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What percentage of flyback smps losses are due to leakage inductance?

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Flyback

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In an isolated , offline smps (flyback) of 20W approx, what percentage of the circuit losses are due to the leakage inductance in the flyback transformer?
 
Depends upon the leakage inductance value. The power wasted is (1/2 LI^2 * f )where I is the peak current, L is the leakage inductance and f is the switching frequency.
 
I can't really read your reference since it is a for-fee download. I would think the losses would depend upon the particular transformer you are using. Some designs (such as toroidal) generally have lower leakage inductance than others.
 
Seems like you have the answers.

There are some fairly inventive 'snubber' circuits that dampen switching parasitic resonances by storing the energy that would normally be absorbed and dumped by a zobel network and dumping it into c out on the next cycle, wasting very little.

The coupling between the pri and sec affects leakage inductance, some chokes have a copper foil around them, not in the magnetic circuit, over the whole item, this serves to reduce emmisions and also they say it improves coupling (which reduces leakage inductance), I've never tried this technique though.
 
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Well I am 80% sure that the flybacks biggest loss mechanism is the leakage...

Flyback problems at high power:
For a single transistor flyback at high power, the main feature standing in its way is the leakage inductance in the transformer, and the losses that it brings about… Each switching cycle, the leakage inductor stored energy (0.5*L*Ipk^2) is dissipated in the clamp resistor.(as you mentioned)
However, when the FET turns off, the leakage energy forces current through the primary and into the clamp, and unfortunately a significant proportion of the energy stored in the core’s mutual inductance also gets dissipated in the clamp.
(..this isn’t so bad with the Two-Switch-Flyback because the energy is returned to the input capacitor).
This latter peril isn’t suffered by eg Bridge or Forward converters, because they transfer power to the output during the FET-ON interval.
One other single-switch-flyback-high-power-problem is again instigated by the transformer leakage inductance……(its FET switching loss)…and relates to the FET drain voltage which transistions higher than V(in) due to it rising to the primary clamp voltage…..as you know the leakage inductor has to discharge somewhere and it does so by charging up the drain node stray capacitance up to higher-than-V(in) voltage and then flowing into the clamp capacitor/resistor.
…All this results in higher-than-normal FET switching losses.
So doing a Flyback at higher power would be helped by reducing the leakage inductance….eg multiple sandwich winding, & always using full-layers of turns, & preferably avoiding short bobbin spindle types.
 
Agreed.
The design of the inductor is critical.
The air gap everyone likes also seems to be an enemy it does improve the capabilities of the choke, but it comes with its own baggage.
The effects you describe catch students out, dissispation can be a lot higher than expected, and as you say a zobel just shifts the dissipation away from the fet onto the zobel resistor.
These are the reasons why a flyback psu tends to be less than 100w, unless the designer is either quite clued up, or has a 100k lab at their disposal.
 
thanks though quoting from the ti article
Solder a good quality COG or film type capacitor from the MOSFET SW node to GND
I thought fet snubber caps should go across the fet, and not to gnd?
 
thanks though quoting from the ti article

I thought fet snubber caps should go across the fet, and not to gnd?
Doesn't the FET source in a flyback normally go to ground?
 
The article is talking about a temporary cap, not the permanent zobel, its just to work out the leakage inductamce by halfing the ring freq.
On a current mode supply you would connect across the fet as you said.
The fet source goes through a resistor to ground on a current mode supply, or sometimes on a voltage mode too that has overcurrent protection.
You knew that zapper.
 
The example circuit in the TI article is of a buck converter, not a flyback. So I highly doubt that the placement of the snubber cap would be the same.
 
Strictly speaking thats correct, assuming the impedance of the + rail is low then you can still adapt the rule to work out a snubber for a flyback.
One thing I've seen and havent worked out yet is on a push pull switcher, the cap and R on some designs are reversed on each side of the primary, the cap to ground one side, and the R on the other, presumably to reduce parasitics from the snubber components themselves.
 
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