STEVAL-ISA192V1 7 W dual output flyback converter with standby managed by capacitive touch using VIPer0P and STM32L

This offline converter key feature is the zero power mode (ZPM), an idle state during which the device is totally shut down. The IC enters ZPM by pulling the OFF pin to SGND for more than 10 ms and exits ZPM (resuming normal switching) by pulling the ON pin to SGND for more than 20 μs. This function can be tested via the tactile switches connected to the ON and OFF pins.

In ZPM the mains power consumption at 230 VAC can be below 5 mW; the STEVAL-ISA192V1 evaluation board consumption is 30 mW due to the MCU polling strategy implemented at firmware level.

The board has a dual output (i.e., - 5 V and + 7 V based on flyback topology). The negative output is obtained by just connecting EAGND to the negative rail and a voltage divider among FB, EAGND and SGND. The positive output derives from the turn ratio of the transformer secondary windings.

To achieve extremely low input power consumption, the IC is biased through a diode coming from the + 7 V, which maintains the VCC voltage high enough (> 4.5 V) to prevent the internal HV current source activation.

The STEVAL-ISA192V1 is equipped with the STM32L0 microcontroller which detects tactile push buttons and capacitive touch interface. The STM32L0 core manages VIPer0P to achieve minimum power consumption in ZPM, while supplying the microcontroller and user interface (capacitive touch and push button) and still maintaining the overall efficiency quite high in standard operation mode.