Micro - Controller Implementation

Nowadays the use of a micro-controller is popular. But control of a circuit switching at 20 kHz tends to be difficult, essentially due to delays involved in A/D conversion and some mathematical operations. Hence the approach to closed-loop control is a combination of rule-based logic and action of a PID controller. The delay that occurs from the instant the A/D converter samples an input signal to the instant when the corresponding digital output is available for use tends to be of the same order as the switching cycle period.

At 20 kHz, the switching cycle period is 50 mS and the delay involved due to the use of an A/D converter is about 40 mS. This means that no more than one sample can be obtained during one switching cycle period. If this sampling is done in an asynchronous manner, closed-loop can be difficult. For example, the inductor current can vary considerably over a cycle period and asynchronous sampling of inductor current may not lead to stable closed-loop control. It is preferable to sample the inductor current at a predetermined instant of the cycle period. For example, at the start of a switching cycle, a sample and hold circuit can be used to store the instantaneous value of inductor current and the A/D converter can convert this value. In the fifth applet displayed below, the A/D converter delay is set to be equal to one switching cycle period. When that is so, the sampling occurs at the beginning of each cycle. The inductor current and the capacitor voltage are sampled once in two cycles, since conversion period for each sample equals one switching cycle period.

If the sampling is to be done at a faster rate, the sampling has to be asynchronous and then an analogue filter has to be used to reduce the variations in the feedback signal corresponding to inductor current before it is used as the input to the A/D converter.

The scheme suggested for controlling the SMPS with a micro-controller is outlined with the help of a pseudo-code presented below.

Initialize:

    Output Voltage Count = 0 // Output of A/D converter
    Inductor Current Count =0 // Output of A/D converter
    Old Current Count = 0
    // Previous current count required for derivative feedback

    swCycleCount = 100
    // Set the period of switching frequency as a number of micro-controller clock frequency.

    atod_delay =100
    // Set the A/D converter's conversion period and computing delay as a number of // micro-controller clock frequency.
 
    lowCount= 5;
    highCount=95;
    dutyCycle=lowCount;
    // Set the limits for duty cycle and set the duty cycle at its lowest value

    on_off=true;
    // boolean value indicating switch is ON when it is true

    rampCount=0;
    // Used with dutyCycle and swCycleCount for setting on_off to either true or false value

    atod_Count=0;
    feedback Voltage=0
    integrator Output=0
    //Output of an integrator used in feedback control
    Go to Main Loop

Main Loop:

    Call Parameters Subroutine
    Increment ramp Count
    If (rampCount<dutyCycle) on_off = true;
    Else on_off = false;
    If (rampCount == swCycleCount)
    {
        on_off = true
        rampCount=0;
    }

    Increment atod_Count
    Increment rampCount
    If (atod_Count>atod_delay) call NextValues subroutine
    Return to Main Loop
 

Parameters Subroutine:
    Begin
        Desired Output Voltage=50.0
        // numerical setting using thumb-wheel switches/KeyPad
        Current Limit = 125%
        // Set in software, using KeyPad or thumb-wheel switches

        kd=20
        // Derivative feedback coefficient in Ohms.
        // Feedback in Volt equals (kd ´ Inductor Current).
 
        dkI=0.1
        // Integrating Coefficient effective when output voltage is within
        // a close band of ± 5% of source voltage from the desired output voltage.

End subroutine

NextValues Subroutine
    Begin
        atod_Count=0;
        Inductor Current Count = output of A/D output
        Ouput Voltage Count = output of A/D output
        if (Inductor Current Count <=Current Limit)
        {
            feedback Voltage= Ouput VoltageCoun +kd*( Inductor Current Count - Old Current Count);
            if (Desired Output Voltage>(feedback Voltage+5%)) dutyCycle++;
            if (Desired Output Voltage<(feedback Voltage-5%)) dutyCycle--;
            if (Desired Output Voltage>(feedback Voltage-5%)) AND (Desired Output Voltage<(feedback Voltage+5
                {
                    integrator output+=
                    dkI*(Desired output Voltage-Ouput Voltage Count)
                    dutyCycle+= integrator output;
                    if (dutyCycle<=lowCount) dutyCycle=lowCount
                    if (dutyCycle>=lowCount) dutyCycle=highCount
                }

            else
                {
                    integrator output =0;
                }
        else
        {
            dutyCycle = dutyCycle-2;
            if (dutyCycle<=lowCount) dutyCycle=lowCount;
        }
    End Subroutine

The fifth applet presented below simulates the operation of the switch mode step-down power supply controlled by a micro-controller. The applet has default values for the parameters listed below.

    Input Voltage,dc avg = 100
    Input Ripple Volt., pk-pk = 0;
    Input Ripple Frequency = 100
    Inductance, microHenry = 500;
    Capacitor, microFarad = 500 ;
    Load Resistance, Ohms = 10;
    Desired Output Voltage = 50;
    Rated Current, Amp = 10;
    Micro's Clock Freq, MHz = 2 ;
    Switching Period:Clock Cycles = 100;
    A/D Delay :Clock Cycles = 100;
    Derivative FeedBack Coef. =20;
    Integrating Coefficient = 0.1;

This applet has the same structure as the fourth applet and is hence not described any further.

click here to open the applet

FIFTH APPLET

 
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