High power adjustable switching power supply (SMPS) 3-60V 40A

[Richiewillmott]

Hi Faux,


I understand what you are saying, and yes I have jumped the gun a bit in assuming the Transformer will work.


This is my first SMPS I've made even though I am competent in Electronics.


So what I can gather from your post is I should of tested the Power side by applying an adjustabe external quare wave to the IGBT gates from an Arduino or something similar to see if the transformer outputs in the way it should, correct me if I'm wrong.



The transformer that was used by the guy who posted the schematic came from an old 56V PSU High power adjustable switching power supply (SMPS) 3-60V 40A thats why I assumed it would work.

[MorbidFractal]

You can initially test the power side by just having the auxiliary supply running without the HV bus connected to the power switches themselves. This will let you look at the gate drive waveforms without the devices carrying any power and confirm that the levels and phasing, both devices are turned on at the same time, are clean and correct. Things may/will get dirtier when you apply the main power. In respect of that a variac is a handy lump of iron to add to your test bench.

The tell from the original author beyond dimensions and winding is...

"Transformation ratio of primary to secondary is about 3:2 to 4:3 and the ferrite core (EE shape) has no air gap."

No air gap. If you can see it, you might have to remove a couple of layers of tape from the outside of your transformer and possibly a screen foil, then if it is gapped there will be a visible shim where the two halves of the core mate with each other. That's not guaranteed because sometimes the gap is placed in the centre leg.

If it is there then you have to think harder about things. Assuming the core has sufficient area and number of primary turns with the gap being shimmed then you might be able to pull it apart, remove the shim and put it back together again with no gap. That will raise the magnetising inductance but leakage will probably still be high.

The hope would be, probably not, that things are much of a muchness. It's certainly the case that two switch forward, ungapped, vs flyback, gapped, designs work out to have more or less an equivalent number of primary turns and core size for a given power throughput from a given input voltage. LLC may not be so forgiving.

Either way it gets you closer to a working result.

[FauxFrench]

Hi Richie,

With our traditional use of net-transformers, we are used mainly to care about the primary voltage and the winding relation to the secondary winding(s). That core saturation is avoided the producer takes care of in his marking of the transformer and the operating frequency of the net (50Hz/60Hz) is substantially constant all over the world. Secondary parameters like leakage inductance are normally negligible and the input signal is necessarily symmetric. Your assumption about the recovered transformer is logical from such experience.

For SMPS transformers, you clean the blackboard and start with all parameters free to be decided though the basic laws of physics evidently still apply. The core has to be chosen for the intended operational frequency, the core size and core type for the operational max. power and EMI circumstances. Then you have to consider how the flux is building up in the core, how to use this energy and in particular how to reset the magnetic flux in the core such that you have no slow flux build-up that in the end will make the core saturate on top of a dynamic flux.
Second order effects like ringing of the leakage inductance and winding capacitance suddenly play an important role. You hinted good electronic knowledge in general but you violated a basic concern in SMPS design because it is your first SMPS.

In case of malfunctioning of an SMPS that has never worked before, the transformer is a frequent suspect.
I know of many good SMPS designers but compared to the number of electrical engineers, it is in general small I believe. Most engineers I know of are not comfortable with how magnetic components behave when switched at high frequency.
Evidently, the most simple Buck and boost converters most can make work.

At the time where I designed SMPS, we used manually variable generators to control the power switches during test of the power stage. No Arduino in those days. If you had to build a constant-speed regulation system for your car, you would check manually that the engine would run stably and predictably before including it in an automatic regulation loop.

My guess is that constructing a correct power transformer (T1) is the only way ahead for you now.
NB: Follow the experienced advice of MorbidFractal first. He has an impressive knowledge also on this transformer.