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mosfet speeds and turn on/off times

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Thunderchild

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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

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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).
 
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eblc1388

Active Member
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

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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.
 
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Thunderchild

New Member
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

New Member
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

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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...

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.

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.
 

dknguyen

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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).
 

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Thunderchild

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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
 
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