Under the condition that the electrical technical indicators meet the requirements for normal use, in order to enable the components of the power supply to work safely and reliably in various harsh environments, a protection circuit must be designed. In the process of designing a protection circuit, how to select the parameters of components is the key to the design. If the parameter selection is unreasonable, the protection circuit will affect the performance of the power supply and even damage the device. Therefore, selecting reasonable parameters for the protection circuit plays a vital role in the reliability of the power supply.

Aiming at the links that affect the reliability of switching power supplies, this article introduces in detail the design of anti-surge soft-start circuits, instantaneous overvoltage suppression circuits, and elimination of transformer DC bias circuits, and provides calculation methods for the selection of components in the protection circuits. . The protection circuit introduced in this article is specifically designed for arc welding power sources with an output no-load voltage of 70V, an output current of 160A, a frequency of 20kHz, and a rated power of 6kW.

Circuit structure of arc welding power supply

The digital arc welding power supply consists of two parts: the main circuit and the control circuit. Among them, the main circuit is composed of rectifier link, filter link, inverter link, transformer rectifier filter link and other parts. The structure of the main loop is shown in Figure 1.

The rectifier part uses a three-phase full-wave rectifier module, and the filter part uses two sets of parallel and two sets of series-connected power frequency filter capacitors. The filtered DC power is sent to the input end of the inverter module. The inverter module adopts intelligent IpM module. From the perspective of circuit form, IpM has the same structure as a full-bridge inverter. The driver drives two diagonal components to conduct at the same time, interleaving the input voltage to the primary of the high-frequency transformer, and the output can be adjusted by changing the duty cycle. Voltage. The output of the high-frequency transformer is rectified and filtered by diodes and reactors to output stable DC.

The DC output voltage Ud after power frequency rectification is 537V. The maximum output current I0=160A. Since the structure of two transformers is connected in series, the secondary output current of each transformer is Id=Io/2, then the input current of the primary side of the transformer is I=N2/N1×Id≈1/5×80=16(A). The input voltage on the side is V=Ud/2≈270V, and the current on the AC side of the rectifier bridge is:

  (1)

Design of arc welding power supply protection circuit

1 Design of anti-surge soft start circuit

The input of the power supply is a capacitor input type, that is, a capacitor is used to filter the DC input. Therefore, once AC ripple is added, current will flow through the capacitor. When the three-phase input current of the power supply is turned on, since the initial voltage on the capacitor is zero, the capacitor will form a large surge current at the moment of charging. Especially for high-power switching power supplies that use larger-capacity filter capacitors, the surge current will reach more than 100A. Such a large surge current occurs at the moment when the power is turned on. In severe cases, it will often cause the input fuse to blow out or the contacts of the closing switch to burn out, causing overcurrent damage to the rectifier bridge; in mild cases, it will also cause the air switch to spark. , the gate cannot be closed. For this reason, a soft-start circuit to prevent surge current must be set up to ensure normal and reliable operation of the power supply.

The value of the surge current increases with the increase of the input voltage, and reaches the maximum value when the input voltage phase on the AC side reaches 90o. Using capacitors for filtering usually results in the peak value Iacp of the input current being approximately 3 to 4 times that of Iac. If the surge current can be effectively suppressed, the surge current can be suppressed to less than 5 times the AC input Iac. However, if the surge current is suppressed excessively, the time it takes for the capacitor to fully charge will increase, oscillation will occur before charging is completed, and secondary inrush current will flow. Therefore, the selection of the resistor in the surge suppression circuit is very important. The soft start circuit is shown in Figure 2.

Figure 2 Input impulse suppression circuit

According to the calculation of equation (1), the AC input current Iac=14A, then the surge current can be suppressed by 4 times the AC input I'=1/4×Iac=3.5A. The input phase voltage is 220V, then the input phase The peak voltage Eip is 311V.

The required resistance value is

R=Eip/Iac=89Ω(2)

The instantaneous power of the resistor is

pR=(Eip)2/R=1087W(3)

The instantaneous overpower of the resistor is large. In order to ensure that the resistor can effectively suppress the surge current, a wire-wound cement resistor should be selected. Its resistance to instantaneous overpower can be as high as 100 to 400 times the rated power. Here, you can choose a current limiting resistor with a resistance of 100Ω and a cement resistor with a power of 10W.

2 DC bias elimination circuit

The principle of the full-bridge inverter is shown in Figure 3.

Figure 3 DC bias suppression circuit

The driver drives two diagonal components to be turned on at the same time. Switch tubes in the same phase cannot be turned on at the same time, otherwise the power supply will be short-circuited. Therefore, the two sets of trigger pulses should have a dead time that is both at a low level, and the dead time must be greater than the longest conduction saturation delay turn-off time of the switching tube. In Figure 3, when T1, T4 and T2, T3 are turned on alternately, the potentials of points a and b float according to the turn-on of the switch tube. If the switching tubes have different switching characteristics, then under the same base pulse width, the voltage waveforms at the a and b contacts will be affected, as shown in Figure 4.

The coupling capacitor C and the reactor at the output end form a series resonant circuit, and its resonant frequency is

(4)

Among them, LR is the secondary inductance value converted to the primary side of the transformer.

In order to make the charging of the coupling capacitor linear, the resonant frequency must be lower than the switching frequency of the inverter. In the design, the resonant frequency is taken to be 1/4 of the inverter switching frequency. According to formula (4), the capacitance value can be calculated as

(5)

The capacitor charges or discharges once every half cycle, and the charging voltage is V. When the charging voltage of the capacitor is V with reverse polarity, if the voltage is too large, it will affect the regulation rate of the inverter voltage.

The charging voltage of the capacitor

(6)

Among them, I is the average current of the primary side of the transformer, and Δt is the capacitor charging time interval.

According to equation (6), the charging voltage of the capacitor VC> (10% ~ 20%) V is calculated. Through calculation, it can be seen that the value of VC is too large, which will have an adverse effect on the inverter, so the capacitor value must be re-determined. Here, we determine the value of the coupling capacitor to be 4µF.

3 Design of instantaneous overvoltage suppression circuit

The full-wave rectifier of the pWM modulated full-bridge circuit is shown in Figure 5. D1 and D2 are fast recovery diodes.

The output voltage of the secondary side of the transformer is Vs, then the diodes D1 and D2 bear a reverse voltage of 2Vs when they are cut off. Since the leakage inductance of the high-frequency transformer and the interjunction capacitance of the rectifier form a resonant circuit when it is turned off, instantaneous overvoltage oscillation breaks down the diode, causing the output of the power supply to be short-circuited. Therefore, an RC buffer circuit should be installed in the output part of the power supply to protect the fast recovery diode and improve the reliability of the circuit. For power supplies with large current output, the buffer RC should be set at both ends of each rectifier tube. The design of the buffer must not only protect the diode, but also minimize losses.

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