Isolation conversion circuit and feedback control circuit thereof

Abstract

The invention discloses an isolation conversion circuit and a feedback control circuit thereof. The isolation conversion circuit comprises a transformer, a main power switch and a feedback control circuit. The feedback control circuit comprises a voltage judgment circuit, a calculation circuit, a first time delay circuit, a second time delay circuit and an adjusting circuit. The isolation conversion circuit provides more accurate feedback voltage to enter a feedback loop, and the adjustment precision of the circuit is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to an electronic circuit, and more particularly, to an isolated conversion circuit and a feedback control circuit thereof. BACKGROUND

[0002] In power conversion, isolated conversion circuits such as flyback conversion circuits, forward conversion circuits, etc. are widely used to convert input voltage into desired output voltage. Since there is isolation between the primary side and the secondary side of the circuit, when primary side control is used, output voltage needs to be sampled for voltage regulation; or even when secondary side control is used, output voltage needs to be sampled when the circuit has requirements such as overvoltage protection or undervoltage protection, etc.

[0003] The prior art usually uses an auxiliary winding (also referred to as a third winding) to sample output voltage: at a fixed time point after the voltage across the auxiliary winding is pulled high, the voltage across the auxiliary winding at the time point is sampled, and the voltage across the auxiliary winding sampled at the time point is taken as the feedback voltage representing the output voltage. Subsequently, the feedback voltage is delivered to the control loop for voltage regulation or circuit protection.

[0004] However, when the secondary side current is large, the feedback voltage changes greatly before the secondary side current crosses zero. Therefore, the sampled feedback voltage does not necessarily reflect the true output voltage. This affects the accuracy of the output voltage control, and may even cause circuit failure. SUMMARY

[0005] According to an embodiment of the present invention, an isolation conversion circuit is proposed, comprising: a transformer having a primary winding for receiving an input voltage; a secondary winding for providing an output voltage to supply a subsequent load; a tertiary winding for generating a feedback voltage representing the output voltage; a main power switch that is periodically turned on and off to convert the input voltage into an output voltage; a feedback control circuit coupled to the third winding, receiving the feedback voltage, and generating a switch control signal for controlling the main power switch to be turned on and off, wherein the feedback control circuit comprises: a voltage determination circuit for determining whether the feedback voltage is pulled high; a calculation circuit for calculating a time interval during which the feedback voltage is greater than a reference voltage to obtain a calculated time interval; a first delay circuit for responding to a determination result of the voltage determination circuit, and when the feedback voltage is pulled high, a first delay circuit for The delay circuit starts delaying and generates a calculated delay signal after delaying for a calculated time period; the second delay circuit responds to the judgment result of the voltage judgment circuit, and when the feedback voltage is pulled high, the second delay circuit starts delaying and generates a fixed delay signal after delaying for a fixed time period; wherein: when the isolation conversion circuit is in the startup phase or in a transient state, after the feedback voltage is pulled high, the feedback voltage is sampled and maintained as a sampling voltage after a time interval of a fixed time period; when the isolation conversion circuit is started or operates in a steady state, after the feedback voltage is pulled high, the feedback voltage is sampled and maintained as a sampling voltage after a time interval of a calculated time period; the regulation circuit responds to the sampling voltage and generates the switching control signal to control the main power switch.

[0006] According to an embodiment of the present invention, a feedback control circuit for an isolated converter circuit is further provided. The isolated converter circuit receives an input voltage and generates an output voltage. The feedback control circuit includes: a voltage determination circuit for determining whether a feedback voltage representing the output voltage is pulled high; a calculation circuit for calculating a time interval during which the feedback voltage is greater than a reference voltage to obtain a calculated time interval; a first delay circuit, responsive to the determination result of the voltage determination circuit, wherein when the feedback voltage is pulled high, the first delay circuit begins delaying and generates a calculated delay signal after a delay of a calculated time interval; and a second delay circuit, responsive to the determination result of the voltage determination circuit, wherein when the feedback voltage is pulled high, the second delay circuit begins delaying and generates a fixed delay signal after a delay of a fixed time interval. When the isolated converter circuit is in a startup phase or in a transient state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as a sampled voltage for a time interval of a fixed time interval; and when the isolated converter circuit is completely started or operating in a steady state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as a sampled voltage for a time interval of a calculated time interval.

[0007] According to the embodiment of the present application, a feedback control circuit for an isolated conversion circuit is also provided, the isolated conversion circuit receives an input voltage and generates an output voltage, the feedback control circuit comprises: a voltage judging circuit, judging whether a feedback voltage representing the output voltage is pulled high; a calculating circuit, calculating a time interval during which the feedback voltage is greater than a reference voltage, and obtaining a calculated time; a delay circuit, in response to the judging result of the voltage judging circuit, starting to delay when the feedback voltage is pulled high, and generating a calculated delay signal after delaying for the calculated time; a sample-and-hold circuit, in response to the calculated delay signal, sampling and holding the feedback voltage, and obtaining a sampled voltage; and an adjusting circuit, in response to the sampled voltage, generating a switch control signal for controlling a main power switch.

[0008] According to the above-mentioned isolated conversion circuit and its feedback control circuit according to the aspects of the present application, the adjusting precision of the circuit is improved. BRIEF DESCRIPTION OF DRAWINGS

[0009] Figure 1 Fig. 1 is a circuit structure schematic diagram of an isolated conversion circuit 100 according to an embodiment of the present application;

[0010] Figure 2 Fig. 2 is a circuit structure schematic diagram of an isolated conversion circuit 200 according to an embodiment of the present application;

[0011] Figure 3 Fig. 3 is a circuit structure schematic diagram of an isolated conversion circuit 300 according to an embodiment of the present application;

[0012] Figure 4 Fig. 4 is a flow chart 400 of a method for an isolated conversion circuit according to an embodiment of the present application. DETAILED DESCRIPTION

[0013] The specific embodiments of the present application will be described in detail below, it should be noted that the embodiments described herein are only used for illustration and do not limit the present application. In the following description, a large number of specific details are set forth in order to provide a thorough understanding of the present application. However, it is obvious to those skilled in the art that the specific details are not necessarily used to implement the present application. In other instances, well-known circuits, materials or methods are not specifically described in order to avoid obscuring the present application.

[0014] Throughout this specification, references to "one embodiment," "an embodiment," "an example," or "an example" mean that a particular feature, structure, or characteristic described in connection with that embodiment or example is included in at least one embodiment of the present invention. Therefore, the phrases "in one embodiment," "in an embodiment," "an example," or "an example" that appear in various places throughout this specification do not necessarily refer to the same embodiment or example. Furthermore, particular features, structures, or characteristics may be combined in one or more embodiments or examples in any suitable combinations and / or subcombinations. Furthermore, those skilled in the art will appreciate that the figures provided herein are for illustrative purposes only and are not necessarily drawn to scale. It should be understood that when an element is referred to as being "coupled to" or "connected to" another element, it can be directly coupled or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly coupled to" or "directly connected to" another element, there are no intervening elements. Identical reference numerals indicate identical elements. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0015] Figure 1 FIG. 1 is a schematic diagram of the circuit structure of the isolation conversion circuit 100 according to an embodiment of the present invention. Figure 1 In the embodiment shown, the isolation conversion circuit 100 includes: a transformer T having a primary winding T1, receiving an input voltage V IN , the secondary winding T2 provides the output voltage V O To supply the subsequent load RL; the third winding T3 generates a representative output voltage V O Feedback voltage V FB The main power switch 101 is periodically turned on and off to set the input voltage V IN Converted into output voltage V O Feedback control circuit 102, coupled to the third winding T3, receives the feedback voltage V FB , generates a switch control signal G that controls the main power switch 101 to be turned on and off 101 .

[0016] The feedback control circuit 102 includes a voltage determination circuit 21 for determining the feedback voltage V FB Is it pulled high? Calculation circuit 22 calculates the feedback voltage V FB Greater than the reference voltage V REF The duration interval is calculated to obtain the calculation duration T C The first delay circuit 23, responding to the judgment result of the voltage determination circuit 21, when the feedback voltage V FB is pulled high, the first delay circuit 23 starts to delay, and delays for the calculated time length T C Then generate the calculation delay signal D TC; a sample-and-hold circuit 24, responsive to the calculation delay signal D TC , samples and holds the feedback voltage V FB to obtain a sample voltage V FBS ; an adjustment circuit 25, responsive to the sample voltage V FBS , generates the switch control signal G 101 to control the main power switch 101. When the feedback voltage V FB is pulled high, after a short time delay, such as a short pulse time delay, the calculation circuit 22 resets the calculation time T C and starts to calculate the time interval during which the feedback voltage V FB is greater than the reference voltage V REF to obtain the calculation time T C , which is provided to the first delay circuit 23 after the feedback voltage V FB is pulled high in the next switching cycle. In the embodiment shown in Figure 1 , the output of the voltage determination circuit 21 is delivered to the calculation circuit 22 via the short pulse circuit TP1, but those skilled in the art should appreciate that the output of the voltage determination circuit 21 can also be delivered to the calculation circuit 22 directly without passing through the short pulse circuit TP1 (as shown in the dashed line box in Figure 1 ), and the short time delay can be achieved by using the transmission delay of the circuit itself.

[0017] In one embodiment of the present application, the calculation circuit 22 multiplies the time interval during which the feedback voltage V FB is greater than the reference voltage V REF by a proportional coefficient k1 to obtain the calculation time T C . In one embodiment of the present application, the proportional coefficient k1 is close to 1, for example, k1 is set to 0.9.

[0018] In one embodiment of the present application, the calculation circuit 22 starts to count when the feedback voltage V FB increases to be greater than the reference voltage V REF , and stops to count when the feedback voltage V FB decreases to be less than the reference voltage V REF to obtain the time interval.

[0019] In one embodiment of the present application, the voltage determination circuit 21 includes a comparator to compare the size of the feedback voltage V FB and a threshold voltage V H , and when the feedback voltage V FB is greater than the threshold voltage V H , it indicates that the feedback voltage V FB is pulled high.

[0020] In one embodiment of the present invention, the sample and hold circuit 24 includes: a short pulse circuit TP2, which responds to the calculation delay signal T DC , generating a short pulse signal; the sampling and holding unit (such as Figure 1 As shown, it has a sampling switch and a holding capacitor, responding to a short pulse signal and adjusting the feedback voltage V FB Sampling and holding are performed to obtain the sampling voltage V FBS .

[0021] In one embodiment of the present invention, the regulating circuit 25 adjusts the sampling voltage V FBS Regulated to the internal voltage reference V RI , to set the output voltage V O Adjust to the desired value.

[0022] In one embodiment of the present invention, the internal voltage reference value V RI and reference voltage V REF There is a proportional relationship, such as V REF =k2*V RI , where k2 is a proportional coefficient close to 1. For example, k2 can be set to 0.95.

[0023] When the isolation conversion circuit 100 is running, in each switching cycle, the calculation circuit 22 calculates the calculation time length T calculated in the previous cycle. C is sent to the first delay circuit 23. When the main power switch 101 is turned off, the feedback voltage V FB On the one hand, the first delay circuit 23 responds to the feedback voltage V FB is pulled high to start the delay, and after the delay is calculated for the time T C Then generate the calculation delay signal D TC The short pulse circuit TP2 responds to the calculated delay signal T DC , that is, the feedback voltage V FB The calculated delay signal D after being pulled high TC After the determined delay time, a short pulse signal is generated, so that the sampling and holding circuit 24 FB Sampling and holding are performed to obtain the sampling voltage V FBS Then the voltage V FBS is sent to the regulating circuit 25 to generate a switch control signal G 101 To control the main power switch 101, to output voltage V O On the other hand, the feedback voltage V FB After being pulled high, after a short time delay, the calculation circuit 22 calculates the calculation time T calculated in the previous cycle C Reset, restart the feedback voltage V FBand reference voltage V REF Compare and calculate the feedback voltage V FB Greater than the reference voltage V REF Then, the new duration interval is multiplied by the proportional coefficient k1 to obtain the calculated duration T of the current cycle. C , to supply the first delay circuit 23, so that the first delay circuit 23 responds to the feedback voltage V in the next switching cycle FB is pulled high to start the delay and delay the calculation time of the current cycle T C The isolated conversion circuit 100 operates in this way in each switching cycle to obtain a sampling time point of the feedback voltage, thereby obtaining an accurate feedback voltage, that is, obtaining accurate information of the output voltage.

[0024] Figure 2 FIG. 2 is a schematic diagram of the circuit structure of the isolation conversion circuit 200 according to an embodiment of the present invention. Figure 2 The isolation conversion circuit 200 is shown with Figure 1 The isolated converter circuit 100 is similar to Figure 1 The difference between the isolated converter circuit 100 and the Figure 2 In the embodiment shown, the feedback circuit 102 further includes: a second delay circuit 26, which responds to the judgment result of the voltage judgment circuit 21 and when the feedback voltage V FB When the signal is pulled high, the second delay circuit 26 starts to delay and generates a fixed delay signal T after delaying for a fixed time. DF , that is, the fixed delay signal D TF Relative to the feedback voltage V FB The selection circuit 27 selects the fixed delay signal D when the isolation conversion circuit 200 is in the startup phase (such as the startup signal SS is 1), or when the load of the isolation conversion circuit 200 suddenly changes and the circuit is in a transient state (such as the state signal ST is 1). TF The feedback voltage V FB After being pulled high, the feedback voltage V is sampled and maintained for a fixed period of time. FB , that is: at this time the feedback voltage V FB After being pulled high, after a fixed period of time, the feedback voltage is sampled and held as the sampling voltage V FBS When the isolation conversion circuit 200 is started (eg, the start signal SS is 0) or the isolation conversion circuit 200 is running in a steady state (eg, the start signal ST is 0), the selection circuit 27 selects the calculation delay signal D TC The feedback voltage V FBAfter being pulled high, the feedback voltage V is sampled and maintained after the calculated delay time. FB , that is: at this time the feedback voltage V FB After being pulled high, the feedback voltage is sampled and held for a calculated time interval as the sampling voltage V FBS .

[0025] Figure 3 FIG. 3 is a schematic diagram of the circuit structure of the isolation conversion circuit 300 according to an embodiment of the present invention. Figure 3 The isolation conversion circuit 300 is shown with Figure 1 The isolated converter circuit 100 is similar to Figure 1 The difference between the isolated converter circuit 100 and the Figure 3 In the embodiment shown, the feedback circuit 102 includes: a voltage determination circuit 21, which determines the feedback voltage V FB Is it pulled high? Calculation circuit 22 calculates the feedback voltage V FB Greater than the reference voltage V REF The duration interval is calculated to obtain the calculation duration T C The first delay circuit 23, responding to the judgment result of the voltage determination circuit 21, when the feedback voltage V FB is pulled high, the first delay circuit 23 starts to delay, and delays for the calculated time length T C Then generate the calculation delay signal D TC The first sample and hold circuit 24, responds to the calculation delay signal D TC , the feedback voltage V FB Sampling and holding are performed to obtain the first sampling voltage V FBS1 The second delay circuit 26 responds to the judgment result of the voltage determination circuit 21, when the feedback voltage V FB is pulled high, the second delay circuit 26 generates a fixed delay signal D after a fixed time. TF , that is, the fixed delay signal D TF Relative to the feedback voltage V FB is pulled high with a fixed time delay; the second sampling and holding circuit 28 responds to the fixed delay signal D TF , the feedback voltage V FB Sampling and holding are performed to obtain the second sampling voltage V FBS2 The feedback circuit 102 further includes a selection circuit 27, which selects the first sampling voltage V when the isolation conversion circuit 300 is in the startup phase (e.g., the startup signal SS is 1), or when the load of the isolation conversion circuit 300 suddenly changes and the circuit is in a transient state (e.g., the state signal ST is 1). FBS1 As the sampling voltage V FBS , that is, the feedback voltage V FBAfter the feedback voltage is pulled high, the feedback voltage is sampled and held after a fixed time interval, and is taken as the sampling voltage V FBS ; when the isolation conversion circuit 200 is started (such as the start signal SS is 0) or the isolation conversion circuit 200 is in a steady state (such as the state signal ST is 0), the selection circuit 27 selects the second sampling voltage V FBS2 as the sampling voltage V FBS , that is, the feedback voltage V FB After the feedback voltage is pulled high, the feedback voltage is sampled and held after a fixed time interval, and is taken as the sampling voltage V FBS .

[0026] When the isolation conversion circuit 200 / 300 is running, when the isolation conversion circuit is in the starting stage or the load appears to jump so that the circuit is in a transient state, after the feedback voltage is pulled high, the feedback voltage is sampled and held after a fixed time interval, and is taken as the sampling voltage, into the subsequent regulation circuit, so that the feedback voltage can quickly enter the feedback loop, and does not affect the reaction time of the circuit; when the isolation conversion circuit is started or the load does not appear to jump and the circuit is in a steady state, after the feedback voltage is pulled high, the feedback voltage is sampled and held after a calculated time interval, and is taken as the sampling voltage, into the subsequent regulation circuit, so that the more accurate feedback voltage enters the feedback loop, and improves the regulation accuracy of the circuit.

[0027] Figure 4 A method flowchart 400 for an isolation conversion circuit according to an embodiment of the present application is shown. The isolation conversion circuit includes a primary winding receiving an input voltage, a secondary winding providing an output voltage, a third winding generating a feedback voltage representing the output voltage, and a main power switch. The method includes:

[0028] Step 401: determine whether the feedback voltage is pulled high. For example, compare whether the feedback voltage is higher than a threshold voltage. When the feedback voltage is greater than the threshold voltage, it indicates that the feedback voltage is pulled high.

[0029] Step 402: determine the state of the isolation conversion circuit: whether the isolation conversion circuit is in a starting stage or in a transient state. If the isolation conversion circuit is in a starting stage or in a transient state, go to step 403; if the isolation conversion circuit is started or in a steady state, go to step 404.

[0030] Step 403: after the feedback voltage is pulled high, sample the feedback voltage after a fixed time interval, as a sampling signal, which is transmitted to a regulation circuit to regulate the turn-on and turn-off of the main power switch.

[0031] Step 404: After the feedback voltage is pulled high, a time interval of the calculated time length is passed, the feedback voltage is sampled as a sampling signal, which is delivered to the regulating circuit to regulate the turn-on and turn-off of the main power switch.

[0032] In one embodiment of the present application, the method further comprises: when the isolated conversion circuit completes starting or is in steady-state operation, calculating a time length during which the feedback voltage is greater than the reference voltage to obtain a time length interval, multiplying the time length interval by a proportional coefficient to obtain the calculated time length, and using the calculated time length for sampling the feedback voltage in the next switching cycle of the isolated conversion circuit: that is, in the next switching cycle of the isolated conversion circuit, after the feedback voltage is pulled high, a time interval of the calculated time length is passed, and the feedback voltage is sampled as a sampling signal.

[0033] Although the present application has been described with reference to several exemplary embodiments, it is understood that the words that have been used are words of description and illustration, rather than words of limitation. As the application can be embodied in many different forms without departing from the spirit or essential characteristics thereof, it is understood that the examples are to be considered in all respects only as illustrative and not restrictive. The scope of the application should be determined, not with reference to the above description, but should be given with reference to the appended claims, and throughout the claims the use of "comprising" and "including" the use of "comprising" and "including" and the like denotes "one or more", "including at least the elements listed after such recitations". The use of "preferably", "preferred", "desired", "may" and "desired" and the like, indicates that although the described feature, technique, method or implementation can be useful and / or preferred, it can not be necessary. The use of "including", "containing" and "having" has the same meaning as "comprising" and "including".

Claims

1. An isolated conversion circuit, comprising: a transformer having a primary winding receiving an input voltage; a secondary winding providing an output voltage for a load; a tertiary winding generating a feedback voltage representing the output voltage; a main power switch periodically turned on and off to convert the input voltage to the output voltage; a feedback control circuit coupled to the tertiary winding, receiving the feedback voltage, generating a switch control signal to control the main power switch to be turned on and off, the feedback control circuit comprising: a voltage judging circuit judging whether the feedback voltage is pulled high; a calculation circuit calculating a time interval during which the feedback voltage is greater than a reference voltage to obtain a calculated time; a first delay circuit, in response to a result of the voltage judging circuit, starting to delay when the feedback voltage is pulled high, and generating a calculated delay signal after delaying for the calculated time; a second delay circuit, in response to the result of the voltage judging circuit, starting to delay when the feedback voltage is pulled high, and generating a fixed delay signal after delaying for a fixed time; wherein: when the isolated conversion circuit is in a start-up phase or in a transient state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as a sample voltage after a time interval of the fixed time; when the isolated conversion circuit is in a steady state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as the sample voltage after a time interval of the calculated time; and an adjustment circuit, in response to the sample voltage, generating the switch control signal to control the main power switch.

2. The isolated conversion circuit of claim 1, further comprising: a sample-and-hold circuit; and a selection circuit, when the isolated conversion circuit is in the start-up phase or in the transient state, selecting to deliver the fixed delay signal to the sample-and-hold circuit to cause the sample-and-hold circuit to sample and hold the feedback voltage after the feedback voltage is pulled high for the fixed time to obtain the sample voltage; and when the isolated conversion circuit is in the steady state, selecting to deliver the calculated delay signal to the sample-and-hold circuit to cause the sample-and-hold circuit to sample and hold the feedback voltage after the feedback voltage is pulled high for the calculated time to obtain the sample voltage.

3. The isolated conversion circuit of claim 1, wherein: the calculation circuit starts to count when the feedback voltage increases to be greater than the reference voltage, and stops to count when the feedback voltage decreases to be less than the reference voltage to obtain the time interval; and the time interval is multiplied by a proportional coefficient to obtain the calculated time.

4. The isolated conversion circuit of claim 1, wherein: when the feedback voltage is pulled high, the calculation circuit resets the calculated time after a short time delay, and starts to calculate a time interval during which the feedback voltage is greater than the reference voltage in a current switching period to obtain the calculated time to be provided to the first delay circuit when the feedback voltage is pulled high in a next switching period.

5. A feedback control circuit for an isolated conversion circuit receiving an input voltage to generate an output voltage, the feedback control circuit comprising: a voltage judging circuit judging whether a feedback voltage representing the output voltage is pulled high; ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ a calculation circuit, which calculates a time interval during which the feedback voltage is greater than the reference voltage to obtain a calculation time; a first delay circuit, which is responsive to a result of the voltage determination circuit, starts to delay when the feedback voltage is pulled up, and generates a calculation delay signal after delaying for the calculation time; a second delay circuit, which is responsive to the result of the voltage determination circuit, starts to delay when the feedback voltage is pulled up, and generates a fixed delay signal after delaying for a fixed time; wherein: when the isolated conversion circuit is in a starting stage or in a transient state, after the feedback voltage is pulled up, the feedback voltage is sampled and held as the sampling voltage after a time interval of the fixed time; when the isolated conversion circuit is started up or runs in a steady state, after the feedback voltage is pulled up, the feedback voltage is sampled and held as the sampling voltage after a time interval of the calculation time.

6. The feedback control circuit of claim 5, further comprising: a sample-and-hold circuit; a selection circuit, which selects the fixed delay signal to be delivered to the sample-and-hold circuit when the isolated conversion circuit is in the starting stage or in the transient state, so that the sample-and-hold circuit samples and holds the feedback voltage after the feedback voltage is pulled up for the fixed time to obtain the sampling voltage; and which selects the calculation delay signal to be delivered to the sample-and-hold circuit when the isolated conversion circuit is started up or runs in the steady state, so that the sample-and-hold circuit samples and holds the feedback voltage after the feedback voltage is pulled up for the calculation time to obtain the sampling voltage.

7. The feedback control circuit of claim 5, wherein: the calculation circuit starts to count when the feedback voltage increases to be greater than the reference voltage, and stops to count when the feedback voltage decreases to be less than the reference voltage to obtain the time interval; the time interval is multiplied by a proportional coefficient to obtain the calculation time.

8. The feedback control circuit of claim 5, wherein: when the feedback voltage is pulled up, the calculation circuit resets the calculation time after a short time delay, starts to calculate the time interval during which the feedback voltage is greater than the reference voltage in a current switching period to obtain the calculation time to be provided to the first delay circuit after the feedback voltage is pulled up in a next switching period.

9. The feedback control circuit of claim 5, further comprising: a first sample-and-hold circuit, which is responsive to the calculation delay signal, samples and holds the feedback voltage to obtain a first sampling voltage; a second sample-and-hold circuit, which is responsive to the fixed delay signal, samples and holds the feedback voltage to obtain a second sampling voltage; a selection circuit, which selects the first sampling voltage as the sampling voltage when the isolated conversion circuit is in the starting stage or in the transient state, and selects the second sampling voltage as the sampling voltage when the isolated conversion circuit is started up or runs in the steady state.

10. A feedback control circuit for an isolated conversion circuit, the isolated conversion circuit receiving an input voltage to generate an output voltage, the feedback control circuit comprising: a voltage determination circuit, which determines whether a feedback voltage indicative of the output voltage is pulled up; a computing circuit, which computes a time interval during which the feedback voltage is greater than the reference voltage to obtain a computed time; a delay circuit, which is responsive to a result of the voltage judging circuit, starts to delay when the feedback voltage is pulled up, and generates a computed delay signal after delaying for the computed time; a sample-and-hold circuit, which is responsive to the computed delay signal, samples and holds the feedback voltage to obtain a sampled voltage; an adjusting circuit, which is responsive to the sampled voltage, generates a switch control signal to control the main power switch.

11. The feedback control circuit of claim 10, wherein: the computing circuit starts to count when the feedback voltage increases to be greater than the reference voltage, and stops to count when the feedback voltage decreases to be less than the reference voltage to obtain the time interval; the time interval is multiplied by a proportional coefficient to obtain the computed time.

12. The feedback control circuit of claim 10, wherein: the computing circuit resets the computed time after a short time delay when the feedback voltage is pulled up, and starts to compute a time interval during which the feedback voltage is greater than the reference voltage in a current switching period to obtain the computed time to provide to the first delay circuit when the feedback voltage is pulled up in a next switching period.

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