Flyback Converter with Output Voltage Calibration

Abstract

The present disclosure relates to a flyback converter with output voltage calibration. Output voltage calibration circuits and techniques are disclosed to increase the accuracy and precision of a constant voltage mode of a flyback converter.

Description

Technical Field

[0001] The present application relates to flyback converters, and more particularly to flyback converters with output voltage calibration. Background Art

[0002] The battery of a battery-powered device is charged by various constant voltage or constant current modes, depending on the battery charge state. As the names imply, the charging voltage remains constant at a certain fixed level during the constant voltage mode, while the charging current can vary. Conversely, the charging current remains constant during the constant current mode, while the charging voltage can vary. Proper sequencing and control of the constant voltage mode and constant current mode are critical to battery life. For example, the battery of a smartphone is usually integrated and non-removable. If the battery of such a device is damaged, the entire smartphone must be replaced. Therefore, conventional mobile devices include battery management circuits that control the charging voltage and charging current of the battery.

[0003] Since the battery management circuit within the mobile device is controlling the charging voltage and charging current applied to the battery, the tolerance of the switching power converter that powers the mobile device is relaxed. An exemplary charging system is shown in FIG. 1. A switching power converter such as a flyback converter 100 converts an input voltage to a specified output voltage V_out during a constant voltage operating mode. A battery-powered device such as a smart phone 105 includes a battery management circuit 110 that uses the output power from the flyback converter 100 to control a constant voltage or constant current charge applied to a battery that powers a system 115. For example, in a constant voltage mode, the battery management circuit 110 regulates a constant output voltage from the flyback converter 100 to a constant charging voltage for the battery. Similarly, in a constant current mode, the battery management circuit 110 regulates a constant output current from the flyback converter 100 to a constant charging current for the battery. This regulation of the battery management circuit 100 provides some tolerance for regulation in the flyback converter 100.

[0004] For example, the output voltage and output current tolerances of the flyback converter 100 can be + / - 5% for both the constant voltage mode of operation and the constant current mode of operation, as shown in FIG2 . For the desired constant voltage mode of 5V, the 5% tolerance means that the output voltage can actually be in the range from 4.75V to 5.25V. In order to maintain regulation during constant voltage operation, the primary side controller (not shown) in the flyback converter 100 requires some means of sensing the output voltage. In a primary feedback only configuration, the output voltage can be sensed through an auxiliary winding (or through the primary winding). Also shown in FIG2 , during the constant current mode of operation, a similar tolerance range occurs for the output current.

[0005] As shown in FIG. 2 , if the output voltage tolerance is fairly large, this indirect sensing of the output voltage is sufficient. However, portable devices have been developed in which the battery management circuit 100 is not present or is bypassed (in this case referred to as a direct charging system herein). In a direct charging system, the power converter itself directly charges the battery of the portable device. However, it should be noted that modern smartphones typically integrate the battery into the phone in an undetachable manner, so that if the battery is defective, the entire smartphone becomes defective. This is particularly problematic in view of the high cost of modern smartphones. Therefore, it is critical that power converters such as the flyback converter 100 regulate constant voltage and constant current modes with fairly high accuracy in order to maintain the health of the battery of the mobile device. Therefore, the tolerance of these operating modes is reduced in the direct charging system (e.g., + / -1% of the required constant current or constant voltage). Despite the reduced tolerance, it should be noted that there is a large tolerance on the component parameters used to indirectly sense the output voltage. In addition, the indirectly sensed output voltage is then compared to a reference voltage in the primary feedback-only system. The setting of the reference voltage also affects the accuracy of the output voltage regulation. Therefore, primary-side regulation of the output current voltage to the necessary tolerance during constant voltage operation is problematic.

[0006] Therefore, there is a need in the art for a flyback converter with improved primary side regulation of the output voltage during constant voltage operation. Summary of the invention

[0007] A flyback converter for direct charging applications has a secondary side output voltage calibration circuit. The calibration circuit senses the output voltage so that a reference voltage used in a constant voltage mode of operation can be adjusted in response to the sensed output voltage. However, this output voltage sensing is performed relatively infrequently compared to the power switch cycle frequency, so that the stability of the control loop for constant voltage operation is not affected by the reference voltage calibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIG. 1 is a diagram of a conventional mobile device charging system, wherein the mobile device includes a power management circuit.

[0009] 2 is a graph showing an increased margin due to a power management circuit of a mobile device for a constant voltage operation mode and a constant current operation mode in a power converter in the system of FIG. 1 .

[0010] Figure 3A is an illustration of a direct charging system according to an aspect of the present disclosure, wherein the mobile device does not include a power management circuit, and wherein the flyback converter includes a secondary-side output voltage calibration circuit.

[0011] Figure 3B yes Figure 3A Block diagram of a flyback converter.

[0012] Figure 4 yes Figure 3B A more detailed view of the constant voltage calibration of a flyback converter.

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

[0014] In order to meet the need in the art for improved primary-side regulation of the output voltage during constant voltage operation, a secondary-side calibration technique is introduced. The calibration is of relatively low frequency and therefore does not destroy or change the loop stability of the primary-side regulation. In order to perform the calibration, a secondary-side calibration circuit is provided that directly measures the output voltage under known operating conditions. For example, it is conventional for charging via a data cable (e.g., via a USB cable) that the flyback converter includes a transistor output switch that disconnects the flyback converter from the power lead in the USB cable. Therefore, the switch can be turned off during calibration so that there is no unknown load problem that affects the output voltage regulation. During calibration, a known resistive load is applied as a load, and the flyback converter is isolated from any other load by turning off the transistor output switch. Although calibration can be performed without using such a known load, its use provides a more stable and accurate calibration compared to a no-load condition.

[0015] Figure 3A An exemplary flyback converter 300 with a calibrated secondary side calibration circuit for a constant voltage mode of operation is shown in FIG. The accuracy provided by the secondary side output voltage calibration enables the flyback converter 300 to safely charge a battery directly to power a system 302 in a mobile device 301. In this direct charging configuration, there is no power management circuitry controlling the charging of the battery in the mobile device 301. Instead, there is the flyback converter 300 that must regulate the constant voltage and constant current charging modes of the battery. Figure 3B 300 is shown in more detail in FIG. The primary side controller 305 controls the switching of the power switch transistor S1 to regulate operation in either the constant voltage or constant current operation mode. During constant voltage regulation, the controller 305 indirectly samples the output voltage Vout by sampling the feedback voltage (VFB) on the auxiliary winding 310 of the transformer T1. The feedback voltage is filtered in the loop filter 335 and compared with the reference voltage (V _REF) to generate an error voltage. Based on the error voltage, the controller 305 adjusts the cyclic modulation of the power switch transistor S1, such as by pulse width modulation or pulse frequency modulation, to maintain the output voltage at a desired level during constant voltage operation.

[0016] When the power switch transistor S1 is cycled on, the input voltage V_IN causes a magnetizing current to flow through the primary winding of the transformer T1. When the power switch transistor S1 is turned on, the output diode D1 prevents the secondary current from flowing in the secondary winding of the transformer T1. Alternatively, the secondary current can be controlled by a synchronous rectification switch known in the synchronous rectification art. When the power switch transistor S1 is cycled off, the secondary current flows to charge the output capacitor C2 with the output voltage (V_OUT). During normal operation (no calibration is performed), the switch S3 is opened to prevent the calibration load 320 (e.g., a resistor) from discharging current. In addition, the output switch S2 is closed during normal operation so that the flyback converter can drive a load (e.g., a mobile device not shown).

[0017] Under predetermined conditions such as startup or in response to a threshold temperature change, the output voltage is measured by the output calibration circuit 325. Based on the measured output voltage, the calibration circuit 325 adjusts the reference voltage that is compared to the feedback voltage in the error amplifier 330. However, the calibration circuit 325 is located on the secondary side of the transformer T1, and therefore cannot directly transmit the new value of the reference voltage to the primary side of the flyback converter 300. Instead, the reference voltage command is driven through a ground isolation channel 315 (e.g., an opto-isolator). Alternatively, a capacitor or the transformer T1 itself can be used to form the ground isolation channel 315. If the calibration indicates that the output voltage is too low compared to the desired output voltage, the calibration circuit 325 will increase the reference voltage. Conversely, if the output voltage is too high compared to the desired output voltage, the calibration circuit 325 reduces the reference voltage.

[0018] With respect to setting the desired output voltage, the setting may be negotiated between the flyback converter and the load, such as is performed in a fast charging system. A data cable (e.g., a USB cable) can only conduct so much current to the load. If the output voltage is maintained at a default level (e.g.,

[0019] 5V), the charging of a relatively large capacity battery may take too long to meet the user's expectations. However, in a fast charging system, the output voltage can be increased from the default level (e.g., from 5V to 9V, or from 5V to 12V, etc.), so that charging can be performed faster. Whether the desired output voltage is the default level or the increased level, the calibration circuit 325 adjusts the reference voltage accordingly.

[0020] After passing through the ground isolation channel 315, the reference voltage command is received at the error amplifier 330. Since the command is digital, the controller 305 only needs an interface (not shown) to receive and decode the digital command for the reference voltage. Since such an interface is known in the field of flyback converters for sending information across the channel 315, the interface will not be discussed further. Sensing the output voltage by only primary feedback is performed by sensing the reflected voltage on the auxiliary winding 310 when the transformer is reset. After filtering in the filter 335 (or before filtering), the feedback voltage is digitized so as to be received at the error amplifier 330 as well. The error amplifier 330 is therefore implemented in the digital domain and is shown separately from the controller 305 for illustration purposes. In an alternative embodiment, error amplification can be performed in the analog domain. In such an embodiment, it is necessary to convert the reference voltage command into an analog reference voltage in a digital-to-analog converter (DAC).

[0021] exist Figure 4 The resulting constant voltage feedback loop for the flyback converter 300 is shown in more detail in FIG. As described above, the output voltage sensing and the resulting adjustment of the reference voltage are performed relatively infrequently, so that the stability of the feedback loop is not affected by the output voltage calibration. As previously described, the voltage calibration circuit 325 senses the output voltage and converts it into a digital version of the output voltage in an analog-to-digital converter. To form a digital command for the reference voltage, the first adder 405 forms the difference between the digital version of the output voltage and a default calibration reference (V_cali_ref). The default calibration reference is a digital version of the default output voltage, such as 5V discussed previously. The difference with the first adder 405 is represented as a calibration error (Err_cali) 410. The calibration error is stored in a memory (not shown) because it will be reused until the next calibration cycle. The desired output voltage for the constant voltage mode of operation is represented herein as Vset. In a fast charging embodiment, the expected value may be equal to the default value, or it may be greater than the default value. To map the calibration error to the desired output voltage, the calibration error 410 is multiplied by the ratio of Vset to the default calibration reference in the multiplier 415. If Vset is the same as the default calibration reference (no increased output voltage in the fast charge embodiment), then multiplier 415 leaves calibration error 410 unchanged. However, if Vset is greater than the default calibration reference due to a fast charge mode of operation requiring an increase from the default output voltage, then multiplier 410 increases the calibration error accordingly.

[0022] The corrected calibration error from the multiplier 415 is then subtracted from Vset in a second adder 420 to form a reference voltage command. The interface (not shown) then drives the reference voltage command through channel 315 so that a digital reference voltage (Vref) can be recovered on the primary side of the flyback converter 300. In order to sense the output voltage by only primary feedback, the auxiliary winding voltage is sensed by, for example, a voltage divider 425. In an alternative embodiment, the primary winding voltage may be sensed alternatively. After voltage scaling 430 and waveform analysis 435 to locate the transformer reset sampling time, the resulting feedback voltage Vfb is subtracted from the reference voltage at the error amplifier 330 to form an error signal used by the controller 305 to modulate the cycle of the power switching transistor (represented as power supply 440).

[0023] Some skilled in the art will now recognize that, depending on the specific application at hand, many modifications, substitutions and changes may be made to the materials, devices, configurations and methods of use of the apparatus of the present invention without departing from the spirit and 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 examples of the present disclosure, but should be fully commensurate with the appended claims and their functional equivalents.

Claims

1. A circuit for a flyback converter, comprising: an output switch for isolating an output node of the flyback converter from a power lead in a data cable; an output voltage calibration circuit configured to directly sense an output voltage of the flyback converter when the output switch is closed to provide a digital version of the output voltage, and the output voltage calibration circuit configured to adjust a reference voltage command in response to a comparison of the digital version of the output voltage with a desired level of the output voltage; a ground isolation channel configured to transmit the reference voltage command from the secondary winding side of the flyback converter to the primary winding side of the flyback converter; an error amplifier configured to form an error signal in response to a difference between a digitized feedback voltage and the reference voltage command; as well as A power switch controller is configured to modulate a cycle of a power switch transistor in response to the error signal, wherein the power switch controller is further configured to form the digitized feedback voltage by sampling an auxiliary winding voltage at a transformer reset time.

2. The circuit according to claim 1, wherein The error amplifier is a digital circuit implemented by the power switch controller.

3. The circuit according to claim 2, wherein: The power switch controller is located on the primary winding side of the flyback converter.

4. The circuit according to claim 1, wherein: The output voltage calibration circuit is also configured to directly sense the output voltage when the flyback converter is driving a known internal load.

5. The circuit according to claim 4, wherein: The output voltage calibration circuit includes a first adder configured to form a difference between the digital version of the output voltage and a default calibration reference to form a calibration error.

6. The circuit according to claim 5, wherein: The output voltage calibration circuit also includes a multiplier for multiplying the calibration error by a ratio of a desired level of the output voltage to the default calibration reference to form an adjusted calibration error.

7. The circuit according to claim 6, wherein: The output voltage calibration circuit also includes a second adder configured to form a difference between the desired level of the output voltage and the adjusted calibration error to form a command for an adjusted reference voltage.

8. The circuit according to claim 7, wherein: The ground isolation channel includes an optical isolator.

9. A method of calibrating a flyback converter operating in a constant voltage mode of operation, the method comprising: sensing an output voltage of the flyback converter to form a sensed output voltage while the flyback converter is isolated from an external load and is driving a known internal load; comparing the sensed output voltage to an expected level of the output voltage to form a calibration error; adjusting a reference voltage command in response to the calibration error to form an adjusted reference voltage command; transmitting the adjusted reference voltage command from the secondary winding side of the flyback converter to the primary winding side of the flyback converter; Providing digital feedback voltage by sampling the auxiliary winding voltage during the transformer reset time; comparing the adjusted reference voltage command to a digitized feedback voltage to form a digital error signal; as well as A cycling of a power switch in the flyback converter is modulated in response to the digital error signal.

10. The method according to claim 9, wherein: Comparing the sensed output voltage with the expected level of the output voltage is performed at startup of the flyback converter.

11. The method of claim 9, further comprising charging a mobile device with the output voltage in a direct charging mode of operation.

12. The method according to claim 9, wherein: Comparing the sensed output voltage to the expected level of the output voltage to form the calibration error includes subtracting a default calibration reference from the sensed output voltage to form the calibration error.

13. The method according to claim 12, wherein: Adjusting the reference voltage command in response to the calibration error includes: multiplying the calibration error by a ratio of the desired level of the output voltage to the default calibration reference to form an adjusted calibration error; and The adjusted calibration error is subtracted from the desired level of the output voltage to form the adjusted reference voltage command.

14. The method according to claim 9, further comprising: sensing the auxiliary winding voltage at the transformer reset time to form a sensed feedback voltage; as well as The sensed feedback voltage is digitized to form the digitized feedback voltage.

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