With the main power on, the motherboard's onboard power regulation circuitry (Voltage Regulator Modules or VRMs) begins to fire up. This is the phase where
Once the main rails are stable, localized buck converters (Voltage Regulator Modules) on the motherboard activate in a strict cascading order: desktop motherboard power sequence pdf
The PSU detects PS_ON# going low and instantly activates its heavy-duty internal switching circuits. With the main power on, the motherboard's onboard