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Buck Regulator

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im stuck on question a iii).
Somebody guide me.

i know Vout=Vin (ton/T)

f=1/T

but i dont think the answer is that simple for 7 marks..............
 

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Hi there,

Given that the transistor turns on in 100ns and takes 8us to turn off but you still have to maintain the correct duty cycle for inputs of 10 to 40vdc, the turn on and turn off time will limit the maximum frequency where you can achieve the right duty cycles for full range operation.
If you can reduce the turn off time, so much the better, but for this question they dont want you to do that, so you will be working with 8us and 100ns.
For example, you can not switch at 250kHz because the fastest the transistor can switch is around 125kHz, and that's even without providing the correct duty cycle, so there is no way you can switch that fast. The question is, what really is the maximum frequency you can obtain. It's your job to figure this out.

Count yourself lucky that they did not include an efficiency constraint in this question too...I would have :)
 
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Hi there,

Given that the transistor turns on in 100ns and takes 8us to turn off but you still have to maintain the correct duty cycle for inputs of 10 to 40vdc, the turn on and turn off time will limit the maximum frequency where you can achieve the right duty cycles for full range operation.
If you can reduce the turn off time, so much the better, but for this question they dont want you to do that, so you will be working with 8us and 100ns.
For example, you can not switch at 250kHz because the fastest the transistor can switch is around 125kHz, and that's even without providing the correct duty cycle, so there is no way you can switch that fast. The question is, what really is the maximum frequency you can obtain. It's your job to figure this out.

Count yourself lucky that they did not include an efficiency constraint in this question too...I would have :)


ton/T=Vout/Vin

@Vin(min)=10V ton(max) / T = 5/10 = 0.5

@Vin(max)=40V ton(min) / T = 5/40 =0.125 (this is the smallest on time ratio)


Now if i get ton min and max I can calculate the range of T & hence the range of frequency...

Im stuck again....im not sure if the ton values i want to use are correct
i want to use ton(min)=100nS
ton(max)=????:confused:
 
Hi,

From this problem description it appears that they are specifying the turn on time, which you can assume is always 100ns.
Thus, there is no min or max, just one single value.

Note that with problems like this if you get too specific, there's never a decent solution. You have to take everything at face value and you may have to consider what the instructor expects after some lecture or another.
I would say with this problem you should go with the simplest interpretation: constant ton and toff.
 
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Ton(min) is no less than 8µs because once the transistor is turned on it takes 8µs to turn off. (Of course Ton can be zero if you never turn it on. ;) )

ton/T=Vout/Vin only if the diode is ideal.
 
Ton(min) is no less than 8µs because once the transistor is turned on it takes 8µs to turn off. (Of course Ton can be zero if you never turn it on. ;) )

ton/T=Vout/Vin only if the diode is ideal.

Hi,

Arent you confusing ton with toff? toff is 8us but ton is only 100ns. That's a total of 8.1us for both.
 
if i just get the ton(min) "problem" sorted out then I know how to solve this Question.
 
Arent you confusing ton with toff? toff is 8us but ton is only 100ns. That's a total of 8.1us for both.
I believe that Ton in this context is defined as the period of conduction, not the delay from base to collector. Similarly, Toff would be the period where the transistor isn't conducting.

So then what is called Ton? Is it the period that the controller is telling the transistor to conduct, or is it the actual period that collector current is flowing? Any hints from the instructor?
 
I think normal convention is that Ton for an off transistor is the time from the start of the transistor base turn-on pulse to the time the transistor is fully on. Similarly, Toff for an on transistor is the time from the removal of the transistor base drive to the time the transistor is fully off.
 
Hi again,

I have to agree with Carl. In the real world they usually use the 10 percent and 90 percent threshold points, but for little problems like this they usually go with the simplest interpretation.
 
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