978-1-4799-0036-7/13/$31.00 ©2013 IEEE 231 36th Int. Spring Seminar on Electronics Technology

A Digitally Controlled Programmable Power Supply Used in a Vaporizer

Radu Etz1), Dorin Petreus1), Tiberiu Frentiu2), Toma Patarau1)

1) Technical University of Cluj–Napoca, Romania 2) Babes–Bolyai University, Cluj–Napoca, Romania

radu.etz@ael.utcluj.ro

Abstract: A digitally controlled programmable switch mode synchronous step down DC-DC power supply for a vaporizer used in a spectrometric instrumentation analyzer is designed taking into consideration the delays caused by the analog to digital converter, digital to analog converter and the discretization process of the control coefficients. The control method used is average current mode control. Because the control method is implemented using a microcontroller the power supply can be transformed from a voltage source to a current source through the PC interface without the need to compute new values for the two proportional integral (PI) controllers. The simulations and experimental results are presented, compared and conclusions are drawn about how the discretization process and the delays affect the phase margin and the design process of the power supply.

1. INTRODUCTION

In this paper a digitally programmable power supply used in a micro plasma equipment is presented. The equipment is used in environmental and foods control for identifying the chemical elements found in a sample. Due to pollution and problems caused by food industries, the use of forbidden by law ingredients or discrepancies between the content and the label, these type of equipments need to become portable and used more intensively.

The plasma microtorch equipment is presented in figure 1. The main elements are the RF generator for plasma generation, the vaporizer chamber where the sample is introduced and vaporized at high temperature using a filament, the DC-DC converter that delivers the power needed by the filament to reach the reference temperature, the spectrometer to analyze the composition of the sample, the microcontroller that controls the entire system and the PC interface used for online measurement and control over the process.

The programmable power supply is used to provide energy to the heating element inside the vaporizer chamber. This element based on a voltage or current pattern vaporizes the sample used for spectrum analysis. The main advantage of using

digital control [1,2] in this kind of applications is the online programmability of the power supply [3].

The chosen control method is the classical average current mode control [4] where two control loops are used, an outer slower voltage loop that generates the current reference and a faster inner current loop [5,6]. Because the temperature variation is a very slow process compared with the switching period of the power supply, the reference for the current or voltage loop can be computed to obtain the desired temperature based on the power provided by the DC-DC converter. By using this control method and a PC interface the voltage loop can be disabled and the reference for the current loop can be programmed through the serial port. With this configuration the voltage programmable power supply can be transformed in a current programmable power supply.

Taking into consideration that in our days the mobility of a system and the miniaturization is a must in all domains the DC-DC converter used in the implementation is a synchronous step down converter with reduced size passive components. The use of a synchronous topology indicates that the power supply is a current source or current sink. The design of the two control loops starts as mentioned in [7-9] from the analog domain and based on the Tustin approximation [7] its digital counterpart is found.

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Fig. 1. The miniaturized plasma microtorch equipment.

By using two control loops with a digital signal controller (DSC), the switching frequency is limited based on the oscillator frequency, the machine cycle period, the set of instruction used to implement the loops, the time of conversion and the other tasks the DSC must accomplish.

In the implementation section these restrictions are presented and the switching frequencies together with the discrete control laws are chosen based on the system necessities [10].

The methodology used to implement the digitally programmable power supply is as it follows: the design of the two proportional integrative (PI) analog control loops, simulations of the analog implementation using Psim software, computing of the digital coefficients used by the numerical PI control laws, simulation of the numerical algorithm by using a generic microcontroller implemented in a C block in the Psim software, C and assembly based language code written for the DSC and testing the prototype in different functioning situations.

The next section describes the system and the implementation of the system.

2. IMPLEMENTATION

As presented in the Introduction, a DC-DC synchronous step down converter was designed based on [11] and the components were computed resulting

the following values: inductance (L) 1 μH, capacitance (C) 680 μF, series resistance of the capacitor (ESR) 20 mΩ, for input voltage (Vin) 5 V, output voltage (Vo) 2.5 V, load (Ro) 0.5 Ω and switching frequency 100 kHz.

The two designed analog control loops are presented in figure 2. For both loops, the fast inner current loop and the outer voltage loop, PI controllers are used and based on the Tustin approximation the coefficients for the control law presented in equation 1 are computed.

11][ 110 ndAnerBnerBnd , (1)

where B[n] and A[n] are the coefficients of the digital control law, d is the output of the control law, and er the difference between the reference value and the measured value.

Fig. 2. Analog average current mode control.

The delays introduced by transforming the analog controllers into their digital counterparts can be observed by analyzing the Bode plots for the analog

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implementation, figure 3 and the Bode plots for the digital implementation figure 4.

Fig. 3. The Bode plots for the analog current loop.

Fig. 4. The Bode plots for the digital current loop.

As one can observe the phase at the crossover frequency in the analog case is –86 degrees resulting a 94 degrees phase margin. For the discrete domain it can be observed form the plot that the phase has an added delay of 25 degrees, resulting a phase margin of 69 degrees.

For this reason the analog loops are designed with a phase margin that even with this phase delay introduced by the discretization, offers a phase margin for the digitally controlled system greater than the stability criterion of 45°.

Figure 5 presents the schematic in Psim with the numerical control loops implemented in the generic

Fig. 5. The Psim schematic of the DC-DC converter with the numerical control method.

microcontroller block using C language. The output voltage and inductor current are read at half of the duty cycle resulting the average value.

The trigger value is computed every switching cycle and the new value is updated at the end of the current loop computation. The switching frequency is chosen based on the time needed by the microcontroller to update the value of the duty cycle every switching cycle.

For implementing the control a TMS320f28027 DSC with 60 MHz clock frequency is used. For this case one PI controller takes 725 ns and the conversion time for two analog to digital channels takes 1 μs. It results a time needed to compute the two control loops and update the duty cycle value of 2.45 μs.

This limits the maximum switching frequency to 400 kHz. For this value even if the duty cycle can be updated every switching cycle, the time left for the background loop is of 50 ns. As one can see in figure 6 the time-line summary of sampling and processing operation are presented for two cases, 400 kHz and 100 kHz switching frequencies.

Fig. 6. The time-line summary of sampling and processing operation.

Because the microcontroller is used for other processes beside the control of the vaporizer power supply, and because a PC user interface implemented in C#, figure 7, is used to update the references for one of the loops using serial communication, the switching frequency is chosen to be 100 kHz, giving a 7.5 μs background loop time.

In figure 8 the flow chart of the firmware implemented in the microcontroller is presented and one can observe that the power supply based on the parameters sent from the user interface can work with both loops being closed and in this case the value sent from the interface is the voltage loop reference, or only with the current loop being active in which case

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the value sent from the interface being the current loop reference.

Fig. 7. The PC user interface.

Fig. 8. The algorithm flow chart.

In the next section the simulations and experimental results for different functioning conditions are presented and compared referring to the dynamic response of the converter. Also the devices used for experimental implementation are presented along with the test stand.

3. RESULTS

3.1. Simulations

Three cases are taken into consideration and for each of them, the simulation result and the experimental one are presented. For the first case the reference current is changed between two values and the simulation, output voltage figure 9 a) and output current figure 9 b) waveforms are presented. The current step has a value of 3 A (2 A to 5 A change) and only the current loop is being active.

Fig. 9. Simulations results for a) the output voltage and b) load current at current reference change.

In the second case both loops are being active, and the voltage reference is changed between two values. The two values of the output voltage are 2.5 V and 3 V and the waveforms for the output voltage figure 10 a) and output current figure 10 b) can be observed. For both cases, the output voltage and output current are stable resulting that the designed control loops can be implemented in the microcontroller and used with the interface.

Fig. 10. Simulation results for a) the output voltage and b) load current at voltage reference change.

In the third case the reference voltage is kept constant at 2.5 V for a load change corresponding to a load current change from 2 A to 5 A with a 5 ms

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period. The output voltage is steady, as one can see in figure 11 a), with a time response of 1.5 ms.

Fig. 11. Simulation results for a) the output voltage and b) load current at output load change.

3.2. Experimental Setup

The experimental setup with the synchronous step down converter is presented in figure 12. A DC power supply and an electronic load are connected at the output when the output load current is changed. For the other two cases a resistor of 0.5 Ω was used as the load. The control laws are implemented using the TMS320F28027 Piccolo DSC from Texas Instruments. The voltage and current are read using two ADC channels from the DSC, and one PWM module is used to trigger the conversion at half the switching period and to control the two MOSFET switches.

The firmware is implemented using C language for the initializations and for the background loop, and assembly language is used for the two control loops called from the ADC interrupt service routine (ISR).

Fig. 12. The experimental setup.

3.3. Experimental Results

The waveforms obtained with the experimental setup in the same cases as for the simulations are presented in figures 14, 15, 16. Comparing the results obtained experimentally with the simulation ones, one can conclude that the power supply is stable and can be successfully used in the application.

Fig. 13. Experimental results for the output voltage Ch. 1 and load current Ch. 2 at current reference change.

Fig. 14. Experimental results for the output voltage Ch. 1 and load current Ch. 2 at voltage reference change.

Fig. 15. Experimental results for the output voltage Ch. 1 and load current Ch. 2 at output load change.

In figure 16 the output voltage is presented at output load change and the second waveform represents the output voltage ripple. The value of the ripple is 200 mV and the voltage reaches a peak value of 2.92 V and a minimum value of 2.18 V.

Fig. 16. Output voltage and output voltage ripple.

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4. CONCLUSIONS

A digitally controlled power supply for a vaporizer was designed starting from the analog domain and transposing it into discrete domain. The delays effects were taken into consideration and the average current mode control was implemented using a DSC. This control method is chosen because the user must be able to control the output voltage or output current through the user interface implemented in C#. The power supply delivers power to the filament and based on the current or voltage over temperature characteristic experimentally obtained, the temperature can be set for different working patterns from the user interface.

Most of the current mode power supplies implemented using a microcontroller are using the peak current mode control method, because it has the advantage of using only one control loop implemented in software. In this case the duty cycle is updated after an analog comparator compares the value of the control law output with the current reference, hence reducing the time needed for the computations.

In this case because the DSC is working at 60Mhz and the two control loops are implemented in assembly language, average current mode control can be used taking into consideration that the application needs to control through the PC interface both the output voltage value and the average output current value.

In the results section the simulations and experimental results were compared and the same values are obtained for the both cases.

In conclusion the method proposed to control the temperature of a filament in a vaporizer chamber offers a low cost solution and the power supply control method offers a stable and steady response.

ACKNOWLEDGEMENT

This work was supported by the project "Miniaturized Equipment with Capacitively Coupled Plasma Microtorch and Analytical Technologies for Simultaneous Elemental Determination used in

Environment and Foods Control (MICROCCP)" CNCS – UEFISCDI, Project No. PN-II-PT-PCCA-2011-3.2-0219.

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