Boost Converters (dc-dc)
Boost converters (DC-DC):
Fig: Boost converters Circuit & Waveform of source current is
- In a 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 load is of the same type as the Buck converter i.e inductive.
- The inductance of the load is small. An inductance, L is assumed in series with the input supply.
- Firstly, the switch, S (i.e., the device) is put ON (or turned ON) during the period, TON ≥ t ≥ 0, the ON period being TON. The output voltage is zero, if no battery (back emf) is connected in series with the load, the load inductance is small. The current from the source (is) flow in the inductance L.
- The value of current increases linearly with time in the interval, with (di/dt) being positive. As the current through L increases, the polarity of the induced emf is taken positive, the left hand side of L being ve. The equation for the circuit is, or, .
- The switch, S is put OFF during the period, T ≥ t ≥ TON. . The OFF period is TOFF = T – TON ,with the time period being T = TON TOFF. As the current through L decreases, with its direction being in the same direction as in the case of the Buck converters, the induced emf reverses, the left hand side of L being -ve.
- So, the induced emf (taken as –ve in the equation given later) is added with the supply voltage, being of the same polarity, thus, keeping the current () in the same direction. The current (is = i0) decreases linearly in the time interval, TOFF, as the output voltage is assumed to be nearly constant at v0 ≈ V0, with (dis/dt) being negative, as Vs < V0.
- The equation of the circuit is given as
- In the source current waveform earlier, the current varies linearly from I1 (Imin) to I2 (Imax) during the time interval TON. Using the expression for dis/dt during this time interval, . Similarly, the current varies linearly from I2 (Imax) to I1 (Imin) during the interval TOFF.
- The ON time interval is TON = kT. The range of is reduced. This is, because the minimum value is higher than the minimum (0.0), and the maximum value is lower than the maximum (1.0), for reasons given there, which are also valid here.
- The source current is assumed to be continuous. The expression for the output voltage can also be obtained. The output voltage is higher than the input voltage, as contrasted with the previous case of buck converter (dc-dc). So, this is called boost converter (dc-dc), when a self- commutated device is used as a switch.
- Instead, if thyristor is used in its place, this is termed as step-up chopper. The variation (range) of the output voltage can be easily computed.