Agilent Technologies E3614A Manual

A SERVICE OF

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1-14

LOAD CONSIDERATIONS

This section provides information on operatin

g

y

our suppl

y

with

various t

y

pes of loads connected to its output.

PULSE LOADING

The power suppl

y

will automaticall

y

cross over from constant-

volta

g

e to constant current operation in response to an increase

(over the preset limit) in the output current. Althou

g

h the preset

limit ma

y

be set hi

g

her than the avera

g

e output current, hi

g

h peak

currents (as occur in pulse loadin

g

) ma

y

exceed the preset cur-

rent limit and cause cross over to occur. If this cross over limitin

g

is not desired, set the preset limit for the peak requirement and

not the avera

g

e.

REVERSE CURRENT LOADING

An active load connected to the power suppl

y

ma

y

actuall

y

deliver a reverse current to the power suppl

y

durin

g

a portion of

its operatin

g

c

y

cle. An external source can not be allowed to

pump current into the suppl

y

without loss of re

g

ulation and possi-

ble dama

g

e to the output capacitor of the power suppl

y

. To avoid

these effects, it is necessar

y

to preload the suppl

y

with a dumm

y

load resistor so that the power suppl

y

delivers current throu

g

h the

entire operatin

g

c

y

cle of the load devices.

Fi

g

ure 16. Reverse Current Loadin

g

Solution

OUTPUT CAPACITANCE

An internal capacitor, connected across the output terminals of

the power suppl

y

, helps to suppl

y

hi

g

h-current pulses of short

duration durin

g

constant volta

g

e operation. An

y

capacitance

added externall

y

will improve the pulse current capabilit

y

, but will

decrease the safet

y

provided b

y

the current limitin

g

circuit. A

hi

g

h-current pulse ma

y

dama

g

e load components before the

avera

g

e output current is lar

g

e enou

g

h to cause the current limit-

in

g

circuit to operate.

The effect of the output capacitor durin

g

constant current opera-

tion are as follows:

a. The output impedance of the power suppl

y

decreases with

increasin

g

frequenc

y

.

b. The recover

y

time of the output volta

g

e is lon

g

er for load

resistance chan

g

es.

c. A lar

g

e sur

g

e current causin

g

a hi

g

h power dissipation in the

load occurs when the load resistance is reduced rapidl

y

.

REVERSE VOLTAGE LOADING

A diode is connected across the output terminals with reverse

polarit

y

. This diode protects the output electrol

y

tic capacitors and

the series re

g

ulator transistors from the effects of a reverse volt-

a

g

e applied across the output terminals. For example, in series

operation of two supplies, if the AC is removed from one suppl

y

,

the diode prevents dama

g

e to the unener

g

ized suppl

y

which

would otherwise result from a reverse polarit

y

volta

g

e.

Since series re

g

ulator transistors cannot withstand reverse volt-

a

g

e, another diode is connected across the series transistor. This

diode protects the series re

g

ulators in parallel or auto-parallel

operation if one suppl

y

of the parallel combination is turned on

before the other.

BATTERY CHARGING

The power suppl

y

's OVP circuit contains a crowbar SCR, which

effectivel

y

shorts the output of the suppl

y

whenever the OVP trips. If

an external volta

g

e source such as a batter

y

is connected across the

output, and OVP inadvertentl

y

tri

gg

ered, the SCR will continuousl

y

sink a lar

g

e current from the source; possibl

y

dama

g

in

g

the suppl

y

.

To avoid this a diode must be connected in series with the output as

shown in Fi

g

ure 17.

Fi

g

ure 17. Recommended Protection Circuit for

Batter

y

Char

g

in

g