mosfet speeds and turn on/off times - Electronic Circuits Projects Diagrams Free

Thunderchild · 13th September 2009, 08:32 AM

When looking for a mosfet (or any other power transistor) and considering its speed how to I relate that to the frequency it can be switched at ? I know it will depend lergely on application maybe ? so if a mosfet has a turn on time of 60 ns and a turn off time of 45 ns thats 105 ns of more or less dead time in a cycle and you'd want this to be no more than about 10 % of each cycle or less meaning a 1050 ns cycle which is 1.05 ms so thats about 500 Hz - 1 KHz ?

dknguyen · 13th September 2009, 08:41 AM

It all boils down to the waveform you want. You are going to have to look at the trapezoidal pulse you want in your square wave and decide what you want it to look like. How much of the rise and fall slope do you want in each pulse relative to the steady state "full-on" part of the pulse? Balance this with your desired frequency to get acceptable power dissipation. THe worst case scenario is the square wave at the smallest non-zero duty cycle that you want, at the highest frequency that you want. Remember- this is the WORST case scenario. Whatever numbers you specify here, you will get a more ideal square wave at any lower frequency or higher duty cycle.

THis means that your 10% transition time criteria is very strict if that is what you want at a worst case scenario. When I was doing calculations, I too used 10% as my desired transition time per pulse in the worst case scenario (10% the on-time of the shortest pulse). But after doing the heat dissipation calculations I realized it was not going to be possible without very large fast transistors with very powerful gate drives. It became a lot more reasonable when I used something like 66% transition time for the worst case scenario (2/3ths of the shortest pulse is transition and only 1/3 of it is steady state-full on.). This meant it would not be a very nice square wave at low duty cycles and it would be lossy...but guess what? At low duty cycles it also dissipates the least power so it wasn't a problem. At higher duty cycles, the on-time got longer but the transition times stayed the same, so the square wave got much nicer as the duty cycle (and power dissipation) increased.

It has very little to do with how fast the MOSFET can switch. You can always make the MOSFET switch faster by using a higher current gate driver and it can almost always switch your MOSFET faster than you actually need it to (making problems like RF, EMI, and ringing).

eblc1388 · 13th September 2009, 08:50 AM

Quote:

Originally Posted by Thunderchild

you'd want this to be no more than about 10 % of each cycle or less meaning a 1050 ns cycle which is 1.05 ms so thats about 500 Hz - 1 KHz ?

About right, gives or takes a few ns.

Reminds me of an old lady whose age is 18 plus a few months.

dknguyen · 13th September 2009, 08:57 AM

Ahh! THat's what was so strange. I was wondering why he was getting low kHz frequencies when he had ns transition times that only took up 10% of the duty cycle. When I was doing my calculations a while back I used much slower transition times and I was getting 10-30kHz frequencies.

Thunderchild · 13th September 2009, 09:02 AM

ok well see this is for the prewar car power reg (another thread) so I also want as little heat dissipation as possible as its a closed box. maybe the not so perfect square is probably fine as I'm driving an inductive load so don't want massive back spikes due to instant rise times maybe.

how exactly do I drive a mosfet faster with more current ? I'm running it on a pic is that enough of a drive ?

Thunderchild · 13th September 2009, 09:03 AM

Quote:

Originally Posted by dknguyen

Ahh! THat's what was so strange. I was wondering why he was getting low kHz frequencies when he had ns transition times that only took up 10% of the duty cycle. When I was doing my calculations a while back I used much slower transition times and I was getting 10-30kHz frequencies.

where did I go wrong ? 30 KHz is 33 ns isn't it ? and for a 45-60 ns on off times thats too fast ?

dknguyen · 13th September 2009, 09:07 AM

Quote:

Originally Posted by Thunderchild

ok well see this is for the prewar car power reg (another thread) so I also want as little heat dissipation as possible as its a closed box.

Lower switching frequency and faster transition times would help this...

Quote:

Originally Posted by Thunderchild

maybe the not so perfect square is probably fine as I'm driving an inductive load so don't want massive back spikes due to instant rise times maybe.

Exactly. Slower fall times would greatly help this. So you have two conflicting criteria that you must balance out. Faster transition are more efficient, but make more ringing.

Quote:

Originally Posted by Thunderchild

how exactly do I drive a mosfet faster with more current ? I'm running it on a pic is that enough of a drive ?

A PIC is definately not enough gate current for high frequency switching. And unless you are using a logic level MOSFET, the voltage will not be high enough to turn on the gate either.

Use a gate driver circuit (or more conveniently, a gate driver IC). It's like a amplifier for the PIC signal that will boost the voltage to proper gate drive levels, as well as provide high current capability to charge/discharge up the gate capacitance faster.

Thunderchild · 13th September 2009, 09:15 AM

hm like a buffer

Thunderchild · 13th September 2009, 09:29 AM

how about a small fast transistor as a buffer ?

dknguyen · 13th September 2009, 09:37 AM

Gate drivers are like digital buffers. Except they are focused on momentary, but very high peak currents. One can be built from transistors. A gate driver built from FETs will suffer the same switching problems you are trying to overcome in the power FET, so they tend to always use some BJTs to get the high speed and get around the gate capacitance. Sometimes they use also use some FETs to overcome this BJT limitation to get closer to the rail.

You need high peak currents to quickly charge/discharge the gate capacitance. BUt since it is momentary the power levels aren't very high so you don't need a very big BJT. But most BJTs saturate at a current lower than optimal so you might need to find BJTs that are designed to sink/source high current levels for their power level, depending on the size of your power FET (or you could use a big BJT that is totally overpowered, but those are also hard to find).

Thunderchild · 13th September 2009, 04:12 PM

Thanks DK I'll give it a read, I'm trying to have as few components as possible as I'm trying to build a saleable device rather than a one off so costs some into it as well as rugged design

Jirka46 · 13th September 2009, 06:04 PM

Quote:

Originally Posted by Thunderchild

1050 ns cycle which is 1.05 ms so thats about 500 Hz - 1 KHz ?

1050 ns= 1.05 microseconds=0.00105 miliseconds
so that is about 500kHz to 1 MHz
Jirka

Thunderchild · 13th September 2009, 06:15 PM

ah yess I see I skipped us and jumped a whole 1000 , thank goodness now any mosfet will do the job !

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar
Title	Starter	Forum	Replies	Latest
mosfet turn on voltage	firewater	General Electronics Chat	4	30th August 2007 02:18 PM
Very low turn-on voltage MOSFET	speakerguy79	General Electronics Chat	6	7th June 2007 05:08 PM
Couldn't turn on and off the logic MOSFET....	devonsc	General Electronics Chat	53	3rd January 2005 12:29 PM
Need Help With Calculating Torque/Speeds	StormRyda	Electronic Projects Design/Ideas/Reviews	13	3rd April 2004 04:53 PM
Internet speeds	herbymcduff	General Electronics Chat	5	15th January 2003 06:23 PM

13th September 2009, 08:32 AM	#1
Thunderchild	mosfet speeds and turn on/off times When looking for a mosfet (or any other power transistor) and considering its speed how to I relate that to the frequency it can be switched at ? I know it will depend lergely on application maybe ? so if a mosfet has a turn on time of 60 ns and a turn off time of 45 ns thats 105 ns of more or less dead time in a cycle and you'd want this to be no more than about 10 % of each cycle or less meaning a 1050 ns cycle which is 1.05 ms so thats about 500 Hz - 1 KHz ? __________________ I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately) www.simonsphotography.org.uk/ - My other hobby www.rushdenrotaract.org.uk/ - make a difference and have fun ! Never buy "Trust" products, all mine broke !!!

13th September 2009, 08:41 AM	#2
dknguyen	It all boils down to the waveform you want. You are going to have to look at the trapezoidal pulse you want in your square wave and decide what you want it to look like. How much of the rise and fall slope do you want in each pulse relative to the steady state "full-on" part of the pulse? Balance this with your desired frequency to get acceptable power dissipation. THe worst case scenario is the square wave at the smallest non-zero duty cycle that you want, at the highest frequency that you want. Remember- this is the WORST case scenario. Whatever numbers you specify here, you will get a more ideal square wave at any lower frequency or higher duty cycle. THis means that your 10% transition time criteria is very strict if that is what you want at a worst case scenario. When I was doing calculations, I too used 10% as my desired transition time per pulse in the worst case scenario (10% the on-time of the shortest pulse). But after doing the heat dissipation calculations I realized it was not going to be possible without very large fast transistors with very powerful gate drives. It became a lot more reasonable when I used something like 66% transition time for the worst case scenario (2/3ths of the shortest pulse is transition and only 1/3 of it is steady state-full on.). This meant it would not be a very nice square wave at low duty cycles and it would be lossy...but guess what? At low duty cycles it also dissipates the least power so it wasn't a problem. At higher duty cycles, the on-time got longer but the transition times stayed the same, so the square wave got much nicer as the duty cycle (and power dissipation) increased. It has very little to do with how fast the MOSFET can switch. You can always make the MOSFET switch faster by using a higher current gate driver and it can almost always switch your MOSFET faster than you actually need it to (making problems like RF, EMI, and ringing). __________________ Tanaka Sensei (avatar) says: Please spell it "ridiculous" correctly! Not "rediculous". ^^ Last edited by dknguyen; 13th September 2009 at 08:53 AM.

13th September 2009, 08:57 AM	#4
dknguyen	Ahh! THat's what was so strange. I was wondering why he was getting low kHz frequencies when he had ns transition times that only took up 10% of the duty cycle. When I was doing my calculations a while back I used much slower transition times and I was getting 10-30kHz frequencies. __________________ Tanaka Sensei (avatar) says: Please spell it "ridiculous" correctly! Not "rediculous". ^^ Last edited by dknguyen; 13th September 2009 at 08:58 AM.

13th September 2009, 09:02 AM	#5
Thunderchild	ok well see this is for the prewar car power reg (another thread) so I also want as little heat dissipation as possible as its a closed box. maybe the not so perfect square is probably fine as I'm driving an inductive load so don't want massive back spikes due to instant rise times maybe. how exactly do I drive a mosfet faster with more current ? I'm running it on a pic is that enough of a drive ? __________________ I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately) www.simonsphotography.org.uk/ - My other hobby www.rushdenrotaract.org.uk/ - make a difference and have fun ! Never buy "Trust" products, all mine broke !!!

13th September 2009, 09:15 AM	#8
Thunderchild	hm like a buffer __________________ I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately) www.simonsphotography.org.uk/ - My other hobby www.rushdenrotaract.org.uk/ - make a difference and have fun ! Never buy "Trust" products, all mine broke !!!

13th September 2009, 09:29 AM	#9
Thunderchild	how about a small fast transistor as a buffer ? __________________ I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately) www.simonsphotography.org.uk/ - My other hobby www.rushdenrotaract.org.uk/ - make a difference and have fun ! Never buy "Trust" products, all mine broke !!!

13th September 2009, 04:12 PM	#11
Thunderchild	Thanks DK I'll give it a read, I'm trying to have as few components as possible as I'm trying to build a saleable device rather than a one off so costs some into it as well as rugged design __________________ I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately) www.simonsphotography.org.uk/ - My other hobby www.rushdenrotaract.org.uk/ - make a difference and have fun ! Never buy "Trust" products, all mine broke !!!

13th September 2009, 06:15 PM	#13
Thunderchild	ah yess I see I skipped us and jumped a whole 1000 , thank goodness now any mosfet will do the job ! __________________ I AM the exeption that disproves the rule in many ways but the rules still apply (unfortunately) www.simonsphotography.org.uk/ - My other hobby www.rushdenrotaract.org.uk/ - make a difference and have fun ! Never buy "Trust" products, all mine broke !!!