White Paper

How to Choose Power FETs for ORing MOSFET
Controllers
Introduction
Many power applications use multiple supplies with fault tolerance to share the load current, or to haveredundancy in the power system for high reliability.
In these power distribution systems, paralleled power supplies contribute equally to the load current throughsome type of output sharing scheme. A common design practice is to include discrete ORing power diodes toprotect against reverse current flow in the event that one of the power supplies suffers a catastrophic output
short-to-ground. Reverse current also can occur if the current sharing scheme fails and an individual power
supply voltage falls significantly below the other power supplies.
Although the discrete ORing diode solution is common and inexpensive to implement, it has somedrawbacks. The primary downside is the increased power dissipation loss in the ORing diodes. Anotherdisadvantage is that it can be difficult to detect a shorted or open ORing diode. These drawbacks canjeopardize power system reliability.
This article examines how using ORing MOSFET controllers and power FETs in place of ORing power diodes
can reduce power consumption in high-current power distribution systems. Presented are the key selection
steps for choosing power FETs, along with proven design tips to ensure engineers meet their target
specifications when developing a redundant power distribution system.
Figure 1 shows the disadvantage of using ORing diodes in a high current supply system: in this system, each
diode is dissipating 10W (20A with a diode drop of 0.5V). Both would need their own heat sink, as the diodes
alone consume 20W in this system.

Figure 1. High current redundant power supply using ORing diodes

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In high load current applications, power consumption can be reduced if the ORing diodes are replaced with
MOSFETs managed by a high voltage MOSFET ORing controller, such as Intersil’s ISL6144. Figure 2 shows
the ISL6144, which includes an on-chip charge pump for driving a power FET. The controller also includes
features that allow it to shut down its output if the input supply fails. This helps prevent transients whenanother redundant system controller sources its output load-current. Using an ORing MOSFET controller
reduces power loss and enhances reliability.

Figure 2. High current redundant power supply using ORing MOSFET controllers

Selecting Power FETs
Selecting the right power FET is necessary to help the ORing MOSFET controller perform its job. You canselect a suitable FET by following these steps:
1) Use an N-channel FET that has a drain-source voltage (Vds) breakdown that is greater than the busvoltage. For example, if the bus voltage is 48V, you will want to use a FET with 60V breakdown voltage.
2) Pick a FET with a current rating that can support the bus load. The Figure 2 example shows a redundant
system where either 48V supply could fail. If a supply failure occurs, the other power supply will need to send
the full 40A load through one ORing FET. Therefore the FET would need to be rated for more than 40A.
3) Pick a FET with an appropriate Rdson for what you are trying to achieve. Several factors will affect thischoice. You will want to focus on FET power dissipation under worst-case conditions, followed by checking
that the FET’s worst case Rdson allows the power supply voltage to maintain supply voltage regulation withmargin.
Be mindful that the amount of power dissipation in the FET dictates how much heat sinking will be required.
The value chosen for Rdson will directly affect the worst-case power dissipation, which in turn dictates theheat sink requirements. The choice of the Rdson value may be an iterative process where you choose the Rdson
value based upon what heat sink or airflow requirements are needed to make the solution fit your system.

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For higher power levels, efficiency may be the biggest concern as lower power dissipation minimizes the heatsinking or air flow requirements. The value of Rdson directly affects the heat sink requirements and whether
airflow will be required for cooling. It can be more cost-effective to choose an expensive FET for lower Rdson(which buys you higher efficiency) as this may save costs on the heat sink and airflow requirements.
During this process, be sure to use the worst case Rdson. The ISL6144 ORing MOSFET controller’s charge
pump enables the gate FET drive to be 10V above its source. Thus, be sure to look at the FET characteristics
when it’s driven with Vgs(on) of 10V. Do not worry about choosing a FET for high frequency switching. In thisapplication, the FETs will slowly turn on-and-off. Therefore, minimizing the gate charge is not required.

FETs vs. Diodes: A Closer Look
To see the true benefit of FETs over diodes in a redundant power distribution application, set a goal ofachieving lower power dissipation in the FET under worst-case conditions – set it lower than diodes canachieve. In the original system highlighted earlier, we saw that each diode dissipated 10W under normal
operating conditions. If one diode fails, the other diode would need to carry the full 40A. This would push thediode system to a forward voltage of 0.6 volts for a worst-case power dissipation of 24W.
In the FET system, the worst-case power dissipation occurs when the system fails and forces one FET tocarry the full 40A load. Your goal should be to use the lowest possible Rdson FET so that efficiency is improved
and the heat sink and airflow requirements are minimized.
When you reach this point in the design, a good next step is to log on to your favorite component distributor’s
web site and go to the FET selection section. Many distributors have search tools that allow you to input avariety of FET parameters and get a listing of appropriate devices that meet your parameters. Since Rdson is a
key parameter for this application, you want to search for the devices with lowest Rdson values at areasonable cost. Since the distributors tools include pricing you can get a good idea what lower Rdson willcost.
By using the distributor’s search tool, you’ll see that a FET with 60V breakdown and 100A can be found withRdson at 125 °C at about 4.3 milliohms – all for a cost of less than $1 at 1K quantities.
If you use this FET, your first task is to check the worst-case power dissipation, which in this case is having
one failed input supply. Under this condition, the I2R power dissipation is: 40A x 40A x 0.0043 = 6.88W.
Under normal (non-fault) conditions, each FET carries 20A and operates at a cooler temperature. This resultsin lower Rdson and even lower losses.
Knowing the wattage prepares you to design the cooling system, which will likely depend upon the heatgenerated by other system components and whether you planned air-cooling for this system. Lastly, you willneed to check the voltage regulation from the selected FET. With 40A flowing through the FET Rdson of0.0043 ohms, the voltage drop will be 40 x 0.0043 = 0.172V. Therefore, the 48V output would change by
only 0.172/48 = 0.3%.

Conclusion
Moving away from the use of standard ORing power diodes to an ORing controller that drives a power FET
can minimize power loss and maintain redundant system reliability. The power FET selection steps anddesign tips provided will guide you in the design of your next redundant power distribution system.
Remember, these FETs lack voltage and overcurrent protection so never chose a FET at its absolute limits.
Give yourself plenty of headroom.

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Next Steps•Learn more about the ISL6144•Download the datasheet•Order samples###© 2018 Renesas Electronics America Inc. (REA). All rights reserved. All trademarks and trade names are those of their respective owners. REA believes theinformation herein was accurate when given but assumes no risk as to its quality or use. All information is provided as-is without warranties of any kind,
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