Current-mode PWM controller for flyback and boost stages
The Texas Instruments UC3845AD8G4 is a current-mode PWM controller for boost, buck, flyback, or forward converters. It delivers a single transistor-driver output with a maximum duty cycle of 48%. The 48% duty cycle ceiling prevents pulse stacking in single-ended flyback converters, keeping the transformer core reset between cycles.
Switching frequency and transformer sizing
The oscillator runs up to 500 kHz, which lets the designer shrink the magnetic components compared to a lower frequency design. There is no clock-sync pin on this variant — the UC3845AD8G4 runs free-running. If synchronisation to an external clock is needed, the UC3845 family does not support it; look at the UCC28C4x series for that feature.
Supply range and bias design
The Vcc range spans 7.6 V to 30 V, covering common bias rails like 12 V and 24 V industrial supplies without a pre-regulator. The undervoltage lockout (UVLO) threshold for the UC3845 variant is 8.4 V (start) / 7.6 V (stop), which matches the lower end of the supply range — the controller starts when Vcc crosses 8.4 V and shuts down if it drops below 7.6 V.
It belongs in indoor power supplies, telecom line cards, and benchtop equipment — not in an engine bay, outdoor enclosure, or any location where the ambient temperature cycles below freezing or above 70°C.
The tube shipment is fine for prototyping and low-volume production.