Detailed explanation of the circuit principle of half-bridge soft switching inverter welding machine

This is a new type of half-bridge soft switching inverter technology, which can make the inverter switch device work under soft turn-on and soft turn-off conditions. Its switch voltage stress and current stress are greatly reduced, the switch loss is also greatly reduced, the device heat generation is greatly reduced, and the electromagnetic interference amplitude is also greatly reduced. Due to the use of half-bridge, the device cost is also reduced accordingly.

To achieve the above purpose, the "half-bridge soft-switch inverter welding machine" includes: an input filter circuit, a primary-side rectifier filter circuit, a half-bridge soft-switch inverter circuit, an isolation transformer and a secondary-side rectifier filter circuit and a main control board circuit, which are connected in sequence according to the direction of the electric power flow of the equipment. The main control board circuit is connected to both the secondary rectifier filter circuit and the half-bridge soft-switch inverter circuit.

As shown in Figure 1: "Half-bridge soft switch inverter welding machine" includes: input filter circuit 1, primary side rectifier filter circuit 2, half-bridge soft switch inverter circuit 3, isolation transformer 4 and secondary side rectifier filter circuit 5 and main control board circuit 6, which are connected in sequence according to the power flow direction of the equipment. The main control board circuit 6 is connected to both the secondary rectifier filter circuit 5 and the half-bridge soft switch inverter circuit 3.

The composition and interconnection relationship of each circuit in Figure 1 are shown in Figure 2.

As shown in Figure 2:

The input filter circuit 1 is composed of a power switch S1, differential mode filter capacitors C27 and C28, common mode filter capacitors C29, C30, C31, C32 and common mode filter inductor L1. The grid interference signal is filtered out by the above filter, so that the welding machine is protected from external electromagnetic interference and the stability is improved; similarly, the interference signal generated by the welding machine will also be filtered out by the above filter, so that the welding will not generate electromagnetic interference to the outside world, and the stability of other equipment is improved.

The primary side rectifier filter circuit 2 is composed of a rectifier bridge BR1 and capacitors C34 and C35. The AC voltage and current sent into the machine are rectified into DC voltage and current by the rectifier bridge BR1, and then sent to the half-bridge soft switch inverter circuit 3 after filtering by capacitors C34 and C35.

The half-bridge soft-switching inverter circuit 3 is composed of two groups of insulated gate field effect power switch devices Q1 and Q2 connected in series in a forward direction, and another two groups of insulated gate field effect power switch devices Q01 and Q02 connected in series in a reverse direction to form an auxiliary switch circuit. R48, R49, R54, and R55 are respectively gate series drive resistors of the four insulated gate field effect power switch devices, and R50 and C38; R51 and C39 are respectively parallel resistor-capacitor absorption circuits of the two poles (D and S poles for MOSFET devices, C and E poles for IGBT devices, and A and K poles for MCT devices) of the two insulated gate field effect power switch devices Q1 and Q2 of the half-bridge main inverter circuit. After the two groups of insulated gate field effect power switch devices Q01 and Q02 of the auxiliary switch circuit are connected in series in a reverse direction, one side is connected to the midpoint of the half-bridge bridge arm formed by the two groups of insulated gate field effect power switch devices Q1 and Q2 connected in series, and the other side is connected to the midpoint of the DC bus series resonant capacitors C36 and C37. The insulated gate field effect power switch devices Q01, Q02, resonant capacitors C36, C37 and saturated inductor L2 form a soft switch auxiliary resonant circuit. This ensures that the main switches Q1 and Q2 are turned on with zero current and turned off with zero voltage. Capacitors C40 and C39 are bridge arm capacitors, and their capacity is large enough so that each time the main switches Q1 and Q2 are turned on, the potential at point C is basically maintained at half of the DC bus voltage. Inductor L3 is a reactive power inductor, and its function is to ensure that the circuit can still meet the soft switching conditions when the welding power supply is unloaded.

The non-modulated (fixed-width) drive pulse signal with a phase difference of 180 degrees output from socket A1 is sent to two groups of insulated gate field effect power switching devices Q01 and Q02; the PWM (modulated-width) drive pulse signal with a phase difference of 180 degrees output from socket A2 is sent to two groups of insulated gate field effect power switching devices Q1 and Q2.

The isolation transformer circuit 4 is provided by a medium frequency transformer T5 having a primary winding and a secondary winding. One end of the primary winding of the medium frequency transformer T5 is connected to the midpoint C of the capacitor bridge arm through the saturated inductor L2, and the other end is connected to the midpoint B of the half-bridge bridge arm of the main inverter circuit after passing through the primary current transformer T4. The secondary side is connected to the secondary side rectifier filter circuit. The primary winding and the secondary winding are safely insulated by insulating materials.

The secondary side rectifying and filtering circuit 5 is composed of fast recovery rectifying diodes D15 and D16, a filtering inductor L4, and RC absorption resistors R52 and R53 and RC absorption capacitors C42 and C43.

The main control circuit 6 is composed of internal current setting, current feedback, PWM pulse width modulation circuit, fixed-width complementary pulse signal circuit and isolation drive circuit.

Realization of half-bridge soft switching inverter function

Referring to FIG. 2, the main switches Q1 and Q2 of the insulated gate field effect power switch device are connected in series in a forward direction to form a half-bridge structure, while the auxiliary switches Q01 and Q02 of the insulated gate field effect power switch device are connected in series in a reverse direction, with one end connected to the midpoint of the series connection of Q1 and Q2, and the other end connected to the midpoint of the series capacitors C36 and C37. The auxiliary switches Q01 and Q02 are driven by a driving signal with a phase difference of 1800 and a non-modulated pulse width (fixed pulse width) sent from the main control board introduced by the socket A1, while the main switches Q1 and Q2 are driven by a driving signal (PWM signal) with a phase difference of 1800 and a modulated pulse width sent from the main control board introduced by the socket A2. The main switch Q1 and the auxiliary switch Q01 are driven to turn on at the same time, and then Q1 is turned off by PWM control, while Q01 is turned off with a fixed pulse width lag; then the main switch Q2 and the auxiliary switch Q02 are driven to turn on at the same time, and then Q2 is turned off by PWM control, while Q02 is turned off with a fixed pulse width lag. This repeated opening and closing forms the switching sequence logic conditions required for soft switching. T5 is an isolation transformer, T4 is a primary current transformer, L2 is a package inductor, and L4 is a secondary smoothing inductor.

The circuit works as follows:

When the last cycle just ends, capacitor C36 has completely discharged its charge and the terminal voltage is zero; capacitor C37 has been fully charged and the terminal voltage is the DC bus voltage U.

At the beginning of this cycle, first, when the main switch Q1 and the auxiliary switch Q01 are turned on, the current will flow along "+" → "Q1" → "B" → "T4 primary side" → "T5 primary side" → "L2" → "C". Transformer T5 transfers the electric energy to the secondary side, the secondary side rectifier diode D16 is turned on, and the inductor L4 stores energy. When the main switch Q1 is turned on, due to the effect of the saturated inductor L2, the current flowing through the saturated inductor L2 and the main switch Q1 will rise linearly from zero, so the main switch Q1 is turned on with zero current. After the main switch Q1 is turned on, the voltage at point B is equal to the voltage U of the DC bus "+" (ignoring the conduction voltage drop of the main switch Q1), so there is no voltage at both ends of the auxiliary switch Q01, and no current flows, so the auxiliary switch Q01 is turned on with zero voltage and zero current. Afterwards, the main switch Q1 will be PWM turned off. Since the current of the secondary side inductor L4 cannot change suddenly, the current of the secondary side rectifier diode D16 will be gradually diverted to the rectifier diode D15. Finally, the two groups of rectifier diodes are turned on at the same time, and the primary and secondary sides of the intermediate frequency transformer T5 are short-circuited.

After the main switch Q1 is turned off, the current in the saturated inductor L2, the leakage inductance of the intermediate frequency transformer T5, and the distributed inductance of the inverter circuit cannot change suddenly, and continues to flow along the path of "A" → "Q01" → "Q02 internal diode" → "B" → "T4 primary side" → "T5 primary side" → "L2" → "C". The capacitor C36 is charged linearly, the capacitor C37 is discharged linearly, and the voltages at points A and B drop slowly. The voltage across the main switch Q1 rises linearly from zero, so the main switch Q1 is turned off at zero voltage. As time goes by, the capacitor C36 is charged with the bus voltage with a voltage value of U, and the capacitor C37 is discharged to a voltage value of zero. At this time, the current transformation path of the saturated inductor L2, the leakage inductance of the transformer T5, and the distributed inductance of the inverter circuit continues to flow along "-" → "Q2 internal diode" → "B" → "T4 primary side" → "T5 primary side" → "L2" → "C", and the auxiliary switch Q01 is turned off at this time. It can be seen that Q01 is turned off at zero voltage and zero current.

After this cycle ends, capacitor C37 has been fully charged and the terminal voltage is zero; capacitor C36 has been fully charged and the terminal voltage is the DC bus voltage U.

Then the next cycle begins, the main switch Q2 and the auxiliary switch Q02 are turned on at the same time, and the current will follow "C" → "L2" → "T5 primary side" → "T4 primary side" → "B" → "Q2" → "-". Transformer T5 transfers the electric energy to the secondary side, the secondary side rectifier diode D15 is turned on, and the inductor L4 stores energy. When the main switch Q2 is turned on, due to the effect of the saturated inductor L2, the current flowing through the saturated inductor L2 and the main switch Q2 will rise linearly from zero, so the main switch Q2 is turned on with zero current. After the main switch Q2 is turned on, the voltage at point B is equal to the voltage of the DC bus "-" (ignoring the conduction voltage drop of the main switch Q2), so there is no voltage across the auxiliary switch Q02, and no current flows, so the auxiliary switch Q02 is turned on with zero voltage and zero current. Afterwards, the main switch Q2 will be PWM turned off. Since the current of the secondary side inductor L4 cannot change suddenly, the current of the secondary side rectifier diode D15 will be gradually diverted to the rectifier diode D16. Finally, the two groups of rectifier diodes are turned on at the same time, and the primary and secondary sides of the intermediate frequency transformer T5 are short-circuited.

After the main switch Q2 is turned off, the current in the saturated inductor L2, the leakage inductance of the intermediate frequency transformer T5, and the distributed inductance of the inverter circuit cannot change suddenly, and continues to flow along the path of "C" → "L2" → "T5 primary side" → "T4 primary side" → "B" → "Q02" → "Q01 internal diode" → "A", capacitor C37 is linearly charged, capacitor C36 is linearly discharged, and the voltage at points A and B rises slowly. The voltage across the main switch rises linearly from zero, so the main switch Q2 is turned off by zero voltage. As time goes by, capacitor C37 is charged with the bus voltage with a voltage value of U, and capacitor C36 is discharged to a voltage value of zero. At this time, the current transformation path of the saturated inductor L2, the leakage inductance of the transformer T5, and the distributed inductance of the inverter circuit continues to flow along "C" → "L2" → "T5 primary side" → "T4 primary side" → "B" → "Q1 internal diode" → "+", and the auxiliary switch is turned off at this time. It can be seen that Q02 is turned off by zero voltage and zero current.

This cycle repeats itself to achieve the half-bridge soft switching inverter function. It can be seen that the two groups of main switches operate in the state of zero current turn-on and zero voltage turn-off, realizing the soft switching function of the main switch, reducing the voltage and current stress of the main switch, reducing the voltage and current change rate during switching that causes electromagnetic interference, and reducing the heat generated by the switching loss of the main switch device. At the same time, the auxiliary switches used to collaboratively create soft switching conditions operate in the zero voltage, zero current turn-on and zero voltage, zero current turn-off states. Therefore, the two groups of auxiliary switches only bear very small switching voltage and current stress, causing electromagnetic interference and heat generated by switching losses are very small. Implementation of the drive pulse circuit that meets the half-bridge soft switching inverter function

See Figure 3

U1 is a current-mode PWM integrated circuit, whose pin 1 is the soft-start terminal, and the external voltage-dividing resistors R13, R26 and capacitor C14 form a soft-start timing circuit; pin 2 is a 5.1V internal reference voltage regulator; pins 3 and 12 are connected to the power ground; pin 4 is the primary side pulse current signal input terminal; pin 5 is the error signal voltage input terminal, and pins 5, 6 and 7 are internally an op amp circuit, pin 5 is the op amp input in-phase terminal, pin 6 is the op amp circuit inverting terminal, and pin 7 is the op amp output terminal. Pins 6 and 7 are connected, and the internal op amp is connected to form an emitter follower with pin 5 as the input terminal; the external capacitor C17 at pin 8 is a PWM fixed-frequency capacitor; the external resistor R31 at pin 9 is a PWM fixed-frequency resistor; pin 10 is the synchronization signal output terminal; pins 11 and 14 are two complementary output terminals of the PWM pulse signal; pins 13 and 15 are the power supply terminals; and pin 16 is the pulse shutdown terminal. The complementary PWM pulse signals with a phase difference of 1800 output from pins 11 and 14 are sent to the power amplifier drive circuit composed of MOSFET tubes M5, M6, M7, and M8, and then isolated by the isolation drive transformer T2. They are divided into two groups and shaped by diodes D5, D13, resistors R22, R41, R42, R43, and capacitors C22 and C23, and then sent to the gates of the main switches Q1 and Q2 through socket A2.

The pulse width of this set of PWM signals is variable, and its pulse width is continuously adjusted and changed according to the error between the given welding current and the actual welding output current.

The sawtooth wave signal taken out from the 8th pin of the PWM integrated circuit U1 and amplified by the emitter follower of the integrated circuit U2B; the pulse synchronization signal taken out from the 10th pin of the PWM integrated circuit U1; the PWM pulse signal taken out from the 11th and 14th pins and the amplified and regulated error signal sent from the PI regulator are sent to the pulse phase-locked and frequency-dividing circuit composed of the integrated circuits U3A, U3B, U4A, U4B, U5A, U5B, U6 and U7 and the resistors R1, R2, R3, R4, R11, R12, R16 and capacitor C1, etc., and its output is isolated by the isolation drive transformer T1 after passing through the power amplifier drive circuit composed of the MOSFET tubes M1, M2, M3, and M4, and is divided into two groups, which are shaped by the diodes D1, D6, resistors R9, R10, R21, R23, and capacitors C2 and C3, and then sent to the gates of the auxiliary switches Q01 and Q02 through the socket A1. The pulse width of this group of drive signals with a phase difference of 1800 is fixed.

In the pulse phase-locked and frequency-dividing circuit, U4A and U4B are D-type flip-flops with RS trigger terminals; U5A and U5B are four-input OR gates; U6 and U7 are co-directional amplifiers (the timing circuit NE555 is used here).

These two sets of driving pulse signals make the main switch Q1 and the auxiliary switch Q01 turn on at the same time, Q1 PWM turns off, and Q01 turns off with a fixed pulse width hysteresis; the main switch Q2 and the auxiliary switch Q02 turn on at the same time, Q2 PWM turns off, and Q02 turns off with a fixed pulse width hysteresis. In this way, a reasonable driving pulse signal is provided for realizing half-bridge soft switching.

Implementation of other welding functions

1. Welding current setting and feedback, PWM regulation and welding current display:

The welding current setting circuit is composed of potentiometers RT2, RT3 (see Figure 2), integrated circuit U2D, resistors R36, R46, and potentiometer RT1. Among them, resistors R36 and R46 are equal in value, and they form an inverter with integrated circuit U2D to convert the positive given signal voltage into negative, and send it to the error comparison point E through potentiometer RT1.

Similarly, the positive current feedback signal collected on the shunt FL1 (see Figure 2) is sent to the error comparison point E through the resistor R45 after high-frequency filtering by the capacitor C25. The error signal is amplified and adjusted by the error amplifier composed of the integrated circuit U2C, the resistors R33, R44, the capacitors C19, C24, the diodes D14, ZD1, etc., and then sent to the 5th pin of the integrated circuit U1.

In addition, the current pulse signal on the primary side is collected by the mutual inductor T4 (see Figure 2), rectified by D9, D10, D11, and D12, and high-frequency filtered by C24. Finally, a pulse voltage signal with an amplitude proportional to the amplitude of the primary side pulse current is obtained on the sampling resistor R34. This signal is sent to the 16th pin of the integrated circuit U1 as an overcurrent shutdown signal after passing through the resistor-capacitor network composed of resistors R19, R18 and capacitor C7; the other signal is sent to the 4th pin of the integrated circuit U1 after passing through the resistor-capacitor network composed of resistors R29, R27, R28, R30 and capacitor C18. After the 4th pin and the sawtooth wave compensation signal are synthesized, the error signal sent from the 5th pin of the integrated circuit U1 is compared with the error signal sent from the inside of the integrated circuit U1 to generate a PWM pulse. After phase locking and frequency division through the internal circuit of the integrated circuit U1, complementary PWM signals are output from its 11th pin and 14th pin respectively. The sawtooth wave output from pin 8 of integrated circuit U1 is amplified by the emitter follower of integrated circuit U2B, and then passes through the resistor-capacitor network composed of resistors R24, R25 and capacitors C13, C15 to serve as a compensation sawtooth wave signal.

The welding current digital display circuit is composed of resistors R39, R40, capacitor C26 and DGM1 (see Figure 2).

2. Realization of undervoltage, overcurrent and overheat protection functions

The undervoltage protection circuit is composed of integrated circuit U2A and resistors R14 and R15. When the grid voltage is too low, the control circuit board +15V

When the voltage is insufficient, the integrated circuit U1 will output a high potential, which is guided by the diode D2. After the capacitor C6 filters out the interference, it is sent to the 16th pin of the integrated circuit U1 through the resistor R17, so the integrated circuit U1 turns off the PWM output; similarly, the signal is also sent to the 2nd pin of the integrated circuit U5A and the 12th pin of the integrated circuit U5B to turn off the drive signal of the auxiliary switch.

When the primary side current exceeds the set value, it is detected by transformer T4, rectified by rectifier diodes D9, D10, D11, and D12, filtered by capacitor C21, and the overcurrent signal is obtained on the sampling resistor R21. It is divided by R19 and R18, filtered by C7, and sent to pin 16 of integrated circuit U1, thereby shutting down the PWM output.

When the temperature of the power switch device of the inverter is too high for some reason, the temperature relay TS1 (see Figure 2) installed on the heat sink of the power switch device will be disconnected (normally it is in the normally closed state), the +5V voltage end of the potentiometer RT3 (see Figure 2) will lose voltage, the current setting voltage will be zero, and the welding machine will stop outputting current until the temperature drops.

The three-terminal integrated voltage stabilizing circuit U8 is composed of rectifier diodes D3, D4, D7, D8, filter capacitors C8, C9, C10, C11, C12,

U9, U10 and other components form a voltage-stabilized power supply circuit, which supplies power to the entire main control board and the welding current digital display meter. The industrial frequency AC transformer T3 (see Figure 2) is the power supply transformer for the entire control board circuit.