Boost Regulator

BOOST REGULATOR

The boost regulator produces an output voltage greater than input voltage. Fig.5.35 shows the circuit diagram of boost regulator using power MOSFET. The circuit operation has 2 modes.

1. Mode 1

At t = 0, Q₁ is switched ON

The input current flows through the inductor L and transistor Q₁.

2. Mode 2

At t = t₁ Q₁ is switched OFF

The current flowing through transistor will not flow through L, C load and diode D₁.

i. The inductor current drops until transistor Q₁ is turned ON again in the next cycle.

ii. The energy stored in inductor L is transferred to the load. Fig.5.35 shows the equivalent circuit for mode 2.

Fig.5.36 shows the equivalent circuit for mode 1.

Fig.5.37 equivalent circuit for Mode 1

Fig.5.38 shows the waveform for voltages and currents for continuous load current.

Assume inductor current rise linearly from I₁ to I₂ in time t₁,

In time t₂, the inductor current falls linearly from I₂ to I₁.

where ∆I - peak-to peak ripple current of inductor L

Equate (3) and (4)

Substitute t₁ = KT and t₂ = (1 - K) T

Assume lossless transistor

Substitute (7)

Switching period T is given by

T = t₁ + t₂

Substitute (2) and (5)

Peak-to-peak ripple current,

Substitute T = 1/f

when the transistor is ON, the capacitor supplies the load current for t = t₁.

The average capacitor current is I_C = I_a

The peak-to-peak ripple voltage of the capacitor is

Substitute I_C = I_a

From Equation (7)

Substitute t₁ in (11)

Advantages

i. It can step up the output voltage without a transformer

ii. High efficiency due to single transistor

iii. Input current is continuous which is very desirable for sources like battery

Disadvantages

1. Output voltage is very sensitive to variations in the duty cycle K and is difficult to stabilize the regulator.

2. It is difficult to protect the output circuit in case of short circuit as the transistor is connected in shunt with the load.

3. Requires larger filter capacitor and inductor compared to buck regulator.

4. There is no isolation from input to output

Applications

1. Hybrid electric vehicles

2. Lighting systems

Comparison of Buck and Boost Converter