Efficient Power Management of AdvancedMCs in MicroTCA
By Jim Bird, Texas Instruments

AdvancedMCs are often used in high-power density environments with demanding power, thermal, and reliability requirements. Shrinking form factors and growing power demands make efficient power management essential. This is especially true in MicroTCA, where the form factor is small and the functionality on the cards is limited (since they were intended as mezzanine add-ons rather than as central functions).

MicroTCA systems must distribute power using separate 12V and 3.3V runs for each module, since the required power management circuits are on the host or socket side. This makes the AdvancedMCs smaller and cheaper, but makes power management more challenging in redundant systems. A standard AdvancedMC is 75mm x 150mm and may consume up to 80 W. Since the maximum current draw allowed on the 3.3V rail is 150mA, the 80 W will be coming from the 12V rail. Most AdvancedMCs today use far less than maximum power, but the average keeps increasing as the number of complex modules such as multicore processors and DSPs grows.

Good power system design and attention to detail throughout development are far less painful than trying to correct an inefficient, overheating product. Here are a few tips for designing an efficient MicroTCA power distribution system.

• Use the biggest FETs you can afford. Hotswap FETs and ORing FETs can have a wide range of RDSON. Power loss in them is directly proportional to RDSON, so every 10 percent drop in its value produces a 10 percent drop in loss from that FET. A module with 5 milliOhm Hotswap and ORing FETs will dissipate 1.3 less Watts than one using 20 milliOhm FETS, when powering an 80 Watt load.
• Make the backplane runs as thick, wide, and short as possible. A separate layer for distribution is always a good idea.
• Choose current sensing and control devices with low threshold voltages. Hotswap controllers and current limiters typically use a series shunt resistor, which develops a voltage proportional to the load current. The lower the threshold voltage, the lower the losses.
• Use the most highly integrated solution possible. Hotswap and ORing systems can be built using individual components, but such implementations invariably are large and consume far more power than a device designed for the job. For example, the TPS2359 from Texas Instruments provides complete 12V and 3.3V power management (including ORing) for two AdvancedMCs with a typical current draw of 3mA. This single chip can replace an entire series of individual components at a surprisingly low cost.
• Test, test, test! If you can think of a test, do it. After all, your customers will, even if they don’t mean to.

AdvancedMCs and MicroTCA are high-power density systems with demanding power and power distribution requirements. A thorough understanding of the specification and attention to detail during design are essential to develop an efficient, reliable system.

About the Author

Jim Bird is a Systems Engineer for TI's Power Interface products group. You can reach him at ti_jimbird@list.ti.com.