DC-DC Converter Design

I'm in the middle of the design phase for a high power DC-DC converter. The input source is 24vdc at about 115A which is supplied by heavy duty batteries. The output is at 180vdc at 10A peak. Peak power is 1800W. The minimum voltage input to the transformer is 16vdc (it will be wound as a 1:11.25 transformer). The problem that I am facing is the power conversion itself. I've reviewed several designs and in the transformer circuit, they have nothing, diodes, or capacitors. Now I understand why a diode would be needed (to dissipate the inductive kickback off the transformer primary), but I am not sure why a capacitor would be used unless it is to form a parallel LC tank circuit. If that is in fact the case, then wouldn't the inductance of the transformer vary based on load current, which would alter the resonant frequency of the circuit? And wouldn't that also have an effect on the skin effect in the transformer windings? I've attached a basic and incomplete schematic where I added both the diodes and the capacitors. Values are currently unspecified.

Also, voltage sensing on the secondary. I would like to maintain galvanic isolation between the primary and secondary due to the high voltages involved. I've though about placing a VF converter on the high side and sending it through an optical isolator to a FV converter on the low side. I have also considered just winding a sense winding on the transformer as well...but I don't think that will be very representative of the actual output voltage under dynamic load conditions.

What do you guys think?

Normally, the transformer is connected in the drain leads of the fets. As you have it, the diode will short out the power when the opposite fet turns on.
Also, to turn on the fet you will need a voltage way above the source, where will you get this from and how will you ensure that the fet is not damaged by excess volts?

There will be some energy stored in the transformer but this will be dumped into the load in normal operation. Under light load, the fet which is OFF passes the current back to the power supply using its internal diode.

The transformer does not run at resonance. The capacitors may be added to reduce the generation of high frequency noise. Skin effect should not be a problem unless running at high frequencies and then you will need special fet drivers and high frequency rectifiers.

If dc to dc inverter running at higher frequencies. You use multiple strand wire instead of single solid conductor for transformer winding to reduce skin effect.

Did you inherit this project and schematic from someone else?
That schematic (odd variation of a push-pull) will not function as drawn.
For example, when Q2 is on the transformer voltage is limited by the voltage drop of D2 reflected through the turns. Likewise for D1 when Q3 is on. As drawn, huge currents will flow through Q2-D2 or Q3-D1. Something isn't right.

The basic topology must be understood first way before getting into advanced analysis details such as skin effect.

This is my own new design. The schematic was something that I put together at 12:30 AM to illustrate my questions, with diodes and caps. I modified it to the basic push-pull design so it should pass muster with the experts here. I have no clue as to what I was thinking when I drew it up. I think it was due to a combination of lack of sleep and the learning curve associated with the use of a new schematic capture program. I have a basic design written down on paper with some of the calculations.

Anyways, the attached file should be better organized. And thanks for the tip on running multiple wires in parallel to avoid skin effect. I was thinking of just using a flat copper sheet wound around the core to mitigate that. I will be winding the transformers myself. Considering the power involved, I plan on using multiple channels for the actual power converter and placing 0.3 ohm 10 watt power resistors in the secondary to balance the load currents.

Now it's looking like a more realistic schematic. The dot for the second winding must be at pin 3, not pin 4. The use of a copper sheet for the primary is not uncommon at 115A IF you can parallel the secondary windings across each primary winding.

just wanna suggest you put zener diodes across the mosfets to absorb the inductive kickback of the transformer...........it should preserve the life of the mosfets

I just glanced at your first post again. 11.25:1 turns ratio will not be sufficient at 16Vin/180Vout. Other factors are involved in order to derive turns ratio. Among these are: maximum duty cycle, output diode drop, switching times and delays, MOSFET voltage drop, xfmr primary voltage drop, input filter voltage drop, output filter voltage drop, just to name a few.

Design of a 1.8kW SMPS is not a trivial task. I just witnessed an engineer who was semi-experienced in power supply design trying to get a 2kW current source running but all the company got after 4 months was a breadboard with ringing, voltage spikes, and a poor loop response. If you are doing this for a hobby/learning project then it sounds like good fun. I've designed DC several converters in this power range. I'll check this forum occasionally to see how you're doing. If you are tasked with this at work however, a good power supply specialist could help get you started.

It's a hobby thing, for now.

Hmm...zener diode snubbers... Instead of using a zener diode to protect the mosfets, why not just use a schottky blocking diode between the mosfet and the transformer winding? Place a capacitor in there to store the inductive kick and then cycle it back through the transformer when the other mosfet conducts. Not sure how well that will work given the possibility of core saturation since both primaries will be in phase at that point. My reasoning for this is that the power source is a battery, so I do not want to waste the energy if I can help it. I think at the very least it sure as heck would remove any DC biasing in the primary windings and negate the possibility of hysterisis causing a problem, and would lighten the transitional load on the opposite mosfet because the core is already changing magnetic polarity when it starts to conduct. How feasable would that be to impliment. I'll post a revised schematic when I'm at my computer and when I have had time to think about it.

I have wound some E type transformers in the past, but nothing like a SMPS transformer. I like the idea of alternating the paralleled primary foil windings with the secondary windings. The more that the flux cuts across the windings, the better the efficiency.

Putting a diode in series with the mosfets will drop the efficiency by at least 10% and I do not see any advantage, it will stop any stored energy being returned to the power source.
Stored energy in the transformer can be returned to the power source through the inherent diode in the fet. A Schottky diode across the fet may help a little.
The Zener is used across the fet to eliminate high voltage transient pulses. A well built transformer with low leakage inductance will minimise these pulses. Fets are reputed to be tolerant of such pulses unlike bjt transistors.