# dV/dt and dI/dt discussion

Discussion in 'Analog & Mixed-Signal Design' started by mishra87, Jan 20, 2017.

1. ### mishra87 Thread Starter Member

Jan 17, 2016
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3
i am a bit new of these terminology !!!

Could anybody enlight me the basic understanding of dV/dt and dI/dt.

What all source creates these parameters and what is effect in the circuit.

Lets say

dI/dt = 100 A/μs
and
dV/dt= 5.5 V/ns

what does it means in application circuit

Thanks in Advance!!!

2. ### MrChips Moderator

Oct 2, 2009
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What do you understand by dV/dt?

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3. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Every microsecond the current is increasing by 100 amperes. If at time zero you have 0 amperes, then at time 1 microsecond you will 100 amperes, then at time 2 microseconds you will have 200 amperes. And. So. On.

Every nanosecond the voltage is increasing by 5.5 volts.

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4. ### wayneh Expert

Sep 9, 2010
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That notation represents instantaneous slopes of voltage and current plotted against time. They are similar to ∆V/∆t and ∆I/∆t (two-points-make-a-line slopes), but are evaluated at the limit where ∆t approaches zero. For a straight line, they are equivalent. For a complex curve, they may be very different depending on where you look.

You would learn all about this in an introductory calculus class.

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5. ### crutschow Expert

Mar 14, 2008
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Some devices, such as SCR's, are sensitive the rate of voltage rise applied to the device (dV/dt) or the rate the current increases through the device when turned on (di/dt), so the devices have limits on these parameters.
If you exceed those limits, the device may malfunction or be damaged.

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6. ### mishra87 Thread Starter Member

Jan 17, 2016
499
3
Thank you all for your intrested and time !!!

Actually I have taken the value of dV/dT and dI/dT in post#1 from Mosfet IRF540 datasheet.

So how these parameters affect to the Mosfet. How we can define these two terminology in terms of MOSFET. !!!

7. ### wayneh Expert

Sep 9, 2010
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A MOSFET cannot change its state instantly - it takes some finite amount of time to go from on to off. See how the timing diagram shows a slope in between states. The dV/dt and dI/dt measures are related to the switching times possible with that device. Steeper slopes = faster transitions.

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8. ### Jony130 AAC Fanatic!

Feb 17, 2009
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In this case (MOSFET) when the voltage at drain changes to fast (dV/dT) the strange things can happen.
For exampel the MOSFET can even be turn into conduction even if the gate voltage are below Vgs(th).
http://www.infineon.com/dgdl/mosfet.pdf?fileId=5546d462533600a4015357444e913f4f (page 11 dV/dt capability)

9. ### MrAl AAC Fanatic!

Jun 17, 2014
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Hi,

In addition to the other posts here already...

dV/dt is a measure of how fast the voltage is changing.

If you think about what you may have learned already, such as y=m*x+b, that involves variables that dont change with time. There are many instances of this in real life such as when we go to the store to buy something like apples. If we buy 1 apple for 50 cents we know that if we buy 2 apples it will cost two times 50 cents which is one dollar (USD). We are therefore dealing with variables that dont really change with time. But it was found a long time ago that if we know about how things change with time we can solve a wider class of problems (more types of problems) and so the derivative came about.

dV/dt is the time rate of change of voltage, and if we knew what the voltage wave was we could find the time rate of change by taking the first derivative. For example if V(t)=5*t^2 then taknig the first derivative we would get dV/dt=10*t.

These kinds of concepts help when dealing with electrical components that are best described by the derivatives with time.
For two common examples:
For the capacitor we have dV/dt=i/C.
For the inductor we have dI/dt=v/L.

These relationships help us solve problems that would be more difficult to solve without having derivatives.

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10. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Such a perfect example tossed away....

A straight line has the formula y = m*x + b. Here the slope m is the rate of change.

It just answers the question "how much does y change for some change in x"?

Calculus extends the concept of rate of change to any equation, but a straight line best explains the basic concept.

If you were to try to compute the rate of change of an equation at some given point you would wind up rediscovering how to find the derivative of an equation.

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11. ### mishra87 Thread Starter Member

Jan 17, 2016
499
3
Thank you all for your valuable time and comments .

12. ### MrAl AAC Fanatic!

Jun 17, 2014
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Hi,

Yes you can take the derivative of y(x) with respect to x there, but i wanted to explore time derivatives not spacial derivatives. If we try to talk about too much at one time it just starts to get confusing. It was actually better to do this in more than one post.
Also, the equation for y in the previous posts has no time variables they are all dependent on space alone.
If we made x depend on time then there would be variables that depend on time, but i did not want to get too complex there.
Of course we can always do y=m*t+b but you see how the possibilities start to add up as we can also do y=m*x(t)+b, etc.
We could also do y=dy/dx*x+b which is what you were referring too, but you see how much more complicated that appears to be already.

13. ### mishra87 Thread Starter Member

Jan 17, 2016
499
3
Actually my intention was to understand these two terminology in terms of electrical ckt.

Let's say if some ele. Component have a very High/Low dV/dT rating so what one can understand. What is impact on real application.

Anyway thank you all for your guidance .

Last edited: Jan 22, 2017
14. ### ErnieM AAC Fanatic!

Apr 24, 2011
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In that respect the term dV/dT is a shorthand way of saying rate of change. A high dV/dT means a high rate of change.

(Aside: in equations like y = m*x + b the "x" and "y" have whatever meaning you want to give them. They may have units of position, time, volts, amps, corn flakes, whatever. The equation itself is a pure abstraction that may be used to describe other phenomena.)