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boost converter

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SeanHatch

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I have a couple of questions about boost converters. I just designed one, tested it, and it works, but not perfectly.

First, the transfer function is NOT 1/(1-D). It's relatively close, but not exact. Is this to be expected? It is about 10 - 15% off, on average. It's really ok though, since the conroller works it out anyway. As long as there's no other side effects.

Second, the transfer function seems to depend on the output impedance.

And, I'm using this boost converter to drive another boost converter. So how do I find the input impedance of the second boost converter? Doesn't it depend on the state of the internal switches?

Thanks;
Sean
 
The transfer function will be off by 10-15% because the efficiency is 90-85%.
If a boost converter produced 12 volts it actually needs to make 12.8 volts because of the loss in the diode. And on the input side; 8 volts in actually works like 7.5 volts because of the voltage lost across the current cense resistor and switch. I am only guessing at your numbers but the transfer function may work at 7.5 to 12.8 and not 8 to12. (volts)
 
SeanHatch said:
I have a couple of questions about boost converters. I just designed one, tested it, and it works, but not perfectly.

First, the transfer function is NOT 1/(1-D). It's relatively close, but not exact. Is this to be expected? It is about 10 - 15% off, on average. It's really ok though, since the conroller works it out anyway. As long as there's no other side effects.

Second, the transfer function seems to depend on the output impedance.

And, I'm using this boost converter to drive another boost converter. So how do I find the input impedance of the second boost converter? Doesn't it depend on the state of the internal switches?
Thanks;
Sean
I'm no expert on boost converters, or any kind of switcher for that matter, but I believe that the input to any switcher looks like a negative resistance (discounting reactive effects). The reason I say this is that, for a fixed load, as the input voltage to a switcher goes up, the current goes down. That's the basic definition of negative resistance. I don't know how this would affect the stability of a boost converter driving another one, but it would cause me some concern, because negative resistance is one of the components of an oscillator.
 
That can be a problem.

If the circuit supplying the converter has some sort of current limiting scheme, it could easilly oscillate.
 
Dear All,

I would like to design 12V to 24V/20A DC-DC Boost converter. If anybody is having schematics please help me. My mail id is gs.srikanth at rediffmail.com

Advance thanks
 
boost converter is just very simple and u can get it anywhere in google, the conversion ratio is depends on ur input and output.

there are actually some more complicated boost converter topology, but try to understand the basic one first :)
 
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