Imagine you're designing a redundant power system: two synchronous buck converters feed the same 1.2 V rail to a server FPGA. One is running, the other is on standby. When the standby unit starts up, it sees the rail already sitting at 1.2 V — held there by its partner. What happens next determines whether your board survives the next ten seconds.

A naive synchronous buck converter will start its PWM at zero duty cycle and ramp up. But with a low-side MOSFET that switches ON during the first dead-time interval, the output capacitor (already charged by the other supply) dumps current backward through the inductor into the low-side FET. The inductor current goes negative, the high-side body diode can conduct on the next cycle, and you get a brutal output droop — sometimes pulling the rail below regulation and resetting the load. In worst cases, the reverse current trips overcurrent protection or damages the FETs.

The solution is pre-bias startup (also called "monotonic startup"). The controller must detect that the output is already above zero and behave like a diode-rectified converter until its internal soft-start ramp catches up to the actual output voltage.

Two common implementations:

• Diode emulation mode: The low-side FET is held OFF during startup. The converter operates in discontinuous mode, allowing the inductor current to free-wheel through the high-side body diode (or low-side body diode in some topologies) but never reverse. Once the soft-start reference exceeds the feedback voltage, normal synchronous switching engages.
• Tri-state PWM: Both FETs are held off whenever the internal ramp is below the FB voltage. Switching begins only when the ramp would command an output higher than what's already present.

Quick design check: If your soft-start time is t_ss = 2 ms and your output cap is 100 µF charged to 1.2 V pre-bias, the maximum allowable reverse current to keep droop under 50 mV is roughly:

I_rev,max = C × ΔV / Δt = 100 µF × 50 mV / 2 ms = 2.5 mA

That's a tiny budget — far less than the inductor ripple current of a typical 5 A converter (~1 A peak-to-peak). Without pre-bias support, you'd blow that budget on the first switching cycle.

When selecting controllers, look for datasheet keywords: "pre-bias startup," "monotonic startup," "diode emulation during soft-start," or "non-inverting startup." Parts like the TI TPS54620, LM27402, and Analog Devices LTC3833 explicitly handle this. Don't trust a generic synchronous buck controller — many older designs silently fail this test.