Help identifying switchmode topology a transformer is for

Thread Starter


Joined Dec 6, 2017

Around 1995 I bought many surplus boxes of flyback transformers that were spare parts for some kind of Japanese printer power supply. ‘Many’ is relative, but I figured they were good for, um, something.

I don’t have a document to upload (yet), but over the years I have had opportunities to analyze (destructively and not!) some and think there is enough info for a magnetics-wise person to recognize what kind of purpose they served or could be applied to.

We identified ferrite core material, likely freq. of use., core and gap dimensions and I think AL #. I have wire diameters and at one point I thought I had counted winding turns but can’t find that.

I understand without a lot of this info it would be a stupid question or blind experiment to just try in a circuit.

I will type or convert the data from spreadsheet into a PDF file.

They look like a simple transformer with a primary and a tapped secondary (or I have it backwards) I presume is a feedback winding. It is not a center-tap so I think that rules out Royer or other push-pull symmetrical applications.

Basically, the untapped winding is about 680 mH, and the larger portion if the tapped winding is about 1.6-1.8 mH (different samples and test voltages), and the ‘feedback’ winding was 4.5-6 uH. They were measured at 25 kHz, 1 V on some kind of Wayne Kerr inductance analyzer. A turns ratio of 20:1 (or 1:20) agrees with a 400:1 (1:400) inductance/impedance ratio if you look at 680 mH:1.7 mH.

The small inductance part of the tapped winding is roughly 5-6% turns ratio of the full winding it is part of. The ferrite core set was squarish, about 30x30 mm and 19 mm tall. The core is gapped with a round cross section and 1.65 mm gap shared equally between the two halves. They appear to be TDK H5A with 25 kHz being typical frequency of use in the early 1990’s.

I have a lot more info I don’t have memorized, like the remaining core dimensions.

What I think is a safe question at this point is identifying or best guessing whether the tapped winding suggests a step-down flyback with a feedback winding for a PWM regulator, or some weird feedback tap on a primary. I have not found this kind of tap in my book & inline searching. I din’t have any perspective for whether the large or small inductance suggest which winding is the primary.

Thank you.