Dynamic discharge current control for improved power supply output regulation

Abstract

A flyback converter controller is provided with a secondary side controller that regulates discharge current from an output voltage rail during a communication period with a primary side controller to maintain the output voltage within a regulation rate.

Description

Technical Field

[0001] The present application relates to switching power converter controllers and, more particularly, to dynamic discharge current control for improving power supply output regulation in a flyback converter. Background Art

[0002] Mobile devices such as smartphones are traditionally charged with a data interface (such as a USB cable) that is subject to a maximum current limit. At a default charging voltage such as 5V, the current limit results in extended charging times. In order to reduce charging times even with the current limit of the USB cable, certain fast charging protocols have been developed in which the output voltage driven from a switching power converter via the data interface is increased from a default level (e.g., from 5V to 9V or 12V, etc.) to deliver higher power without increasing the current beyond the charging cable limit. At these increased charging voltages, although the current that can be conducted through the data interface is limited, the charging time is reduced.

[0003] In a flyback converter implementing a fast charging protocol, a transformer electrically isolates the primary side from the secondary side. The secondary side drives a data interface to charge the mobile device. From this, the secondary side controller enumerates the mobile device and determines whether the increased supply voltage is supported. In order to maintain electrical isolation, the secondary side controller cannot simply communicate the generated information to the primary side controller of the flyback converter through wires or leads. Instead, the secondary side controller is traditionally caused to send messages to the primary side controller via a ground isolation channel. The message notifies the primary side controller of the increased supply voltage. For example, the secondary side controller is traditionally caused to send the message via a pulse sent through an optoisolator. The primary side controller responds to the received pulse by pulsing the primary side power switching transistor. The pulsing of the primary side power switching transistor causes the supply voltage to rise undesirably above the expected value of the supply voltage, especially during light load conditions.

[0004] Therefore, there is a need in the art for improved regulation of the supply voltage during primary-side to secondary-side communication as achieved by pulsing of power switching transistors. Summary of the invention

[0005] According to a first aspect of the present disclosure, a flyback converter is provided, which includes: a current source configured to conduct a discharge current from an output voltage rail for the flyback converter in response to a discharge command; and a secondary side controller configured to enumerate mobile devices for whether the mobile devices support a fast charging mode, and configured to send a message to the primary side controller during a communication period to confirm that the mobile device supports the fast charging mode, the secondary side controller including: a discharge control logic circuit configured to adjust the discharge command in response to a deviation of a power supply voltage from an expected value of the power supply voltage during the communication period.

[0006] According to a second aspect of the present disclosure, a communication method during a communication period for a flyback converter is provided, the communication method comprising the following steps: sending a message as a series of pulses from a secondary side controller to a primary side controller; in the primary side controller, responding to the series of pulses by cycling a power switch through a series of cycles; measuring a deviation of a power supply voltage from a desired value in response to the power switch cycling through the series of cycles; and adjusting a discharge current from an output voltage rail supplying the power supply voltage to ground to maintain the power supply voltage within a regulation range including the desired value.

[0007] According to a third aspect of the present disclosure, a flyback converter is provided, comprising: a plurality of selectable resistor circuits coupled between output voltage rails for the flyback converter, each selectable resistor circuit comprising a resistor connected in series with a switch; and a secondary side controller configured to enumerate mobile devices for whether the mobile devices support a fast charging mode, and configured to send a message to a primary side controller during a communication period to confirm that the mobile devices support the fast charging mode, the secondary side controller comprising: a discharge control logic circuit configured to select from the plurality of selectable resistor circuits in response to a deviation of a power supply voltage from an expected value of the power supply voltage during the communication period to adjust a discharge current conducted from the output voltage rail to ground.

[0008] These advantageous features may be better understood by considering the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 A flyback converter according to one aspect of the present disclosure is illustrated in which a current source is dynamically controlled to minimize disturbances in a power supply voltage during primary-side to secondary-side communication.

[0010] Figure 2 The power supply voltage and discharge current waveforms during a conventional communication period are illustrated.

[0011] Figure 3 is a more detailed view of a secondary side controller including an analog-to-digital converter for measuring deviations of a supply voltage from a desired value according to one aspect of the present disclosure.

[0012] Figure 4 Illustrated in Figure 3 Power supply voltage and discharge current waveforms during the communication period of the secondary-side controller.

[0013] Figure 5 is a more detailed view of a secondary side controller including a plurality of comparators for measuring deviations of a supply voltage from a desired value according to one aspect of the present disclosure.

[0014] Embodiments of the present disclosure and their advantages will be 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

[0015] Figure 1 An example flyback converter 100 is illustrated in FIG. 1 , which reduces disturbances in the power supply voltage during primary-side to secondary-side communication. A diode bridge (not illustrated) rectifies the AC main voltage into a rectified input voltage V_IN carried on an input voltage rail 105. An input capacitor C1 coupled between the input voltage rail and ground is used to smooth the input voltage. Transformer T1 includes a primary winding P coupled between the input voltage rail and a power switching transistor S1. A primary-side controller U1 controls the cycling of the power switching transistor to regulate the power supply voltage carried on a voltage bus V_BUS on a USB cable 110.

[0016] In an embodiment with primary side control, it is the primary side controller U1 that regulates the supply voltage. For example, the primary side controller U1 can sense the Vsense voltage on the auxiliary winding Aux at the transformer reset time in each cycle of the power switching transistor S1 to sense the supply voltage. Based on this sensing, the primary side controller U1 can then calculate the peak primary current of the next cycle of the power switching transistor S1. When the power switching transistor S1 is turned on, the primary current flows through the primary winding P and flows through the power switching transistor S1 to establish a sense resistor voltage Isense across the sense resistor R, which is coupled between the power switching transistor S1 and ground. When the desired peak current has been reached as determined by Isense, the primary side controller U1 turns off the power switching transistor S1.

[0017] However, it will be appreciated that in an alternative embodiment, the cycling of the power switch transistor S1 is controlled by the secondary side controller U2, as implemented by the primary side controller U1. Regardless of how the cycling of the power switch transistor is controlled, it is the secondary side controller U2 that enumerates the mobile devices (not illustrated) via data channels (such as D+ and D-) in the USB cable 110. It will be appreciated that in an alternative embodiment, the USB cable 110 may be replaced with other types of data cables, such as a lightning data cable. After enumerating the mobile devices as to whether the increased power supply voltage is supported, the secondary side controller U2 alerts the primary side controller U1 of the new power supply voltage level via an opto-isolator (COMM) 120. In an alternative embodiment, other types of ground isolated communication channels may replace the opto-isolator 120, such as a capacitor or a signaling transformer.

[0018] The resulting secondary-side to primary-side message sent through the optoisolator 120 includes a series of pulses. The primary-side controller U1 acknowledges receipt of the pulses by cycling the power switch transistor S1. Just like the power cycle, this communication cycle of the power switch transistor S1 results in magnetic energy being stored in the transformer T while the power switch transistor S1 is on. During this on-time of the power switch transistor S1, the output diode D1 connected to the secondary winding S for the transformer T prevents secondary current from flowing in the secondary winding S. In an alternative embodiment, the output diode D1 can be replaced with a synchronous rectifier switching transistor.

[0019] Once the power switch transistor cycling stops, the magnetic energy stored in the transformer T forward biases the output diode D1, causing the secondary winding current to flow through the output diode D1 to charge the output capacitor C2 coupled between ground and the output voltage rail 115 with the supply voltage. Thus, the primary-side to secondary-side communication that occurs via the communication cycle of the power switch transistor S2 increases the supply voltage. During normal operation, the secondary-side controller U2 keeps the output switch transistor S2 turned on, causing the output voltage rail 115 to be connected to the V_BUS rail in the USB cable 110.

[0020] If the secondary side controller U2 senses via the data channel that the mobile device has been disconnected from the USB cable 110, it is possible that a new mobile device may be connected to the USB cable 110, and the new mobile device does not support the increased power supply voltage. In order to reduce the power supply voltage to a default level (e.g., to 5V), the secondary side controller U2 turns on a current source I1 coupled between ground and the output voltage rail 115. The secondary side controller U2 also utilizes the current source I1 to handle the increase in power supply voltage that occurs during primary-to-secondary communication. Although it is conventional to use the current source I1 to handle the increase in power supply voltage, the current conducted by the current source I1 during such conventional use is constant. This can be referred to Figure 2 To better understand, Figure 2 The power supply voltage Vbus and the current source current (discharge current) waveform during the primary-to-secondary communication period are illustrated. Communication starts at time t1 and ends at time t2. During this communication period, the discharge current is conducted at a constant level. The power supply voltage Vbus unexpectedly rises during the communication period and then experiences a large amount of interference after time t2, which is also undesirable.

[0021] The secondary side controller U2 handles this undesired disturbance in the supply voltage by dynamically controlling the amount of current discharged by the current source I1 during the communication period. This dynamic control of the amount of current is responsive to deviations of the supply voltage from its desired level and may be accomplished with respect to either digital or analog sensing of the supply voltage. For example, the secondary side controller U2 may include a Figure 3 The analog-to-digital converter (ADC) shown in FIG. ADC digitizes the power supply voltage Vbus that drives the mobile device 305. For the sake of illustration, Figure 3 The USB cable is shown in simplified form in FIG. The discharge control logic circuit 310 in the secondary side controller U2 determines the deviation between the digitized power supply voltage and the digitized expected power supply voltage. The deviation (which can be positive or negative) is itself a digital value. Based on the digital deviation, the discharge control logic circuit 310 changes the discharge current conducted by the current source I1. For example, the discharge control logic circuit 310 generates a 3-bit control signal that is used to control the current source I1 so that the current source I1 can conduct one of eight different levels of discharge current in response to the control signal. Alternatively, the discharge control logic circuit 310 can generate an analog control signal that continuously changes the discharge current.

[0022] The secondary side controller U2 includes a communication module 315 for controlling the sending of messages to the primary side controller U1 through the optical isolator. In some embodiments, the discharge control logic circuit 310 responds to the initiation of communication by the communication module 315 by commanding the current source I1 to discharge a default level of current while the communication is active. The default level of current is then adjusted while the communication is active in response to the deviation of the power supply voltage from its expected value.

[0023] Figure 4A discharge current waveform in response to a digital control signal is shown together with a corresponding power supply voltage waveform. The communication period starts at time t1 and ends at time t4. In response to the start of the communication period, the discharge current begins to conduct at its default level, which is used whenever communication is valid. Starting from time t1, the power supply voltage Vbus begins to increase from its desired level (Vdesired). Thus, the discharge current discharges at the default level from time t1 until the power supply voltage rises to the upper limit 405 of the power supply voltage regulation range. In response to the power supply voltage reaching its upper limit at time t2, the discharge current increases to a second level, which causes the power supply voltage to drop until it reaches the lower limit 410 of the power supply voltage regulation range at time t3. The discharge current then decreases to the default level until the communication period ends at time t4. With Figure 3 Compared with the static discharge current control, the power supply voltage disturbance generated is significantly reduced.

[0024] In an alternative embodiment, the supply voltage is not digitized, e.g. Figure 5 In this embodiment, the power supply voltage is compared with a plurality of threshold voltages by corresponding multiple comparators. For example, the first comparator 505 compares the power supply voltage with a first reference voltage (Vref1) (eg Figure 4 The second comparator compares the power supply voltage with a second reference voltage (Vref2) (eg, Figure 4 The lower limit voltage 410 of the discharge current is compared. Based on the output of the two comparators, the discharge control logic circuit 310 can determine whether the power supply voltage is within the desired adjustment range and control the discharge current accordingly. Note that the current source can be replaced by an optional resistor such as resistors R1 and R2. Resistor R1 is coupled to ground from the output voltage rail by means of switch S1. Similarly, resistor R2 is coupled to ground from the output voltage rail by means of switch S2. It will be understood that additional resistors can be implemented to increase the dynamic range of the discharge current adjustment. By changing the selected resistance by actuating the corresponding switch, the discharge control logic circuit thereby adjusts the discharge current. Each resistor connected in series with its corresponding switch can be represented here as a selectable resistor circuit.

[0025] As will now be understood by those skilled in the art and depending on the particular application at hand, many modifications, substitutions and changes may be made in and to the materials, devices, constructions and methods of use of the apparatus of the present disclosure without departing from the scope thereof. In view of this, the scope of the present disclosure should not be limited to the scope of the specific embodiments illustrated and described herein, as they are only carried out in some exemplary manner thereof, but the scope should fully correspond to the scope of the appended claims and their functional equivalents.

Claims

1. A flyback converter, comprising: a current source configured to conduct a discharge current from an output voltage rail for the flyback converter in response to a discharge command; and a secondary-side controller configured to enumerate a mobile device as to whether the mobile device supports a fast charging mode and to send a message to the primary-side controller during a communication period confirming that the mobile device supports the fast charging mode, the secondary-side controller comprising: A discharge control logic circuit is configured to adjust the discharge command in response to a deviation of a power supply voltage from an expected value of the power supply voltage during the communication period.

2. The flyback converter according to claim 1, characterized in that: The secondary side controller further comprises: an analog-to-digital converter configured to digitize the supply voltage to form a digitized measurement of the supply voltage, and wherein the discharge control logic circuit is further configured to adjust the discharge command in response to a deviation of the digitized measurement of the supply voltage from a digitized version of the expected value of the supply voltage.

3. The flyback converter according to claim 2, characterized in that: The discharge command is a digital discharge command.

4. The flyback converter according to claim 3, characterized in that: The discharge control logic circuit is configured to adjust the digital discharge command from a default level, and wherein the current source is further configured to conduct a default value for the discharge current in response to the digital discharge command being equal to the default level.

5. The flyback converter according to claim 1, characterized in that: The secondary side controller further comprises: a first comparator configured to compare the power supply voltage with a first reference voltage; and A second comparator is configured to compare the supply voltage with a second reference voltage, and wherein the discharge control logic circuit is further configured to adjust the discharge command in response to a first output signal from the first comparator and a second output signal from the second comparator.

6. The flyback converter according to claim 2, characterized in that: The secondary side controller is further configured to send the message to the primary side controller via an optical isolator.

7. The flyback converter according to claim 1, characterized in that: The message comprises a series of pulses sent through an optoisolator to the primary side controller.

8. The flyback converter according to claim 7, characterized in that: The primary side controller is configured to cycle the power switching transistor through a series of cycles in response to the series of pulses.

9. The flyback converter according to claim 1, characterized in that: The current source is arranged in parallel with a secondary winding of a transformer for the flyback converter.

10. The flyback converter according to claim 1, characterized in that: The secondary-side controller is also configured to enumerate the mobile device via the USB cable.

11. A method during a communication period for a flyback converter, the method comprising the steps of: sending a message from the secondary side controller to the primary side controller as a series of pulses; In the primary-side controller, responding to the series of pulses by cycling a power switch through a series of cycles; measuring a deviation of a power supply voltage from an expected value for the power supply voltage in response to the power switch cycling through the series of cycles; as well as adjusting a discharge current from an output voltage rail supplying the supply voltage to ground to maintain the supply voltage within a regulation range that includes the desired value for the supply voltage; The method further comprises enumerating mobile devices to determine whether the mobile devices support a fast charging mode, wherein sending the message comprises sending a confirmation that the mobile device supports the fast charging mode.

12. The method according to claim 11, characterized in that Measuring the deviation of the power supply voltage from an expected value comprises: digitizing the supply voltage to form a digital supply value; and A digitized version of the desired value for the supply voltage is subtracted from the digital supply value.

13. The method according to claim 11, characterized in that The method further comprises the following steps: A default value of the discharge current is discharged from the output voltage rail to ground at the beginning of the communication period, wherein adjusting the discharge current includes: in response to the deviation of the power supply voltage from the expected value of the power supply voltage exceeding a first threshold, increasing the discharge current from the default value to a first increased value.

14. The method according to claim 11, characterized in that Adjusting the discharge current maintains the supply voltage within a desired regulation range that includes the desired value of the supply voltage.

15. The method according to claim 11, characterized in that Adjusting the discharge current includes adjusting a current source coupled to the output voltage rail.

16. The method according to claim 13, characterized in that The method further comprises the following steps: In response to the deviation of the power supply voltage falling below a second threshold, the discharge current is reduced from the first increased value to the default value.

17. A flyback converter, comprising: a plurality of selectable resistor circuits coupled between output voltage rails for the flyback converter, each selectable resistor circuit comprising a resistor in series with a switch; and a secondary-side controller configured to enumerate a mobile device as to whether the mobile device supports a fast charging mode and to send a message to the primary-side controller during a communication period confirming that the mobile device supports the fast charging mode, the secondary-side controller comprising: A discharge control logic circuit is configured to select from the plurality of selectable resistor circuits to adjust a discharge current conducted from the output voltage rail to ground in response to a deviation of a supply voltage from an expected value of the supply voltage during the communication period.

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