Buck

Used for: Stepping a voltage down in value with high efficiency

Literature references

Pros:

  • Simple step-down circuit
  • High level of support with control ICs and application info
  • Switch SW1 voltage limited to Vin + SW2 drop + spike
  • Current limiting/short circuit protection relatively easy because of series switch
  • Low output ripple current into C1

Cons:

  • Needs high-side gate drive or P-channel switch for SW1
  • Step-down in voltage only
  • High input ripple current from input cap C2  - needs to be low ESR
Buck

Basics:

  • SW1 and SW2 are alternately on and off at high frequency
  • SW1 and SW2 are never both on together
    Vout = Vin x D where D is the switching duty cycle (0 -1)
  • Input ripple current is high and must be sourced from C2
  • Output ripple current into C1 can be low, set by L1 value
  • SW2 can be a diode - the action of SW1 turning off automatically forward biases the diode causing it to conduct. Action of SW1 turning on reverse biases the diode causing it to block current

CONTROLLING A BUCK CONVERTER WITH CURRENT MODE CONTROL WITH SLOPE COMPENSATION

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Vout is compared with a reference using the error amplifier producing a voltage Ve proportional to the difference. This is compared with a representation of inductor current Is as a voltage dropped across RCS. R2 adds in a small sawtooth signal to achieve slope compensation. Inductor current is a sawtooth so the PWM stage compares this with Ve to produce pulses:  wider = output too low, narrower  =  output too high. See HERE for detail of PWM.

See HERE for explanation of slope compensation. Details such as error amplifer compensation have been omitted. See below

CONTROL LOOP PARAMETERS

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Vramp = amplitude of sawtooth ramp  from inductor current and slope compensation
Ri  = Rcs = Rsense x Current gain eg. Rsense x 100 for 100:1 sense transformer
Optimum slope compensation assumed in each case
D = Duty cycle
f = operating frequency