flayback transformer

[alireza7]

hi
i am working on a flyback transformer and voltage multiplier (diode and capacitor network)  to make 12vdc to 1600vdc
at the flyback transformer stage i need 400k p-p to feed voltage multiplier network

 since my voltage source was 12v dc i considered 400/12 for transformer turn ratio and i wound up primary wounding 4 turns and secondary wounding 170 turns .
but in practice i got only 44 v p-p at the output of  flyback transformer.
when i looked at the input voltage of primary wounding i figured out its just about 1 v p-p !! i supposed it to be about 10 v

then i realized mosfet draws a lot of current and the 12v voltage in uper terminal of primary wounding drops and also in the other terminal of primary wounding mosfet do not pull down the voltage in its on state and the drain voltage just goes down to about 7v during on state and mosfet becomes too hot
mosfet is irf820s
what is the problem?
what should i do to get  more voltage p-p in primary wounding and also 400v on secondary terminal

[Prehistoricman]

You're either not driving the gate of your MOSFET hard enough (check the datasheet for on characteristics, what gate voltage you need for full conduction), or your frequency is too high, and therefore there is not enough time for the coil to draw current and build a magnetic field.

[strawberry]

Wrong direction of windings and might be missing gap in core. All this makes transformer in extreme saturation and definitely fast blow up MOSFET.
I can see transformer turn ratio might be 1:10..20. And that's not how Flyback works
Missing voltage RCD clamp might over voltage MOSFET...

Try IR2153 as 12V Push-Pull with two 55V MOSFET..

[Buriedcode]

For flybacks, the transformer is more a coupled inductor, and you really need to know its primary (and secondary) inductance and saturation current.  Best to measure these rather than just calculate the values based on assumptions. Turns ratio isn't as important as it would be for other topologies since the primary voltage will be boosted anyway. 

With 7 voltages across the drain whilst on, no wonder its getting toasty.  How are you driving the MOSFET? (on-time, off-time, voltage, source impedance/current capability etc..)

[Benta]

How do you propose making a voltage multiplier with a flyback producing DC? Just wondering.

[strawberry]

How do you propose making a voltage multiplier with a flyback producing DC? Just wondering.

CRT TV Flyback transformer had multipliers

[capt bullshot]

Looks like the core goes into saturation.
You need an air gap to get higher allowable primary current and you may want to try to increase the switching frequency.
It is very important to get the MOSFET to turn on fully (say, less than 0.1 ... 1V voltage drop across drain and source while it is conducting). A snubber network on the primary side is also highly recommended.

A resonant royer style converter might be easier to get to work, like the circuit I've attached

[Benta]

How do you propose making a voltage multiplier with a flyback producing DC? Just wondering.

CRT TV Flyback transformer had multipliers

Yeah, but they don't have output rectifier and filter cap.

[Buriedcode]

I believe his initial diagram was just an example taken from the web of the sort of primary side he has.

[alireza7]

thanks to every one for replying
i'm suspected on core saturation and i will try a bigger core with more primary winding turns and tell you the result

[Neilm]

Remember the core stores the energy before discharging it into the secondary. Adding a small gap in the core will allow it to store more and help prevent saturation so you don't need to change cores.

[Buriedcode]

thanks to every one for replying
i'm suspected on core saturation and i will try a bigger core with more primary winding turns and tell you the result

Without knowing the inductance, how will you know what frequency to drive it at, at what peak current?  You can calculate inductance form the core specs and number of turns, but its easier just to measure it.  Blindly using different cores and increasing windings is a hap hazard way of getting the circuit to work.

I often drop this link in whenever people talk about flyback converters:
http://www.dos4ever.com/flyback/flyback.html

The trouble is people still insist on thinking in terms of turns ratio, and treating it as a 'transformer', when its really an inductor with two windings - it stores energy.  Essentially treat it as a single inductor in a boost converter.  Once you have a working boost converter that boosts your 12V to say 40V, then just add another winding with 10x more turns for your secondary.  There is a reason why people use formulas to calculate things.

[Benta]

thanks to every one for replying
i'm suspected on core saturation and i will try a bigger core with more primary winding turns and tell you the result

Designing inductors for power electronics is an iterative process. Your overall design will dictate inductance and current and power.
When that's fixed, you'll need to consider:

Core size/shape
Core material
Saturation point
Air gap
Copper fill
Isolation
Interwinding capacitance
Core loss
Copper loss
etc...

An experienced designer will arrive at a good result after 10 iterations. Beginners might need 30 or 50.

Main problem: there is no "closed loop" solution, it's try, try and try again...

[strawberry]

Iron Powder toroid core (yellow color material 26 is for DC applications ) have higher saturation compared to Ferrite and no gap needed.

[alireza7]

although other aspects need work but the main problem was mosfet which had a high Ron , i used a low Ron mosfet and i get 600v

[alireza7]

in some pages i have read polarity in flyback is important but in my flyback transformer which powers a voltage multiplier i do not see any difference in output voltage when i change the secondary winding output terminals together.
is it really important? if yes why i do not  see any obvious change?
my output voltage in positive voltage multiplier is 1600v and in negative voltage multiplier is -1600v and load at each of them is 260k ohm

[Neilm]

On a flyback transformer the polarity is very important. The transformer works by current flowing through the primary charging the magentic core. The diode in the secondary stops that winding discharging the core. When the current in the primary stops flowing the effects of the magentics mean current will start flowing in the secondary. This should be calculated so the core discharged before current starts flowing in the primary again.
If the windings are in the wrong polarity then the secondary will conduct as the current flows in the primary. This will transfer some of the power into the secondary but you will not see real voltage rise.

What I would suggest is you check your secondary diodes. I have had issues in the past where slow diodes were fitted in a secondary and the circuit simply didn't work. You need good (fast)rectifier diodes in this position.

[alireza7]

i became confused
you said i will not get rise in voltage if the polarity be wrong but i change the polarity and i get same 1600 voltage and 6ma current on both posetive and negetive terminal of my voltage multiplier circuit.
also if you look at my schematic you can see there is a +1600v and a -1600v voltage multiplier circuit and the secondary winding feeds +1600v rail in positive peak of secondary winding and feeds -1600v rail in negative peak of secondary winding but they are exactly the same and symmetric . according to your description one of them should work properly and voltage in other one should not rise as much as other one.

[Buriedcode]

What makes a "flyback" converter is that current does not flow in the primary and secondary at the same time - the transformer is really a coupled inductor. 

The reason current does not flow in both windings at the same time is because of the diode on the secondary, which is why the polarity of windings is important - as current flows through the primary when it is energized, the diode blocks induced current in the secondary, effectively removing it from the circuit so it appears as one is simply charging an inductor.

As you have added two multipliers, current can flow in both directions in the secondary.  Coupled (pun intended) with the fact you have wound your transformer with a 12:400 ratio means you will get roughly +/- 1600 out of your 4-stage multipliers.  But... it isn't a 'flyback' converter.  It seems its more of half a push-pull.

One way to check what is going on is to scope the waveforms on the primary and secondary, the current on the primary, and also measure its efficiency.  The fact you're getting your desired voltage is a good sign, but how efficient is it?  With a fixed load on the output, measure the input voltage/current.

All this is assuming you have a working circuit and you're actually measuring your result rather than simulating.

[alireza7]

the ideal input current is 3200*0.006/12=1.6A
 my Idc is about 2A and Ipeak is about 18A

considering Idc efficiency is 1.6/2=80%

i use a 3329 ferrite core and my primary winding is 21 turn and secondary winding is 700 turn and pwm frequency 4.8khz and duty cycle 19%
i figure out the core become too hot after a while

[Buriedcode]

the ideal input current is 3200*0.006/12=1.6A
 my Idc is about 2A and Ipeak is about 18A

18A peak?  That's a pretty big transformer..  and with 2A average, that means the primary switch spends most of its time off.  Also 18A is a lot to switch through a MOSFET.  Even with say 0.05ohm on resistance, without switching it'll have to dissipate ~16W..

considering Idc efficiency is 1.6/2=80%

I'm not sure where you got these numbers from.  Efficiency is power out/power in.  You say you have two charge pump outputs, one positive and one negative, both 1600V with a 260k ohm load.
That means either output is drawing 1600^2 / 260k = 9.85W.   For two outputs = 19.7W. 

Your input is 12v @ 2A (I doubt this value). Which is 24W.  Eff. 19.7/24 = 0.81 which is indeed 80%  where did the 1.6 come from?

i use a 3329 ferrite core and my primary winding is 21 turn and secondary winding is 700 turn and pwm frequency 4.8khz and duty cycle 19%
i figure out the core become too hot after a while

A quick google says thast roughly 35mm x 24mm E core?  Fine for 20W but if you have 18A on the pimrary of one of those - its saturating, unless it has a monstrous air gap in it.

Ok, the turns ratio is 700/21 = 1:33.3 exactly as you wanted for 400V out from 12V in (for a forward transformer, not a flyback).  But the duty cycle... for a step up, normally the duty is >50%, but can be much less for discontinuous mode.

4.8kHz = 208us period.  At 19% duty that is an on-time of: 0.19 * 208 = 39.6us.  If your peak is 18A, with an input voltage of 12V, then the inductance is:

I(peak) = V/L * T  Rearranged for L.   L = V/I(peak) *T = 12/18 * 39.6u = 26.4uH.  Again this is based on your numbers, so if they are measured, then that's roughly what your primary inductance is.

However yours isn't a flyback converter. I suspect what is happening is you're either your frequency is too low, or because it is a push-pull converter being driven asymmetrically,  the core is saturating.

As was mentioned many posts ago, go for a royer push-pull configuration. It will require another primary winding, and a feedback winding, but is much more suited to what you're after - plus its output voltage is pretty much down to the turns ratio.   If you insist on driving it with one transistor, then scope the primary waveform, and reduce the ON-time to get the peak current down, and see what happens when the switch turns off.
 

[alireza7]

as i said :
the ideal input current is 3200*0.006/12=1.6A
so 1.6A is the current that circuit will draw if it's efficiency be 100%
so with compare to Practical 2A we have: 1.6/2=80% if you do some math and simplification you will see that your calculation and my calculation are the same:

 3200*0.006/12=1.6A =>  3200*0.006/12/2 =3200*0.006/(24) and (approximately) 3200*0.006 = 2 * 1600^2 / 260K = 2*1600*1600/260k =3200 / 0.00615

the difference is just because i approximately considered a 260K ohm load on 1600v draws 6 ma which actually draws 6.15ma so you got 81% and i got 80%