
Home  Forums  Rules  Articles  diyAudio Store  Gallery  Wiki  Blogs  Register  Donations  FAQ  Calendar  Search  Today's Posts  Mark Forums Read  Search 
Solid State Talk all about solid state amplification. 

Please consider donating to help us continue to serve you.
Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving 

Thread Tools  Search this Thread 
16th December 2007, 08:55 AM  #1 
diyAudio Member

Lateral MOSFET, How do I calculate power output
Hello's
Wondering if I can gain some insight on either rules of thumb calculations or if there are some pretty straight forward ways of calculating the power output of Lateral MOSFET based amplifiers. I realize that there are different losses at different load impedances. Typically it is assumed that the power output can be doubled by simply halving the load impedance. This is untrue however, no? I guess my question is.. Based on transformer secondary voltage, what kind of power should I expect to see in the real world at either 8,4 or 2 Ohm? Are these powers affected by more or less paralleled output devices? Typically, is efficiency lost with more output parallels? Appreciate any clarification Scott 
16th December 2007, 10:32 AM  #2 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Hi,
start with the power capability of the output stage devices. 1pair of 100W FET devices can produce 60W into a severe load, or 100W into a moderate load. BJTs tend to give a little less (~80%) due to derating for secondary breakdown. Let's use 2pair of 100W L FETs. and set the target at 100W into a severe load. That's 40Vpk and 5Apk into an 8r0 resistive load and upto 14Apk into a highly reactive 8ohm load. To get 40Vpk into your load you will need supply rails that hold up above 45Vdc when heavily loaded. You can start with +50Vdc and this may just give 100W into 8r0 but could be just short of target power. A 35+35Vac 160VA (mono) or 300VA (stereo) transformer will achieve close to target. The severe load capability (a real speaker) is a worse case than driving half the load resistance you are designing for. This implies that your amp should be able to deliver at least 180% of it's maximum power into a 4r0 load, but not continuously, since the heatsink will heat up quite rapidly. This 180% power into 4ohms cannot be achieved with the transformers mentioned earlier. I find that putting in a larger transformer than required for the design helps with extending the bass response. I would probably fit a 250VA 35+35Vac for a 100W into 8ohms knowing that this should be able to give about 180W into 4r0 on test. But keep in mind this is a design for driving a severe 8ohm load, not a 4ohm amplifier.
__________________
regards Andrew T. Sent from my desktop computer using a keyboard 
18th December 2007, 12:43 AM  #3 
diyAudio Member
Join Date: Dec 2006

also, you have a higher bias voltage for MOSFETS, which means your drivers have to swing another 310V for full output close to railtorail. this means you actually need a higher rail voltage for at least the VAS and driver stage, so for 100W output, you will want at least +/ 70v for the VAS/drivers. to simplify things, you can make this your output rail voltage as well, but your transistors need to be 150V devices.
__________________
Vintage Audio and ProAudio repair ampz(removethis)@sohonet.net spammer trap: spammers must die 
18th December 2007, 12:55 AM  #4 
diyAudio Member
Join Date: Jan 2006
Location: Sydney

I go through a bit of a calculation on the webpage describing my lateral MOSFET amp, at http://www.littlefishbicycles.com/poweramp/index.html.
I derived a simple formula to give Rth(max) for the heatsink, but you can easily solve for power, given Rth etc. To get the dissipated power, I found the easiest way was just to use LTspice. I designed the amp with LTspice, and simply measured the power dissipation in the output transistors. 
18th December 2007, 07:54 AM  #5 
diyAudio Member

Very nice! Thank You
The designs I use are based on Randy Slone's work. I will be using the same Hitachi output devices. I see you are in the process of making an antithump circuit. Have you considered a circuit that controls the mains power based on an input signals presence? THX again 
Thread Tools  Search this Thread 


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
10 Watt Single End, JFET input  Lateral NMOSFET output  lineup  Solid State  86  4th April 2015 04:54 PM 
Lateral Power MOSFET collection, PDF  lineup  Solid State  1  28th August 2008 05:29 PM 
hitachi lateral power mosfet in pspice  JBnl  Solid State  2  12th March 2005 05:49 PM 
New To Site?  Need Help? 