Controller+MOSFET Equals Smaller ORing Solutions

Aug 1, 2008 12:00 PM
By David Morrison, Editor in Chief

The use of Schottky diodes to connect redundant power supplies in parallel for load sharing — a technique known as ORing — offers a simple means of protecting the system against a power-supply failure. However, because Schottky diodes introduce voltage drops and power losses, they are replaced in some applications with power MOSFETs, which are controlled in such a manner to act like ideal diodes. Because the MOSFETs selected have low R_DSON, their conduction losses are a fraction of a Schottky's losses, and their voltage drop is typically negligible. Consequently, heatsinking requirements are reduced with a MOSFET.

Naturally, the tradeoff in using a MOSFET for ORing power supplies is added complexity because an additional controller is needed. However, with that added complexity, designers not only get reduced power losses, voltage drops and easier thermal management, they also have the benefit of additional forms of circuit protection that are easily built into the MOSFET controllers, also known as ideal diode controllers or ORing controllers. Two recent product announcements illustrate how ORing controllers are evolving to shrink the controller and MOSFET footprint, while also providing greater functionality.

Linear Technology's LTC4358 is a single high-voltage ideal diode controller with a co-packaged 5-A (20-mΩ) n-channel MOSFET. The LTC4358 regulates the forward voltage drop across the internal MOSFET to ensure smooth switchover from one path to another without oscillation. If a power source fails or is shorted, the LTC4358 ensures a fast 500-ns turn-off to minimize reverse current transients. The device is offered in 4-mm × 3-mm 14-pin DFNs and 16-lead TSSOPs.

Designed to operate with a 9-V to 26.5-V supply, the LTC4358 shrinks pc-board footprint versus two-chip controller+MOSFET designs. Alison Steer, product marketing manager for mixed-signal products at Linear Technology, describes the space savings versus an existing solution.

“Our single ideal diode controller, LTC4357, with an external SO-8 FET takes at least 0.125 in²,” Steer explains. “The LTC4358 TSSOP package consumes about one-third that space, and the DFN, half that again. This is only for the components — extra area is needed for heat dissipation. Choosing an external FET with similar R_DSON as the LTC4358 results in the same thermal area requirement. For 5 A, a single-sided board required 2 in. x 2 in. of area.”

Pricing for the LTC4358 begins at $2.10 each in 1000-piece quantities.

Picor, a subsidiary of Vicor, has introduced the Cool-ORing family of active-ORing controller ICs and full-function active-ORing solutions. Offered in various IC package styles, the controller chips are designed to drive industry-standard n-channel MOSFETs. Meanwhile, the modules co-package the company's controllers with optimized MOSFETs in 5-mm x 7-mm land-grid arrays (LGAs).

The full-function modules (PI2121, PI2122, PI2123 and PI2125) provide a compact alternative to separate controller+MOSFET combinations as well as competing ORing modules. According to Picor, the 35-mm² footprint of the LGA is up to 70% smaller than existing solutions. Moreover, these devices are said to require less thermal derating (no derating up to 60°C ambient) than some of the existing modules.

The small size of the new modules is partly attributed to their use of proprietary MOSFET die developed for Picor. For example, the PI2121, an 8-V, 24-A device suitable for ≤ 5-V bus applications, contains a MOSFET with an R_DSON of 1.5 mΩ typ. To achieve that value of on-resistance in a discrete MOSFET, it may be necessary to select a FET that is larger than Picor's controller+MOSFET module, according to Carl Smith, director of strategic marketing and business development at Picor.

The two other modules include the PI2123, a 15-V, 15-A solution suitable for ≤ 9.6-V bus applications, and the PI2125, a 30-V, 12-A solution suitable for ≤ 12-V bus applications. Those two devices feature MOSFETs with on-resistances of 3 mΩ and 5.5 mΩ, respectively.

The controller ICs (PI2001, PI2002 and PI2003) and the modules target a range of bus voltages, provide scalability for different current levels and feature fast dynamic response to input power-source failures. Reverse-current turn-off delay time for the PI2121, PI2123 and PI2125 modules and for the PI2001 controller is 160 ns typical. Programmable undervoltage and overvoltage detection, overtemperature detection and active low-fault flag output are common functions to all seven devices. But then several other features such as load disconnect and performance levels are specific to certain family members.

The full-function modules start at $1.98 each in 10,000-piece quantities. The controllers are available in 3-mm x 3-mm 10-lead TDFNs and 8-lead SOICs with prices starting at $0.76 each in 10,000-piece quantities.

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