High current, low voltage pulsed DC transformer

Question

I'm trying to understand the feasibility of a DC transformer that takes a low voltage, high current pulse and converts it to a lower voltage, higher current pulse (with a resistive load on the output).

Ex: 12V, 100A -> 4V, 300A @ ~1ms pulse duration with >1sec between pulses

Transformer ratings and calculations that I've found typically assume AC, and pulse transformers are assumed to have high frequencies, so I'm not sure where to look for reference.

A simple LTspice simulation shows the general idea:

Assuming a frequency of ~1Hz, what I've found so far suggests that there would be close to zero hysteresis losses, but eddy and ohmic losses could be a concern due to the high field strength and current.

It looks like the main problem would be saturation of the magnetic field of the transformer(?). For reference, assuming a simple solenoid with a length of 50mm, two turns of wire carrying 300A, and with a relative permeability of 200 the field strength could reach 3T.

Per Wikipedia:

"...high permeability iron alloys used in transformers reach magnetic saturation at 1.6–2.2 teslas (T),[4] whereas ferrites saturate at 0.2–0.5 T.[5] Some amorphous alloys saturate at 1.2–1.3 T.[6] Mu-metal saturates at around 0.8 T"

Does this just mean that higher currents would have to be split between cores to reduce the field strength below saturation? My current idea is to throw a 3:1 ratio of thick cables on a microwave oven transformer (or two).

Is a transformer even the best device for something like this? It seemed like the simplest solution.

Answer 1

I'm trying to understand the feasibility of a DC transformer

Not feasible; transformers require that the average voltage applied to the primary is zero volts. No wiggle room here.

Transformer ratings and calculations that I've found typically assume AC

That's because transformers need a zero average voltage at the primary and, AC fits the bill.

Ex: 12V, 100A -> 4V, 300A @ ~1ms pulse duration with >1sec between pulses

In your diagram you show the primary as being 9 μH and, over a period of 1 ms, the current into the primary will rise to: -

$$di = dt\cdot\dfrac{V}{L} = 0.001\times 12 \div 0.000009 = 1333\text{ amps}$$

That's the magnetization current and nothing to do with your load current so, even if you could find a way of making the input voltage have an average value of zero, your magnetization inductance is orders of magnitude too low.

_{Regarding your simulation, you appear to have an invisible 6 mΩ in your primary circuit that you are not disclosing.}