Standby power supply circuit, control method thereof and power supply system
By comparing the inductor current sampling signal and the reference signal, and combining the maximum conduction time, the undervoltage of the supercapacitor is determined, which solves the problems of limited undervoltage detection range and energy consumption in the existing technology, and realizes efficient energy management.
Patent Information
- Application Number
- CN202510228848.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-27
- Publication Date
- 2026-02-17
AI Technical Summary
In existing backup power circuits, undervoltage detection is difficult to set to a low detection range and is affected by process fluctuations, resulting in additional energy consumption.
By comparing the sampled signal and reference signal of the inductor current, and combining them with the preset maximum conduction time, it is determined whether the supercapacitor voltage is undervoltage, and the switching converter is turned off, thus avoiding direct detection of the supercapacitor voltage.
It enables timely shutdown of the switching converter when the supercapacitor voltage is low, avoiding additional energy consumption. The detection method is simple and unaffected by process fluctuations, maximizing the utilization of supercapacitor stored energy.
Smart Images
Figure CN121546748A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of power electronics, and in particular to a backup power supply circuit, its control method, and a power supply system. Background Technology
[0002] Many line-powered smart IoT devices require backup power to maintain communication during unexpected power outages. For example, smart meters can use backup power to send information about the time and location of a power outage after the main power circuit fails. Supercapacitors, as a new type of energy storage device, have many advantages such as rapid charging and discharging, high power density, and long cycle life, and are widely used in the field of backup power.
[0003] like Figure 1 The diagram illustrates a commonly used backup power supply circuit. When the main power supply circuit (not shown) is operating normally, the supercapacitor Csc is charged to its withstand voltage. In the event of an unexpected power failure in the main power supply circuit, the backup power supply circuit, consisting of the supercapacitor Csc and a switching converter, continuously provides the necessary power to the downstream load. To achieve a longer operating time, the supercapacitor needs to be discharged from full charge to the lowest possible voltage. When the voltage on the supercapacitor is too low for the switching converter to supply energy to the downstream load, the switching converter needs to be shut down promptly to reduce the power consumption of the supercapacitor and the energy consumption of the backup battery (not shown) (if the switching converter is operating at this time, the backup battery will supply power to it, thus consuming energy). A commonly used undervoltage detection circuit needs to receive the supercapacitor voltage to determine if the supercapacitor voltage is too low.
[0004] Figure 2 This is a commonly used undervoltage detection circuit, including a switch M1, a resistor R1, and an inverter O1. The gate of switch M1 receives the voltage Vsc from the supercapacitor, the drain receives the supply voltage Vcc through resistor R1, and the source is grounded. The input of inverter O1 receives the drain voltage V1 of switch M1. When the voltage Vsc of the supercapacitor is less than the turn-on threshold Vth, switch M1 is turned off, and the drain voltage V1 becomes the supply voltage Vcc. After passing through two stages of inverter O1, a high-level active undervoltage signal UVLO is obtained, which is used to characterize the supercapacitor voltage undervoltage. When the voltage Vsc of the supercapacitor is greater than or equal to the turn-on threshold Vth, switch M1 is turned on, and the drain voltage of switch M1 is close to ground voltage. After passing through two stages of inverters, a low-level inactive undervoltage signal UVLO is obtained, indicating that the supercapacitor is not undervoltage. Because switch M1 is limited by the turn-on threshold Vth, it is difficult to set the detection range very low, and the distribution range is also difficult to control with changes in temperature and process. Another commonly used voltage detection circuit uses a comparator for voltage detection. Although it can achieve a lower detection range, it consumes more chip area and quiescent current. Summary of the Invention
[0005] The purpose of this invention is to provide a backup power supply circuit, its control method, and a power supply system. This invention does not perform direct undervoltage detection on the supercapacitor voltage. The detection method is simple and reliable, and the detection results are not affected by process fluctuations. When the supercapacitor voltage is too low, it can avoid additional energy consumption.
[0006] The present invention also provides a backup power supply circuit, including a supercapacitor and a switching converter, wherein the supercapacitor provides an input voltage to the switching converter, and the output voltage of the switching converter supplies power to the load.
[0007] The switching converter includes an inductor, a first switching transistor, and a control circuit. The inductor receives the input voltage, and the first switching transistor is connected to the first inductor.
[0008] The control circuit includes a comparator, a control unit, and an undervoltage detection circuit.
[0009] The comparator compares the current sampling signal, which characterizes the inductor current, with a reference signal to generate a comparison signal;
[0010] The control unit controls the turn-off time of the first switch transistor based on the comparison signal and the preset maximum on-time.
[0011] The undervoltage detection circuit is used to determine whether the input voltage is undervoltage based on whether the conduction time of the first switch reaches the maximum conduction time.
[0012] Optionally, if the conduction time of the first switching transistor reaches the maximum conduction time in each of N consecutive switching cycles, the undervoltage judgment circuit determines that the input voltage is undervoltage.
[0013] N is an integer greater than or equal to 1.
[0014] Optionally, when the current sampling signal reaches the reference signal, the comparison signal changes from invalid to valid;
[0015] During the maximum conduction time, when the comparison signal is valid, the control unit controls the first switch to turn off;
[0016] If the comparison signal is invalid, the control unit controls the first switch to turn off when the on-time of the first switch reaches the maximum on-time.
[0017] Optionally, the control circuit further includes a timing module, which starts timing from zero when the first switch is turned on.
[0018] When the timing module has not reached the maximum conduction time, and the comparison signal is valid, the timing module outputs an invalid timing signal.
[0019] If the comparison signal is invalid, when the timing time reaches the maximum conduction time, the timing module outputs a valid timing signal to control the first switch to turn off.
[0020] Optionally, the undervoltage detection circuit includes a counter. When the conduction time of the first switching transistor reaches the maximum conduction time, the counter counts once. When the counter counts N times consecutively, the undervoltage detection circuit determines that the input voltage is undervoltage.
[0021] Optionally, when the undervoltage detection circuit determines that the input voltage is undervoltage, it outputs a valid undervoltage signal to control the switching converter to stop working.
[0022] Optionally, when the undervoltage signal is valid, the undervoltage signal controls the switching converter to stop working at the end of the current switching cycle or after M switching cycles, where M is an integer greater than or equal to 1.
[0023] Optionally, when the undervoltage signal is valid, the undervoltage signal is used to control the input terminal of the switching converter to be in an open circuit state with the supercapacitor, so as to control the switching converter to stop working.
[0024] Optionally, when the undervoltage signal is valid, the undervoltage signal is used to control the switching devices in the switching converter to remain off, so as to control the switching converter to stop working.
[0025] Optionally, the switching converter includes a boost converter, a buck-boost converter, or a flyback converter.
[0026] The present invention also provides a control method for a backup power circuit, including a supercapacitor and a switching converter, wherein the supercapacitor provides an input voltage to the switching converter, the output voltage of the switching converter supplies power to a load, the switching converter includes an inductor and a first switching transistor, the inductor receives the input voltage, and the first switching transistor is connected to the first inductor;
[0027] The sampled signal characterizing the inductor current is compared with a reference signal, and the turn-off time of the first switch is controlled according to the comparison result and the preset maximum conduction time.
[0028] Whether the input voltage is undervoltage is determined based on whether the conduction time of the first switch reaches the maximum conduction time.
[0029] Optionally, if the on-time of the first switching transistor reaches the maximum on-time in each of N consecutive switching cycles, the input voltage is determined to be undervoltage.
[0030] N is an integer greater than or equal to 1.
[0031] Optionally, during the maximum conduction time, when the current sampling signal reaches the reference signal, the first switch is controlled to turn off;
[0032] If the comparison signal is invalid, the first switch is turned off when the on-time of the first switch reaches the maximum on-time.
[0033] Optionally, when the input voltage is determined to be undervoltage, a valid undervoltage signal is generated to control the switching converter to stop working.
[0034] The present invention also provides a power supply system, including any of the above-described backup power supply circuit, input power supply, main power supply circuit and load, wherein the input power supply charges the supercapacitor of the backup power supply circuit and supplies power to the load through the main power supply circuit;
[0035] When the main power supply circuit loses power, the backup power supply circuit supplies power to the load. When the voltage of the supercapacitor is low, the backup power supply circuit stops working.
[0036] Compared with existing technologies, this invention has the following advantages: This invention compares a current sampling signal characterizing the inductor current with a reference signal, and controls the turn-off time of the first switch transistor based on the comparison result and a preset maximum conduction time; and determines whether the input voltage is undervoltage based on whether the conduction time of the first switch transistor reaches the maximum conduction time. This invention does not directly detect undervoltage of the supercapacitor voltage, but utilizes the principle of the switching converter itself to detect whether the supercapacitor voltage is undervoltage. When the supercapacitor is undervoltage, this invention can promptly turn off the switching converter, avoiding additional energy consumption. Attached Figure Description
[0037] Figure 1 This is a schematic diagram of a backup power supply circuit.
[0038] Figure 2 The schematic diagram is of an existing undervoltage detection circuit.
[0039] Figure 3 This is a schematic diagram of one embodiment of the backup power supply circuit of the present invention;
[0040] Figure 4 This is a schematic diagram of the control circuit of the present invention;
[0041] Figure 5 This is a waveform diagram of the backup power supply circuit of the present invention.
[0042] Figure 6 This is the circuit schematic of the power supply system. Detailed Implementation
[0043] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of the present invention.
[0044] To provide the public with a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the invention, but those skilled in the art can fully understand the invention without these details.
[0045] The invention is described more specifically in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the drawings are in a simplified form and use non-precise proportions, in order to facilitate and clearly illustrate the purpose of the embodiments of the invention.
[0046] The backup power circuit of this invention includes a supercapacitor and a switching converter. The switching converter receives the input voltage (i.e., the voltage of the supercapacitor) provided by the supercapacitor and converts the input voltage into an output voltage to supply power to the load when the main power supply circuit fails. The inductor of the switching converter receives the input voltage provided by the supercapacitor. The first switching transistor of the switching converter is connected to the inductor. After the first switching transistor is turned on, the inductor is charged. This type of switching converter can be a boost converter, a buck-boost converter, a flyback converter, etc.
[0047] like Figure 3 The diagram illustrates a schematic of one embodiment of a backup power supply circuit. Taking a boost converter as an example, the boost converter includes an inductor L, a first switch M1, a second switch M2, and a control circuit. The first terminal of inductor L receives the voltage Vsc from the supercapacitor Csc as the input voltage of the boost converter. The second terminal of inductor L is connected to the first terminals of the first and second switches M1 and M2. The second terminal of the first switch M1 is grounded, and the second terminal of the second switch M2 is connected to the output capacitor Cout. The load is connected in parallel with the output capacitor Cout, and the voltage across the load is the output voltage Vout. The resistors Res and Rdc connected in series with inductor L in the diagram represent the equivalent resistance of the supercapacitor Csc and the equivalent DC resistance of inductor L, respectively. The switching states of the first switch M1 and the second switch M2 are complementary. When the first switch M1 is on and the second switch M2 is off, inductor L charges, and the inductor current rises. The slope of the rising inductor current is Vsc - I. L*(Res+Rdc+Rds1)] / L, where Rds1 is the on-resistance of the first switching transistor M1. As the inductor current increases, the slope of the inductor current's rise will gradually decrease and eventually drop to zero. If the on-time of M1 is unrestricted, the maximum value of the inductor current can be obtained as Imax = Vsc / (Res+Rdc+Rds1).
[0048] When the supercapacitor voltage Vsc is high, the inductor current rises rapidly. The control circuit samples the inductor current, and when the inductor current reaches a preset current limit, it controls the first switch M1 to turn off. When the supercapacitor voltage Vsc is low, the inductor current is consistently / for a long time less than the preset current limit. Therefore, when the first switch M1's on-time reaches the preset maximum conduction time, the control circuit controls the first switch M1 to turn off. When the first switch M1's on-time is the preset maximum conduction time, it can be determined that the supercapacitor voltage is undervoltage, and undervoltage protection is activated, such as controlling the boost converter to stop working and disconnecting the boost converter from the supercapacitor. To avoid false triggering and misjudging the supercapacitor voltage undervoltage, the supercapacitor voltage undervoltage is only determined when the first switch's conduction time is the maximum conduction time in each of several consecutive switching cycles. This invention does not require obtaining the supercapacitor voltage; it can detect whether the supercapacitor voltage is undervoltage using the working principle of the switching converter. The detection method is simple and unaffected by process fluctuations. This invention maximizes the utilization of energy stored in supercapacitors by setting a reasonable maximum on-time. When the voltage of the supercapacitor is too low to provide energy to the load, the switching converter can be shut down in time to avoid additional energy loss.
[0049] like Figure 4 As shown, the present invention is illustrated. Figure 3 The schematic diagram of the control circuit includes comparator 01, timing module 02, control unit 03, undervoltage judgment circuit 04 and drive unit 00. Comparator 01 compares the sampling signal CS, which represents the inductor current, with the reference signal Vref to generate comparison signal Bon1. When the sampling signal CS reaches the reference signal Vref, the comparison signal Bon1 changes from low level invalid to high level valid.
[0050] The timing module 02 starts timing from zero when the first switch is turned on. When the timing time of the timing module 02 reaches the preset maximum conduction time, the timing module 02 outputs a high-level valid timing signal Bon2. The control unit 03 is used to control the turn-off time of the first switch based on the comparison signal Bon1 and the timing signal Bon2. Specifically, the control unit 03 includes an OR gate logic circuit 05 and a flip-flop 06. The OR gate logic circuit 05 receives the comparison signal Bon1 and the timing signal Bon2 and outputs a reset control signal Bon_reset. When the comparison signal Bon1 is valid and the timing signal Bon2 is invalid, the reset control signal Bon_reset is high-level valid, and controls the first switch to turn off / on through the drive unit 00. That is, when the sampling signal CS reaches the reference signal Vref, the conduction time of the first switch has not yet reached the maximum conduction time, and the high-level valid comparison signal Bon1 controls the first switch to turn off / on. When the comparison signal Bon1 is invalid and the timing signal Bon2 is valid, the reset control signal Bon_reset is active high, controlling the first switch to turn off / on / off via the drive unit 00. This means the sampling signal never reaches the reference signal. When the first switch's on-time reaches its maximum, the active high-level timing signal Bon2 controls the first switch to turn off. When both the comparison signal Bon1 and the timing signal Bon2 are valid, the reset control signal Bon_reset is active high, controlling the first switch to turn off / on / off via the drive unit 00. This means the first switch's on-time reaches its maximum when the sampling signal CS reaches the reference signal Vref. The undervoltage detection circuit 03 determines whether the supercapacitor's voltage is undervoltage based on whether the conduction time of the first switching transistor reaches its maximum conduction time. This undervoltage detection circuit can be implemented using a counter 07. When the timing signal Bon2 is valid (the conduction time of the first switching transistor reaches its maximum conduction time), the counter 07 counts once. If the counter counts N times consecutively within N consecutive switching cycles (N is a positive integer greater than zero), the supercapacitor's voltage is determined to be undervoltage, and a valid undervoltage signal UVLO is output to indicate this. The counter 07 is then reset. If N is 1, the supercapacitor's voltage is determined to be undervoltage the first time the timing signal Bon2 is valid, allowing for rapid determination of whether the supercapacitor's voltage is undervoltage. However, to avoid false triggering due to disturbances, N can be set to an integer greater than 1. For example, if N is 3, the supercapacitor's voltage is only determined to be undervoltage and a valid undervoltage signal UVLO is generated when the counter 07 counts 3 times within 3 consecutive switching cycles.When the undervoltage signal UVLO is valid, the control switch converter stops working. Optionally, the control switch converter can stop working after the end of the switching cycle in which the undervoltage signal UVLO is valid. If it stops working before the inductor current reaches zero, it can easily damage the switching transistors of the switch converter. Alternatively, when the undervoltage signal UVLO is valid, the control switch converter can stop working after M (M is an integer greater than 0) switching cycles. If the preset maximum on-time is not set large enough, even when the undervoltage signal UVLO is valid, the switch converter will not stop working immediately after the end of the current cycle. For example, when M equals 3, the switch converter will stop working after 3 switching cycles after the undervoltage signal UVLO is valid, in order to further consume the supercapacitor's charge before stopping working. There are several ways to control the switch converter to stop working, such as keeping all the switching transistors of the switch converter off, disconnecting the input of the switch converter (i.e., disconnecting the connection between the input terminal of the switch converter and the supercapacitor), or stopping the power supply to the switch converter. After the switch converter stops working, all its switching transistors will eventually turn off.
[0051] like Figure 5 The diagram illustrates the operating waveforms of the backup power supply circuit of this invention. During cycle T1, the voltage Vsc across the supercapacitor is relatively large, and the inductor current I... L The reference current Iref is quickly reached. When the inductor current reaches the reference current, the first switch is turned off, the inductor charging is cut off, and the inductor freewheels, causing the inductor current to decrease. As the charge of the supercapacitor is gradually consumed, the voltage Vsc of the supercapacitor gradually decreases, and the inductor current I... LAs the voltage Vsc of the supercapacitor gradually increases, the slope of the inductor current gradually decreases during the rising phase. In cycle T2, as the voltage Vsc of the supercapacitor gradually decreases, the time it takes for the inductor current to reach the reference current becomes longer compared to cycle T1; that is, time t2 in the diagram is longer than time t1. In cycle T3, as the voltage Vsc of the supercapacitor continues to decrease, the average rate of rise of the inductor current also decreases. The inductor current fails to reach the reference current for a long time. When the on-time of the first switch reaches its maximum on-time Ton_max, the first switch is turned off. In cycle T4, the voltage Vsc of the supercapacitor continues to decrease, and the average rate of rise of the inductor current also continues to decrease. The inductor current fails to reach the reference current. When the on-time of the first switch reaches its maximum on-time Ton_max, the first switch is turned off. During two consecutive switching cycles (T3 and T4), when the on-time of the first switching transistor is at its maximum on-time (Ton_max), a high-level valid undervoltage signal (UVLO) is generated. Typically, the switching converter is turned off after the inductor freewheeling current ends, i.e., after cycle T4. All switching transistors in the switching converter are turned off, the inductor current remains zero, and the output voltage gradually decreases to zero after the energy stored in the output capacitor is consumed by the load. Alternatively, a high-level valid undervoltage signal can be generated as soon as the on-time of the first switching transistor reaches its maximum on-time in cycle T3. However, to avoid false triggering or other reasons causing the first switching transistor to reach its maximum on-time, several more cycles of on-time detection are performed after the maximum on-time is detected. After generating the high-level valid undervoltage signal, several more switching cycles can be allowed to further consume the energy of the supercapacitor before the switching converter is turned off, maximizing the utilization of the energy stored in the supercapacitor.
[0052] like Figure 6 The diagram illustrates the schematic of the power supply system of this invention, including a supercapacitor Csc, a switching converter, an output capacitor Cout, a load, a main power supply circuit, and an input power supply. Under normal circumstances, the input power supply powers the load through the main power supply circuit, which also charges the supercapacitor until its voltage reaches its withstand voltage. The load can be electrical equipment such as an electricity meter. When the main power supply circuit experiences an unexpected power outage, the supercapacitor Csc continues to supply power to the load through the switching converter and the output capacitor Cout to ensure a longer operating time. When the energy of the supercapacitor Csc is continuously consumed and its voltage becomes low, the switching converter stops working, and the supercapacitor ceases to supply power to the load.
[0053] Although the embodiments are described and illustrated separately above, some common technologies are involved. Those skilled in the art can replace and integrate them between the embodiments. If there is any content not explicitly described in one embodiment, then another embodiment that is described can be referred to.
[0054] The embodiments described above do not constitute a limitation on the scope of protection of this technical solution. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the above embodiments should be included within the scope of protection of this technical solution.
Claims
1. A backup power supply circuit, comprising a supercapacitor and a switching converter, wherein the supercapacitor provides an input voltage to the switching converter, and the output voltage of the switching converter supplies power to a load, characterized in that: The switching converter includes an inductor, a first switching transistor, and a control circuit. The inductor receives the input voltage, and the first switching transistor is connected to the first inductor. The control circuit includes a comparator, a control unit, and an undervoltage detection circuit. The comparator compares the current sampling signal, which characterizes the inductor current, with a reference signal to generate a comparison signal; The control unit controls the turn-off time of the first switch transistor based on the comparison signal and the preset maximum on-time. The undervoltage detection circuit is used to determine whether the input voltage is undervoltage based on whether the conduction time of the first switch reaches the maximum conduction time.
2. The backup power supply circuit according to claim 1, characterized in that: If the conduction time of the first switching transistor reaches the maximum conduction time in each of N consecutive switching cycles, the undervoltage judgment circuit determines that the input voltage is undervoltage. N is an integer greater than or equal to 1.
3. The backup power supply circuit according to claim 1, characterized in that: When the current sampling signal reaches the reference signal, the comparison signal changes from invalid to valid; During the maximum conduction time, when the comparison signal is valid, the control unit controls the first switch to turn off; If the comparison signal is invalid, the control unit controls the first switch to turn off when the on-time of the first switch reaches the maximum on-time.
4. The backup power supply circuit according to claim 3, characterized in that: The control circuit also includes a timing module, which starts timing from zero when the first switching transistor is turned on. When the timing module has not reached the maximum conduction time, and the comparison signal is valid, the timing module outputs an invalid timing signal. If the comparison signal is invalid, when the timing time reaches the maximum conduction time, the timing module outputs a valid timing signal to control the first switch to turn off.
5. The backup power supply circuit according to claim 2, characterized in that: The undervoltage detection circuit includes a counter. When the conduction time of the first switch reaches the maximum conduction time, the counter counts once. When the counter counts N times consecutively, the undervoltage detection circuit determines that the input voltage is undervoltage.
6. The backup power supply circuit according to claim 2, characterized in that: When the undervoltage detection circuit determines that the input voltage is undervoltage, it outputs a valid undervoltage signal to control the switching converter to stop working.
7. The backup power supply circuit according to claim 6, characterized in that: When the undervoltage signal is valid, the undervoltage signal controls the switching converter to stop working at the end of the current switching cycle or after M switching cycles, where M is an integer greater than or equal to 1.
8. The backup power supply circuit according to claim 6, characterized in that: When the undervoltage signal is valid, the undervoltage signal is used to control the input terminal of the switching converter to be in an open circuit state with the supercapacitor, so as to control the switching converter to stop working.
9. The backup power supply circuit according to claim 6, characterized in that: When the undervoltage signal is valid, the undervoltage signal is used to control the switching devices in the switching converter to remain off, so as to control the switching converter to stop working.
10. The backup power supply circuit according to claim 1, characterized in that: The switching converter includes a boost converter, a buck-boost converter, or a flyback converter.
11. A control method for a backup power circuit, comprising a supercapacitor and a switching converter, wherein the supercapacitor provides an input voltage to the switching converter, and the output voltage of the switching converter supplies power to a load, characterized in that: The switching converter includes an inductor and a first switching transistor, the inductor receiving the input voltage, and the first switching transistor being connected to the first inductor; The sampled signal characterizing the inductor current is compared with a reference signal, and the turn-off time of the first switch is controlled according to the comparison result and the preset maximum conduction time. Whether the input voltage is undervoltage is determined based on whether the conduction time of the first switch reaches the maximum conduction time.
12. The control method for the backup power supply circuit according to claim 11, characterized in that: If the on-time of the first switching transistor reaches the maximum on-time in each of N consecutive switching cycles, then the input voltage is determined to be undervoltage. N is an integer greater than zero.
13. The control method for the backup power supply circuit according to claim 11, characterized in that: During the maximum conduction time, when the current sampling signal reaches the reference signal, the first switch is controlled to turn off; If the comparison signal is invalid, the first switch is turned off when the on-time of the first switch reaches the maximum on-time.
14. The control method for the backup power supply circuit according to claim 11, characterized in that: When the input voltage is determined to be undervoltage, a valid undervoltage signal is generated to control the switching converter to stop working.
15. A power supply system, characterized in that: The system includes a backup power supply circuit, an input power supply, a main power supply circuit, and a load as described in any one of claims 1-10, wherein the input power supply charges the supercapacitor of the backup power supply circuit and supplies power to the load through the main power supply circuit. When the main power supply circuit loses power, the backup power supply circuit supplies power to the load. When the voltage of the supercapacitor is low, the backup power supply circuit stops working.