alternation betwen transformator magnetization inductance & switch capacitance

[veladimir]

hello friends
I design a single switch forward switching power supply.
when mosfet switched off magnetization inductane starts to charging the D-S capacitance of mosfet but it takes long time therefor off-time of transistor is not enough for clearing magnetic flow of core and starts continous mode.
In the theory when mosfet switched off magnetization current of core forces the third wiring to
flow the current instantaneously and in a time that is equal to on-time of transistor , reverse voltage applyed to trans and claer the magnetic flow .
what can i do for decreasing this time and power dissipation in transistor?
note that i remove the load !
note that by adding a gap magnetization inductance reduces and Cds charging time reduces
but other problem appears in turning on of transistor!

 

[veladimir]

help !
can one help me?

 

[MrAl]

Hi,

Im not sure i totally understand what you are asking, but it sounds like your MOSFET is not switching fast enough. To get the maximum switch speed of a MOSFET you need to use a gate driver IC chip. This is a chip made especially for driving mosfet gates. For fast switching times, this is very important.

 

[veladimir]

I use a gate drive IC 7667 but because of big core magnetization inductance, magnetization current is very low and
and needed time for charging mosfet Cds capacitance is very high.
L = 0.1H
Cds= 100pf
Ton = 5us
V= 300v
Im=V*Ton/L=6mA
ω=1/√LC =316000=2π50000 ==> f= 50KHz ==> T = 20us
mosfet is on for 5us and is turning off in 10us
when mosfet switched off magnetization current start to charge the Cds and resonant start for half priod and
then diode conduct and ...
I should say that load is not connected .
how I can reduce this time . adding Air gap make other problem in turning on procedure.
thanks for attention.

 

[MrAl]

Hi,

10us is totally unacceptable for the turn off time of the MOSFET.
The first thing i can think of is to add some resistive load referenced to whatever rail works best. Hopefully your app can stand a little loss here and there. If not, perhaps you can design a little active load that turns on and off when you need it. That would be a little more involved.
Of course reducing inductance would help, but you'd have to go pretty low i think in your case so that's probably not practical.

 

[johansen]

find a switch with lower capacitance.
you will probably have to use a small transistor.
even parasitic capacitance is significant here, and i think you will find the inter turn capacitance of your inductor dominates.
(keep in mind you are trying to get a .1H inductor to resonate faster than 100Khz!)

however, your turn on time now is 5us, that is far too long, they make igbt's that can turn on in 200nS

 

[veladimir]

Hi
can one introduce a mosfet or IGBT with prameters
800v
1A
Rds < 20ohm
I use 2n80