- Hewlett-Packard Power Supply User Manual

Installation - 3

Fast/Normal Operation

A switch on the rear of the unit lets you switch between operating in either Fast mode or Normal

mode. When set to Fast mode, this switch disconnects the output capacitor that is located inside the

unit. Fast mode lets you improve or enhance certain operating characteristics; while at the same

time degrading other operating characteristics.

1. In Fast mode, the programming time for voltage programming is faster than for normal

operation, however, output noise is greater.

2. In Fast mode, the absence of the internal output capacitor results in increased output

impedance and therefore, greater stability when driving inductive loads. Conversely, the

addition of external capacitive loads in Fast mode will reduce the stability of the unit during

constant voltage operation.

3. In Normal mode, the internal output capacitor helps control peak voltage excursions away

from the the nominal value for sudden changes in load current. In Fast mode, larger peak

voltage excursions will show up at the output of the unit during sudden load current changes.

Capacitive Loading

In Normal mode, the dc source will be stable for many load capacitances, however, large load

capacitances may cause ringing in the dc source’s transient response. If this occurs, the problem

may be solved by increasing or decreasing the total load capacitance.

In Fast mode, the dc source can maintain stability only for small capacitive loads. These limits are:

HP 6631B

2.2 µF.

HP 6632B and 66322A

1.0 µF.

HP 6633B

0.22 µF.

HP 6634B

0.10 µF.

Inductive Loading

Fast mode is recommended for driving inductive loads, although you will typically experience

stable conditions in Normal mode. Inductive loads present no loop-stability problems in constant

voltage operation, but may cause problems in constant current operation if the load impedance is

outside the boundaries mentioned below.

The ratio of the load inductance (L) to the sum of the Equivalent Series Resistance (ESR) for the

load (R1) and dc source (Rs), determines whether the source can drive a load without becoming

unstable. To help determine this ratio, the value of Rs is listed as:

HP 6631B

0.1 ohm.

HP 6632B and 66322A

0.2 ohm.

HP 6633B

1.0 ohm.

HP 6634B

2.0 ohm.

If the ratio L/(R1 + Rs) is less than 0.005, the dc source can reliably drive the load.

INH/FLT Connections