A protection circuit of an LLC resonant converter and a display device
By designing power-down detection, shutdown control, and delayed discharge modules in the protection circuit, the problem of common damage to the upper and lower power transistors caused by rapid power-on and power-off of the LLC resonant converter was solved, thus achieving safe operation of the LLC resonant converter.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHENZHEN SKYWORTH DISPLAY TECH CO LTD
- Filing Date
- 2025-12-31
- Publication Date
- 2026-05-29
AI Technical Summary
During rapid power-on and power-off cycles, LLC resonant converters suffer damage to both upper and lower power transistors due to the failure of the resonant cavity to release energy in a timely manner, and there is no effective solution in existing technologies.
Design a protection circuit including a power-down detection module, a shutdown control module, a delayed power-on module, and a discharge module. By monitoring the reference voltage of the LLC control chip in real time, the circuit detects power failure, forces shutdown, and delays the discharge of resonant cavity energy to ensure that the resonant cavity energy is cleared upon the next restart.
This effectively prevents damage to the MOSFET under rapid power-on and power-off conditions, ensuring the safe operation of the LLC resonant converter.
Smart Images
Figure CN122118626A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of electronic circuit technology, and in particular to a protection circuit and display device for an LLC resonant converter. Background Technology
[0002] In the field of switching power supplies, LLC resonant converters are widely used due to their high efficiency and soft-switching characteristics. However, when the AC input is rapidly switched on or off due to poor contact, the energy stored in the resonant cavity cannot be released in time. If the control chip restarts according to its inherent timing, the residual energy in the resonant cavity will induce a resonant current during the conduction phase of the lower transistor and continue flowing through the body diode of the lower transistor during the dead time. If the energy is too large, the body diode cannot fully recover before the upper transistor turns on next, resulting in common conduction of the upper and lower transistors, generating high-voltage and high-current spikes, and damaging the power MOSFET. Currently, no effective solution has been proposed to address the problem of common damage to the upper and lower power transistors due to the failure to release energy in the resonant cavity during rapid switching of LLC resonant converters.
[0003] Therefore, the damage to the MOS caused by the rapid switching on and off of the LLC resonant converter has become a technical problem that needs to be solved. Summary of the Invention
[0004] This application provides a protection circuit and display device for an LLC resonant converter to solve the aforementioned technical problem of "MOS damage caused by rapid power-on and power-off of the LLC resonant converter".
[0005] According to one aspect of the embodiments of this application, this application provides a protection circuit for an LLC resonant converter, comprising: the LLC resonant converter including an LLC control chip and a half-bridge power circuit, the protection circuit including: a power-down detection module connected to the output terminal of the reference voltage of the LLC control chip, configured to output a power-down detection signal when the reference voltage is detected to be lower than a preset threshold; a power-off control module connected to the protection pin of the LLC control chip, configured to pull down the voltage of the protection pin to below a shutdown threshold in response to the power-down detection signal, so that the LLC control chip stops outputting a drive signal; a delayed power-on module coupled to the power-off control module, configured to generate a delay signal when the reference voltage is not lower than the preset threshold, so as to maintain the power-off control module's pull-down state of the protection pin during the delay period; and a discharge module connected in parallel with the resonant cavity in the half-bridge power circuit, configured to discharge the energy in the resonant cavity after the LLC control chip stops outputting the drive signal.
[0006] Optionally, the power failure detection module includes a voltage divider circuit and a reference voltage source; the voltage divider circuit, consisting of a first voltage divider resistor and a second voltage divider resistor connected in series, is configured to divide the reference voltage to obtain a voltage divider signal; the reference voltage source is configured to compare the voltage divider signal with an internal reference threshold, and enter a cutoff state when the voltage divider signal is lower than the internal reference threshold, so as to output a power failure detection signal.
[0007] Optionally, the shutdown control module includes a first switching transistor and a second switching transistor; the control terminal of the first switching transistor is turned on in response to a power failure detection signal to pull down the potential of the target node; the control terminal of the second switching transistor is connected to the target node and is configured to turn on when the potential of the target node is pulled down, thereby pulling down the voltage of the protection pin.
[0008] Optionally, the power-off control module further includes a bias and logic conversion unit, which includes a first bias resistor, a third voltage divider resistor, and a fourth voltage divider resistor. One end of the first bias resistor is connected to the power supply voltage, and the other end is connected to the anode of the reference voltage source and one end of the third voltage divider resistor. The other end of the third voltage divider resistor is connected to one end of the fourth voltage divider resistor and the control terminal of the first switching transistor. The other end of the fourth voltage divider resistor is grounded.
[0009] Optionally, the bias and logic conversion unit is configured to provide a working bias to the reference voltage source when the reference voltage source is turned on, and to generate a turn-on voltage at the control terminal of the first switch transistor through the voltage division effect of the third and fourth voltage divider resistors when the reference voltage source is turned off.
[0010] Optionally, the shutdown control module further includes a current-limiting resistor, a second bias resistor, and a fifth voltage-dividing resistor. The current-limiting resistor is connected in series between the control terminal of the first switching transistor and ground. One end of the second bias resistor is connected to the power supply voltage, and the other end is connected to the control terminal of the second switching transistor. One end of the fifth voltage-dividing resistor is connected to the collector of the first switching transistor, and the other end is connected to the control terminal of the second switching transistor and one end of the bias resistor. The second bias resistor and the fifth voltage-dividing resistor are configured to jointly control the on and off states of the second switching transistor.
[0011] Optionally, the shutdown control module also includes a Zener diode, the cathode of which is connected to the output terminal of the delayed power-on module, and the anode is connected to the control terminal of the second switching transistor.
[0012] Optionally, the delayed power-on module includes a charging resistor and a capacitor, and the delay signal is generated by charging the capacitor through the charging resistor.
[0013] Optionally, the circuit also includes a discharge diode, the anode of which is connected to one end of the capacitor and the cathode of which is connected to the supply voltage. The discharge diode is configured to rapidly discharge the capacitor when power is lost.
[0014] According to another aspect of the embodiments of this application, this application provides a display device including the protection circuit of the aforementioned LLC resonant converter.
[0015] Compared with related technologies, the technical solutions provided in this application have the following advantages: This application provides a protection circuit for an LLC resonant converter, comprising: an LLC resonant converter including an LLC control chip and a half-bridge power circuit; the protection circuit including: a power-down detection module connected to the output terminal of the reference voltage of the LLC control chip, configured to output a power-down detection signal when the reference voltage is detected to be lower than a preset threshold; a power-off control module connected to the protection pin of the LLC control chip, configured to pull down the voltage of the protection pin to below a shutdown threshold in response to the power-down detection signal, so that the LLC control chip stops outputting drive signals; a delayed power-on module coupled to the power-off control module, configured to generate a delay signal when the reference voltage is not lower than the preset threshold, so as to maintain the low state of the protection pin by the power-off control module during the delay period; and a discharge module connected in parallel with the resonant cavity in the half-bridge power circuit, configured to discharge the energy in the resonant cavity after the LLC control chip stops outputting drive signals. The power-down detection module monitors the chip's reference voltage in real time. Upon detecting an abnormal power failure, it immediately sends a signal, which is then responded to by the shutdown control module. This signal forces the chip's protection pin voltage down, stopping all drive outputs. Subsequently, the delayed power-on module generates a delay after power is restored, maintaining the shutdown state. During this delay, the discharge module discharges the residual energy in the resonant cavity, ensuring that the next restart will occur in a safe state with the resonant cavity energy depleted. This safeguards the MOSFET under rapid power-on and power-off conditions and solves the problem of MOSFET damage caused by rapid power-on and power-off of LLC resonant converters. Attached Figure Description
[0016] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.
[0017] To more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the accompanying drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without creative effort.
[0018] Figure 1 This is a schematic diagram of a protection circuit for an optional LLC resonant converter according to an embodiment of this application; Figure 2 This is a structural diagram of an optional LLC resonant converter and its protection circuit provided according to an embodiment of this application.
[0019] Figure labeling: HV+ - High voltage DC bus input positive terminal; U1 - LLC control chip; RVCC - Reference voltage; BLK - Protection pin; VCC - Supply voltage; Q5 - Upper power transistor (MOSFET); Q6 - Lower power transistor (MOSFET); U3 - Reference voltage source; R17 - First voltage divider resistor; R24 - Second voltage divider resistor; Q4 - First switching transistor; Q3 - Second switching transistor; R18 - First bias resistor; R23 - Third voltage divider resistor; R25 - Fourth voltage divider resistor; R20 - Current limiting resistor; R21 - Fifth voltage divider resistor; R30 - Second bias resistor; Z1 - Zener diode; R16 - Charging resistor; C8 - Resonant capacitor; C10 - Capacitor; R11, R12, R13 - Bleeding resistors; D2, D3 - Secondary rectifier diodes; D4 - Discharge diode; T1 - Transformer; CE2 - Filter capacitor. Detailed Implementation
[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.
[0021] In the following description, the use of suffixes such as "module," "part," or "unit" to denote elements is solely for the purpose of illustration and has no specific meaning in itself. Therefore, "module" and "part" may be used interchangeably.
[0022] In the field of switching power supplies, LLC resonant converters are widely used due to their high efficiency and soft-switching characteristics. However, when the AC input is rapidly switched on or off due to poor contact, the energy stored in the resonant cavity cannot be released in time. If the control chip restarts according to its inherent timing, the residual energy in the resonant cavity will induce a resonant current during the conduction phase of the lower transistor and continue flowing through the body diode of the lower transistor during the dead time. If the energy is too large, the body diode cannot fully recover before the upper transistor turns on next, resulting in common conduction of the upper and lower transistors, generating high-voltage and high-current spikes, and damaging the power MOSFET. Currently, no effective solution has been proposed to address the problem of common damage to the upper and lower power transistors due to the failure to release energy in the resonant cavity during rapid switching of LLC resonant converters.
[0023] Therefore, the damage to the MOS caused by the rapid switching on and off of the LLC resonant converter has become a technical problem that needs to be solved.
[0024] To address the problems mentioned in the background art, according to one aspect of an embodiment of this application, a protection circuit for an LLC resonant converter is provided. The LLC resonant converter includes an LLC control chip and a half-bridge power circuit, such as... Figure 1 As shown, the protection circuit includes: The power failure detection module 11 is connected to the output terminal of the reference voltage of the LLC control chip and is configured to output a power failure detection signal when the reference voltage is detected to be lower than a preset threshold. The power-off control module 12 is connected to the protection pin of the LLC control chip and is configured to pull the voltage of the protection pin down below the shutdown threshold in response to a power-down detection signal, so that the LLC control chip stops outputting drive signals. The delayed power-on module 13 is coupled to the power-off control module and is configured to generate a delay signal when the reference voltage is not lower than a preset threshold, so as to maintain the power-off control module's pull-down state of the protection pin during the delay period. The discharge module 14, connected in parallel with the resonant cavity in the half-bridge power circuit, is configured to discharge the energy in the resonant cavity after the LLC control chip stops outputting the drive signal.
[0025] Figure 2 This is a schematic diagram of the LLC resonant converter and its protection circuit provided in this application. As shown in the figure, HV+ is the positive input terminal of the high-voltage DC bus of the LLC resonant converter. The main control system is based on the LLC control chip U1, which drives the half-bridge power circuit composed of Q5 and Q6. The half-bridge output excitation is composed of transformer T1, T1's built-in integrated resonant inductor and resonant capacitor (such as C8), forming an LLC resonant network. After the energy is isolated and transformed by the transformer, it is output as DC voltage by the secondary rectifier diodes (such as D2, D3) and the filter network (including filter capacitor CE2).
[0026] The protection circuit is independent of the main control logic and is used to provide active protection for the LLC system. Its working process and core components include: 1. Power-down detection: The circuit monitors the internal reference voltage (RVCC) generated by the LLC control chip U1 in real time. This voltage is sampled by a voltage divider circuit composed of R17 and R24. The sampled signal is sent to the reference terminal of the reference voltage source U3 (such as TL431) and compared with the 2.5V precision threshold inside U3. When the AC input power fails, causing RVCC to drop and the voltage divider signal to fall below 2.5V, U3 switches from the on state to the off state. The off state of U3 itself is the first key "power-down detection signal".
[0027] 2. Logic transition and shutdown control, including logic transition and two-level shutdown execution: Logic conversion: The first bias resistor R18, voltage divider resistors R23 and R25 together constitute the logic conversion unit. When U3 is turned on, this unit provides bias for U3; when U3 is turned off, VCC passes through R18 and is divided by R23 and R25, generating a high-level drive signal at the base of the first switching transistor Q4 (NPN type).
[0028] Two-stage shutdown execution: After Q4 is turned on, its collector potential (i.e., the target node) is pulled low. This node is directly connected to the base of the second switching transistor Q3 (PNP type). Q3 turns on when its base potential is pulled low. R30 and R21 provide bias voltage for Q3, and its emitter is connected to the protection pin (BLK) of U1. The turn-on of Q3 is equivalent to forcefully pulling the BLK pin low to ground potential, thereby forcing U1 to immediately stop outputting all drive signals, and the LLC main circuit stops working. R20 is the base current limiting resistor of Q4, used to protect Q4.
[0029] 3. Delayed power-on and reset: Delay generation: The charging resistor R16 and capacitor C10 constitute a classic RC delay circuit. When AC is re-energized and VCC recovers, VCC charges C10 through R16, and the voltage across C10 rises slowly.
[0030] Delay maintenance: Before the voltage of C10 is charged to exceed the voltage regulation value of the Zener diode Z1, Z1 is turned off, the base of Q3 remains at a low potential, thereby maintaining the conduction of Q3 and the low level state of the BLK pin, and the system is forced to remain in the shutdown delay stage.
[0031] Fast reset: The cathode of discharge diode D4 (or the corresponding diode in the diagram) is connected to VCC, and the anode is connected to C10. When the AC power fails and VCC drops, D4 conducts in the forward direction, providing a fast discharge path for C10. This ensures that C10 can be quickly reset after each power failure, preparing for the next accurate delay.
[0032] 4. Energy Dissipation: After the main circuit stops working, the discharge resistors R11, R12, and R13 connected in parallel with the resonant cavity (both ends of the resonant capacitor) start working to safely dissipate the dangerous energy stored in the resonant cavity as heat.
[0033] As an optional embodiment, the power failure detection module includes a voltage divider circuit and a reference voltage source; the voltage divider circuit, consisting of a first voltage divider resistor and a second voltage divider resistor connected in series, is configured to divide the reference voltage to obtain a voltage divider signal; the reference voltage source is configured to compare the voltage divider signal with an internal reference threshold, and enter a cutoff state when the voltage divider signal is lower than the internal reference threshold, so as to output a power failure detection signal.
[0034] This application provides the TL431 as a reference voltage source.
[0035] The input of the voltage divider circuit is connected to the output of the reference voltage (RVCC) generated internally by the LLC control chip. The reference voltage is a stable voltage (usually 13V) obtained by internal regulation of the supply voltage VCC, and its stability directly reflects the state of the AC input power supply.
[0036] The reference voltage is divided by a first voltage divider resistor and a second voltage divider resistor, resulting in a proportionally reduced voltage signal across the second voltage divider resistor. By setting the resistance ratio of the first and second voltage divider resistors, the voltage signal can be made equal to a preset value, such as 2.5V, when the reference voltage is normal (e.g., 13V).
[0037] The voltage divider signal is sent to the reference terminal (Ref pin) of the reference voltage source TL431. The TL431 has an internal reference threshold preset (e.g., 2.5V).
[0038] For example, when the power supply is normal, the reference voltage is stable at 13V, and the voltage of the voltage divider signal is 2.5V. At this time, the voltage divider signal is equal to the internal reference threshold (2.5V) of the reference voltage source, the reference voltage source is turned on, and its anode-cathode (AK) is in a low-resistance state.
[0039] For example, during an abnormal power outage, the AC power failure causes a rapid drop in both the supply voltage and the reference voltage. When the reference voltage drops to a certain value, causing the voltage of the voltage divider signal to fall below 2.5V, the reference voltage source is cut off, and its AK terminals become in a high-impedance state.
[0040] The change in state from on to off of the reference voltage source can output a power-down detection signal. When the reference voltage source is off, the anode voltage increases, and the high-level signal at this time is the valid power-down detection signal.
[0041] By changing the resistance ratio of the first voltage divider resistor and the second voltage divider resistor, the RVCC voltage drop point of the trigger protection can be flexibly set to adapt to different design requirements.
[0042] As an optional embodiment, the power-off control module includes a first switch and a second switch; the control terminal of the first switch is turned on in response to a power-down detection signal to pull down the potential of the target node; the control terminal of the second switch is connected to the target node and is configured to turn on when the potential of the target node is pulled down, thereby pulling down the voltage of the protection pin.
[0043] The first switching transistor provided in this application is an NPN transistor, and the second switching transistor is a PNP transistor.
[0044] When the reference voltage source is turned off, its anode voltage rises, and the high-level signal at this time is a valid power-down detection signal.
[0045] A high-level signal is applied to the base (i.e., the control terminal) of the first switching transistor. The first switching transistor, being an NPN transistor, quickly turns on under the high-level drive. After the first switching transistor turns on, its collector current will drive the target node (corresponding to...) Figure 2 The potential of the connection point (the junction of the collector of the first switching transistor, one end of the current-limiting resistor R20, the anode of the Zener diode Z1, and the base of the second switching transistor) is pulled low to near ground potential. At this point, the first switching transistor has completed one signal current amplification and logic inversion (input high level, output low level).
[0046] The target node that is pulled low is directly connected to the base (i.e., the control terminal) of the second switching transistor. As a PNP transistor, when its base potential is pulled low, its conduction condition (emitter voltage > base voltage) will be met, thus enabling it to conduct rapidly.
[0047] After the second switching transistor is turned on, its emitter is connected to the protection pin (BLK) of the LLC control chip, and its collector is grounded. This is equivalent to connecting a low resistor between the BLK pin and ground, thereby forcibly pulling the voltage of the BLK pin down to a level far below its turn-off threshold (such as 2.15V), causing the LLC control chip to immediately stop outputting all drive signals.
[0048] By connecting two transistors in series, a very high current amplification factor can be provided, which can quickly and reliably drive the subsequent load (i.e., pull the BLK pin low) to ensure that the shutdown command is executed quickly.
[0049] As an optional embodiment, the power-off control module further includes a bias and logic conversion unit, which includes a first bias resistor, a third voltage divider resistor, and a fourth voltage divider resistor. One end of the first bias resistor is connected to the power supply voltage, and the other end is connected to the anode of the reference voltage source and one end of the third voltage divider resistor. The other end of the third voltage divider resistor is connected to one end of the fourth voltage divider resistor and the control terminal of the first switching transistor. The other end of the fourth voltage divider resistor is grounded.
[0050] The third and fourth voltage divider resistors are connected in series between VCC and ground (GND). Their voltage division point, that is, the connection point of the third and fourth voltage divider resistors, is directly connected to the base (i.e., the control terminal) of the first switching transistor.
[0051] As an optional embodiment, the bias and logic conversion unit is configured to provide an operating bias to the reference voltage source when the reference voltage source is turned on, and to generate a turn-on voltage at the control terminal of the first switch transistor through the voltage division effect of the third and fourth voltage divider resistors when the reference voltage source is turned off.
[0052] When the AC power supply is normal and the reference voltage is normal, the reference voltage source is turned on. At this time, the anode-cathode of the reference voltage source is in a low-resistance state, which makes the current flowing through the third and fourth voltage divider resistors very small. The voltage drop generated across the fourth voltage divider resistor is very low, that is, the base voltage of the first switching transistor is very low, which is insufficient to turn on the first switching transistor, and the first switching transistor remains off.
[0053] When AC power fails, the reference voltage drops, causing the voltage at the Ref pin of the reference voltage source to fall below 2.5V, at which point the reference voltage source is cut off. The anode-cathode of the reference voltage source becomes a high-resistance state. At this time, the supply voltage passes through the first bias resistor, and then through the third and fourth voltage divider resistors for voltage division. By adjusting the resistance values of the third and fourth voltage divider resistors, the voltage across the fourth voltage divider resistor can be increased sufficiently to forward bias and turn on the first switching transistor.
[0054] By cleverly utilizing the same resistor network (first bias resistor, third voltage divider resistor, and fourth voltage divider resistor) through the bias and logic conversion unit, the reference voltage source is provided with bias current during normal operation, and automatically converted into a voltage divider network when power is off (the reference voltage source is cut off) to generate the turn-on voltage to drive the first switching transistor. This functional multiplexing achieves efficient utilization of hardware resources.
[0055] As an optional embodiment, the shutdown control module further includes a current-limiting resistor, a second bias resistor, and a fifth voltage-dividing resistor. The current-limiting resistor is connected in series between the control terminal of the first switching transistor and ground. One end of the second bias resistor is connected to the power supply voltage, and the other end is connected to the control terminal of the second switching transistor. One end of the fifth voltage-dividing resistor is connected to the collector of the first switching transistor, and the other end is connected to the control terminal of the second switching transistor and one end of the bias resistor. The second bias resistor and the fifth voltage-dividing resistor are configured to jointly control the on and off states of the second switching transistor.
[0056] When the bias and logic conversion unit outputs a high level, driving the first switch to turn on, the current-limiting resistor limits the current flowing into the base of the first switch to turn on.
[0057] Based on the characteristics of a transistor, the base current needs to be controlled within a safe range to prevent overcurrent from burning out the emitter junction of the first switching transistor. The maximum value of the base current of the first switching transistor can be set by adjusting the resistance value of the first current-limiting resistor.
[0058] After the first switching transistor is turned on, its collector potential is pulled down to near ground. This low potential is transmitted to the base of the second switching transistor through the fifth voltage divider resistor (acting as a pull-down resistor), strongly pulling down the base potential of the second switching transistor. At this time, the second bias resistor and the fifth voltage divider resistor form a voltage divider network.
[0059] Since turning on the first switch is equivalent to grounding the lower end of the fifth voltage divider resistor, the result of this voltage division is that the base of the second switch receives a sufficiently low voltage (far lower than its emitter voltage), satisfying the conduction condition of the PNP transistor, thereby enabling the second switch to turn on quickly.
[0060] After the second switch is turned on, a low-impedance path is formed between its emitter (connected to the BLK pin) and collector (grounded), which pulls the voltage of the protection pin (BLK) down to far below its turn-off threshold, forcing the LLC control chip to stop outputting all drive signals.
[0061] By cooperating with the fifth voltage divider resistor and the second bias resistor, a stable and controlled drive current and voltage determined by the resistor voltage divider are provided to the base of the second switch when it needs to operate. This makes the conduction state of the second switch both rapid and stable, avoiding the current surge that may be generated by direct drive.
[0062] As an optional embodiment, the shutdown control module also includes a Zener diode, the cathode of which is connected to the output terminal of the delayed power-on module, and the anode of which is connected to the control terminal of the second switching transistor.
[0063] The cathode (K) of the Zener diode is connected to the output terminal of the delayed start-up module, and the anode (A) is connected to the base of the second switching transistor.
[0064] Due to the reverse breakdown characteristics of Zener diodes, they are used as a voltage threshold or threshold device in protection circuits.
[0065] During the initial power-on phase or the delay phase after a power outage and subsequent power-on, the output voltage of the delayed-start module is low and has not yet charged to the Zener diode's regulated voltage (e.g., 5.1V). At this time, the Zener diode is in reverse cutoff and has no effect on the circuit. Whether the second switch is turned on or off is entirely determined by the state of the first switch (i.e., the potential of the target node).
[0066] When the delay time ends, the voltage across the capacitor is charged to a level exceeding the Zener diode's regulation voltage. At this point, the Zener diode undergoes reverse breakdown, maintaining a stable voltage (5.1V) between its cathode and anode. This voltage is applied between the base and emitter of the second switching transistor. Since the second switching transistor is a PNP transistor, its emitter voltage is approximately the normal voltage of the BLK pin (e.g., above 5V). The base voltage is clamped by the Zener diode at a potential 5.1V lower than the emitter voltage. This ensures that the emitter junction of the second switching transistor is reliably reverse biased, thereby forcibly turning off the second switching transistor and preventing false turn-on.
[0067] Without a Zener diode, simply turning off the first switch might not be sufficient to ensure the base of the second switch is absolutely off during normal system operation, posing a risk of mis-turn-on due to interference. The Zener diode provides a clear voltage threshold; as long as the capacitor voltage exceeds its Zener value, it can forcibly and stably turn off the second switch, eliminating the risk of accidental turn-off of the LLC chip.
[0068] As an optional embodiment, the delayed power-on module includes a charging resistor and a capacitor, and the delay signal is generated by charging the capacitor through the charging resistor.
[0069] The charging resistor and capacitor are connected in series between the supply voltage and ground to form an RC (resistor-capacitor) series circuit. The supply voltage charges the capacitor through the charging resistor.
[0070] The delayed signal is the voltage across the capacitor. According to the characteristics of an RC circuit, when the power supply voltage is applied, the voltage Vc across the capacitor does not jump instantaneously, but starts from 0V and gradually increases exponentially over time.
[0071] A gradually increasing voltage Vc is fed into the circuit (such as the base circuit connected to the second switching transistor in the shutdown control module). When the Vc voltage is below a certain threshold (such as the voltage regulation value of the Zener diode plus the conduction voltage of the second switching transistor), the subsequent circuit remains in a shutdown state; when Vc charges to above the threshold, the state of the subsequent circuit flips, allowing the system to power on. The time elapsed from the start of charging to the voltage reaching the threshold is the delay time.
[0072] According to the formula It can be seen that the delay time is mainly determined by the charging resistor and capacitor (i.e., the time constant τ). Specifically, by adjusting the values of the charging resistor and capacitor, the time constant τ is changed, thereby adjusting the required delay time proportionally.
[0073] By selecting charging resistors and capacitors with different resistance and capacitance values, the required delay length can be set as needed, offering high design flexibility.
[0074] As an optional embodiment, the circuit also includes a discharge diode, the anode of which is connected to one end of the capacitor and the cathode of which is connected to the supply voltage. The discharge diode is configured to rapidly discharge the capacitor when power is lost.
[0075] This embodiment adds a discharge diode to the delay circuit. Its anode is connected to one end of the capacitor (i.e., the connection point between the capacitor and the charging resistor), and its cathode is connected to the power supply voltage network.
[0076] Because diodes have unidirectional conductivity, discharge diodes utilize this characteristic to create an asymmetric charging and discharging path for capacitors.
[0077] When the system is powered on normally or in a stable operating state, the supply voltage is a stable high voltage (e.g., 15V). At this time, the cathode voltage of the discharge diode is higher than the anode voltage, and the discharge diode is in a reverse bias state (cutoff), which is equivalent to an open circuit. Therefore, the path of the supply voltage charging the capacitor through the charging resistor is unaffected, and the charging current flows through the supply voltage, charging resistor, capacitor, and ground.
[0078] When the AC input is disconnected and a power outage occurs, the supply voltage drops rapidly. The instant the supply voltage falls below the voltage across the capacitor, the anode voltage of the discharge diode becomes higher than the cathode voltage, and the discharge diode immediately switches to a forward-biased state (conducts). At this moment, the charge stored in the capacitor flows through the discharge diode and the load circuit of the supply voltage (…). Figure 2 (Not shown) forms a low-impedance discharge path, rapidly releasing electrical energy. This allows the capacitor's voltage to drop quickly with the supply voltage, rather than discharging slowly only through the charging voltage.
[0079] By setting a discharge diode, it can be ensured that the capacitor can be quickly reset after each power failure, providing a complete and accurate delay for the next power-on.
[0080] This application provides a protection circuit for an LLC resonant converter, comprising: an LLC resonant converter including an LLC control chip and a half-bridge power circuit; the protection circuit including: a power-down detection module connected to the output terminal of the reference voltage of the LLC control chip, configured to output a power-down detection signal when the reference voltage is detected to be lower than a preset threshold; a power-off control module connected to the protection pin of the LLC control chip, configured to pull down the voltage of the protection pin to below a shutdown threshold in response to the power-down detection signal, so that the LLC control chip stops outputting drive signals; a delayed power-on module coupled to the power-off control module, configured to generate a delay signal when the reference voltage is not lower than the preset threshold, so as to maintain the low state of the protection pin by the power-off control module during the delay period; and a discharge module connected in parallel with the resonant cavity in the half-bridge power circuit, configured to discharge the energy in the resonant cavity after the LLC control chip stops outputting drive signals. The power-down detection module monitors the chip's reference voltage in real time. Upon detecting an abnormal power failure, it immediately sends a signal, which is then responded to by the shutdown control module. This signal forces the chip's protection pin voltage down, stopping all drive outputs. Subsequently, the delayed power-on module generates a delay after power is restored, maintaining the shutdown state. During this delay, the discharge module discharges the residual energy in the resonant cavity, ensuring that the next restart will occur in a safe state with the resonant cavity energy depleted. This safeguards the MOSFET under rapid power-on and power-off conditions and solves the problem of MOSFET damage caused by rapid power-on and power-off of LLC resonant converters.
[0081] According to another aspect of the embodiments of this application, this application provides a display device including the protection circuit of the aforementioned LLC resonant converter.
[0082] Optionally, specific examples in this embodiment can refer to the examples described in the above embodiments, and will not be repeated here.
[0083] In specific implementation, the embodiments of this application can be referred to the above embodiments and have corresponding technical effects.
[0084] It is understood that the embodiments described herein can be implemented in hardware, software, firmware, middleware, microcode, or a combination thereof. For hardware implementation, the processing unit can be implemented in one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), general-purpose processors, controllers, microcontrollers, microprocessors, other electronic units for performing the functions described herein, or combinations thereof.
[0085] For software implementation, the techniques described herein can be implemented by units that perform the functions described herein. The software code can be stored in memory and executed by a processor. The memory can be implemented in the processor or external to the processor.
[0086] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.
[0087] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.
[0088] In the embodiments provided in this application, it should be understood that the disclosed apparatus and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative. For instance, the division of modules is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple modules or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be an indirect coupling or communication connection through some interfaces, devices, or units, and may be electrical, mechanical, or other forms.
[0089] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.
[0090] In addition, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit.
[0091] If the aforementioned function is implemented as a software functional unit and sold or used as an independent product, it can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the embodiments of this application, or the part that contributes to the prior art, or a part of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this application. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, ROM, RAM, magnetic disks, or optical disks. It should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. In the absence of further restrictions, an element defined by the phrase "comprising a..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.
[0092] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.
Claims
1. A protection circuit for an LLC resonant converter, the LLC resonant converter comprising an LLC control chip and a half-bridge power circuit, characterized in that, The protection circuit includes: The power failure detection module is connected to the output terminal of the reference voltage of the LLC control chip and is configured to output a power failure detection signal when the reference voltage is detected to be lower than a preset threshold. The power-off control module, connected to the protection pin of the LLC control chip, is configured to pull the voltage of the protection pin down below the shutdown threshold in response to the power-down detection signal, so that the LLC control chip stops outputting drive signals; A delayed power-on module, coupled to the power-off control module, is configured to generate a delay signal when the reference voltage is not lower than the preset threshold, so as to maintain the power-off control module's pull-down state of the protection pin during the delay period. The discharge module, connected in parallel with the resonant cavity in the half-bridge power circuit, is configured to discharge the energy in the resonant cavity after the LLC control chip stops outputting the drive signal.
2. The circuit according to claim 1, characterized in that, The power failure detection module includes a voltage divider circuit and a reference voltage source; The voltage divider circuit, consisting of a first voltage divider resistor and a second voltage divider resistor connected in series, is configured to divide the reference voltage to obtain a voltage divider signal. The reference voltage source is configured to compare the voltage divider signal with an internal reference threshold, and to enter a cutoff state when the voltage divider signal is lower than the internal reference threshold, so as to output the power failure detection signal.
3. The circuit according to claim 1, characterized in that, The shutdown control module includes a first switching transistor and a second switching transistor; The control terminal of the first switching transistor is turned on in response to the power failure detection signal to pull down the potential of the target node; The control terminal of the second switching transistor is connected to the target node and is configured to turn on when the potential of the target node is pulled low, thereby pulling the voltage of the protection pin low.
4. The circuit according to claim 3, characterized in that, The shutdown control module further includes a bias and logic conversion unit, which includes a first bias resistor, a third voltage divider resistor, and a fourth voltage divider resistor. One end of the first bias resistor is connected to the power supply voltage, and the other end is connected to the anode of the reference voltage source and one end of the third voltage divider resistor. The other end of the third voltage divider resistor is connected to one end of the fourth voltage divider resistor and the control terminal of the first switching transistor. The other end of the fourth voltage divider resistor is grounded.
5. The circuit according to claim 4, characterized in that, The bias and logic conversion unit is configured to provide a working bias to the reference voltage source when the reference voltage source is turned on, and to generate a turn-on voltage at the control terminal of the first switch transistor through the voltage division effect of the third and fourth voltage divider resistors when the reference voltage source is turned off.
6. The circuit according to claim 3, characterized in that, The shutdown control module further includes a current-limiting resistor, a second bias resistor, and a fifth voltage-dividing resistor. The current-limiting resistor is connected in series between the control terminal of the first switching transistor and ground. One end of the second bias resistor is connected to the power supply voltage, and the other end is connected to the control terminal of the second switching transistor. One end of the fifth voltage-dividing resistor is connected to the collector of the first switching transistor, and the other end is connected to the control terminal of the second switching transistor and one end of the bias resistor. The second bias resistor and the fifth voltage-dividing resistor are configured to jointly control the on and off states of the second switching transistor.
7. The circuit according to claim 3, characterized in that, The shutdown control module also includes a Zener diode, the cathode of which is connected to the output terminal of the delayed power-on module, and the anode is connected to the control terminal of the second switching transistor.
8. The circuit according to claim 1, characterized in that, The delayed power-on module includes a charging resistor and a capacitor, and the delay signal is generated by charging the capacitor through the charging resistor.
9. The circuit according to claim 8, characterized in that, The circuit also includes a discharge diode, the anode of which is connected to one end of the capacitor and the cathode of which is connected to the power supply voltage. The discharge diode is configured to rapidly discharge the capacitor when power is lost.
10. A display device, characterized in that, The protection circuit includes the LLC resonant converter as described in any one of claims 1 to 9.