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Old 9th August 2012, 02:52 PM   #461
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Hi Terry,

I read the app-note and can appreciate your suggestion attempting to minimize the phase difference between voltage and current hence PF = 1 which can only be considered during the charge cycle when the diodes actually turn on.

Then again in my opinion that load would seem very capacitive or near inductive depending on the choice of the reservoir capacitor size, series resistance and series inductance as well as the complex signal being applied to the load.

We are back at the start - what capacitor? Or are you suggesting that this methodology could make the choice of capacitor size virtually irrelevant? Now that may be interesting.
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Last edited by Nico Ras; 9th August 2012 at 02:59 PM.
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Old 9th August 2012, 03:00 PM   #462
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Nico,

My last few posts have really just been a distraction. as you rightly point out the objective is to formulate some rules re. cap size, and I've been playing silly buggers with the transformer leakage inductance, which is quite unhelpful and probably ought to be ignored.

a summary is that the transformer coupling is jolly important, and should be maximised (IOW the leakage inductance should be minimised).

Andrew (and Nico) #457 is talking about putting a series-resonant L-C circuit in between the transformer and the rectifier. If this is tuned to resonate at the AC line frequency it will draw sinusoidal current (its a bandpass filter) and turns the xfmr leakage inductance (in this case referred to the secondary) from an inconvenient parasitic (that can be minimised but never eliminated) into a useful component.

and the rough numbers I calculated dont look that unreasonable. its very hard work designing a transformer for a specific leakage, but it can be done - eg an EE core with a split bobbin will have a well controlled leakage inductance. Its not really a good idea though, I was just waiting for a sweep to finish.

Andrew if you want I can email you a copy of the paper.

A unity power factor boost rectifier is an entirely different beast. take a standard boost converter (series L, FET to 0V, diode to Vout) and place it between the rectifier and the filter cap. then stick a multiplier in the SMPS controller, between the error amplifier and the current comparator. feed the multiplier from the full-wave rectified (but not smoothed) input. that forces the boost FET current to have a |sin(wt)| envelope, so the xfmr winding currents are sinusoidal.

http://www.st.com/internet/com/TECHN...CD00004002.pdf

is for one operating off-line, but there is no reason why one cant run one from the secondary of a transformer (its a lot safer). HTH
This all is nonsence, Just use a normal transformer and use a Cap as bigh as you can get, you can place and you can effort.
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Old 9th August 2012, 04:32 PM   #463
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Hi Nico,
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Originally Posted by Nico Ras View Post
...hence PF = 1 which can only be considered during the charge cycle when the diodes actually turn on.
with an ordinary rectifier-capacitor this is correct. But here is where a PFC boost converter is different. as long as the controller as running and the secondary voltage is higher than the rectifier drop (2*Vd), the boost converter is transferring power. When the rectified secondary voltage |Vs_pk*sin(wt| is small the boost converter draws a small current....and when |Vsec| is large it draws a large current such that the envelope of the boost FET current is (nearly) sinusoidal.

the problem with a rectifier-capacitor filter is that because the conduction angle is small (Andrew is quite right, its usually about 10%) the peak current is much higher than you would expect (IOW the VA rating is > the real power rating), so the voltage drop across the transformer leakage inductance is likewise larger.

making the rectifier capacitance larger reduces the 100Hz ripple, thereby decreasing the conduction angle, which raises the peak current even higher - if its say 10%, then doubling the capacitor will roughly halve the conduction time (a sine is pretty flat near the peak), and therefore double the peak input current. This in turn will roughly double the voltage dropped across the transformer leakage inductance, making the LF droop worse. its not nice to the diodes either.

for any particular transformer there is probably an optimum value of rectifier capacitance that minimises both the 100Hz ripple and the LF droop
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Old 9th August 2012, 05:37 PM   #464
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Thanks for the explanation Terry.
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Old 9th August 2012, 07:43 PM   #465
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Would you concur that 100 Farad would be a reasonable value reservoir cap for a 1 watt class AB amp. We are trying to establish when is bigger big enough. Even the argument of the battery has been touched early in the thread, according to you should we consider the battery CCC or just capacity. Does impedance play a role....?

I am not convinced that your M.Sc is standing you in good stead regarding the challenges being discussed and solved in this thread.
Are you serious? 100Farad? For a 1 Watt amp everthink higher then 1000uF is overkill. Yes impedance is important, with high C the low impedance for low frequencies is guaranteed.
For high frequencies put paralel a 100nF low inductance C.
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Old 9th August 2012, 08:28 PM   #466
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Its up to you to take me serious or not. As a matter of fact most Swiss citizen doesnt take Dutch serious met hun grote mond.
At least we have a "grote mond" that makes sounds that other Europeans can understand
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Old 9th August 2012, 09:02 PM   #467
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to be clear we mean lot of sound without any meaning.
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Old 9th August 2012, 09:25 PM   #468
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Yep "SchwyzerdŁŁtsch". I am beginning to doubt that you are teaching at a university.
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Old 9th August 2012, 09:31 PM   #469
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thats what I mean, lot of noise. Look at your fellows when they walk around here. Without shoes but sandalets, without proper pants but short ones, making lot of noise and spending nothink and always complaining.
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Old 9th August 2012, 09:33 PM   #470
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We're not to keen on uniforms but there is a reason for that.
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