Buck Regulator

BUCK REGULATOR

Buck converter is also known as step down converter and produces a lower output voltage than input voltage. The average output voltage is less than the input voltage. Fig.5.31 shows the circuit diagram of a buck regulator using power BJT.

i. The switch is implemented by using BJT, MOSFET or IGBT. The duty cycle varies from 0 to 1.

ii. The output voltage has harmonics which requires DC filter for smoothing out ripples.

iii. Gating signal is obtained by comparing the gating signal with de signal and hard limiting the difference signal to obtain a square wave pulse.

iv. The control circuit switches the transistor Q₁ between ON and OFF states. The indicator current forward biases the diode D₁ and D₁ conducts for the remaining switching period. This diode is known as Free Wheeling Diode.

Operation

The circuit operates in 2 modes.

Mode 1

At t = 0, Q₁ is switched ON.

i. The input current flows through filter (L and C) inductor and capacitor and load resistor. Fig.5.32 shows the equivalent circuit for Mode 1.

Mode 2

At t = t₁, Q₁ is Switched OFF.

i. The free wheeling diode D₁ conducts due to energy stored in inductor.

ii. The inductor current flows through L, C, Load and diode D_, until Q₁ is switched ON in the next cycle.

Fig.5.33 shows the equivalent circuits for Mode 2.

For continuous current flow in the inductor L, The voltage and current waveform are shown in Fig.5.34. The inductor current may be discontinuous depending on the switching Frequency, filter inductance and Capacitance.

The voltage across the inductor L is

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

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

where ΔI - peak to peak ripple current of inductor L Equate (3) and (6)

Assume lossless transistor,

Average input current,

I_S = K I_a

Switching period T is given by

Using KCL,

Load current is written as

i_L = i_C + i_o ...(9)

Δi_o is very small and negligible

Δi_L = Δi_C

Average capacitor current is given by

I_C = ΔI/4 ...(10)

Capacitor voltage is given by

Peak to Peak ripple voltage of the capacitor is

ΔV_C = V_C - V_C (t=0)

Substitute (11)

Substitue (10)

Substitute (8) in (12)

The converter produces a dc output voltage which is controllable through the duty cycle D.

The conversion ratio is defined as the ratio of the dc output voltage V₀ to dc input voltage V_in under steady-state conditions.

For bulk converter,

M(D) = K

Advantages

1. Requires only one transistor

2. Simple construction

3. High efficiency

4. Less expensive

Disadvantages

1. di/dt of the load current is limited by inductor

2. Smoothing input filter is required

3. Need protection circuit to avoid short circuits

4. Input current and charging current of output capacitor is discontinuous which require large filter size.

Applications

1. Used to drive high current loads, used in PCs, mother boards

2. Battery chargers

3. Solar chargers