Flyback Converter with No-Load Power Control for Reduced Power Dissipation

Abstract

The present disclosure provides a flyback converter with no-load power control for reducing power consumption. The flyback converter has a secondary side low power mode controller that detects whether a mobile device has been disconnected from the flyback converter. In response to the detection, the low power mode controller initiates a low power operating mode in which the primary side controller is disabled to improve efficiency.

Description

Technical Field

[0001] The present application relates to switching power converters, and more particularly to a flyback converter with no-load power control for reducing power consumption. Background Art

[0002] Flyback switching power converters are commonly used to charge mobile devices such as smartphones and tablet computers because their transformers provide safe isolation from the AC household current. The flyback converter operates efficiently when the mobile device provides the load. However, during no-load conditions (e.g., when the flyback converter remains connected to the AC power source but no mobile device is connected for charging), this efficiency tends to drop significantly. These efficiency issues can be better understood by considering the example flyback converter 100 shown in FIG. 1 .

[0003] In the flyback converter 100, when the controller 105 causes the NMOS power switch transistor S1 to cycle on, the rectified input voltage (V_IN) drives a magnetizing current into the first terminal of the primary winding of the transformer T1. The source of the power switch transistor S1 is coupled to ground, and its drain is coupled to the remaining second terminal of the primary winding T1. The controller 105 regulates the output voltage (V_OUT) generated on the secondary side of the transformer T1 by controlling the cycle of the power switch transistor S1. However, the controller 105 cannot directly monitor the output voltage, such as through a wire or lead, because then the isolation through the transformer T1 will be destroyed. In the flyback converter 100, the feedback voltage (V_OUT) reflected at the transformer reset time is sensed on the auxiliary winding 110 of the transformer T1. FB ), the controller 105 uses the primary-only feedback technique to monitor the output voltage. The auxiliary winding voltage can also be rectified by the diode D1 and stored across the capacitor C1 to generate the supply voltage Vcc for the controller 105.

[0004] When the magnetizing current flows through the primary winding, the secondary-side synchronous rectification (SR) controller 115 turns off the SR switch transistor S2 connected in series with the secondary winding of the transformer T1 to prevent the secondary winding current from flowing. When the power switch transistor S1 is turned off, the SR controller 115 turns on the SR switch transistor S2 in a cycle to allow the magnetic energy stored in the transformer T1 to drive the secondary winding current to charge the output voltage stored across the output capacitor C2.

[0005] The efficiency of the flyback converter 100 is affected by the controller power consumption in the primary side controller 105 and the SR controller 115. During periods of relatively high load, the controller power consumption is a relatively minor factor in the efficiency of the flyback converter. However, as the load decreases, the controller power consumption becomes a significant factor in reducing efficiency. In extremely low load or no-load conditions, the primary side controller 105 enters a burst mode of operation or a skip mode of operation, in which power pulses are sent relatively infrequently. During this no-load operation, the primary side controller 105 continues to draw an operating current (I_Vcc). Similarly, the SR controller continues to draw an operating current (I_SR). Therefore, the total controller power consumption is Vcc*I_Vcc+V_OUT*I_SR. This controller power consumption continues unabated because it is common practice to leave the flyback converter connected to the AC power source even though the mobile device is disconnected from the flyback converter. Therefore, the controller power consumption is a significant factor in reducing efficiency, especially at higher levels of the output voltage.

[0006] Therefore, there is a need in the art for a flyback converter with improved low load efficiency or no load efficiency. Summary of the invention

[0007] To improve efficiency during no-load operating conditions, a secondary-side low-power mode controller is provided for the flyback converter, which cooperates with a primary-side watchdog circuit. The low-power mode controller is configured to detect a no-load condition that occurs when the mobile device is disconnected from the flyback converter. In response to the detection, the low-power mode controller shuts down the secondary-side SR controller (in SR embodiments) and signals the watchdog circuit to enter a low-power operating mode. In the low-power operating mode, the watchdog circuit commands the primary-side controller, which controls the cycling of the power switch transistor, to enter a sleep operating mode in which the primary-side controller consumes no power.

[0008] Since the primary side controller does not cycle the power switch transistor during the low power mode of operation, the output capacitor maintains the power supply voltage of the low power mode controller. Due to the (minimum) power consumption and leakage loss of the low power mode controller, the output voltage stored across the output capacitor will then slowly decrease during the low power mode of operation. The low power mode controller is also configured to monitor the output voltage and trigger the end of the low power mode of operation in response to the output voltage dropping below the threshold voltage. In response to detecting that the output voltage has dropped below the threshold voltage, the low power mode controller enables the operation of the SR controller (in the SR embodiment) and triggers the watchdog circuit to enable the operation of the primary side controller. The primary side controller can then continue to fully cycle the power switch transistor so that the output voltage is restored. If the no-load condition persists, the low power mode controller will trigger the low power mode of operation again in response to the recovery of the output voltage. Since the combined power consumption of the watchdog circuit and the low power mode controller during the low power mode of operation is less than the combined controller power consumption of the primary side controller and the SR controller, the efficiency is greatly improved during no-load operation.

[0009] These advantageous features may be better understood through consideration of the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is a diagram of a conventional flyback converter.

[0011] Figure 2 is a diagram of a flyback converter configured for low power operation during no-load or reduced load operating periods according to aspects of the present disclosure.

[0012] Figure 3 yes Figure 2 A more detailed diagram of the implementation of a flyback converter.

[0013] Embodiments of the present disclosure and their advantages are best understood by referring to the following detailed description.It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the accompanying drawings. DETAILED DESCRIPTION

[0014] To improve efficiency, the flyback converter has a secondary side low power mode controller circuit and a primary side watchdog circuit. The secondary side low power mode controller is configured to detect when the mobile device is decoupled from the flyback converter. If the secondary side low power mode controller detects the decoupling, the secondary side low power mode controller triggers a low power operating mode. During the low power operating mode, both the primary side controller and the SR controller are isolated from their supply voltages so that both controllers are turned off and do not consume power. The power consumption of the low power mode controller and the watchdog circuit is much lower than the controller power consumption, so the no-load efficiency is significantly improved. Some example embodiments will now be discussed in more detail.

[0015] exist Figure 2 2 shows an example flyback converter 200 including a low power mode controller 205 and a watchdog circuit 210. As discussed with respect to the flyback converter 100, the flyback converter 200 includes a primary side controller 105, a diode D1, a capacitor C1, a transformer T1, an auxiliary winding 110, a power switch transistor S1, an SR switch transistor S2, an output capacitor C2, and an SR controller 115. During normal operation when a mobile device (not shown) is coupled to the flyback converter 200, the primary side controller 205 operates by responding to a feedback voltage V from the auxiliary winding 110, as practiced in the primary-only feedback technique. FB The output voltage V_OUT is regulated by controlling the cycle of the power switch transistor S1. Alternatively, a feedback voltage can be obtained from the output voltage and transmitted from the secondary side of the transformer T1 to the primary side controller 105 through an optical isolator. When the power switch transistor S1 is turned on, the SR controller 115 keeps the SR switch transistor S2 turned off. When the primary side controller 105 causes the power switch transistor S1 to cycle off, the SR controller 115 causes the SR switch transistor S2 to cycle on, so that the magnetic energy stored in the transformer T1 can drive the output current (I_OUT) and output voltage of the flyback converter 200.

[0016] The power consumption of the primary side controller 105 is the product of the current it consumes (I_Vcc) and the power supply voltage Vcc it maintains through the diode D1 and the capacitor C1. Similarly, the power consumption of the SR controller 115 is the product of the current it consumes (I_SR) and the output voltage used as the power supply voltage Vcc_SR of the SR controller 115. Therefore, the total controller power consumption during normal operation is (I_Vcc*Vcc)+(I_SR*Vcc_SR). In contrast, the power consumption of the low power mode controller is the product of its input current I_SEC and its power supply voltage Vcc_Sec. Since the low power mode controller only monitors the low power operating mode, its power consumption (I_SEC*Vcc_Sec) is much less than the controller power consumption. The watchdog power consumption is even lower than the low power mode controller power consumption, so the total power consumption during the low power operating mode is much lower than the controller power consumption that would be consumed in a conventional flyback converter such as the flyback converter 100.

[0017] To detect whether a mobile device is connected to the flyback converter 200, the low power mode controller 205 can monitor the voltage across the sensing resistor Rs connected in series with the output power rail, which provides the output voltage to such a mobile device. If the voltage across the sensing resistor Rs drops below the disconnection threshold voltage (such as 50mv or another appropriately low threshold voltage), the low power mode controller 205 detects the disconnection of the mobile device. Note that it is conventional to charge the mobile device through a data cable such as a USB cable or an Apple lightning cable. In such an embodiment, the low power mode controller 205 can monitor the data connection (such as the D+ and D- interfaces in the USB cable) to determine whether the mobile device is disconnected. Regardless of how the low power mode controller 205 detects that the mobile device is disconnected, it is configured to continue to trigger the low power operating mode accordingly. In addition, the watchdog circuit 210 may include a timer.

[0018] exist Figure 3, the flyback converter 200 is shown in more detail in . The EMI filter 305 filters the EMI noise from the flyback converter to prevent it from affecting the AC power supply. The diode bridge 310 rectifies the AC voltage from the AC power supply to store the rectified input voltage across the input capacitor C3. When the primary side controller 105 turns on the power switch transistor S1, the rectified input voltage drives the magnetizing current into the primary winding of the transformer T1. When the power switch transistor S1 is cycled off, the RCD snubber network 315 protects the drain of the power switch transistor S1 from overvoltage. The primary controller 105 is configured to monitor the current conducted through the power switch transistor S1 by monitoring the voltage across the sense resistor Rsense. When the sense resistor voltage indicates that the desired peak current has been reached in the primary winding of the transformer T1, the primary side controller 105 cycles the power switch transistor S1 off. The SR controller 115 and the SR switch transistor S2 are as described with respect to Figure 2 The SR controller 115 is powered from the output voltage via switch S4.

[0019] The low power mode controller 205 responds to detecting that the mobile device has been disconnected from the output terminal 301 by opening switch S4 to shut down the SR controller 115. In addition, the low power mode controller 205 signals the watchdog circuit 210 to shut down the primary side controller 105. Note that the low power mode controller 205 cannot directly signal the watchdog circuit 210 through a lead or wire because such a connection would destroy the primary and secondary side isolation provided by the transformer T1. Therefore, the low power mode controller 205 signals the watchdog circuit 210 through an opto-isolator 325 such as compensated by the compensation network 320. During normal operation, the secondary side of the opto-isolator receives power through switch S4 such as coupled through the compensation network 320. To trigger the low power operating mode, the low power mode controller opens switch S4, which turns off the primary side transistor in the opto-isolator 325. The watchdog circuit 210 responds to this open circuit state of the opto-isolator 325 by opening switch S3. Since the primary side controller 105 receives its power supply voltage Vcc through switch S3, the primary side controller 105 is turned off during the low power mode of operation. Therefore, during the low power mode, the power switch transistor S1 is not cycled. The watchdog circuit 210 is powered by the power supply voltage Vcc. Similarly, the low power mode controller 205 is powered by the output voltage. The output voltage will subsequently drop during the low power mode because the power switch transistor S1 is not cycled to replenish the output voltage. In order to prevent the low power mode controller 205 from shutting down due to lack of power, the low power mode controller 205 is configured to compare the output voltage with a first threshold voltage, which can be a programmable reference voltage. If the output voltage drops below the first threshold voltage, the low power mode controller 205 closes switch S4 so that the optical isolator and the SR controller 115 are powered. The watchdog circuit 210 responds to the generated output voltage from the optical isolator 325 by closing switch S3 so that the primary side controller 105 can cycle the power switch S1.

[0020] The output voltage of the flyback converter 200 will then rise above the first threshold, to the desired level. To resume the low power operating mode, the low power mode controller 205 can compare the output voltage to a second threshold voltage (which can also be programmable and greater than the first threshold voltage). If the mobile device is still disconnected and the output voltage rises above the second threshold voltage, the low power mode controller again initiates the low power operating mode.

[0021] Those skilled in the art will recognize by now that many modifications, substitutions and changes can be made in and to the materials, devices, configurations and methods of use of the devices of the present disclosure without departing from the scope of the present disclosure. 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, as they are merely intended to be some examples thereof, but should be fully commensurate with the scope of the appended claims and their functional equivalents.

Claims

1. A flyback converter, comprising: a synchronous rectification switch coupled to the secondary winding of the transformer; a synchronous rectification controller configured to control a cycle of the synchronous rectification switch; switch; as well as A low power mode controller is configured to detect a no-load condition of the flyback converter, wherein the low power mode controller is further configured to disconnect the switch to shut down the synchronous rectification controller in response to detecting the no-load condition of the flyback converter, and to connect the switch to power the synchronous rectification controller in response to detecting a load condition of the flyback converter.

2. The flyback converter according to claim 1, wherein: The low power mode controller is also configured to monitor a voltage across an output sense resistor to detect the no load condition.

3. The flyback converter according to claim 2, wherein: The low power mode controller is further configured to detect the no load condition in response to a voltage across the output sense resistor dropping below a disconnect threshold voltage.

4. The flyback converter according to claim 1, wherein: The low power mode controller is also configured to monitor data terminals of a data and charging cable to detect the no-load condition.

5. The flyback converter according to claim 4, wherein: The low power mode controller is further configured to monitor the data terminal to determine whether a mobile device is connected to the flyback converter via the data and charging cable, and wherein the low power mode controller is further configured to detect the no-load condition in response to detecting that the mobile device is disconnected from the data and charging cable.

6. The flyback converter according to claim 4, wherein: The data and charging cable is a USB cable.

7. The flyback converter according to claim 1, wherein: The low power mode controller is also configured to determine whether the output voltage of the flyback converter has dropped below a first threshold voltage during the low power operating mode, and wherein the low power mode controller is also configured to terminate the low power operating mode in response to detecting that the output voltage of the flyback converter has dropped below the first threshold voltage.

8. A method for a flyback converter, comprising: cycling a synchronous rectification switching transistor during a period when no-load condition exists in the flyback converter, wherein a synchronous rectification controller controls the cycling of the synchronous rectification switching transistor; detecting a no-load condition of the flyback converter; as well as In response to detecting the no-load condition of the flyback converter, a switch coupled between a power supply voltage node and the synchronous rectification controller is opened to shut down the synchronous rectification controller.

9. The method of claim 8, further comprising: A disconnection of a mobile device from the flyback converter is detected to detect the no-load condition.

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