Hi, I'm designing a flyback converter in where a MOSFET not connected to ground must be driven. Please check the following circuit:

The MOSFET needs to be on the high side and could be n or p (preferably n). The duty cycle of operation goes from 0% to 95%.
Could a pulse transformer be used in such a high range of duty cycle?
I'm NOT looking for a fully integrated IC (Mosfet + driver) solution.
How to start-up the power supply? a driver with HV source current?
Whatever thoughts you have on this are appreciated.

Thanks

 

You're better using a Pfet mosfet with the Drain to the pulse transformer.

 

You cannot organize the Q12 in that way. Reason - main transformer will become oversaturated, or gap will be huge. For solve this problem exists half-bridge or H bridge circuits.
For driving one possibility is CD4013, one half generates CLK, another divides it to 2 to obtain a supersymmetrical signal. That is non-adjustable generating on all 100% (what about through-put currents), but You can use it in start-stop regime, (what is speaking truth, the IR2153 greatest weakness).
Another `teachy` is NE555, what allows to realize the PWM. Apply just mechanical restriction to PWM max to avoid throughput.
More another of simplest for beginners is IR2153, what is just ideal for any simple low power solution without of any sophisticated services, Instead of pwm use a star-stop. Bit more advanced is UC3845 permitting CCPS and CVPS regimes both, and having inherent anti-throughput time shift circuit inside. For power up to few kW that is just excellent, but uses specific `strange` topology what s exclusively well described at danyk.cz (there in, is English mirror too, read it obligeously).
If Your aim is more heavy, like 100 kW or megawatt, then read my article beforehand to buy anything, it will be in ijiar journal autumn number soon (see for my surname). I not recomend to start with any monster for beginner.
Most of them all are capable to drive mosfet gate straightly, IF ONLY the mosfet is relatively small, like IRF530, 630, 730. If mosfet or igbt is stronger, like 50 Amperes and more, then driver (or say gate booster) is needed. Avoid from `too good` form on the gate, otherhow will see the devil via dV/dt between the D and S. Use a proper RC snubbering there. Use a non-inductive wires between rectifyer capacitors and mosfets. The best way - two sandwich PLATES, weaker - bifillary line, more weaker - honeycomb structure, hexa-angle, four-angle, triangle wiring etc.
High end drivers may cost hundreds, You need no such
Middle sector costs some 10 bucks, and You need no even such, however it saves the time in comparison with self-made driver.
Most cheapest (cents) are separate for upper end and for grounded end drivers with or without optrons, that are fine, however when larger currents (eg 30 Amps in gate) are needed, then prost smd transistor booster cascade is just like what one may need. However its not for beginners.
The most worst case is small toro transformer in gate. I`m not feel happy while winding. But if one is - why not. Only never do that mistake my college did, He produced a core what never de-saturates and wondered why signal `is not going through`.
Other-how optron is solution for upper transistor or alternatively the`Volt-addition` circuit like IR2153 do.

 

You're better using a Pfet mosfet with the Drain to the pulse transformer.

Hi, thanks for answering, something like this (forget about component values)?

You cannot organize the Q12 in that way. Reason - main transformer will become oversaturated, or gap will be huge. For solve this problem exists half-bridge or H bridge circuits.

Oh, I see, I have both windings in the same orientation or the diode reversed. This is more like a forward converted without the 3rd winding.
I'm aiming at low power, something like 10W.
Thanks for pointing out so many component references.
How would the system start if the driver works at 12V? do I need a HV current source in the IC?
For example, with the IR2153, I can't do bootstrapping unless I use a half bridge or I am missing something? How do I get Vcc - 15V to drive the P Mosfet?

Thanks

 

The pulse transformer provides isolation from the line to the load. Caution must be exercised on the primary side.

@Jaimesg
The thread can stay open as long as circuitry on the primary side remains isolated from the load terminals.
Comments are invited.

 

RE:""How would the system start""
Normally all smps has own separate "on duty" low power PS.
For example, look how it is organized at ATX circuitry (Google-> ATX circuit->pictures)
Or look how it is organized elegantly at Danyk.cz with unique IC of TNY267, 268, 269 etc.
Or apply just an analog PS with small winded 50 Hz transformer, or better reuse the old
mobile phone charger for 2 bucks the dozen.

P.S. - if the goal is SO SMALL than 10W, why not to use that TNY thing alone??

 

RE:""How would the system start""
Normally all smps has own separate "on duty" low power PS.
For example, look how it is organized at ATX circuitry (Google-> ATX circuit->pictures)
Or look how it is organized elegantly at Danyk.cz with unique IC of TNY267, 268, 269 etc.
Or apply just an analog PS with small winded 50 Hz transformer, or better reuse the old
mobile phone charger for 2 bucks the dozen.

P.S. - if the goal is SO SMALL than 10W, why not to use that TNY thing alone??

I've been checking for the TNY267 and it's actually pretty good. I think i'm going to give it a try.
I found a very interesting document:
http://www.onsemi.com/pub/Collateral/AN-6093.pdf.pdf
My goal is to make this as small as possible.

On a flyback configuration if I use a full bridge rectifier in the secondary, will I get a bit more current out of the same core, by getting current out of the secondary in both ON and OFF state of the switch?:

Would a ZVS forward configuration have more power density than a flyback one?

Regards

 

Actually I am surprised to hear of ZVT/ZVS in the context beyond the at least 50 000 Watts, and so more about buck/boost or any other non-HBridge converter (probably the lack of my experience?). Never say `never`, but I feel it like the cannon trying to shoot the sparrows.