Specifications

668xA: Sharing Current at Low Output in Auto-Parallel Mode
Question: Why don't my 668xA 5 kW power supplies share current at low output current when operated in auto-
parallel?
Answer: It's normal. They will share current at full load. Here's a complete explanation. Using 668xA Series power
supplies in auto-parallel, a maximum of five 668xA Series power supplies, with the same model number, may be
configured for auto-parallel operation. The 668xA power supplies were designed with a deliberate external
programming offset so the master will output current before the slaves output current. Therefore, slave(s) supplies
will always sink current when low output current values are desired.
Auto-parallel procedure:
Connect the 668xA power supplies with Im from the master going to +Ip of each slave and comP going to -Ip of
each slave (refer to the figure on the next page).
Each load lead (wire) should be same wire gauge and length.
Turn on all power supplies.
Program each slave supply for zero output current either by pressing [CURRENT] [0] [ENTER] from the front panel
keypad or sending the command "CURR:LEV 0" via the GPIB. See note 2 and 3.
Program each slave supply's output voltage to a value at least 2 volts greater than the programmed output voltage of
the master supply.
Program the master supply's output current to one-half the total output current if there is one slave supply or one-
third the total output current if there are two slave supplies.
Enable all power supplies by pressing [Output On/Off] key.
Increase the master supplies output voltage. At low output currents the slave(s) will sink current. This is normal. At
low output current the master will be supplying all the load current and sink current to the slave(s). At maximum
output current each supply will be delivering an equal amount of current. It is normal when operating at less than
maximum current to have unequal current sharing between the master and slave supplies. The master supply provides
more current at low output currents but current sharing will become equal when they reach maximum output current.
If remote sensing, only the master supply +S/-S lines are connected remotely. The slave supplies should be
connected for local sensing, connected at the rear of their respective output terminals.
Notes:
The current division between the master and slave(s) can be determined from the following formula:
Iout = Im [ 1 + Ns ( 1 + 0.127V / 5V ) ] - Ns Ifs ( 0.127V/ 5V )
The amount of current the master must output before the slave(s) outputs current can be determined from the
following formula.
Ns * Ifs ( 0.127V / 5V )
where
Im = master current
Ns = number of slaves
Ifs = full scale current
Example: 1 master 2 slaves Model hp6680A 5V 875A
Iout = Im ( 3.0508 ) - 44.4A
Master current limit must be set above 44.4A / 3.0508 = 14.55A to obtain any output current.
For no-load condition: