Current-mode PWM controller for multi-topology power supplies
The UC3843N: Its 97% maximum duty cycle suits flyback and forward converters operating near continuous conduction mode, where the on-time must extend beyond 90% without pulse skipping. The supply range of 12 V to 28 V on Vcc/Vdd means the controller can be biased directly from a regulated auxiliary winding or a downstream linear regulator without an additional pre-regulator.
Temperature grade and package — prototype bench and repair bench fit
Packaged in a tube (standard DIP shipping format), the part is suited for small-quantity procurement and hand-loading into sockets or through-hole boards.
Output drive and control — external switch, no synchronous rectification
The transistor-driver output (single output, single phase) drives an external power MOSFET or IGBT gate directly — the controller does not integrate the power switch, so the designer selects the external FET based on the rail voltage and load current. No synchronous rectification is built in — the output stage uses a diode for the freewheeling path, which adds forward-drop loss compared to a synchronous buck controller but simplifies the bill of materials for lower-power designs. Frequency control via an external RC network sets the oscillator; the part does not support clock synchronization, so multiple UC3843N controllers in a system run at independent frequencies unless externally synchronized through a separate clock injection circuit. Output configuration is positive, isolation capable — the error amplifier and reference are referenced to ground, and an optocoupler can close the feedback loop across the isolation barrier in flyback and forward converters.
RoHS3 compliant, so it meets current European substance restrictions for legacy BOMs.