Nomogram designs discontinuous fly-back converters

Power supply company Pulsiv has developed a graphical technique – a nomogram – to determine critical transformer parameters for fly-back converters operating in discontinuous conduction mode.

“This intuitive tool can replace more complex equation-based methods, providing a fast and accurate visual process for identifying primary side inductance and minimum duty cycle,” according to the company.

How to use it:

Find the switching frequency and maximum duty cycle of the chosen fly-back controller – some controllers change switching frequency based on the fly-back operating voltage, take this into account.

Select the required peak current based on the output power, and the minimum fly-back operating voltage.

Draw straight lines across the nomogram to identify suitable values for converter primary side inductance, and minimum duty cycle based on the maximum operating voltage.

Check the fly-back controller datasheet to ensure that it can meet the minimum duty cycle requirement just found.

Why has Pulsiv published a design chart?

Pulsiv has a patented front-end power factor correction method that can be paired with a DCM fly-back converter in isolated low and medium power ac-dc power supplies, as an alternative to a boost-style PFC stage – for USB-C devices, LED drivers and battery chargers, for example.

The novel front-end (whose working can be found in this article), uses active valley-filling controlled by a microprocessor, and for it the company claims:

97.5% front-end efficiency (99.5% peak, 90% at 2W)
Flat efficiency profile across load range
No need for input voltage derating
No in-rush current
Single-switch fly-back operation up to 180W

As peak operating voltage is reduced by moving from boost to valley-fill PFC, it also claims: reduced primary side inductance, lower leakage inductance and higher total winding area sometimes allowing a smaller core.

Click and copy the nomogram from the image above to get a high-resolution version, or find it here alongside a video on how to use it

A note on nomograms by Pulsiv:

The field of nomography was invented in 1884 by French engineer and mathematician, Philbert Maurice d’Ocagne to provide engineers with a precise and practical method for calculating complicated formulas using a parallel coordinate system. Results from a nomogram are obtained quickly and reliably by simply drawing one or more lines, without needing to solve algebraic equations.

Pulsiv is a University of Plymouth spin-out, now with sites in Cambridge and on the Plymouth Science Park.