MC34063A Switch Regulator: Questions & Answers

[Jiggy-Ninja]

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 V_out 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 V_out affects the calculation for t_on/t_off, which in turn affects the calculation for I_pk(switch), which in turn affects the calculation for R_sc, and in general seems to have a more complicated relationship with the whole thing than I originally thought.

Furthermore, the C_T formula implies that t_on 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 R_sc 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 R_sc 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.

 

[Antiphon]

This is a very big subject but you have the right outlook for investigating it.

In the datasheet, the circuit first connects the inductor across the input supply. Current begins to flow through it inceasing as a ramp in time. When the current reaches a certain value set by the sensing resistor and the chip, the transistor in the chip turns off. The current in the inductor keeps flowing though so it makes the voltage on the inductor "fly up" to the voltage on the 330uF capacitor. The inductor then releases it's current on a decreasing ramp into this capacitor and the result is a voltage-boosting power supply.

The other parts are involved in the regulation.

 

[Jiggy-Ninja]

This is a very big subject but you have the right outlook for investigating it.

In the datasheet, the circuit first connects the inductor across the input supply. Current begins to flow through it inceasing as a ramp in time. When the current reaches a certain value set by the sensing resistor and the chip, the transistor in the chip turns off. The current in the inductor keeps flowing though so it makes the voltage on the inductor "fly up" to the voltage on the 330uF capacitor. The inductor then releases it's current on a decreasing ramp into this capacitor and the result is a voltage-boosting power supply.

The other parts are involved in the regulation.

The basics for step-up and inverting seem simple enough. I just need to take a second look at step-down.

That still leaves the question (along with all the others in my post): For step-up, what happens if V_in becomes higher than V_out?

 

[Bob S]

The basics for step-up and inverting seem simple enough. I just need to take a second look at step-down.

That still leaves the question (along with all the others in my post): For step-up, what happens if V_in becomes higher than V_out?

In SEPIC (Single Ended Primary Inductance Converter) converters, the output voltage can be either higher or lower than the input voltage. See

http://www.national.com/an/AN/AN-1484.pdf

Bob S

 

[Jiggy-Ninja]

In SEPIC (Single Ended Primary Inductance Converter) converters, the output voltage can be either higher or lower than the input voltage. See

http://www.national.com/an/AN/AN-1484.pdf

Bob S

The same configuration can have an input voltage higher or lower than the output?

That still leaves a couple questions (in addition to the several others I raised in my post):

1) What class of regulator is the MC34063 (and others like it)?

2) Why does the MC34063 need two separate configurations for step-up and step-down? What happens if you set it up in the wrong configuration?

I decided to get a couple http://www.onsemi.com/pub_link/Collateral/NCP3063-D.PDF" from Digikey and conduct some breadboard experiments with them (datasheet says they are an upgrade to the one I posted, so they should work similarly). Even so, if anyone has the answers now that'd be great, so I can know more about what to look for in the experiments and interpret the results better.