The Texas Instruments UC3844D8 is a current-mode PWM controller with a maximum duty cycle of 48%, intended for single-ended topologies — flyback, forward, boost, buck, and buck-boost. The 48% ceiling is the hard limit for the on-time of the external power switch; in a flyback design it sets the maximum volt-seconds the transformer can deliver before the core resets. The controller drives a single transistor output and includes frequency control for oscillator synchronisation.
Switching frequency up to 500 kHz — transformer sizing trade-off
The oscillator runs up to 500 kHz, which lets the designer shrink the magnetic components — a 200 kHz flyback transformer is about half the core volume of a 100 kHz design. The trade-off is higher switching losses in the MOSFET and snubber, so the actual frequency choice depends on the thermal budget of the power stage. The controller does not include clock synchronisation, so multiple converters on the same board must be free-run or use external sync injection.
12V to 28V supply rail — bias winding planning
The Vcc supply range of 12V to 28V means the controller needs a regulated auxiliary winding or a separate bias supply within that window. Below 12V the undervoltage lockout holds the output off; above 28V the internal regulator clamps. In a typical offline flyback, the auxiliary winding is designed to deliver ~16V–18V after startup, which sits comfortably inside the range. The output is a single transistor driver, positive polarity, with isolation capability through an external optocoupler feedback loop.