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Registered Member #3925
Joined: Fri Jun 03 2011, 10:50AM
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Posts: 121
hi,
would it be better to drive a flyback transformer with a sine-wave, than a square wave, correct me if im wrong, but isnt using a square wave extremely inefficient? because a perfect square wave goes from 0-?V in 0 time, so the waveform is just direct current, and driving a transformer with direct current will just cause it to heat up, and will be a waste of almost all of the energy in the square wave?
im explaining this very badly,but i think youll get it :D
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
A square wave is not DC (depending on how you define 'DC'), the voltage is applied to the transformer winding for an extremely short time and then removed. For a 40kHz driver at 50% duty cycle it is only applied for 13 microseconds. A DC would be if you applied the input voltage constantly to the winding without ever removing it, and this would be bad because the core would saturate eventually and the transformer would not transfer power from primary to secondary.
Because of the fast transitions involved in a square wave, frequency dependent losses in the core material will be somewhat larger than if driven with a sine wave.
Practically driving a transformer with a sine wave from a DC bus is somewhat tricky. You can use resonant or quasi-resonant techniques which includes Mazilli (I think). You can use an even higher frequency modulated square wave with a filter to efficiently produce a sine wave to drive the transformer. Or the simplest way with a linear driver would end up being ridiculously lossy.
I'm not familiar with the Mazilli driver specifically, but in theory you can have a resonant driver driving the primary with a sine wave and end up with a flyback mode converter. The trick is how the output of the transformer is rectified, a flyback has diodes to block any current leaving the secondary while the primary is driven, and the secondary currentl only flows when the primary winding turns off. The output comes from the stored magnetic flux in the core you built up while the primary was driven, then it turns off and this stored energy flows out of the secondary.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Mattski wrote ...
I'm not familiar with the Mazilli driver specifically, but in theory you can have a resonant driver driving the primary with a sine wave and end up with a flyback mode converter. The trick is how the output of the transformer is rectified, a flyback has diodes to block any current leaving the secondary while the primary is driven, and the secondary currentl only flows when the primary winding turns off. The output comes from the stored magnetic flux in the core you built up while the primary was driven, then it turns off and this stored energy flows out of the secondary.
With all due respect, Mattski, while I agree with the other points you made above, I'd argue that flyback mode requires a square wave to drive it. While the voltage waveform is a square wave, the current waveform (if that is the correct terminology) rises gradually to a maximum (due to inductance), and is then cut off abruptly, giving the characteristic 'saw tooth' waveform. I would also argue that it is this 'abrupt cut off', where the current 'flies back' to zero (resulting in the almost instantaneous collapse of the field, which, in turn, causes a high voltage spike on the output) that defines the 'flyback mode topology'.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Well it wouldn't work with a sine wave that goes negative, it would have to be more of a half-wave rectified sinusoid shape. I'm not sure exactly how this could be implemented, or if it would be a good idea, but if this is the voltage waveform applied to the primary then you will have a current waveform which goes from zero to a maximum in sinusoidal fashion, then when the primary voltage goes to zero the energy stored in the inductance of the transformer can be transferred to the load side.
A quick and dirty simulation shows that such a converter is possible. In my mind what distinguishes the flyback among other isolated converters is that the primary conducts current for a time as energy is stored up in the core. Then the primary stops conducting current, and the stored flux in the core then decreases by current flowing through the secondary winding. In all other transformer isolated converters I'm familiar with the secondary winding conducts current at the same time as the primary winding.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Mattski wrote ...
. In my mind what distinguishes the flyback among other isolated converters is that the primary conducts current for a time as energy is stored up in the core. Then the primary stops conducting current, and the stored flux in the core then decreases by current flowing through the secondary winding. In all other transformer isolated converters I'm familiar with the secondary winding conducts current at the same time as the primary winding. .
While I agree that that is what is generally taught, etc. I'd still argue that an 'AC flyback' (ie, a flyback transformer without any diodes) will produce an output while the primary is conducting, albeit a much smaller voltage than when the field collapses.
All inductors store energy and then release it. Think of how a resonant LC tank circuit works, for example.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
wrote ... While I agree that that is what is generally taught, etc. I'd still argue that an 'AC flyback' (ie, a flyback transformer without any diodes) will produce an output while the primary is conducting, albeit a much smaller voltage than when the field collapses.
Absolutely that's correct, but an 'AC flyback' is a just a two-winding transformer which can be driven in any of many possible configurations. If it's driven in flyback mode then there will be some rectifying circuit on the secondary side which blocks secondary current from flowing while primary current is flowing. In a DC flyback this diode is integrated into the transformer package, with an AC transformer the diode is external.
If this diode isn't here then the current through the secondary winding actually cancels some of the magnetic flux generated by the primary winding. So the primary winding doesn't just look like an inductance, it looks like an inductance in parallel with the load on the secondary reflected to the primary, generally with most of the primary current being the reflected load current rather than the magnetizing current which adds to the magnetic flux in the core. This is what happens in an ordinary 60Hz AC transformer, or a full-bridge, half-bridge, or push-pull circuit, and in all of these the output voltage is never more than turns ratio times the input voltage. But since the flyback is deliberately storing energy in the magnetic flux of the core it acts like a buck-boost converter, and it's output voltage can be higher by virtue of the inductance forcing current "uphill" into a larger voltage than the turns ratio alone would allow.
The transformer could be driven in other converter modes (push pull, half bridge, full bridge) where secondary and primary current flow at the same time, but then the output voltage would not be more than the turns ratio of the transformer times the input voltage because it's not making use of the inductance of the transformer to get an additional voltage rise like the flyback can.
P.S. Alf, did your question get answered? Ash and I have veered off a bit from your original question, though hopefully you have found our discussion interesting for understanding flybacks slightly more.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
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Posts: 4245
I think I agree with all of your last post, Mattski.
To go back to your proposed half-wave rectified sine wave input you reffered to in an earlier post, the shorter the switch-off time from maximum current, the greater the voltage spike on the output (there is also a reverse voltage spike on the primary, but I'll ignore that for now). This is why a mosfet is more efficient than an igbt, which tails off more gradually.
The half-wave rectified sine wave will produce a much smaller voltage on the output than a square wave would, will it not?
(ie, is it not 'rate of change in current over time' is proportional to 'voltage produced'?)
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
wrote ... The half-wave rectified sine wave will produce a much smaller voltage on the output than a square wave would, will it not?
The magnitude of the voltage spike depends on the magnitude of the peak current and the load's characteristics, not the speed of the turn-off, nor the voltage waveform which produced the peak current.
An inductor tries to maintain a continuous current through itself because the magnetic flux cannot be discontinuous in time, so when current is interrupted the voltage rises to try to keep forcing the current through. When a voltage is applied to an inductor, the current would ramp up with I=1/L*integral(V(t)) which as you're aware is a linear ramp for a square wave voltage.
So when the primary current goes to zero when a switch of some sort opens, the inductor fights back. In the case of the flyback the continuity of current can be fulfilled by the secondary winding since it shares the same flux path. So the voltage only rises until in can forward bias the rectifier on the output side. If there is a spark gap on the output side then the voltage will rise until it reaches the breakdown voltage of the gap. However the stored energy in the core will take path of least resistance - if the spark gap is too wide then the transistor would break down instead of the spark gap.
So based on that, the speed of transistor switch-off doesn't affect the output voltage, since there is just a certain amount of stored energy in the core which is transferred to the load after switch-off. The stored energy is proportional to the peak current right when the transistor switches off, no matter what shape the current waveform was before that, or how quickly the primary current goes to zero, as long as the turn-off time is short. If you had an exceptionally slow turn-off then some of this stored energy would turn to heat in the transistor, but it will typically be negligible. So that energy is transferred to the load, and energy/time is power, and for a well-behaved load like a resistor that trivially relates to output voltage, it's a bit more complex with badly behaved loads like spark gaps. (Note: this assumes discontinuous conduction mode where flux decays to zero with every switching cycle, it's slightly different for continuous mode).
wrote ... (ie, is it not 'rate of change in current over time' is proportional to 'voltage produced'?)
Better to think of it as rate of change in magnetic flux over time, since the current under consideration can come from the primary OR secondary winding, but it's one flux that they share in the ferromagnetic core.
wrote ... This is why a mosfet is more efficient than an igbt, which tails off more gradually.
IGBT's are slower, and there is a spike in power dissipation called switching loss which gets larger when a device takes longer to turn on or off, so IGBT's have more switching loss than a comparable MOSFET. IGBT's have a more or less fixed voltage drop when fully on, while MOSFETs look like a resistor, so conduction loss for an IGBT is Vce,sat*Ids but for a MOSFET it's Rds,on*(Ids)^2, so at high currents a MOSFET has more conduction loss.
(Hope that all makes sense, I tend to ramble on late at night)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Most of it makes sense, Mattski, but I'm still re-reading the bit about IGBT's and MOSFET's.
Are you saying that, at high currents, an IGBT would be more efficient (or produce a higher voltage), despite the slower turn off time, because the current in the primary is higher, due to the greater resistance of the MOSFET?
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drive flyback transformer with a sine wave?
