Pulse Transformer Replacements
Replacing pulse transformers is a little bit trickier than it could seem. Thus, choosing a component that can be pin-to-pin replaceable takes some importance.
In previous design, a three or even four windings transformers where used. For this board, two 1:1 coupled inductors do the job. This is a little big thing in terms of ease-of-replacement: where we had to be dealing with particular ratios and several windings is now a simple 1:1 ratio conversion (apart from the choice of the correct primary inductance and taking care of the saturation current).
Factors to take care of
Saturation Current
It is the maximum current an inductor can drive without suffering damage.
Primary Inductance
The value of the primary inductance determines, given a value of the saturation current, the maximum time a flyback configuration can be pulling a high value in the output without suffering damage. Primary inductance is directly proportional to the maximum pulse width.
It can be thought that the best option is choosing a model of coupled inductors with high primary inductance. However, primary inductance is directly proportional with the series inductance that will slow down the transitions to/from high to low.
In our base design a value of 100uH was choosen as the reference, which yields a maximum pulse width around 2us (enough for the Blocking needs).
Coupling Ratio
For the case of this design the coupling ratio is 1:1, yielding an increased ease-of-replacement.
Vus product
This value is comparable to the ones above described. The Vus product is value commonly used in pulse transformers. It represents the product of the outputted voltage with respect of the length of the pulse generated.
Pin-to-pin compatible models
• Pulse Electronics PF0552 and PF0553 series
• Coilcraft MSD1278 series
• Coiltronics DR74 and DR125 series
Considerations
In the case that the inductors with 100uH would be out-of-stock, it is possible to select others inductors of different primary inductance. In that case, it will be necessary to take into account the saturation current the coupled inductors can withstand and, then, check against the maximum pulse width for the desired application.