Current-mode PWM controller for isolated and non-isolated topologies
The Texas Instruments UC2845D8 is a current-mode PWM controller designed for step-up, step-down, and step-up/step-down converter topologies including boost, buck, flyback, and forward converters. It operates from a 12V to 28V supply rail and delivers a single transistor-driver output with a maximum duty cycle of 48%. The switching frequency reaches up to 500kHz, allowing the designer to shrink magnetics and reduce output capacitor size in medium-power isolated supplies.
48% duty cycle — what it means for flyback and forward designs
The 48% max duty cycle prevents transformer saturation in single-ended topologies. In a flyback converter, this cap sets the maximum volt-second product across the primary winding.
Switching frequency up to 500 kHz — magnetics and EMI trade-offs
The oscillator can be programmed to switch at frequencies up to 500 kHz. Running at the upper end shrinks the transformer core size and reduces output filter capacitance.
Supply voltage range and bias rail design
The UC2845D8 requires a Vcc supply between 12V and 28V. In a flyback converter, this bias is typically derived from an auxiliary winding on the transformer. The 12V minimum means the controller will not start until the auxiliary rail has built up to at least 12V, which sets the startup resistor and the Vcc capacitor sizing. The 28V upper limit accommodates 24V industrial bus supplies directly, but if the auxiliary winding produces a higher voltage under light load, an external zener clamp or a linear regulator may be needed to keep Vcc within the 28V absolute maximum.
Package and footprint: 8-SOIC
Housed in an 8-pin SOIC package (body 3.90 mm wide), the UC2845D8 is a surface-mount part that fits standard SOIC-8 land patterns. The tube shipping option is typical for engineering samples and low-volume production; for higher volumes the part is also available in tape-and-reel (check the full ordering code suffix).
The part is ROHS3 compliant.