8877 Linear Amplifier Power Supply
Plate Supply -- This is a switching design with some unusual features.
Bridgeless Power Factor Correction (PFC) --
This geometry eliminates the typical bridge rectifier found in a PFC. The phase of the incoming AC line is detected and the appropriate low side FET is switched on for the duration of the half cycle. The input current sense is also switched to this side of the input. The other FET is then controlled in a normal manner by the PFC IC. When the AC line input reverses, the assignments are swapped. The FETs are very low on resistance devices for minimal power loss in this part of the circuit. The boost rectifiers are Silicon Carbide devices to minimize recovery losses.
Power Inverter --
The inverter is a phase modulated H bridge which regulates the output voltage.
Transformer & Rectifiers --
The power transformer is a coaxial design. The primary winding consists of tubular copper braid. Seven individually insulated twin run secondaries are contained within the primary. This construction makes for an easy to wind unit with very low leakage inductance. Each secondary feeds a bridge rectifier, the outputs of which are then combined in series to create the output voltage via a two stage LC filter.
Soft Start --
A soft start is implemented by means of a power FET in the return leg of the bulk storage capacitors. This FET is driven by a comparator which keeps the FET disabled until the capacitors have been charged to 250 volts. The comparator also disables the PFC IC until this threshold is exceeded.
Smart Bleed --
When the low voltage power supply turns off, a 100mA bleed current is drawn from the bulk storage capacitors.