Slow Start Circuit; Supply Voltage Controller; Switching Circuit; Shutdown Circuit - Agilent Technologies 4284A Service Manual

[ Slow Start Circuit ]

The slow start circuit, composed of A1 01 , A1 02, A1 03, A 1 09, and A1 0 1 0, lengthens the
rise time of the supply voltage by limiting the maximum switching pulse width at power u p.
When the voltage from the primary rectifier becomes g reater than about 240 V, th e slow
start circuit is enabled.

[ Supply Voltage Controller ]

A1 U2 (supply voltage controller), controls the switching circuit. The switching duty cycle is
adjusted by A 1 R 1 9 (FREO-ADJ).

[ Switching Circuit ]

The switching circuit used to convert the DC primary voltage to a 40 k Hz AC voltage con­
sists of A 1 01 1 , and A 1 01 2. The duty cycle is the parameter varied to regulate the output
voltage, and it is controlled by A 1 U2 (supply voltage controller), by comparing the 5 V Vref
(reference voltage) to the 9 V feedback voltage produced by A 1 C R21 , A 1 CR22, A 1 L4, and
A1 C25.

[ Shutdown Circuit ]

The shutdown ci rcuit, consisting of A1 06, A1 07, A1 C R 1 8, and A1 C R 1 9 , stops both the
A1 U 2 's oscillator and the switching circuit if one of the following situations occur.
1 . The FAN STOP signal form A1 U6 is received by the shutdown circuit.
2. The over voltage signal from A1 U5 is received by the shutdown circuit.
[ Secondary Rectifier ]
The secondary rectifier rectifies the output of the secondary windings of A 1 T3 and ou tputs
+5 V, +8 V, ± 1 2 V, ± 1 5 V, -21 V, and ± 48 V.
[ Over Voltage Detector ]
A1 CR40, A1 CR41 , A1 CR43 , A1 C R44, A1 CR45, and A1 C R47 make u p the over voltage
detector. If any voltage exceeds its limit, the over VOltage detector will send a shutdown
signal to A 1 U5 (opto-coupler) which will transfer the signal to the shutdown circuit.
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