Realization of Switching Power Supply Protection Circuit for CNC

The reliability of the switching power supply directly affects the reliability of the electronic product system. Starting from the various protection circuits of the switching power supply of the relevant CNC system, this paper analyzes and designs the soft start circuit of the CNC switching power supply, the working principle and specific design method of the overvoltage protection, overcurrent protection, undervoltage protection and other circuits, solves the problem of the working reliability of the CNC switching power supply, and provides a guarantee for the mass production of the CNC system. The CNC system of precision machine tools has a stable guarantee for the accuracy of the processed parts, and is being used more and more widely in mechanical processing.

introduction

The switching power supply circuit is responsible for providing power to all parts of the entire machine tool CNC system. This article mainly introduces the working principle and implementation method of various protection circuits of a switching power supply for a machine tool CNC system. Through actual research, the stability of the switching power supply of the system is greatly improved, and the protection function is stable and reliable, meeting the requirements of mass production.

1 Analysis of the working principle of the protection circuit

The switching power supply for machine tool CNC includes circuits such as soft start protection, overvoltage protection, overcurrent protection, and undervoltage power-off protection.

(1) Soft start circuit

Since the input rectifier circuit of the switching power supply is mostly filtered by a capacitive filter circuit, a surge current with a current amplitude of tens or even hundreds of amperes is often generated at the moment of power on. This surge current is very harmful and can cause startup failure or even damage to the switching power supply. Commonly used soft start circuits include surge protection composed of thyristors and current limiting resistors, soft start protection composed of relay contacts, and soft start protection circuits composed of negative temperature coefficient resistors.

The switching power supply of this system adopts a soft start protection circuit composed of negative temperature coefficient resistors, which is simple, practical and reliable. As shown in Figure 1, after the 220 V AC is filtered by the coil L1 to remove the common mode interference, the rectification generates a DC voltage of about 300 volts. The RT resistor is a negative temperature coefficient thermistor, model M02-7Ω. When the power is turned on, the surge current causes the thermistor to heat up, the resistance value decreases rapidly, and the output DC voltage is gradually established, which can effectively prevent the surge current from impacting the power circuit, making the entire power half-bridge conversion circuit stable and reliable.

Figure 1 Input soft-start circuit composed of negative temperature coefficient resistors

When the switching power supply starts, since the pulse width modulator has not yet established a stable driving pulse, measures need to be taken to gradually establish the driving pulse. The switching power supply pulse width modulator adopts the pulse width modulator TL494 with a high cost performance. As shown in Figure 2, the fourth pin of TL494 is dead zone control, which can not only provide safe dead zone time control for the conversion power tube, but also serve as soft start control for the driver chip. At the moment of power on, no voltage is established on capacitor C1, and +5 V is sent to TL494: 4 pin through capacitor C1, blocking the output pulse of the pulse width modulator. As the voltage across capacitor C1 gradually increases, the voltage at TL494: 4 pin gradually decreases, and the driving pulse width gradually widens. When the auxiliary power supply +15 V fails, the transistor V1 is quickly turned on, and the +5 V voltage is sent to TL494: 4 pin through transistor V1, cutting off the driving pulse, so that the switching power supply stops working without being damaged.

Figure 2 Using TL494: 4 pin for soft start and power protection

(2) Overvoltage protection circuit

Common digital signal processing circuits mostly use TTL or CMOS series integrated gate circuits. For TTL integrated gate circuits, the operating voltage is usually not greater than 5.5 V. The switching power supply output of this numerical control system has multiple outputs, including +5 V, +15 V, -15 V, +24 V, etc. In the switching power supply system, the main conversion voltage +5 V is protected from overvoltage. The specific circuit is shown in Figure 3.

Figure 3 CNC switching power supply overvoltage protection circuit

Working principle: The digital control switch power supply provides the anode working voltage of the thyristor V4 tube from the auxiliary power supply +15V, and the actual output sampling voltage is sent to the voltage regulator tube V5. When it exceeds the protection voltage threshold +5.5V, the output voltage is divided by the voltage regulator tube, resistors R3 and R4 to trigger the thyristor V4 to turn on, and the auxiliary power supply +15V is grounded through the resistor R1, and the power supply of the 8-pin is cut off through the diode V2. By adjusting the RP potentiometer, the output voltage protection threshold point can be set.

(3) Overcurrent protection circuit

The working principle of the overcurrent protection circuit of the switching power supply is shown in Figure 4. The primary side of the transformer T1 is connected in series in the primary side loop of the main transformer of the switching power supply. Through experiments, a reasonable transformer primary-to-secondary turns ratio is selected to sense the primary current value during the switching power supply conversion, which is rectified by diodes V1 ~ V4, filtered by R1 and C1 and sent to the potentiometer RP. The larger the primary current, the larger the voltage rectified by the current sampling transformer, the lower the center point voltage of the potentiometer RP, and the voltage of TL494:2 pin decreases accordingly, which makes the voltage of TL494:3 pin increase and sent to the pulse width modulator to gradually reduce the driving pulse width of TL494, thereby achieving the purpose of overcurrent protection. In the figure, capacitors C4, C5, and R10 are feedback components of the TL494 error amplifier, making the amplifier circuit stable and reliable.

Figure 4 Circuit diagram of overcurrent protection circuit of CNC switching power supply

(4) Undervoltage protection circuit

The +5 V and PF (POWER FAIL) signals are used for comparison. When the +5 V power fails, the PF signal must be maintained for at least 10 ms to store relevant information. The undervoltage protection circuit is shown in Figure 5.

(a) Power-off protection using LM339 voltage comparator

(b) Power-on timing and power-off protection timing diagram

Figure 5 Undervoltage protection circuit of digital control switching power supply

2 Protection circuit debugging and implementation

(1) Soft start circuit debugging

Thermistor soft start circuit can be used to bake the negative temperature coefficient thermistor with a soldering iron, and its resistance value change can be measured with a multimeter. At the same time, the resistance change rate can be estimated by timing. Thermistors with different resistance values ​​are installed in the circuit respectively, and the high voltage waveform output by the rectifier circuit when the power is turned on is tested with an oscilloscope high voltage probe. The voltage build-up time can be compared to select the appropriate negative temperature coefficient thermistor.

(2) Overvoltage protection circuit debugging

Requirements for the thyristor trigger circuit in the overvoltage protection circuit: 1) It is required to provide sufficient trigger voltage and current when triggered. 2) When not triggered, the trigger terminal voltage should be less than 0.15 V ~ 0.2 V. To prevent false triggering, a negative bias of 1~2 V is generally added. 3) The rising edge of the trigger pulse should be steep, preferably less than 10us, so that the trigger voltage is accurate. 4) The trigger pulse must have a sufficient width. Since the turn-on time of the thyristor is generally less than 6us, the pulse width should be greater than 6us, preferably 20us~50us.

Overvoltage protection circuit debugging: gradually adjust the output voltage to 5.5 V, use a multimeter to test the triggering pole voltage of the thyristor, and use an oscilloscope to observe the waveforms of the driver chip TL494: 8-pin and 11-pin, and adjust the overvoltage protection multi-turn potentiometer RP until the protection circuit is activated and the driving waveform disappears. At this time, keep the multi-turn potentiometer RP knob position unchanged. Gradually lower the output voltage, and the protection circuit will not act because the thyristor is not triggered. If the output voltage is increased to 5.5 V, the protection circuit will act. Repeat the test until the protection circuit works stably and reliably.

(3) Overcurrent protection circuit debugging

The overcurrent protection circuit selects high-frequency ferrite core EE12, the primary inductance is 0.013 mH, and the secondary inductance is 0.74 mH. The maximum output current of the switching power supply + 5 V is 25 A. Cut a section of enameled wire with a diameter of 1.2 mm, and measure its resistance value to be 0.2Ω. Connect this simulated load in the circuit, measure the overcurrent rectifier output voltage Ui, adjust the overcurrent protection multi-turn potentiometer, and the circuit starts protection when Ui=-0.57 V. Change the output simulated load and repeatedly debug the overcurrent protection circuit parameters until the overcurrent protection circuit is stable and reliable.

3 Conclusion

This paper proposes a specific practical circuit for soft start protection and overvoltage and overcurrent protection. Finally, the working parameters of each protection circuit are reasonably set, so that the protection function of the switching power supply of the CNC system is stable and reliable, the performance of the whole machine is improved, and the foundation is laid for the mass production of the CNC system.