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Buck Converters (Non-isolated)


Used for:

Stepping a voltage down in value with high efficiency


Pros:




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Cons:

BASICS:

SW1 and SW2 are alternately on and off at high frequency

SW1 and SW2 are never both on or both off 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


Buck Literature References

CONTROL LOOP PARAMETERS

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    


CONTROLLING A BUCK CONVERTER WITH CURRENT MODE CONTROL WITH SLOPE COMPENSATION



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