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Sunday, January 28, 2024

Transformer snubber design using Quasimodo test-jig

This is a little post about the transformer snubber design tool using the Quasimodo test-jig, designed by Mark Johnson.

The link to the overall post is here and the pcb's can be ordered here.

I've had this jig for almost 10 years and used it every time I need to add a transformer to my designs.

The test-jig helps you to create an optimum snubber configuration for transformer windings having critical damping, without requiring any calculations and without measuring the transformer's inductance or capacitance.

From the website: 
A power transformer snubber is a wonderful thing for reducing or eliminating RFI from rectifier-induced LCR ringing. Unfortunately it's a huge pain to design and optimize a snubber. First you have to measure the transformer's leakage inductance and secondary capacitance, at about 100 kHz, which is not especially easy. Then you have to estimate the capacitance of your rectifier(s), which does not always appear in datasheets. Finally you plug these numbers into a formula that spits out snubber values -- and then you hope it's all correct.

Here is the schematic of the Quasimodo test-jig:

The name Quasimodo is used because it was the bell ringer of the Notre Dame. I like that name, it rings a bell! In essence the test-jig creates a 555 generated frequency and a fast MOSFET to create a pulse with a very sharp edge, ringing the transformer winding. The snubber is used to limit the ringing.

Here is how you need to connect the transformer winding connections/shorts for various transformers. Make sure you short all windings except the one you want to measure:

Some real results on a Triad PP28-180 transformer with two independent primary and two independent secondary windings. See the first picture of the post for the setup.

First, banging one of the secondary windings without a snubber:

With a 10nF and 1K resistor snubber:

After tuning the snubber to 680 Ohm with 10nF:

No more ringing, the bell is silent now. 

An additional 150nF across the winding results in more damping:

Here is the result in my application (AC/DC Load):

Highly recommended!

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