate in a second operation mode (e.g., a constant voltage operation mode).
PWM controller 205 can generate a PWM control signal based on constant voltage/constant current control signal Vcomp to control main switch QM. For example, when enable signal EN1 is inactive, the PWM control signal can be used to turn on main switch QM. Also, when enable signal EN1 is active, main switch QM may remain off, in order to reduce power loss. Enable signal EN1 can be generated by comparing constant voltage/constant current control signal Vcomp against reference voltage Vref1. Enable signal EN1 can be configured to represent for a load state (e.g., light load, no load, heavy load, etc.) of the main circuit. Constant voltage/constant current control signal Vcomp can be received by enable signal generating module 204 coupled to select controller 303. Therefore, when the load of the main circuit is reduced to a certain extent or even becomes no-load, main switch QM of the flyback SMPS can be turned off to stop energy from being transferred from the input side, and thus to avoid power loss.
When control signal Vctrl1 is at high level, select controller 303 can control the entire circuit operating in the first operation mode (e.g., a constant current operation mode). Select controller 303 can generate constant current control signal Vcomp based on error signal Verr. PWM controller 205 can receive constant current control signal Vcomp and generate the PWM control signal to control main switch QM, so as to maintain output current Iout as substantially constant.
When control signal Vctrl1 is a pulse signal with a certain duty cycle, select controller 303 can control the flyback SMPS to operate in a second operation mode (e.g., a constant voltage operation mode). Under this circumstance, select controller 303 can generate constant voltage control signal Vcomp based on error signal Verr. Also, PWM controller 205 can receive constant voltage control signal Vcomp and generate the PWM control signal. For example, when enable signal EN1 is inactive, main switch QM can be turned on to maintain voltage Vout as substantially constant. When first enable signal EN1 is active, main switch QM may be off. That is, when the load of the main circuit is reduced to a light load or even becomes no-load, enable signal EN1 can be utilized to remain main switch QM off to stop energy from being transferred from the input side to avoid associated power loss.
In particular embodiments, a CVCC control circuit can collect voltage and current signals through signal collection portion E, and generate control signal through CVCC control portion D to control main switch QM. In this way, the output voltage or current can be maintained as substantially constant with a relatively simple circuit structure also, when the CVCC control circuit is operating under a light load or even a no-load state, enable signal EN1 can be configured to turn off main switch QM to stop energy from being transferred from the input side, thus avoiding associated power loss.
Referring now to FIG. 4, shown our specific example implementations and example operation procedures of the current controller, the voltage controller, the select controller, and the first enable signal generating module. Current controller 301 of the CVCC control circuit can include transconductance amplifier 401. A non-inverting input terminal of transconductance amplifier 401 can receive reference current Iref1, an inverting input terminal can receive output current feedback signal IFB, and an output terminal can output error signal Verr.
Voltage controller 302 of the CVCC control circuit can include comparator 402 and logic controller 403. A non-inverting input terminal of comparator 402 can receive reference voltage Vref2, an inverting terminal can receive output voltage feedback signal VFB, and an output terminal can output middle signal Vsig. Logic controller 403 can receive middle signal Vsig, and in one example the PWM control signal can be a clock signal. At each rising edge of the PWM control signal, control signal Vctrl1 can be updated as necessary based on middle signal Vsig.
For example, at each rising edge of the PWM control signal, middle signal Vsig can be sampled to decide if there is a change in its detected state (e.g., detected as a high or a low level based on appropriate thresholds). If there is no change in state the detected state of middle signal Vsig, control signal Vctrl1 can be maintained in its same state. However, when middle signal Vsig is detected or sampled as a different state, control signal Vctrl1 can accordingly change on a next rising edge of the PWM control signal. First logic controller 403 can be implemented by a D type flip-flop, where input terminal D can receive middle signal Vsig, and terminal CLK can receive PWM control signal as its clock signal. Therefore, at each rising edge of the PWM control signal, the D flip-flop can effectively output middle signal Vsig as control signal Vctrl1.
Select controller 303 of the CVCC control circuit can include OR-gate 405, switch Q1, switch Q2, discharge circuit 406, and capacitor C1, as shown in FIGS. 3 and 4. OR-gate 405 can receive control signal Vctrl1, and enable signal EN1, and can provide an output to control switch Q1. Enable signal EN1 can be generated by enable signal generating module 204. Enable signal generating module 204 can include hysteresis comparator 404. A non-inverting input terminal of hysteresis comparator 404 can receive reference voltage Vref1, an inverting input terminal can receive constant voltage/constant current control signal Vcomp, and an output terminal can output enable signal EN1.
A first input terminal of switch Q1 can receive error signal Verr, and a second input terminal can be series connected to switch Q2 and discharge circuit 406. For example, switch Q2 may only be turned on when enable signal EN1 and control signal Vctrl1 are both inactive. Otherwise, switch Q2 may remain off. Also, one terminal of capacitor C1 can be coupled to a common junction of switches Q1 and Q2, while the other terminal can be coupled to ground. The voltage across capacitor C1 can be configured as constant voltage/constant current control signal Vcomp. The discharging time of discharge circuit 406 can be constant or flexible. As such, discharging circuit 406 can be implemented by suitable components (e.g., a constant resistor, a variable resistor, a constant current source, or a variable current source, etc.).
FIG. 4 only shows and describes specific example implementations and operation procedure of current controller 301, voltage controller 302, select controller 303, and enable signal generating module 204. Other circuit structures like voltage feedback module 201 and current feedback module 202 are not shown. Voltage feedback module 201 and current feedback module 202 can be configured by sensors with conversion processing function and feedback function. Also, the data collection portion can have a same or a similar structure as described above, and PWM controller 205 can be configured by a PWM control circuit of a traditional SMPS.
One example operation procedure and operating principles of the CVCC control circuits in FIGS. 3 and 4 can be described below in conjunction with the signal waveform diagram shown in FIG. 5. When the main circuit is under a heavy-load condition, output voltage Vout may decrease. Also, output voltage feedback signal VFB obtained by voltage feedback module 201 may be relatively low. Since reference voltage Vref2 can be larger than output voltage feedback signal VFB during this time (e.g., between t2 and t4), middle signal Vsig output by comparator 402 may remain at a high level. Consequently, control signal Vctrl1 may also remain at a high level, and select controller 303 can control the flyback SMPS operating in the first operation mode (e.g., a constant current operation mode).
Under this circumstance, switch Q1 may remain on and switch Q2 may can remain off. Error signal Verr output by current controller 301 can be utilized to charge capacitor C1. When the output current of the main circuit is changing, output current feedback signal IFB may also change. Error signal Verr may increase or decrease accordingly to cause the charging current for capacitor C1 to increase or decrease. As such, constant current control signal Vcomp may change and this effect can be transferred to PWM controller 205. The PWM control signal generated by PWM controller 205 can control main switch QM, so as to maintain the output current as substantially constant.
For example, when the main circuit is under light load conditions, control signal Vctrl1 can be a pulse signal with a certain duty cycle. Select controller 303 can control operation of the flyback SMPS in a second operation mode (e.g., a constant voltage operation mode). Under the constant voltage control mode, when enable signal EN1 is inactive, the PWM control signal can be configured to control main switch QM to maintain output voltage Vout of the flyback SMPS as substantially constant. When enable signal EN1 is active, main switch QM may remain off. In practical applications, enable control signal EN1 can be inverted by an inverter, and inversion of enable control signal EN1 and the PWM control signal can be input to two terminals of AND-gate 407. An output signal of AND-gate 407 can be configured as control signal DRV for main switch QM.
In the constant voltage mode, when the load is relatively light, enable signal EN1 may remain at a low level, and the operation waveform diagram of the CVCC control circuit can be seen in FIG. 5. Since enable signal EN1 may be at a low level inactive state, switches Q1 and Q2 may only be controlled by control signal Vctrl1. During the period of t1 to t2, output voltage feedback signal VFB can be larger than reference voltage Vref2, so middle signal Vsig generated by comparator 402 can be low. Also, control signal Vctrl1 can be low to control switch Q1 to remain off, and switch Q2 to remain on. In addition, capacitor C1 may discharge through discharge circuit 406, and constant voltage control signal Vcomp may decrease accordingly. Further, the duty cycle of the corresponding PWM control signal may also decrease as well as output voltage feedback signal VFB.
From time t2 on, output voltage feedback signal VFB may be smaller than reference voltage Vref2, and middle signal Vsig output by comparator 402 may go to a high level. By this time, since the rising edge of the PWM control signal has not yet occurred, control signal Vctrl1 can remain low. Thus, the switch states of switches Q1 and Q2 may remain the same. Also, constant voltage control signal Vcomp can continue decreasing until time t3. At time t3, the rising edge of the PWM control signal can arrive, and control signal Vctrl1 output by D flip-flop may thus go high.
After control signal Vctrl1 goes to high level, switch Q1 can be turned on and switch Q2 can be turned off. Also, capacitor C1 can be charged by output current feedback signal IFB based on error signal Verr. Thus, constant voltage control signal Vcomp may increase, and the duty cycle of the PWM control signal may also increase as well as output voltage feedback signal VFB. By repeating such, output voltage Vout of the main circuit can be substantially constant.
When the load of the main circuit further decreases, enable signal EN1 can be a pulse signal. The waveform diagram of the CVCC control circuit under this circumstance can be shown as FIG. 6. As the load becomes lighter, output voltage Vout of the main circuit may increase. Also, output voltage feedback signal VFB is relatively large and may be larger than reference voltage Vref2. Thus, control signal Vctrl1 output by comparator 402 can remain low, and the switch states of switches Q1 and Q2 may only be determined by enable signal EN1.
During the period of t1 ̃t2, constant voltage control signal Vcomp may decrease to lower limit voltage VL from upper limit voltage VH, enable signal EN1 can remain low, and the output of OR-gate 405 can also remain low. Also, switch Q1 can remain off and switch Q2 can remain on. Capacitor C1 can discharge through discharge circuit 406, and constant voltage control signal Vcomp may decrease slowly. Therefore, control signal DRV of main switch QM can be kept consistent with the PWM control signal, and output voltage feedback signal VFB may increase gradually.
At time t2, constant voltage control signal Vcomp may decrease to lower limit voltage VL, and enable signal EN1 output by hysteresis comparator 404 may go high. Also, the output of OR-gate 405 may also go high to turn on switch Q1. At the same time, switch Q2 can be turned off. As enable signal EN1 goes high, the transconductance of transconductance amplifier 401 may decrease to generate a smaller error signal Verr.
During the period of t2 ̃t3, capacitor C1 can receive error signal Verr and can be charged. The charging time can be extended since error signal Verr is smaller or less than, and constant voltage control signal Vcomp may increase gradually. Control signal DRV of main switch QM can remain low, so main switch QM can remain off, and output voltage feedback signal VFB can decrease gradually.
The illustrated implementations of all the example modules or circuits can be replaced by other circuit structures or other components with the same or a similar function. For example, the switches of the CVCC control circuit can be configured by MOSFET transistors, or any other appropriate transistors or switching devices. The CVCC control circuit of particular embodiment can control the circuit operation mode by applying a select controller. When the load of the main circuit is reduced to a certain predetermined level, enable signal EN1 can maintain main switch QM at an off state to stop energy from being transferred from the input side of the main, thus avoiding associated power loss.
In particular embodiments, output voltage feedback signal VFB can decrease to reference voltage Vref2 when constant voltage control signal Vcomp rises to upper limit voltage VH. However, if the load becomes lighter, and possibly becomes no-load, output voltage feedback signal VFB may not reduce to reference voltage Vref2. Yet when constant voltage control signal Vcomp has risen to upper limit voltage VH and enable signal EN1 has gone low, output voltage feedback signal VFB may start to gradually increase as control signal DRV of main switch QM can remain consistent with the PWM control signal. In order to extend the off time of main switch QM when the main circuit is under a light load or no-load state, adjustments on the CVCC control circuit described above can be made, as will be discussed further below with reference to FIG. 7.
Referring now to FIG. 7, compared to the above described example (see, e.g., FIG. 4), the present example CVCC can convert two-input AND-gate 407 to three-input AND-gate 704. Also, two input signals of three-input AND-gate 704 can remain unchanged to be an inverted version of enable signal EN1, and the PWM control signal. The third input terminal of AND-gate 704 can receive enable signal EN2 generated by enable signal generating module 720. Enable signal generating module 720 can include comparator 701, AND-gate 702 and RS flip-flop 703, as shown in FIG. 7.
A non-inverting input terminal of comparator 701 can receive output voltage feedback signal VFB, an inverting input terminal can receive reference voltage Vref3, and an output terminal can be coupled to a first input terminal of AND-gate 702. A second input terminal of first AND-gate 702 can receive an inverted version of the PWM control signal, and an output terminal can be coupled to a reset terminal of RS flip-flop 703. A set terminal of RS flip-flop 703 can receive enable signal EN1, and an output terminal can output enable signal EN2.
When enable signal EN2 is active high and enable signal EN1 is inactive low, the PWM control signal can be used to control main switch QM. When enable signal EN2 is inactive low or enable signal EN1 is active high, main switch QM may remain off. As shown in FIG. 7, enable signal EN1 can be used to obtain an inverted version of signal enable signal EN1 through an inverter. The inverted version of enable signal, enable signal EN2, and the PWM control signal can be input to terminals of AND-gate 704. The output signal of AND-gate 704 can be configured as control signal DRV for main switch QM.
Example operation procedure and operation principles of the example CVCC control circuit of FIG. 7 can be described in detail below in conjunction with the specific waveform diagram. Referring now to FIG. 8, during the period of t1 ̃t2, output voltage feedback signal VFB may be smaller than reference voltage Vref3, so the output signal of second comparator 701 can be at a low level to cause enable signal EN2 to go high. During this period, the operation procedure and operation principle of the CVCC control circuit can be substantially the same as the CVCC control circuit shown in FIG. 4.
When the load of the main circuit is even lighter than in the example operation discussed above, or even becomes no-load, output voltage Vout of the main circuit may increase to cause output voltage feedback signal VFB to not fully decrease to second reference voltage Vref2 by time t2. However, at time t2, constant voltage control signal Vcomp may already have risen to upper limit voltage VH to result in enable signal EN1 going low. Thus, the PWM control signal can still control main switch QM. From time t2 on, output voltage feedback signal VFB can start rising, and constant voltage control signal Vcomp may decrease due to the discharging of capacitor C1.
At time t3, output voltage feedback signal VFB may have risen to reference voltage Vref3, so the output signal of comparator 701 may go high and be input to first AND-gate 702. At the same time, PWM control signal may be at a low level, AND-gate 702 can generate a high signal to the reset terminal of RS flip-flop 703 to reset enable signal EN2 to low. From time t3 on, control signal DRV of main switch QM may go low to turn off main switch QM. Also, output voltage feedback signal VFB may decrease gradually as the energy transferred from the input side of the main circuit is stopped.
During the period of t3 ̃t4, enable signal EN2 can stay at a low level. Since enable signal EN1 remains at a low level, the discharging of capacitor C1 may cause constant voltage control signal Vcomp to continue to decrease. At time t4, constant voltage control signal Vcomp can decrease to lower limit voltage VL to cause enable signal EN1 to go high. Because enable signal EN1 can be input to the set terminal of RS flip-flop 703, enable signal EN2 output by RS flip-flop 703 can go high.
During the period of time t4 ̃t5, as enable signal EN1 goes high, main switch QM can remain off. Also, output voltage feedback signal VFB may continue to decrease, capacitor C1 may begin to charge, and constant voltage control signal Vcomp may start rising until reaching upper limit voltage VH at time t5. When the load of the main circuit is reduced to a certain extent or even becomes no-load, the example CVCC control circuit can turn off main switch QM by enable signal EN2 to stop energy from being transferred from the input side, thus avoiding associated power loss.
In particular embodiments, the output voltage signal can be collected through auxiliary winding NT to obtain output voltage feedback signal VFB. Since output voltage Vs of auxiliary winding NT may be a discontinuous signal, output voltage feedback signal VFB may be pulled down to avoid the entire circuit possibly suffering from undesirable effects due to signal discontinuity, such as entering a locked state. In view of this, particular embodiments can also provide another specific implementation of the CVCC control circuit, as will be discussed below with reference to FIG. 9.
Referring now to FIG. 9, the example CVCC control circuit can also include AND-gate 901 and recovery circuit 902, as compared to the above example. AND-gate 901 can receive control signal Vctrl1 and enable signal EN2. Also, an output terminal of AND-gate 901 can connect to an input terminal of OR-gate 405 of select controller 303. When the output of comparator 701 is high, recovery circuit 902 can pull down output voltage feedback signal VFB. As a result, the output of comparator 701 can quickly recover to a low state.
In this example, recovery circuit 902 can include switches Q3 and Q4. A first terminal of switch Q3 can connect to the non-inverting input terminal of comparator 701. A second terminal of switch Q3 can be series connected with switch Q4 to ground. Also, a control terminal of switch Q3 can connect to a first input terminal thereof, in a diode-connected transistor configuration. A control terminal of switch Q4 can connect to the output terminal of comparator 701.
When output voltage feedback signal VFB is larger than reference voltage Vref3, the output of comparator 701 can go high to turn on switch Q4. Thus, output voltage feedback signal VFB can be pulled down by switches Q3 and Q4 to result in the output of comparator 701 going low again. Therefore, when output voltage feedback signal VFB is larger than reference voltage Vref3, the output of comparator 701 can be a single pulse signal.
At the same time, since output voltage feedback signal VFB can be pulled down, it may be smaller than reference voltage Vref2. Control signal Vctrl1 output by voltage controller 302 can go high to ensure that switch Q1 cannot be turned on by mistake. Also, switch Q2 cannot be turned off by mistake. Control signal Vctrl1 and enable signal EN2 can be coupled to the input terminal of AND-gate 901. When control signal Vctrl1 goes high because output voltage feedback signal VFB is pulled down, comparator 701 can output a low level to keep enable signal EN2 at a low level. Thus, the output of AND-gate 901 can remain inactive low, and may be input to OR-gate 405.
In particular embodiments, output voltage feedback signal VFB can be pulled down via AND-gate 901 and recovery circuit 902 to avoid the entire circuit entering a locked state. In addition, when the output of the SMPS is controlled as substantially steady, main switch QM can be controlled along with the load change to avoid power losses, as discussed above.
In one embodiment, a method can include: (i) obtaining an output current feedback signal and an output voltage feedback signal; (ii) generating a constant voltage/constant current control signal based on the output current feedback signal and the output voltage feedback signal; (iii) generating a first enable signal based on the constant voltage/constant current control signal and a first reference voltage; (iv) controlling a main switch of the flyback SMPS based on the first enable signal, where the main switch is turned off by the first enable signal when the first enable signal is active; (v) generating, in a first operation mode, a PWM control signal based on the constant current control signal to control the main switch to maintain substantially constant output current; and (vi) generating, in a second operation mode, the PWM control signal based on the constant voltage control signal, where the PWM control signal is used to control the main switch when the first enable signal is inactive.
Referring now to FIG. 10, particular embodiments can provide a CVCC control method with improved load regulation for flyback SMPS. For example, the CVCC control method can include obtaining output current feedback and output voltage feedback signals at S1001. For example, a signal collection circuit can be configured to collect output current and output voltage signals of the SMPS. Also, voltage and current feedback modules can process the output current and voltage signals to obtain output current and voltage feedback signals.
At S1002, a constant voltage/constant current control signal can be generated based on the output current feedback signal and the output voltage feedback signal. At S1003, an enable signal can be generated based on the constant voltage/constant current control signal and a reference voltage. At S1004, a main switch of the flyback SMPS can be controlled based on the enable signal.
At S1005, in the first operation mode, a PWM control signal under constant current state can be generated based on the constant current control signal to control the main switch, so as to remain the output current as substantially constant. At S1006, in the second operation mode, a PWM control signal under constant voltage state can be generated based on the constant voltage control signal. If the enable signal is active, the main switch can be turned off by the enable signal. If the enable signal is inactive, the main switch can be controlled (e.g., turned off/on) by the PWM control signal. Further, the enable signal can be configured to represent a load state of the main circuit. For example, when under light load or no-load condition, the main switch can be turned off to stop energy from being transferred from the input side, thus avoiding associated power loss.
In addition, the above described control method can also include generating an error signal and a control signal based on the output current feedback signal and the output voltage feedback signal, and calculating the difference between the output current feedback signal and a reference current to obtain the error signal. Also, the output voltage feedback signal can be compared against a second reference voltage to generate the control signal. Also, the operation mode of the flyback SMPS can be selected based on the error signal and the control signal.
When the SMPS is working operates in the second operation mode, the above described control method can also include obtaining a second enable signal based on the output voltage feedback signal, a third reference voltage, the PWM control signal, and the first enable signal. When the second enable signal is active and the first enable signal is inactive, the main switch can be controlled based on the PWM control signal. When the second enable signal is inactive or the first enable signal is active, the main switch can be turned off.
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 various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

## Claims

1. A constant voltage/constant current (CVCC) control circuit for load regulation for a flyback switch mode power supply (SMPS), the CVCC comprising:
a) a voltage feedback circuit configured to generate an output voltage feedback signal;
b) a current feedback circuit configured to generate an output current feedback signal;
c) a control signal generating circuit configured to receive said output voltage feedback signal and said output current feedback signal, and to generate a CVCC control signal;
d) a first enable signal generating circuit configured to compare a first reference voltage and said CVCC control signal to generate a first enable signal;
e) said control signal generating circuit comprising a current controller configured to calculate a difference between said output current feedback signal and a reference current to generate an error signal, a voltage controller configured to compare said output voltage feedback signal against a second reference voltage to generate a first control signal, and a select controller configured to control whether said flyback SMPS operates in a constant voltage mode or a constant current mode based on said first control signal and said error signal;
f) said select controller comprising an OR-gate configured to receive said first control signal and said first enable signal, a first switch configured to receive said error signal and having a control terminal coupled to said OR-gate, a second switch coupled to said first switch and a discharge circuit and being configured to turn on when said first enable signal and said first control signal are inactive, and a capacitor coupled to ground and a common node of said first and second switches, wherein a voltage across said capacitor is configured as said CVCC control signal; and
g) a pulse-width modulation (PWM) controller configured to generate a PWM control signal based on said CVCC control signal to control a main switch of said flyback SMPS, wherein said PWM control signal is configured to turn on said main switch when said first enable signal is inactive, and wherein said main switch remains off when said first enable signal is active.

2. The CVCC control circuit of claim 1, wherein said first enable signal generating circuit comprises a first hysteresis comparator having a non-inverting input terminal configured to receive said first reference voltage, an inverting input terminal configured to receive said CVCC control signal, and an output terminal configured to output said first enable signal.

3. The CVCC control circuit of claim 1, wherein said voltage controller comprises:
a) a first comparator having a non-inverting input terminal configured to receive said second reference voltage, an inverting input terminal configured to receive said output voltage feedback signal, and an output terminal configured to output a middle signal; and
b) a logic controller configured to receive said middle signal, and to generate said first control signal, wherein said PWM control signal is used as a clock signal for said logic controller.

4. The CVCC control circuit of claim 1, further comprising a second enable signal generating circuit having:
a) a second comparator configured to receive said output voltage feedback signal and a third reference voltage;
b) a first AND-gate coupled to receive an output of said second comparator and an inverted version of said PWM control signal; and
c) an RS flip-flop, wherein a reset terminal of said RS flip-flop is coupled to an output of said first AND-gate, a set terminal of said RS flip-flop is coupled to said first enable signal, and an output terminal of said RS flip-flop is configured to output a second enable signal,
d) wherein said PWM control signal is configured to turn on said main switch when said second enable signal is active and said first enable signal is inactive, and
e) wherein said main switch remains off when said second enable signal is inactive or said first enable signal is active.

5. The CVCC control circuit of claim 4, further comprising:
a) a second AND-gate configured to receive said first control signal and said second enable signal, and having an output coupled to said control signal generating module; and
b) a recovery circuit configured to pull down said output voltage feedback signal to fast recover an output of said second comparator to a low level when said output of said second comparator is active.

6. A constant voltage/constant current (CVCC) control method for load regulation for a flyback switch mode power supply (SMPS), the method comprising:
a) obtaining an output current feedback signal and an output voltage feedback signal;
b) generating a CVCC control signal based on said output current feedback signal and said output voltage feedback signal;
c) generating a first enable signal based on said CVCC control signal and a first reference voltage;
d) controlling a main switch of said flyback SMPS based on said first enable signal, wherein said main switch is turned off by said first enable signal when said first enable signal is active;
e) generating, in a first operation mode, a pulse-width modulation (PWM) control signal based on said constant current control signal to control said main switch to maintain substantially constant output current;
f) generating, in a second operation mode, said PWM control signal based on said CVCC control signal, wherein said PWM control signal is used to control said main switch when said first enable signal is inactive;
g) generating an error signal by calculating a difference between said output current feedback signal and a reference current;
h) comparing said output voltage feedback signal against a second reference voltage to generate a first control signal; and
i) selecting, by a select controller, one of said first and second operation modes based on said error signal and said first control signal, wherein said select controller comprises an OR-gate configured to receive said first control signal and said first enable signal, a first switch configured to receive said error signal and having a control terminal coupled to said OR-gate, a second switch coupled to said first switch and a discharge circuit and being configured to turn on when said first enable signal and said first control signal are inactive, and a capacitor coupled to ground and a common node of said first and second switches, wherein a voltage across said capacitor is configured as said CVCC control signal.

7. The method of claim 6, further comprising:
a) obtaining a second enable signal based on said output voltage feedback signal, a third reference voltage, said PWM control signal, and said first enable signal;
b) controlling said main switch based on said PWM control signal when said second enable signal is active and said first enable signal is inactive; and
c) turning off said main switch when said second enable signal is inactive or said first enable signal is active.