As an Electronics and Communication Engineering Student... I have worked on a lot of switching PSUs.. From Flybacks to Full Bridge LLC Resonant Converters.. Come to your question.. am not going to talk about cheap ass Half bridge push pull PSUs.. 1)Dual Switch Forward - In this PSU.. there are 2 switching FETs.. and both FETs are controlled together at the same time with Isolated Gate Driving Circuitry.. (Using A tiny Gate Transformer) that drivers both MosFETs together but.. have isolation from low side switch and high side switch and Driving Circuitry.. 2) In Half Bridge LLC Resonant Converters (an Example Coolermaster MWE 450 bronze V2).. also uses a Gate Driving Transformer. But this Gate transformer sits between primary and secondary side.. (secondary side actually has PSU FM Controller that controls primarily FETs using gate driving transformer and current amplifier Circuitry to drive From Low side of The PSU.. (this controlling circuit for primary Rails works 5Vsb Powersupply.. 3) In Full Bridge LLC Resonant Converters.. It's the same as Half bridge but uses 4 separate driving coils to drive 4 MosFETs in bridge configuration. To Which at a time only 2 FETs conduct at a time.. In Flyback Topology (like on phone chargers).. Transformers are usually switched from the low side.. (-ve side of Rectified mains) and its Gate is controlled from a very small resistive dropper later fulfilled by Flyback coil from transformer. It is used to power the controlling circuitry. Hope this Answers the question..

1 N channel MosFET Means its either a Flyback Topology or Single Switch forward topology..

@@ArifKhan-bp9zx very well and nicely explained dear brother... but I haven't asked for this. you're talking all about smps power supply topologies and design principles but let's come to my question ? The lecturer in this video has said that we need to consider the drain voltage of P/N channel mosfet to provide the required voltage to gate to enable the mosfet. (I guess he is wrong) that's why asked him or his institute, why we have 5v or around 7-8 volt. in gate of smps switching mosfet (N-Channel) ? Whether we have 320 to 340v on it's drain pin.... as per his statement we should then get high voltage of 340v on it's gate to enable it for switching right. Then why we have 5 to 8v on it's gate ? As per my knowledge, whether it is N-channel or P-channel we always need to consider it's Source pin to select a required supply of gate.

@@atifservicecenter Main Question ki baat kare ki kaise MosFET on hota hai... Pehle circuit ke bahar MosFET ko on karne ke lea... Hame Threshold (Vgs)voltage se zada ka Voltage gate pe Dena hai.. most MosFETs pe maximum Open Drain to Source Voltage bhi hota hai.. Like GP13N50 MosFET ke baat kare to.. it's 500volts (Vds) hota hai.. usse Gate ko koi farak nahi padta.. bas Vds should be low.. (MOSFET non conditioning ke time) . Agar 550 Voltage hoga to MOSFET automatically short ho jayega.. MOSFET ko on karne ke lea Voltage dene hota hai gate pe.. threshold voltage se zada.. but Vgs max se kam hona chahea.. otherwise.. MOSFET fir se kharaab ho jayega.. Voltage Source ke refrence me lagana hai.. na ki floating Voltage dena hai gate pe... Vds kitna bhi ho Vgs should be under limit... Ha PMos me Gate voltage negative likha hota hai.. like -20 volts.. it means.. agar ham multimeter se naape.. from gate to source.. to -20 voltage show hoga... But actually me.. vo Source se Gate measure kare to 20 volts positive voltage show hoga... MOSFET driving me Vgs should be under the specified value.. Either it's n mos or p mos... Vgs should be more than Thresholds Gate voltage.. andd less than max vgs..

N-channel MosFET Tab conduct karta hai jab.. Gate to source voltage.. Threshold voltages se equal ya usse zada hota hai.. to Gate and source pin ke beech kuch to lagana hoga.. P channel MosFET Ko enable karne ke lea Source to gate voltage chahea hota hai... (Source pe input already jab a raha hai from +ve rail. To Source to gate, means gate should be 0 voltage.. means connected to ground.. enable hone ke lea.. I guess utna samajh aa gata hoga

BJT is little harder to get the idea at first.. but baad me easy hai.. MosFETs starting me easy hai but baad me bahut hard hai.. to totally understand them..