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Ash Small

Fri Jan 17 2014, 11:12PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Peter, I certainly am confused over that point, and have been for some time. I have been reading exactly what you suggest, as well as some old posts on this forum, and I've also read it is possible to 'recover' energy from the primary, so that the only energy 'used' is that which flows in the secondary. I'm still confused over this point, and I'll look into it further.

But yes, the point about primary and secondary having, for example, the same number of turns, and the same current flows in each, if the secondary has, say, a small capacitor, the voltage will be higher. (I suppose I should say 'if the secondary load has a higher impedance the voltage will be greater'.

This contrasts with the 'general rule' that a 1:1 turns ratio will have the same voltage in the secondary as in the primary.

EDIT: Could you post a copy of the, presumably, LTSpice file, to save me having to start from scratch on a simulation please? I spent most of last night reading up on how to enter coupled inductors into LTSpice.

Andy

Fri Jan 17 2014, 11:57PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Ash, not sure if this is helpful, but if you have say two primarys and one secondary all with 1 turn, one of the primarys is driven by ac, the second primary has a switch, close it,it will collpase the primarys field quickly and make more voltage onto the secondary.

Peter02

Sat Jan 18 2014, 01:04PM

Registered Member #1488 Joined: Sat May 17 2008, 10:41AM
Location: Germany
Posts: 18

Hi,
i have attached the LTspice file, and 2 additional links, one explaining flyback operation and converter design in great detail and the other discussing one method of providing a lossless snubber to improve eficciency and lower the voltage stress to the primary switch.

Greetings

Ash Small

Sat Jan 18 2014, 07:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Thanks Peter. I've had a quick look at the snubber and noticed it has an inductor in it (at least from first impressions). I don't think I've come accross one like it before. I'm more familiar with the diode capacitor resistor types.

I'll read it in more detail later.

EDIT: In my earlier post above, the confusing one, I should have put 'high impedance conected to secondaty and low impedance connected to primary' I think. Maybe that would have made more sense. It was 3:30am and I was tired.

Ash Small

Mon Jan 20 2014, 02:53AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

post deleted

Ash Small

Tue Jan 21 2014, 07:09PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I think I've now 'cracked' tjhe whole 'flyback transformer' mystery.

From Switching Power Supply Design Third Edition
Abraham I. Pressman
Keith Billings
Taylor Morey

"The rules for an inductor with more than one winding are as follows:
The primary to secondary ampere-turns ratios are conserved (not the
voltage ratios, as was the case with a true transformer). For example,
if the primary is, say, 100 turns and the current when Q1 turns â€œoffâ€ is
1 amp, then we have developed 100 ampere-turns in the primary. This
must be conserved in the secondaries. With, say, a single secondary
winding of 10 turns, the secondary current will be 10 amps (10T Ã—
10A = 100 ampere-turns). In the same way, a single turn will develop
100 amps or 1000 secondary turns will develop 0.1 amps.
So where do we stand with regard to voltage?Well, to the first order,
there is no correlation between primary and secondary voltages. The
secondary voltage is simply a function of load. Consider the 10-turn
10-amp (100 ampere-turns) secondary winding example mentioned
above. If we terminate the winding with a 1-ohm load, we will get
10 volts. What is more striking because the 10 amps must be conserved
is that if we terminate it with 100 ohms, we will get 1000 volts!
This is why the flyback topology is so useful for generating high voltages
(donâ€™t try to open circuit this winding because it will destroy
the semiconductors).With several secondary windings conducting at
the same time, then the sum of all the secondary ampere-turns must
be conserved.
So the lesson we learn here is that flyback â€œtransformersâ€ actually
operate as inductors and must be designed as such. (In Chapter 7, I
use the term choke instead of inductor because the core must support
both DC and AC components of current.) If flyback â€œtransformersâ€
had originally been called by their correct functional name, â€œflyback
chokes,â€ then a lot of confusion could have been avoided.
We must not forget that voltage transformation is still taking place
between primary and secondary windings even if they are not conducting
at the same time. Taking the above example of 10 turns terminated
in 100 ohms, the 1000 volts thus developed on this secondary
winding will reflect back to the primary as 10,000 volts; this added
to the supply of 100 volts will stress Q1 in its â€œoffâ€ state with 10,100
volts (where did I put that 11,000 volt transistor?). Hardly practical,
but the theory holds."

I take it this means that the only reason why you need a 'turns ratio' between primary and secondary is so that the voltage reflected from secondary to primary doesn't blow the silicon.

Also, if you have several one turn primaries, each with their own switch, in parallel, for example, the total reflected voltage would be shared by the switches as if they were in series, I think.

If my assumptions are true I think I've got the beginnings of my circuit.

Any comments would be welcome.

Shrad

Tue Jan 21 2014, 08:18PM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

maybe a split primary with a central copper ribbon connected to several ribbon coils distributed along the core would be a nice model to start with...

if the primary is in the outer layer you can even solder the primary terminals directly on the PCB

Ash Small

Wed Jan 22 2014, 12:48AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Shrad wrote ...

maybe a split primary with a central copper ribbon connected to several ribbon coils distributed along the core would be a nice model to start with...

if the primary is in the outer layer you can even solder the primary terminals directly on the PCB

Yes, I think something along those lines would be worth trying. I'm still reading up on gapped cores, etc. and I'm thinking of running some test using a car battery, to get some huge currents flowing. I'll have to use HV rated wire for the secondary (or secondaries, I'm considering using multiple cores with the secondaries in series, although I've not decided yet.), and I'm also contemplating using a separate mosfet for each primary turn (or set of primary turns).

I do think foil primaries is the way to go, though.

Ash Small

Wed Jan 22 2014, 02:25AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

post deleted

Peter02

Wed Jan 29 2014, 05:49PM

Registered Member #1488 Joined: Sat May 17 2008, 10:41AM
Location: Germany
Posts: 18

Hi,
that explanation of flyback transformer operation is very good i think.
Splitting the primary is a good way to share the load between several primary switches. This method is extensively used in particle accelerators and higher power pulsed laser supplies.
here are two links on the subject, these and many more can be found with search terms like "inductive Voltage Adder" or "Linear Transformer Driver"

Also, if you have several one turn primaries, each with their own switch, in parallel, for example, the total reflected voltage would be shared by the switches as if they were in series, I think.

I think that should be right, the reflected voltage is also the same if you use only one one-turn primary. The benefit is mostly the current sharing between the primaries and their switches.

Greetings

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