Power switches/mosfets please help

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Yeah, I didn't check it but I'm sure it's probably just a square wave.

If anyone is interested...

Upon further research, it seems like the most common method is to use a push-pull topology to convert the 12VDC to 12VAC at high frequency, use a hf transformer to step-up to ~110VAC, then rectify to ~180VDC. Then this is inverted using H-bridge to 110VAC at 60Hz. Also, a LPF is used either after the rectification or the H-bridge to get a sine. So the high frequency operation is in the front-end to step up the input voltage. Later, in the H-bridge the desired output frequency (50/60Hz) is used. Otherwise, a large transformer is needed if not using a high frequency front-end.
 

Have a look at,

https://www.edaboard.com/threads/202321/#post855461

I'll reproduce it here..



This is based on the UC3827

https://focus.ti.com/lit/ds/symlink/uc3827-1.pdf
**broken link removed**

It 'looks' complex and in some respects I suppose it is. However all the control at the bottom is contained within the IC.

What you will need is a sine generator and precision active rectifier. Might just be a couple of quad op-amps. As you have noted there is the push-pull transformer with switches.

There is the penalty of the extra buck switch/mosfet on the primary but you do not need the output filter inductor.

All the sine generation is done on the primary side. As far as the bridge is concerned you do not have to 'PWM' it to get the sine wave out. You just 'chop' it at the required 50/60Hz output frequency.

Hopefully the following waveforms make sense. Do you have LTSPice?



Genome.
 
https://obrazki.elektroda.pl/96_1298468483_thumb.png
Click to expand...

This is the output of the above circuit?

The schematic is almost too sophisticated for me. I do have LT Spice. If you have the files for LT Spice can you please provide them to me so I can play with your circuit? Is that how you simulate a transformer in LT Spice?

This is based on the UC3827
Click to expand...

So all the labeled signals in the schematic are coming from a UC3827, signals which you have simulated?

Thanks!
 

It probably is 'sophisticated' but it is the best way I can think of coming up with a 'pure sine wave'. Perhaps it looks complicated but as suggested much of the control is buried in the UC3827.

Not all of the signals come from it. Just the drive and control for the primary side switches. You will still need to generate the sine/half sine and the 50/60Hz drive for the secondary side bridge.

That's how I model 'ideal' transformers' in spice. Coupled inductors with values set as inductances according to squares of turns ratios.

Here is the model.. I've just realised I have done silly things elsewhere.

Code: 
Version 4
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TEXT -192 32 Left 0 !K1 LPA LPB LS 1
TEXT -872 1016 Left 0 !.MODEL MSW SW(RON=3m ROFF=1E9 VT=5)
TEXT -872 1064 Left 0 !.MODEL DID D(RON=10m ROFF=1E9)
TEXT -872 1088 Left 0 !.tran 0 110m 10m uic
TEXT -872 992 Left 0 !.LIB OPAMP.SUB
TEXT -872 1040 Left 0 !.MODEL CMP SW(RON=10m ROFF=1E9 VT=0)

copy the text into your text editor and save it as a .asc file.

Be afraid... as FvM has suggested you get some 'wicked' currents flying about especially if you want 200W from 12V

Genome.
 
There seems to be something wrong with the control loop polarity. I also don't get the purpose of summing current and voltage process values. If a current limiting action is intended, there should be a kind of analog "OR" respectively max() operation.

Why don't you simply post a zipped *.asc file?
 
This is 'average current mode control'. There is an internal current loop that sets the primary side inductor current which gets reflected through to the secondary by the push-pull stage and transformer. Effectively it is a buck converter but the transformer provides for step up. The voltage loop sets the current demand for the current loop. There is an internal peak current limit within the UC3827 otherwise it is possible to limit the output of the voltage error amplifier to achieve a similar function which might be preferable.

The loop polarity is correct, at least for the model and I do believe it matches what the UC3827 implements, but being what it is it is 'hard' to follow things through.

Why don't you simply post a zipped *.asc file?
Click to expand...

Thanks.. just worked out how to do it.

Genome.
 

Attachments

  • sin123a.zip
    2.9 KB · Views: 101
I didn't analyze the circuit, but saw the controller output wind-up to kV and MV while the buck converter stays off when running your analysis setup. Of course, there may be other reasons for the behaviour as well, e.g. missing voltage limitation for the integrator that the real device has.

I understand know, that the current control loop is simply representing UC38xx current mode, I didn't think about it sufficiently.
 

Downloads.. Quick/Slow check.. As delivered it does what it says on the 'tin'.



You've been having a 'play', haven't you.

Genome.
 
Genome, thanks! I had a hang up but I'm back at it. Just downloaded your circuit and I'm going to take a look at it.
 

Downloads.. Quick/Slow check.. As delivered it does what it says on the 'tin'.
Click to expand...
Yes, everythink O.K. Unfortunately I have been simulating the text from the code window before you managed to provided a zip file. It has a few "µ" symbols replaced by "?".
 
Genome, your circuit is pretty awesome. I really only need a modified sine, and I understand the H-bridge. Where I am having trouble is the input stage. I need to decide how I am going to convert 12Vdc to ~155Vdc for the H-bridge. My original plan was to use a half-bridge converter. My problem is I have to design all of this by myself, including the transformer. I wanted to keep it as simple as possible, like only 4-pins on the transformer, such as a 2-switch forward converter.

What is the easiest way to get the dc bus I need for the H-bridge? I think your push-pull buck is more than I'm willing to design. I don't think I would get the transformer right.
 

The center-tapped design is optimal in terms of primary voltage drop. And push-pull results in the smallest possible transformer size. But the 2-switch forward converter can work as well. In this case, I would move the power inductor providing the buck operation to the secondary, as it's usually done in the arc welder circuits based on this topology. The duty cycle of the primary switch then sets the output voltage.
 
I'm able to get the ~155VDC by simulating a 2-switch forward converter in LT Spice, by Genome's methods. I just don't know how realistic it is to assume, with the right components, that it will work in the real world. Also, (stupid question) how can I determine the frequency of the simulated pulse voltage?
 

In the simulation schematic, all switching frequencies are defined by the period of various pulse voltages. You'll find them also in the waveform. For the real circuit, it's set by the switching controller RC oscillator.

I just don't know how realistic it is to assume, with the right components, that it will work in the real world.
Click to expand...
It's realistic in my opinion.
 
Thanks FvM!

The center-tapped design is optimal in terms of primary voltage drop. And push-pull results in the smallest possible transformer size.
Click to expand...

How significant are these differences going to be? In my case (low cost, running out of time, modest efficiency), is it going to be worthwhile to design for a topology which uses a more sophisticated transformer?
 

The transformer size increase should be moderate.

I didn't yet care for the details of your specification, and most likely don't need to know it for an answer. I think, that the 2-switch-converter can be a good compromise. Some aspects, e.g the perfect recovery of leak inductance energy are really convenient. That's why the topology is favourite for robust arc welder inverters. Current mode control as provided by Genomerics' simulation model would be preferred as well.
 
Thanks! I don't want to bore you with details nor am I asking for a schematic, calculations, etc. I want to do the work on my own and learn, but I fear that divulging too deep into the world of smps is not what I need right now. I need a quick, somewhat solid design soon, and just need experienced and knowledgeable people like yourself and Genome to provide some guidance, which I greatly appreciate. I'm sure you get some kind of satisfaction helping people out otherwise I wouldn't see you on here so often (yes I have looked at many posts on these topics).

With that said, I have been looking at controllers, any suggestions? Unlike the UC3827, it needs to be widely available and fairly cheap. I have been looking but there are a lot. I wish for minimal externals, possibly one that I can use with both the 2-switch and the H-bridge (1 part#, 2 ICs). Any suggestions would be helpful.
 

For a single output current mode controller UC3842/3 is one of the mostly used types. UC3846 is fine for a push-pull inverter, you can also use SG3525 if no current mode is intended.
 
I'm a little confused... With the UC3842/3 it has a single output. Does that mean I need two? One for each fet? Thanks!


ADDED: Can I use a push-pull PWM controller that has 2 outputs (1 for each switch) with a 2-switch forward converter? The way I simulated it (like Genome's push-pull buck), the switches are operating the same as if in push-pull, I'm pretty sure.

---------- Post added at 17:43 ---------- Previous post was at 16:01 ----------

Okay so the defined operation of a 2-switch forward converter is for the switches to be simultaneously turned on and off. When I simulate this there is a voltage spike of ~340VDC before it slowly levels off to ~155VDC. When I simulate it with pulses 180 degrees out of phase, the voltage quickly rises from 0V to ~160V, then quickly levels to ~155V.

---------- Post added at 18:00 ---------- Previous post was at 17:43 ----------

Disregard what i said about the pulses being 180 degrees out of phase... this is incorrect for a 2-switch forward converter. I was simulating incorrectly.

The correct switching method for a 2-switch forward converter is simultaneous switching.
 
Last edited:

Rather than using a two-switch forward, if you wish to go down that route, you may be better off doing a single switch forward converter..



Using either you run into possible problems in that duty cycle is limited to 50% so say you wanted 120W output your would be drawing 10A average from a 12V battery but it would be as a 20A square wave. The push-pull circuit would relax that. In this case power losses are increased but I suppose you just scale accordingly.

With the two switch topology you have ... two switches in series and therefore you will double the losses when compared to the single switch version. In the single switch version you still have to allow for transformer reset which will place 2VIN on the drain of the switch so its voltage rating needs to be higher. Since you are operating with low input voltages that need not be such an issue...

Of course since you are no longer using the upper switch you don't have the added complexity of generating a level shifted supply for it.

What you will need is a clamp winding, L4 in the above diagram, to allow for transformer reset back to the input supply. This needs to be tightly coupled to L3, the main primary and whilst it seems like the complication of an added winding it carries little to no power and you can almost get it for 'free'.

Assuming you are not forced to use foil for the primary and stick with Enamelled Copper Wire then it is likely that you will be winding the primary as a twisted rope. That is a number of strands of wire in a twisted bundle. You would pick off one of those wires and use it for your clamp winding. So for example, 7 in the bundle, 6 for the primary and one for the clamp.

You may still require a dissipative clamp or you might rely on using an avalanche rated Mosfet.

The basic circuit model would be,



Start up,



Regulation



I've sort of picked some values out of my hat on that one but the model is attached if you wish to play. Current sensing might be an issue that would be simplified with a current transformer. Otherwise to avoid the 1V limit imposed by the controller if you were to use resistive sensing you would pre-bias the ISNS pin from the reference. You may still end up with excessive losses or noise problems though.

Genome.
 

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