Current-mode PWM controller for boost, flyback, and forward converters
The onsemi TL3843BD-8 is a current-mode PWM controller designed for step-up (boost) and step-up/step-down (flyback, forward) topologies. It integrates a transistor driver output, frequency control, and operates from a 7.6 V to 30 V supply rail. Switching frequency reaches up to 500 kHz, and the maximum duty cycle extends to 96% — unusually high for a current-mode part, giving headroom in wide-input-ratio designs where the off-time is compressed. The commercial temperature range (0°C to 70°C) suits indoor power supplies, appliance PSUs, and office-equipment converters. Housed in an 8-SOIC package, it mounts on standard surface-mount boards.
96% duty cycle — what it buys you
A 96% maximum duty cycle means the controller can keep the switch on for nearly the entire switching period. In a boost converter with a high step-up ratio — say 12 V in to 48 V out — the required duty cycle can exceed 85%. Many current-mode controllers clip at 50% or 85%, forcing a two-stage solution or a coupled-inductor topology. The TL3843BD-8 handles those ratios in a single stage, saving inductor count and board area. The trade-off: the off-time is only 4% of the period, so the transformer reset time in a flyback or forward converter is tight; plan the turns ratio and leakage inductance accordingly.
Switching frequency up to 500 kHz — transformer sizing
The TL3843BD-8 switches up to 500 kHz. At that frequency, a flyback transformer for a 30 W design can use a ferrite core roughly half the volume of one designed for 100 kHz. The controller does not include clock synchronisation (no SYNC pin), so it runs free-running. If your system needs to synchronise multiple converters to a common clock, consider a part with clock sync capability. For single-converter designs, the 500 kHz ceiling lets you reduce magnetics size without pushing into the MHz range where gate-drive losses and snubber dissipation climb steeply.
Supply range and start-up behaviour
The Vcc range is 7.6 V to 30 V. The 7.6 V minimum means the controller will not start from a 5 V rail — a common bias in telecom or USB-powered designs. If your input rail is 5 V, you need a separate boost stage or a different controller with a lower UVLO threshold. On a 12 V or 24 V industrial bus, the TL3843BD-8 runs directly without a pre-regulator. The upper 30 V limit covers 24 V systems with transients up to 30 V, but a 48 V bus requires an external dropping resistor or a linear pre-regulator to stay inside the abs-max rating.
Lifecycle and sourcing posture
The TL3843BD-8 carries an Active lifecycle status and is ROHS3 compliant. It is sourced through independent distribution and quoted to order against an RFQ.