Design of Control Circuit for Ultrasonic Power System Based on DSP

This paper uses the high-speed TMS320LF2407A DSP control chip to design the control circuit of the system, and uses a full-bridge inverter as the main power conversion circuit of the ultrasonic vibration system to solve the drift of the resonant frequency caused by load temperature changes and other reasons, and ensure the high efficiency of the system. Here, a coarse-fine composite frequency tracking scheme is studied, using a sweep frequency method to achieve coarse frequency tracking, and a hardware phase-locked loop to achieve fine tracking. The combination of these two methods not only ensures automatic frequency tracking within a wider frequency variation range, but also ensures fast and accurate tracking. In order to meet the requirements of load changes, a soft-switching PS-PWM control method is used to make the output power of the system continuously adjustable.

1 Main circuit topology analysis

The main circuit of the ultrasonic power supply adopts a full-bridge inverter topology, as shown in Figure 1. Among them: Z1~Z4 are the main power switches; D1~D4 are the internal anti-parallel parasitic diodes of Z1~Z4; C1~C4 are external parallel capacitors or parasitic capacitors of power tubes; T is a high-frequency pulse transformer; L0 is a series tuning matching inductor; PZT is an ultrasonic transducer.

The inverter part uses the parasitic capacitance and parallel capacitance of the power tube, as well as the leakage inductance of the transformer to achieve soft switching zero voltage phase shift control (ZVS-PSP-WM). Zero voltage switching relies on the conduction of the anti-parallel diode of the power switch tube to achieve zero voltage turn-on of the power device; and the zero voltage turn-off of the power device is achieved through the charging process of the power resonant capacitor.

In one switching cycle, phase-shift control has 12 switching modules. Before analysis, the following assumptions are made:

(1) All the switching devices Z1~Z4 and their anti-parallel diodes D1~D4 in the circuit are ideal switching devices;

(2) All inductors and capacitors are ideal components and the stray inductance of the circuit is not considered;

(3) The impact of adding dead zone on inverter operation is not considered;

(4) The input voltage of the inverter is a constant voltage source.

The driving waveforms of the four switch tubes of the phase-shift control inverter are shown in Figure 2. The two power tubes of each bridge arm of the inverter are complementary and turned on at 180°, and the conduction angles of the two bridge arms differ by one phase, that is, the phase shift angle. Z1 and Z2 are phase-fixed arms, and Z3 and Z4 are phase-shifted arms. Among them, Z1 and Z2 are turned on before Z3 and Z4 respectively, and the phase shift angle is φ. By adjusting the size of φ, the output voltage of the inverter can be changed, thereby adjusting the output sinusoidal current amplitude, so that the output power can be adjusted.

During the operation of the inverter, the on and off time of the power switch tube is constant. The on and off of the two switch tubes in the same bridge arm require a certain delay time to prevent the upper and lower bridge arms from being directly connected and ensure the safety of the switch tube.

2 Control strategy

The working process of the main circuit control strategy is further analyzed below. During the working process of the inverter, the on and off time of the power switch tube is constant. The turn-on sequence is Z1→Z4→Z2→Z3. The turn-on and turn-off of the two switch tubes in the same bridge arm require a certain delay time to prevent the upper and lower bridge arms from being directly connected and ensure the safety of the switch tube.

The inverter circuit of PS-PWM power control has the following main working modes within one cycle, as shown in Figure 3.

(1) Working mode 1 [time t0] (see Figure 3(a)): At time t0, Z1 and Z4 are turned on at the same time, and the current i flows from: Z1→R→L→C→Z4.

(2) Operation mode 2 [t0, t1] (see Figure 3(b)): Z1 is turned off at t0, the current i charges C1, and the charge of C3 is removed. The voltage of C1 rises linearly from zero, and the voltage of C3 decreases linearly from E. Z1 is turned off in ZVS.

(3) Working mode 3 [t, t2] (see Figure 3(c)): At time t1, the voltage of C3 drops to zero, D3 turns on naturally, clamping Z3 at zero. At this time, Z3 is turned on and Z3 is turned on at ZVS. At this time, no current flows through Z3.

(4) Operation mode 4 [t, t3] (see Figure 3 (d)): Z4 is turned off at t2, and the current i removes the charge of C2 and charges C4 at the same time. The voltage of Z4 starts to rise from zero, and Z4 is turned off in ZVS. At t3, the voltage on C4 rises to E, that is, when the charge on C2 is zero, D2 is naturally turned on.

(5) Operation mode 5 [t3, t4] (see Figure 3(e)): At t3, D2 is turned on, clamping Z2 at zero. At this time, Z2 is turned on, so Z2 is turned on in ZVS. Although Z2 is turned on, no current flows. At t4, D2 and D3 are naturally turned off, and current flows through Z2 and Z3.

(6) Operation mode 6 [t4, t5] (see Figure 3(f)): At t4, the current crosses zero from the positive direction and increases in the negative direction. The flow direction of the current i is: Z2→C→L→R→Z3. At t5, Z3 is turned off and the inverter starts to work in the other half cycle. The operation is similar to the above half cycle.

3 Software Design

A novel ultrasonic power control system is designed in combination with high-performance DSP digital chips. The hardware design block diagram of the whole system is shown in Figure 4. DSP uses TMS320LF2407A, and the external expansion FLASH uses CY7C1021V33-122 chip. PWM is a pulse output, which is respectively led by PWM1, PWM2, PWM3, and PWM4, and sent to IGBT through integrated driver isolation to control its conduction and shutdown. Iset is a given circuit, Io, Id, and Udt are load current, inverter DC input current and voltage respectively. These three signals are sent to their respective conditioning circuits, and then sent to the A/D interface of DSP after conditioning. In case of external faults, such as overheating, an interrupt request is sent to DSP to implement protection.

Here, TMS320LF2407A is used to implement the PS-PWM algorithm, and its EV is used to generate the PWM control signal. The function of the power control program is to compare the current value detected from the load with the power setting value, and the difference is processed by the digital PI control algorithm to obtain the phase shift angle θ value that needs to be adjusted. The result is returned to the main program to affect the setting value of the comparison unit 1 (CMPR1). The PS-PWM power control algorithm is shown in Figure 5.

In order to ensure the normal operation of the ultrasonic power supply, in addition to designing hardware protection circuits for various faults, software protection is also used. Protection is achieved by hardware and software to ensure reliable operation of the system. Software protection is achieved by filtering and sampling the detected signal and connecting it to XINT2, the highest interrupt level of DSP. When a fault occurs, the software interrupt program is entered to block all PWM pulse outputs to achieve protection. The interrupt protection program flow is shown in Figure 6.

4 Simulation and Experimental Results

Based on the above theoretical analysis and the hardware and software design of the system, the PSpice software is used to simulate the phase-shift power control ultrasonic power supply, as shown in Figures 7 and 8.

The selected ultrasonic transducer model is DH-6160F-15S-3, with a resonant frequency of 25 kHz, a resonant impedance of 15Ω, and a static capacitance of 27000 pF. By calculation, its matching inductance is 0.75 mH. Figures 7 and 8 show the simulated waveforms of output voltage u and output current i when the phase shift angle is φ=0° and φ=45° respectively. By comparing and analyzing the simulated waveforms, when the phase shift angle φ gradually increases, the output voltage pulse width gradually decreases, and the current amplitude gradually decreases. It can be seen that the output power can be adjusted by adjusting the size of φ. In addition, the power tube works in the ZVS soft switching state, which reduces the switching loss and voltage and current stress. The inverter always works in the load resonance state, the power factor on the load side is high, the control is simple, and the reliability of the power supply is improved. According to the previous design, the 3 kw/30 kHz ultrasonic generator is experimented. The following gives the drive waveform of the inverter bridge, the PS-PWM control output waveform, and the frequency tracking experimental waveform. FIG9 is the driving waveform of Z1 and Z4 when θ=60°, FIG10 is the output voltage and current waveform when θ=60°; FIG11 is the output voltage and current waveform in the steady state after frequency tracking.

5 Conclusion

Since the traditional switch tube trigger circuit is controlled by pulse phase shift by hardware, its circuit is complex, the components are easy to age, and the output waveform is prone to distortion to varying degrees, which greatly affects the symmetry of the trigger pulse. The control system composed of a microprocessor can complete the control task in real time and accurately under the premise of meeting the accuracy. The use of software to realize phase shift control can greatly improve the symmetry of the trigger pulse and improve the signal accuracy. DSP is used here to realize the PS-PWM control of power. By changing the phase shift angle, power regulation in a wider range can be achieved, and the power switch device works in a soft switching state, which greatly improves the system efficiency, is more flexible, and operates more reliably.