10.2 System Design and Behavior with Transients

The power system, perhaps consisting of multiple supplies, interconnections, and loads, is unique for every product. A power distribution has low impedance, and low loss, which yields high Q by its nature. While the addition of lossy capacitors helps at low frequencies, their benefit at high frequencies is compromised by parasitics. Transient events with rise times in the 10-ns range may be caused by inserting or removing units, load fluctuations, switched loads, supply fluctuations, power supply ripple, and shorts. These transients cause the distribution to ring, creating a situation where ORing controllers may trip off unnecessarily. In particular, when an ORing device turns off due to a reverse current fault, there is an abrupt interruption of the current, causing a fast ringing event. Since this ringing occurs at the same point in the topology as the other ORing controllers, they are the most likely to be effected.

The ability to operate in the presence of noise and transients is in direct conflict with the goal of precise ORing with rapid response to actual faults. A fast response reduces peak stress on devices, reduces transients, and promotes un-interrupted system operation. However, a control with small thresholds and high speed is most likely to be falsely tripped by transients that are not the result of a fault. The power distribution system should be designed to control the transient voltages seen by fast-responding devices such as ORing and hotswap devices.

The TPS2410 was designed with several features to help tune its speed and sensitivity to individual systems. The FLTR pin provides a convenient place to filter the bus voltage before it causes undesired tripping (see Fast Comparator Input Filtering – C(FLTR)). Some applications may find it possible to use RSET to advantage by setting the reverse turn-off threshold more negative. Last, the STAT pin may be used to desensitize the turnoff threshold of an on-line TPS2410 when a redundant TPS2410 has turned off. This is especially attractive in dual redundant systems (see Input ORing and STAT). Ultimately, the performance may have to be tuned to fit the characteristics of each particular system.