Output Power And Control Circuits (Figure 4-4); Output Power; Control Circuits - Agilent Technologies 6541A Service Manual

Output Power and Control Circuits (Figure 4-4)

Output Power

The output power circuits are shown across the top of Figure 4-4. They consist of: power rectifiers, SCR (crowbar), filter
capacitors, a current-monitoring resistor on the main board, and regulator and downprogramming stages (on the
A4A1-A4A4 tunnel boards for the 500 watt models, and on the main board for the 200 watt models).
For the 500 watt Agilent 655xA and 665xA models, there are two (top and bottom) left tunnel circuits (A4A1 and A4A3)
and two (top and bottom) right tunnel circuits (A4A2 and A4A4). The 200 watt Agilent 654xA and 664xA models use one
left and one right tunnel circuit. Table 4-3 summarizes these model differences.
Table 4-3. A1 Main Board and Heat Sink Assembly Model Differences
Item
Heat sink assembly
Tunnel circuits
Regulator stages
Downprogrammers
Each left tunnel circuit has three regulator stages and one downprogramming stage. Each right tunnel circuit has four
regulator stages. Thus, there are a total of fourteen regulator stages and two downprogramming stages for the 500 watt
models, and half this number for the 200 watt models. Each regulator stage consists of an amplifier driver, and one NPN
series regulator. Models 6645A, 6545A, 6555A, and 6655A use a MOSFET regulator and no driver.
The output NPN transistor (or MOSFET) of each stage is mounted on the heat sink assembly and is connected between the
+RAIL and the inboard side of the current sampling resistor in the +OUT line. The conduction of these output transistors is
increased, or decreased, by the OUTPUT CONTROL signal from the CV/CC control circuits in order to regulate the output
voltage (CV mode), or the output current (CC mode).
Each downprogramming stage consists of a comparator, transistor driver, and a downprogramming transistor. Each NPN
downprogramming transistor is connected between the inboard side of the +OUT line and the -RAIL. The conduction of the
downprogramming transistors is controlled by the DP CONTROL signal from the CV/CC control circuits. Conduction is
increased when the output is downprogrammed to shunt current away from the load, thus allowing faster downprogramming.
The SCR, connected across the output, will fire and short the output when an overvoltage condition is detected. The SCR is
controlled by the OV signal from the SCR control circuit as described under, "Control Circuits."
Resistor R657 monitors the output current.

Control Circuits

The control circuits are shown across the bottom of Figure 4-4 and consist of the CV/CC control, output voltage/current
monitor, bias supplies, and SCR control. All of these circuits are located on the Al main board.
The CV/CC control circuits provide a CV control loop and a CC control loop. For any value of load resistance, the supply
must act either as a constant voltage (CV) or as a constant current (CC) supply. Transfer between these modes is
accomplished automatically by the CV/CC control circuit at a value of load resistance equal to the ratio of the programmed
voltage value to the programmed current value. A low level CV* or CC* signal is returned to the secondary interface to
indicate that the corresponding mode is in effect.
86 Principles of Operation
500 Watt Models 655xA & 665xA
External to main board
Two left and two right circuits
Total of 14 stages
Total of two
200 Watt Models 654xA & 664xA
Located on main Board
One left and one right circuit
Total of seven stages
One total