# Patent

## Title
Control circuit of isolated converter and switching power supply, and control method for the same

## Abstract

Controlling an isolated converter can include: controlling a first voltage signal across the secondary winding according to a wake-up signal to reflect change of an output voltage of the isolated converter; obtaining a second voltage signal representing the first voltage signal at the primary side; controlling the main power switch according to a detection result of the second voltage signal; generating the wake-up signal according to the output voltage and a first threshold voltage when the isolated converter is in a dynamic loading state of a normal operating mode; generating the wake-up signal according to the output voltage and a second threshold voltage when the isolated converter is in a load steady state of a standby operating mode; and generating the wake-up signal according to the output voltage and a third threshold voltage when the isolated converter is in a dynamic loading state of the standby operating mode.

## Background
RELATED APPLICATIONS
This application claims the benefit of Chinese Patent Application No. 201510233160.5, filed on May 8, 2015, which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
The present disclosure generally relates to the field of switching power supplies, and more particularly to control circuits and methods of operation of an isolated converter and a switching power supply.
BACKGROUND
Primary controlled switching power supplies are widely used in applications, such as LED drivers, chargers, adapters, and so on, due to fewer components, a simplified structure, and lower product costs. As shown in FIG. 1, an output voltage may be sampled by detecting a voltage across an auxiliary winding of a transformer. Such a primary controlled system may sample the output voltage by detecting a voltage across the auxiliary winding of the transformer in every switching cycle, instead of directly detecting the output voltage. Thus, drawbacks of this approach may be in failing to timely obtain the output voltage change information when an output load switches from a light-load or no-load state to a heavy-load or full-load state, resulting in relatively poor dynamic performance. Also, the system frequency may be unable to be further decreased due to dynamic response characteristics of the primary controlled system, which can limit the decrease of power losses.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an example of a conventional primary-side control circuit.
FIG. 2 is a schematic block diagram of an example control circuit of an isolated converter, in accordance with embodiments of the present invention.
FIG. 3 is a waveform diagram of example operation of the control circuit of FIG. 2 when the isolated converter is in a dynamic loading state of a normal operating mode, in accordance with embodiments of the present invention.
FIG. 4 is a waveform diagram of example operation of the control circuit of FIG. 2 when the isolated converter is in a load steady state of a standby operating mode, in accordance with embodiments of the present invention.
FIG. 5 is a waveform diagram of example operation of the control circuit of FIG. 2 when the isolated converter is in a dynamic loading state of a standby operating mode, 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.
In one embodiment, a control circuit of an isolated converter comprising a transformer having a primary winding and a secondary winding, a main power switch coupled with the primary winding, and a freewheeling diode coupled with the secondary winding, can include: (i) a secondary-side controller configured to generate a wake-up signal according to an output voltage of the isolated converter to control a first voltage signal across the secondary winding to represent change of the output voltage of the isolated converter; (ii) the secondary-side controller being configured to generate the wake-up signal according to the output voltage and a first threshold voltage when the isolated converter is in a dynamic loading state of a normal operating mode; (iii) the secondary-side controller being configured to generate the wake-up signal according to the output voltage and a second threshold voltage when the isolated converter is in a load steady state of a standby operating mode; (iv) the secondary-side controller being configured to generate the wake-up signal according to the output voltage and a third threshold voltage when the isolated converter is in a dynamic loading state of the standby operating mode; (v) a primary-side feedback circuit configured to obtain a second voltage that represents the first voltage signal; and (vi) a primary-side controller configured to detect the second voltage signal and to control the main power switch according to a detection result in order to maintain the output voltage of the isolated converter at a predetermined value.
Referring now to FIG. 2, shown is a schematic block diagram of an example control circuit of an isolated converter, in accordance with embodiments of the present invention. In this particular example, the isolated converter may be a flyback converter, but other topologies can also be supported in certain embodiments. Here, the isolated converter can receive input voltage signal VIN, and may provide stable output voltage VO for a load (e.g., an LED). For example, the flyback converter can include a transformer having primary winding Np and secondary winding Ns, main switch S1 coupled to primary winding Np, and a freewheeling diode coupled to secondary winding Ns. In this example, the freewheeling diode may be diode D1 having a cathode connected to the secondary winding, and an anode connected to a reference potential. One skilled in the art will recognize that the freewheeling diode with one terminal coupled to the secondary winding and the other terminal coupled to a positive pole of an output capacitor, while a negative pole can connect to ground, can also be supported in certain embodiments.
In this particular example, the control circuit of the isolated converter can include secondary-side controller 01, primary-side feedback circuit 02, and primary-side controller 03. Secondary-side controller 01 can generate wake-up signal VTS for controlling voltage signal VTSEC across the secondary winding to represent the change of the output voltage of the isolated converter. The process of generating wake-up signal VTS by secondary-side controller 01 can include generating wake-up signal VTS according to the relationship of the output voltage and a first threshold voltage by the secondary-side controller when the isolated converter is in a dynamic loading state of a normal operating mode. Wake-up signal VTS can be generated according to the relationship of the output voltage and a second threshold voltage by the secondary-side controller when the isolated converter is in a load steady state of a standby operating mode. Wake-up signal VTS can be generated according to the relationship of the output voltage and the third threshold voltage by the secondary-side controller when the isolated converter is in a dynamic loading state of a standby operating mode.
From the generation of the wake-up signal, it can be seen that the wake-up signal is a signal that represents the change of the output voltage according to the output voltage, such that voltage signal VTSEC is controlled to represent the change of the output voltage. In this particular example, secondary-side controller 01 can also include a switching circuit coupled in parallel with diode D1 in parallel. Wake-up signal VTS can control voltage signal VTSEC to represent the change of the output voltage by controlling the conduction (on) and shutdown (off) states of the switching circuit. Principles of controlling voltage signal VTSEC to represent the output voltage by the wake-up signal will be described in more detail below in conjunction with example operating waveforms (see, e.g., FIGS. 3-5).
As shown in FIG. 2, secondary-side controller 01 can include a first secondary-side detection circuit, a second secondary-side detection circuit, a third secondary-side detection circuit, a secondary mode detection circuit, and a wake-up signal generator. The first secondary-side detection circuit can detect if output voltage VO is less than a first threshold voltage, and may generate secondary-side detection signal VDS1. The first secondary-side detection circuit can include a second voltage bleeder, a sample and hold circuit, a third voltage bleeder, and a first comparison circuit. The second voltage bleeder can divide output voltage VO, in order to obtain output voltage feedback signal VOFB. The sample and hold circuit can receive output voltage feedback signal VOFB, and may obtain sample and hold voltage VOSH by sampling the output voltage feedback signal at a fixed time in each switching cycle.
For example, the sample and hold circuit can receive voltage VD at a cathode of the freewheeling diode, and may generate a time point of sampling based on voltage VD, in one example, when voltage VD indicates that freewheeling diode D1 has stopped working, the sample and hold circuit may begin sampling output voltage feedback signal VOFB and generating sample and hold voltage VOSH, so as to further obtain reference signal VREFN via the third voltage bleeder. The first comparison circuit can compare the output voltage feedback signal against the reference signal, and may generate secondary-side detection signal VDS1. For example, the ratio between the reference signal and the first threshold voltage can be equal to the ratio between output voltage feedback signal VOFB and output voltage VO. Thus, the first comparison circuit can compare the output voltage against the first threshold voltage by comparing the output voltage feedback signal VOFB and reference signal VREFN.
In this particular example, the first threshold voltage may be represented by the reference signal which is determined by the sampling voltage of the output voltage at a fixed moment in a previous switching cycle. So, the first threshold voltage may actually change along with the output voltage, so as to control the decrease of the output voltage to be a relatively low value. For example, by setting the second voltage bleeder and the third voltage bleeder, reference signal VREFN can be, e.g., about 97% of the output voltage feedback signal. If the first threshold voltage is 97% of output voltage VO, the decrease of output voltage VO can be at most about 3% when the isolated converter is in the dynamic loading state of the normal operating mode. In addition, wake-up signal VTS can be active, and thus the switching state of the main power switch at the primary-side may be fast controlled by indirectly sensing the decrease of the output voltage through detecting voltage signal VZCS which is affected by the wake-up signal, such that output voltage VO may increase and be maintained at a predetermined value. In this way, the first threshold voltage can change along with the output voltage when the isolated converter is in the normal operating mode, so as to improve the dynamic response speed of the isolated converter.
In this particular example, the second voltage bleeder can include R3 and R4, and the third voltage bleeder can include R5 and R6. In other examples, the first secondary-side detection circuit may not include the second voltage bleeder and the third voltage bleeder, the sample and hold circuit may directly sample the output voltage to obtain the first threshold voltage, and the first comparison circuit may directly compare the output voltage against the first threshold voltage. The first comparison circuit can be implemented by comparator A with a non-inverting terminal that receives reference signal VREFN, an inverting terminal that receives output voltage feedback signal VOFB, and an output terminal that generates secondary-side detection signal VDS1.
The second secondary-side detection circuit can detect if the output voltage is smaller or less than the second threshold voltage, and may generate secondary-side detection signal VDS2. For example, the second secondary detection circuit can include comparator B with a non-inverting terminal that receives fixed reference voltage VREFS, an inverting terminal that receives output voltage feedback signal VOFB, and an output terminal that generates secondary-side detection signal VDS2. For example, the ratio between reference voltage VREFS and threshold voltage may be equal to the ratio between the output voltage feedback signal and the output voltage.
The third secondary-side detection circuit can detect if the output voltage is smaller or less than the third threshold voltage, and may generate secondary-side detection signal VDS3. For example, the third secondary-side detection circuit can include a fourth voltage bleeder and comparator C. The fourth voltage bleeder can divide reference voltage VREFS in order to obtain reference voltage VREFA. Comparator C may have a non-inverting terminal that receives reference voltage VREFA, an inverting terminal that receives output voltage feedback signal VOFB, and an output terminal that generates secondary-side detection signal VDS3. The ratio between reference voltage VREFA and the third threshold voltage may be equal to the ratio between the output voltage feedback signal and the output voltage. Thus, reference voltage VREFA can represent the third threshold voltage, which may be less or smaller than the second threshold voltage. The output voltage can be determined by reference voltage VREFS when the isolated converter is in the load steady state, and output voltage feedback signal VOFB can be maintained at reference voltage VREFS, when the isolated converter is in the dynamic loading state of the standby operating mode. The decrease of output voltage feedback signal VOFB may be determined by reference voltage VREFA.
Thus, reference voltage VREFA can equal to about 97% of reference voltage VREFS by setting the fourth voltage bleeder, so as control the decrease of output voltage VO to be about 3% when the isolated converter is in the dynamic loading state of the standby operating mode. Once output voltage feedback signal VOFB is decreased to be lower than reference voltage VREFA (e.g., output voltage feedback signal VOFB is about 97% of the predetermined output voltage when the isolated converter is in the load steady state of the standby operating mode), the secondary-side controller can activate wake-up signal VTS, and the primary-side controller can control the operating state of the main power switch, so as to maintain the output voltage at a predetermined value, and to improve the dynamic response speed in the standby operating mode.
The secondary-side mode detection circuit can detect the present operating mode of the isolated converter, and may generate secondary-side mode detection signal VDM. The secondary-side mode detection signal can be used to indicate if the isolated converter is in the normal operating mode or in the standby operating mode. For example, the secondary-side mode detection circuit can detect the operating mode of the isolated converter according to the voltage at the cathode of freewheeling diode D1. When the secondary-side detection signal indicates the isolated converter is in the standby operating mode, secondary-side controller 01 may enter into a sleep mode to reduce the static operational current of the secondary-side controller. The secondary-side controller entering the sleep mode can mean that the unrelated circuits besides the second secondary-side detection circuit, the third secondary-side detection circuit, and the wake-up signal generator are disabled when the isolated converter is in the standby operating mode, while only the circuits relevant to the sleep mode can be enabled for operation.
The wake-up signal generator can receive secondary-side detection signal VDS1, secondary-side detection signal VDS2, secondary-side detection signal VDS3, and secondary-side mode detection signal VDM, and may generate wake-up signal VTS. The wake-up signal generator can be controlled by the secondary-side mode detection signal, in order to generate the wake-up signal VTS according to secondary-side detection signal VDS1, secondary-side detection signal VDS2, and/or secondary-side detection signal VDS3.
In this particular example, the freewheeling diode of the isolated converter can be diode D1, and secondary-side controller 01 also can include a switching circuit coupled in parallel with the diode. Also, wake-up signal VTS can control voltage signal VTSEC by controlling the switching states of the switching circuit. For example, the switching circuit can include switch S2 coupled between the anode and cathode of the diode D1, and may be controlled by wake-up signal VTS. For example, when wake-up signal VTS is high, switch S2 can be turned on, and voltage signal VTSEC may go high to represent the change of the output voltage. Primary-side feedback circuit 02 can be set at the primary-side of the isolated converter to obtain voltage signal VZCS at the primary-side, which represents voltage signal VTSEC.
For example, primary-side feedback circuit 02 can include primary-side auxiliary winding NA coupled with the primary winding NP and secondary winding NS of transformer T, and a first voltage bleeder. Primary-side auxiliary winding NA can obtain a primary-side auxiliary voltage that is proportional to voltage signal VTSEC, and the first voltage bleeder may obtain voltage signal VZCS by dividing the primary-side auxiliary voltage. Also, the first voltage bleeder can include resistors R1 and R2, but the configuration can include forms in different cases. Voltage signal VTSEC may represent wake-up signal VTS, and voltage signal VZCS may represent voltage signal VTSEC. Thus, wake-up signal VTS can be represented by voltage signal VZCS at the primary side, after being transmitted to the primary-side of the isolated converter because of the coupling relationship of the secondary winding and the primary-side auxiliary winding. Because voltage signal VTSEC can represent the change of the output voltage, it may be able to reflect the change of the output voltage by detecting voltage signal VZCS at the primary side, in order to control the isolated converter.
Primary side controller 03 at the primary side can detect voltage signal VZCS, and may control the switching state of the main power switch according to the detection result, in order to maintain the output voltage of the isolated converter be the predetermined value. In this example, primary side controller 03 can include a pulse signal detection and decoder circuit to detect voltage signal VZCS, and generate primary-side detection signal VDP1 for controlling the switching state of the main power switch. The pulse signal detection and decoder circuit can receive voltage VZCS, and may obtain the change of the output voltage after decoding voltage signal VZCS. When the output voltage is lower than the predetermined voltage (e.g., wake-up signal VTS is active), primary-side detection signal VDP1 can be activated to turn on main power switch S1, such that more energy can be transmitted to the secondary side from the primary side, and the output voltage may be maintained at the predetermined voltage.
The primary-side controller can also include a primary-side feedback control circuit, a pulse width frequency circuit, and a driving circuit. The primary-side feedback control circuit can detect the primary-side feedback signal of the output voltage, and may generate primary-side detection signal VDP2 for controlling the switching state of the main power switch. In this example, the primary-side feedback signal can be voltage signal VZCS, and the primary-side feedback control circuit can detect if the primary-side feedback signal is the predetermined value, such that the output voltage is maintained at the predetermined value when the isolated converter is in the load steady state of the normal operating mode.
The pulse width frequency circuit can generate pulse frequency control signal VPW according to primary-side detection signal VDP1 and primary-side detection signal VDP2, such that the pulse width and the frequency of the driving signal of the main power switch Are increased to further increase the response speed of the system in the dynamic loading state when the isolated converter is in the dynamic loading state of the normal operating mode or the standby operating mode. The driving circuit can receive primary-side detection signal VDP1, primary-side detection signal VDP2, and pulse width frequency signal VPW, and may generate driving signal VGS. Driving signal VGS can control the turn on/off of the main power switch. For example, driving signal VGS can be generated when the isolated converter is in the dynamic loading state of the normal operating mode (or the standby operating mode). Driving signal VGS may also have a higher frequency and a larger pulse width than when in the load steady state of the normal operating mode (or the standby operating mode), and when the frequency is very low and pulse width is very small in the load steady state of the standby operating mode.
Primary-side controller 03 also can include a primary-side detection circuit to detect the present operating mode of the isolated converter, and the primary-side controller can the sleep mode when the isolated converter is in the standby operating mode. The primary-side controller entering the sleep mode may mean that remaining signals besides the signal for generating primary-side detection signal VDP1 in the primary-side controller are inactive when the isolated converter is in the standby operating mode, so as to decrease the standby loss of the isolated converter. In this case, the load steady state can refer to that in which the load of the isolated converter is essentially in a stable state, and the dynamic loading state can refer to that in which the load fast switches to heavy-load or full-load from no-load or light-load.
In this way, the control circuit of the isolated converter in certain embodiments can generate different wake-up signals by setting different threshold values when the isolated converter operates in different modes. Thus, the decrease of the output voltage can be relatively small when the isolated converter is in the dynamic loading state. Also, the dynamic response speed of the system can be improved, and the operating frequency may be relatively low when the isolated converter is in the load steady state of the standby operating mode, in order to effectively reduce standby losses.
Referring now to FIG. 3, shown is a waveform diagram of example operation of the control circuit of FIG. 2 when the isolated converter is in a dynamic loading state of a normal operating mode, in accordance with embodiments of the present invention. In FIG. 3, Io is an output current of the isolated converter, VD is a voltage at the cathode of freewheeling diode D1, TS is a sense signal obtained by sampling the voltage VD through the sample and hold circuit (e.g., the output voltage is sampled when sense signal Ts is high). During the time interval t1 ̃t4, switching control signal VGS can be controlled by detection signal VDP2. At time t2, main power switch S1 may be off, freewheeling diode D1 can be turned off, and the freewheeling operation can end. The sample and hold circuit can activate sense signal Ts based on voltage VD at time t3 (e.g., the sampling operation can begin when the freewheeling diode is turned on). Here, the sample and hold circuit can obtain sample and hold voltage VOSH by sampling and holding output voltage feedback signal VOFB. Also, the third voltage bleeder can make reference voltage VREFN to be about 97% of sample and hold voltage VOSH, and comparator A can compare output voltage feedback signal VOFB against reference voltage VREFN.
At time t4, the isolated converter may enter a dynamic loading state, and the output current may rapidly rise and the output voltage can begin to decrease. At time t5, output voltage feedback signal VOFB can be reduced to reference voltage VREFN; that is, output voltage VO can be about 3% smaller/less than at time t3. Then, comparator A can activate secondary-side detection signal VDS1 to control the wake-up signal generator to generate wake-up signal VTS with a certain pulse width (e.g., the width lasts from t5 to t6). Wake-up signal VTS can control voltage signal VTSEC of the secondary winding to be high and voltage signal VZCS to be high, when the pulse signal detection and decoder circuit detects that voltage signal VZCS is larger than a predetermined value, and the pulse width of voltage signal VZCS is larger than a predetermined value. Detection signal VDP1 can be generated at time t6 for controlling the pulse width frequency circuit to generate pulse width frequency signal VPW, such that the driving circuit can generate a group of driving signals with relatively larger pulse widths and higher frequencies, to control the output voltage. In this way, the system dynamic response can be improved.
Referring now to FIG. 4, shown is a waveform diagram of example operation of the control circuit of FIG. 2 when the isolated converter is in a load steady state of a standby operating mode, in accordance with embodiments of the present invention. For example, as the isolated converter remains in the sleep mode during standby operating period, mode detection signal VDM generated at the secondary side may indicate if the isolated converter is in the standby operating mode (e.g., sleep mode). When mode detection signal VDM indicates the isolated converter is in the sleep mode (e.g., VDM is high level), output voltage feedback signal VOFB may take reference voltage VREFS as a reference, and when output voltage feedback signal VOFB is decreased to reference voltage VREFS, wake-up signal VTS can be activated so as to accordingly generate switching control signal VGS. That is, switching control signal VGS can be generated according to the relationship of output voltage feedback signal VOFB and reference voltage VREFS in the sleep mode, such that the output voltage is maintained at the second threshold voltage. It can be seen from the example of FIG. 4 that switching control signal VGS may have a relatively smaller pulse width and lower frequency in the sleep mode, so as to reduce standby losses.
Referring now to FIG. 5, shown is a waveform diagram of example operation of the control circuit of FIG. 2 when the isolated converter is in a dynamic loading state of a standby operating mode, in accordance with embodiments of the present invention. During the time interval f1-f4, mode detection signal VDM can be active to indicate that the isolated converter is in the sleep mode. Thus, main power switch S1 can be controlled according to the operation shown in FIG. 4, such that the output voltage feedback voltage can be maintained at reference voltage VREFS. In the sleep mode, at time t2, the load of the isolated converter can begin increasing, the output current may rapidly rise, and the output voltage can decrease. Thus, the isolated converter can be in the dynamic loading state of the sleep mode after time f2. At time f3, output voltage feedback signal VOFB may be decreased to reference voltage VREFA, which can be about 97% of reference voltage VREFS (e.g., the output voltage can be about 3% smaller/less than in the load steady state of the standby operating mode).
In such case, one or more wake-up signals VTS having pulse widths larger than in the load steady state of the standby operating mode can be generated at the secondary side, in order to reflect the change of the output voltage. Then, wake-up signals VTS can be transmitted to the primary side, such that that switching control signal VGS with relatively larger pulse widths and high frequencies can be generated at the primary side for controlling the output voltage to be rapidly increased to the predetermined voltage. This can improve system dynamic response characteristics.
Therefore, the control circuit of the isolated converter in certain embodiments can generate different wake-up signals by setting different threshold values when the isolated converter operates in different modes. Thus, the decrease of the output voltage can be relatively small when the isolated converter is in the dynamic loading state. This can improve the dynamic response speed of the system, and the operating frequency can be relatively lower when the isolated converter is in the load steady state of the standby operating mode, in order to effectively reduce the standby loss.
In one embodiment, a method of controlling an isolated converter with a transformer having a primary winding and a secondary winding, a main power switch coupled with the primary winding, and a freewheeling diode coupled with the secondary winding, can include: (i) controlling a first voltage signal across the secondary winding according to a wake-up signal to reflect change of an output voltage of the isolated converter; (ii) obtaining a second voltage signal that represents the first voltage signal at the primary side of the isolated converter; (iii) detecting the second voltage signal and controlling the main power switch according to a detection result in order to maintain the output voltage at a predetermined value; (iv) generating the wake-up signal according to the output voltage and a first threshold voltage when the isolated converter is in a dynamic loading state of a normal operating mode; (v) generating the wake-up signal according to the output voltage and a second threshold voltage when the isolated converter is in a load steady state of a standby operating mode; and (vi) generating the wake-up signal according to the output voltage and a third threshold voltage when the isolated converter is in a dynamic loading state of the standby operating mode.
Particular embodiments also include a control method for an isolated converter (e.g., a flyback converter). The flyback converter can include a transformer having a primary winding and a secondary winding, a main power switch coupled with the primary winding, and a freewheeling diode coupled with the secondary winding. The control method can include controlling a first voltage signal across the secondary winding according to a wake-up signal generated at the secondary side of the isolated converter to reflect the change of an output voltage of the isolated converter. The control method can also include obtaining a second voltage signal which represents the first voltage signal at the primary side of the isolated converter.
The control method can also include detecting voltage signal VZCS, and controlling the main power switch according to the detection result, in order to maintain the output voltage at a predetermined value. When the isolated converter is in a dynamic loading state of a normal operating mode, the wake-up signal can be generated at the secondary side according to the relationship of the output voltage and the first threshold voltage. When the isolated converter is in a load steady state of the standby operating mode, the wake-up signal can be generated at the secondary side according to the relationship of the output voltage and the second threshold voltage. Also, when the isolated converter is in a dynamic loading state of the standby operating mode, the wake-up signal is generated at the secondary side according to the relationship of the output voltage and the third threshold voltage.
The control method can also include detecting a primary-side feedback signal of the output voltage at the primary side to control the switching state of the main power switch. The control method can also include the primary-side controller and/or the secondary-side controller entering the sleep mode when the isolated converter is in the standby operating mode. Also, the pulse width and frequency of the driving signal of the main power switch can be increased when the isolated converter is in the dynamic loading state.
Particular embodiments can also include a switching power supply that includes a transformer having a primary winding and a secondary winding, a power switch being coupled with the primary winding, and a freewheeling diode being coupled with the secondary winding, where the switching power supply also includes a control circuit of an isolated converter, as described above. 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.

## Claims

1. A method of controlling an isolated converter comprising a transformer having a primary winding and a secondary winding, a main power switch coupled with said primary winding, and a freewheeling diode coupled with said secondary winding, the method comprising:
a) controlling a first voltage signal across said secondary winding according to a wake-up signal to reflect change of an output voltage of said isolated converter;
b) obtaining a second voltage signal that represents said first voltage signal at said primary side of said isolated converter;
c) detecting said second voltage signal and controlling said main power switch according to a detection result in order to maintain said output voltage at a predetermined value;
d) generating said wake-up signal according to said output voltage and a first threshold voltage when said isolated converter is in a dynamic loading state of a normal operating mode;
e) generating said wake-up signal according to said output voltage and a second threshold voltage when said isolated converter is in a load steady state of a standby operating mode; and
f) generating said wake-up signal according to said output voltage and a third threshold voltage when said isolated converter is in a dynamic loading state of said standby operating mode.

2. The method of claim 1, further comprising detecting a primary-side feedback signal of said output voltage at said primary side for controlling said main power switch.

3. The method of claim 2, wherein at least one of a primary-side controller and a secondary-side controller enter a sleep mode when said isolated converter is in said standby operating mode.

4. The method of claim 2, wherein a pulse width and frequency of said driving signal of said main power switch are increased when said isolated converter is in said dynamic loading state.

5. A control circuit of an isolated converter comprising a transformer having a primary winding and a secondary winding, a main power switch coupled with said primary winding, and a freewheeling diode coupled with said secondary winding, the control circuit comprising:
a) a secondary-side controller configured to generate a wake-up signal according to an output voltage of said isolated converter to control a first voltage signal across said secondary winding to represent change of said output voltage of said isolated converter;
b) said secondary-side controller being configured to generate said wake-up signal according to said output voltage and a first threshold voltage when said isolated converter is in a dynamic loading state of a normal operating mode;
c) said secondary-side controller being configured to generate said wake-up signal according to said output voltage and a second threshold voltage when said isolated converter is in a load steady state of a standby operating mode;
d) said secondary-side controller being configured to generate said wake-up signal according to said output voltage and a third threshold voltage when said isolated converter is in a dynamic loading state of said standby operating mode;
e) a primary-side feedback circuit configured to obtain a second voltage that represents said first voltage signal; and
f) a primary-side controller configured to detect said second voltage signal and to control said main power switch according to a detection result in order to maintain said output voltage of said isolated converter at a predetermined value.

6. The control circuit of claim 5, wherein said secondary-side controller comprises:
a) a first secondary-side detection circuit configured to detect if said output voltage is smaller than a first threshold voltage, and to generate a first secondary-side detection signal;
b) a second secondary-side detection circuit configured to detect if said output voltage is smaller than a second threshold voltage, and to generate a second secondary-side detection signal;
c) a third secondary-side detection circuit configured to detect if said output voltage is smaller than a third threshold voltage, and to generate said third secondary-side detection signal;
d) a secondary-side mode detection circuit configured to detect a present operating mode of said isolated converter, and to generate a secondary-side mode detection signal; and
e) a wake-up signal generator configured to receive said first secondary-side detection signal, said second secondary-side detection signal, said third secondary-side detection signal, and said secondary-side mode detection signal, and to generate said wake-up signal.

7. The control circuit of claim 6, wherein said secondary-side controller further comprises a switching circuit coupled with said diode in parallel, and said wake-up signal is configured to control said first voltage signal by controlling the switching state of said switching circuit.

8. The control circuit of claim 6, wherein said secondary-side mode detection circuit is configured to control said secondary-side controller to enter a sleep mode when said isolated converter is in said standby operating mode.

9. The control circuit of claim 5, wherein said primary-side controller comprises a pulse signal detection and decoder circuit configured to detect said second voltage signal, and to generate a first primary-side detection signal to control said main power switch.

10. The control circuit of claim 9, wherein said primary-side controller comprises:
a) a primary-side feedback control circuit configured to detect a primary-side feedback signal of said output voltage, and to generate a second primary-side detection signal to control said main power switch;
b) a pulse width frequency circuit configured to generate a pulse width frequency control signal according to said first primary-side detection signal and said second primary-side detection signal, in order to increase a pulse width and frequency of said driving signal of said main power switch when said isolated converter is in said dynamic loading state; and
c) a driving circuit configured to receive said first primary-side detection signal, said second primary-side detection signal, and said pulse width frequency signal, and to generate said driving signal.

11. The control circuit of claim 10, wherein said primary-side controller further comprises a primary-side detection circuit configured to detect a present operating mode of said isolated converter, wherein said primary-side controller is configured to enter said sleep mode when said isolated converter is in said standby operating mode.

12. A switching power supply, comprising said transformer and said control circuit of claim 5.