Good MOSFET selection

[gdylp2004]

Hi guys,

I thought of upgrading my buck converter's high side MOSFET. Previously, I was using IRF640NPbF (datasheet: http://www.irf.com/product-info/datasheets/data/irf640n.pdf).

Do anyone know if the following are the main criteria for searching a good mosfet (please let me know if I've missed some important considerations out):

Rds(on) - Commonly know for power dissipation since high Rds(on) represent higher voltage drop, hence more power loss
Gate-charge - Lesser gate charge the better? Since the charge required for the MOSFET to turn on and off directly related to the gate charge?
If I am choosing this Toshiba MOSFET (datasheet: **broken link removed**) with a Rds(on) of only 38mΩ (10x reduction compared to IRF640N), is it a good choice?

If yes, what about the gate-charge? For a Gate-source voltage about 12V & Vds of ~100V, IRF640N has ~50nC but the Toshiba FET has ~150nC! (3X difference). Does this mean the Toshiba FET is not better than the IRF640? Or I can't made such comparison because the test Id is not the same, then what should we do?

Thanks.

 

[billnaylor]

have a look at this excellent tutorial:
Buck Converter Design

YOu need to balance ON resistance with Qg to get the switching losses and conduction losses down. If the duty cycle is low, ON resistance becomes less important

 

[gdylp2004]

have a look at this excellent tutorial:
Buck Converter Design

YOu need to balance ON resistance with Qg to get the switching losses and conduction losses down. If the duty cycle is low, ON resistance becomes less important

Thanks for your reply. I've actually already build the non-sync Buck and has been posted in the "Power Electronics" subforum: https://www.edaboard.com/threads/233047/. Unfortunately, not many are replying to my thread.

Do you know, by chance, that why does my V(gs), hence o/p voltage has transient spikes? I've attempted to increase the o/p or smoothing cap to 300uF (x2 150uF cap in parallel).

I've also try to increase Rgate to 22ohms, but instead of reducing the spikes, I got my MOSFET burnt. That is logical as I believe it was due to the longer discharge time during high MOSFET = OFF causes the nMOS to be in the linear region for too long, hence becoming too hot, thus burnt.

For those who could help me, the attached is my full schematic. Thanks in advance.

 

[gdylp2004]

Hi guys,

I've changed my nMOS from IRF640NPbF to IRFB4227PbF as intended. Was glad because initially, the nMOS was having a fever of >160 °C after running for only a min or so, but now the replaced MOSFET is only at 155 °C (steady state ~15mins). Also, the new MOSFET has a higher max. operating temperature range is 175 °C compared to the former's max. 150 °C.

However, the V(gs) waveform seems to tell me that the charging of the gate terminal of the FET is overdamped by looking at the knee of the rising edge. I've removed 5.6 Ω in hope to improve the dv/dt response but to no avail.

Does anyone has clue why removing Rgate has not much effect, and are there any tricks that I can do to improve this? Or I would need to re-select a nMOS with a lower gate capacitance (which means choosing another nMOS with lower gate charge).

Thank you.

 

[gdylp2004]

HI,



I would like to know is there a distinct difference between IRFB4620 and IRFB5620.



Datasheets: https://www.farnell.com/datasheets/91452.pdf

https://www.farnell.com/datasheets/356287.pdf



I've literally compared each and every parameters and realised almost most of them are the same yet the applications as suggested by the individual datasheet are different. One is for SMPS while the other is a Class D amplifier?



What I would like to know is if they are interchangeable?



Thank you!

 

[billnaylor]

to speed up the switching of the FET you might need a MOSFET driver. I cannot see the difference between the 2 MOSFETs you have picked in terms of switched mode power supply operation

 

[gdylp2004]

exactly! I wonder why they would describe different operations in their respective datasheets where almost all of the parameters are the same! Misleading!