bias current
i forgot,if you put 330 ohms as 220 for r20 and r22 ,you will need always 5 volts across the resistors
so i=u/r : i=4/330 i=12 ma (not 6 ma as you said )
in fact,it s special to understand an all-n-channel mosfet amp
i ve build many amp but it was all the time complementary devices.
the bias stage is more easy to understand.
i will post picture of my work
i ve checked why it's barely impossible to run a power amp with igbt
even if the control is the same,i have read igbt is unstable (thermal instability ),it need more stage to stabilize temp and quiescent current
see you
i forgot,if you put 330 ohms as 220 for r20 and r22 ,you will need always 5 volts across the resistors
so i=u/r : i=4/330 i=12 ma (not 6 ma as you said )
in fact,it s special to understand an all-n-channel mosfet amp
i ve build many amp but it was all the time complementary devices.
the bias stage is more easy to understand.
i will post picture of my work
i ve checked why it's barely impossible to run a power amp with igbt
even if the control is the same,i have read igbt is unstable (thermal instability ),it need more stage to stabilize temp and quiescent current
see you
Sorry Thierry,
English problem. I meant 6 mA less. The circuit normally runs 18 mA so 18 mA - 6 mA = 12 mA.
I did not sim my amp, but I made some measurments when it was finished. The amp is very good in terms of noise, distortion and square wave response. I posted square wave response pictures for 1khz and 10khz here; http://www.diyaudio.com/forums/showthread.php?postid=530814#post530814
Your amp will still work with the changes you made of course and it should still be good.
Cheers
English problem. I meant 6 mA less. The circuit normally runs 18 mA so 18 mA - 6 mA = 12 mA.
I did not sim my amp, but I made some measurments when it was finished. The amp is very good in terms of noise, distortion and square wave response. I posted square wave response pictures for 1khz and 10khz here; http://www.diyaudio.com/forums/showthread.php?postid=530814#post530814
Your amp will still work with the changes you made of course and it should still be good.
Cheers
Re[03]: Power Supply and Vceo Calculations
Hi Quasi,
Thanks for your reply.
I had a hunch there was specific reasons for your PSU layout. I will likely hard wire my PSU even though the caps are PC mount. Easier and more flexible to fine tune if need be. I intend to use the PSU design www.tnt-audio.com as a reference base for my PSU. The major difference will be altering the:
http://www.tnt-audio.com/clinica/ssps3_e.html
to work with a centre tap transformer. I may see what effect small caps across the bridge rectifier segments may or may not provide. Like you my preference is for one PSU per amplifier. If the transformer VA rating is suitable for two or more amps to be driven by the transformer that I will do so.
One element I like to add is some status LEDs. One LED for each side of the PSU rail outputand again one LED for each side of the amp rail after the fuse. Do you or anyone else know if the LED and related voltage drop resister will have any negative impact to the amp performance by so doing?
With respect to the voltage measurement points I asked about I really was not implying you do so and take more of your time on my account. I posted the request as you had suggested measuring something by others that may have the cover off their amp. In essence I was trying to piggy nack you measurement request.
As FYI, remember my ask for a PCB version spaced for TO-220 devices? Cancel that request. Basically I have decided any TO-220 devices I use is better off with the extra space between. In small manner this makes up for the smaller die/device footprint rahter than concentrating the heat into a smaller contact area.
BTW, who is the cat? Yours, friend or pic you found and like. I like the pic and personality in pic.
Regards,
John L. Males
Willowdale, Ontario
Canada
19 January 2005 10:47
Hi Quasi,
Thanks for your reply.
I had a hunch there was specific reasons for your PSU layout. I will likely hard wire my PSU even though the caps are PC mount. Easier and more flexible to fine tune if need be. I intend to use the PSU design www.tnt-audio.com as a reference base for my PSU. The major difference will be altering the:
http://www.tnt-audio.com/clinica/ssps3_e.html
to work with a centre tap transformer. I may see what effect small caps across the bridge rectifier segments may or may not provide. Like you my preference is for one PSU per amplifier. If the transformer VA rating is suitable for two or more amps to be driven by the transformer that I will do so.
One element I like to add is some status LEDs. One LED for each side of the PSU rail outputand again one LED for each side of the amp rail after the fuse. Do you or anyone else know if the LED and related voltage drop resister will have any negative impact to the amp performance by so doing?
With respect to the voltage measurement points I asked about I really was not implying you do so and take more of your time on my account. I posted the request as you had suggested measuring something by others that may have the cover off their amp. In essence I was trying to piggy nack you measurement request.
As FYI, remember my ask for a PCB version spaced for TO-220 devices? Cancel that request. Basically I have decided any TO-220 devices I use is better off with the extra space between. In small manner this makes up for the smaller die/device footprint rahter than concentrating the heat into a smaller contact area.
BTW, who is the cat? Yours, friend or pic you found and like. I like the pic and personality in pic.
Regards,
John L. Males
Willowdale, Ontario
Canada
19 January 2005 10:47
Re: bias current
It depends upon your circuit topology..not on the IGBT's
Adequate Thermal Compensation
Active Gate Turn-OFF
Topology used
Dont drive the IGBT with circuits which are designated to drive bipolars by simply increasing the biasing voltage to their threshold voltages, unacceptable rail loss would occur with lots of instabilities.
K a n w a r
thierry said:
It depends upon your circuit topology..not on the IGBT's
Adequate Thermal Compensation
Active Gate Turn-OFF
Topology used
Dont drive the IGBT with circuits which are designated to drive bipolars by simply increasing the biasing voltage to their threshold voltages, unacceptable rail loss would occur with lots of instabilities.
K a n w a r
bias current
hi keypunch,
all the mosfet design amp can work with igbt with little changes.
elektor had design hexfet 60,with irf 540,9540
few month later they printed an issue with the same drawing and topology with complementary igbt,it s working well
but people who tried this way send complain to the designer
offset of the output voltage unstable ( quiescent current in the same way )
that's why i tried to run the amp with igbt (it s working well ) but unstable.
i ve read it 's hard to stabilze igbt in power amp,devices must have low gain (Id vs Vgs ).
the igbt i ve used are big one (top3 55 amp 500 v 230 w,q=120 c )
transconductance is too important.
irg4pc50ud ( IR)
hi keypunch,
all the mosfet design amp can work with igbt with little changes.
elektor had design hexfet 60,with irf 540,9540
few month later they printed an issue with the same drawing and topology with complementary igbt,it s working well
but people who tried this way send complain to the designer
offset of the output voltage unstable ( quiescent current in the same way )
that's why i tried to run the amp with igbt (it s working well ) but unstable.
i ve read it 's hard to stabilze igbt in power amp,devices must have low gain (Id vs Vgs ).
the igbt i ve used are big one (top3 55 amp 500 v 230 w,q=120 c )
transconductance is too important.
irg4pc50ud ( IR)
igbt instead of mosfet
sorry workhorse,last was for you...
i forgot to tell that output balancing resistors in my case are only 0.2 ohms
i think with 0.47 ohms per branch will improve thermal and quiescent stability.
but i didnt try,anyway the mosfet i'm using are quite good.
( fqa 44n30: 43 A,300 v ,310 w,0.069 mohms, low gate charge)
the amp seems to work correctly but i will get some real measurment
i've take a look to irfp450 datasheet,transconductance have a curve at low current (0 to 4 A )
i think high voltage mosfet aren't a good choice for hifi application
they need a important bias cuurent to avoid crossing distortion.
you can get a look a audio device,the curve a the beginning is very low,and that's why they used high quiescent current (100 mA minimum per fet,it's depend of the transconductance )
sorry workhorse,last was for you...
i forgot to tell that output balancing resistors in my case are only 0.2 ohms
i think with 0.47 ohms per branch will improve thermal and quiescent stability.
but i didnt try,anyway the mosfet i'm using are quite good.
( fqa 44n30: 43 A,300 v ,310 w,0.069 mohms, low gate charge)
the amp seems to work correctly but i will get some real measurment
i've take a look to irfp450 datasheet,transconductance have a curve at low current (0 to 4 A )
i think high voltage mosfet aren't a good choice for hifi application
they need a important bias cuurent to avoid crossing distortion.
you can get a look a audio device,the curve a the beginning is very low,and that's why they used high quiescent current (100 mA minimum per fet,it's depend of the transconductance )
Re: Re[03]: Power Supply and Vceo Calculations
The LEDs will not have any effect on the amp. These will draw a constant DC current of around 10mA each and so there should ne be any audible noise. If you aree worried about any possible noise put a 0.1uF capacitor as clos as possible to LED feed and to the noisy ground (remember the discussion about noisy v quiet ground?)
It's our family cat and the photo is of her taken as a kitten. Further information about the cat can be gained here;
http://www.diyaudio.com/forums/showthread.php?postid=820538#post820538
Cheers
keypunch said:
The LEDs will not have any effect on the amp. These will draw a constant DC current of around 10mA each and so there should ne be any audible noise. If you aree worried about any possible noise put a 0.1uF capacitor as clos as possible to LED feed and to the noisy ground (remember the discussion about noisy v quiet ground?)
It's our family cat and the photo is of her taken as a kitten. Further information about the cat can be gained here;
http://www.diyaudio.com/forums/showthread.php?postid=820538#post820538
Cheers
Hi Quasi,
I think I understand your logic for adopting the PSU layout you have posted. I don't agree.
The somewhat pulsing currents passing between the caps and the rectifier and to the centre taps are full of high frequency components that induce very high voltage drops on the short PCB track from the pair of CT to MAIN GND.
I think it would be better to swap CT with MAIN GND and this then isolates the PSU common connection (MAIN GND) from the rectifier and it's pulsing currents.
Using a similar argument the PSU output connections (AMP POS & NEG) should also be moved away from the main charging route from the rectifier. Yes, we do agree that the bypass should be located close to the output. I would also ceramic bypass the rectifer + to - direct across the rectifier terminals to reduce at source the peak impulses.
I think I understand your logic for adopting the PSU layout you have posted. I don't agree.
The somewhat pulsing currents passing between the caps and the rectifier and to the centre taps are full of high frequency components that induce very high voltage drops on the short PCB track from the pair of CT to MAIN GND.
I think it would be better to swap CT with MAIN GND and this then isolates the PSU common connection (MAIN GND) from the rectifier and it's pulsing currents.
Using a similar argument the PSU output connections (AMP POS & NEG) should also be moved away from the main charging route from the rectifier. Yes, we do agree that the bypass should be located close to the output. I would also ceramic bypass the rectifer + to - direct across the rectifier terminals to reduce at source the peak impulses.
Hi AndrewT,
Hmm my PSU layout seems to differ from my reasoniing as well. Funny thing is when you look at the photo I posted of my completed amp I actually have the CT of the transformer secondary going to where you say it should; http://www.diyaudio.com/forums/attachment.php?s=&postid=586005&stamp=1109588438 so I think I labelled this in a tired moment.
Yes the +ve and -ve amp should come of seperate short tracks leading away from the caps (just as they do from the fuses on the amp PCB). I'll re-do the layout to do this.
As far as the ceramics on th rectifier goes, yes this could be done easily, but the rectifier is very slow, wouldn't hurt though.
Cheers
Hmm my PSU layout seems to differ from my reasoniing as well. Funny thing is when you look at the photo I posted of my completed amp I actually have the CT of the transformer secondary going to where you say it should; http://www.diyaudio.com/forums/attachment.php?s=&postid=586005&stamp=1109588438 so I think I labelled this in a tired moment.
Yes the +ve and -ve amp should come of seperate short tracks leading away from the caps (just as they do from the fuses on the amp PCB). I'll re-do the layout to do this.
As far as the ceramics on th rectifier goes, yes this could be done easily, but the rectifier is very slow, wouldn't hurt though.
Cheers
This PCB of the Power supply corrects an error and includes AndrewT's suggestions regarding the capacitor charging paths.
Under the board a 200v ceramic capacitor of around 47 - 100 nF can be soldered across the rectifiers +ve and -ve terminals. In addition 100v ceramics can be soldered across each effective diode in the bridge if desired. I havn't bothered but it's up to the individual.
Cheers
Under the board a 200v ceramic capacitor of around 47 - 100 nF can be soldered across the rectifiers +ve and -ve terminals. In addition 100v ceramics can be soldered across each effective diode in the bridge if desired. I havn't bothered but it's up to the individual.
Cheers
Hi Quasi,
that revised layout seems better.
to all,
is there any benefit:-
by relocating the bypass caps closer to the outputs?
by relocating the outputs and bypass caps at the extreme far end of the PSU PCB?
making the discharge resistor values higher and drawing less current from the PSU?
that revised layout seems better.
to all,
is there any benefit:-
by relocating the bypass caps closer to the outputs?
by relocating the outputs and bypass caps at the extreme far end of the PSU PCB?
making the discharge resistor values higher and drawing less current from the PSU?
Re[03]: LEDs
Hi Quasi,
Thanks for your reply about the LEDs. For the LED post Fuse I would connect it just after the fuse on amp module near the bypass caps so I believe the existing 0.1uF bypass cap will accomodate. For the pre fuse LEDs I will place near the PSU 0.1uF bypass cap. As these LEDs will need to be chassis mounted their wires could induce some artifacts over and above the LED itself so your suggestion of placing a bypass cap makes good sense.
I asked the question as I had no idea if "innocent" LEDs could introduce some artifact I had no idea about. I know red LEDs are at times used in amp circuits as a strickly a diode, not for status, due to the specific voltage rating red LEDs have. In that case I had no clue if other circuit elements directly or indirectly addressed such possible artifacts.
My my, your wife gave kitty a long name. I will not ask if you have to use a long name when you use your wife's name nor if such long names has your wife decided on for your kids
. I had one cat who passed away a year ago October past. Her name was Chicago "White Sox". She was mostly black except for most of nose and the white feet with part of leg from foot all white. Ergo the name. That was her name when I adopted her. I wold often use "Miss" ahead of her full name, of "Miss White Sox". She also had a nickname of "Chico".
Regards,
John L. Males
Willowdale, Ontario
Canada
20 January 2006 12:15
Hi Quasi,
Thanks for your reply about the LEDs. For the LED post Fuse I would connect it just after the fuse on amp module near the bypass caps so I believe the existing 0.1uF bypass cap will accomodate. For the pre fuse LEDs I will place near the PSU 0.1uF bypass cap. As these LEDs will need to be chassis mounted their wires could induce some artifacts over and above the LED itself so your suggestion of placing a bypass cap makes good sense.
I asked the question as I had no idea if "innocent" LEDs could introduce some artifact I had no idea about. I know red LEDs are at times used in amp circuits as a strickly a diode, not for status, due to the specific voltage rating red LEDs have. In that case I had no clue if other circuit elements directly or indirectly addressed such possible artifacts.
My my, your wife gave kitty a long name. I will not ask if you have to use a long name when you use your wife's name nor if such long names has your wife decided on for your kids
. I had one cat who passed away a year ago October past. Her name was Chicago "White Sox". She was mostly black except for most of nose and the white feet with part of leg from foot all white. Ergo the name. That was her name when I adopted her. I wold often use "Miss" ahead of her full name, of "Miss White Sox". She also had a nickname of "Chico".Regards,
John L. Males
Willowdale, Ontario
Canada
20 January 2006 12:15
For Zeonrider - Quasi amp using 2N3773
From other thread
This is the schematic modified to take high power bi-polar NPN transistors. Over the next few weeks I'll draft a PCB and post it in a new thread.
The PCB will limited to TO-3 devices because many DIY's seem to have plenty of these laying around.
Cheers
From other thread
zeonrider said:
This is the schematic modified to take high power bi-polar NPN transistors. Over the next few weeks I'll draft a PCB and post it in a new thread.
The PCB will limited to TO-3 devices because many DIY's seem to have plenty of these laying around.
Cheers
LEDS
Hi John,
Sometimes LEDs are used in constant current sources in amplifiers.
The LED forward voltage (approx 1.8v for red LEDs) is used as the reference voltage for ccs transistor.
Cheers
All other members of my family are limited to 2 or 3 names and we generally only use the first name (except when more expressive names are required, like today if the kids don't get off their computer now, ......I mean right now!!!!).
Cheers again
Hi John,
Sometimes LEDs are used in constant current sources in amplifiers.
The LED forward voltage (approx 1.8v for red LEDs) is used as the reference voltage for ccs transistor.
Cheers
All other members of my family are limited to 2 or 3 names and we generally only use the first name (except when more expressive names are required, like today if the kids don't get off their computer now, ......I mean right now!!!!).
Cheers again
Soma questions about Quazi's amplifier
Hello!
I could buy IRFP260 for a very good price, so I plan to use them in Quazi's amplifier. The problem is, that the Gate-Source capacity of theese FETs is about 5nF.
If I paralell theese devices, the gate-capacitance will be so much, that the previous stage can't drive properly at high frequency. How can I solve this problem? If I "insert" a small current-amplifier stage just before the MOSFETs, will that be OK? Maybe, an emitter-follower?
Just for comparison, if it's required for my question:
IRFP260: Pd=280W, Cg=5.2nF, Id=46A
IRFP240: Pd=180W, Cg=1.3nF, Id=20A
IRFP450B: Pd=205W, Cg=2.9nF, Id=14A
And another Q:
What power ratings should be the "small" resistors? (not the ones series with the Drain, or Source)
I see only 0.6W types on theese pictures:
http://www.diyaudio.com/forums/attachment.php?s=&postid=511828&stamp=1100430955
http://www.diyaudio.com/forums/attachment.php?s=&postid=520811&stamp=1101456107
Can I use 1206 SMD resistors?
And what about the TO92 case transistors? Can I replace them with their SMD-version? (I hate drilling PCBs.... )
Is it possible, to implement a short-circuit-protection, without too much pain ?
Thanks in advance!
Hello!
I could buy IRFP260 for a very good price, so I plan to use them in Quazi's amplifier. The problem is, that the Gate-Source capacity of theese FETs is about 5nF.
If I paralell theese devices, the gate-capacitance will be so much, that the previous stage can't drive properly at high frequency. How can I solve this problem? If I "insert" a small current-amplifier stage just before the MOSFETs, will that be OK? Maybe, an emitter-follower?
Just for comparison, if it's required for my question:
IRFP260: Pd=280W, Cg=5.2nF, Id=46A
IRFP240: Pd=180W, Cg=1.3nF, Id=20A
IRFP450B: Pd=205W, Cg=2.9nF, Id=14A
And another Q:
What power ratings should be the "small" resistors? (not the ones series with the Drain, or Source)
I see only 0.6W types on theese pictures:
http://www.diyaudio.com/forums/attachment.php?s=&postid=511828&stamp=1100430955
http://www.diyaudio.com/forums/attachment.php?s=&postid=520811&stamp=1101456107
Can I use 1206 SMD resistors?
And what about the TO92 case transistors? Can I replace them with their SMD-version? (I hate drilling PCBs....
)Is it possible, to implement a short-circuit-protection, without too much pain ?
Thanks in advance!
Hi Danko,
Your problem could well end up being heat. Let me explain.
Whatever you do to drive the output stage harder will mean a lower impedance on the gates and this means more current. I.e. the current through the pre-output stage in the amp is normally around 18mA. With a rail to rail voltage of 150v this equals 2.7 watts, most of which is dissipated by the two transistors.
You could easily increase the current here (lower the impedance) by reducing the value of R20 and R22. Lets say you used 100 ohms. The current will now be about 40mA and the heat will be 6 watts, so a large heatsink is required plus T9 & T10 will need to be upgraded.
Adding another stage will not get away from this problem, you would just shift the heat somewhere else. In any case the second stage has plenty of gain and it could control a few hundred milliamps in the pre-drive stage so you do not need the extra stage.
If I were you I would just build it using the high power FETs and examine the waveform. I think you will be pleasantly surprised.
With respect to the resistors, 0.6 watt types are used throughout the amp except for the output source resistors. To check the actual dissipation for each resistor use ohms law calculations. The main current flows are shown here; http://www.diyaudio.com/forums/showthread.php?postid=818662#post818662 ; post #325.
You can replace the TO92 transistors with SMD types, if you can find some with the ratings needed.
It is fairly easy to implement short cct protection. My idea is an optocoupler where the LED is used across an output source resistor and the optocoupler transistor is used to control protection, i.e. second stage shutdown, load disconnection, amp-off etc.
Cheers
Your problem could well end up being heat. Let me explain.
Whatever you do to drive the output stage harder will mean a lower impedance on the gates and this means more current. I.e. the current through the pre-output stage in the amp is normally around 18mA. With a rail to rail voltage of 150v this equals 2.7 watts, most of which is dissipated by the two transistors.
You could easily increase the current here (lower the impedance) by reducing the value of R20 and R22. Lets say you used 100 ohms. The current will now be about 40mA and the heat will be 6 watts, so a large heatsink is required plus T9 & T10 will need to be upgraded.
Adding another stage will not get away from this problem, you would just shift the heat somewhere else. In any case the second stage has plenty of gain and it could control a few hundred milliamps in the pre-drive stage so you do not need the extra stage.
If I were you I would just build it using the high power FETs and examine the waveform. I think you will be pleasantly surprised.
With respect to the resistors, 0.6 watt types are used throughout the amp except for the output source resistors. To check the actual dissipation for each resistor use ohms law calculations. The main current flows are shown here; http://www.diyaudio.com/forums/showthread.php?postid=818662#post818662 ; post #325.
You can replace the TO92 transistors with SMD types, if you can find some with the ratings needed.
It is fairly easy to implement short cct protection. My idea is an optocoupler where the LED is used across an output source resistor and the optocoupler transistor is used to control protection, i.e. second stage shutdown, load disconnection, amp-off etc.
Cheers
Thank you Quazi, for the fast reply!
I gonna try this! Unfortunately tomorrow will start the spring-semester at the high school, so I will have almost no time for this
I will calculate the dissipation of the resistor's, thanks for the picture! I'm just busy now, with PCB manufacturing...
I will think about using an optocoupler as current-limit-detector. But, to tell the truth I have not seen schematics, using an optocoupler at this place.
Thanks, Quazi!
I gonna try this! Unfortunately tomorrow will start the spring-semester at the high school, so I will have almost no time for this
I will calculate the dissipation of the resistor's, thanks for the picture! I'm just busy now, with PCB manufacturing...
I will think about using an optocoupler as current-limit-detector. But, to tell the truth I have not seen schematics, using an optocoupler at this place.
Thanks, Quazi!
Hi!
I calculated the dissipation of all the resistors. I think, the 1206 SMD-components will be OK.
Here's the PCB layout:
http://sziget.mine.nu/~danko/aramkor/testbed/15/gkrellShoot_02-05-06_171834.png
The blue line is an insulated wire. The copper is on the component side, so I can easily measure everything, and I can experiment with this stuff.
Here's the PCB:
http://sziget.mine.nu/~danko/aramkor/testbed/15/dscn8489.jpg
I calculated the dissipation of all the resistors. I think, the 1206 SMD-components will be OK.
Here's the PCB layout:
http://sziget.mine.nu/~danko/aramkor/testbed/15/gkrellShoot_02-05-06_171834.png
The blue line is an insulated wire. The copper is on the component side, so I can easily measure everything, and I can experiment with this stuff.
Here's the PCB:
http://sziget.mine.nu/~danko/aramkor/testbed/15/dscn8489.jpg
Hi Danko,
Nice PCB you have designed. I see you go for the curve traces and round not square pads.
I have a few questions based on your PCB that are parts related if I may. I am not questioning any part of your decision in the below questions. I am asking in part to learn some things and/or to expand on what I have read/researched.
1) I see the "+" symbol about many of the parts on the silk screen. What is the significance of the "+" on the silk screen?
2) I notice you seem to use two capacitors to do PSU bypassing - 1uF with a 100nF in parallel. May I ask why you added the 1uF in parallel to the 100nF/0.1uF capacitor?
3) For T1/T5 you chose to use MPSA42's rather than 2SC1845s. Basic reason for choice?
4) I see you use two 0R47 resistors in parallel for the output device source resistor per output device. I know quasi has a preference to using 0R47 for these resistors, but indicated 0R33 on the schematic based on many comments. Of course with parallel 0R47 the effect is using 0R237. Why did you choose to use two parallel resistors for source, as opposed to using a single 0R22 resistor? Also, what is your reasoning to use 0R22 vs many who feel 0R33 is lower value to use in such suitations?
5) You have chosen to use 220uF for C11/C13, rather than 330uf? May I ask why? I am inclinded to use 680uF or 1000uF instead.
6) YOu have chosen to use two 1uF capacitors in parallel for C1? Your reason to parallel tow 1uF rather than use a 2uF (or 2.2uF)? Reason you perfer 2uF rather than 1uF?
As FYI I am likley to use APT2045BN's for at least the initial versions I make as they seem to have a nice balance of PD vs Ciss. I have been trying to select output decices with the best Ciss vs PD I can. For this reason I have some IRFP450 so I can benmark the stock design to using devices like the APT4025BN, and as happens mostly TO-220 devices. The latter of which have lower Ciss, but lower power output handling which for some casues is more than enough for the application needs. Like you I look at the overall Ciss loading of the number of parallel devices. I have been using the 3xCiss of the IRFP450 as my benchmark total of the output devices as well as a few other key parameters of a MOSFET to determine what will be on the shortlist to test and see if there is any improvement.
I do not have anywhere the knowledge quasi and others about here have. I do like to dabble in some inventive theory of my own for better or worse. I am patient sort, so I move along at tugboat speed The theory first of my own inventiveness and asking questions is the time consuming part. Then test out what I have learned and wish to validate or invalidate from my theory .
I am at the research/determine/find some parts still before I can do the test out part. The parts of focus for are not output devices, and for all intents are transistors. One major issue is sourcing some of the transistors used and/or alternates. I have already gathered a few different MOSFET types for initial comparison.
Danko, though the IRFP260 has some good Pd ability, but the T(off) time is really slow compared to T(on), about a 4/1 ratio that may lead to cross conduction issues in the upper audio frequency range. Most MOSFETs seem to have more of a 2/1 ratio of T(on)/T(off). The IRFP450 also has this 4/1 ratio Also the rise and fall times of the IRFP260 are about twice of the average MOSFET used for audio purposes, which may or may not be of concern for your intended use of the devices for this amp. The IRFP450 on other hand has rise and fall times about average for most MOSFETs which is twice as fast as the IRFP260. Just some thoughts to think about, not a negative at all to your choice or application/price/performance elements you have intended.
Danko, all the best to your chosen design varient. It will be interesting to hear what your thoughts and experiences are once you have the amp working.
Regards,
John L. Males
Willowdale, Ontario
Canada
06 February 2006 20:43
Nice PCB you have designed. I see you go for the curve traces and round not square pads.
I have a few questions based on your PCB that are parts related if I may. I am not questioning any part of your decision in the below questions. I am asking in part to learn some things and/or to expand on what I have read/researched.
1) I see the "+" symbol about many of the parts on the silk screen. What is the significance of the "+" on the silk screen?
2) I notice you seem to use two capacitors to do PSU bypassing - 1uF with a 100nF in parallel. May I ask why you added the 1uF in parallel to the 100nF/0.1uF capacitor?
3) For T1/T5 you chose to use MPSA42's rather than 2SC1845s. Basic reason for choice?
4) I see you use two 0R47 resistors in parallel for the output device source resistor per output device. I know quasi has a preference to using 0R47 for these resistors, but indicated 0R33 on the schematic based on many comments. Of course with parallel 0R47 the effect is using 0R237. Why did you choose to use two parallel resistors for source, as opposed to using a single 0R22 resistor? Also, what is your reasoning to use 0R22 vs many who feel 0R33 is lower value to use in such suitations?
5) You have chosen to use 220uF for C11/C13, rather than 330uf? May I ask why? I am inclinded to use 680uF or 1000uF instead.
6) YOu have chosen to use two 1uF capacitors in parallel for C1? Your reason to parallel tow 1uF rather than use a 2uF (or 2.2uF)? Reason you perfer 2uF rather than 1uF?
As FYI I am likley to use APT2045BN's for at least the initial versions I make as they seem to have a nice balance of PD vs Ciss. I have been trying to select output decices with the best Ciss vs PD I can. For this reason I have some IRFP450 so I can benmark the stock design to using devices like the APT4025BN, and as happens mostly TO-220 devices. The latter of which have lower Ciss, but lower power output handling which for some casues is more than enough for the application needs. Like you I look at the overall Ciss loading of the number of parallel devices. I have been using the 3xCiss of the IRFP450 as my benchmark total of the output devices as well as a few other key parameters of a MOSFET to determine what will be on the shortlist to test and see if there is any improvement.
I do not have anywhere the knowledge quasi and others about here have. I do like to dabble in some inventive theory of my own
for better or worse. I am patient sort, so I move along at tugboat speed The theory first of my own inventiveness and asking questions is the time consuming part. Then test out what I have learned and wish to validate or invalidate from my theory .I am at the research/determine/find some parts still before I can do the test out part. The parts of focus for are not output devices, and for all intents are transistors. One major issue is sourcing some of the transistors used and/or alternates. I have already gathered a few different MOSFET types for initial comparison.
Danko, though the IRFP260 has some good Pd ability, but the T(off) time is really slow compared to T(on), about a 4/1 ratio that may lead to cross conduction issues in the upper audio frequency range. Most MOSFETs seem to have more of a 2/1 ratio of T(on)/T(off). The IRFP450 also has this 4/1 ratio
Also the rise and fall times of the IRFP260 are about twice of the average MOSFET used for audio purposes, which may or may not be of concern for your intended use of the devices for this amp. The IRFP450 on other hand has rise and fall times about average for most MOSFETs which is twice as fast as the IRFP260. Just some thoughts to think about, not a negative at all to your choice or application/price/performance elements you have intended. Danko, all the best to your chosen design varient. It will be interesting to hear what your thoughts and experiences are once you have the amp working.
Regards,
John L. Males
Willowdale, Ontario
Canada
06 February 2006 20:43