Phase-shift control Method

In this control, the duty cycle varies by phase-shifting the PWM pulses respect to each-other with fixed frequency and turn-on time of each pulse.

duty = Function (phase-shift)

Full-bridge Inverter:

In general, normal full-bridge inverter shown below which is also called as 2-leg converter consist of 4 switches.

Leg-1 switches : S₁, S₄

Leg-2 switches : S₃, S₂

Diagonal switch pairs : (S₁, S₂) & (S₃, S₄)

1. Each leg should operate with complimentary (i.e. 180^o phase) pulses with 50% duty cycle.
2. Each diagonal switch pair shall operate simultaneously.

G₁ – G₄ are gating signals of switches S₁ – S₄ respectively.

The Inverter output voltage generates with positive input voltage when S₁ and S₂ are conducting (t₀ – t₁) and with negative input voltage when S₃ and S₄ are conducting (t₁ – T_s) in one full-switching cycle (0 – T_s) as shown in below figure and it repeats. This is a normal inverter operation.

Phase-shift Control:

The phase-shift control is implemented by delaying the phase in-between diagonal switch pairs i.e. between S₁ and S₂; and between S₃ and S₄.

The leg-1 switches (S₁, S₄) are at original positions, leg-2 switches (S₂, S₃) shift the starting point some angle respectively as shown in below figure.

• When S₁ and S₂ are conducting @ (t₁ – t₂), positive input voltage (+V_s)

• When S₃ and S₄ are conducting @ (t₃ – T_s), negative input voltage (-V_s)

• When both high-side switches (S₁, S₃) @ (t₀ – t₁) or both low-side switches (S₂, S₄) @ (t₂ – t₃) are conducting, ZERO voltage

Due to this phase-shift, the time period (pulse width or duty) of generating inverter output voltage will be reduced with introducing the zero-voltage stages.

Hence, the output voltage can be controlled using phase-shift control method in this manner.