CODEWITHSUNDEEP
microcontrollermikroc
Misal313
Misal313

Reputation: 179

How to Generate PWM with delay?

I am implementing an isolated boost converter. I have to generate a PWM signal for the switches given in the figure below. I have difficulty understanding the pattern. The PWM pattern is as follows: At the beginning all four switches are kept on, then switches 1, 4 are kept on while switches 2, 3 are closed as shown in the figure. Please, help me to get started on this problem. How can I generate this type of PWM? Then, at a later time the PWM should be shifted with some duty cycle time for Q2, Q3. I am confused. How can I add some delay or shift the PWM? I am using a pic18f45k22 micro controller, and the programming tool is MikroC.

Isolated Boost Convertor

PWM for Switches

Upvotes: 1

Views: 2156

Answers (1)

Weather Vane
Weather Vane

Reputation: 34585

I don't know whether the length of the "off" time is critical, but suppose the mark/space ratio is 1:3 as suggested in your timing diagram, 

Q1,Q4   1011101110111
Q2,Q3   1110111011101

Configure a free-run timer to interrupt at one quarter of your required cycle period. At each interrupt it performs one of four tasks in sequence, such as this pseudo code

void timer_interrupt() {
    static int operation = 0;               // is initialised only once
    clear_timer_status();               // acknowledge the interrupt
    switch (operation) {
        case 0: Q14_off();
                break;
        case 1: Q14_on();
                break;
        case 2: Q23_off();
                break;
        case 3: Q23_on();
                break;
    }
    operation = (operation + 1) % 4;    // advance to next operation
}

If you want a smaller mark/space ratio, you can do it in a similar way. Suppose you want a ratio of 1:7 represented by

Q1,Q4   101111111011111110
Q2,Q3   111110111111101111

now in this case the timer rate should be one eighth of the cycle, but not every interrupt will have an action

void timer_interrupt() {
    static int operation = 0;               // is initialised only once
    clear_timer_status();               // acknowledge the interrupt
    switch (operation) {
        case 0: Q14_off();
                break;
        case 1: Q14_on();
                break;
        case 4: Q23_off();
                break;
        case 5: Q23_on();
                break;
    }
    operation = (operation + 1) % 8;    // advance to next operation
}

There are other ways to do this: such as an array of output bit patterns that you look up as pattern[operation]

Upvotes: 1

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