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I heard someone say the QR (quasi-resonant) flyback has a smaller primary side current than an ACF (active-clamp flyback) under the same circumstances. Is this correct?

I tried to search for comparable tables between QR flyback and ACF, but I didn't find any useful information about this.

ocrdu
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Magnetic778
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2 Answers2

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You usually need a slightly stronger magnetizing current with an active-clamp flyback (ACF) than in a classical QR converter. This is because of the need to swing negative in the case of the ACF so that enough energy is stored in the leakage inductance and in the magnetizing inductance before the high-side switch turns off.

The simplest way to check this point is to run two simulations of an ACF and a QR converter tweaked at the same operating point (19 V) from a 120-V dc source et delivering roughly 60 W (same turns ratios):

enter image description here

These circuits are freely downloadable from my webpage and are part of the 80+ ready-made simulation templates you can use with the free demo version of SIMPLIS. Once the periodic operating point (POP) is found (a few fractions of seconds) then you can measure the rms currents and see that the ACF one is slightly higher than in the QR case:

enter image description here

Verbal Kint
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  • Thanks for your answer. Although the ACF has a higher current than QR. but ACF can achieve the fully ZVS but QR not. does this mean ACF's efficiency is usually better than QR? In what kind of situation we will choose QR flyback or ACF flyback? – Magnetic778 Sep 27 '22 at 22:22
  • https://electronics.stackexchange.com/questions/636201/why-doesnt-the-divider-ratio-appear-in-the-closed-loop#comment1684933_636201 – G36 Sep 28 '22 at 03:30
  • @Magnetic778, ACF can indeed achieve full ZVS in certain conditions and it recycles the energy stored in the leakage inductance. The QR converter can meet ZVS but at the cost of a massive output voltage reflection which is paid by a higher \$BV_{DSS}\$ for the MOSFET. Cost-wise, a QR is less expensive than an ACF and easier to control but I believe QR will show a slightly lower efficiency than ACF. So if a fraction of percent of efficiency matters in your project, then ACF is probably the way to go. – Verbal Kint Sep 28 '22 at 06:42
  • Under the same condition, the turn ratio is the same why QR converter need to pay for a higher BVDss? recycles the energy mean the efficiency will be higher? – Magnetic778 Sep 28 '22 at 11:43
  • In a QR converter, the depth of the drain-source valley at turn off is proportional to the reflected voltage so if you have a 320-V dc input voltage and want true ZVS, the transformer turns ratio needs to be selected so that you reflect 320 V. These reflected 320 volts add up to the input voltage at turn off and, together with the clamp value which is usually 30% above the reflected value, it starts to build up quite a breakdown for the MOSFET. Yes, some energy goes back to the source in an ACF. – Verbal Kint Sep 28 '22 at 12:32
  • @Magnetic778, please have a look at my [APEC 2011 seminar](https://cbasso.pagesperso-orange.fr/Downloads/PPTs/Chris%20Basso%20APEC%20seminar%202011.pdf) *The Dark Side of Flyback Converters* which discusses many of these elements. – Verbal Kint Sep 28 '22 at 12:34
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Is this correct?

Active clamp flyback converters have higher efficiency (or even the highest amongst all the flyback configs) because the dissipative snubber-clamper of a QR flyback is replaced with a "recycling" unit.

So, due to the fact that the ACF has higher efficiency the primary side currents cannot seem to be higher. For some reason I thought "input" current rather than primary current.

Although ACF seems to be the most not sure but quite possibly efficient variant of the flybacks, the clamp capacitor brings a slight issue: During de-magnetising of the core the clamp current flows through the primary winding and this effectively increases the primary current, but "during demagnetisation" only.

If this is meant with "higher primary currents" then yes, it's true.

Rohat Kılıç
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  • Counterpoint: ACF connects additional capacitance to the winding, thus increasing current flow. This is reactive current, so it is conservative, but also requires high-quality reactive components to maintain efficiency. Alas I don't have enough experience with both to give a convincing answer. – Tim Williams Sep 27 '22 at 17:34
  • @TimWilliams I missed a point here. I'm editing my answer. – Rohat Kılıç Sep 27 '22 at 18:19
  • Hi Rohat, Thank you for your answer. How do you make sure the ACF's efficiency is better than QR flyback? – Magnetic778 Sep 27 '22 at 22:23
  • @Magnetic778 first paragraph. Also ZVS (Zero voltage switching) can be achieved to increase the efficiency further. – Rohat Kılıç Sep 28 '22 at 02:39