# Ton and Toff

Discussion in 'Homework Help' started by haalamri, Jul 5, 2011.

1. ### haalamri Thread Starter New Member

Jun 22, 2011
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I want to know how to calculate Rising time and Falling time Toff, Trise.
I know if I want to calculate Ton I use the following equation:

Ton= Duty cycle(D)/switching Freq(F). In my case, the Ton= (0.333/50KHz)= 7us.
Is there any equations that I can use to figure out those two elements?

P.S. This is for a Buck converter simulation. I need it to control the switch using LTspice

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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One definition might be the time taking for a signal to change between two percentage values of the final steady-state value of the signal - say from 5% to 95% of the final value.

3. ### simo_x Member

Dec 23, 2010
200
6
As I know your equation is not correct..
Dc = Ton / T
where T = Ton + Toff & off course F = 1 / T

If You know your the duty cycle, it's a simple equation
Bye
Simo

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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The OP's equation looked OK to me. Its expressed in a different form to the one you have given.

It appears the OP was asking about rise time in relation to Buck converter design / simulation.

Rise & fall time is more related to device switching speed and the rise and fall time values will give rise to actual duty cycles that differ from the expected values based on the assigned primary excitation duty cycle.

5. ### simo_x Member

Dec 23, 2010
200
6
Ops.. Sorry, I got confused! Excuse me, please

6. ### haalamri Thread Starter New Member

Jun 22, 2011
12
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I'm looking at the MOSFET I'm supposed to use, the Trise=65ns, the Ton=16nS, Toff=47nS.

I'm assuming my total T=63nS, this will lead me to find the duty cycle=16/63= 0.2539

Looks OK so far, but when I calculate the switching frequency it looks HUGE! 15.87MHz??

Please correct me if I'm wrong!

P.S. Here's a link of the datasheet of my MOSFET
http://www.irf.com/product-info/datasheets/data/irf9540ns.pdf

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
You are confusing duty cycle with the potential effect of device rise and fall times on duty cycle.

Normally, the rise (fall) times are only important when they become significant compared with the switching frequency. In your case, with a switching frequency of 50KHz, the effect of the stated rise & fall times on overall duty cycle for the particular device you mention will probably be insignificant.

That said, you should pay particular attention to ensuring the gate drive circuitry you use achieves the optimum switching performance.