Agilent Technologies 667xA, 665xA, 664xA Local Voltage Sensing, Inductive Loads, Battery Charging

	Series 664xA/665xA Power Supplies, Maximum OVP External Capacitance (∝F)
6641A	6642A	6643A	6644A	6645A	6651A	6652A	6653A	6654A	6655A
700,000	35,000	15,000	7,000	3,000	1.6 (F)	100,000	50,000	18,000	8,000

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 external capacitance limit of 8,000 ∝F, this total resistance must be not less than 56 milliohms. For smaller values of external capacitance, this resistance may be derated linearly.

Inductive Loads

Inductive loads provide no loop stability problems in CV mode. However, in CC mode inductive loads will form a parallel resonance network with the power supply's output capacitor. Generally, this will not affect the stability of the supply, but it may cause ringing of the current in the load. Ringing will not occur if the Q (quality factor) of the parallel resonant network is ≤ 0.5. Use the following formula to determine the Q of your output.

					1	L
				Q = Rint + Rext C
where: C = model-dependent internal capacitance (see below); L = inductance of the load; Rext = equivalent series
	resistance of the load; Rint = model-dependent internal resistance (see below):
C =	6641A	6642A	6643A	6644A	6645A	6651A	6652A	6653A	6654A	6655A
	4,200 ∝F	550 ∝F	180 ∝F	68 ∝F	33 ∝F	10,000 ∝F	1100 ∝F	440 ∝F	120 ∝F	50 ∝F
Rint =	7 mΩ	30 mΩ	50 mΩ	125 mΩ	300 mΩ	4 mΩ	20 mΩ	30 mΩ	80 mΩ	250 mΩ

Battery Charging

The power supply's OVP circuit contains a crowbar SCR that effectively shorts the output of the supply whenever 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.

Note that if the OVP trips, you must remove the external current source in order to reset the internal SCR as part of clearing the OVP circuit (see Clearing the OV Condition in “Chapter 5 - Front Panel Operation”).

Local Voltage Sensing

Your power supply was shipped set up for local sensing. This means that the supply will sense and regulate its output at the output terminals, not at the load. Since local sensing does not compensate for voltage drops across screw terminals, bus bars, or load leads, local sensing should only be used in applications that require low output current or where load regulation is not critical.

Local sensing is obtained by placing the SENSE switch (see Figure 4-3a) in the Local position. The power supply is shipped with the switch in this position.

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