LLC resonant converter

Abstract

The LLC resonant converter comprises two or more LLC half-bridge resonant conversion circuits, the input ends of the LLC half-bridge resonant conversion circuits are connected in series, and the output ends of the LLC half-bridge resonant conversion circuits are connected in parallel; each LLC half-bridge resonant conversion circuit comprises a half-bridge unit, an LLC resonant unit, a transformer and a rectifying unit; the half-bridge unit comprises a pair of switching tubes which are connected in series; the LLC resonance unit comprises two LLC resonance networks, the two LLC resonance networks are in one-to-one correspondence with the pair of switch tubes, and the LLC resonance networks and the corresponding switch tubes form a loop. The circuit topology that the input ends of the LLC half-bridge resonant conversion circuits are connected in series and the outputs of the LLC half-bridge resonant conversion circuits are connected in parallel is adopted, the input voltage range of the converter can be effectively widened, the stress of a single device is reduced, and input and output ripples are reduced.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, specifically to power conversion, and particularly to an LLC resonant converter. Background Art

[0002] The power converter realizes power conversion through the high-frequency operation of the switching tubes. Energy storage power supplies in application fields such as new energy vehicles and spacecraft have put forward higher requirements for the efficiency and power density of the power converter. With the increase of the switching frequency, the switching loss has become a key problem to be solved in the design of high-power density converters. The LLC resonant converter can reduce the switching loss because the voltage or current periodically crosses zero during resonance. In the traditional asymmetric half-bridge LLC resonant converter, the energy transfer from the primary side to the secondary side is completed during the on-time period of the upper tube in one cycle. If the converter power is relatively large, it will lead to too large a peak value of the input current within half a cycle and current discontinuity. In addition, multiple filter capacitors need to be connected in parallel at the output end of the single-phase LLC resonant converter for filtering, which reduces the power density of the converter and limits its application in high-power occasions. In addition, due to the limited voltage and current stress and power capacity of a single device, under the requirements of high efficiency in a wide input voltage and full load range, higher requirements are put forward for the selection of power devices, the improvement of switching frequency and power density, the determination of resonant parameters, and the optimization of control methods. In addition, limited by the device process and manufacturing errors, the parameter errors of the devices participating in series and parallel can only be minimized as much as possible during the actual circuit implementation and cannot be completely avoided. Summary of the Invention

[0003] To solve the defects of the prior art, the present invention provides an LLC resonant converter, including an LLC half-bridge resonant conversion circuit; the LLC half-bridge resonant conversion circuit includes a half-bridge unit, an LLC resonant unit, a transformer, and a rectification unit; the half-bridge unit includes a pair of switching tubes connected in series; the LLC resonant unit includes an LLC resonant network; the LLC resonant network includes a resonant capacitor, a resonant inductor, and an exciting inductor; the exciting inductor is the exciting inductor of the primary side of the transformer.

[0004] The LLC resonant unit includes two LLC resonant networks. The two LLC resonant networks correspond to the pair of switching tubes one by one, and the LLC resonant network forms a loop with the corresponding switching tube.

[0005] The resonant capacitors of the two LLC resonant networks are resonant capacitors with evenly divided capacities, the resonant inductors of the two LLC resonant networks are equal-value resonant inductors, and the exciting inductors of the two LLC resonant networks are equal-value exciting inductors; the coils used for the exciting inductors of the two LLC resonant networks are wound in parallel on the primary side of the transformer.

[0006] Preferably, there are at least two LLC half-bridge resonant conversion circuits, and the input ends of the LLC half-bridge resonant conversion circuits are connected in series, and the output ends of the LLC half-bridge resonant conversion circuits are connected in parallel.

[0007] Preferably, a voltage-dividing capacitor is connected in parallel to the input end of each LLC half-bridge resonant conversion circuit.

[0008] Preferably, the LLC resonant converter further includes: a driving unit for driving each switch tube to operate, a monitoring unit for monitoring the operating state of the converter, and a control unit for controlling the operation of the driving unit according to the operating state of the converter monitored by the monitoring unit.

[0009] Preferably, the monitoring unit includes one or more of a resonant current acquisition circuit, an output current acquisition circuit, an output voltage acquisition circuit, and a temperature acquisition circuit.

[0010] Preferably, the output current acquisition circuit includes a switch circuit for controlling an acquisition frequency; and the control unit performs low-frequency time-sharing acquisition of the output current of the converter through the output current acquisition circuit.

[0011] Preferably, the control unit determines the size of the resonant current through the resonant current acquisition circuit, and performs two-level protection according to the determined size of the resonant current: when it is determined that the resonant current increases to the first-level limit value, the switching frequency is increased to reduce the output energy of the converter; if it is determined that the resonant current continues to increase to the second-level protection limit value, the output of the converter is shut down, and a resonant current over-limit signal is output to the outside through protocol communication.

[0012] Preferably, the control unit collects the voltage across a resonant capacitor in a certain LLC half-bridge resonant conversion circuit through a resonant current collection circuit, and determines the magnitude of the resonant current based on the voltage across the resonant capacitor.

[0013] Preferably, the LLC resonant converter further includes a heat dissipation device for actively dissipating heat from the converter; the heat dissipation device is activated according to the temperature level; the heat dissipation level of the converter is controllable. In a specific embodiment, heat dissipation is activated when the temperature reaches 60 degrees and is deactivated when the temperature is below 55 degrees. In other embodiments, the heat dissipation level can be specifically controlled according to the temperature level, such as controlling the fan speed; and a self-test program for the heat dissipation device is added to the converter's power-on self-test program. In a specific embodiment, the converter starts the heat dissipation device and runs it for 10 seconds.

[0014] Preferably, the control unit includes a controller (the controller can be a DSP) with multiple ADs, multiple PWMs, and a CAN communication module; the control unit communicates with the upper-level control unit through the communication module, and the operating state of the converter is set by the upper-level control unit through a communication interface (the communication interface can be a CAN interface), including the operating voltage, current, power, and whether to start the output, and the converter detects its own state (the operating state of the converter monitored by the monitoring unit) and feeds it back to the upper-level control unit through the same communication interface.

[0015] The advantages and beneficial effects of the present invention are as follows:

[0016] For the two LLC resonant networks of the LLC resonant unit of the present invention, a symmetric dual-resonant network resonant cavity structure is adopted. Compared with the asymmetric resonant cavity structure, the input current is continuous and the ripple is smaller.

[0017] The present invention adopts two or more LLC half-bridge resonant conversion circuits, and a circuit topology in which the input ends of each LLC half-bridge resonant conversion circuit are connected in series and the output ends are connected in parallel. The output current is evenly divided on the secondary side, which can effectively increase the input voltage range of the converter, reduce the stress on a single device, and reduce the input and output ripple.

[0018] Voltage-dividing capacitors are respectively connected in parallel at the input ends of each LLC half-bridge resonant conversion circuit of the present invention. When the power allocated to one of the circuits is relatively large, the power extracted from the voltage-dividing capacitor at its front end is relatively large, and the voltage of the corresponding voltage-dividing capacitor will drop, resulting in an increase in gain. The increase in gain will cause the power allocated to this circuit to decrease. Therefore, after adding the voltage-dividing capacitor, current sharing among the LLC half-bridge resonant conversion circuits can be achieved.

[0019] The present invention adopts the design of series connection at the input of each LLC half-bridge resonant conversion circuit, staggered turn-on of the switching tubes of each circuit, and parallel connection at the output end, which can reduce the device stress, lower the difficulty of device selection, reduce the output ripple, reduce the volume of the output filter capacitor, expand the input voltage range and the output power level, and is suitable for the DC / DC circuit topology of high-voltage DC power supply systems.

[0020] The present invention can perform low-frequency (such as 100 Hz) time-sharing acquisition of the output current of the converter; compared with continuous current acquisition, it can reduce the power consumption of the sampling circuit and improve the overall efficiency.

[0021] The control unit of the present invention collects the voltages at both ends of a resonant capacitor in a certain LLC half-bridge resonant conversion circuit through a resonant current acquisition circuit, and judges the magnitude of the resonant current based on the voltages at both ends of the resonant capacitor; since the two symmetric resonant capacitors inside each LLC half-bridge resonant conversion circuit of the present invention evenly divide the resonant current, and the power is evenly divided among the LLC half-bridge resonant conversion circuits, as long as the voltages at both ends of one resonant capacitor are sampled, the overall resonant current level can be reflected.

[0022] The present invention also adopts a heat dissipation device (such as a fan) to actively dissipate heat from the converter, and the heat dissipation device is controlled by a control unit, so that the heat dissipation device starts according to the temperature level, and further the heat dissipation level of the converter is controllable, which can improve the efficiency of the converter and reduce power consumption.

[0023] The converter of the present invention also communicates with the upper-level control unit. The working state of the converter is set by the upper-level control unit, including operating voltage, current, power, and whether to start output. Moreover, the converter detects its own state and feeds it back to the upper-level control unit, which can realize remote monitoring and setting of the converter. Description of the Drawings

[0024] Figure 1 is the circuit diagram of the resonant converter;

[0025] Figure 2 is a schematic diagram of the driving unit and the half-bridge unit;

[0026] Figure 3 is a schematic diagram of the output current acquisition circuit;

[0027] Figure 4 is a schematic diagram of the resonant current acquisition circuit;

[0028] Figure 5 is the flowchart for monitoring the working state of the converter;

[0029] Figure 6 is the circuit diagram for controlling the heat dissipation device (specifically a fan) by the control unit;

[0030] Figure 7 is the control flowchart for actively dissipating heat from the converter;

[0031] Figure 8 is the main signal waveform of the two-way LLC half-bridge resonant conversion circuit;

[0032] Figure 9 is the main signal working wave of an LLC resonant network in a one-way LLC half-bridge resonant conversion circuit. Detailed Embodiments

[0033] The following further describes the detailed embodiments of the present invention in conjunction with the drawings and embodiments. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and cannot be used to limit the protection scope of the present invention.

[0034] The technical solutions for the specific implementation of the present invention are as follows:

[0035] As Figure 1As shown in the figure, the present invention provides an LLC resonant converter, which includes an inverter unit 1, a resonant unit group 2, a control unit 3, a rectifier unit group 4, and a sampling unit 5.

[0036] Specifically, the LLC resonant converter of the present invention includes two or more ( Figure 1 in this case, two) LLC half-bridge resonant conversion circuits; the input ends of each LLC half-bridge resonant conversion circuit are connected in series, and the output ends of each LLC half-bridge resonant conversion circuit are connected in parallel;

[0037] Specifically, each LLC half-bridge resonant conversion circuit respectively includes a half-bridge unit, an LLC resonant unit, a transformer, and a rectifier unit; the half-bridge unit includes a pair of switching tubes connected in series; the LLC resonant unit includes two LLC resonant networks, the two LLC resonant networks correspond to the pair of switching tubes one by one, and the LLC resonant network forms a loop with the corresponding switching tube; more specifically, the two LLC resonant networks respectively include a resonant capacitor, a resonant inductor, and an exciting inductor; the resonant capacitors of the two LLC resonant networks are resonant capacitors with evenly divided capacitances, the resonant inductors of the two LLC resonant networks are equal-value resonant inductors, and the exciting inductors of the two LLC resonant networks are equal-value exciting inductors; the coils used for the exciting inductors of the two LLC resonant networks are wound in parallel on the primary side of the transformer;

[0038] Taking one LLC half-bridge resonant conversion circuit as an example, as Figure 1 shown, C r1 , C r2 are resonant capacitors with evenly divided capacitances, L r1 , L r2 are equal-value resonant inductors, L m1 , L m2 are equal-value exciting inductors; Q1 and Q2 are power switching tubes, and the phase difference between the two switching tubes is 180°, and they conduct complementarily; the transformer T3 is composed of two parts, the two sets of coils on the primary side are wound in parallel, the secondary windings are connected in parallel, and the turn ratio from the two sets of primary sides to the secondary side is N; the series resonant capacitor C r1 , the transformer exciting inductor L m1 and the series resonant inductor L r1 constitute the No. 1 LLC resonant network, and the series resonant capacitor C r2 , the transformer exciting inductor L m2 and the series resonant inductor L r2 constitute the No. 2 LLC resonant network. During operation, the two LLC resonant networks equally divide the resonant current, and the resonant currents of the two LLC resonant networks are staggered by 180° in phase; when Q1 is on and Q2 is off, the input voltage passes through Q1, L m1 , L r1 , C r1 and then reaches point B; when Q1 is off and Q2 is on, the input voltage passes through Cr2 , L r2 , L m2 , after Q2 reaches point B; D1 and D2 are secondary rectifier diodes, and their outputs are connected in parallel to evenly divide the output current;

[0039] The two LLC resonant networks of the LLC resonant unit of the present invention adopt a symmetric double-resonant network resonator structure. Compared with the asymmetric resonator structure, the input current is continuous and has smaller ripple.

[0040] The input-output voltage transfer ratio (voltage gain) of a single-channel LLC half-bridge resonant conversion circuit is where f n = f s / f r is the normalized frequency, f s is the switching frequency, is the resonant frequency; is the quality factor; λ is the ratio of the resonant inductor to the exciting inductor, λ = L r / L m ; combined with R ac = (8n 2 R out ) / π 2 (Formula 2), the output impedance expression of the single-channel LLC half-bridge resonant conversion circuit is obtained: In the formula is the resonant impedance;

[0041] When multiple LLC half-bridge resonant conversion circuits with the same structure are directly connected in series at the input end, R out can represent the power allocated to one LLC. It can be seen from Formula 3 that R out is a function of the resonant parameters L r , L m , C r and the gain M; since the output ends of multiple ( Figure 1 two in this case) LLC half-bridge resonant conversion circuits are connected in parallel, the output voltages of multiple LLC half-bridge resonant conversion circuits are the same. Due to the inevitable differences in the device parameters of multiple LLC half-bridge resonant conversion circuits in practical applications, the power allocated to multiple LLC half-bridge resonant conversion circuits will be uneven. In severe cases, it is possible that one of them will bear all the output power and damage the power supply;

[0042] To solve this problem, the present invention connects voltage-dividing capacitors in parallel at the input ends of multiple LLC half-bridge resonant conversion circuits (that is, adding series voltage-dividing capacitors at the input stage). Taking the two LLC half-bridge resonant conversion circuits in Figure 1 as an example, the instantaneous voltage expression of the input voltage-dividing capacitor is: V C2 (s) = Vin -V C1 (s), where Z1(s) and Z2(s) are the resonant impedances of the two LLC half-bridge resonant converter circuits, V in is the total input voltage. When the power distributed to one path is large, the power extracted from its front-end voltage-divider capacitor is large, and the corresponding voltage-divider capacitor voltage will decrease, and the gain will increase. From Formula 3, it can be seen that the increase in gain will reduce the power distributed to this path. Therefore, adding the voltage-divider capacitor can achieve current sharing on both paths.

[0043] The present invention adopts a design in which each LLC half-bridge resonant conversion circuit input is connected in series, the switching tubes of each circuit are turned on alternately, and the output ends are connected in parallel. This design can reduce device stress, ease the difficulty of device selection, reduce output ripple, reduce the volume of the output filter capacitor, expand the input voltage range and output power level, and is suitable for the DC / DC circuit topology of a high-voltage DC power supply system.

[0044] According to the working principle of LLC resonant converter, when overloaded or output short-circuited, the converter loop current increases sharply, which may cause device burnout in a short time. Therefore, a current detection circuit must be added. For LLC resonant converter, the duty cycle is kept at 50%. When the load changes, it first adjusts the switching frequency instead of turning off the switch tube immediately. If you want to observe the effect of the frequency change, you have to wait until the next switching cycle at least. To solve this problem, the present invention adopts an output current acquisition circuit (such as Figure 3 In addition to the resonant current collection circuit (as shown in Figure 4 shown).

[0045] like Figure 3 In the output current acquisition circuit shown, CT can be a current transformer with a turn ratio of 1:100; Q OC1 , Q OC2 , Q OC3 and other resistor and capacitor components form a sampling control network, which collects the output current in a time-sharing manner under the control signal. OC1 The current is converted into voltage and then sent to the control unit (AD acquisition input terminal of the main control chip); the control signal can be a square wave signal with a frequency of 100Hz and a pulse width of 30uS. The control unit (controller) controls the control signal and the AD acquisition circuit.

[0046] like Figure 4 The resonant current collection circuit shown, C S1 is the DC blocking capacitor, C S2 With R S2 Composed of RC filter network, O C1A linear optocoupler can be adopted; the collected signal of the resonant current is output and connected to the AD input interface of the main controller. There are two levels of protection inside the controller. The first level: when the detected input voltage exceeds 2V (in one embodiment), the operating frequency is increased to limit the power output. If the increase in frequency cannot suppress the continuous increase of the current, when the detected input voltage exceeds 2.4V (in one embodiment), the drive output is turned off. Since the two symmetrical resonant capacitors inside each LLC half-bridge resonant conversion circuit in the present invention evenly divide the resonant current, and the power is evenly divided among the LLC half-bridge resonant conversion circuits, so as long as the voltage across one of the resonant capacitors is sampled, the overall resonant current level can be reflected. In actual use, the protection voltages of the two levels of protection can be determined according to the actual input and output parameters of the converter. Here, only two voltage values in one implementation form are given as examples.

[0047] To ensure the stability and dynamic performance of the converter system, closed-loop control is implemented. The output voltage sampling signal and the output current sampling signal are used as the inputs of the system closed-loop control. The control circuit generates two sets (in one embodiment) of complementary logic signals with a dead zone and a duty cycle of 50%, and the two sets of signals are phase-shifted by 90°. After passing through the drive unit (drive amplifier circuit) composed of a gate driver and a pulse transformer, the switching tubes are driven.

[0048] Figure 5 It is a flowchart for monitoring the working state of the converter; the monitoring of the working state of the converter includes: monitoring the input voltage, output voltage, output current, resonant current, temperature, power, etc. of the converter. The monitoring results are fed back to the front-end control unit through a communication interface (in one embodiment), and at the same time, power control instructions are received through this interface to set the working state of the power supply.

[0049] Such as Figure 6 、 Figure 7As shown in the figure, the present invention also uses a heat dissipation device (such as a fan) to actively dissipate heat from the converter. The DC output of the converter serves as the power supply for the heat dissipation device. To improve the efficiency of the converter and reduce power consumption, the present invention also controls the heat dissipation device through a control unit to start the heat dissipation device according to the temperature level. When the monitoring unit (the converter temperature acquisition circuit) monitors that the converter temperature is greater than 60 degrees (in one embodiment), the heat dissipation device is started. After the heat dissipation is started, if the monitored temperature is less than 55 degrees, the heat dissipation is turned off; otherwise, the heat dissipation is maintained. Regarding the monitoring unit, to ensure that the ambient temperature can be correctly acquired, two temperature acquisition circuits are used for temperature acquisition. The acquisition results of the two temperature acquisition circuits are respectively connected to the control unit (two AD input interfaces of the controller). The temperature processing part program segment converts the two temperature acquisition results. If any acquisition result exceeds the limit, the heat dissipation is started. In addition, to verify the correctness of the heat dissipation device and its control logic, a heat dissipation device startup program is added to the power-on initialization program segment in the present invention for self-check. If the temperature monitoring result does not exceed the limit during the self-check process, the heat dissipation device automatically shuts down after running for 10S (in one embodiment). If the temperature exceeds the limit, the heat dissipation continues normally.

[0050] Figure 1 The main signal waveforms of the two-way LLC half-bridge resonant conversion circuit are as Figure 8 shown, V gs_Q1 ~V gs_Q4 are the gate control signals of the switching tubes Q1~Q4. Among them, the phase difference between V gs_Q1 , V gs_Q3 , V gs_Q2 , V gs_Q4 is 90°. The dead time between V gs_Q1 , V gs_Q2 and between V gs_Q3 , V gs_Q4 is fixed and equal; i D1 ~i D4 are the rectifier diode current signals; i Lm1 , i Lm3 are the current signals of the two exciting inductors, and i Lr1 , i Lr3 are the current signals of the two resonant inductors.

[0051] The main signal working waveforms of an LLC resonant network in one-way LLC half-bridge resonant conversion circuit are as Figure 9 shown. Specifically, at the resonant frequency f m <f s <f r when one of the LLC resonant networks (C r1 , L m1 and L r1) working mode; in the stage from t0 to t1, both the switching transistors Q1 and Q2 are turned off. At the moment t0, the drain-source voltage V of the switching transistor Q2 ds_Q2 drops to 0, and the resonant cavity current flows through the internal diode of Q2 for freewheeling, and Q2 can enter the zero-voltage turn-on state;

[0052] At the moment t1, the switching transistor Q2 is turned on with zero voltage. In the stage from t1 to t2, the resonant current is in phase with the current flowing through the resonant capacitor L r2 and is out of phase with the current flowing through L r1 ;

[0053] In the stage from t2 to t3, at the moment t2, the resonant current is equal to the exciting current. At this time, six-element resonance occurs among the two exciting inductors, two resonant capacitors, and two resonant inductors, and the transformer is no longer clamped, and the secondary rectifier diode D2 completes ZCS;

[0054] In the stage from t3 to t4, at t3, both the switching transistors Q1 and Q2 are turned off, and the dead time is entered. During this period, the resonant current charges the parasitic capacitor of the switching transistor Q2 and discharges the parasitic capacitor of Q1. At the moment t4, Q1 can enter the zero-voltage turn-on state. The states of t4 - t5, t5 - t6, t6 - t7, and t7 - t8 are the same as those of the aforementioned t0 - t1, t1 - t2, t2 - t3, and t3 - t4.

[0055] The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An LLC resonant converter, comprising an LLC half-bridge resonant conversion circuit; the LLC half-bridge resonant conversion circuit includes a half-bridge unit, an LLC resonant unit, a transformer, and a rectification unit; the half-bridge unit includes a pair of switching tubes connected in series; the LLC resonant unit includes an LLC resonant network; the LLC resonant network includes a resonant capacitor, a resonant inductor, and an exciting inductor; characterized in that: The LLC resonant unit includes two LLC resonant networks, which correspond to the pair of switching tubes one by one, and the LLC resonant network forms a loop with the corresponding switching tube; the resonant capacitance capacities of the two LLC resonant networks are evenly divided, the resonant inductances of the two LLC resonant networks are equal in value, and the magnetizing inductances of the two LLC resonant networks are equal in value.

2. The LLC resonant converter according to claim 1, wherein The LLC half-bridge resonant conversion circuit has at least two paths, and the input ends of each path of LLC half-bridge resonant conversion circuit are connected in series and the output ends are connected in parallel.

3. The LLC resonant converter according to claim 2, wherein Voltage-dividing capacitors are respectively connected in parallel to the input ends of each path of LLC half-bridge resonant conversion circuit.

4. The LLC resonant converter according to claim 2 or 3, characterized in that, It further includes: A driving unit for driving each switching tube to work, a monitoring unit for monitoring the working state of the converter, and a control unit for controlling the working of the driving unit according to the working state of the converter monitored by the monitoring unit.

5. The LLC resonant converter according to claim 4, wherein The monitoring unit includes one or several of a resonant current acquisition circuit, an output current acquisition circuit, an output voltage acquisition circuit, and a temperature acquisition circuit.

6. The LLC resonant converter according to claim 5, characterized in that, The output current acquisition circuit includes a switching circuit for controlling the acquisition frequency; the control unit performs low-frequency time-division acquisition on the output current of the converter through the output current acquisition circuit.

7. The LLC resonant converter according to claim 5, wherein The control unit judges the magnitude of the resonant current through the resonant current acquisition circuit, and executes two-level protection according to the judged magnitude of the resonant current: when it is judged that the resonant current increases to the first-level limit value, the switching frequency is increased to reduce the output energy of the converter; If it is judged that the resonant current continues to increase to the second-level protection limit value, the output of the converter is turned off, and a resonant current overlimit signal is output externally.

8. The LLC resonant converter according to claim 7, wherein, The control unit acquires the voltage across both ends of a resonant capacitor in a certain path of LLC half-bridge resonant conversion circuit through the resonant current acquisition circuit, and judges the magnitude of the resonant current according to the voltage across both ends of the resonant capacitor.

9. The LLC resonant converter according to claim 5, wherein It further includes a heat dissipation device for actively dissipating heat from the converter; the heat dissipation device is started according to the temperature level; and a self-check program of the heat dissipation device is added to the power-on self-check program.

10. The LLC resonant converter according to claim 4, characterized in that, The control unit has a communication module; the control unit communicates with the upper-level control unit through the communication module, the working state of the converter is set by the upper-level control unit, and the converter detects its own state and feeds it back to the upper-level control unit.

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