Motor brake system power supply design based on SG3525

In recent years, the application of power electronic devices and PWM technology in electronic technology and power supply systems has become more and more mature. This paper uses the SG3525 controller to design a pulse power supply for the motor brake system, which is currently an ideal power supply for the motor brake system. The circuit has the characteristics of small size, easy control, stable system, and reliable performance.

1 Structure and main performance of SG3525

SG3525 is a current-controlled PWM pulse width modulation controller. It can not only adjust the dead time by adjusting the size of the external resistor, but also has functions such as soft start mode and pulse control blocking protection. By adjusting the capacitance of CT on the 5th pin of SG3525 and the resistance of RT on the 6th pin, the frequency of the output control signal PWM can be changed, and the output pulse width can be changed by adjusting the voltage of COMP on the 9th pin. The above functions can greatly improve the dynamic performance of the switching power supply and simplify the design of the control circuit. Its external circuit structure is simple and easy to use. The output drive is a pull output form, which increases the driving capacity. It contains undervoltage lockout circuit, soft start control circuit, and latch. In addition, it also has overcurrent protection function, adjustable frequency, and can limit the maximum duty cycle. Its internal structure is shown in Figure 1.

The output voltage Vc of pin 13 is used as the input voltage of pin 15 of the reference voltage regulator. When Vc>8 V, the output amplitude of pin 16 of the reference voltage regulator is 5.1 V and the accuracy is +1%. When Vc<8 V, the output of the reference voltage regulator decreases and the accuracy also decreases. Due to the existence of an undervoltage lockout inside the chip, when pin 15 is undervoltage, the undervoltage lockout outputs a high level. After passing through an NOR gate, it becomes a low level input to the base of T1 and T5, turning off T1 and T5, and the output of pin 13 is Vc, and the pulses of pins 11 and 14 are disabled. The control pulse output from the power drive circuit to the power field effect tube disappears, and the converter has no voltage output, thereby achieving the purpose of undervoltage lockout protection.

The base of transistor T3 inside SG3525 is connected to pin 10 of the chip through a resistor. When the system has an overcurrent signal, a high level is output and connected to the NOR gate of the undervoltage lockout output through the resistor. As a result, the output of pins 11 and 14 is disabled, achieving the purpose of protection.

The soft start function of the chip is mainly realized by the external capacitor C3 of pin 8 and transistor T3. When the system is undervoltage, the undervoltage lockout of the system outputs a high level to the base of T3, turning on T3, and C3 starts to discharge. When the voltage of C3 is reduced to zero, the PWM comparator outputs a high level. After passing through the PWM latch, this high level is connected to two NOR gates, turning off T1 and T5, and prohibiting the pulses of pins 11 and 14. When the overvoltage signal is normal, the low level output by the undervoltage lockout turns off T3, and C3 charges slowly. When C3 is charged to a high level, the PWM comparator outputs a low level, and after passing through the PWM latch, T1 and T5 resume normal working state.

2 System Structure Design

The power supply current required by the SG3525 chip is relatively small, which is more than ten mA, so its power supply is supplied by a 24 V power supply that is converted from 24 V to 15 V through a voltage regulator chip 78M15. The output frequency of pins 11 and 14 is set by pins 5, 6 and 7. The two square wave signals generated control two MOSFET tubes respectively and are connected to the input ends of the pulse transformer. The middle contact at the input end of the pulse transformer is connected to a 15 V power supply converted from 24 V, and the two outputs of the pulse transformer are connected to two IGBTs respectively, so as to chop the 540 V DC voltage rectified by the rectifier bridge from the 380 V AC voltage to form the rated voltage required by the motor. After the motor is powered on, the brake system is released and the motor runs; when the power is off, it is in a locked state and the motor stops running. The system structure is shown in Figure 2.

2.1 Driver Design

As shown in Figure 2, the system uses SG3525 as the main control chip. By setting the parameters of the resistors and capacitors at pins 5, 6, and 7, the chip's output square wave frequency is controlled. The square wave output frequency at pins 11 and 14 is 1/2 of the sawtooth wave of the oscillator at pin 3. The oscillator output frequency is f, and the frequency of the output at pins 11 and 14 is f0.

f=1/[CT(0.7RT+3RD)] (1)

fo=f/2 (2)

Among them, CT, RT and RD are respectively the external capacitor C9 of pin 5, the external resistor R20 of pin 6 and the resistor R39 between pin 5 and pin 7. Considering the switching loss of IGBT and the size of pulse transformer, C9=2.4 nF, R20=10 kΩ, R39=100 Ω are selected, and the output frequency f0=30 kHz of pins 11 and 14 can be obtained. [page]

The soft start capacitor C10 of the circuit is 4.7μF. The chip can only work normally when this capacitor is charged to the high level of the 8th pin of the control chip. The 11th pin and the 14th pin form a push-pull output, and the two MOSFET tubes are turned on and off in turn. When the gate of Q2 is at a high level, Q2 is turned on, and Q3 is turned off at this time. The 15 V power supply of the middle contact of the pulse transformer forms a loop through the upper half of the transformer and Q2. At this time, the output of the pulse transformer, the same-name terminals 5 and 1, generate a voltage of about 15 V, which can drive the IGBT to turn on after filtering; when the gate of Q2 is at a low level, Q2 is turned off, and the gate of Q3 is at a high level, and Q3 is turned on. The middle contact of the pulse transformer forms a loop through the lower half of the pulse transformer and Q3. At this time, the output of the pulse transformer, the 6th pin and the 2nd pin generate a voltage of about 15 V, which is added to the emitter of the IGBT, so that the IGBT can be reliably turned off.

2.2 Rectification Circuit

The system rectifier circuit is shown in Figure 3. After the three-phase AC power is filtered by the EMI filter inductor, the electromagnetic interference generated by the AC component is greatly reduced, and harmonic interference is also prevented. A small-capacity, high-voltage-resistant, non-inductive capacitor is connected to the EMI output end to eliminate high-frequency interference. In order to increase the capacitance and withstand voltage value of the capacitor, two large high-voltage-resistant capacitors are connected in parallel at the output end of the rectifier bridge. The capacity of C20 and C27 must be large enough to reduce the output voltage ripple and low-frequency oscillation. However, C20 and C27 cannot be infinitely large. In order to meet the system output voltage ripple output requirements, this capacitor should meet

In the formula, C is the filter capacitor, in F; f is the pulse frequency of the rectifier circuit, in Hz; U is the maximum output voltage of the rectifier circuit, in V; I is the maximum output current of the rectifier circuit, in A. ACv is the ripple factor, in %.

2.3 BUCK Circuit

The motor brake system has an electromagnetic brake system at the tail of the motor. When the electromagnetic brake system is powered, the brake pads are separated and the motor runs; when the power is off, the brake pads brake the motor under the action of the spring. The rated voltage of the selected brake excitation coil is 190 V DC and the rated current is 0.55 A. The power supply of the system is AC380V AC, so it must be rectified and stepped down to make the output voltage about 190 V DC to ensure the normal operation of the motor.

In the system structure diagram 2, the switch tube selected for the chopper step-down circuit is IGBT, and two IGBTs are selected to improve the reliability of the system. The 380 V AC power supply is rectified to 540 V DC voltage through a three-phase silicon bridge and then connected to the collector of the IGBT, the emitter is connected to the filter inductor and capacitor, and the gate is connected to the output end of the pulse transformer as the driver of the IGBT.
 

(1) Filter inductor. In this design, it must be ensured that the inductor current is continuous under any circumstances. The smaller the current change △I, the smaller the minimum average current, and the larger the required inductance L. Under certain ripple voltage requirements, the output capacitor can be reduced. In engineering, △I=0.2L is usually selected, and the inductance size needs to meet

(2) Filter capacitor. The ripple current of the inductor flows into the output filter capacitor, causing output voltage ripple. In order to reduce the output voltage ripple and the low-frequency oscillation of the DC voltage, the selection of the filter capacitor is particularly important. The product of the output filter capacitor and its equivalent series resistance satisfies CResr ≥ T/2, where T is the switching period. The output ripple is mainly determined by Rers. If the peak-to-peak value of the output voltage ripple is △Upp, the capacitor size must satisfy C ≥ 65 × △I × 10-6/Upp (6)

2.4 Overcurrent protection circuit

The overcurrent protection circuit of the system is to connect a 0.2Ω/2 W high-precision resistor in series on the main circuit of the system as current detection, sample at the input end of this resistor, input the sampled signal into the linear optocoupler PC817 (G1), and connect the output end of PC817 to the 10th pin of SG3525. When the system current is too large, the light-emitting diode of the optocoupler PC817 is turned on, and a high level is output to the 10th pin of SG3525 to prohibit the square wave output of the 11th and 14th pins, so that the pulse transformer and IGBT both stop working. The motor load coil loses power, and the brake pad locks the motor rotor under the action of the spring, thus achieving the purpose of protection. The designed overcurrent protection circuit omits the current detection element and the current transformer, greatly simplifies the design circuit, and isolates the strong and weak parts of the system, better prevents the interference between the strong and weak parts, and enhances the reliability of the system.

2.5 Voltage stabilization output circuit

The voltage stabilization loop of the system is sampled at both ends of the load coil, as shown in Figure 2. The sampling signal is input into the optocoupler PC817 (G2), and the output is passed through the voltage divider circuit to obtain a voltage signal equal to the reference end of SG3525, which is input into the in-phase input end of SG3525. When the load voltage increases, the brightness of the optocoupler light-emitting diode increases, the optocoupler is turned on, and the current transmitted to the output end increases linearly. The voltage signal at the sampling point of the output end decreases as the optocoupler increases, and the voltage at the in-phase input end of the internal voltage comparator of SG3525 decreases, outputting a low level, controlling the duty cycle of PWM, reducing the duty cycle of the output square wave at pins 11 and 14, and reducing the average voltage of the load coil, forming negative feedback, and returning the voltage at both ends of the load coil to the rated value.

3 Simulation Results

The simulation circuit was built on the simulation software Saber. The input voltage was three-phase AC380 V+5%, the output voltage was DC190 V, the output current was 0.55 A, the output power was rated at 105 W, and the switching frequency was 30 kHz. The simulation waveforms of the 11th and 14th pins of SG3525 are shown in Figure 4. The phase difference of the square waves output from the 11th and 14th pins is 180°. These two outputs directly drive the MOS tube, and then drive the IGBT through the pulse transformer. The simulation output waveform is shown in Figure 5. After a period of time, the output voltage of the system basically stabilizes at 190 V, and the voltage fluctuation is small. When the actual measurement of the normal operation of the motor to the brake system coil voltage, the normal operating voltage of the motor is between 150 and 280 V, so this voltage fluctuation can be ignored, achieving the expected effect.

At the same time, when designing the system PCB, since this system involves 380 V AC and the power supply system of the system main control chip, and the control signal part of the system is weak electricity, the interference between strong electricity and weak electricity must be considered when laying out the PCB. Try to layout the strong power supply part on one side of the PCB board and the weak electricity on the other side. The middle can be made by punching holes or slots on the board to reduce the interference between strong and weak electricity. In the sampling part, the optocoupler PC817 is used for both signal transmission and isolation, and its layout is at the boundary between strong and weak electricity. When wiring, the power line and ground line should be widened as much as possible. The middle also involves analog ground and digital ground, and these two also need to be wired separately. The routing of the main control signal should also be widened appropriately to ensure accurate signal transmission.

4 Conclusion

This paper introduces the hardware design of a push-pull motor brake system switching power supply based on the SG3525 controller. Through the push-pull circuit design, it not only overcomes the problem of complex peripheral hardware drive, but also effectively improves the system's driving ability, making the system's stability performance good and the output stable. In addition, its hardware design is simple, and the layout and wiring on the printed circuit board are convenient, so it has a high practical potential.