Isolated power converter and the feedback control circuit thereof

Abstract

This application discloses an isolated power converter and its feedback control circuit. The isolated power converter includes a transformer, a main power switch, and a feedback control circuit. The feedback control circuit includes a voltage determination circuit, a calculation circuit, a first delay circuit, a second delay circuit, and an adjustment circuit. The isolated power converter provides a more accurate feedback voltage into the feedback loop, improving the circuit's adjustment accuracy.

Description

Technical Field

[0001] This invention relates to an electronic circuit, and more specifically, to an isolated power converter and its feedback control circuit. Prior Technology

[0002] In power conversion, isolated power converters, such as flyback converters and forward converters, are widely used to convert input voltage into the required output voltage. Because the primary and secondary sides of the circuit are isolated, when primary-side control is used, the output voltage needs to be sampled for voltage regulation; or even when secondary-side control is used, the output voltage needs to be sampled when the circuit has requirements such as output voltage overvoltage protection or undervoltage protection.

[0003] Existing technology typically uses an auxiliary coil (also called a third coil) to sample the output voltage: at a fixed time point after the voltage across the auxiliary coil is pulled high, the voltage value across the auxiliary coil is sampled, and this sampled voltage value is used as a feedback voltage characterizing the output voltage. Subsequently, this feedback voltage is sent to the control loop for voltage regulation or circuit protection.

[0004] However, when the secondary current is large, the feedback voltage changes significantly before the secondary current crosses zero. Therefore, the sampled feedback voltage may not accurately reflect the true output voltage. This affects the accuracy of output voltage control and may even cause circuit malfunctions. Summary of the Invention

[0005] According to an embodiment of the present invention, an isolated power converter is provided, comprising: a transformer having a primary winding for receiving an input voltage; a secondary winding for providing an output voltage to supply a downstream load; a third winding for generating a feedback voltage characterizing the output voltage; a main power switch that is periodically turned on and off to convert the input voltage into an output voltage; and a feedback control circuit coupled to the third winding for receiving the feedback voltage and generating a switch control signal to control the main power switch to turn on and off. The feedback control circuit includes: a voltage determination circuit for determining whether the feedback voltage is pulled high; a calculation circuit for calculating the duration for which the feedback voltage is greater than a reference voltage to obtain a calculation duration; and a first delay circuit that, in response to the determination result of the voltage determination circuit, delays the first delay when the feedback voltage is pulled high. The delay circuit starts delaying and generates a calculated delay signal after the calculated delay time. The second delay circuit starts delaying when the feedback voltage is pulled high, based on the judgment result of the voltage determination circuit, and generates a fixed delay signal after the fixed delay time. Specifically: when the isolated power converter is in the startup phase or in a transient state, after the feedback voltage is pulled high, the feedback voltage is sampled and held after a fixed time interval, serving as the sampled voltage. When the isolated power converter has finished starting up or is operating in a steady state, after the feedback voltage is pulled high, the feedback voltage is sampled and held after a calculated time interval, serving as the sampled voltage. The adjustment circuit generates the switch control signal in response to the sampled voltage to control the main power switch.

[0006] According to an embodiment of the present invention, a feedback control circuit for an isolated power converter is also proposed. The isolated power converter receives an input voltage and generates an output voltage. The feedback control circuit includes: a voltage determination circuit for determining whether a feedback voltage characterizing the output voltage is pulled high; a calculation circuit for calculating the duration interval during which the feedback voltage is greater than a reference voltage to obtain a calculation duration; a first delay circuit for starting a delay when the feedback voltage is pulled high, based on the determination result of the voltage determination circuit, and generating a calculation delay signal after the calculation duration; and a second delay circuit for starting a delay when the feedback voltage is pulled high, based on the determination result of the voltage determination circuit, and generating a fixed delay signal after the fixed duration. Specifically: when the isolated power converter is in the 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 after a fixed time interval; when the isolated power converter has completed startup or is operating in a steady state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as a sampled voltage after a calculated time interval.

[0007] According to an embodiment of the present invention, a feedback control circuit for an isolated power converter is also proposed. The isolated power converter receives an input voltage and generates an output voltage. The feedback control circuit includes: a voltage determination circuit, which determines whether a feedback voltage characterizing the output voltage is pulled high; a calculation circuit, which calculates the duration for which the feedback voltage is greater than a reference voltage to obtain a calculation duration; a delay circuit, which, in response to the determination result of the voltage determination circuit, starts to delay when the feedback voltage is pulled high, and generates a calculation delay signal after the calculation duration has been delayed; a sample and hold circuit, which, in response to the calculation delay signal, samples and holds the feedback voltage to obtain a sampled voltage; and an adjustment circuit, which, in response to the sampled voltage, generates a switch control signal to control the main power switch.

[0008] The isolated power converter and its feedback control circuit according to various aspects of the present invention improve the adjustment accuracy of the circuit. [ ] Simple Explanation of the Diagram

[0009] [Figure 1] is a schematic diagram of the circuit structure of the isolated power converter 100 according to an embodiment of the present invention;

[0010] [Figure 2] is a schematic diagram of the circuit structure of the isolated power converter 200 according to an embodiment of the present invention;

[0011] [Figure 3] is a schematic diagram of the circuit structure of the isolated power converter 300 according to an embodiment of the present invention;

[0012] [Figure 4] shows a flowchart 400 of a method for an isolated power converter according to an embodiment of the present invention. Implementation

[0013] Specific embodiments of the present invention will now be described in detail. It should be noted that the embodiments described herein are for illustrative purposes only and are not intended to limit the invention. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that these specific details are not necessary to practice the invention. In other instances, well-known circuits, materials, or methods have not been specifically described to avoid obscuring the invention.

[0014] Throughout this specification, references to "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 invention. Therefore, the phrases "in an embodiment," "in an embodiment," "an example," or "an example" appearing in various places throughout the specification do not necessarily refer to the same embodiment or example. Furthermore, specific features, structures, or characteristics can be combined in one or more embodiments or examples in any suitable combination and / or sub-combination. Moreover, those skilled in the art will understand that the accompanying drawings provided herein are for illustrative purposes and are not necessarily drawn to scale. It should be understood that when an element is referred to as "coupled to" or "connected to" another element, it can be directly coupled to or coupled to the other element, or there may be intermediate elements. Conversely, when an element is referred to as "directly coupled to" or "directly connected to" another element, there are no intermediate elements. The same reference numerals indicate the same elements. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0015] Figure 1 is a schematic diagram of the circuit structure of an isolated power converter 100 according to an embodiment of the present invention. In the embodiment shown in Figure 1, the isolated power converter 100 includes: a transformer T having a primary winding T1 that receives an input voltage VIN and a secondary winding T2 that provides an output voltage VO to supply a downstream load RL; a third winding T3 that generates a feedback voltage VFB characterizing the output voltage VO; a main power switch 101 that is periodically turned on and off to convert the input voltage VIN into the output voltage VO; and a feedback control circuit 102 coupled to the third winding T3, receiving the feedback voltage VFB, and generating a switch control signal G101 to control the main power switch 101 to turn on and off.

[0016] The feedback control circuit 102 includes: a voltage determination circuit 21, which determines whether the feedback voltage VFB is pulled high; a calculation circuit 22, which calculates the duration for which the feedback voltage VFB is greater than the reference voltage VREF, and obtains the calculation duration TC; a first delay circuit 23, which, in response to the determination result of the voltage determination circuit 21, starts to delay when the feedback voltage VFB is pulled high, and generates a calculation delay signal DTC after the calculation duration TC is delayed; a sample and hold circuit 24, which, in response to the calculation delay signal DTC, samples and holds the feedback voltage VFB, and obtains a sampled voltage VFBS; and an adjustment circuit 25, which, in response to the sampled voltage VFBS, generates the switch control signal G101 to control the main power switch 101. In this circuit, when the feedback voltage VFB is pulled high, after a short delay, such as the delay of a short pulse, the calculation circuit 22 resets the calculation duration TC and begins to calculate the duration during which the feedback voltage VFB is greater than the reference voltage VREF within the current switching cycle. The calculation duration TC is then provided to the first delay circuit 23 after the feedback voltage VFB is pulled high in the next switching cycle. In the embodiment shown in Figure 1, the output of the voltage determination circuit 21 is sent to the calculation circuit 22 via the short pulse circuit TP1. However, those skilled in the art should realize that the output of the voltage determination circuit 21 can also be sent directly to the calculation circuit 22 without passing through the short pulse circuit TP1 (shown as a dashed box in Figure 1), utilizing the circuit's own transmission delay to achieve a short-time delay effect.

[0017] In one embodiment of the present invention, the calculation circuit 22 multiplies the calculated duration interval during which the feedback voltage VFB is greater than the reference voltage VREF by a scaling factor k1 to obtain the calculation duration TC. In one embodiment of the present invention, the scaling factor k1 is close to 1, for example, k1 is set to 0.9.

[0018] In one embodiment of the present invention, the calculation circuit 22 starts timing when the feedback voltage VFB increases to be greater than the reference voltage VREF; and stops timing when the feedback voltage VFB decreases to be less than the reference voltage VREF, thereby obtaining the duration interval.

[0019] In one embodiment of the present invention, the voltage determination circuit 21 includes a comparator that compares the magnitude of the feedback voltage VFB and the threshold voltage VH. When the feedback voltage VFB is greater than the threshold voltage VH, it indicates that the feedback voltage VFB is pulled high.

[0020] In one embodiment of the present invention, the sample-and-hold circuit 24 includes: a short pulse circuit TP2, which generates a short pulse signal in response to the calculation delay signal DTC; and a sample-and-hold unit (as shown in FIG1, having a sampling switch and a holding capacitor), which responds to the short pulse signal, samples and holds the feedback voltage VFB, and obtains the sampled voltage VFBS.

[0021] In one embodiment of the present invention, the adjustment circuit 25 adjusts the sampling voltage VFBS to the internal voltage reference value VRI in order to adjust the output voltage VO to the desired value.

[0022] In one embodiment of the present invention, the internal voltage reference value VRI and the reference voltage VREF are proportionally related, such as VREF=k2*VRI, where k2 is a proportionality coefficient close to 1. For example, k2 can be set to 0.95.

[0023] During the operation of the isolated power converter 100, in each switching cycle, the calculation circuit 22 sends the calculation duration TC calculated in the previous cycle to the first delay circuit 23. When the main power switch 101 is turned off, the feedback voltage VFB is pulled high. On one hand, the first delay circuit 23 starts to delay in response to the pull-high feedback voltage VFB, and generates a calculation delay signal DTC after the delay of the calculation duration TC. The short pulse circuit TP2 generates a short pulse signal in response to the calculation delay signal DTC, that is, after the delay time determined by the calculation delay signal DTC after the feedback voltage VFB is pulled high, so that the sample and hold circuit 24 samples and holds the feedback voltage VFB to obtain the sampled voltage VFBS. Subsequently, the sampled voltage VFBS is sent to the adjustment circuit 25 to generate a switch control signal G101 to control the main power switch 101 to adjust the output voltage VO or protect the circuit. On the other hand, after the feedback voltage VFB is pulled high, after a short delay, the calculation circuit 22 resets the calculation duration TC obtained in the previous cycle and restarts the comparison between the feedback voltage VFB and the reference voltage VREF. It then times the interval where the feedback voltage VFB is greater than the reference voltage VREF to obtain a new duration interval. Subsequently, this duration interval is multiplied by the proportional coefficient k1 to obtain the calculation duration TC of the current cycle, which is supplied to the first delay circuit 23. This causes the first delay circuit 23 to begin delaying in the next switching cycle in response to the pull-up of the feedback voltage VFB, and to delay the calculation duration TC of the current cycle. The isolated power converter 100 operates in this way in each switching cycle to obtain the sampling time point of the feedback voltage, thereby obtaining accurate feedback voltage information, i.e., accurate information about the output voltage.

[0024] Figure 2 is a schematic diagram of the circuit structure of an isolated power converter 200 according to an embodiment of the present invention. The isolated power converter 200 shown in Figure 2 is similar to the isolated power converter 100 shown in Figure 1. However, unlike the isolated power converter 100 shown in Figure 1, in the embodiment shown in Figure 2, the feedback circuit 102 further includes: a second delay circuit 26, which, in response to the judgment result of the voltage determination circuit 21, starts delaying when the feedback voltage VFB is pulled high, and generates a fixed delay signal TDF after a fixed delay period. That is, the fixed delay signal TDF has a fixed delay period relative to the pull-up of the feedback voltage VFB; and a selection circuit 27, which selects to send the fixed delay signal TTF to the isolated power converter 200 when the isolated power converter 200 is in the startup phase (e.g., startup signal SS is 1), or when a sudden change in the load of the isolated power converter 200 causes the circuit to be in a transient state (e.g., status signal ST is 1). The sample-and-hold circuit 24 samples and holds the feedback voltage VFB for a fixed period of time after it is pulled high. That is, after the feedback voltage VFB is pulled high, the feedback voltage is sampled and held for a fixed period of time, which is used as the sample voltage VFBS. When the isolated power converter 200 completes startup (e.g., startup signal SS is 0) or the isolated power converter 200 is operating in a steady state (e.g., status signal ST is 0), the selection circuit 27 selects to send the calculation delay signal DTC to the sample-and-hold circuit 24, so that after the feedback voltage VFB is pulled high, the sample-and-hold circuit 24 samples and holds the feedback voltage VFB for a calculated delay period of time. That is, after the feedback voltage VFB is pulled high, the feedback voltage is sampled and held for a calculated time interval, which is used as the sample voltage VFBS.

[0025] Figure 3 is a schematic diagram of the circuit structure of an isolated power converter 300 according to an embodiment of the present invention. The isolated power converter 300 shown in Figure 3 is similar to the isolated power converter 100 shown in Figure 1. However, unlike the isolated power converter 100 shown in Figure 1, in the embodiment shown in Figure 3, the feedback circuit 102 includes: a voltage determination circuit 21, which determines whether the feedback voltage VFB is pulled high; a calculation circuit 22, which calculates the duration for which the feedback voltage VFB is greater than the reference voltage VREF, obtaining a calculation duration TC; and a first delay circuit 23, which, in response to the determination result of the voltage determination circuit 21, starts delaying when the feedback voltage VFB is pulled high, and generates a calculation delay signal DTC after the calculation duration TC is delayed. The first sample-and-hold circuit 24 responds to the calculation delay signal DTC, samples and holds the feedback voltage VFB to obtain the first sample voltage VFBS1; the second delay circuit 26, in response to the judgment result of the voltage determination circuit 21, generates a fixed delay signal DTF after a fixed time when the feedback voltage VFB is pulled high, that is, the fixed delay signal DTF has a fixed delay time relative to the pull-up of the feedback voltage VFB; the second sample-and-hold circuit 28 responds to the fixed delay signal DTF, samples and holds the feedback voltage VFB to obtain the second sample voltage VFBS2. The feedback circuit 102 further includes: a selection circuit 27, which selects a first sampling voltage VFBS1 as the sampling voltage VFBS when the isolated power converter 300 is in the startup phase (e.g., startup signal SS is 1), or when a sudden change occurs in the load of the isolated power converter 300, causing the circuit to be in a transient state (e.g., status signal ST is 1). That is, after the feedback voltage VFB is pulled high, the feedback voltage is sampled and held after a fixed time interval and is used as the sampling voltage VFBS; when the isolated power converter 200 has completed startup (e.g., startup signal SS is 0) or the isolated power converter 200 is operating in a steady state (e.g., status signal ST is 0), the selection circuit 27 selects a second sampling voltage VFBS2 as the sampling voltage VFBS. That is, after the feedback voltage VFB is pulled high, the feedback voltage is sampled and held after a calculated time interval and is used as the sampling voltage VFBS.

[0026] When the isolated power converter 200 / 300 is running, during the startup phase or when the load changes and the circuit is in a transient state, the feedback voltage is pulled up. After a fixed time interval, the feedback voltage is sampled and held as the sampling voltage before entering the subsequent adjustment circuit. This allows the feedback voltage to enter the feedback loop quickly without affecting the circuit's response time. When the isolated power converter has finished starting up or the circuit is in a steady state without a load change, the feedback voltage is pulled up again. After a calculated time interval, the feedback voltage is sampled and held as the sampling voltage before entering the subsequent adjustment circuit. This results in a more accurate feedback voltage entering the feedback loop, improving the circuit's adjustment precision.

[0027] Figure 4 shows a flowchart 400 of a method for an isolated power converter according to an embodiment of the present invention. The isolated power converter includes: a primary coil for receiving an input voltage, a secondary coil for providing an output voltage, a third coil for generating a feedback voltage characterizing the output voltage, and a main power switch. The method includes:

[0028] Step 401: Determine if the feedback voltage is pulled high. Compare the feedback voltage to the threshold voltage. If the feedback voltage is greater than the threshold voltage, it indicates that the feedback voltage is pulled high.

[0029] Step 402: Determine the status of the isolated power converter: Is the isolated power converter in the startup phase or in transient operation? If the isolated power converter is in the startup phase or in transient operation, proceed to step 403; if the isolated power converter has completed startup or is in steady-state operation, proceed to step 404.

[0030] Step 403: After the feedback voltage is pulled high, the feedback voltage is sampled after a fixed time interval. The sampled signal is sent to the adjustment circuit to adjust the on and off of the main power switch.

[0031] Step 404: After the feedback voltage is pulled high, the feedback voltage is sampled after a calculated time interval. This sampled voltage is then sent to the regulation circuit as a sampling signal to regulate the on and off states of the main power switch.

[0032] In one embodiment of the present invention, the method further includes: when the isolated power converter completes startup or is in steady-state operation, calculating the duration for which the feedback voltage is greater than the reference voltage to obtain a duration interval, multiplying the duration interval by a proportional coefficient to obtain the calculated duration, and using it for sampling the feedback voltage in the next switching cycle of the isolated power converter: that is, in the next switching cycle of the isolated power converter, after the feedback voltage is pulled high, the feedback voltage is sampled as a sampling signal after the time interval of the calculated duration.

[0033] Although the invention has been described with reference to several exemplary embodiments, it should be understood that the terminology used is descriptive and exemplary, and not restrictive. Since the invention can be embodied in many forms without departing from the spirit or essence of the invention, it should be understood that the above embodiments are not limited to any of the foregoing details, but should be interpreted broadly within the spirit and scope defined by the appended claims. Therefore, all variations and modifications falling within the scope of the claims or their equivalents should be covered by the appended claims.

[0034] 21: Voltage Determination Circuit 22: Calculation Circuits 23: First Delay Circuit 24: Sample and hold circuit, first sample and hold circuit 25: Adjustment circuit 26: Second Delay Circuit 27: Selection Circuit 28: Second Sample and Hold Circuit 100: Isolated Power Converter 101: Main power switch 102: Feedback control circuit, feedback circuit 200: Isolated Power Converter 300: Isolated Power Converter 400: Method Flowchart 401: Steps 402: Steps 403: Steps 404: Steps DTC: Calculate Delay Signal DTF: Fixed Delay Signal G101: Switch control signal k1: Proportionality coefficient RL: Downstream load SS: Startup Signal ST: Status Signal T: Transformer T1: Primary coil T2: Secondary coil T3: Third coil TC: Calculation duration TDF: Fixed Delay Signal TP1: Short pulse circuit TP2: Short pulse circuit VFB: Feedback Voltage VFBS: Sampling Voltage VFBS1: First sampling voltage VFBS2: Second sampling voltage VH: Critical Voltage VIN: Input voltage VO: Output voltage VREF: Reference Voltage VRI: Internal Voltage Reference Value

Claims

1. An isolated power converter, comprising: A transformer has a primary winding that receives the input voltage. The secondary coil provides the output voltage to supply the subsequent load. The third coil generates a feedback voltage that characterizes the output voltage; The main power switch is periodically turned on and off to convert the input voltage into the output voltage; A feedback control circuit, coupled to the third coil, receives the feedback voltage and generates a switch control signal to control the main power switch to turn on and off. The feedback control circuit includes: a voltage determination circuit to determine whether the feedback voltage is pulled high; a calculation circuit to calculate the duration for which the feedback voltage is greater than a reference voltage, and obtain a calculation duration; and a first delay circuit, which, in response to the determination result of the voltage determination circuit, starts to delay when the feedback voltage is pulled high, and generates a calculation delay signal after delaying the calculation duration. The second delay circuit, in response to the judgment result of the voltage determination circuit, starts delaying when the feedback voltage is pulled high, and generates a fixed delay signal after a fixed delay period; wherein: when the isolated power converter is in the startup phase or in a transient state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as the sampled voltage after the fixed time interval; when the isolated power converter has completed startup or is operating in a steady state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as the sampled voltage after the calculated time interval; the adjustment circuit generates the switch control signal in response to the sampled voltage to control the main power switch.

2. The isolated power converter as claimed in claim 1, further comprising: A sample-and-hold circuit; a selection circuit, wherein when the isolated power converter is in the startup phase or in the transient state, the selection circuit selects to send the fixed delay signal to the sample-and-hold circuit, so that the sample-and-hold circuit samples and holds the feedback voltage for the fixed duration after the feedback voltage is pulled high, thereby obtaining the sampled voltage; when the isolated power converter has completed startup or is operating in the steady state, the selection circuit selects to send the calculated delay signal to the sample-and-hold circuit, so that the sample-and-hold circuit samples and holds the feedback voltage for the calculated delay duration after the feedback voltage is pulled high, thereby obtaining the sampled voltage.

3. The isolated power converter as claimed in claim 1, wherein: The calculation circuit starts timing when the feedback voltage increases to be greater than the reference voltage; it stops timing when the feedback voltage decreases to be less than the reference voltage, thus obtaining the duration interval; the duration interval is multiplied by a proportional coefficient to obtain the calculation duration.

4. The isolated power converter as claimed in claim 1, wherein: When the feedback voltage is pulled high, after a short time delay, the calculation circuit resets the calculation duration and starts calculating the duration interval during which the feedback voltage is greater than the reference voltage in the current switching cycle, so as to provide the calculation duration to the first delay circuit after the feedback voltage is pulled high in the next switching cycle.

5. A feedback control circuit for an isolated power converter, the isolated power converter receiving an input voltage and generating an output voltage, the feedback control circuit comprising: The voltage determination circuit determines whether the feedback voltage, which represents the output voltage, has been pulled high. The system includes a calculation circuit that calculates the duration during which the feedback voltage is greater than the reference voltage, obtaining a calculation duration; a first delay circuit that, based on the judgment result of the voltage determination circuit, starts delaying when the feedback voltage is pulled high, and generates a calculation delay signal after the calculation duration; and a second delay circuit that, based on the judgment result of the voltage determination circuit, starts delaying when the feedback voltage is pulled high, and generates a fixed delay signal after the fixed duration. Specifically: when the isolated power converter is in the startup phase or transient state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as a sampled voltage after a fixed time interval; when the isolated power converter has completed startup or is operating in a steady state, after the feedback voltage is pulled high, the feedback voltage is sampled and held as the sampled voltage after the calculation duration.

6. The feedback control circuit as described in claim 5 further includes: Sample and hold circuit; When the isolated power converter is in the startup phase or in the transient state, the selection circuit selects to send the fixed delay signal to the sample-and-hold circuit, so that the sample-and-hold circuit samples and holds the feedback voltage for the fixed duration after the feedback voltage is pulled high, thereby obtaining the sampled voltage; when the isolated power converter has completed startup or is operating in the steady state, the selection circuit selects to send the calculated delay signal to the sample-and-hold circuit, so that the sample-and-hold circuit samples and holds the feedback voltage for the calculated delay duration after the feedback voltage is pulled high, thereby obtaining the sampled voltage.

7. The feedback control circuit as described in claim 5, wherein: The calculation circuit starts timing when the feedback voltage increases to be greater than the reference voltage; it stops timing when the feedback voltage decreases to be less than the reference voltage, thus obtaining the duration interval; the duration interval is multiplied by a proportional coefficient to obtain the calculation duration.

8. The feedback control circuit as described in claim 5, wherein: When the feedback voltage is pulled high, after a short time delay, the calculation circuit resets the calculation duration and starts calculating the duration interval during which the feedback voltage is greater than the reference voltage in the current switching cycle, so as to provide the calculation duration to the first delay circuit after the feedback voltage is pulled high in the next switching cycle.

9. The feedback control circuit as described in claim 5 further includes: The first sample-and-hold circuit responds to the calculation delay signal by sampling and holding the feedback voltage to obtain the first sample voltage; The second sample-and-hold circuit responds to the fixed delay signal by sampling and holding the feedback voltage to obtain a second sample voltage; the selection circuit selects the first sample voltage as the sample voltage when the isolated power converter is in the startup phase or in the transient state; when the isolated power converter has finished startup or is operating in the steady state, the selection circuit selects the second sample voltage as the sample voltage.

10. A feedback control circuit for an isolated power converter, the isolated power converter receiving an input voltage and generating an output voltage, the feedback control circuit comprising: The voltage determination circuit determines whether the feedback voltage, which represents the output voltage, has been pulled high. The system includes: a calculation circuit that calculates the duration during which the feedback voltage is greater than the reference voltage, obtaining a calculation duration; a delay circuit that, based on the judgment result of the voltage determination circuit, starts delaying when the feedback voltage is pulled high, and generates a calculation delay signal after the calculation duration has elapsed; a sample-and-hold circuit that, in response to the calculation delay signal, samples and holds the feedback voltage, obtaining a sampled voltage; and an adjustment circuit that, based on the sampled voltage, generates a switch control signal to control the main power switch.

11. The feedback control circuit as described in claim 10, wherein: The calculation circuit starts timing when the feedback voltage increases to be greater than the reference voltage; it stops timing when the feedback voltage decreases to be less than the reference voltage, thus obtaining the duration interval; the duration interval is multiplied by a proportional coefficient to obtain the calculation duration.

12. The feedback control circuit as described in claim 10, wherein: When the feedback voltage is pulled high, after a short time delay, the calculation circuit resets the calculation duration and starts calculating the duration interval during which the feedback voltage is greater than the reference voltage in the current switching cycle, so as to provide the calculation duration to the first delay circuit after the feedback voltage is pulled high in the next switching cycle.

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