Resistorless Precharging of Boost PFCs by Phase Angle Control of Thyristors

Most AC/DC Power Supply Units (PSUs) and automotive on-board chargers (OBCs) which operate directly from the AC mains employ a PFC boost converter as front-end stage. The PFC stage regulates the AC side power factor close to one and at the same time it also controls the DC-link voltage, i.e. the input voltage for subsequent converter stages, to an adjustable constant value. Most PFC stages operate in boost mode and therefore require a precharge circuit for limiting the initial current required to charge up the DC-link capacitor (from 0V to a value close to AC voltage amplitude) prior to the regular PFC boost operation. Conventionally current limiting during the precharge interval is facilitated by very bulky resistors which are bypassed by a kind of switch (relay or semiconductor based) when the regular boost operation starts. Both, switches and precharge resistors are extra components which are exclusively used for initial precharging only and do require a considerable volume and cost effort. This paper proposes a method to totally omit the precharge resistors and moreover simplify the switch implementation. With just replacing two rectifying diodes by thyristors and providing an appropriate phase angle control precharge functionality is reached. The proposed concept is finally verified by measurements on a 3.3kW prototype.