Current-mode PWM controller for multi-topology power supplies
The Texas Instruments UC3843D8 is a current-mode PWM controller designed for step-up, step-down, and step-up/step-down topologies including boost, buck, buck-boost, flyback, and forward converters. It operates from a 12 V to 28 V supply rail and delivers a single transistor-driver output with a maximum duty cycle of 97%. The switching frequency reaches up to 500 kHz, allowing the designer to trade off transformer/inductor size against switching losses. This is a commercial-temperature-grade part (0°C to 70°C), suited for indoor power supplies, AC-DC adapters, and isolated DC-DC modules where the ambient stays within that range.
A 97% maximum duty cycle is notably high for a current-mode controller. It gives the designer headroom in low-dropout or high step-down ratio designs where the on-time must stay wide even at minimum input voltage. The trade-off is that the controller does not include clock synchronization (no external sync input), so the switching frequency is set by an external RC network and runs free-running. For multi-phase or noise-sensitive systems that require phase-locked switching, a synchronizable controller would be needed.
The UC3843D8 is supplied in an 8-pin SOIC package (8-SOIC, 0.154" body width, 3.90 mm width) and ships in Tube form. The supply voltage range of 12 V to 28 V (Vcc/Vdd) means the controller needs a bias rail that stays above 12 V — it is not a wide-Vin part that starts from 5 V. The output configuration is positive and isolation-capable, so it can drive a primary-side MOSFET in an isolated flyback or forward converter with an optocoupler feedback loop.
The UC3843D8 is listed as Active (current production) with ROHS3 compliance.