Why would you choose this FET over another? - Page 1
Forum section: electronics / projects

author: MattHollands
author_rank: Frequent Contributor
timestamp: May 02, 2016, 05:06:20 am
content: Hi,So I have a design project for school designing a SMPS. And I am trying to understand why you would every choose a particular MOSFET over another. We have a limited selection of FETs to choose from.So an example is we have the IRFZ24N (http://www.irf.com/product-info/datasheets/data/irfz24n.pdf) and the IRFZ34N (http://www.irf.com/product-info/datasheets/data/irfz34n.pdf). They are both very similarly priced (7c difference in quantities of 1000), both rated at 55V.IRFZ34N: Idsmax = 29A, Rds(on) = 40mR.IRFZ24N: Idsmax = 17A, Rds(on) = 70mR.So, if you had a requirement for less than say 12A, why would you every choose the '24 over the '34? It seems you get a lower Rds(on) and therefore lower power dissipation on the '34.The ratings on the '24 are specified at 10A, while the ratings on the '34 are specified at 16A which I suppose may be advantageous if you are operating around 10A.I would naturally assume that higher current FETs would have a higher Rds(on), so this surprises me.Is it purely a money thing (just a couple cents)? What am I missing? Thank!

author: Someone
author_rank: Super Contributor
timestamp: May 02, 2016, 05:42:00 am
content: As with many fets you have a tradeoff between on resistance and gate charge, so you calculate a figure of merit based on your specific design.

author: Circlotron
author_rank: Super Contributor
timestamp: May 02, 2016, 11:48:10 pm
content: Simplifying part of evb14's answer - At DC, low on-resistance is (often) king. If the mosfet is being rapidly switched on and off though, you have to charge and discharge it's internal capacitances every switching cycle. The lower resistance mosfet with it's bigger chunk of silicon and probably higher internal capacitances, and therefore quantity of energy to be wasted every cycle, may well end up running hotter than the smaller, higher resistance device. If you have a range of devices to choose from, there will be a "best" device for lowest *overall* dissipation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 03, 2016, 02:31:34 am
content: Neither: they're both ancient and their specs suck!You did mention it's an SMPS application, but didn't mention what kind (topology; range of frequency, current and voltage). Nor what your "limited selection" consists of... it's impossible to conclude anything from this.Tim

author: MattHollands
author_rank: Frequent Contributor
timestamp: May 04, 2016, 06:35:43 pm
content: Thanks evb and circlotron, that's a lot of information to process!T3sl4co1l: I specifically didn't add to much detail because I want to do the work myself so was trying to put up a hypothetical situation. But the answer is that it is a buck converter operating around the 30V input mark and 15V out. Inductor current of about 10A with 1.2App current. I'm going to operate at about 100kHz. There is no opportunity to build and test, this is an exercise in choosing a modelling parts.

author: nctnico
author_rank: Super Contributor
timestamp: May 05, 2016, 12:35:25 am
content: When choosing a MOSFET I try to find the sweet spot where the gate charge and RDSon are the lowest given the constraints (maximum voltage and current). In my experience the MOSFET should have a current and voltage rating which is just right for the design limits.