Buck-Boost Converters (dc-dc)
Buck-Boost converters (DC-DC):
Fig: Buck-Boost converter (dc-dc) Fig: Inductor current (iL) waveform
- In a buck-boost converter (dc-dc) only a switch is shown, for which a device belonging to transistor family is generally used. Also, a diode is used in series with the load.
- The connection of the diode may be noted, as compared with its connection in a boost. The inductor, L is connected in parallel after the switch and before the diode.
- The load is of the inductive type. A capacitor, C is connected in parallel with the load. The polarity of the output voltage is opposite to that of input voltage here.
- When the switch, S is put ON, the supply current flows through the path, Vs, S and L, during the time interval, TON. The currents through both source and inductor increase and are same, with (diL/dt) being positive.
- The polarity of the induced voltage is same as that of the input voltage. The equation for the circuit is,
- Then, the switch, S is put OFF. The inductor current tends to decrease, with the polarity of the induced emf reversing. (diL/dt) is negative now, the polarity of the output voltage,V0 being opposite to that of the input voltage, Vs.
- The path of the current is through L, parallel combination of load & C, and diode D, during the time interval, TOFF. The output voltage remains nearly constant, as the capacitor is connected across the load.
- The equation for the circuit is, .
- The inductor current waveform is shown in Fig. As stated earlier, the current varies linearly from IL1 to IL2 during the time interval, TON. IL1 to IL2 are the minimum and maximum values of the inductor current respectively.
- So, using the expression for diL/dt during this interval, IL2 – IL1 = (VS/L)TON.
- Similarly the current varies linearly from IL2 to IL1 during the time interval, TOFF.
- So using the expression for diL/dt during the interval, IL2 – IL1 = (VS/L)TOFF.
- Equating the two equations, (VS/L)TON = (V0/L)TOFF , from which the average value of the output voltage is obtained as
- For the range, 0 ≥ k ≥ 0.5, the output voltage is lower than the input voltage, thus, making it a buck converter (dc-dc).
- For the range 0.5 ≥ k ≥ 1.0, the output voltage is higher than the input voltage, thus, making it a boost converter (dc-dc).
- For k = 0.5, the output voltage is equal to the input voltage.
- So, this circuit can be termed as a buck-boost converter. Also it may be called as step-up/down chopper. It may be noted that the inductor current is assumed to be continuous.
- The range of k is somewhat reduced. The expression for the output voltage can be obtained by using other procedures.