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Control of DC-DC converters by PWM



  1. Generate triangular waveform of period T as the carrier signal vcr, and a DC reference signal vr.
  2. Use comparator to compare signals vr with vcr so that:
v r < vcr   where vg = high
v r < vcr   where vg = low

    Apply vg as IGBT gate pulse (gives vge).
    {v_r} = \frac{{{V_r}}}{T}t
    k = \frac{{{V_{cr}}}}{{{V_r}}}

    k is also called the modulation index, M
    Power Electronic Converter: Control of DC-DC converters by PWM
    Power Electronic Converter: Control of DC-DC converters by PWM
    Example-1
    For a buck converter, R=1 ohm, Vd = 40 V, V0=5 V, fs = 4 kHz. Find the duty ratio and “on” time of the switch.


    Power Electronic Converter: Control of DC-DC converters by PWM
    Solution 1
    k = V0 /Vd = 5/40 = 0.125 = 12.5%
    Ts = 1/fs = 0.25 μs = 250 μs
    Ton = kTs = 31.25 μs
    Toff = Ts – ton = 218.75 μs
    When the switch is “on”: VL = Vd - V0 = 35 V
    When the switch is “off”: VL = -V0 = - 5 V
    I0 = IL = V0 / R = 5 A
    Id = k I0 = 0.625 A

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