Sorensen DCS Series 3kW Supplies
Theory of Operation
5.3.1
5.3.2
5.3.3
AC Input and RFI Filter Circuit (A5 PCB)
The AC input is connected to the power supply via a four-pole connector in the rear panel. For
three phase operation, three (3) phase lines, X, Y, and Z (190-250 Vac at 14A), and a safety
ground line are required. For single phase operation, two (2) phase lines (200-250 Vac at 20A),
and a safety ground line are required. Single phase input lines are connected to the X and Z
inputs of the connector so that each line is connected to a separate rectifier on the A6 PCB. For
continuous use with single phase inputs, the output power from the supply must be limited to
2500W to avoid tripping the input circuit breaker. The safety ground connection for both single
and three phase operation is essential both to ensure that there is no shock hazard present and
to ensure proper operation of the RFI filter and other bypass circuits.
The RFI filter, used to attenuate both common mode and differential mode noise, is an LC filter
formed by X (line-to-line) capacitors C1-3 and C7-9, inductor L1, Y (line-to-ground) capacitors
C4-6, and resistors R1-3 which serve as discharge resistors for the X capacitors.
Input Rectifier and Inrush Limiting Circuit (A6 PCB)
The AC input is rectified by the heatsink-mounted rectifiers CR2 and CR2A and then filtered by
inductor L1 and capacitor C2 to provide the raw high voltage DC (approximately 250-350V) for
the main switching transistors and the A2 PCB auxiliary supply circuit. Resistor R2 provides a
discharge path for the bulk capacitor C2 on power down. Capacitors C1, C3, and C4 provide
noise filtering. The initial power up inrush current to capacitor C2 is limited by resistors R1.
When the DC voltage reaches approximately 240V, the coil of relay K1 is energized by the
auxiliary supply circuit on the A2 PCB and the inrush resistor is shorted by the relay contacts.
Input current then flows through the relay contacts.
PWM, Sync Circuit, Power Transformer Drive Circuit, and
Power Transistor Bridge (A3 PCB)
The A3 PCB consists of a PWM circuit, a sync circuit, a power transistor drive circuit, and a
power transistor bridge.
Pulse width modulator (PWM)
U1 is a fixed-frequency current mode controller which drives
the main switching transistors Q8-Q11 through drive transistors Q2-Q7 and transformer T1. The
PWM varies its output pulse width (and therefore the ON time of the power transistors and the
power supply output) in response to feedback signals from the A2 PCB voltage and current
control circuits, and current sense transformer T2 which monitors the current in the primary of
the power transformer.
The PWM consists of six (6) main blocks: an error amplifier, a current sense amplifier, a
comparator circuit, an oscillator circuit, a 5.1V
REFERENCE
circuit, and an output stage.
The error amplifier monitors the feedback signal from the A2 PCB voltage and current control
circuits at pin 5 of U1 and provides a reference for the PWM comparator inverting input. A
current limit signal at pin 1 of U1, derived by resistors R1 and R2 from the PWM
5.1V
REFERENCE
, limits the maximum output swing of the error amplifier thereby setting a
maximum limit for the primary current in the power transformer.
Operation Manual
5-5
