A switch-mode power supply controller controls a circuit that includes a flyback-based, switch-mode power supply in the context of an input voltage source, a USB Type-C PD controller and an output load. The switch-mode power supply controller may be configured to estimate input voltage based on a measured magnetizing inductance discharge time. Furthermore, the switch-mode power supply controller may be configured to estimate output voltage based on the measured magnetizing inductance discharge time and the estimated input voltage. Still further, the estimated voltages may be used by the switch-mode power supply controller to limit certain currents and optimize power efficiency. Even further, the estimated and measured value may be employed by the switch-mode power supply controller to estimate and indicate brownout conditions.