Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

In this article, design design an intelligent, digital adjustable switching DC power supply by using ARM7 chip of SAMSUNG SC344BOX ARM7, to achieve fully digital control for the main circuit, adjustable output voltage ,and improves accuracy and stability of output voltage. Control algorithm can upgrade system through software programming, and be convenient for user to neatly choose different control functions according to their needs .

Overall design of power system
Design requirements of power system are: power frequency AC 220 V input, adjustable output voltage DC 10 ~ 2 000 V, output current less than 100mA, the user can change the output voltage by using the keyboard anytime, the display shows the current operating state. According to design requirements of the power system , it’s composed of two parts ,the power circuit and control circuit, shown in Figure 1.

The power circuit is mainly included rectify rectifier, BUCK converter, push-pull DC-DC converter, filter, transform the frequency power into required DC power. Control circuit is included ARM7 control unit, the voltage divider feedback sampling, keyboard / display, adjust the output DC voltage according to users’ input parameters ,and show the current operating state .EMC protection eliminate noise interference of power frequency power supply ,protect system circuit from being damaged.

work principle and design of power circuit
rectifying filtering and design of BUCK convertor

Rectifier changes the power frequency AC power into DC power, the circuit is shown in Figure 2, waveform of input / output is shown in Figure 3. In order to improve the rectification efficiency, use full-bridge rectifier, rectifier bridge silicon should use high cost performance KBP3510, 2 个 47μF capacitor in parallel with 400 V, output DC voltage U0 is about 300 V.

BUCK converter, is also known as step-down converter, which ARM based on the current input parameters and the feedback voltage, resulting in PWMl , using IGBT integrated driver to drive IGBT of main circuit, because the changes in duty cycle control the output voltage value, the circuit is shown in Figure 4. BUCK converter must work in the continuous state, the marginal value of the inductor L    As the output power of power system is 150 W, then P01> 150 W, take P01 = 200W and  . f0 = 100 kHz; U01 is variable between 2 ~ 200 V, you can get , The relation curve is shown Figure 5. So when L> 340μH, capacitor C3 is 500 V 470μF,then ensure that the converter can maintain a continuous state.

Design of push-pull DC converter and filter design

As the power output is less than 200 W, using push-pull DC-DC converter to meet this requirement. Shown in Figure 6, S1 and S2 is the IGBT, they alternately turn, conduction ratio of each switch is 50%, S1, S2 is controlled separately by PWM2, PWM3 ,PWM2, PWM3 is the control signal given by ARM. S1, S2's role is to produce high-frequency transformer T primary symmetrical alternating square wave, when the S2 conduction (S1 cut-off) when, T increased the flux in the core, when S1 turns on (S2 cutoff), the T's core flux decline. Produce a change in the secondary voltage, the D1, D2 rectified, they will have a DC output voltage U02, respectively, in the ideal state of primary and secondary transformer winding turns. Because the primary winding 01 connected to the voltage reflected to the primary the other half, so S1 or S2 in a non-conducting, the voltage across the IGBT voltage is greater than it 2U01 2U01, that is, the pressure is greater than the IGBT 400 V.

In the high-frequency, the transformer is usually higher magnetic ferrites or other magnetic materials beryllium Mo alloy core, its purpose is to obtain a large magnetizing inductance, reducing the power loss in the magnetic circuit, so that it can with minimal loss and distortion broadband transmission with pulse energies. In this paper, PQ ferrite core, inner core is cylindrical, but also facilitate easy routing around the coil into the honeycomb to reduce distributed capacitance, and no edges, high pressure when the ignition is not easy to breakdown, secondary side turns not more than 2000 ring, primary and secondary turns ratio of 1:12.

Because of many turns of the high transformer , degree of coupling between first-class are strong and parasitic capacitance is large, then its output waveform is not continuous, the waveform shown in Figure 7. Therefore, the output voltage should have filters to get more stable voltage. Filter design shown in Figure 6, capacitor voltage 3 kV with capacity of 470 pF.

ARM control system and its software implementation function

The system uses the ARM which is SC344BOX chip of SAMSUNG, the ARM chips are of low-cost, high-performance, with excellent peripheral modules , apply for industrial control, biometrics, multimedia control, streaming media network and smart appliances. Its main features are:
(1) ARM7TDMI core supports Thumb (16 bit) / ARM (32 bit) dual instruction set, can be well compatible with 8-bit / 16-bit devices; with 8 KB cache, clocked speed at up to 66 MHz;
(2) LCD controller can support 256-color STN, and the LCD has dedicated DMA;
(3) 5 PWM timers, a channel internal timer;
(4) 16-bit watchdog timers;
(5) 8 channels 10-bit ADC;
(6) 71 general purpose I / O ports, 8-channel external interrupt source.

Using the system designed by SC344BOX, almost all of the instructions can be completed within 20 ns, with powerful computing capabilities of command, it is easy to implement various control algorithms and high-speed sampling, in order to reduce the static difference of system, using a closed loop to achieve the control of whole system.

The main function and software of the ARM in the power system are as follows:
1. Generate PWM wave. PWMl is used to drive IGBT in the BUCK circuit. According to the output sampling, set and adjust the period register value of timer and compare the register value to change cycle and pulse width of output PWMl wave. The cycle of PWM2, PWM3 is setting to interactive square wave of 50 kHz.

2. Real-time sampling. Using 8-rout 10-bit ADC integrated in SC34480X to convert circuit for achieving voltage, current real-time sampling, the minimum conversion time per channel is 500ns, through the sampling module MAXl22, convert sampling signal into 0 ~3.3V level required by 2407 ADC, in one switching cycle, will do sample 80 times (switching frequency of 50kHz), after sampling, adjust PWMI waveform of IGBT tube in BUCK driver circuit through the software programming ,to achieve the purpose of voltage stabilization, and when the output voltage, current or voltage is too high . ARM call the appropriate subroutine to handle unexpected events, play a protective role.

3. Set the watchdog circuit for prevent crashes when programming software.

Conclusion

Based on analysis of article as above ,design a switching power supply with ARM-based intelligent digital control technology,. Power indicators meet the performance requirements: output voltage continuously adjustable, ripple factor less than 0.5% output voltage less than O. 3%. The power supply has been put into use, works well.