Power adapter with ultra-low standby power

Abstract

The invention is entitled "Power adapter with ultra-low standby power." A standby power system for a flyback converter is disclosed. The flyback converter includes a primary side, a secondary side, an output terminal located at the secondary side, and a secondary side controller, wherein the output terminal is configured to be electrically connected to a load. The standby power system includes a comparator located at the secondary side, an optocoupler in signal communication with the primary side, the secondary side, and the comparator, and a cable disconnect detector (or load detector). The cable disconnect detector is configured to determine whether a device is electrically connected to the flyback converter through a charging cable and to set the flyback converter to a standby mode if the device is disconnected from the charging cable.

Description

Technical Field

[0001] This application relates to switching power supply converters, and more specifically, to systems for reducing the power consumption of switching power supply converters in standby mode. Background Technology

[0002] A flyback converter, such as one used to charge the battery of a mobile device, includes a feedback loop for regulating the output voltage in both normal operating mode and standby operating mode. In both operating modes, the flyback converter is connected to an AC power line (i.e., the AC trunk). In normal operating mode, the mobile device is connected to the flyback converter. However, in standby operating mode, the flyback converter is disconnected from the mobile device (e.g., a smartphone). To regulate the output voltage in both operating modes, the flyback converter includes a feedback loop with an error amplifier that generates an error voltage based on the difference between the output voltage and a desired value. A loop filter filters the error voltage to generate a control voltage. Because the feedback loop must be active during standby mode to regulate the output voltage, the flyback converter still consumes power during standby mode; this power is often referred to as standby power or standby loss.

[0003] To reduce standby losses, existing techniques typically focus on reducing integrated circuit (IC) current. Known methods for reducing IC quiescent current include shutting down various functional blocks of the circuit when the flyback converter detects a no-load condition. However, the feedback loop must remain on, which consumes most of the power during standby operation. Furthermore, in embodiments where a control voltage is generated on the secondary side of the flyback converter and transmitted across an optocoupler, the optocoupler subsequently conducts continuous current, which also increases losses.

[0004] Therefore, there is a need in the art for flyback converters that have reduced power consumption during standby mode. Summary of the Invention

[0005] To provide reduced standby mode power consumption, a standby power system is provided that can be implemented on either side (or both sides) of the flyback converter transformer. This standby power system uses a comparator to control the switching of power switching transistors after a cable disconnection detector triggers a standby operation mode. The cable disconnection detector is configured to determine whether a device (e.g., a mobile device or a wireless charger for a mobile device) is electrically connected to the flyback converter via a charging interface such as a USB cable or a Lightning cable, for example, by monitoring the data path in that interface. If the cable disconnection detector detects a device disconnection, it sets the flyback converter to standby operation mode, in which the feedback loop used for normal operation is de-energized. For example, the cable disconnection detector triggers the de-energization of the error amplifier and loop filter in the feedback loop in response to a device disconnection. In an alternative embodiment, the cable disconnection detector may be replaced by a load detector that determines whether a load is being applied to the device. If the load detector detects that the load is absent or below a threshold load level, it sets the flyback converter to standby operation mode.

[0006] In an implementation where the comparator is a secondary-side comparator, this comparator operates during standby mode to compare the flyback converter output voltage with a threshold voltage. If the secondary-side comparator detects that the output voltage is below the threshold voltage, it drives a digital signal through a ground-isolated channel. For example, the secondary-side comparator can allow current to flow through a photodiode in an optocoupler. However, if the output voltage is above the threshold voltage, no current flows in the optocoupler. In response to the photodiode current, the primary-side controller triggers a power switching transistor to cycle and charge the output voltage. Since there is no load during standby mode, the output voltage will charge very quickly above the threshold voltage. Therefore, the duty cycle for signaling through the optocoupler is relatively small because the optocoupler does not conduct current except for the relatively short period when the output voltage is below the threshold voltage.

[0007] A primary-side comparator can be used in place of or in conjunction with a secondary-side comparator. A cable disconnection detector (or load detector) remains on the secondary side to signal the standby power system, for example, via an opto-isolator, that the device is disconnected from the charging cable. In response to this detection, the normal-mode feedback loop is de-energized. The primary-side controller is powered by a supply voltage generated by the auxiliary winding in the flyback converter transformer. To maintain the output voltage during regulation, the primary-side comparator compares this supply voltage to a threshold voltage. Thus, the supply voltage acts as a proxy for the output voltage relative to the regulation of the primary-side comparator. If the supply voltage drops below the threshold voltage, the primary-side comparator causes the power switching transistor to begin cycling until the desired peak primary current is reached. Consequently, the resulting standby mode advantageously consumes very low power because the primary-side comparator does not trigger any optocoupler signaling.

[0008] These and other aspects of the invention will be more fully understood by reading the following detailed description of specific exemplary embodiments. Other aspects, features, and embodiments will become apparent to those skilled in the art when reading the following description of specific exemplary embodiments in conjunction with the accompanying drawings. Although features may be discussed with respect to certain embodiments and the following drawings, all embodiments may include one or more of the advantageous features discussed herein. In other words, although one or more embodiments may be discussed as having certain advantageous features, one or more of such features may also be used according to the various embodiments discussed herein. Similarly, although exemplary embodiments may be discussed below as embodiments of devices, systems, or methods, it should be understood that such exemplary embodiments may be implemented in various devices, systems, and methods. Attached Figure Description

[0009] The invention can be better understood by referring to the following accompanying drawings. The embodiments and advantages of this disclosure are best understood by referring to the following detailed description. The components in the drawings are not necessarily drawn to scale, but rather the emphasis is on illustrating the principles of the invention. Throughout the drawings, similar reference numerals denote corresponding parts.

[0010] Figure 1A A system block diagram of a standby power system for a flyback converter according to one aspect of the present disclosure is shown, the standby power system utilizing a comparator on the secondary side of the flyback converter.

[0011] Figure 1B A normal mode portion of the feedback loop within a secondary-side controller according to one aspect of this disclosure is shown.

[0012] Figure 2 Some exemplary waveforms of a flyback converter during standby mode are shown according to one aspect of this disclosure.

[0013] Figure 3 An example of one aspect of this disclosure is shown. Figure 1A The flowchart shown illustrates the method executed by the standby power system.

[0014] Figure 4 A system block diagram of a flyback converter utilizing a primary-side comparator is shown according to one aspect of this disclosure. Detailed Implementation

[0015] To provide reduced standby mode power consumption, a standby power system including a secondary-side comparator and / or a primary-side comparator is provided. A cable disconnection detector is used to determine whether a device (e.g., a mobile device such as a smartphone, or a wireless charger for a mobile device) is electrically connected to the flyback converter through the charging interface, for example, by monitoring the data path in the charging interface leading to the charging cable (such as a USB cable or a Lightning cable). In an alternative embodiment, the cable disconnection detector may be replaced by a load detector that detects whether the device is drawing a load. If the cable disconnection detector detects that the device is disconnected, the cable disconnection detector sets the flyback converter to a standby operating mode, in which the feedback loop used for normal operating mode is powered down. For example, the cable disconnection detector triggers the power-down of the error amplifier and loop filter in the feedback loop in response to the device disconnection. As used herein, the term "device" refers to a mobile device or a wireless charger for a mobile device. The load detector is similarly used to set the flyback converter to standby mode in response to detecting that there is no load below a threshold load value.

[0016] Standby power systems with secondary-side comparators will be discussed first, followed by standby power systems with primary-side comparators. However, it should be understood that a standby power system may include both primary-side and secondary-side comparators. The comparator consumes relatively less power compared to the feedback loop used during normal operation, making it easier to achieve “zero standby power” (5mW or less standby power consumption). In contrast, conventional standby mode power consumption does not provide zero standby power because the standby power consumption of the feedback loop and associated circuitry exceeds 5mW.

[0017] During normal operation, the feedback loop uses, for example, an error amplifier and a loop filter to generate the control voltage. If the control voltage is generated on the secondary side of the flyback converter, the flyback converter can be considered to use secondary-side regulation. In normal operation, the secondary-side controller regulates the output voltage from the output terminal by sending an analog signal representing the control voltage through an optocoupler to the primary side of the flyback converter. The primary-side controller receives this analog signal and uses it as an input to control the cycling of the power switching transistors. Note that this control signal communication has a 100% duty cycle. This is problematic for the conventional standby operation mode where the feedback loop is used to regulate the output voltage, as the optocoupler continuously burns power.

[0018] However, in the standby operation mode using secondary-side regulation disclosed herein, the secondary-side comparator monitors the output voltage to send a digital (on / off) signal via an optocoupler. This digital signal is generated by the secondary-side comparator by comparing the output voltage with a threshold voltage. When the output voltage drops below the threshold voltage, the secondary-side comparator turns on the current through the photodiode in the optocoupler (the "on" portion of the digital signal). However, when the output voltage is greater than the threshold voltage, the secondary-side comparator turns off the photodiode current (the "off" portion of the digital signal). Generally, the threshold voltage is preset or limited such that the supply voltage used for the secondary-side controller is greater than an undervoltage lockout (UVLO) value. The undervoltage lockout value is the voltage necessary to keep the integrated circuit (IC) functioning in response to reconnection of the device to the flyback converter's charging interface. This allows the flyback converter to exit standby mode and resume normal operation almost immediately.

[0019] The "on" digital signal transmitted via the optocoupler is received by the primary side of the standby power system, allowing the primary side to cycle the power switching transistor. For example, the primary side can turn on the power switching transistor until the primary current, as measured by the voltage across the sensing resistor, exceeds the voltage corresponding to the desired peak primary current. When the primary side senses that the peak primary current has been reached, it turns off the power switching transistor. The resulting power transmitted to the secondary side of the flyback converter causes the output voltage to increase rapidly because there is no load during standby operation. The secondary-side comparator then responds to the output voltage exceeding a threshold voltage by turning off the photodiode current. Therefore, the "on" portion of the digital signal transmitted by the optocoupler has a relatively small duty cycle. For example, in one embodiment, the "on" portion of the digital signal can be 200 microseconds to 400 microseconds, while the "off" portion is approximately 0.5 seconds. This very low duty cycle results in the optocoupler consuming relatively little power. In other embodiments, the optocoupler can be replaced by other types of ground-isolated channels, such as signal converters or capacitors. However the ground isolation channel is implemented, the secondary-side comparator will advantageously drive it with a relatively low duty cycle digital on / off signal, thus saving standby power. The following discussion will concern implementations where the ground isolation channel is an optocoupler without sacrificing generality. The charging cable can be a Universal Serial Bus (USB) power adapter or a Lightning cable.

[0020] Figure 1A An exemplary flyback converter 102 according to this disclosure is shown, which uses secondary-side regulation and has a standby power system 100 including a secondary-side comparator 104. The flyback converter 102 includes a transformer T having a primary-side winding 110 and a secondary-side winding 112. The primary winding 110 is connected in series with a primary-side power switching transistor SW. During normal operating mode, a primary-side controller 114 drives the gate of the power switching transistor SW to control the cycling of the power switching transistor SW in response to a control current transmitted by a secondary-side controller 116 via an optocoupler 122 (e.g., driven through the optocoupler (OPTO) terminal). The secondary-side controller 116 may also act as a synchronous rectifier (SR) controller to control the cycling of the SR switching transistor. In an alternative embodiment, the SR switching transistor may be replaced by an output diode to rectify the secondary-side current.

[0021] At input terminal 118, a rectified input voltage V, such as that generated by diode bridge rectification of the AC trunk, is received. IN And the rectified input voltage is rectified by the input capacitor C. INThe filter drives the primary winding current through the power switching transistor SW when SW is turned on. When the power switching transistor SW is on, the SR switching transistor remains off. This is achieved by a sensing resistor R coupled between the source and ground of the power switching transistor SW. S When the Vipk voltage across the terminals detects that the primary winding current has reached the required peak, the power switching transistor SW stops cycling and the SR switching transistor starts cycling, causing the secondary winding current to flow as indicated by the output capacitor C. OUT Output voltage (V) is generated at the supported output terminal 120. OUT ).

[0022] Cable disconnection detector 123 monitors data channels (e.g., CC1 / CC2 pins in USB interface 124) to determine whether device 144 is electrically connected to a USB cable (or other suitable charging interface), and if device 144 is disconnected, sets flyback converter 102 to standby mode. In this example, cable disconnection detector 123 may be implemented as part of secondary-side controller 116, which includes input / output (I / O) terminals for connecting to data channels in the charging interface to charge device 144.

[0023] During standby mode, the secondary-side comparator 104 receives the output voltage V via a voltage divider, for example. OUT The voltage divider includes a first resistor R1, a second resistor R2, and a ground portion 146. The output terminal of the secondary-side comparator 104 drives the gate of the transistor 150, which is connected in series with the photodiode 152 in the optocoupler 122. The secondary-side comparator 104 reduces the output voltage V as proportionally reduced by the voltage divider. OUT With threshold voltage (V) LOW ) for comparison. If V OUT The scaled-down version is greater than V. LOW If V is turned off, the secondary comparator 104 shuts down transistor 150, preventing the optocoupler 122 from consuming power. However, if V OUT The scaled-down version is less than V LOW Then, the secondary-side comparator 104 turns on transistor 150 to allow the photodiode current to conduct. The following discussion will assume that the secondary-side comparator 104 directly compares the output voltage with the threshold voltage; however, it should be understood that this comparison can be indirect, such as by comparing a scaled-down version of the output voltage. Since photodiode 152 is coupled between output terminal 120 and transistor 150, when transistor 150 is turned on, the output voltage drives photodiode current through photodiode 152.

[0024] Therefore, optocoupler 122 can be considered to transmit a digital on / off signal, which is "on" when the secondary-side comparator 104 activates its output signal and "off" when the secondary-side comparator 104 grounds its output signal. In response to this digital signal transmission, standby power system 100 includes a primary-side portion that may include a second comparator 156 that communicates with the photodetector 158 of optocoupler 122. In this example, photodetector 158 is a bipolar junction transistor (BJT) having a voltage V electrically connected to the primary-side power supply voltage V via resistor R3. DD The collector of the photodetector 158 and the emitter electrically connected to the ground. The second comparator 156 is configured to compare the voltage at the emitter of the photodetector 158 with a predetermined turn-on voltage (V). ON The system compares the current with the current and, in response, generates an output signal (also known as an enable signal EN) to control the cycle of the power switching transistor SW. More generally, the primary side portion of the standby power system 100 is used to detect whether current flows through the photodetector 158.

[0025] In this example, the second comparator 156 communicates with a sequential logic circuit or storage element such as, for example, a set-reset flip-flop (also known as an SR flip-flop or SR latch) 170, wherein the output signal of the second comparator 156 drives the set terminal of the SR flip-flop 170. The second comparator 156 may also communicate with the SR latch via an oscillator 172, wherein the oscillator 172 is configured to generate a pulse signal 174 in response to the activation of the output signal from the second comparator 156. The pulse signal 174 drives the set input of the SR flip-flop 170 to set the flip-flop 170.

[0026] The Q output 180 of the SR flip-flop 170 drives the gate of the power switching transistor SW. Therefore, when the SR flip-flop 170 is set, the power switching transistor SW is turned on to conduct. To reset the flip-flop 170 (which can also be represented as an SR latch), the primary side of the standby power system 100 may also include a third comparator 176 that communicates with the reset input (R) of the SR flip-flop 170. The third comparator 176 is configured to sense the resistor R... S The voltage across the sensing resistor is compared to a predetermined peak current voltage reference value (V). ipk The system compares the voltage across the sense resistor and generates a reset signal 178 that drives the reset input R of the SR flip-flop 170. Therefore, when the voltage across the sense resistor is equal to Vipk, the power switching transistor SW is turned off by resetting the flip-flop 170.

[0027] Those skilled in the art will understand that the term "signal communication" refers to any type of communication and / or connection between circuits, components, modules, and / or devices that allows the circuit, component, module, and / or device to transmit and / or receive signals and / or information from another circuit, component, module, and / or device. This communication and / or connection can be any signal path that allows signals and / or information to be transmitted from one circuit, component, module, and / or device to another and includes wireless or wired signal paths. Signal paths can be physical, such as, for example, wires, electromagnetic waveguides, cables, terminals with attachments and / or electromagnetic or mechanical coupling, semiconducting or dielectric materials or devices, or other similar physical connections or couplings. Additionally, signal paths can be non-physical, such as free space (in terms of electromagnetic propagation) or information paths via digital components, where communication information is transmitted from one circuit, component, module, and / or device to another in a different digital format without a direct electromagnetic connection.

[0028] Figure 1B A portion of the normal-mode feedback loop in the secondary-side controller 116 is shown in more detail. Error amplifier 200 compares its output voltage with a reference voltage Vref to form an error signal. The output of secondary-side comparator 104 is a digital signal, while the error signal is an analog signal. Loop filter 205 filters the error signal into a control voltage that drives optocoupler 122. Note that this control voltage has a 100% duty cycle, causing its transmission during normal standby operation to result in constant power consumption by optocoupler 122. However, during the standby operation mode disclosed herein, both error amplifier 200 and loop filter 205 are powered down. Secondary-side comparator 104 then controls the feedback via optocoupler 122. Not only is the power consumption of secondary-side comparator 104 relatively low compared to the normal-mode feedback loop, but the near-zero duty cycle of the "on" portion of the digital control signal transmitted through optocoupler 122 also significantly reduces optocoupler power consumption. During normal operation, the primary-side controller 114 processes the control signals received from the optocoupler 122, such as through a proportional-integral-derivative (PID) control algorithm, to control the cycle of the power switching transistor SW accordingly.

[0029] Some operating waveforms used in flyback converter 102 are Figure 2The diagram shows graphs of the primary-side supply voltage VCC, output voltage, and gate voltage of the power switch SW during standby mode. As will be discussed further herein, the primary-side supply voltage VCC is generated at the auxiliary winding and filtered by a VCC capacitor, which may have a significantly smaller capacitance than the output capacitor (e.g., reduced to 1 / 100th of its capacitance). The ripple in the primary-side supply voltage VCC can therefore be relatively noticeable compared to the ripple in the output voltage. Since there is no load, the power switch transistor SW in this example cycles approximately every 0.5 seconds. The output voltage then recovers to the desired value (approximately 5.5V in this example). The output voltage then gradually decreases until it triggers another pulse from the power switch transistor SW. However, the supply voltage VCC may drop by several volts from one pulse to another during this approximately 0.5-second dead time. The low duty cycle of the digital control signal transmitted through optocoupler 122 is confirmed by the gate voltage of the power switch transistor SW. The duty cycle in this example is approximately 300 to 400 microseconds divided by 0.5 seconds, which is quite low, thus correspondingly reducing the power consumption of the optocoupler.

[0030] Now refer to Figure 3 The flowchart shown discusses an exemplary method of operation for the flyback converter 102. Method 300 begins by determining 302 whether device 144 is electrically connected to the charging port of the flyback converter 102. If device 144 is electrically connected, then refer to Figure 1B The feedback loop under discussion operates in normal operating mode. In normal operating mode, the primary-side controller 114 and the secondary-side controller 116 are fully powered. Conversely, if device 144 is not connected to the charging interface, the method sets flyback converter 102 to standby operating mode 306, in which the normal mode feedback loop is powered down. During standby mode, the method determines the output voltage V in step 308. OUT Is it greater than or equal to the threshold voltage V? LOW And in response, the current of the 310 photodiode remains off. If V OUT Less than V LOW This method then switches on the current of photodiode 312, and a digital control signal is transmitted to the primary side of flyback converter 102, causing power switching transistor SW to cycle in step 314. The method then returns to 308, monitoring V from output terminal 120. OUT .

[0031] See you again Figure 2 Ripple in the primary-side supply voltage VCC can potentially trigger undervoltage lockout. For a flyback converter 402 with a standby power system 400, Figure 4The diagram illustrates the standby operation mode of a primary-side comparator 403 that monitors the primary-side power supply voltage VCC. The function of the primary-side comparator 403 is similar to that discussed with reference to comparator 156, except that comparator 403 compares the primary-side power supply voltage VCC with a threshold voltage (VLow). This comparison can be performed indirectly, for example, by using a reduced version of the power supply voltage, such as a voltage divider formed by a first resistor R1 and a second resistor R2. The remainder of the primary-side portion of the standby power system 400 is as discussed with reference to standby power system 100. The primary controller 114 and the primary-side portion of the standby power system 400 can be considered to form the primary-side system of the flyback converter 402. Comparators 404 and 156 can be considered to compare a feedback signal with a threshold voltage. For comparator 403, the feedback signal is the primary-side power supply voltage VCC. For comparator 156, the feedback signal is a voltage generated by the received optocoupler signal.

[0032] Those skilled in the art will now recognize that various modifications, substitutions, and variations can be made to the materials, apparatus, configuration, and methods of use of the device of the present invention without departing from the scope of this disclosure. Therefore, the scope of this disclosure should not be limited to the specific embodiments shown and described herein, as they are merely examples, but should be fully commensurate with the scope of the appended claims and their functional equivalents.

Claims

1. A secondary side controller for a flyback converter, the secondary side controller comprising: a feedback loop configured to operate during a normal operating mode to generate a control signal based on an error between an output voltage of the flyback converter and a reference voltage; a cable disconnect detector configured to control the feedback loop during the normal operating mode in response to detecting a device coupled to the flyback converter, and to power down the feedback loop during a standby operating mode in response to detecting the device disconnected from the flyback converter; and a comparator configured to turn on a photodiode current through a photodiode in an optocoupler during the standby operating mode in response to the output voltage being below a threshold voltage, and to turn off the photodiode current in response to the output voltage being greater than the threshold voltage, wherein a primary side controller is configured to be powered down during the standby operating mode, and wherein the power switch transistor is cycled in response to conduction of the photodiode current.

2. The secondary side controller of claim 1, further comprising: a transistor in series with the photodiode, wherein the comparator is configured to turn on the transistor to turn on the photodiode current in response to the output voltage being below a threshold voltage.

3. The secondary side controller of claim 1, further comprising: a data lane; wherein the cable disconnect detector is configured to monitor the data lane to detect whether the device is disconnected from the flyback converter.

4. The secondary side controller of claim 3, wherein the data lane is a data lane to a USB cable.

5. The secondary side controller of claim 1, wherein the secondary side controller comprises a synchronous rectifier controller.

6. The secondary side controller of claim 1, wherein the feedback loop comprises a loop filter configured to drive the optocoupler with the control signal during the normal operating mode.

7. A primary side system for a flyback converter, the primary side system comprising: a controller configured to process a control signal to control cycling of a power switch transistor during a normal operating mode, and configured to be powered down during a standby operating mode; and a standby mode system comprising: a storage element configured to start cycling of the power switch transistor in response to being set, and to stop cycling of the power switch transistor in response to being reset; a first comparator configured to set the storage element in response to a feedback signal being less than a threshold voltage; and a second comparator configured to reset the storage element after the storage element has been set, wherein the feedback signal is a primary side supply voltage or a voltage generated from a received optocoupler signal. ​ 8. The primary side system of claim 7, wherein the first comparator is configured to set the storage element in response to an optocoupler current being less than a threshold current.

9. The primary side system of claim 7, wherein the first comparator is configured to set the storage element in response to a primary side supply voltage being less than the threshold voltage.

10. The primary side system of claim 7, further comprising: an oscillator, wherein the first comparator is configured to set the storage element by turning on the oscillator.

11. The primary side system of claim 10, wherein the storage element is a flip-flop.

12. The primary side system of claim 7, wherein the controller is further configured to generate the control signal using primary-side feedback only.

13. The primary side system of claim 7, wherein the controller is further configured to receive the control signal from an optocoupler.

14. A method for powering down portions of a flyback converter having an optocoupler in a standby mode, the method comprising: setting the flyback converter to a standby mode in response to a device being disconnected from a charging interface to the flyback converter; conducting a photodiode current in an optocoupler in response to an output voltage of the flyback converter being less than a first threshold voltage during the standby mode; and when the output voltage is greater than the first threshold voltage during the standby mode, turning off the photodiode current; circulating a power switch transistor in response to the conducting of the photodiode current, wherein the circulation of the power switch transistor comprises: turning on an oscillator to generate a pulse signal at a set input of a flip-flop to set the flip-flop; turning on the power switch transistor in response to the setting of the flip-flop; resetting the flip-flop in response to a sense resistor voltage exceeding a peak current voltage; and turning off the power switch transistor in response to the resetting of the flip-flop.

15. The method of claim 14, further comprising: detecting the device being disconnected from the charging interface by monitoring a data lane in the charging interface.

16. The method of claim 14, further comprising: comparing a supply voltage to a second threshold voltage during the standby mode; and circulating a power switch transistor in response to the supply voltage being less than the second threshold voltage during the standby mode.

17. The method of claim 16, wherein the circulation of the power switch transistor in response to the supply voltage being less than the second threshold voltage during the standby mode comprises setting a flip-flop to turn on the power switch transistor.

18. The method of claim 17, wherein the setting of the flip-flop is in response to a pulse signal from an oscillator. ​ ​

Images