- #1
Jiggy-Ninja
- 309
- 1
Specifically, this switching reg as an example, though these questions may apply to others: http://www.sparkfun.com/datasheets/IC/MC34063A.pdf
Switching regulators weren't covered very well in my class, and they're significantly more complicated than linear regulators (which also makes them a bit more interesting), so I'm trying to go over the datasheet and take advantage of this forum to learn some more. Brace yourselves, for this is a long one.
First off, is there a website that has a series of pictures of an animation of some kind that shows, in detail, how a switching reg like this works? How the currents flow and voltages work to keep it operating? I'm a very visual person, and many of these questions might be able to be answered if I had a better spatial/chronological picture of how these things work. Or is there a part like this in Multisim? I haven't foundone, or I would have run some of these experiments myself.
Inductor values in the datasheet are listed as L(min). The use of (min) implies that any inductance larger than the calculated value would work. How far can this be taken? If I calculate L(min) as 100uH, would 100mH work just as well? Or, taking things even more extreme, 100H? From a theoretical and practical standpoint, what drawbacks (aside from cost/size) are there for choosing an inductor that is too large, if any?
There are separate configurations for step-up, step-down, and inverting configurations.
For the step-up and step-down configurations, I assume that the voltages and currents act "transformerish" (for lack of a more eloquent description). To simplify, let's assume 100% efficiency with a smooth action (infinite frequency with no pulsing or ripple).
For a step-up configuration, 5V input to 10V output, the regulator will draw 2A from its source for every 1A it pushes out to the load.
For step-down, 10V input to 5V output, the opposite. 1A of current is drawn for every 2A pushed to the load.
For the inverting configuration (page 8), how do the currents for that flow? Say it has 5V input with -5V output. Will 1A be drawn from the source, 1A will be drawn from the load, and 2A flow out of the ground? Bump the input up to 10V. Will it then draw 1A from the load, 0.5A from the source, with 1.5A flowing out of ground?
Staying on the inverting configuration for now, a naive interpretation of the Vout formula shown on page 10 suggests that I could use a potentiometer for R2 to make a variable output inverting switching regulator that could provide an adjustable output from -1.25V on up (or down, depending on your perspective) in the same style as the LM137 and 337 linear regulators.
On further inspection, the value of Vout affects the calculation for ton/toff, which in turn affects the calculation for Ipk(switch), which in turn affects the calculation for Rsc, and in general seems to have a more complicated relationship with the whole thing than I originally thought.
Furthermore, the CT formula implies that ton remains constant, so adjusting the output voltage in that way will affect the switching frequency, and therefore the output ripple. The ripple problem can be compensated for by having a larger output capacitor, so I'm not too worried about that.
The Rsc thing seems like it might be a problem, but I'm not entirely sure how it relates to all this. It's connected to the oscillator, but I don't really know what it's doing to the oscillator. All I know is that it's a current sensing resistor. What effects would an inaccurately chosen Rsc value (too high and too low) have?
Almost finished, I promise. Moving on to the step-up and step-down configurations, they too, at a naive first glance, look like they could be made adjustable, but a similar tangle of calculation interconnections exist that complicate the whole thing. And the most obvious problem:
What happens if, with a step-up configuration, the input voltage becomes higher than the output voltage? (Ex. 5V out, 10V in).
Opposite scenario with a step-down configuration? (Ex. 5V out, 3.3V in)
Thanks for your time.
Switching regulators weren't covered very well in my class, and they're significantly more complicated than linear regulators (which also makes them a bit more interesting), so I'm trying to go over the datasheet and take advantage of this forum to learn some more. Brace yourselves, for this is a long one.
First off, is there a website that has a series of pictures of an animation of some kind that shows, in detail, how a switching reg like this works? How the currents flow and voltages work to keep it operating? I'm a very visual person, and many of these questions might be able to be answered if I had a better spatial/chronological picture of how these things work. Or is there a part like this in Multisim? I haven't foundone, or I would have run some of these experiments myself.
Inductor values in the datasheet are listed as L(min). The use of (min) implies that any inductance larger than the calculated value would work. How far can this be taken? If I calculate L(min) as 100uH, would 100mH work just as well? Or, taking things even more extreme, 100H? From a theoretical and practical standpoint, what drawbacks (aside from cost/size) are there for choosing an inductor that is too large, if any?
There are separate configurations for step-up, step-down, and inverting configurations.
For the step-up and step-down configurations, I assume that the voltages and currents act "transformerish" (for lack of a more eloquent description). To simplify, let's assume 100% efficiency with a smooth action (infinite frequency with no pulsing or ripple).
For a step-up configuration, 5V input to 10V output, the regulator will draw 2A from its source for every 1A it pushes out to the load.
For step-down, 10V input to 5V output, the opposite. 1A of current is drawn for every 2A pushed to the load.
For the inverting configuration (page 8), how do the currents for that flow? Say it has 5V input with -5V output. Will 1A be drawn from the source, 1A will be drawn from the load, and 2A flow out of the ground? Bump the input up to 10V. Will it then draw 1A from the load, 0.5A from the source, with 1.5A flowing out of ground?
Staying on the inverting configuration for now, a naive interpretation of the Vout formula shown on page 10 suggests that I could use a potentiometer for R2 to make a variable output inverting switching regulator that could provide an adjustable output from -1.25V on up (or down, depending on your perspective) in the same style as the LM137 and 337 linear regulators.
On further inspection, the value of Vout affects the calculation for ton/toff, which in turn affects the calculation for Ipk(switch), which in turn affects the calculation for Rsc, and in general seems to have a more complicated relationship with the whole thing than I originally thought.
Furthermore, the CT formula implies that ton remains constant, so adjusting the output voltage in that way will affect the switching frequency, and therefore the output ripple. The ripple problem can be compensated for by having a larger output capacitor, so I'm not too worried about that.
The Rsc thing seems like it might be a problem, but I'm not entirely sure how it relates to all this. It's connected to the oscillator, but I don't really know what it's doing to the oscillator. All I know is that it's a current sensing resistor. What effects would an inaccurately chosen Rsc value (too high and too low) have?
Almost finished, I promise. Moving on to the step-up and step-down configurations, they too, at a naive first glance, look like they could be made adjustable, but a similar tangle of calculation interconnections exist that complicate the whole thing. And the most obvious problem:
What happens if, with a step-up configuration, the input voltage becomes higher than the output voltage? (Ex. 5V out, 10V in).
Opposite scenario with a step-down configuration? (Ex. 5V out, 3.3V in)
Thanks for your time.