Mosfet Package and Thermal characteristics  - Page 1
Forum section: electronics / general

author: arslan43711
author_rank: Contributor
timestamp: July 16, 2024, 04:36:42 am
content: Hi everyone,I am working on DC to DC converter design. The input voltages are 12v and the output is 5V 10A.I have used the TI power designer tool to create the schematic design, But it suggest to use the SMD MOSFET.Is those tine tiny MOSFET can handle this amount of current without heat sink?What if I change the MOSFET to TO220 while trying to keep the gate charge and rds on similar to suggested MOSFET. Is that going to work fine as per the generated design(In term of performance or Emi perspective)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 16, 2024, 06:06:29 am
content: MOSfets can have a very low Rds-on and often the main cause of heating is during the switching time, when there is both voltage over the FET and current through it. By switching fast, the average heat is also kept low.It is also not true that SMT fets are "without heat sink". The idea is to add a considerable amount of copper plane to the PCB, (copper is a very good heat conductor) and this makes the PCB itself a heatsink. How good this heat sink is, varies a lot with it's design. More copper results in a better heat sink. If you put a bunch of thermal via's under the FET, you can use both sides of the PCB as a heat sink. It is very doable to have a 10A smps with SMT fets and without an extra aluminum heatsink, but the design has to be carefully chosen.It's also common to use SMT fets with thermal via's through the PCB, and then clamp an aluminium heat sink to the other side of the PCB. This is not optimal from a heat transfer point of view, but it's done anyway because it allows for fully automated assembly. There are also heatsinks that you can place over the FET, and then directly solder to the PCB. Davey Jones has shown a few of those in a teardown of one of his Rhode & Schwartz power supplies.Yet another option is to use "thermal bridges" (or whatever they are called). These look exactly like SMT resistors, but they do not have resistive material on them. Their only purpose is to transfer heat from one copper net to another copper net. As a result, you don't have to use a big copper area directly connected to the tab of the SMT fet, but you can also use other nets with a lot of copper area (GND plane, wide power tracks) as part of the heat sinking solution.It is also common to use two FET's, even if one FET could handle the current. With two FET's, each FET has half the current, and thus 1/4 of the DC power dissipation, so overall power dissipation is halved. It also spreads power dissipation over a wider area, which makes the heat sink more effective.So there are plenty of options. TO220 is easy from an DIY point of perspective (If it gets too hot just slap on more aluminium). With the PCB as a heat sink, you may need more then one iteration of get the heat sinking right. There is a lot of variation in effectiveness on heat sinking with a PCB. Things like copper with or without a solder mask has an influence. There are also options for the PCB itself to be made out of aluminium or ceramic. LED panels on an aluminum PCB are common for example.

author: thm_w
author_rank: Super Contributor
timestamp: July 16, 2024, 09:06:23 pm
content: What FET did TI designer say to use? They should tell you the expected dissipation right in the calculator.Or you can look up the on resistance and make a rough guess

author: arslan43711
author_rank: Contributor
timestamp: July 17, 2024, 02:54:41 am
content: For high side it suggested CSD15571Q2 and for low side CSD17578Q3A.I did the calculations and the total switching + conduction losses add up to around 2 watts. Which is not good for this package size.If I use TO220 package does that going affect any. If rest of the design remans the same?

author: thm_w
author_rank: Super Contributor
timestamp: July 17, 2024, 09:49:39 pm
content: CSD17578Q3A looks ok, 6mohm at 10A. Max dissipation of 2.5WBut you can definitely find better SMD FETs if needed. These are tiny SON parts. Doesn't the TI calculator let you optimize for size/cost/etc? could try some other option.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 18, 2024, 12:47:37 am
content: SMDs are also better for switching performance: the loop inductance can be much smaller, allowing faster switching, and higher Fsw to use smaller parts, or lower Fsw and larger parts for higher efficiency.SMDs in DFN3x3 and 5x6 are good up to a couple watts with little effort; maybe 5 or 10 tops, using a multilayer PCB, heavyish copper, and good heatsinking (e.g. thermal pads clamping the board between heatsinks).Mind they are often rated to ludicrous power levels (D(2)PAK moreso than DFN)when Tc = 25C, but this is only achievable with fluid cooling, or at least, like, being soldered to a copper heat spreader, but how are you going to do that along with a PCB, right.A 50W converter would have to be designed pretty poorly to cost more than 10% of that, or 5W. Bettering 95% should be easy enough.Tim