Flyback converter with improved dynamic load response

Abstract

A secondary side control loop for a flyback converter is provided that uses a compensator to generate a secondary side control signal during normal operation. During periods of significant load variation, the compensator is bypassed and an open loop signal is used as the secondary side control signal.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of U.S. Provisional Patent Application No. 62 / 867,663, filed June 27, 2019, entitled "Flyback Converter With Improved Dynamic Load Response", the contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to flyback converters, and more specifically to flyback converters with improved dynamic load response. Background Technology

[0004] Isolated switching power converters, such as flyback converters, are commonly used in AC-DC and DC-DC conversions, where there is electrical isolation between the input power supply and one or more output terminals. A flyback converter is a buck-boost converter in which inductors are separated to form a transformer, thus both changing the output voltage magnitude and achieving primary-to-secondary isolation. The controller in a flyback converter cycles through the power switching transistors to regulate the output voltage. Figure 1 A prior art flyback converter is shown, comprising a power switching transistor S1 coupled to the primary winding T1 of a transformer T. The on / off cycle of the power switching transistor S1 is controlled by a primary-side controller U1. When the primary-side controller U1 cyclically turns the power switching transistor on, an input voltage V_IN connected to the input terminal of the primary winding T1 causes a primary winding current to flow through the primary winding T1. The peak primary winding current at the end of the on-time of the power switching transistor S1 depends on the input voltage V_IN, the magnetizing inductance of the transformer T, and the on-time of the power switching transistor S1. When the power switching transistor S1 is on, an output diode D1 coupled to the secondary winding T2 of the transformer prevents the secondary winding current from flowing. Alternatively, the output diode D1 can be replaced by a synchronous rectifier switching transistor.

[0005] When the power switching transistor S1 is turned off, the magnetic energy stored in the transformer causes current to flow in the secondary winding and charges the output capacitor C1 with the output voltage (V_OUT). The secondary-side controller U2 detects the difference between the output voltage and a preset reference voltage and generates a control signal based on this difference. This information needs to be transmitted to the primary-side controller U1 while maintaining electrical isolation. Optical isolators are typically used in this application.

[0006] Figure 2 The image shows the use of Figure 1the feedback loop of an SR flyback converter. The secondary-side controller includes an error amplifier (EA) that compares an output voltage V_OUT to a reference voltage V_REF to generate an error signal. A compensator with an integral filter filters and smooths the error signal to form a control signal that is driven into an opto-isolator. On the primary side, which is electrically isolated by the transformer, a filter is placed at the output of the opto-coupler to smooth the received control signal. Alternatively, the filtering can be created by the parasitic effects of the opto-coupler itself. The primary-side controller controls the cycling of the power switch transistor in response to the filtered received control signal. In Figure 2 In the middle, the transformer and the power switch transistor and any associated components such as the output capacitor CI are represented symbolically by the circuit. The regulation of the output voltage by the feedback loop adjusts the output voltage to equal the desired value set by the reference voltage. However, the filtering at the output of the opto-coupler reduces the feedback loop bandwidth and delays the received control signal response. This delay reduces the dynamic load response speed, which results in unusual overshoots and undershoots of the output voltage for transient load changes.

[0007] Accordingly, there is a need in the art for a flyback converter with secondary-side output voltage detection and improved dynamic response. SUMMARY

[0008] According to a first aspect of the disclosure, there is provided a secondary-side controller for a flyback converter with secondary-side regulation, the secondary-side controller comprising: an error amplifier configured to generate an error signal in response to a difference between an output voltage and a reference voltage; a compensator configured to compensate the error signal to produce a closed-loop control signal; an output state monitor configured to determine whether a threshold load change has occurred for the flyback converter, wherein the secondary-side controller is configured to drive an opto-coupler with the closed-loop control signal in response to an absence of the threshold load change, and to drive the opto-coupler with an open-loop control signal in response to the threshold load change.

[0009] According to a second aspect of the disclosure, there is provided a method of secondary-side regulation of a flyback converter, the method comprising: during a period in which a load change of the flyback converter is less than a threshold load change: comparing an output voltage to a reference voltage to generate an error signal, and processing the error signal in a compensator to produce a closed-loop control signal in a secondary-side controller, and transmitting the closed-loop control signal to a primary-side controller; and during a period in which a load change of the flyback converter is greater than the threshold load change: generating an open-loop control signal that does not involve the use of a compensator in the secondary-side controller, and transmitting the open-loop control signal to the primary-side controller.

[0010] According to a third aspect of the disclosure, there is provided a flyback converter with secondary side regulation, the flyback converter comprising: a secondary side controller configured to generate an open loop control signal during an open loop period in response to a load change of the flyback converter being greater than a threshold level, and configured to generate a closed loop control signal after termination of the open loop period, the secondary side controller comprising a multiplexer configured to select the open loop control signal or the closed loop control signal to provide a selected control signal; an optical isolator configured to transmit the selected control signal; and a primary side controller configured to control cycling of a power switch transistor in response to receiving the selected control signal from the optical isolator.

[0011] These advantageous features can be better understood by considering the following DETAILED DESCRIPTION. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 A SSR conventional flyback converter with secondary side output voltage detection is shown.

[0013] Figure 2 A feedback loop for a conventional SSR flyback converter of Figure 1 is shown.

[0014] Figure 3A A SSR flyback converter with improved dynamic response according to an aspect of the disclosure is shown.

[0015] Figure 3B A control loop for a SSR flyback converter of Figure 3A according to an aspect of the disclosure is shown.

[0016] Figure 4 Some exemplary input signals for an output state monitor in an improved control loop for Figure 3B is shown.

[0017] Embodiments of the disclosure and their advantages are best understood by referring to the following detailed description along with the accompanying drawings. It should be understood that like reference numerals refer to like elements in the several figures. DETAILED DESCRIPTION

[0018] Figure 3A A SSR flyback converter 300 with improved dynamic response is shown in FIG. 3. Some features of the SSR flyback converter 300 are as described for the SSR flyback converter 100 of FIG. 1. Figure 1The SSR flyback converter 300 includes a primary side controller U1 that controls the on and off cycles of the power switch transistor S1 to regulate the output voltage V OUT. When the power switch transistor S1 is on, a secondary side controller U2 keeps the synchronous rectification (SR) switch transistor off to prevent secondary winding current from flowing in the secondary winding T2 of the transformer T. When the power switch transistor S1 is off, the magnetic energy stored in the transformer T causes the secondary winding current to flow and charge the output capacitor C1 at the output voltage (V OUT) to power the load at the output current (I OUT). The primary winding peak current through the primary winding T1 depends on the input voltage (V IN), the magnetizing inductance of the transformer T, and the on time of the power switch transistor S1.

[0019] The secondary side controller U2 includes a port for detecting the drain voltage at the drain D of the SR switch transistor. Similarly, the secondary side controller U2 includes a port for detecting the source voltage at the source S of the SR switch transistor. The source voltage is the ground voltage of the secondary side controller, so that the drain voltage of the SR switch transistor is also its drain-to-source voltage. To detect when the power switch transistor S1 is on or off, the secondary side controller U2 compares the drain-to-source voltage of the SR switch transistor to a corresponding threshold voltage known in the art of synchronous rectification. The secondary side controller U2 also includes a port for detecting the output voltage, so that an error signal can be generated.

[0020] To provide an improved dynamic response, the secondary side controller U2 is configured to generate the control signal in either a feedback loop according to load usage or in an open loop operating mode. If the application of the load exceeds a threshold level, the secondary side controller U2 generates the control signal in the open loop operating mode. In contrast, if the load variation does not satisfy the threshold level, the secondary side controller U2 generates the control signal in the closed loop operating mode.

[0021] The closed loop operating mode is discussed with respect to Figure 2 . In particular, the closed loop operating mode is modified for the SSR flyback converter 300 as shown with respect to Figure 3B Figure 2 ​The conventional feedback loop discussed to improve the speed of dynamic response to transient load changes. During closed loop mode of operation (no significant load transients), the secondary side error amplifier (EA) generates an error signal in response to the difference between the output voltage (V OUT ) and the reference voltage (V REF ). The compensator and integrator compensate and integrate the error signal, for example, with proportional-integral-derivative (PID) control, to produce a closed loop control signal that is selected by a multiplexer (MUX) to drive the opto-isolator. A filter filters the control signal received from the opto-isolator so that the primary side controller can accordingly control the cycling of the power switches in the circuit. In Figure 3B , the transformer and power switch transistors, and any associated components such as the output capacitor CI, are represented symbolically by the circuit as discussed with respect to Figure 2 .

[0022] Thus, it can be appreciated that, in addition to including a multiplexer, the feedback loop for the SSR flyback converter 300 in closed loop mode of operation operates as discussed with respect to the conventional feedback loop discussed with respect to Figure 2 . To detect whether a transient load change meets a threshold level, the secondary side controller U2 includes an output state monitor that monitors whether a load transient, whether from an increase in load or a decrease in load, is large enough. If the load transient is large enough, the output state monitor drives a preset (open loop) control signal as represented by a preset setting module to present the open loop control signal to the multiplexer. For example, the preset setting module can include a lookup table that maps a particular output voltage change into an open loop control signal. The output state monitor also controls the selection through the multiplexer so that the multiplexer selects the open loop control signal in response to the threshold change. In this way, the received control signal for the primary side controller can be quickly adjusted in response to a threshold change in the output voltage despite the filter delay.

[0023] It is noted that the dynamic load change can be caused by a sudden application of load or a sudden removal of load. In response to a full application of load, the output voltage will drop below a first threshold level. Similarly, in response to a sudden removal of load, the output voltage will rise above a second threshold level. The output state monitor responds to both of these conditions by causing open loop generation of the control signal. The open loop value of the control signal depends on the implementation. For example, in one implementation, the output monitor sets the control signal to a maximum value in response to a threshold application of load. In such an implementation, the control signal increases in response to an increase in load. But in an alternative implementation, the control signal decreases in response to an increase in load. In such an implementation, the output state monitor sets the control signal to a minimum value in response to a threshold increase in load. The setting of the control signal in response to a threshold removal of load is obviously the inverse of the setting of the control signal in response to a threshold application of load. In the implementation where the control signal increases in response to an increase in load, the output state monitor sets the control signal to a minimum value in response to detecting a threshold removal of load. Conversely, for the implementation where the control signal decreases in response to an increase in load, the output state monitor sets the control signal to a maximum value in response to detecting a threshold removal of load. The setting of the control signal by the output state monitor is by way of a pre-set setting module.

[0024] If the output state monitor detects a threshold increase, the output state monitor can also instruct the compensator to adjust a pole, zero, or gain in its frequency response such that the compensator has a temporary increase in its response bandwidth. When the multiplexer eventually selects again the closed loop control signal generated by the compensator after termination of the open loop mode, the compensator can thus quickly react to the transient response due to its frequency response adjustment. The transient period for which the multiplexer selects the open loop control signal can be fixed or can be adaptively dependent on the magnitude of the output voltage variation. For example, the output state monitor can have several thresholds for detecting an application of load. Depending on the threshold level, the output state monitor can detect different amounts of load transients. The open loop mode can also be referred to as a bypass mode, as the compensator is bypassed when the open loop mode is active.

[0025] With respect to selecting the open loop control mode, the output state monitor can respond to a number of different factors. For example, as Figure 4As shown, the output state monitor can select the bypass operating mode using the rate of change of the output voltage, the rate of change of the output current, the magnitude of the output voltage, the switching frequency of the power switch transistor S1, or any combination of these factors. For example, the output state monitor can compare the output voltage to a corresponding threshold voltage to determine whether the output voltage has changed by a threshold amount (positive or negative) due to a transient change in the load. To detect the output current, the output state monitor can detect the voltage on a sense resistor (not shown) connected between the source of the SR switch transistor and the ground output port of the SSR flyback converter 300. The output state monitor can compare these various factors to corresponding thresholds to determine whether a load transient of sufficient magnitude has occurred.

[0026] With respect to resuming from the bypass mode to closed loop control, the output state monitor can use a watchdog timer that establishes a minimum activation time for the bypass mode. Further, the watchdog timer can be adaptive based on the severity of the transient condition that has been detected. Alternatively, it can monitor whether the output voltage has stabilized as an indication to return to regular closed loop operation. The switching frequency can also be used as a criterion for this transition. As previously discussed, the output state monitor can also adjust the dynamic response speed of the compensator. For example, such closed loop adjustments can be performed for less significant transient load changes where the bypass mode is not invoked.

[0027] As those skilled in the art will now appreciate, numerous modifications, substitutions and changes can be made to the materials, arrangements, configurations and usage of the devices of the application without departing from the scope thereof as expressed in the claims hereafter. In view of this, the scope of the present disclosure should not be limited to the scope of the specific embodiments shown and described herein, since they are only examples of some of the many possible embodiments, but should be in accordance with the full scope of the claims below and their functional equivalents.

Claims

1. A secondary-side controller for a flyback converter having secondary-side regulation, comprising: an error amplifier configured to generate an error signal in response to a difference between an output voltage and a reference voltage; a compensator configured to compensate the error signal to produce a closed-loop control signal; an output state monitor configured to determine whether a threshold load change has occurred for the flyback converter, wherein the secondary-side controller is configured to drive an optocoupler with the closed-loop control signal in response to an absence of the threshold load change, and to drive the optocoupler with an open-loop control signal in response to the threshold load change, the open-loop control signal not involving use of the compensator; and a multiplexer configured to select the open-loop control signal or the closed-loop control signal to provide the selected control signal to drive the optocoupler, wherein the output state monitor is configured to select an open-loop mode using a rate of change of the output voltage, a rate of change of an output current, a magnitude of the output voltage, a switching frequency of a power switch transistor, or a combination thereof.

2. The secondary-side controller of claim 1, wherein the output state monitor is configured to determine the threshold load change using the rate of change of the output voltage.

3. The secondary-side controller of claim 1, wherein the output state monitor is configured to determine the threshold load change using the rate of change of the output current.

4. The secondary-side controller of claim 1, wherein the output state monitor is configured to transition from driving the optocoupler with the open-loop control signal to again driving the optocoupler with the closed-loop control signal in response to a threshold change in the switching frequency of the power switch transistor.

5. The secondary-side controller of claim 1, wherein the secondary-side controller is further configured to increase a response speed of the compensator in response to the threshold load change.

6. The secondary-side controller of claim 1, further comprising: a lookup table configured to generate the open-loop control signal.

7. The secondary-side controller of claim 1, wherein the output state monitor is further configured to control the multiplexer to select the open-loop control signal in response to the threshold load change, and to select the closed-loop control signal in response to an absence of the threshold load change.

8. A method of secondary-side regulation for a flyback converter, for the secondary-side controller of any of claims 1-7, the method comprising: during a period in which a load change for the flyback converter is less than a threshold load change: comparing an output voltage to a reference voltage to generate an error signal, and processing the error signal in a compensator to produce a closed-loop control signal in a secondary-side controller, and transmitting the closed-loop control signal to a primary-side controller; and a multiplexer configured to select the open-loop control signal or the closed-loop control signal to provide the selected control signal to drive the optocoupler, wherein the output state monitor is configured to select an open-loop mode using a rate of change of the output voltage, a rate of change of an output current, a magnitude of the output voltage, a switching frequency of a power switch transistor, or a combination thereof. generating an open loop control signal that does not involve use of the compensator in the secondary side controller and transmitting the open loop control signal to the primary side controller during a period when the load variation of the flyback converter is greater than the threshold load variation, wherein the open loop mode is selected by an output state monitor in the secondary side controller using a rate of change of output voltage, a rate of change of output current, a magnitude of output voltage, a switching frequency of a power switch transistor, or a combination thereof.

9. The method of claim 8, further comprising: detecting whether the load variation of the flyback converter is greater than the threshold load variation by using the rate of change of output voltage.

10. The method of claim 8, further comprising: detecting whether the load variation of the flyback converter is greater than the threshold load variation by using a rate of change of output current.

11. The method of claim 8, further comprising: switching from generating the open loop control signal to generating the closed loop control signal in response to a change in a switching frequency of a power switch transistor.

12. The method of claim 8, further comprising: adjusting a cycling of a power switch transistor in response to receiving the open loop control signal at the primary side controller.

13. The method of claim 8, wherein the threshold load variation is a threshold load variation for an applied load.

14. The method of claim 8, wherein the threshold load variation is a threshold load variation for a removed load.

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