WO2017099768A1

WO2017099768A1 - Short circuit protection for data interface charging

Info

Publication number: WO2017099768A1
Application number: PCT/US2015/064829
Authority: WO
Inventors: Jianming Yao; Yong Li
Original assignee: Dialog Semiconductor Inc
Current assignee: Renesas Design North America Inc
Priority date: 2015-12-09
Filing date: 2015-12-09
Publication date: 2017-06-15
Anticipated expiration: 2018-06-09

Abstract

A switching power converter is provided that monitors the switching frequency of a power switch while a device to be charged through a data cable is disconnected from the data cable. Should the switching frequency exceed a threshold value, the switching power converter detects a soft-short condition in the data cable.

Description

Short Circuit Protection for Data Interface Charging

TECHNICAL FIELD

[0001 ] This application relates to power converters, and more particularly to a fault monitor for a power converter charging a device through a data interface cable.

BACKGROUND

[0002] The explosive growth in mobile electronic devices such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. A flyback switching power converter is typically provided with a mobile device as its transformer provides safe isolation from AC household current. It is conventional for the switching power converter to couple to the device being charged through a standard interface such as a Universal Serial Bus (USB) interface.

[0003] The USB interface includes a differential pair of signals (D+ and D-) for signaling and also provides power and ground. With regard to the delivery of power, a USB cable can only provide a certain amount of current. For example, the USB 2.0 standard allows for a maximum output current of 500 mA whereas the USB 3.0 standard supports a maximum output current of 900 mA. Traditionally, the delivery of power through a USB cable used a voltage of 5.0 V. But modern mobile device batteries typically have a storage capacity of several thousand milliamps. The charging of such batteries, even at the increased output currents allowed in the USB 3.0 standard, while thus be delayed if the power is delivered using a 5 volt power supply voltage. This is particularly true in that the switching power supply, the cable, and the receiving device all present a resistance to the output current. [0004] To enable a rapid charge mode in light of the output current limitations and also the associated losses from device resistances, it is now becoming conventional to use markedly higher output voltages over the USB cable. For example, rather than use the conventional USB output voltage of 5 V, rapid charging modes have been developed that use 9V, 12V, or even 1 V. The increased voltages allow the switching power supply to deliver more power over the USB cable without exceeding the maximum output current limitations. However, many legacy devices can only support the standard 5 V from a USB cable. A rapid-charge switching power supply will thus engage in an enumeration process with the device being charged to determine if the higher output voltages are supported. This enumeration may occur over the differential D+ and D- pins. Through the enumeration, the switching power converter and the enumerated device may change the USB output voltage to an increased level that is supported by the enumerated device. The result is considerably reduced charging time, which leads to greater user satisfaction.

[0005] Although rapid charging modes are thus advantageous, problems have arisen in their implementation. For example, the USB cable may get dirty such that a dust particle or other slightly conductive ;object couples between the VCC pin (the pin delivering the output power) and one of the differential signaling pins D+ and D-. Alternatively, the USB cable itself may become frayed from user twisting such that a slightly conductive path exists between the VCC wire and one of the wires for the D+ and D- signals. The result is a "soft short" between VCC and one of the differential data signals. It is denoted as a soft short in that the impedance for the coupling between the corresponding pins (or wires) is relatively high compared to a true short circuit. In that regard, it is conventional for a switching power converter driving a USB interface to include an over-current protection circuit that will shut down the charging through the USB interface if a short circuit is detected. In this fashion, the maximum output current levels for the USB interface are not exceeded. But a soft short will not result in such a large increase in current. A conventional switching power converter with overcurrent protection will thus not respond to a soft short in that the increase in output current is negligible or minor such that it does not trigger an over-current state,

[0006] This lack of response is problematic in that users will often leave a US B cable connected to the switching power converter after removing their portable device. The switching power converter will then waste power by driving the soft short in the cable, which reduces system reliability and safety. Moreover, even if the user removes the cable, the USB interface on the switching power converter itself may be contaminated with dust so as to still experience a soft short.

[0007] Accordingly, there is a need in the art for improved power converters that protect against soft shorts over data interfaces used to deliver power.

SUMMARY

[0008] To detect faults such as a soft short circuit, a switching power converter controller is provided that monitors the data channels on a data cable that is also used for charging battery-powered devices. For example, the data cable may comprise a universal serial bus (USB) cable, a mini-USB cable, or an Apple Lightning cable. At the interface to such data cables, there are data terminals through which the switching power converter controller may enumerate the battery-powered device such as to determine whether the battery-powered device supports increased voltages as used in rapid charging modes. The switching power converter controller disclosed herein exploits these data terminals to determine whether a battery-powered device is connected to the data cable. Should the switching power converter controller determine that no load is present on the data cable (the battery-powered device being

disconnected), the switching power converter controller implements a switching frequency detection mode to detect soft short conditions. In particular, the switching power converter controllers disclosed herein are configured to compare the power switch cycling frequency to a threshold value while operating in the switching frequency detection mode. Should the switching frequency exceed the threshold value, the switching power converter detects the presence of a soft short condition on the data cable. In response to this detection, the switching power converter may trigger a reset or reduce the power switching cycling frequency. In addition, the switching power converter may alert a user of the soft-short condition such as through the illumination of an LED or other display device. The resulting soft short detection is quite advantageous as it protects against soft-short conditions that cannot be detected through conventional over-current threshold techniques. In particular, the relatively high impedance of a soft- short circuit results in a relatively small amount of current being conducted that does not trigger an over-current threshold. But the resulting soft-short circuit is detected through the switching power converter controller described herein.

[0009] These advantageous features may be better appreciated from the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a diagram of an example flyback converter with synchronous rectification and including a secondary-side controller configured to detect soft short circuits in accordance with an aspect of the disclosure. [0011] Figure 2 is a diagram of an example flyback converter without synchronous rectification and including a secondary-side controller configured to detect soft short circuits in accordance with an aspect of the disclosure.

[0012] Figure 3 is a diagram of an example secondary-side controller in accordance with an aspect of the disclosure.

[0013] Figure 4A illustrates an embodiment of the power switch cycling frequency detector and comparator of Figure 3 including a counter in accordance with an aspect of the disclosure.

[0014] Figure 4B illustrates an embodiment of the power switch cycling frequency detector and comparator of Figure 3 including an RC filter in accordance with an aspect of the disclosure.

[0015] Figure 5 illustrates the waveforms for a power switch cycling that exceeds a short-circuit detection frequency in accordance with an aspect of the disclosure.

[0016] Figure 6 is as flowchart of an example method of detecting a soft-short condition in a data cable driven by a switching power converter.

[0017] 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 figures.

DETAILED DESCRIPTION

[0018] An improved fault monitor is provided for switching power converters used to charge mobile devices through data interfaces such as a Universal Serial Bus (USB) cable, a mini-USB cable, a micro-USB cable, or an Apple Lighting cable. The following discussion will assume that the cable interface is a Universal Serial Bus (USB) interface but it will be appreciated that any interface that combines power delivery with data signaling may be protected as discussed herein. With regard to such data interfaces, it is conventional for a switching power converter such as a flyback converter to include a second-side controller that is configured to monitor the data pins in the data interface cable to determine whether a device such as a mobile device is coupled to the data interface cable. The fault monitor disclosed herein exploits this no- device-attached detection capability by comparing the power-switch cycling frequency to a threshold frequency while a no-load condition is detected. Should the fault monitor determine that the power-switch cycling frequency exceeds the threshold frequency during a no-load condition, a soft-circuit fault is determined. The following discussion will be directed to a flyback converter embodiment but it will be appreciated that the fault monitor may be widely applied to other types of switching power converters such as buck or boost converters.

[0019] An example flyback power converter 100 is shown in Figure 1. A bridge rectifier 105 rectifies an AC input voltage from an AC main 1 10 and outputs the resulting rectified input voltage into an input capacitor CI. This rectified input voltage drives a magnetizing current into a primary winding 1 15 of a transformer 120 when a power switch such as an NMOS power switch transistor Ml is driven on by a primary- side controller Ul . The primary-side controller Ul modulates the cycling of the power switch Ml to regulate an output voltage Vout produced at a secondary winding 125 of transformer 120. Since the primary-side controller Ul is isolated from a USB cable having a pair of a D+ and a D- data pins or terminals 135, a secondary-side controller U2 interfaces with the device being charged (not illustrated) through data pins 135. In flyback power converter 100, secondary-side controller U2 controls the cycling of a synchronous rectifier (SR) switch transistor such as an NMOS transistor. The secondary-side controller U2 switches on the SR switch transistor in response to primary-side controller Ul switching off the power switch Ml . The resulting synchronous rectification is conventional and improves efficiency over the use of a diode on the secondary side to perform analogous synchronous rectification. It will be appreciated, however, that the advantageous soft-circuit detection techniques and systems disclosed herein may be practiced without synchronous rectification in alternative embodiments as discussed further below.

[0020] An auxiliary winding 130 for transformer 120 couples to ground through a voltage divider formed by a serial pair of resistors Rl and R2 to produce a sense voltage VSENSE that is received by primary-side controller Ul . For example, primary- side controller Ul may sample VSENSE at the transformer reset time to sense the output voltage. To modulate the output voltage in response to this sensing, primary-side controller Ul may adjust the frequency or pulse width for the cycling of power switch. For example, primary-side controller Ul may monitor the magnetizing current magnitude (CS) through a voltage divider formed by a resistor Rl and a cable drop compensation resistor (RCDC) coupled to the source of power switch transistor Ml . When the current magnitude CS reaches a desired level for a given power switching cycle, primary-side controller Ul may proceed to switch off power switch transistor Ml .

[0021] Secondary-side controller U2 is configured to monitor the voltage on the D+ terminal in USB interface 135 to determine if a load such as a mobile device is attached to another end of the USB cable (not illustrated). In response to this detection, secondary-side controller U2 may enumerate the attached device to, for example, determine if the device supports a rapid-charge mode of operation in which the output voltage may be increased from a nominal level such as 5 V to a higher level such as 12V or 19V. Secondary-side controller U2 may then signal the enumeration data to primary-side controller Ul by grounding an optocoupler 140 coupled to an anode of a load capacitor CL. Load capacitor CL couples between the output voltage node and ground (RTN) to smooth the output voltage. Primary controller Ul detects the voltage change across optocoupler 140 as a detect voltage (DET) to decode the enumeration data.

[0022] To control the cycling of the SR FET, secondary-side controller U2 monitors its drain voltage (DRAIN). While the power switch Ml is conducting, the drain voltage for the SR FET will be grounded or near zero but will then swing high when the power switch Ml is cycled off. As known in the synchronous rectification arts, secondary-side controller U2 responds to this voltage change by driving SR FET on through an OUT terminal. Due to the relatively-low on resistance of the SR FET, the resulting synchronous rectification saves power as compared to the use of a secondary- side diode.

[0023] Should a soft-short circuit exist between the Vout terminal and ground, energy is wasted and the danger of system unreliability increased. To guard against this potentially unsafe condition, secondary-side controller U2 is configured to compare then power-switch cycling frequency to a threshold level during a device-detached condition as detected through the grounding of the D+ terminal. The secondary-side controller U2 thus uses the power-switch cycling frequency as a proxy for the output power of flyback converter 100. Should the power-switch cycling frequency exceed the threshold level with no device attached to the USB interface, secondary-side controller U2 detects a soft-short condition. [0024] The soft-short circuit detection techniques and systems disclosed herein include embodiments in which a diode replaces the SR switch transistor. An example flyback converter 200 is shown in Figure 2 that includes a rectifying diode Dl on the secondary-side. Primary-side controller Ul , power-switch Ml, transformer 120, load capacitor CL, and USB interface 135 operate as discussed with regard to flyback converter 100. The optocoupling between a secondary-side controller 205 and primary- side controller Ul is represented by a comm signal. A user has detached a mobile device from a USB cable 210 but left USB cable 210 still attached to USB interface 135. A soft-short circuit 215 causes a non-zero output current T_OUT to flow from the V_OUT terminal to the ground (GND) terminal in USB interface 135. Like secondary- side controller U2, secondary-side controller 205 is configured to compare the power- switch cycling frequency to a threshold level during a no-device-attached state as detected through the absence of voltage on the D+ terminal. For example, secondary- side controller 205 may monitor the anode voltage for rectifying diode D l to sense the power-switch cycling frequency.

[0025] To perform the threshold comparison discussed herein, a secondary-side controller such as U2 or 205 may include a power switch cycling frequency detector and comparator 300 as shown in Figure 3. A processor 305 responds to a short-circuit frequency detection signal from power switch cycling frequency detector and comparator 300 when processor 305 determines that there is no device coupled to the USB interface as discussed above. For example, processor 305 may trigger a reset of the primary-side controller through the optocoupling signal. In addition, processor 305 may trigger an optional display so that a user is notified of the soft-short circuit condition in the corresponding USB cable. [0026] Power switch cycling frequency detector and comparator 300 may be implemented as shown in Figure 4A. The drain voltage for the SR FET of Figure 1 and for the anode voltage of the rectifying diode will swing between ground when the power switch is conducting and a relatively high voltage such as 60 to 100 V when the power switch is opened. This drain/anode voltage will thus have both a rising edge and a falling edge after each cycle of the power switch. A voltage divider formed such as formed by resistors R4 and R5 as shown in may thus reduce the drain voltage of the SR FET (or the anode voltage of the rectifying diode) so that the reduced voltage may be compared against a suitable reference voltage (Vref) from a reference voltage source 410 at a comparator 400 to form a binary comparator output signal 405. A counter 420 may then count the number of binary transitions for comparator output signal 405 within a given period of time to count the rising and falling edges for the drain/anode voltage. This count is directly proportional to the power switch cycling frequency. A digital comparator 415 compares a resulting count from counter 420 to a threshold to form a soft-short circuit detection signal that is asserted when the count exceeds the threshold.

[0027] In an alternative embodiment detector/comparator 300, an RC filter such as the low-pass filter formed from a resistor R6 and a capacitor C2 converts the SR FET drain voltage (or anode voltage of the rectifying diode) into an analog voltage that may be compared to a threshold at a comparator 430 as shown in Figure 4B. If the threshold exceeds the analog voltage, a soft-short circuit detection signal may be asserted. It will be appreciated that other types of RC filters such as a high-pass filter may be used in alternative embodiments.

[0028] Regardless of how the frequency threshold comparison is made, the resulting soft-short circuit detection as a function of the power switch cycling frequency is shown in Figure 5 with regard to a threshold power switch cycling frequency for a power switch SI . It is assumed with regard to Figure 5 that the secondary-side controller has detected that there is no device attached to the USB cable by sensing that the D+ terminal voltage is grounded. The secondary-side controller will thus monitor for a soft-short condition as discussed with regard to Figures 3 and 4. Since the power switch SI cycling frequency (which is the inverse of the power switch cycle period T) exceeds the threshold value, the corresponding secondary-side controller will assert the soft-short circuit detection signal. In response, the secondary-side controller may signal to the primary-side controller through, for example, an optocoupler signal to trigger a reset of the corresponding flyback converter.

[0029] A method of operation for a soft-short detection will now be discussed with regard to the flowchart of Figure 6. The method includes an act 600 of detecting whether a device is disconnected from a data cable for charging the device. The method also includes an act 605 that is responsive to the detection that the device is

disconnected and comprises comparing a power switch cycling frequency in a switching power converter coupled to the data cable to a threshold frequency to detect whether a soft-short circuit exists in the data cable.

[0030] As those of some skill in this art will by now appreciate and depending on the particular application at hand, many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of use of the devices of the present disclosure without departing from the scope thereof. In light of this, the scope of the present disclosure should not be limited to that of the particular embodiments illustrated and described herein, as they are merely by way of some examples thereof, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents.

Claims

CLAIMS We claim:

1. A switching power converter controller, comprising:

a power switch cycling frequency detector configured to determine whether a power switch cycling frequency has exceeded a threshold value and to assert a soft- short circuit detection signal responsive to the determination; and

a processor configured to trigger a reset of a switching power converter responsive to the assertion of the soft-short circuit detection signal.

2. The switching power converter controller of claim 1, wherein the power switch cycling frequency detector includes:

a voltage divider configured to divide an input voltage into a divided voltage; a first comparator configured to compare the divided voltage to a threshold voltage; and

a counter configured to count binary transitions of an output signal from the first comparator.

3. The switching power converter controller of claim 2, wherein the power switch cycling frequency detector further includes a digital comparator configured to assert the soft-short detection signal in response to a count from the counter to a threshold value exceeding a threshold value.

4. The switching power converter controller of claim 2, wherein the power switch cycling frequency detector further includes a band gap reference configured to provide the threshold voltage.

5. The switching power converter controller of claim I , wherein the processor is a secondary-side controller processor configured to trigger the reset of the switching power converter through a cycling of an optocoupler signal.

6. The switching power converter controller of claim 5, wherein the secondary-side controller processor is configured to monitor a data terminal voltage to determine whether a mobile device is disconnected from a data interface, and wherein the secondary-side controller processor is further configured to respond to the assertion of the soft-short circuit detection signal only when mobile device is disconnected from the data interface.

7. The switching power converter controller of claim 1 , wherein the power switch cycling frequency detector includes:

an C filter configured to filter an input voltage into a filtered voltage; and a comparator configured to compare the filtered voltage to a threshold value to assert the soft-short detection signal.

8. The switching power converter controller of claim 7, wherein the RC filter is a low-pass filter.

9. The switching power converter controller of claim 7, wherein the RC filter is a high-pass filter.

10. The switching power converter controller of claim 6, wherein the data interface is a Universal Serial Bus (USB) interface.

1 1. A soft-short circuit detection method, comprising:

detecting whether a device is disconnected from a data cable for charging the device; and

responsive to the detection that the device is disconnected, comparing a power switch cycling frequency in a switching power converter coupled to the data cable to a threshold frequency to detect whether a soft-short circuit exists in the data cable.

12. The soft-short circuit detection method of claim 11, further comprising resetting a switching power converter responsive to the detection of the soft-short circuit.

13. The soft-short circuit detection method of claim 11, further comprising alerting a user that the soft-short circuit exists.

14. The soft-short circuit detection method of claim 11 , wherein comparing the power switch cycling frequency comprises filtering a drain voltage of a synchronous rectification transistor switch through an RC filter to produce a filtered voltage and comparing the filtered voltage to a threshold value.

15. The soft-short circuit detection method of claim 14, wherein filtering the drain voltage through an RC filter comprises filtering the drain voltage through a low-pass RC filter.

16. The soft-short circuit detection method of claim 1 1, wherein comparing the power switch cycling frequency comprises:

comparing an anode voltage of a rectifying diode to a reference voltage to produce a comparator output signal;

counting binary transition in the comparator output signal to produce a count; and

comparing the count to a threshold value.

17. The soft-short circuit detection method of claim 11, wherein detecting whether thedevice is disconnected from the data cable for charging the device comprises determining whether a D+ terminal of a Universal Serial Bus (USB) cable is grounded.

18. A switching power converter comprising:

a transformer including a primary winding and a secondary winding;

a primary-side controller configured to control a cycling of power switch coupled to the primary winding to regulate an output voltage from the secondaiy winding; and

a secondary-side controller configured to determine whether a frequency for the cycling of the power switch has exceeded a threshold value and to assert a soft-short circuit detection signal responsive to the determination

19. The switching power converter of claim 18, further comprising an optocoupler, wherein the secondary-side controller is configured to ground a terminal of the optocoupler to force a reset of the primary-side controller responsive to the determination that the frequency for the cycling of the power switch has exceeded the threshold value.

20. The switching power converter of claim 19, wherein the secondary-side controller is further configured to monitor a voltage of a data terminal to determine whether a device to be charged is disconnected from a data cable.