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Full Bridge SMPS's have different output voltages, but same NP/NS value?

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Flyback

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Hello,
The following two LTspice simulations are almost exactly the same (both full bridge SMPS’s)…They have the same NP/NS value, but slightly different primary and secondary inductance values.
Everything else is the same for these Full Bridge SMPS’s.
So why does the one with lower primary inductance have a higher output voltage? (316V versus 301V)
Obviously its due to the extra magnetising inductance which keeps current flow into the secondary after the FETs have turned OFF, but why isn’t this documented anywhere?
By the way, the actual turns ratio, NS/NP = 1.288, and the “official” Vout value is Vout = NS/NP*D*VIN = 294V.
 

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ronsimpson

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You are right on the edge of continues/discontinues. You should not be operating there. The dutycycle on the L3 is different. Make L3 much higher in inductance and see if that helps. (2x or 3x)
 

Flyback

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Thanks, but sorry , surely neither of the full bridge simulations is ("genuinely") on the border of discontinuous operation (but please see below), if you look in the L3 current, it is continuous in both cases....*however*, your point is extremely valid, because even though the output inductor is well inside continuous operation, in the primary, it does indeed appear to be bordering on the discontinuous......As you know, this is due to the magnetising current in the primary being negative for a little bit of each switching cycle, which brings down the primary current and makes it look discontinuous.
 
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ronsimpson

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I looked at the current waveform on the voltage side of the graph. You are right about the current.
Look at the two red circles. The voltage and current have something happening at this time. The output duty cycle looks different in the two graphs. 316/301 is not much difference. The time the diodes are pushing current is probably different.
upload_2015-6-3_6-3-2.png
 

Flyback

Well-Known Member
Thanks, also, the full bridge with 316v output has a lower primary inductance, and since the coupling value in the transformer is 0.995 for both full bridges, the one with higher primary inductance has a higher leakage term, which explains partly why it has a lower output voltage, (because as you know, the leakage delays power transfer to the secondary), but whats a little interesting is that both full bridges have a vout which is greater than the standard formula gives.... Vout = (NS/SP)*D*VIN
 
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