AMETEK DCS-E 1KW Series - Manual (Page 52)

Theory of Operation Sorensen DCS Series 1kW and 1.2kW Supplies
3-6 M362500-01 Rev H
V OLTAGE C ONTROL C IRCUIT
The output voltage feedback is monitored by the voltage control op amp U5 pin 3 via the
positive sense line (pin 13 of connector J3 on 8V to 100V models and on pin 1 of the Amp high
voltage connector) and the resistor divider formed by R21, R22, R53, R85 and R147. This
feedback voltage is compared with a 0-5V reference voltage generated by the front panel
voltage control pot to U5 pin 2 through the normally closed CMOS switch U7-2 to provide an
error signal for the PWM. If the output voltage tries to rise above the selected level the voltage
at U5 pin 3 rises and the output of U5 becomes more positive. This increase is applied to the
inverting input of the PWM error amplifier through diode CR18 and resistor R69 causing the
amplifier output to decrease. This reduces the PWM output drive waveform pulse width,
lowering the output voltage and regulating the output at the desired level. Similarly, if the output
voltage tries to fall below the selected level the voltage at U5 pin 3 decreases, U5 output
decreases, the output from the PWM error amplifier increases and the drive waveform pulse
width increases which raises the output voltage to the desired level. During voltage mode
operation the output of the current control circuit at U4-1 pin 1 remains low. Resistor R70 and
capacitor C54 provide frequency compensation for the op amp and diode CR21A limits
saturation. Capacitor C20, resistor R54 and FET Q22 provide a soft start for the power supply
during initial startup and recovery from shut down conditions by ramping the voltage rise at pin
2 of U5. Q22 acts as a voltage-controlled resistor whose resistance increases as C20 is
discharged by R54. Buffer amplifier U2 pin 1 provides for remote monitoring of the output
voltage at pin 19 of connector J3. The output voltage monitor default connection provides a 0-
5Vdc proportional to the full-scale output voltage. With the addition of a jumper added to the
pins of JP1, U2 along with resistors R419 and R420 allow the voltage monitor to provide 0-
10Vdc proportional to the full-scale output voltage. Potentiometer R53 is used to adjust the full-
scale output of the power supply and potentiometer R52 is used to adjust the offset on op amp
U5. Resistor R21 supplies the front panel voltmeter with a voltage proportional to the output
voltage through pin 6 of the J1 connector.
C URRENT C ONTROL C IRCUIT
The output current is monitored by current shunt resistor R91 which develops a voltage across
it proportional to the output current. This voltage is amplified and conditioned by the differential
op amp U15 and associated components to provide a control ground referenced 0-2V signal at
pin 3 of the current control op amp U4-1. This signal is compared to a reference level
generated from the front panel current control pot to pin 2 of U4-1 through the normally closed
CMOS switch U7-1. As the output current increases, the voltage at pin 3 rises until it reaches
the reference level set at U4 pin 2. At this time, the output of U4-1 goes high and the unit
switches from voltage mode to current mode operation. The output current is maintained at the
desired level by providing negative feedback to the PWM error amplifier as described in the
voltage control circuit description above. Resistor R27 and capacitor C27 provide frequency
compensation for U4-1 and diode CR26A limits saturation. Capacitor C20, resistor R54, and
FET Q23 provide a soft start for the power supply during initial startup and recovery from shut
down conditions by ramping the voltage rise at pin 2 of U4. Q23 acts as a voltage-controlled
resistor whose resistance increases as C20 is discharged by R54. Op amp U4-2 and related
components scale the current feedback signal to provide a calibrated 0-5V proportional to the
full scale output current to buffer amplifier U2-2. U2-2 provides a low impedance output at pin 7
of connector J3 for external monitoring of the output current. With the addition of a jumper
added to the pins of JP2, U2-2 along with resistors R422 and R423 allow the current monitor to
provide 0-10V proportional to the full scale output current. The shunt voltage is also fed to the
front panel ammeter via pins 8 and 9 of connector J1 in order to display the output current.