User Connections and Considerations
51
Programming Series 657xA Supplies
Wiring Considerations.
The input impedance of the analog input is over 30 kΩ. If the output impedance of your programming source is not
negligible with this, programming errors will result. Larger output impedances result in proportionally greater errors.
Note from Figure 4-3 that you have three options for programming the current. You can use a voltage source that is positive,
negative, or floating with respect to Common P. Do not exceed ±19 V with respect to Common P.
Make sure that the common connection for your voltage programming source is isolated from the load.
Failure to do this may cause damage to the power supply.
OVP Considerations
Remote Sensing
The OVP circuit senses the voltage near the output terminals and not at the sense terminals. Depending on the voltage drop
between the output terminals and the load, the voltage sensed by the OVP circuit can be significantly higher that that
actually being regulated at the load. You must program the OVP trip high enough to compensate for the expected higher
voltage at the output terminals.
Battery Charging
The power supply's OVP circuit contains a circuit that discharges the output of the supply whenever the OVP trips. If a
battery (or other external voltage source) is connected across the output and the OVP is inadvertently triggered or the output
is programmed below the battery voltage, the power supply will continuously sink a large current from the battery. This
could damage the supply. To avoid this, insert a reverse blocking diode in series with the + output of the supply. Connect
the diode cathode to the + battery terminal and the diode anode to the supply + output terminal. The diode may require a
heat sink.
Series 654xA and 655xA Load Capacitance
For Series 654xA and 655xA power supplies, the OVP circuit has been designed to discharge fully-charged capacitances up
to a specified limit for each model. These limits are as follows:
6541A 6542A 6543A 6544A 6545A 6551A 6552A 6553A 6554A 6555A
700,000µF 35,000µF 15,000µF 7,000µF 3,000µF
1.6F
100,000µF 50,000µF 18,000µF 8,000µF
If a load capacitance approaches the specified limit, it is recommended that you do not make it a normal practice of tripping
the OVP circuit and discharging the load capacitance through that circuit. This could cause long-term fatigue in some circuit
components.
Because of its high output voltage, the Agilent 6555A generates very high currents when discharging
the load capacitor under overvoltage conditions. Excessive currents can damage the supply. The peak
discharge current is limited by the sum of the external capacitor's ESR (equivalent series resistance) and
the series resistance of the external circuit. For the Agilent 6555A's external capacitance limit of 8,000
microfarads, this total resistance must be not less than 56 milliohms. For smaller values of external
capacitance, this resistance may be derated linearly.
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