Flyback Converter with Output Current Calibration

Abstract

The present disclosure relates to a flyback converter with output current calibration. An output current calibration is disclosed to increase the accuracy and precision of a constant current mode of a flyback converter.

Description

Technical Field

[0001] The present application relates to a flyback converter, and more particularly to a flyback converter with output current calibration. Background Art

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

[0003] Because the battery management circuitry within the mobile device is controlling the charge voltage and charge current applied to the battery, the tolerances of the switching power converters powering the mobile device are relaxed. Figure 1 An exemplary charging system is shown in . A switching power converter such as a flyback converter 100 converts an input voltage to a regulated output voltage V_OUT during a constant voltage mode of operation. A battery-powered device such as a smart phone 105 includes a battery management circuit 110 that controls a constant voltage or constant current charge applied to the battery to power a system 115 using the output power from the flyback converter 100. For example, in the constant voltage mode, the battery management circuit 110 regulates the constant output voltage from the flyback converter 100 to a constant charging voltage for the battery. Similarly, in the constant current mode, the battery management circuit 110 regulates the 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 a degree of tolerance for regulation in the flyback converter 100.

[0004] For example, for constant voltage and constant current operation modes, such as Figure 2 As shown, the output voltage and output current tolerance of the flyback converter 100 can be + / - 5%. For a desired constant voltage mode of 5V, a 5% tolerance means that the output voltage can actually be in the range of 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 the auxiliary winding (or through the primary winding). Similarly, Figure 2As shown, a similar tolerance range occurs for the output current during the constant current mode of operation. Just like the output voltage, the primary side controller cannot sense the output current directly, but must instead sense the output current indirectly. To sense the output current indirectly, the primary side controller can use a sense resistor to sense the peak current through the power switch during the switching cycle. The output current can then be estimated because the output current is proportional to the peak current. Specifically, this proportionality depends on many factors including the transformer turns ratio, the switching period, the sense resistor resistance, and the transformer reset time.

[0005] Such as Figure 2 As shown, if the output current tolerance is quite large, this indirect sensing of the output current is sufficient. However, portable devices have been developed in which the battery management circuit 100 is either absent or bypassed in what is herein referred to as a direct charging system. In a direct charging system, the power converter itself directly charges the battery of the portable device. Note, however, that modern smartphones typically have the battery non-removably integrated into the phone, so if the battery is defective, the entire smartphone becomes defective. This is particularly problematic in view of the high cost of modern smartphones. It is therefore critical that power converters such as the flyback converter 100 regulate constant voltage and constant current modes with considerable accuracy in order to keep the battery of the mobile device healthy. Therefore, the tolerances of these operating modes are reduced in direct charging systems (e.g., + / -1% of the desired constant current or constant voltage). Despite the reduced tolerances, note that there are considerable tolerances in the component parameters that control the proportionality between the peak current and the output current (such as the sense resistor and the transformer reset time). Therefore, it is problematic for the primary side regulation of the output current to achieve the necessary tolerance during constant current operation.

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

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

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

[0009] Figure 2It is shown in Figure 1 Graph of the increased tolerance of constant voltage and constant current operation modes in a power converter in a system with FIG. 1 due to the presence of a power management circuit of a mobile device.

[0010] Figure 3A is a diagram of a direct charging system according to one 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 current calibration circuit.

[0011] Figure 3B yes Figure 3A Block diagram of the flyback converter in FIG.

[0012] Figure 4 yes Figure 3B Figure 2 is a graph of the power switching waveform and peak voltage regulation waveform of the flyback converter.

[0013] Figure 5 yes Figure 3B A more detailed illustration of the constant current calibration of the flyback converter.

[0014] Embodiments of the present disclosure and their advantages are best understood by referring to the detailed description that follows.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] To address the need in the art for improved primary side regulation of output current during constant current operation, a secondary side calibration technique has been introduced. The calibration is relatively low frequency and therefore does not disrupt or change the loop stability of the primary side regulation. To better appreciate the advantages of this calibration, the factors that affect the accuracy of primary side regulation during the constant current operation mode of the flyback converter will first be discussed. The output current of a flyback converter operating in discontinuous conduction mode is given by the following equation (1):

[0016]

[0017] Where I0 is the output current, I sec_pk is the secondary peak current, T p is the switching cycle of the power switch, and T rst is the transformer reset time. The transformer reset time is determined by the cycle of the power switch turning off and on from its peak value (I sec_pk ) and the subsequent ramp-down of the secondary current to zero. The secondary peak current I sec_pk Equal to the primary peak current I pk Multiply by the turns ratio N of the flyback transformer ps Since the sensing resistor R senseIt is used to measure the peak voltage (V ipk ) to measure the primary peak current (I pk ), the output current relationship in equation (1) can be rewritten as shown in equation (2) below:

[0018]

[0019] Examination of equation (2) illustrates the difficulty in maintaining precise regulation of the output current. For example, T p 、T rst 、N ps and R sense The measurement of is subject to manufacturing tolerances, parasitic effects, and the resulting uncertainties. Therefore, the traditional regulation of the output current during the constant current mode of operation will not meet the stringent requirements of the direct charging system. From equation (2), it can be seen that during the constant current mode, the following equation (3) is conventionally used to set the peak voltage V ipk :

[0020]

[0021] where k CC is a proportional constant that is traditionally static. By comparing equations (2) and (3), it can be seen that by controlling the peak voltage V according to equation (3) ipk Keep the output current constant, which is suitable for constant current operation mode. As the proportional constant k CC The output current as a function of is given by the following equation (4):

[0022]

[0023] where η x is the transformer conversion rate. The switching period T p is usually constant (although it may be subject to jitter), so the transformer reset time T rst The control makes most of the cycle-to-cycle regulation based on peak voltage. In conventional primary-only regulation of constant output current, the factor T in one switching cycle p and T rst is applied to the subsequent switching cycle to determine the desired peak voltage. The voltage across the sense resistor is then compared to the desired peak voltage to determine the switch on time by cycling the power switch off once the voltage across the sense resistor reaches the desired peak voltage. However, this conventional primary side regulation of the output current is problematic in terms of providing the desired degree of accuracy and tolerance required by direct charging systems, for example.

[0024] In order to provide improved regulation accuracy and tighter tolerances for constant current mode operation, a secondary side calibration circuit is provided that directly measures the output current under known operating conditions. For example, the secondary side calibration circuit can use a secondary side sense resistor. Since the resistance of the secondary side sense resistor is known, the secondary side calibration circuit can measure the output current by detecting the voltage across the secondary side sense resistor and dividing by the resistance of the secondary side sense resistor, as governed by Ohm's law. It should be noted that the proportionality constant k in equation (3) is CC There is a proportionality between the desired value of the output current as governed by equation (4). Therefore, the secondary-side calibration circuit commands the primary-side controller to adjust its proportionality constant k based on the difference between the desired output current and the measured output current. CC In this way, constant current mode regulation accuracy is advantageously improved. Although a secondary side calibration circuit is used, the primary side regulation loop is not affected because the calibration frequency is relatively low. In contrast, the switching frequency of the power switch is often relatively high (e.g., 50 KHz or higher). Since calibration is performed relatively infrequently, it has no practical effect on the primary side regulation of the output current. Therefore, there are no stability issues caused by the inclusion of a secondary side calibration circuit.

[0025] Figure 3A An exemplary flyback converter 300 with secondary side output current calibration is shown in FIG. The accuracy provided by the secondary side output current calibration enables the flyback converter 300 to safely and directly charge a battery used to power a system 302 in a mobile device 301. In this direct charging configuration, there is no power management circuitry in the mobile device 301 that controls the charging of the battery. Instead, the flyback converter 300 must regulate a constant voltage and constant current charging mode of the battery. Figure 3B3. The flyback converter 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 a constant voltage or constant current mode of operation. During constant voltage regulation, the controller 305 samples the output voltage Vout indirectly by sampling the feedback voltage (VFB) on the auxiliary winding 310 of the transformer T1. The feedback voltage is filtered in the loop filter 315 and compared with the reference voltage (V_REF) at the error amplifier (EA) to generate an error voltage. Based on the error voltage and the desired output voltage, the controller 305 adjusts the modulation of the cycle 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. However, depending on the charge state of the battery (not shown) being charged in the mobile device, the controller 305 may not be able to regulate the output voltage to the desired level. The controller 305 then switches to a constant current mode of operation as represented by the CC mode controller 320, which is shown separately from the controller 305 for illustration purposes because it will typically be integrated with the controller 305.

[0026] In the constant current mode of operation, the controller 320 monitors the current flowing through the sense resistor R in series with the power switch transistor S1. sense The peak voltage V generated at both ends ipk , which is in turn connected in series with the primary winding of transformer T1. During the on-time of each power switching cycle, the rectified input voltage V_IN causes a magnetizing current to flow through the power switching transistor S1 and thus through the sensing resistor. Once the peak voltage V ipk When the desired level is reached, the controller 305 turns off the power switch transistor S1. The peak secondary current then flows through the secondary winding of the transformer T1 to produce an output current I_OUT that flows to the load (not shown). The output current charges the smoothing capacitor C2 to produce an output voltage. The output diode D1 rectifies the secondary current, but it should be appreciated that such rectification can be performed using a synchronous rectification switch as is known in the synchronous rectification art.

[0027] The controller 320 can only indirectly control the output current, such as by using equation (3) above, because the controller 320 is isolated from the secondary side of the flyback converter 300 by transformer T1. In the conventional use of equation (3), the proportionality constant k CC is a constant. However, V ipk The calibration circuit 325 directly measures the output current, so that the proportionality constant k CCcan be adjusted to maintain the output current at a desired level. In this way, inaccuracies caused by parasitic effects such as the sense resistor and component tolerances are reduced so that the output current can be regulated to the tight tolerances (e.g., + / -1%) required for direct charging applications in constant current mode. V ipk The measurement of the output current by the calibration circuit 325 occurs on the secondary side of the transformer T1. However, this measurement cannot be transmitted directly to the primary side of the transformer T1, such as by wires or leads, because the isolation between the grounds of the primary and secondary sides of the transformer T1 will be destroyed. Therefore, V ipk The calibration circuit 325 communicates the necessary calibration information through an isolation channel, such as an optoisolator 330. In alternative embodiments, other types of isolation channels may be used, such as the use of capacitors or the transformer T1 itself.

[0028] Given the calibration information about the measured output current, the controller 320 changes the proportionality constant k accordingly. CC In this way, V ipk The level of is changed to be different from that which would be provided by conventional application according to equation (3). Figure 4 Some example waveforms of the power switch cycle and sense resistor voltage are shown in Figure 1. For each on-time of the power switch transistor, the sense resistor voltage rises from ground to V ipk , so the power switch transistor is cyclically turned off. Referring again to equation (3), the switching period T p is basically constant, so it is the transformer reset time T rst Controlling V ipk The conventional cycle-to-cycle adjustment of CC The calibration makes V ipk The output current is more tightly regulated by increasing or decreasing it accordingly.

[0029] exist Figure 5 Additional details of the proportional constant calibration in the flyback converter 300 are shown in FIG. 1 . To measure the output current directly, a secondary-side sense resistor is inserted into the ground path return of the output voltage. The output current I out During constant current operation, it is passed to the load (not shown) and returns along the ground path to produce a voltage across the secondary side sense resistor that is proportional to the output current. The current sensing circuit and analog-to-digital converter (ADC) 500 in the secondary side output calibration circuit 325 sense the output current I by sensing the sense resistor voltage and converting it proportionally to a digital value representing the output current. out. The constant current mode detection and filtering circuit 501 detects whether the load is being driven in constant current mode by, for example, comparing the output current to the output current limit. If the regulation of the output voltage drives the output current to reach the output current limit, the flyback converter 300 can no longer operate in the constant voltage mode, but must instead operate in a constant current mode so that the output current does not exceed the output current limit. In addition, the constant current mode detection and filtering circuit 501 filters the output current calibration so that the output current calibration is completed relatively infrequently compared to the power switch cycling frequency. In this way, the gain and phase margin of the constant current mode of operation are not affected by the output current calibration.

[0030] During the constant current mode of operation, the controllers 320 and 305 regulate the cycling of the power switch transistors to maintain the output current at a desired level, which is designated herein as Iset. The calibration circuit 325 includes an adder 505 that forms the difference between the measured output current and Iset. The difference is then subtracted from Iset in another adder 510 to form a calibrated desired output current (designated herein as iset). The adjusted desired output current is mapped to an adjusted proportional constant k CC (e.g. from iset to k CC The conversion circuit 520 defines CC It is then transmitted to the primary side of the flyback converter 300 through an isolated communication channel such as an opto-isolator 330. If the output current calibration indicates that the measured output current is greater than the expected level, iset to k CC Convert k CC iset to k if the output current calibration indicates that the measured output current is less than the desired level. CC Convert k CC Increased from its default value which would otherwise be used to achieve the desired level of constant current operation.It will be appreciated that adders 505 and 510, conversion circuit 520, and constant current detection and filtering circuit 501 may be implemented on either the secondary side or the primary side of the transformer.

[0031] The constant current mode controller 320 uses the transformer reset time T as determined by sensing the reflected voltage on the auxiliary winding 310. rst , and using the adjusted proportionality constant k CC Using equation (3) to form a digital version of the desired peak voltage (V ipk_d ). The digital-to-analog converter (DAC) converts the digital peak voltage into an analog peak voltage V ipk, so that it is compared with the primary side sense resistor voltage in comparator 515. When the sense resistor voltage reaches the peak voltage V ipk When the comparator output signal changes to a binary one level (e.g., the supply voltage) to trigger the controller 305 to turn off the power switching transistor to terminate its on-time in the current power switching cycle. The controller 305 can regulate the power switching cycle in either a pulse width modulation (PWM) or pulse frequency modulation (PFM) mode of operation. Note, however, that since the sensing of the output current is adjusted, the resulting regulation of the output current is now much more tightly controlled.

[0032] As those skilled in the art will recognize by now and depending on the particular application at hand, many modifications, substitutions and changes may be made in and to the materials, apparatus, configurations and methods of use of the apparatus of the present disclosure 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 intended merely as some examples thereof, but should be fully commensurate with the scope of the appended claims and their functional equivalents.

Claims

1. A circuit for a flyback converter, comprising: a peak voltage determination circuit configured to determine a desired peak voltage for a current switching cycle of the power switching transistor in response to a transformer reset time, a switching period of a cycle of the power switching transistor, and an adjusted proportionality constant, wherein the adjusted proportionality constant is adjusted in response to a determination of an output current of the flyback converter; a comparator configured to compare the expected peak voltage to a primary side sense resistor voltage; and A power switch controller is configured to turn off a power switch transistor in response to an output signal from the comparator, the output signal indicating that the primary side sense resistor voltage is equal to the desired peak voltage.

2. The circuit of claim 1, wherein: The peak voltage determination circuit and the power switch controller are both implemented in a microcontroller.

3. The circuit of claim 1 , further comprising: a current sensing circuit configured to sense a secondary side sense resistor voltage; as well as An analog-to-digital converter is configured to convert the secondary side sense resistor voltage into a digital signal.

4. The circuit of claim 3, further comprising: A first summer is configured to determine a difference between the digital signal and a desired level of the output current.

5. The circuit of claim 4, further comprising: A second summer is configured to determine a difference between a desired level of the output current and the difference from the first summer to provide an adjusted output current level.

6. The circuit of claim 5, further comprising: A conversion circuit is configured to convert the adjusted output current level into the adjusted proportionality constant.

7. The circuit of claim 1 further comprising an isolated communication channel, wherein: The peak voltage determination circuit is also configured to receive the adjusted proportionality constant from the isolated communication channel.

8. The circuit of claim 7, wherein: The isolated communication channel includes an optical isolator.

9. The circuit of claim 7, wherein: The isolated communication channel includes a transformer for the flyback converter.

10. The circuit of claim 1, wherein: The peak voltage determination circuit is further configured to determine the desired peak voltage during a constant current mode of operation of the flyback converter.

11. A method for calibrating a flyback converter during a constant current mode of operation, comprising: sensing an output current of the flyback converter to provide a sensed output current; adjusting a proportionality constant in response to a difference between a desired output current and the sensed output current to provide an adjusted proportionality constant; determining a peak voltage of a primary side sense resistor during a current cycle of the power switch transistor in response to the adjusted proportionality constant, a transformer reset time, and a cycle period of the power switch transistor; as well as The peak voltage is compared to a voltage across the primary side sense resistor.

12. The method of claim 11, further comprising: In response to the voltage of the primary side sense resistor being equal to the peak voltage, the power switch transistor is turned off during a current cycle of the power switch transistor.

13. The method of claim 11, wherein: The sensing of the output current includes sensing a voltage of a secondary-side sense resistor to provide a sensed voltage.

14. The method of claim 13, further comprising digitizing the sensed voltage to provide a digitized sensed voltage.

15. The method of claim 14, further comprising determining a difference between the desired output current and the digitized sense voltage to provide a first difference.

16. The method of claim 15 further comprising determining a difference between the desired output current and the first difference to provide a second difference, and wherein, Adjusting the proportionality constant in response to a difference between a desired output current and the sensed output current includes adjusting the proportionality constant in response to the second difference.

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