A desk review of the MiniPa100 kit – #1: characterise the output transformer

This article is one in a series of a desk review, a pre-purchase study if you like, of the MiniPa100 kit widely sold on eBay and elsewhere online.

One of the first questions to mind is whether it is likely to deliver the rated power, so let's review the MOSFET output circuit design from that perspective.

Sellers mostly seem to need to obscure the MOSFET type in their pics, so essentially you buy this with no assurance as to what is supplied, no comeback if the supplied MOSFET is not up to the task. Online experts suggest the MOSFET is probably a MRF9120 (or 2x IRF640 in a 70W build). The amplifier claims 100W from 12-16V DC supply.

Note that this module does not include the necessary output filter which will lose 5-10% of the power from this module.

In this case Carlos, VK1EA, connected a sample output transformer (T2) core from a recently purchased MiniPa100 kit to a EU1KY antenna analyser. The fixture is critically important, it is at my specification.

We also need to know the geometry parameter ΣA/l.

Above, from the measured dimensions of a sample core, ΣA/l=0.003415/m.

The saved S parameter file was processed as described at nanoVNA-H – measure ferrite core permeability described a method for characterising an unknown ferrite material and a complex permeability curve produced.

Above, the results are fairly good and fairly much as expected, but let's remove the noise by digitising the plots.

Above, the points sampled for the digitised output. Though there is a lot less data in the result, when points are obtained by interpolation, noise is greatly reduced.

The above pic from an eBay advertisement of the 2020 version of the PA would suggest very strongly that there are three turns on the secondary of the output transformer, and a half turn on each drain. Interestingly the 70W versions also appear to use three turns, alarm bells ring!

From all this, we can produce an approximation in Simsmith that captures most of the expected behavior of a practical transformer, including core loss.

Above is the RUSE block schematic used for Core which models the frequency dependent magnetising admittance of the transformer and sets the frequency dependent inductances of the Tfmr element.

(The model assumes that k is independent of frequency which is not strictly correct, but for medium to high µ cores, measurement suggests it is a fairly good assumption.)

More to come…