A current complementing bidirectional inverter circuit
By designing a bidirectional inverter circuit with supplemental current, the inverter state is dynamically adjusted, solving the overload problem caused by insufficient mains current, and realizing automatic compensation of load current and normal operation of the equipment.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Utility models(China)
- Current Assignee / Owner
- DONGGUAN LONGSHENG ELECTRONICS CO LTD
- Filing Date
- 2025-08-08
- Publication Date
- 2026-07-21
AI Technical Summary
Single-function inverters cannot meet the power load requirements in situations where the mains current is limited, leading to overload protection and the inability to simultaneously meet the power demand of the equipment.
Design a bidirectional inverter circuit for current compensation, including a main control module, a filter module, a buck-boost module, an H-bridge or bridgeless PFC module, and a mains power module. The main control module dynamically adjusts the state of the switching transistors to achieve voltage conversion and current compensation, ensuring the load current requirements are met.
It enables automatic current replenishment when the mains current is insufficient, dynamically adjusts the inverter's operating status to meet load requirements, avoids overload protection, and ensures normal equipment operation.
Smart Images
Figure CN224538073U_ABST
Abstract
Description
Technical Field
[0001] This utility model relates to the field of power supply, specifically a bidirectional inverter circuit for compensating for current. Background Technology
[0002] As the power demand of equipment (such as RVs) increases, in many practical applications, there are limitations on the current drawn from the mains power. For example, if an RV is currently in a situation where it is only allowed to use 15A of mains power, but the RV's load requirement is 20A, the power supply will be insufficient and overload protection will be triggered.
[0003] Therefore, inverters with only one function of inverter operation cannot meet the needs of some applications where the use of mains current is limited. Using high-power loads can easily lead to overload protection of the inverter circuit, which requires improvement. Utility Model Content
[0004] The purpose of this invention is to provide a bidirectional inverter circuit for compensating for current, so as to solve the problems mentioned in the background art.
[0005] To achieve the above objectives, this utility model provides the following technical solution:
[0006] A current-compensating bidirectional inverter circuit includes:
[0007] The main control module is used to control the conduction state of the switching transistors of the buck-boost module, H-bridge or bridgeless PFC module, thereby controlling the voltage conversion. It also receives the sampled voltages of the buck-boost module, H-bridge or bridgeless PFC module and mains module, adjusts the conduction state of the switching transistors, and controls whether the relays of the mains module are working.
[0008] The filter module is used to smooth the current during battery charging or discharging;
[0009] The buck-boost module is used to boost the battery discharge voltage to 400V after passing through the filter module when the battery is discharging, and to supply the H-bridge or bridgeless PFC module. When the battery is charging, it is used to step down the voltage output by the H-bridge or bridgeless PFC module and then pass it through the filter module to charge the battery.
[0010] H-bridge or bridgeless PFC modules are used as H-bridge circuits during battery discharge to convert 400V DC to 220V AC to supply the load (at which time the mains current is insufficient to meet the load's power requirements). During battery charging, they are used as bridgeless PFC circuits to convert part of the 220V AC mains current (at which time the mains current, while meeting the load's power requirements, still has excess current) to 400V DC, which then supplies power to the battery through buck-boost circuits and filtering modules.
[0011] The AC power module is used to supply 220V AC AC power.
[0012] The filter module is connected to the buck-boost module, the buck-boost module is connected to the H-bridge or bridgeless PFC module, the H-bridge or bridgeless PFC module is connected to the mains power module, and the main control module is connected to the buck-boost module, the H-bridge or bridgeless PFC module, and the mains power module.
[0013] As a further embodiment of this utility model: the main control module includes a chip U1, the chip U1 model is TMS320F280039, pins 74, 75 (or 32, 36), 88 to 93 of the chip U1 are connected to a buck-boost module, pins 58, 59 (or 30, 31), 66 to 69 of the chip U1 are connected to an H-bridge or a bridgeless PFC module, and pins 20, 21, 39, and 40 of the chip U1 are connected to a mains power module.
[0014] As a further embodiment of this utility model: the buck-boost module includes MOSFETs M8, M9, M10, M11, M12, M15, M16, M17, M18, and M19, a transformer T2, capacitors E3 and E4, resistors R1 and R4. The first terminal of the transformer T2 is connected to the drain (D) of MOSFETs M10, M11, and M12. The second terminal of transformer T2 is connected to the filter module. The third terminal of transformer T2 is connected to the drain (D) terminals of MOSFETs M17, M18, and M19. The source (S) terminals of MOSFETs M10, M11, M12, M17, M18, and M19 are grounded. The fourth terminal of transformer T2 is connected to the source (S) terminal of MOSFET M9 and the drain (D) terminal of MOSFET M15. Terminal 5 of transistor T2 is connected to the source (S) of MOSFET M8 and the drain (D) of MOSFET M16. The source (S) of MOSFET M15 is grounded, and the source (S) of MOSFET M16 is grounded. The drain (D) of MOSFET M8 is connected to the drain (D) of MOSFET M9, one end of capacitor E3, one end of capacitor E4, one end of resistor R1, and an H-bridge or bridgeless PFC module. The other end of resistor R1 is connected to the main control module. The other end of capacitor E3 is grounded, the other end of capacitor E4 is grounded, one end of resistor R4 is grounded, and the other end of resistor R4 is connected to the main control module. The gate (G) of MOSFET M8, the gate (G) of MOSFET M9, the gate (G) of MOSFET M10, the gate (G) of MOSFET M11, the gate (G) of MOSFET M12, the gate (G) of MOSFET M15, the gate (G) of MOSFET M16, the gate (G) of MOSFET M17, the gate (G) of MOSFET M18, and the gate (G) of MOSFET M19 are connected to the main control module.
[0015] As a further embodiment of this invention: the H-bridge or bridgeless PFC module includes IGBTs M4, M5, M6, and M7, an inductor L3, resistors R2 and R3, and a capacitor C4. The collector of IGBT M4 is connected to the collector of IGBT M5, a step-up / step-down module, and the emitter of IGBT M4 is connected to the collector of IGBT M6. One end of the inductor L3 is connected to the emitter of IGBT M6, which is grounded. The other end of the inductor L3 is connected to one end of the resistor R3. One end of capacitor C4 is connected to the mains power module, the other end of capacitor C4 is grounded, the other end of resistor R3 is connected to the main control module, the emitter of IGBT M5 is connected to the collector of IGBT M7, one end of resistor R2 is connected to the main control module, the other end of capacitor C4 is connected to the main control module, the emitter of IGBT M7 is grounded, the gate of IGBT M4 is connected to the main control module, the gate of IGBT M5 is connected to the main control module, the gate of IGBT M6 is connected to the main control module, and the gate of IGBT M7 is connected to the main control module.
[0016] As a further embodiment of this utility model: the mains power module includes a transformer T1, relays RE1 and RE2, resistors R7, R8, R11, and R12, and a transformer T4. The third terminal of relay RE1 is connected to the second terminal of transformer T1. The first terminal of transformer T1 is connected to an H-bridge or bridgeless PFC module and a load. The third and fourth terminals of transformer T1 feed back the mains voltage signal to the main control module. The fourth terminal of relay RE1 is connected to the live wire L and one end of resistor R7. The other end of resistor R7 is connected to the second terminal of transformer T4 in sequence through resistors R8, R11, and R12. The first terminal of transformer T4 is connected to the neutral wire N and the third terminal of relay RE2. The fourth terminal of relay RE2 is grounded. The third and fourth terminals of transformer T4 feed back the mains voltage signal to the main control module.
[0017] Compared with the prior art, the beneficial effects of this utility model are: This utility model detects the output load demand current and automatically supplements the current exceeding the mains power supply. It transmits a synchronous voltage signal to the main control module through physical electrical connection, thereby dynamically adjusting the inverter's working state (inverting or charging). When the load demand is greater than the current that the mains can provide, it draws power from the inverter. When the load demand is less than the current that the mains can provide, it charges the battery. In addition, for loads with large starting current and small operating current, the battery supplies power during startup and charges the battery after startup. Attached Figure Description
[0018] Figure 1 This is the circuit diagram of the main control module.
[0019] Figure 2 This is the circuit diagram for the filter module and the buck-boost module.
[0020] Figure 3 This is a circuit diagram for an H-bridge or bridgeless PFC module.
[0021] Figure 4 This is the circuit diagram for the mains power module. Detailed Implementation
[0022] The technical solutions of the present utility model will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present utility model. Obviously, the described embodiments are only some embodiments of the present utility model, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present utility model without creative effort are within the protection scope of the present utility model.
[0023] Please see Figures 1 to 4 A current-compensating bidirectional inverter circuit includes:
[0024] The main control module is used to control the conduction state of the switching transistors of the buck-boost module, H-bridge or bridgeless PFC module, thereby controlling the voltage conversion. It also receives the sampled voltages of the buck-boost module, H-bridge or bridgeless PFC module and mains module, adjusts the conduction state of the switching transistors, and controls whether the relays of the mains module are working. Chip U1 adjusts the output based on feedback voltage signals, which is a common technology, so it does not involve any innovation in the method.
[0025] The filter module is used to smooth the current during battery charging or discharging;
[0026] The buck-boost module is used to boost the battery discharge voltage to 400V after passing through the filter module when the battery is discharging, and to supply the H-bridge or bridgeless PFC module. When the battery is charging, it is used to step down the voltage output by the H-bridge or bridgeless PFC module and then pass it through the filter module to charge the battery.
[0027] H-bridge or bridgeless PFC modules are used as H-bridge circuits during battery discharge to convert 400V DC to 220V AC to supply the load (at which time the mains current is insufficient to meet the load's power requirements). During battery charging, they are used as bridgeless PFC circuits to convert part of the 220V AC mains current (at which time the mains current, while meeting the load's power requirements, still has excess current) to 400V DC, which then supplies power to the battery through buck-boost circuits and filtering modules.
[0028] The AC power module is used to supply 220V AC AC power.
[0029] The filter module is connected to the buck-boost module, the buck-boost module is connected to the H-bridge or bridgeless PFC module, the H-bridge or bridgeless PFC module is connected to the mains power module, and the main control module is connected to the buck-boost module, the H-bridge or bridgeless PFC module, and the mains power module.
[0030] In this embodiment: Please refer to Figure 1 The main control module includes chip U1, model TMS320F280039. Pins 74, 75 (or 32, 36), 88 to 93 of chip U1 are connected to the buck-boost module. Pins 58, 59 (or 30, 31), 66 to 69 of chip U1 are connected to the H-bridge or bridgeless PFC module. Pins 20, 21, 39, and 40 of chip U1 are connected to the AC power module.
[0031] The main control module's integrated control circuit is working.
[0032] In this embodiment: Please refer to Figure 2 The step-up / step-down module includes MOSFETs M8, M9, M10, M11, M12, M15, M16, M17, M18, and M19, a transformer T2, capacitors E3 and E4, and resistors R1 and R4. The first terminal of transformer T2 is connected to the drain (D) terminals of MOSFETs M10, M11, and M12. The second terminal of transformer T2 is connected to... In the filter module, the third terminal of transformer T2 is connected to the drain (D) terminals of MOSFETs M17, M18, and M19. The source (S) terminals of MOSFETs M10, M11, M12, M17, M18, and M19 are grounded. The fourth terminal of transformer T2 is connected to the source (S) terminal of MOSFET M9 and the drain (D) terminal of MOSFET M15. The fifth terminal of transformer T2... Connect the source (S) of MOSFET M8 to the drain (D) of MOSFET M16. Ground the source (S) of MOSFET M15 and the source (S) of MOSFET M16. Connect the drain (D) of MOSFET M8 to the drain (D) of MOSFET M9. Connect one end of capacitor E3, one end of capacitor E4, one end of resistor R1, and the H-bridge or bridgeless PFC module. Connect the other end of resistor R1 to the main control module. Ground the other ends of capacitors E3 and E4. Ground one end of resistor R4 and connect the other end of resistor R4 to the main control module. The gate (G) of MOSFET M8, the gate (G) of MOSFET M9, the gate (G) of MOSFET M10, the gate (G) of MOSFET M11, the gate (G) of MOSFET M12, the gate (G) of MOSFET M15, the gate (G) of MOSFET M16, the gate (G) of MOSFET M17, the gate (G) of MOSFET M18, and the gate (G) of MOSFET M19 are connected to the main control module.
[0033] The battery power supply is connected to BAT+1 / BAT-1. The power supply passes through a filter module (capacitors C6, C7, C15, C14, E1, E2, C11, C12, C13, C9), and after filtering, it supplies the buck-boost module to boost the voltage to 400V. Resistors R1 and R4 are sampling resistors for detecting the 400V voltage, which are fed back to pins 74 and 75 (or 32 and 36) of chip U1. Based on the detected voltage, chip U1 adjusts the duty cycle and switching state of MOSFETs M8, M9, M10, M11, M12, M15, M16, M17, M18, and M19 to achieve a 400V voltage output. The DC 400V is then output as a stable current to the H-bridge or bridgeless PFC module after passing through capacitors E3 and E4.
[0034] In this embodiment: Please refer to Figure 3 The H-bridge or bridgeless PFC module includes IGBTs M4, M5, M6, and M7, inductor L3, resistor R2, resistor R3, and capacitor C4. The collector of IGBT M4 is connected to the collector of IGBT M5 and the buck-boost module. The emitter of IGBT M4 is connected to the collector of IGBT M6 and one end of inductor L3. The emitter of IGBT M6 is grounded. The other end of inductor L3 is connected to one end of resistor R3 and one end of capacitor C4. The AC power module is connected to the main control module. The other end of capacitor C4 is grounded, and the other end of resistor R3 is connected to the main control module. The emitter of IGBT M5 is connected to the collector of IGBT M7. One end of resistor R2 and the other end of capacitor C4 are connected to the main control module. The other end of resistor R2 is connected to the main control module. The emitter of IGBT M7 is grounded. The gates of IGBT M4, M5, M6, and M7 are connected to the main control module.
[0035] With a 400V DC input, the H-bridge or bridgeless PFC module operates as an H-bridge circuit. Chip U1 controls the duty cycle of IGBTs M4 and M6, and the switching frequency of IGBTs M5 and M7, outputting 220V AC. Inductor L3 and capacitor C4 filter out noise from the 220V sinusoidal AC voltage. Simultaneously, the voltage passing through resistors R2 and R3 is used as a sampling voltage and fed back to chip U1, thereby adjusting the duty cycle of IGBTs M4 and M6, and the switching frequency of IGBTs M5 and M7 to maintain the 220V AC output.
[0036] After the 220V AC power is filtered out for interference by the EMI inductor L4, it supplies power to the load through the current transformer T3. At the same time, the magnitude of the output current to the load (combined power supply from the mains and the battery) is obtained through R9 and R10 and fed back to the chip U1 to realize the output current detection. If an abnormality is detected, the chip U1 controls the IGBT transistors M4, M5, M6 and M7 to turn off, realizing over-power / short-circuit protection.
[0037] In this embodiment: Please refer to Figure 4 The mains power module includes transformer T1, relays RE1 and RE2, resistors R7, R8, R11, and R12, and transformer T4. The third terminal of relay RE1 is connected to the second terminal of transformer T1. The first terminal of transformer T1 is connected to an H-bridge or bridgeless PFC module and the load. The third and fourth terminals of transformer T1 feed back the mains voltage signal to the main control module. The fourth terminal of relay RE1 is connected to the live wire L and one end of resistor R7. The other end of resistor R7 is connected to the second terminal of transformer T4 in sequence through resistors R8, R11, and R12. The first terminal of transformer T4 is connected to the neutral wire N and the third terminal of relay RE2. The fourth terminal of relay RE2 is grounded. The third and fourth terminals of transformer T4 feed back the mains voltage signal to the main control module.
[0038] When AC mains power is input, resistors R7, R8, R11, and R12 sample and divide the AC power, and the transformer T4 feeds back the AC input voltage signal. After the value, frequency, and phase of the AC input voltage are detected by pins 20 and 21 of chip U1 and are found to be normal, the relays RE1 and RE2 are activated, and AC mains power is output. The AC output current signal is detected by current transformer T1 and fed back to pins 39 and 40 of chip U1 to monitor the AC output current in real time. The AC output current and the battery output current together power the load.
[0039] The above describes the battery output current; the following section will discuss battery charging.
[0040] When pins 22 and 23 of chip U1 detect that the current required by the load is less than the mains output current through resistors R9 and R10, the excess current simultaneously supplies power to the H-bridge or bridgeless PFC module through EMI inductor L4. At this time, the H-bridge or bridgeless PFC module acts as a bridgeless PFC circuit. After being boosted by the bridgeless PFC circuit, the voltage is then passed through capacitors E3 and E4 to power the buck-boost module. At this time, the buck-boost module acts as a buck circuit, and after being stepped down, it charges the battery, realizing battery charging and mains bypass output.
[0041] In the H-bridge circuit's buck mode, IGBT M4, inductors L3 and M7 complete the positive half-cycle buck waveform output of a sine wave, while IGBT M6, inductors L3 and M5 complete the negative half-cycle buck waveform output. These two waveforms are output alternately to complete the sine wave output. IGBTs M4 and M6 operate in PWM pulse width adjustment mode, while IGBTs M7 and M5 operate in switching mode. The switching frequency depends on the frequency required for the inverter output. In the bridgeless PFC circuit's boost mode, inductors L3, M6, M4, and M7 complete the positive half-cycle boost of the input sine wave. Inductor L3 stores energy, IGBT M6 performs PWM adjustment, IGBT M4 performs synchronous rectification, and IGBT M7 switches the circuit from IGBT M6 to the input neutral (N) line. Inductor L3, IGBT M4, IGBT M6, and IGBT M5 complete the boost of the negative half-cycle of the input sine wave. Inductor L3 stores energy, IGBT M4 performs PWM adjustment, IGBT M6 performs synchronous rectification, and IGBT M5 switches the circuit from IGBT M4 to the input L line.
[0042] The working principle of this utility model is as follows: The main control module controls the conduction state of the switching transistors of the buck-boost module, H-bridge or bridgeless PFC module, thereby controlling voltage conversion. It also receives sampled voltages from the buck-boost module, H-bridge or bridgeless PFC module, and mains module, adjusts the conduction state of the switching transistors, and controls whether the relays of the mains module operate. The filter module smooths the current during battery charging or discharging. The buck-boost module boosts the battery discharge voltage to 400V after passing through the filter module to supply the H-bridge or bridgeless PFC module during battery discharge. During battery charging, it outputs the voltage from the H-bridge or bridgeless PFC module. The voltage is stepped down and then filtered to charge the battery. The H-bridge or bridgeless PFC module is used as an H-bridge circuit when the battery is discharging to convert 400V DC to 220V AC to supply the load (at this time, the mains current is insufficient to meet the load's power requirements). When the battery is charging, it is used as a bridgeless PFC circuit to convert part of the 220V AC mains current (at this time, the mains current has excess current while meeting the load's power requirements) to 400V DC, which is then used to supply power to the battery through a step-up / step-down circuit and a filter module. The mains module is used to supply 220V AC mains power.
[0043] It will be apparent to those skilled in the art that this invention is not limited to the details of the exemplary embodiments described above, and that it can be implemented in other specific forms without departing from the spirit or essential characteristics of this invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects.
[0044] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.
Claims
1. A current-compensating bidirectional inverter circuit, characterized in that, The current-compensating bidirectional inverter circuit includes: The main control module is used to control the conduction state of the switching transistors of the buck-boost module, H-bridge or bridgeless PFC module, thereby controlling the voltage conversion. It also receives the sampled voltages of the buck-boost module, H-bridge or bridgeless PFC module and mains module, adjusts the conduction state of the switching transistors, and controls whether the relays of the mains module are working. The filter module is used to smooth the current during battery charging or discharging; The buck-boost module is used to boost the battery discharge voltage to 400V after passing through the filter module when the battery is discharging, and to supply the H-bridge or bridgeless PFC module. When the battery is charging, it is used to step down the voltage output by the H-bridge or bridgeless PFC module and then pass it through the filter module to charge the battery. H-bridge or bridgeless PFC modules are used as H-bridge circuits to convert 400V DC to 220V AC to supply the load when the battery is discharging. They are also used as bridgeless PFC circuits when the battery is charging to convert part of the 220V AC AC to 400V DC while meeting the power demand of the load. The DC power is then supplied to the battery through a step-up / step-down circuit and a filter module. The AC power module is used to supply 220V AC AC power. The filter module is connected to the buck-boost module, the buck-boost module is connected to the H-bridge or bridgeless PFC module, the H-bridge or bridgeless PFC module is connected to the mains power module, and the main control module is connected to the buck-boost module, the H-bridge or bridgeless PFC module, and the mains power module.
2. The bidirectional inverter circuit for compensating current according to claim 1, characterized in that, The main control module includes chip U1, model number TMS320F280039. Pins 74, 75, 88 to 93 of chip U1 are connected to the buck-boost module, pins 58, 59, 66 to 69 of chip U1 are connected to the H-bridge or bridgeless PFC module, and pins 20, 21, 39, and 40 of chip U1 are connected to the AC power module.
3. The bidirectional inverter circuit for compensating current according to claim 2, characterized in that, The step-up / step-down module includes MOSFETs M8, M9, M10, M11, M12, M15, M16, M17, M18, and M19, transformer T2, capacitors E3 and E4, and resistors R1 and R4. The first terminal of transformer T2 is connected to the drains (D) of MOSFETs M10, M11, and M12. The second terminal of transformer T2 is connected to a filter. In the wave module, the third terminal of transformer T2 is connected to the drain (D) terminals of MOSFETs M17, M18, and M19. The source (S) terminals of MOSFETs M10, M11, M12, M17, M18, and M19 are grounded. The fourth terminal of transformer T2 is connected to the source (S) terminal of MOSFET M9 and the drain (D) terminal of MOSFET M15. The fifth terminal of transformer T2... Connect the source (S) of MOSFET M8 to the drain (D) of MOSFET M16. Ground the source (S) of MOSFET M15 and the source (S) of MOSFET M16. Connect the drain (D) of MOSFET M8 to the drain (D) of MOSFET M9. Connect one end of capacitor E3, one end of capacitor E4, one end of resistor R1, and the H-bridge or bridgeless PFC module. Connect the other end of resistor R1 to the main control module. Ground the other ends of capacitors E3 and E4. Ground one end of resistor R4 and connect the other end of resistor R4 to the main control module. The gate (G) of MOSFET M8, the gate (G) of MOSFET M9, the gate (G) of MOSFET M10, the gate (G) of MOSFET M11, the gate (G) of MOSFET M12, the gate (G) of MOSFET M15, the gate (G) of MOSFET M16, the gate (G) of MOSFET M17, the gate (G) of MOSFET M18, and the gate (G) of MOSFET M19 are connected to the main control module.
4. The bidirectional inverter circuit for compensating current according to claim 2, characterized in that, The H-bridge or bridgeless PFC module includes IGBTs M4, M5, M6, and M7, inductor L3, resistors R2 and R3, and capacitor C4. The collector of IGBT M4 is connected to the collector of IGBT M5 and the buck-boost module. The emitter of IGBT M4 is connected to the collector of IGBT M6 and one end of inductor L3. The emitter of IGBT M6 is grounded. The other end of inductor L3 is connected to one end of resistor R3, one end of capacitor C4, and the AC power module. The other end of capacitor C4 is grounded. The other end of resistor R3 is connected to the main control module. The emitter of IGBT M5 is connected to the collector of IGBT M7, one end of resistor R2, and the other end of capacitor C4. The other end of resistor R2 is connected to the main control module. The emitter of IGBT M7 is grounded. The gates of IGBTs M4, M5, M6, and M7 are connected to the main control module.
5. The bidirectional inverter circuit for compensating current according to claim 2, characterized in that, The mains power module includes transformer T1, relays RE1 and RE2, resistors R7, R8, R11, and R12, and transformer T4. The third terminal of relay RE1 is connected to the second terminal of transformer T1. The first terminal of transformer T1 is connected to an H-bridge or bridgeless PFC module and the load. The third and fourth terminals of transformer T1 feed back the mains voltage signal to the main control module. The fourth terminal of relay RE1 is connected to the live wire L and one end of resistor R7. The other end of resistor R7 is connected to the second terminal of transformer T4 in sequence through resistors R8, R11, and R12. The first terminal of transformer T4 is connected to the neutral wire N and the third terminal of relay RE2. The fourth terminal of relay RE2 is grounded. The third and fourth terminals of transformer T4 feed back the mains voltage signal to the main control module.