GW Instek PSS-Series User Manual - Manual (Page 13)

PSS-3203/2005 PROGRAMMABLE POWER SUPPLY
USER MANUAL
⎯ 20 ⎯
7-2.The Operation of the whole circuit
Each output channel of PSS-series has a reference voltage circuit at
around 2.5V output voltage which can be amplified to 2.5 (1+R314
/(R315+Vr))=2.5(1+12.4k/(7.68k+Vr)) by a non-inverter amplifier.
When Vr=0~500, take its middle value to convert above formula into
6.41V, and treat this as the reference voltage of DAC AD7541. The
AD7541 is a 12-bit DAC with 6.41V/4095=1.56mV/bit of the resolution.
When the instrument is in C.V. Mode, the MPU will send 3300
counts, 33.00V of output voltage, to DAC, output through analog switch
and Sample Hold circuit, the voltage now is at around -5.148V(-
1.56mVx3300=-5.148V). Then, compare this voltage with the real output
voltage which is retrieved by the voltage sense circuit. As the entire
circuit is a close loop, therefore, the retrieved voltage value will be
followed with the reference voltage of Sample Hold. Besides, the
Comparator output terminal will output a related voltage value to control
the whole output circuit through a driver circuit, and generate the
required output voltage. The attenuation of the voltage sense circuit is
A=R342/(R342+R335)=4.99K/(4.99k+27.0k) =0.156, and the output
voltage is Vout=5.148/A=5.148/0.156=33.00V. If the output is affected
by the offset of materials, can use the Vr to make adequate adjustment.
PSS-3203/2005 PROGRAMMABLE POWER SUPPLY
USER MANUAL
⎯ 21 ⎯
The operation in C.C. Mode is similar to that in C.V. Mode. The
MPU will send 2100 counts, 2.1A output current, to DAC, output through
analog switch and Sample Hold circuit, the voltage now is at around
-3.276V (-1.56mVx2100=-3.276V). Then, compare this voltage with the
real output voltage which is retrieved by the current sense circuit. As the
entire circuit is a close loop, therefore, the retrieved voltage value will be
followed with the reference voltage of Sample Hold. Besides, the
Comparator output terminal will output a relative voltage value to control
the whole output circuit through a driver circuit, and generate the
required output voltage. The current sense circuit for detecting the
voltage value from current sense resistor is mainly composed by
differential amplifier TL071. The multiple of the differential amplifier is
A=-R355/R356=-22.0k/1.91k=-11.518, the voltage on the both sides of
current sense resistor is -3.276/-11.518=0.285V, so the output current is
basis on Iout=(0.285/R374)*2=(0.285/0.3)*2=1.9A, and 2.1A making
multiple operand to get a real output current by using the software.
The Displayed Value of Voltage and Current
The voltage/current sense circuit retrieves a voltage value which will
be sent to Comparator through Analog Switch, and joined with the output
voltage of DAC in dichotomy approaching method to get the same
voltage as the real output value of the instrument.