# How to calculate the maximum output current of a flyback design?

I've searched and researched but to no avail. I understand how to calculate the turner ratio, the output voltage, but it seems that no one cares about how to calculate the current as both this site and various other sites literally do not have ANYTHING related to this aspect of flyback design, those which touched this issue simply skimmed through it without putting any emphasis at all. So please indulge me a bit if I asked a completely illegitimate question(as in, no one should care about what the Iomax is).

I just want to calculate the maximum output current and if possible, the average one but since this is SE, I'll stick to only one question .

More specifically, I want to know how can you calculate the max output current of this particular design:

Please note that I'm not trying to get you to do my homework, so feel free to link me to pages dealing specifically with such a problem, but I doubt you'll show me something I haven't read and gave up before.

Also, the reason I want to know this is because I want to tweak both the voltage and current a bit to tune the output power to what I want, as you can see, this design supports an output of up to 50W, I want to tune it all the way up to 90W with sink.

• You haven't searched this site enough then because I've answered plenty of questions that relate to how the output power of a SMPS is determined. Putting a heatsink on something is unlikely to get you more current for a given voltage due to transformer saturation. What is "the turner ration" btw? – Andy aka Mar 15 '17 at 8:36
• Truns ratio BTW. I've searched using this pattern: "output current flyback" but nothing close showed up, I was way more surprised than you were. – Matt Cox Mar 15 '17 at 9:09
• Also I was talking about the flux density of the core, I reckon the maximum current allowed has something to do with the density as well as the wire resistance of the secondary side. – Matt Cox Mar 15 '17 at 9:21
• Please have a look at this series of articles on the maximum power a flyback converter can transmit. Part 1 is here how2power.com/pdf_view.php?url=/newsletters/1010/articles/…. You will learn that the maximum available power is related to the operating mode (DCM or CCM), the amount of leakage inductance and of course, the input voltage. Regarding the turns ratio, there are plenty of ways to consider it. Have a look at cbasso.pagesperso-orange.fr/Downloads/PPTs/…. – Verbal Kint Jul 1 '17 at 13:42

I just want to calculate the maximum output current

Max output current is wholly related to maximum output power and boost regulators (single inductor) and flyback regulators (transformer) are virtually the same analytically. The cycle of switching is split into two halves: -

• Store energy in the inductor (boost) or primary (flyback)
• Release energy into the output (load and capacitor)

For a simple discontinuous analysis (inductor or primary current allowed to deplete to zero amps), the peak current (prior to release) represents energy stored. If this is dumped into the output then there is so-much energy transferred every switching cycle.

This means that power output is energy per cycle stored (and released) multiplied by switching frequency. This ignores losses but should give you a general feel.

Energy is stored in the inductor or primary inductance and, during that storing process, nothing is transferred to the secondary due to the way the dots on the transformer are positioned. So, for a flyback design, the transformer primary attains a peak current and the energy stored is: -

$\dfrac{LI^2}{2}$

If you want more power you have to attain a higher value of primary current and, because current rises linearly (for a given input voltage rail) you have to have a longer MOSFET on duty. This is where transformer saturation problems can happen so you have to be careful about this.

More specifically, I want to know how can you calculate the max output current of this particular design

There isn't enough information available about the transformer to help you on this and, noting that you don't want someone else to do your homework, I expect you can now work through this. You have to: -

• determine the primary inductance
• determine the peak current
• look at details on the transformer core (material and shape)
• Figure out the H field (maximum)
• Estimate the B field
• See if the core isn't saturating too much