What determines maximum switching frequency of a mosfet? - Page 1
Forum section: electronics / beginners

author: sjd.aliyan
author_rank: Contributor
timestamp: May 12, 2016, 12:23:01 pm
content: Explain tome plz,what makes a mosfet driving at mhz high speeds?Is mosfet with lower rds(on) means it can reach higher frequencies?How to calculate max achievable frequency for a mosfet based on datasheet's information?for example IRLZ44N

author: lem_ix
author_rank: Regular Contributor
timestamp: May 12, 2016, 12:55:33 pm
content: A mosfet has input/output capacitance that limits it's operation speed. Modeling the mosfet with them lets you calculate the time constants of interest. Maybe just simulate it in some spice program for a start? Maximum frequency will of course also depend on the load. There are various circuits for improving switching operation.

author: Kalvin
author_rank: Super Contributor
timestamp: May 12, 2016, 01:00:58 pm
content: First approximation: MOSFET's speed is related to how fast you can charge and discharge the gate's inherent capacitance which is typically in range of nanofarads for the power MOSFETs. Rule of a thumb: You need to supply one ampere of current to a one nanofarad capacitor in order to rise its voltage by one volt in one nanosecond.

author: macgeorge
author_rank: Newbie
timestamp: May 12, 2016, 01:13:31 pm
content: As the previous members have already said, a MOSFET has a finite time that is needed for it to switch on and off, which is caused by its intrinsic capacitors. Therefore, if i.e. the rise and fall time are 50ns each (50ns+50ns=100ns), then you can't drive the MOSFET in frequencies near 10MHz (100ns).However, even if theoretically you could reach those frequencies, in reality you can't, because of the switching losses that occur in common hard switching circuits and are proportionate to the switching frequency. They are also proportionate to the rise and fall times so a slower MOSFET (larger intrinsic capacitors) will have higher switching losses. Therefore, even if you could operate the MOSFET at such a high frequency because of the produced heat it would be damaged.A MOSFET has two types of losses, switching losses and conduction losses (meaning heat because of its ON resistance Rdson). Therefore, even with a MOSFET with a low Rdson (->low conduction losses), the switching losses would still be high.Usually manufacturers produce MOSFETs with either higher Rdson but lower rise and fall times or vice versa, but this is not always the case.

author: Kalvin
author_rank: Super Contributor
timestamp: May 12, 2016, 01:42:54 pm
content: When you are trying to achieve nanosecond switching times, the MOSFET's package inductance will start slowing things down even further and make things oscillate quite easily. Typically a power MOSFET can have a package inductance from a few nanohenrys to over 10 nanohenrys (check the datasheet), which combined with the intrinsic gate capacitance can make the gate voltage oscillate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2016, 05:33:15 pm
content: The gate terminal looks like a capacitor in series with a resistor. The RC time constant limits switching speed.The source terminal inductance is also a big limitation, on packages that have substantial lead length, like TO-220.The load also has to be able to commutate the drain capacitance, which can require considerable minimum load current.Switching losses are dependent on load current, so there comes a point when a transistor is physically unable to switch fast enough, while carrying enough load current to switch the load as such.Tim

author: danadak
author_rank: Super Contributor
timestamp: May 12, 2016, 09:30:38 pm
content: Is mosfet with lower rds(on) means it can reach higher frequencies?Generally speaking lower Rdson = higher Cgd = slower speed.http://www.infineon.com/dgdl/an-937.pdf?fileId=5546d462533600a40153559ea1481181http://www2.st.com/content/ccc/resource/technical/document/application_note/68/cd/c6/ab/ef/17/41/06/CD00003900.pdf/files/CD00003900.pdf/jcr:content/translations/en.CD00003900.pdfhttp://www.radio-sensors.se/download/gate-driver2.pdfhttp://www.ixys.com/documents/appnotes/ixan0010.pdfhttps://www.fairchildsemi.com/application-notes/AN/AN-6069.pdfhttp://www.ti.com/lit/an/slua054/slua054.pdfhttp://ww1.microchip.com/downloads/en/AppNotes/00898a.pdfhttp://www.onsemi.com/PowerSolutions/supportDoc.do?type=appNotes&category=809Regards, Dana.