Design Feature: June 9, 1994

By providing for "hot," or "live," insertion of power supplies, you can improve system serviceability, reliability, and availability. But these improvements don't come without a price; they require extra design effort to select the right connectors, prevent contact damage, avoid system-bus glitches, and balance power distribution.

Start with the connector

One of the first things to consider is the power-supply connector, which you can attach to the supply in one of three ways - with cables, with short wires, or directly to the supply's pc board. Each approach has its advantages and disadvantages.

If you use cables to attach connectors to a power supply and its load, you can remove the supply with part of the cabling in case the supply needs to be replaced. This approach somewhat improves serviceability but lowers overall reliability.

In the second approach, you can attach connectors to the power supply terminals by a set of short wires and then mechanically attach the body of the connector to the supply. Then it's possible simply to insert the power supply into the system rack and engage a connector on the backplane. A set of wires on the backplane runs from the connectors to the load. Because "connectorized" versions of power supplies are uncommon, this approach offers the advantage of using an off-the-shelf power supply with minor modifications. Disadvantages include the need for additional space and the increased cost of repackaging.

In a custom design, you can attach connectors directly to a power supply's pc board. This approach provides the most compact design and the highest reliability. If you select the correct connectors, this way provides the lowest cost for adding connectors. Note, though, that pc-board connectors cannot handle a significant amount of current, so you have to take special precautions in your design.

Don't neglect path resistance

When you use power connectors with only a few contacts, it's not difficult to select the rating and the number of contacts to accommodate any deviation in current-path resistance. However, if large numbers of pins have low current capacity, as is the case with board-mounted connectors, then path resistances become a real concern.

The current in each connector pin or contact is inversely proportional to the resistance of the path (and not the contact itself). In a way, that's good news, because contact resistance is harder to control than any other part of the circuit. Some connectors - for example, the multirow pin-and-socket type - have distinct resistance differences that result from contact-length differences. You can control the resistances of individual contact circuits by providing separate runs, with individual wires or traces, that connect to a common bus. If differences in contact resistances aren't very obvious, it is sufficient to equalize all path resistances (using traces with the same length and the same width) and to make those resistances sufficiently higher than the contact resistance (Fig 1).

Often, pc boards have copper layers or fields for power distribution. In such cases, the currents from connector contacts take the shortest paths to the load, and these paths may be different for different contacts and may differ from the geometrical shape of the conductor. Therefore, it may be necessary to isolate the paths for individual pins or groups of pins (Fig 2).

An engineer who generates the design specification for a power supply must also address current distribution over connector contacts. The current source in a power supply is usually a single point (a device lead), so it is important to minimize or compensate for resistance difference between the source point and the connector contacts.

Guard against contact arcing

The issue of contact arcing is always a concern in live-circuit replacement, but it is much more critical for pc-board connectors with small contacts. Arcing results either from high voltage breaking the air gap between contacts during contact engagement or disengagement or from an interruption of the high current in low-voltage circuits. You should try to minimize the effects of both factors.

Limiting the available current minimizes the effect of high voltage, which is present on the contacts carrying the input ac power to the power supply. During contact engagement, the input current - called "inrush current" in this situation - results from the charging of the power supply's input capacitance. The duration of charging is usually less than 30 to 50 msec, and after this transitional period, any input-current increase is not critical because the contacts are fully engaged and do not produce arcing. The power supply's specified maximum inrush current should not exceed 10A. During contact disengagement, the fully charged input capacitance limits the input current.

Limiting the current can also minimize the effect of high current at low voltage - the usual output of the power supply. The power supply's output current should be significantly lower by the time contact disengagement occurs. To achieve this, you can disable the outputs in advance by the manual switch or, better, by an early disengagement of a connector pin that is shorter than the power-output pins.

Avoid power-bus disturbances

A design aspect that is critical to live power-supply insertion or removal involves the conditions on the common output bus during the transitional period. The common bus should not experience any disturbances that could affect system performance. Two aspects to consider are output capacitance and transient response.

If the power supply's output capacitance comes in contact with the common bus before it gets charged, sagging of the bus is inevitable. You can prevent this by precharging the output capacitance via dedicated pins or by isolating the capacitance with diodes. The isolating diodes may also prevent total system failure if one supply in a multiple-supply system gets a short circuit on its output.

Another source of disturbance on the common bus is a current transient that results from a change in the number of power supplies on the bus. To avoid this kind of disturbance, which may affect system operation, specify and design power supplies with a transient response adequate for the worst-case current-amplitude change and the maximum speed of the change.

In one situation, however, proper transient response is inadequate. If you yank an on-line power supply from a live system, no power supply can compensate for the change. Only the disabling of the output before removing the unit can do the job.