AMETEK DCS-E 1KW Series - Manual (Page 53)

Sorensen DCS Series 1kW and 1.2kW Supplies Theory of Operation
M362500-01 Rev H 3-7
O PERATING M ODE I NDICATOR C IRCUIT
Op amp U8-2 monitors the polarity of the voltage across diode CR18. CR18 is forward biased
during voltage mode and reverse biased during current mode to provide a mode indication
signal at U8 pin 7. This output is used to drive the back to back mode indicator LEDs CR1 and
CR2 on the front panel through pin 1 of connector J1. In voltage mode U8 pin 7 is high and the
green voltage mode led is illuminated. In current mode U8 pin 7 is low and the red current
mode led is illuminated.
A UXILIARY S UPPLY AND 6.2V R EFERENCE C IRCUIT
Transformer T4, rectifier CR42 and capacitors C9 and C10 provide the raw dc supply voltages
from which three terminal regulators U10 and U11 derive the +12V and +5V auxiliary supplies
respectively. Two additional regulators (U12 and U13) provide the regulated +12V to drive
FETs Q7-Q12, Q18 and Q19 on the primary of T2, and the +12V feed to PWM U6. Diodes
CR40 and CR41 with capacitors C7 and C11 provide the negative voltage input to regulator U9
which supplies the -5V auxiliary output. Resistor R2 and temperature compensated reference
zener diode CR1 are used to derive a stable 6.2V reference from the +12V auxiliary supply for
use on the front panel voltage and current meters (via pin 5 of connector J1) and the Remote
indicator circuitry on the A2 PCB. Transistor Q5, diode CR32, resistors R60-62 and zener
diode CR20 form a low voltage lock out which disables the power supply output by shutting
down the PWM should the auxiliary raw supply fall below the threshold set by CR20.
V OLTAGE AND C URRENT C ONTROL C URRENT S OURCES
Op amp U3-1 and transistors Q1, Q2 and Q4, and related components are used to provide
three separate 1mA current sources for the front panel voltage and current control pots. These
current sources are also used for remote resistive programming of the output voltage and
current limit. The circuit operates by comparing the voltage generated by 2.5V reference CR60
to the voltage drop across Q4 emitter resistor R31. The current is regulated at 1mA by
maintaining the drop across R31 at 2.5V. Q1 and Q2 act as current mirrors by having their
bases in common and using identical emitter resistors. The 1mA current source from the
transistor collectors goes to J3 pins 16, 21 and 22 for OVP, voltage and current control
functions.
O VER V OLTAGE P ROTECTION C IRCUIT
The OVP 1mA current source is fed to the front panel OVP adjusting pot via the external jumper
connecting pins 3 and 16 of connector J3, switch SW1-8 and pin 17 of connector J1. This
current source and the OVP adjusting pot provide a 0-5V reference at pin 6 of U3-2, the OVP
control op amp. The power supply output voltage is monitored at pin 5 of U3-2 through the
resistor divider formed by R80, R89, R148 and OVP sense link R412. When the output voltage
increases such that the voltage at pin 5 becomes higher than that at pin 6, the output of U3-2
goes high activating MOSFET Q17 (via drive FET Q2A) which discharges the power supply
output through resistor R93. Diode CR25 latches U3-2 on while diodes CR23 and CR24 gate
the high signal to the shutdown pin of the PWM. The OVP indicator on the front panel is biased
on via pin 15 of connector J1 when the OVP is activated. The OVP may be reset by cycling the
power switch off and then back on to release the latch provided by CR25 or by momentarily
activating the remote shutdown circuit which causes pin 6 to be pulled high through diode CR10
and Q3.