Soft Start is always used when the unit is switched on and off in order to reduce stress for the components and improve the output response. When the system is switched on, the output voltage should rise from zero to the final value following a smooth ramp. When the system is switched off, the output voltage should fall from the initial value to zero following a smooth ramp.
Soft start can be implemented digitally. In a closed-loop system, the output voltage will follow the voltage value (Vref). The dsPIC® DSC will then increase the Vref value from zero to the final value in steps to create the desired ramp.
Soft Start Ramp
To generate a soft start ramp, there is a background timer that generates equally-spaced in-time ticks (for instance these ticks repeat every 200 us). In correspondence with each tick, the dsPIC firmware increments the reference voltage by a preset value. This value is a function of the output steady state voltage and desired ramp duration. The resulting ladder voltage appears at the output of the converter as a smooth voltage because of the “low pass-filter” effect produced by the PID and the short duration of each tick.
Soft start is implemented by the state machine as a number of states. The transition from one state to the following also defines the output voltage waveform.
The states are:
- Stand-by: the initial state where the system is waiting for the signal to switch on the unit. This same state is reached at the end of the switch-off procedure.
- Ramp-up delay: the user can select a programmable delay before the output starts ramping
- Ramp-up: the voltage ramps from zero to the final steady-state value
- Run: normal state when the system is working
- Ramp-down delay: when the unit is switched off, the user can add a programmable delay
- Ramp-down: the voltage ramps down from the steady state to zero. At the end of the ramp, the system goes back to the stand-by state.