6.2 Power supplies

A power supply is the source of a power interface: for a row, or for the single product it is mounted on, such as the bus made on the back of a module from a DC rail (6.1). A bus supply feeds the row's bus board with the rails of 6.4: +12 V and −12 V, and +5 V if it provides one. A supply may be built into an enclosure or sold on its own.

The rules of this clause bind every outlet of a supply, whatever its interface; the limits that differ by interface are tabulated under Output quality. A supply that takes 12 V DC at a DC entry also keeps the entry rules of 8.1.

A supply of the DC rail

A power supply may provide the 12 V DC rail of 8.1 alongside or instead of the bus; it then has PWR-DC12 outlets, publishes the current of each outlet, and meets the outlet rules of 8.1 (SUP-009): an outlet to a module on a DC-BAR21 jack or a DC-VH2 header (DC-009), each protected separately (DC-012); an outlet to a distribution product's input may be an M4 row wiring terminal (PWR-005). Its claim file lists each outlet and each entry with its figures, including the DC entry of a supply fed from an external adapter; its claim line is EIS draft, power supply. Otherwise the 12 V of PWR-DC12 comes from an external adapter, which is outside EIS.

What a supply publishes

A power supply shall publish its ripple and noise on each rail it provides, peak-to-peak, measured as stated under Output quality (SUP-001). It also publishes the current it delivers on each rail, in total and at each outlet (PWR-002, 6.1).

Output connector and protection

An output on a plug-and-socket connector other than a power header, such as a wire-to-board housing or a DC plug, shall not mate with reversed polarity, a power-header outlet meeting the marking and keying of 6.4 instead (SUP-002), since an unkeyed ribbon socket enters any header. An output on row wiring terminals, the M4 terminals of 6.1, cannot be keyed and is exempt; each such terminal shall be marked with its rail or 0 V (SUP-003).

Each output rail of a power supply shall stay within ±5 % of its nominal voltage at its output terminals, from the minimum load the supply publishes on each rail to its rated current, whatever the load on its other rails within those limits (SUP-010): a cross-regulated triple-output supply drifts out of it when one rail is lightly loaded, and then publishes the minimum load that holds it.

Each output rail of a power supply shall survive an indefinite short to 0 V without damage, and shall return to its rated output when the short is removed, after an input cycle if it latches off (SUP-004). During power-up, power-down, or a short on another rail, no output rail shall be driven more than 0.7 V toward the opposite polarity (SUP-005); a Schottky clamp across each rail holds it at rated current. At start-up, no output rail shall overshoot its nominal voltage by more than 10 % (SUP-006). EIS recommends that each rail also survive a short to another rail, and that a supply with +5 V survive +12 V forced onto +5 V, which a 16-pin ribbon reversed at the bus end does (6.4).

A row is a large capacitive load, and a current limiter that cannot charge it within its on-time never starts. EIS recommends constant-current limiting, or a hiccup or foldback limiter whose maker states the capacitance it starts into, published with the supply's ratings. EIS recommends bringing the rails up in an order that avoids reverse stress on modules; SUP-005 bounds the reverse swing whatever the order.

Output quality

Each output rail of a power supply shall have ripple and noise below the limit in the table below for its power interface, 150 mV peak-to-peak on PWR-BUS, measured as stated below (SUP-007):

EIS requires no particular topology. EIS recommends a setpoint within ±1 % and load regulation within ±1 % from 10 % to 100 % load. A linear or post-regulated supply is typically under 5 mV, which suits a row of modules with VIDEO ports.

Ripple and noise are measured peak-to-peak on each rail:

• every rail at its rated current;
• a 300 mm (12 in) twisted pair from the rail's output terminals, or from test points the product publishes;
• the pair terminated at its far end by a 0.1 µF ceramic capacitor in parallel with a 47 µF electrolytic;
• the probe across those capacitors with a short ground spring, at 20 MHz oscilloscope bandwidth.

This is the method of common supply datasheets, Mean Well's among them, so one maker's figure compares with another's; the pair's inductance against the capacitors is part of it.

A supply for one product

A power supply mounted on the product it powers shall publish the heat it dissipates at its rated output and how far it extends behind that product (SUP-008). Like every power entry, it publishes the current it takes at its input (PWR-001); for a linear regulator on the bus, that is its output current plus its own. A module sold with such a supply publishes the currents and the depth of the pair. A supply that takes the bus keeps the power-header rules of 6.4 as a module does (BUS-008, BUS-009, BUS-019).

EIS recommends that such a supply's board fit the outline of a 1U module board (5.2.2): a parallel board no taller than 21.10 mm, with the 1U mounting-hole span of 33.65 mm (PANEL-009) behind a 1U panel. The same supply can then be mounted as a 1U module in a row that has one, or behind the product it feeds.