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Old 27th Sep 2008, 6:05 pm   #61
dominicbeesley
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Default Re: SMPS experiments

Thanks Peter,

I've just been a bit sidetracked recently, trying to sort out car engines, working and playing about with NBTV stuff....

Thanks for the sheets, they look to make sense but I'll have a proper read through tomorrow when I get a while to concentrate and have recovered from this hangover!

Does the same thing go for ferrite torroids with the air-grap / stability thing, I suppose the cores you sent through are fine ungapped for the relatively low number of turns?

If the choke is after the bridge then will the doubler still work, or should I have a seperate choke for HT and EHT? Or would it be possible to have the choke in the "cold" side of the circuit (i.e between HT- and the Rect-)?

Cheers

Dom
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Old 27th Sep 2008, 10:41 pm   #62
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Default Re: SMPS experiments

Hi Dom,

I've not thought much about the EHT side of things. As it's only a small percentage of the HT power (750V @ 1mA versus 250V @ 50mA if I recall your requirements correctly) then you can get away with a slightly naughty circuit. They key points are, Maintaining symmetry in the push-pull circuit; and Ensuring regulation works. (A circuit which peak rectifies the transformer secondary voltage will take no notice of changing the pulse width).

As for ferrite toroids, YES they will saturate very easily unless they are gapped. An air gap in the magnetic circuit dominates the performance (in my last post a couple of words were missing, I blame this keyboard...). However, many toroidal choke cores are not ferrite! This includes the ones I sent you. They are made of iron powder, or molybdenum-nickel-iron alloy powder, ground up fine, mixed with a non-magnetic binder, pressed into shape, and then medium-temperature fired. This gives a homogenous substance which behaves like a nice, linear, low-permeability material (really its a high permeability alloy with thousands of minute air gaps).

Ferrite toroids do exist, they make super transformer cores, but as the permeability is highly temperature dependent and even at room temperature has a tolerance of +/- 30%, they are rarely used where specific inductance values are required.
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Old 28th Oct 2008, 2:05 pm   #63
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Default Re: SMPS experiments

Hi Dom,

As promised here is the sketch of how to get a low power auxiliary output for not much effort.

Basically, during the 'dead' time, when neither of the primary switching transistors are conducting, the current in the output choke will be ramping down towards zero, and so the voltage across the choke is going to be equal to the output voltage (plus a diode drop or two). So, winding an extra winding on the choke, and connecting a rectifier so as to be conducting during the dead time, allows one to pick off some current, at this well-defined voltage.

It doesn't work too well if the primary switching transistors each approach the 50%-on duty factor, because then there's almost no dead time for the auxiliary diode to conduct (so peak currents in this diode would be high and losses could get significant). But then, you'd only need to be running near maximum duty cycle if the primary input voltage is low.

It does need a minimum load on the main output, so that there's enough current through the choke to steal some. Assuming that the pulse widths are constant, if you load the auxiliary output too heavily, it simply starts dropping in voltage, and the main output will rise in voltage. Increasing the load on the main output will restore the status quo.

Have fun!
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Old 30th Oct 2008, 12:37 pm   #64
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Default Re: SMPS experiments

Hello Peter,

Thanks for the information, and now as ever the inevitable question.

When I try this am I best off using a gapped transformer or would it make more sense to use one of the torroids you sent?

Also is it acceptable to connect the three outputs in series to give an extra 26V to the HT and an extra 275V to the EHT? I'm guessing there's no real problem as each higher voltage has lower current requirements but not sure there won't be some problem with the way the EHT choke is stealing current.

Also would it be acceptable to have a lower number of N2 turns and make a doubler or tripler - just to save on the number of turns so I can use thicker wire?

Not got round to making a coil winder yet, but hopefully in the next couple of weeks...

Cheers

Dom
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Old 30th Oct 2008, 2:00 pm   #65
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Default Re: SMPS experiments

Hi Dom,

For the HT output choke, yes you'll need a gapped component, I'd sugest a ferrite transformer core. You could use one of the toroid cores I sent (which for the benefit of Forum members reading this, are Moly-Permalloy Powder cores, which are low-permeability because the powder/resin mix has effectively a 'distributed' gap). However, there will be a lot of turns needed, and putting lots of turns on a toroid is a right pain...

There's absolutely no problem in connecting all your power supply outputs in series. They are all isolated from each other, so go ahead!

Finally, the question about a doubler or tripler. This will use BOTH polarities of the waveform across the choke. Now, one portion has amplitude which is dependent only on the output voltage, but the other portion has amplitude which depends on both the input and the output. So, the output of your tripler will vary with input voltage, even if you have a control loop which stabilises the main outputs. There will also be, in theory, high and unlimited brief current pulses drawn through the main switching transistors, because the main transformer will be feeding the tripler rectifier directly into its peak-rectifying capacitive load. Having said that, there are many power supplies 'out there' which crudely peak-rectify the output from a small auxiliary winding, to provide a low-power 'housekeeping' supply, and the world hasn't ended yet!
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Old 20th Dec 2013, 9:47 pm   #66
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Default SMPS Experiments - part 2

Hello all,

It's been over 5 years since I got half way through building an SMPS power supply with the help of forum members. At the time my car needed an engine swap and I ended up getting distracted, all the parts of my experiment got put in a box and the buried under all the other workshop stuff.

Well, I've dusted off the parts, and started looking at this again. This time the requirements are for a simple audio project that I'm building. I need a 10-40mA supply for the HT at around 190V and 6.3V for LT at 800-1200mA. I've adjusted the thing to run at 100kHz and decided to "cheat" this time and simulate the circuit in LTSpice.

I read, re-read and re-re-read the thread, including kalee's excellent crib sheets and a few application notes and tried to remember where I'd got to. One of the first things I spotted was that we were trying to run the TL494's output transistors as emitter followers to drive the HEXFET gates. The trouble is that the TL494's internal circuits all run at 5V and the output from the followers was only about 4.5V, barely enough to turn on the FETs. So I've bitten the bullet and added some extra drive transistors (after all they're only for pennies!). I've not added any short circuit protect to this as it seems to be a bit OTT for this project.

So here's my first go at "designing a circuit" and some simulation results. The simulation places a varying load of between 4.5k and 19k to roughly simulate 10-40mA begin drawn at ~190V from the HT (switching every 2ms) and 7.8 ohms on the LT to simulate ~800mA.

In the simulation the LT supply is pretty solid but the HT has a lot of swing on it so cross-regulation doesn't look too great! Is there any way to improve this? The period of the overshoot/ripple of the HT is affected by the size of the HT smoother. I've not found anything that affects the amplitude much though!

I do apologise to everyone who was helping last time for leaving such a long break in this!

D
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Last edited by Station X; 20th Dec 2013 at 10:04 pm. Reason: Threads merged.
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Old 21st Dec 2013, 12:45 pm   #67
kalee20
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Default Re: SMPS experiments

You can add downstream filtering for the HT, Dom.

The HT line has GOT to change in amplitude somewhat, where you tap off for voltage sensing, else the control loop won't do anything. However, re-reading your post |I'm puzzled about your reference to cross-regulation, as the bvarying load is on the HT rail, and you are viewing the same rail - so it's not cross-regulation, it's just simple transient load regulation.

I guess by the time I get on-line again, you may have clarified this - good to see the project isn't dead!
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Old 21st Dec 2013, 1:55 pm   #68
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Default Re: SMPS experiments

Hi Peter, sorry my description was a bit rubbish. The LT line is the one where the feedback is taken from as I don't want to overrun the heaters.

The application (this time) is for a VU meter and pre-amp using small Russian valves and linear neon indicators. The indicators can draw the full current for some time (during a loud movement) and so the LT can't be allowed to rise for too long as the heaters would get unhappy. I did try the feedback from the HT line but the LT voltage was all over the shop then...

I'm not sure whether there is another was of arranging the outputs (mutual inductors) where the LT and HT can share the regulation somehow? I'm not even sure if it is poor regulation or just overshoot/ringing on the HT line...I think I might just have to buckle down and cut a PCB and hook it up to a scope. This simulation business is all very well but I like having burnt fingers. Trouble is I'm away travelling for the next fortnight!

D
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Old 21st Dec 2013, 2:07 pm   #69
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Default Re: SMPS experiments

Ah, I just ran the simulation for longer and the regulation is actually ok (about 2V for 30mA change). It is ringing and the ringing is at about 350Hz....the resonant frequency of the inductor and capacitor after the rectifiers. So I'm guessing to combat that I need some kind of RC snubber?
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Old 21st Dec 2013, 7:02 pm   #70
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Default Re: SMPS experiments

It's actually quite normal behaviour, if you are taking feedback from the LT rail, and varying the LT load, to see disturbances on the other rails.

When the load on the LT increases, the voltage starts to drop, and the feedback loop increases the pulse width to compensate. The other rail, whose load hasn't changed, thus sees a surge of energy at the input to its filter, and so you get a blip on the output.

You won't remove this completely, but there are some things you can do. One is to damp the ringing - use a series RC across the filter output (C being the same order as the existing filter capacitor; R being approx √(L/C) as a starting point). For good damping, it would be better to connect R directly across the output of the filter, but that would of course cause a steady and significant DC load, so the series capacitor is necessary.

Another is to wind the HT filter choke on the same core as the LT choke. Doing this, it is important to keep the ratio of the turns the same as the ratio of the turns on the transformer. Diode voltage drops and resistive drops don't enter into things for this.

Have fun!
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Old 21st Dec 2013, 9:19 pm   #71
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Default Re: SMPS experiments

Quote:
Originally Posted by kalee20 View Post
Another is to wind the HT filter choke on the same core as the LT choke. Doing this, it is important to keep the ratio of the turns the same as the ratio of the turns on the transformer.
..... and reverse the connections to one of the windings, if it gets worse instead of better.
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Old 23rd Dec 2013, 2:42 pm   #72
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Default Re: SMPS experiments

Thanks both, I added the snubbers and the shared choke (to the simulation) and it is well within my design aim of 5V max drop on the HT line (it's about 1.5V) with some slight overshoot and ringing but essentially fine....can't wait to get back in the new year and warm the soldering iron up...no doubt something will come along to keep me from it though!
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