# High current, low voltage pulsed DC transformer

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), whereas ferrites saturate at 0.2–0.5 T. Some amorphous alloys saturate at 1.2–1.3 T. 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.

• In a transformer, primary and secondary currents largely cancel out. A small difference remains: the magnetizing current, which only depends on applied voltage. You'll have a harder time with leakage inductance, which may require more interleaving. Sep 20 at 6:43
• After 1 ms, with a 12 V pulse, your transformer core flux will have risen to 12 mVs. The core has to have enough area to handle this. You'll have to reset it during the next second, which will be easy with one second of 12 mV. You could use half the core area if you preset it to -6 mVs, then your pulse uses the -6 to +6 change for its 12 mVs. Sep 20 at 8:07

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.

• Aha a downvote. Maybe the person can explain the reason? Sep 20 at 20:07
• Thanks for the feedback. Looks like it's less simple than I was hoping. I could provide alternating polarity pulses via an H-bridge for the average zero voltage(?). Could you expand on the magnetization inductance requirement, please? Sep 21 at 16:57
• In what direction do you require expansion and by how much? Please also tell me what you know about magnetization inductance in transformers so I don't cover ground you already know @Mandias Sep 21 at 17:14
• I don't know much. From what I've read "magnetization current is typically less than 5% of the full load current of the transformer", which in this case would be 5A, so ~2.4mH is required for the primary inductance? Sep 21 at 17:37
• Well, it has to be seriously bigger than 9 uH that's for sure but, it truly depends on avoiding too much flux density in the core and this is a tricky calculation that for beginners is not straightforward. I've answered questions on it so try looking in that direction. Sep 21 at 18:01