2 switch flyback
2 switch flyback
(OP)
Hello,
I am requesting your assistance if i may as i must make a DCDC converter to have V(in) = 400 Volts and V(out) = 12 Volts.
The high V(in) makes me want to use a 2 switch flyback, as in the following article......
http://www.edn.com/contents/images/47173.pdf
...the two switches, and the clamping to V(in) mean less voltage on the MOSFETs.
...However, i am uneasy as when the secondary is conducting, it will refer back to the primary a voltage to the primary of V(sec) * Np/Ns....
...However, the primary at this (off) time, is clamped to V(in).....so i am wondering , which voltage wins out and ends up across the primary when the secondary is conducting?......by Faraday's Law the primary (should)experience a voltage of N*d(phi)/dt .....the d(phi)/dt being same for both primary and secondary.
So i wonder how the primary can be clamped to V(in) under these circumstances?
I am requesting your assistance if i may as i must make a DCDC converter to have V(in) = 400 Volts and V(out) = 12 Volts.
The high V(in) makes me want to use a 2 switch flyback, as in the following article......
http://www.edn.com/contents/images/47173.pdf
...the two switches, and the clamping to V(in) mean less voltage on the MOSFETs.
...However, i am uneasy as when the secondary is conducting, it will refer back to the primary a voltage to the primary of V(sec) * Np/Ns....
...However, the primary at this (off) time, is clamped to V(in).....so i am wondering , which voltage wins out and ends up across the primary when the secondary is conducting?......by Faraday's Law the primary (should)experience a voltage of N*d(phi)/dt .....the d(phi)/dt being same for both primary and secondary.
So i wonder how the primary can be clamped to V(in) under these circumstances?





RE: 2 switch flyback
RE: 2 switch flyback
The thing is, the primary can only have one voltage across it at any one time......not two different ones at the same time.
By Faraday's Law, we must believe that during secondary conduction, the voltage on the primary coil is given by...
Np/Ns * V(secondary output)
However, by Kirchoff's Law, during secondary conduction, we can see that the voltage across the primary coil is
V(in).
These are most likely not the same voltages..
The mystery is, which voltage does appear across the primary during secondary conduction?...or....who wins out of messr's Kirchoff and Faraday in this case?
RE: 2 switch flyback
calculated reflected voltage smaller than Vin --> Vout as desired
calculated reflected voltage larger than Vin --> this will limit vout below the desired value
RE: 2 switch flyback
Np = 100
Ns = 10
Vin = 400V
Vout = 20V
..................................................
When mosfets go off.....
Secondary starts conducting.
Vs / Ns = d(phi) / dt (1)
At the same time the primary is conducting through the diodes.
Vp / Np = d(phi) / dt (2)
d(phi) / dt must be the same for primary and secondary as they are on the same core.
From (1) and (2) we can say that during the mosfets OFF time.....
Vs / Ns = Vp / Np (3)
Substituting into (3).....
20 / 10 = 400 / 100 (4)
.....(4) is not correct.
This is proof that the 2 switch Flyback converter has disobeyed Faraday's Law.
Does any reader know how it works?
RE: 2 switch flyback
-200 volts should be reflected back to the primary.
(Ignoring parasitics).
ie. 400 volts is across the primary during the on time and
-40v on the secondary (no conduction due to rectifiers
being reverse biased). During the off time the secondary
conducts +20 and reflects -200 on the primary (no
conduction on the primary because the fets are off and the
body diodes are reverse biased).
Hope this helps
RE: 2 switch flyback
Madcow, i agree with you that -200v is reflected back to the primary.
However, if diodes D1 and D2 of 2nd page, fig 5 of following
http://www.edn.com/contents/images/47173.pdf
...were on during the off-time of the Mosfet switch's then this would not be possible -since it would be 400V across the primary and not 200V.
I think that the only way that figure 5 of the above link can work is if diodes D1 and D2 only conduct for say less than 200ns after the MOSFETs are switched OFF ?
-This means that the primary leakage inductance must be small enough that it can discharge its current in this short space of time ?
If diodes D1 and D2 conduct throughout the off time of the two mosfets, then this converter with....
Np = 100
Ns = 10
Vin = 400V
Vout = 20V
....would be only able to give an output voltage of 40 Volts ?
Any thoughts appreciated.
I have finished the PFC stage now, and i hope to switch the 400V with an IR2110 (hi and lo gate driver IC) and hopefully a two switch flyback, if i can understand how it works.
I believe its advantageous for me as instead of the 2 switch forward converter it doesnt need the output inductor.
RE: 2 switch flyback
RE: 2 switch flyback
discontinuous mode at a lower voltage (e.g Vin(min)=200V;D.C=0.4); ypu will rich by calcul. a N1:N2~10, trafo ratio.
So, the reflected input voltage will be:10*12V=120V<<Vin=400V. No current feedback will occur during OFF periodes.500V mosfets will suits the application (for more safety: 600V).