SLUAAY8 October 2024 BQ25960H
First, users need to know how to calculate the efficiency for a battery charger. For the purposes of this example, assume there is no system load and that the only output from the battery charger is going to charge the battery. Efficiency is measured as the output power divided by the input power. For the case of the battery charger, the efficiency is the battery voltage multiplied by the battery current divided by the input voltage and input current. This is summarized in Equation 1.
The best place to measure any voltage that is also in a high current path, whether in a battery application or not, is through a Kelvin sense point. A Kelvin sense point is a thin trace that extends to a measurement point and goes directly to the pin on the IC that corresponds to the desired voltage. The trace cannot be in the conduction path of the voltage being measured. The trace also needs to be narrow and not meet the high current path except for at, or as close as possible, to the IC. Do not use the Kelvin sense path as the main power path for the power being routed, as the trace is not designed for the current being transferred. In general, the ADC of the charger IC must not be used to calculate the efficiency of the charger.
Another issue to keep in mind when measuring efficiency is how to measure the current. There are two ways to measure current, either with a sense resistor or with a direct current measurement from a sourcemeter or equivalent. There are tradeoffs to both, however the best way is to use a sourcemeter. When using a sense resistor, careful calibration is required to get accurate results. With multiple Printed Circuit Boards (PCBs), the calibration can be different from resistor to resistor. In addition, using a multimeter with even 1mV of error can make a huge difference for the efficiency. A sourcemeter is calibrated once and can work with multiple PCBs without additional steps.