Comments on What is the difference between rise/fall time and Turn-On/off Delay Time?
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What is the difference between rise/fall time and Turn-On/off Delay Time?
Hi guys so recently i was looking for a mosfet and a gate drivers and between a lot of their specification the rise/fall time and Turn-On/off Delay Time sometime it called propagation.
My questions is what is the difference between them? And how does this affect the signals or just the switching?and my last question what is the perfect proportion between them should they be the same maybe?
Thank you.
Edit:
Some explanation is needed here, so to drive a mosfet we need a PWM source and a gate driver in case if the potential source is low or if we are driving multiple mosfets like in case of H-bridge. Now the complexity begins when we try to put all that together "Synchronisation" so we have to choose well and specially when we deal with a high frequencies and to do that we need to have a good understanding of some timing specs.
Here an example of some specs of mosfets and gate drivers:
Edit 2:
" In general, the on/off delay is how long it takes from changing the input until the output starts to do something. "
Very clear so the ON/OFF DELAY its the time that the FET driver or the mosfet take to replicate the input signal and the RISE/FALL time is the time that it take to change from the ON state to the OFF state and vice versa.
That drive me to ask another question,
It's okay that the input and the output have a delay ON/OFF time as long as the ON is equal to the OFF time, but what happen if they are not equal?
Another question about the RISE/FALL time, which is the more important as Mr. Olin mentioned in his answer, the lower it take the better, less power losses and heat to disipitate. So the RISE/FALL time should be identical to the input and must not be higher, but what happens if it is lower than the input? For example the RISE/FALL time of the input is 15ns/15ns respectivly and the output can be 10ns/10ns in the datasheet.
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In general, the on/off delay is how long it takes from changing the input until the output starts to do something. The rise/fall time is how long it takes to do the something, once it starts doing it.
Your first FET driver datasheet is a good example:
The switching characteristics are idealized as up to 300 ns where nothing happens after you change the input, then another 30 ns for the output to transition on/off or off/on once it starts doing something. The total worst case time from switching the input until the output is fully in its new state is therefore 330 ns.
So why would anyone care about the details and not just the overall 330 ns delay? Because sometimes the overall delay isn't as important as a fast transition. An ideal switch dissipates no power when off because the current is zero, and no power when on because the voltage across it is zero. The power dissipated in-between can be significant. You want to have the FET transition thru the in-between region as fast as possible.
This FET driver guarantees that it won't leave the FET gate in the in-between region for more than 30 ns each transition. If you're switching a high power heater, for example, the 330 ns delay from when the thermostat says to switch until the load is actually switched is inconsequential. However, transitioning the FET quickly can be the difference between requiring a heat sink or not.
The specs for a FET are interpreted similarly:
The delay time in this case is how long it takes for the channel to change its conduction characteristics from a gate voltage change.
The rise and fall times is how long it takes for the drain voltage to change in a particular circuit. Note how it explicitly says the off/on gate voltages are 0 and 18 V, and that the load on the drain is 2 Ω. These are probably the conditions under which the manufacturer knows the response will be fastest.
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