Buck Converters (dc-dc)
Buck Converters (DC-DC):
Fig: Buck Converter Circuit Diagram & Waveforms
- A diode (termed as freewheeling) is used to allow the load current to flow through it, when the switch (i.e., a device) is turned off. The load is inductive (R-L) one.
- In some cases, a battery (or back emf) is connected in series with the load (inductive).
- Due to the load inductance, the load current must be allowed a path, which is provided by the diode; otherwise, i.e., in the absence of the above diode, the high induced emf of the inductance, as the load current tends to decrease, may cause damage to the switching device.
- If the switching device used is a thyristor, this circuit is called as a step-down chopper, as the output voltage is normally lower than the input voltage.
- Similarly, this dc-dc converter is termed as buck one, due to reason given later. The output voltage and current waveforms of the circuit.
- The output voltage is same as the input voltage, i.e. v0 = Vs ,when the switch is ON, during the period, TON ≥ t ≥ 0. The switch is turned ON at t= , and then turned OFF at t= TON. This is called ON period. During the time interval, T ≥ t ≥ TON, the output voltage is Zero, i.e., v0=0, as the diode DF now conducts. The OFF period is TOFF = T – TON ,with the time period being T = TON TOFF. The frequency is f = 1/T. With T kept as constant, the average value of the output voltage is
- The duty ratio is k= (TON/T) = [TON/(TON TOFF)], its range being 1.0 ≥ k ≥ 0.0 . Normally, due to turn-on delay of the device used, the duty ratio (k) is not zero, but has some positive value.
- Similarly, due to requirement of turn-off time of the device, the duty ratio (k) is less than 1.0. So, the range of duty ratio is reduced. It may be noted that the output voltage is lower than the input voltage.
- Also, the average output voltage increases, as the duty ratio is increased. So, a variable dc output voltage is obtained from a constant dc input voltage. The load current is assumed to be continuous.
- The load current increases in the ON period, as the input voltage appears across the load, and it (load current) decreases in the OFF period, as it flows in the diode, but is positive at the end of the time period, T.