Need to estimate buck converter efficiency in portable apps?

In this article, characteristics of PFM operation are examined. System efficiency is then calculated accordingly. Predicted efficiency within the whole operating range is then compared to test results.

Efficiency analysis of buck converter

PWM mode

Duty cycle calculation at PWM mode

Refer to Figure 2-1 for circuit schematic.

When top PFET is on, using KVL, the following equation is obtained:

Equation (2-1) can be simplified as:

When PFET is off and the NFET is on, using KVL, the following equation is obtained:

Equation (2-3) can be simplified as:

Equation (2-2) minus equation (2-4), we have:

Efficiency estimation:

Definition of several terms:

Efficiency estimation under PFM mode

Burst and Pulse frequencies

Typical inductor current and output voltage waveforms are shown in Figure 2-4.

During PFM, a nonlinear bang-bang control is applied. There are three boundary conditions to regulate output voltage: peal inductor current, Vout upper threshold, Vout lower threshold.

When PFET turns on, the inductor current would increase until it reaches current limit. Note: this current limit is set specifically for PFM, not the over current limit for circuit protections.

When PFET turns off, NFET turns on, the inductor current would decrease until it reaches zero. The following equation is satisfied:

On the other hand, when Vout reaches its upper threshold, the converter shuts off. Only Cout supports the load current until Vout decreases to lower threshold.