THEORY OF OPERATION SORENSEN DCS SERIES MANUAL
3-4
For 230Vac operation rectifier CR35 and filter capacitors C32(A,B,C) and C72(A,B,C) are configured as a full
wave bridge. For 115Vac operation the input voltage selector P1 is switched so that CR35, C32(A,B,C) and
C72(A,B,C) form a voltage doubler. In addition to determining the input rectifier configuration P1 also switches
the connections to the primary of auxiliary transformer T4.
Pulse Width Modulator
Pulse width modulator U6 is a current mode controller which drives the main switching FETs Q13-Q16 through
drive FETs Q7-Q12, Q18 and Q19 and drive transformer T2. Capacitor C69 and resistors R111-R113 form a
snubber on the primary of transformer T1 limiting switching transients. Transformer T3 is used to monitor T1
primary current and provide a feedback signal for the current sense amplifier of U6. Diodes CR29 and CR30
rectify T3 output while resistors R66 and R95-R97 and capacitors C60-C62 and C64A scale and condition the
signal before it is input to the PWM. An internal PWM comparator compares the output of the current sense
amplifier with a limited error signal derived from the output voltage and current control circuits. The output of the
PWM comparator controls the pulse duration of the drive signals from pins 11 and 14 of the PWM thereby
controlling the primary current in T1 and the output power. Resistor R100 and capacitor C63 set the internal
oscillator of U6 to approximately 140kHz, resulting in a 70kHz output switching frequency. A 5.1V reference
developed at pin 2 of the PWM is divided down by resistors R64 and R67 to provide a reference at the non
inverting input of the PWM error amplifier (pin 5). The 5.1V reference voltage is also divided by resistors R65
and R68 to provide a reference at pin 1 of the PWM which limits the swing of the error amplifier output placing an
upper limit on the primary current in transformer T1.
Output Rectifier Circuit and Output Filter
The DCS series uses two different configurations for the output rectifier. The 8V to 80V models use a full wave
center tap configuration while the 150 to 600V models employ a full wave bridge. On 8V and 20V models the
output rectifiers are one piece power tap units mounted on the rectifier heatsink with the secondary snubber
components connected directly to the rectifier input terminals. The 33V to 600V models have their rectifier and
snubber components on a separate printed circuit board which is mounted on a rectifier heatsink. Capacitors
C51-C53, power inductor L3 and common mode inductor L4 form the main output filter on all models. On 8V to
80V models the secondary filter capacitors C401-C407 are on a separate PCB which is mounted on the output bus
bar assembly. On 150V to 600V models the secondary filter capacitors C50, C50A, C50B, C50C, C48, C48A,
C48B, C49, C49A, and C49B are mounted directly on the A2 PCB. Resistor R90 is an output pre-load which
allows the unit to operate under no load or light load conditions.
Models DCS 8-125 and DCS 20-50
On these models diodes CR301 and CR302 rectify the output of power transformer T1. Resistors R301-R304 and
capacitors C301 and C302 form snubbers on the secondary of T1 to limit switching transients while resistors R305,
R305A, R306 and R306A and capacitors C303 and C304 are used to provide additional rectifier snubbing on some
models.
Models DCS 33-33, DCS 40-25, DCS 50-20, DCS 60-18 and DCS 80-13
On these models diodes D1 and D2 rectify the output of power transformer T1. Resistors R2 and R3 and capacitors
C3 and C4 form snubbers on the secondary of T1 to limit switching transients. Resistors R1 and capacitors C2 and
C5 are used to provide additional rectifier snubbing.
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