5 Measuring Efficiency in a Switch-Cap Charger

A switch-cap charger has a ratio of input to output voltage that must be maintained for the converter to work. In a 2:1 switch-cap charger, the input voltage is always slightly higher than 2x the battery voltage. For a switch-cap charger or inductor-less charger, there is usually not a system rail as the output is not regulated. Switch-cap chargers run open-loop, entirely dependent on the input voltage applied, the battery voltage applied, the resistance between the power supply and the converter, and the resistance between the converter and the battery simulator. However, like the buck charger, in addition to input and output current being measured, the Kelvin sense for the input voltage and output voltage need to be measured. The issue of efficiency accuracy is even higher for switch-cap chargers due to the high current output. The BQ25960H can power up to 8A with a single charger, and there can be a significant drop from the output of the converter to the edge of the EVM or the battery. Therefore, this is critical to measure the output voltage of the converter to accurately represent the efficiency of a switch-cap charger.

Take the BQ25960HEVM as an example of a switch-cap charger EVM. The BQ25960H is a 2:1 switch-cap charger, meaning the input voltage is twice the output voltage. TP2 marks the Kelvin sense for the input voltage, or VBUS_M. TP5 marks the Kelvin sense for the output voltage, or VOUT_M. While the input power is physically applied through J1 and the battery simulator is physically applied through J2, the input voltage for the efficiency measurement must be taken at TP2 and the output voltage for the efficiency measurement must be taken at TP5. An example of how to connect the power supply, battery simulator, input voltage measurement, and output voltage measurement for the BQ25960HEVM is shown in Figure 5-1.

Figure 5-1 BQ25960HEVM Efficiency Measurement Setup