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Flyback converter (110-220v line power to 5v)

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Did you have a look at the Tiny Switchers of Power Integration?


Well it's true that a gap in the bobbin will increase the leakage inductance and that this is not optimal but this is only a small power supply and the negative feedback via the opto should minimise the effect so it's no big deal.


--- Quote from: Hero999 on December 14, 2010, 07:55:46 pm ---
--- Quote from: qno on December 14, 2010, 03:20:37 pm ---You may need power factor correction.
--- End quote ---
I don't think power factor correction is a requirement for small power supplies.

--- End quote ---
In the EU/EFTA area you need PFC if the power consumption is 75W or more. Though I guess this only apply if you plan to have the power supply in some kind of commercial product.

Exactly, he only needs 2.5W so even if it's a commercial product, that won't be an issue.

Another thing I've realised about having a high leakage inductance is that you'll need more suppression on the primary side as less of the stored energy will be transferred to the secondary, yes it's not efficient but it's a small SMPS powered off the mains so it's not an issue.

Leakage inductance can be particularly troublesome when operating your flyback at full load. It has a tendency of breaking your circuit (in particular, the pulldown switch). As an example, if your coupling coefficient is k=95%, then the leakage inductance Le is 10% of the magnetizing inductance Lm. Therefore, 90% of the energy gets delivered to the load, and 10% of the energy is delivered straight into the switch's parasitic capacitance. Since this capacitance is small, the voltage tends to spike up very high—above the switch's voltage rating, and causing dielectric breakdown in the silicon. Time to warm up the soldering iron, because your circuit won't work after that.

This is often handled by using an RCD snubber, which clamps the switch voltage and burns the leakage inductance's energy (it decreases efficiency, but at least your circuit doesn't break). Additionally, people often buy or make special flyback transformers which maximize Lm and minimize Le. An alternative to the snubber is to use an active clamp, which prevents the switch from breaking and then recycles the leakage inductor's energy back into the transformer—thereby keeping high efficiency even if the transformer has high leakage.

Much of the reason people are moving toward rectifying the mains immediately using high voltage diodes and then performing DC-to-DC conversion (instead of the old way of transforming it first, then rectifying it like in wall warts) is because of the frequency involved. Remember that fundamentally, transformers are coupled inductors—they rely on inductance. At low frequencies (such as 50 and 60Hz), a small-valued inductor behaves more like a resistor than an inductor. At those low frequencies, a small transformer's primary also behaves like a resistor, and very little energy is delivered to the secondary. Efficiency is low, and lots of heat is generated. Therefore, for low frequencies, you need to use a big transformer (high inductance, low DC resistance). In order to miniaturize the transformer, you need to operate at higher frequencies. DC-to-DC converters often work in tens or hundreds of kHz, allowing the use of much smaller parts (smaller transformers and filtering capacitors)... at the cost of requiring diodes and transistors that can handle high voltage.


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