Boost Converter Duty Cycle and Constant Current Mode

[bayjelly]

Hello,

I'm trying to verify my intuition about how boost converters work:

Is it true that if I boost to at least twice the voltage (ignoring the diode drop for simplification), then if I want to stay in continuous current mode, I need a duty cycle of at least 50% (which would be the break even point, not really boosting)?

My reasoning: When the switch is on (i.e. the inductor having a path to ground), the inductor has the input voltage across, defining the upward slope of the current. But with the switch in the off position, the voltage across the inductor is at least as high in magnitude (but reversed) as the input voltage, meaning the current's downward slope will be at least as steep. So if the total current difference during a cycle is not to be negative, we have to spend at least as much time in the upward slope as in the downward slope (to break even).

So, for example with boost converters converting from 5V to 20V, I will always see duty cycles somewhere over 50%, no matter the load. Does that make sense?

[bayjelly]

So, for example with boost converters converting from 5V to 20V, I will always see duty cycles somewhere over 50%, no matter the load. Does that make sense?

No. I just explained before, due to the third period, you will have to have a minimum load to keep the inductor from going into discontinuous mode.

I misstated this part, actually. As per the title and earlier paragraph, I meant boost converters under the restriction to always keep them in continuous current mode, but overall my intuition seems confirmed. Thanks a lot for the thorough explanation!

Now, I experimented a little with a self-made boost converter using an inductor that is tool small for the given load and voltage ratio, and have no trouble keeping my desired output voltage with closed loop controlling, using duty cycles way below 50%. Obviously, it's operating in DCM (and that's what I ultimately wanted to confirm), but now I'm wondering under what circumstances I need to care about DCM's disadvantages? If I'm happy with the lower efficiency and don't see too much ripple, is there anything else? EMI? (This is only a hobby project.)