On 7/25/05, Russell McMahon wrote: > > I agree that maiby one of the transistors is not conducting as I am > > drooping 10V on the top transistor the question is why. > > Basically, you don't understand FET drive requirements. > But don't worry, you will :-) > > To turn on an N channel FET the gate has to be driven positive > relative to the source. The FET gate-source voltage is written Vgs. > The volts positive relative to the source required to JUST turn it on > depend on the FET used. This voltage is called the "threshold voltage" > and is written as Vth. ie turn on occurs when Vgs >= Vth. This is the > voltage at which the FET JUST turns on. You need more than Vth to turn > it on or "enhance" it fully. Traditionally FETS have Vth of about 6v > and for full enhancement need Vgs of 12 to 15 volts !!!. However, > there are FETS termed "logic FETs" which are fully enhanced at Vgs = > 5v. This allows them to be driven from a 5 volt "logic" supply or > device, such as a 5v FET. Some FETs are available with a fully on Vgs > of 1volt, but these are rare. > > In your case the bottom FET sees a maximum Vgs of 5v. > The ZVN4306 > http://www.solarbotics.net/library/datasheets/Zetex_ZVN4306.pdf > has a maximum Vth of 4 volts and a maximum allowed Vgs of 20 volts. > Above 20 V Vgs the FET will probably die. > > SO your bottom FET is switched on properly. At Vgs = 5v it will > conduct about 3A before starting to want more drive. (See 1st graph on > referenced data sheet). > > BUT > The top FET will only turn on when its gate is 1 to 3v (depends on > production spread) above its source. Even if Vth = 1v the MOST the > drain can get to is 5vgate-1v = 4v. So, with a 12v supply you will > drop 8v across the FET,. > > SOLUTION: > > No details here. Read up on this and then ask more questions. > > - Gate swing should be 0 - 12v > > - Top FET should be P Channel with source to +12v and drain to load. Hi Rusell, I belived this too since a few days ago when a friend shows me a full N MOS FET bridge driven from one transformer. I'm guess you'll imagine how it works and which is the price difference. cheers, Vasile -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist