Post by thread:[EE]: Driving a BUZ10 Mosfet

Hi Jeethu, this link posted the other day http://www.web-ee.com/primers/primers_frm.htm has a MOSFET section, including "Interfacing power MOSFETs to logic devices", which is actually a pdf of Motorola AN1102. This deals extensively with just your question. You don't give any specs for your application, but I'm sure you'll find what you're looking for in that pdf, good luck -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> Can I directly drive the MOSFET from the NAND Gate? > > Please suggest In general.. you must drive the gate arround 10~12V above source level in order to switch on the mosfet. If you are novice with mosfets, you may try an OMNIFET (from sgs-thomson) that works at TTL levels, and is fully protected (short circuit, thermal, clamp voltages... etc.) please see http://eu.st.com/stonline/products/selector/314.htm -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> Can I directly drive the MOSFET from the NAND Gate? That depends on what the gate voltage specs of the FET are. Some FETs are specifically called "logic level" FETs which work with 0-5V gate drive. However, these tend to cost more and have a slightly higher on resistance than their "normal" counterparts. Typical "normal" FETs require around 10V gate drive to fully turn on. Check the data sheet. ******************************************************************** Olin Lathrop, embedded systems consultant in Littleton Massachusetts (978) 742-9014, spam_OUTolinTakeThisOuTembedinc.com, http://www.embedinc.com -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Olin Lathrop wrote: > > > Can I directly drive the MOSFET from the NAND Gate? > > That depends on what the gate voltage specs of the FET are. Some FETs are > specifically called "logic level" FETs which work with 0-5V gate drive. > However, these tend to cost more and have a slightly higher on resistance > than their "normal" counterparts. Typical "normal" FETs require around 10V > gate drive to fully turn on. Check the data sheet. Beware, it is not uncommon for a "logic" FET to be spec'ed for on resistance with the standard 10 volt drive! A CMOS gate does not have enough drive for the FET capacitance if you want fast turn on and turn off times. Remember that the driver sees the gate to source capacitance (typically several hundred picofarads) in parallel with the Miller capacitance. The Miller capacitance can be in the thousands of picofarads when switching high voltages quickly. -- Rich > > ******************************************************************** > Olin Lathrop, embedded systems consultant in Littleton Massachusetts > (978) 742-9014, .....olinKILLspam@spam@embedinc.com, http://www.embedinc.com > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

part 1 1408 bytes content-type:text/plain; (decoded 7bit) Thanks Peter, But due to design constrains, i'll have to Power the CD4093 by 5 Volts. I've included a small piece of the schematic. So, the work around I've devised is to do it was to use a BC 547 NPN Transistor to drive the Mosfets.The Output from one of the gates of CD4093 will drive the base of the BC547 and the 547 will drive the gate of the MosFet. I've also incorporated the 10 Ohms Resistors you had told about. I'm using two Mosfets in parallel because the load is around 35 Amps and BUZ10s can only drive around 23 amps each. Actually, its used to Drive a Push-Pull Step Up Transformer. And I did'nt connect freewheeling diodes in Anti-Parallel to the Mosfets because BUZ10s are supposed have an internal Diode. Have I made any mistakes ?(I make em so often!) Please see the included schematic (MOS.gif) and suggest. Thnx, Jeethu Rao {Original Message removed} part 2 12240 bytes content-type:image/gif; (decode) part 3 136 bytes -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

See my thoughts below. -- Rich Jeethu Rao wrote: {Quote hidden}> > Thanks Peter, > > But due to design constrains, i'll have to Power the CD4093 by 5 Volts. > I've included a small piece of the schematic. So, the work around I've > devised is to > do it was to use a BC 547 NPN Transistor to drive the Mosfets.The Output > from one of the > gates of CD4093 will drive the base of the BC547 and the 547 will drive the > gate of the > MosFet. It looks like that you are using the BC 547 as an emitter follower. When you do this you lose about 0.7 volts of the drive needed to turn on the FET. Even a logic FET needs as much gate-source voltage as you can give it (up to 12 volts) to get the on resistance low. How fast are you pulsing the FET? The 10K emitter resistor will be very slow discharging the capacitances of the FET. I would guess that it could take as much as 100us to turn the FET off. I estimated this by using the time constant of 5000pf (for the FET) and the 10K resistor. > I've also incorporated the 10 Ohms Resistors you had told about. I'm > using two > Mosfets in parallel because the load is around 35 Amps and BUZ10s can only > drive around > 23 amps each. You must always be careful when paralleling devices like this because they do not share the currents equally. FET's are better than bipolar transistors doing this. Adding some resistors from the source of each FET to ground will help the sharing. I would guess that about 0.1 ohms would work at your 17 amps per FET. > Actually, its used to Drive a Push-Pull Step Up Transformer. Are you putting the transformer winding in the source of the FET? This is not good at high currents because the FET will have the entire gate to source across it causing very high heat dissipation -- about 100 watts per FET(!). In fact, unless you are using a negative power supply for the transformer there will be almost no drive to the coil at all because of the gate to source threshold voltage. > And I did'nt > connect freewheeling diodes in Anti-Parallel to the Mosfets because BUZ10s > are supposed > have an internal Diode. The transformer winding should be in the drain circuit. Unfortunately, the internal diodes are then in the wrong place in the circuit to quench the back EMF of the coil. The back EMF goes above the power supply voltage, not below ground. You do need to add the freewheeling diodes from the FET drain to the power supply. > > Have I made any mistakes ?(I make em so often!) > > Please see the included schematic (MOS.gif) and suggest. > > Thnx, > > Jeethu Rao > > {Original Message removed}