AT7120 Owner Manual Rev. 1.1
– March 2009 41/76
The resistances R1 and R2 are the loads for the capacitors C9 and C10 when the power supply is
powered ON. R3 and R4 provoke a fast discharge of C9 and C10 when the power supply is OFF. The
control of the switching function over the PWM modulator is performed by the components TF-3, D5,
D6, D7, R9, T3, C20, C18 and C46.
Switch and Power
The switching function over the DC input voltage is performed by MOSFETs, T1 and T2. The trigger
pulses are generated by integrated circuit CI-1, through the transformer TF-4 that also isolates the
modulation circuit from the AC network.
The switching voltage created by the transistors is applied across the transformer TF-5 to generate the
secondary voltage.
The capacitors C15 and C16 divide the entrance voltage, reducing the voltage over the transistors [seems
unlikely as described]. C14 prevents the direct-current flow towards TF-5.
The noise reduction formed by R7, C17, R34, C31, R37, C35, R43 and C42, help to lower high
frequency noise generated by the fast and short time switching pulses present on the MOSFET
transistors as in TF-4.
Rectification and Filtering
The pulses delivered by switching power devices, are rectified by D8 to D14 and filtered by C32 the
C47. The inductor L2 has multiple sections and stores the transferred energy helping to filter the voltage
provided.
The resistors at the output act as bleeders avoiding undesirable transients and oscillations.
Pulse Width Modulation
This block is composed basically by the integrated circuit CI-1 that performs the PWM switching
process control.
CI-1 output (pins 11 and 14) supply the switching pulses that are applied to the transformer TF-4. The
“Soft-Start" system prevents a fast rise of the output voltage. In this circuit, this function is carried
through by C23.
Feedback
The resistors R10, R13, R12 and TPO-1 form a voltage divider that takes a sample of +32V output of
the power supply and delivers it to CI-1, which compares that with its reference of +5V via R39.
As the result of this sample dictates, the modulation circuit will create a variation in the width of the
switching pulses, thus adjusting the output to maintain a n 32 Vdc.
Protection and Readings
In the event of over-current, in any one of the outputs, the voltage in the inverting input of comparators
CI-3 and CI-4, transferred by the electrical current readings, will be greater that the voltage of reference
in the non inverting entrance.