Timeline for How do I size the coupling capacitance for a SEPIC converter?
Current License: CC BY-SA 3.0
15 events
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| Jan 6, 2019 at 10:47 | comment | added | Andy aka | I’m going round in circles here. If you have a specific question about capacitor types used in a sepic then maybe raise a new question. For the few instances I’ve designed a sepic, I’ve probably used an X7R and not had problems. | |
| Jan 6, 2019 at 10:40 | comment | added | J. Joly | Sorry I made a mistake in my previous comment ... I meant: @Andyaka is it totally true that a SEPIC without decoupling capacitor is a flyback? Because for a flyback you need some space in your coil to store the power. Does a SEPIC also has an airgap for doing that? And choosing this capacitor, what kind does it need to be. With decoupling I think of ceramic, but if you use an X7R, your ESR is rather low, so you can have an unstable circuit. | |
| Jan 6, 2019 at 10:40 | comment | added | Andy aka | Ask yourself, does the sepic store energy in the primary that is converted to energy in the secondary? If the answer is yes, then some designs will require an air gap to make maximum use of energy storage whilst minimising core saturation (energy wastage). | |
| Jan 6, 2019 at 10:31 | comment | added | Andy aka | Many flyback designs use an air gap in the transformer to reduce the onset of core saturation for a given primary inductance. In fact I would say more often than not an air gap is used @J.Joly | |
| Jan 6, 2019 at 8:53 | comment | added | J. Joly | @Andyaka is it totally true that a SEPIC without decoupling capacitor is a flyback? Because for a flyback you need some space in your coil to store the power. Does a flyback also has an airgap for doing that? And choosing this capacitor, what kind does it need to be. With decoupling I think of ceramic, but if you use an X7R, your ESR is rather low, so you can have an unstable circuit. | |
| Jan 6, 2019 at 8:51 | comment | added | J. Joly | @StephenCollings The sepic can also work as a buck, depending on the duty cycle. | |
| Oct 27, 2015 at 17:46 | vote | accept | Stephen Collings | ||
| Oct 14, 2015 at 0:24 | comment | added | Russell McMahon♦ | FWIW (moot) I've seen a SEPIC 'expert' rail on about a coupled inductor implementation NOT being a SEPIC. (And another offering a coupled version :-) ). | |
| Oct 12, 2015 at 17:38 | comment | added | Andy aka | Hmmm... still thinking about that one - been a long day!! | |
| Oct 12, 2015 at 17:31 | comment | added | Stephen Collings | Does that mean an SEPIC with uncoupled inductors is nothing more than a boost converter followed by a complex voltage divider? | |
| Oct 12, 2015 at 17:11 | comment | added | Andy aka | Yeah different words but they mean the same. The smaller your cap is, the higher your input voltage needs to be to get regulation. | |
| Oct 12, 2015 at 17:10 | comment | added | Stephen Collings | Ah! I think I get it now! The capacitor, output inductor, and load form a voltage divider. Regardless of what the regulator may try to do, I can never get a voltage higher than what's created by that divider. So the cap, like you say, sets the minimum possible input voltage. | |
| Oct 12, 2015 at 14:40 | comment | added | Andy aka | If you are using uncoupled inductors then all the energy reaching the load has to pass thru the capacitor and although efficiency doesn't significantly drop with a lower value capacitor the Vin range that can sustain the output voltage does. So I would consider Xc should be one-tenth (or lower) than Rload (min). I liken that capacitor to the one that "loses" voltage in one of those transformerless power supplies that keep cropping up every now and then. | |
| Oct 12, 2015 at 14:31 | comment | added | Stephen Collings | Good answer. But what if the inductors aren't coupled? | |
| Oct 12, 2015 at 14:29 | history | answered | Andy aka | CC BY-SA 3.0 |