For a battery charging application, i have to build a synchronous buck converter (i/p voltage = 12V , o/p Voltage = 4.5V) which would be driven by a PWM signal from the micro-controller. This Buck converter's operating frequency would be around 62kHz. I am planning to use both the NMOS for high and low transistors, which would be driven by a suitable gate driver.

My question is regarding the selection of NMOS device w.r.t their Drain current rating. For e.g lets say my maximum current to charge the battery would be 4A. So i have to select a MOS device whose Id >= 4A . And here are two power MOSFET devices who have a quite high current rating : IRLB8748 (comparatively high Drain current capability) and IRL2703 (comparatively low Drain current capability).

Now both of these devices fulfill my requirement w.r.t drain current (and they dont differ much in pricing). Lets say if i use IRLB8748 (having RdsonMAX = 4.8 mOhms), for 4A (maximum current) the power being dissipated will be IxIxR = 0.0768 Watts. And if i use IRL2703 (having RdsonMAX = 40 mOhms) the dissipated power for 4A current = 0.64 Watts. As it can be seen that for 4A, IRLB8748 dissipates almost 10 times less power than IRL2703 does.

So if i want my transistors to dissipate lesser power (less power dissipation = lesser heat produced = better), should i go for IRLB8748 ? Or can I ? I mean i might be wrong here, but can we use high current rating power transistors in our low current applications to dissipate lesser power (in some cases that can may be also let us avoid using heat sinks) ? Also is it true for my case (i mean can i use IRLB8748 so that my circuit may dissipate less power) ?

Your helpful comments and suggestions would be appreciated.



Absolutely no problem at all. The high current FET does not know to behave any differently just because you are running less current through it. Provided that you have correctly calculated Rds(on) at the correct gate voltage, it is up to you to decide what to use.

Generally speaking, higher performance (lower Rds(on)) devices are more expensive, and higher current devices are both more expensive and physically larger. If neither of those applies or bothers you, crack on.

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    \$\begingroup\$ Though in general switching losses will be higher for higher current rated devices due to the higher total gate charge. If you're looking to minimize losses the high side FET should be a low gate charge device and the low side FET should be a low RDSon device, with both chosen as appropriate for the current requirements. At the relatively low switching frequency of 62kHz switching losses probably aren't going to be a big issue. \$\endgroup\$ – John D Sep 2 '15 at 19:00

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