What this controller brings to an offline PSU design
The L5991D is a current-mode PWM controller from STMicroelectronics that handles both boost and flyback topologies in isolated offline power supplies. It doesn't have an internal power switch, so you pair it with an external MOSFET — that gives you control over the FET's Rds(on) and gate charge to match your efficiency target. The 93% maximum duty cycle lets it run near continuous conduction, which matters when you need a high step-up ratio from a low input voltage without losing regulation headroom. Switching frequency goes up to 1 MHz, which means the transformer and output filter caps shrink compared to a 100 kHz design — useful when board space is tight. The trade-off is higher switching losses in the external FET, so the gate drive layout and snubber design need attention.
Control features and fault handling
The controller includes frequency control, soft start, and synchronization pins — the sync input lets you lock the switching frequency to an external clock, which keeps the noise spectrum predictable in a multi-rail system. Built-in fault protection covers current limiting and over-voltage on the output side. These are basic protections that prevent a runaway condition from taking out the secondary-side components, but they don't replace a proper OCP/OVP supervisor in a safety-critical supply. Output isolation is supported — the controller drives a transformer with an optocoupler feedback loop, so the primary and secondary sides are galvanically separated. The 10 V to 20 V Vcc range covers the typical bias rail from an auxiliary winding or a separate bias supply.
Package, temperature grade, and lifecycle
The L5991D is in a 16-SOIC package, supplied in Tube form — not Tape & Reel. If your pick-and-place line expects reeled parts, the alternate order code L5991D013TR is the same die in Tape & Reel packaging. Operating junction temperature range is -40°C to 150°C, which qualifies as industrial grade. The 150°C TJ limit means you have thermal margin in a hot enclosure as long as the junction stays below that peak — the actual case temperature will be lower depending on the PCB copper area and airflow. Sourced to order against your BOM quantity.