Philips Semiconductors

PIP401

OR-ing Intelligent Switch

Product data

Rev. 03 — 5 November 2002

10 of 16

9397 750 10578

© Koninklijke Philips Electronics N.V. 2002. All rights reserved.

of line inductance. The VRS connection may be connected directly to VO if the circuit

inductance is negligible. In general, this means that the distance between the PIP401

and the load must be less than 25 mm.

condition, with no voltage between input and output, both PIP401 devices will be off.

If one of the 5 V supplies is turned on, the comparator output in the associated

PIP401 will go HIGH, slowly turning on the series power switch (soft start). The series

power switch in the other PIP401 will remain off, preventing current from flowing from

output to input.

If both 5 V supplies are on, and the power supplies have forced current sharing

capability, then both PIP401 devices will be on.

If the power supplies do not have forced current sharing capability then the supply

with the highest output voltage will tend to supply all the load current. If the difference

in output voltage is greater than a few millivolts, then the PIP401 device connected to

the power supply with the highest output voltage will be on, and the PIP401 device

connected to the power supply with the lowest output voltage will be off.

Remote voltage sensing is not usually a feature of this topology of parallel redundant

operation. This is because the remote sense lines provide an alternative path for fault

currents when one of the power supplies fails. Therefore, in order to maintain good

load regulation, the PIP401 must have a low forward voltage drop under normal

operating conditions. The typical on-state voltage drop of the PIP401 is 70 mV at

40 A. Two or more PIP401 devices may be connected in parallel in order to reduce

this further.

11.2 Fault condition

With reference to Figure 7, assume that both 5 V power supplies are supplying

current to the load. Assume that one of the supplies fails and its output voltage

collapses to zero. The voltage on the output terminal of the PIP401 drops from 5 V to

zero and goes slightly negative as the voltage across the line inductance reverses in

order to maintain current flow.

At the instant that the power supply fails, the PIP401 detects that the voltage on VI is

less than the voltage on the remote sense connection VRS. The drive voltage to the

series power switch is immediately turned off, but the series power switch takes up to

a further 300 ns to switch off completely. During this delay time, the current

decreases from its initial value to zero at a rate determined by the bus voltage and the

line inductance.

If the PIP401 turns off completely before the current reaches zero, then there is no

reverse current flow. On the other hand, if the rate of fall of current is very high,

current will flow from VO to VI until the PIP401 turns off completely. The peak reverse

fault current depends on the magnitude of the line inductance and the initial load

current.