Current-mode PWM controller for flyback and boost converters
The Texas Instruments UC3843AD8TRG4 is a current-mode PWM controller designed for step-up, step-down, and step-up/step-down topologies including boost, buck, flyback, and forward converters. It delivers a single transistor-driver output with a maximum duty cycle of 96% and switches at frequencies up to 500 kHz. The supply range of 7.6 V to 30 V lets it run directly off a rectified 24 V industrial rail without a pre-regulator, and the 8-SOIC package keeps the footprint compact for space-constrained power supplies.
96% duty cycle — near-continuous on-time for high step-up ratios
The 96% maximum duty cycle is the defining spec of the UC3843A family. In a flyback converter operating in continuous conduction mode, this allows a near-continuous on-time, which directly translates to a higher step-up ratio from a low input voltage. For a 5 V to 48 V boost or a universal-input offline flyback, the 96% ceiling gives the designer headroom to avoid pulse-skipping at the high-line minimum load condition. The UC3842/3/4/5 family uses a duty-cycle clamp: the UC3843A is the 96% variant, while the UC3842A clamps at 50% and the UC3845A at 50% with a lower UVLO threshold.
Switching up to 500 kHz — magnetics size vs. gate-drive loss
The oscillator frequency is set by an external RT/CT network and runs up to 500 kHz. At 500 kHz the transformer or inductor shrinks significantly compared to a 100 kHz design — useful for a 10 W to 50 W flyback in an adapter or auxiliary supply. The trade-off is gate-drive current: the UC3843A's totem-pole output sources and sinks a peak of 1 A, so above 200 kHz the gate charge of a large MOSFET (e.g., 30 nC to 50 nC) will dissipate several hundred milliwatts in the driver. The datasheet's frequency-compensation and slope-compensation guidance should be followed above 300 kHz to avoid subharmonic oscillation at duty cycles above 50%.
That means no last-time-buy deadline, no forced redesign for a current BOM line. The base product number UC3843A is a long-running industry-standard PWM controller, so supply continuity is well established. For dual-sourcing flexibility, the pin-compatible UC3843BD8TRG4 is a direct cross — same function, same 96% duty cycle, same 8-SOIC footprint.
This is the commercial-grade envelope: indoor power supplies, office equipment, appliance control boards, and telecom line cards in a climate-controlled rack. For a motor drive in an unventilated cabinet or an outdoor base station that sees -20°C start-up, the industrial-grade UC3843AD8TR (same function, -40°C to 85°C) is the correct choice. The 0°C lower limit means the controller may not start reliably below freezing — a common trap when a commercial-rated part is specified into a marginally ventilated enclosure.
Positive output configuration with isolation capability
The output configuration is positive, isolation capable — meaning the single transistor-driver output drives the primary-side MOSFET directly, and the feedback loop can be closed through an optocoupler for galvanic isolation. This is the standard arrangement for offline flyback and forward converters where the secondary side must be isolated from the mains. The frequency control feature allows the oscillator to be synchronised to an external clock (when the RT/CT pin is driven above the oscillator ramp), useful for multi-phase or noise-sensitive designs.