Lars
The following are some unanswered questions from my eirlear posts
which I presume you overlooked .
I the Zeta I have made , on turn ON two Irfp250 one from each rail failed
ie ( the drain source are showing short ) , I disconnected these and the
amp is working fine , I do not understand WHY two mosfets failed , I had
used closely matched mosfets two in each rail out of which one from
each rail failed .
How do we decide what is the optimum power requirement of T14 as
per the wattage of the ZETA version we decide to make ,until now you
have said that it is on us if we use Irf9640 or Irfp9240 and as per the
dissipation of these devices we select the drain resistor of 14 anywhere
between 10 ohms and 47ohms .
I also want to make the 900w version of the zeta soon , but I am a little
scared regarding the use of so many parallel mosfets , could anybody
give a feedback if he has made the same . Lars when are YOU going to
make a prototype of the PA version of the ZETA with 6 to 10 pairs of
Irfp250n devices .
With regards
Rajeev
The following are some unanswered questions from my eirlear posts
which I presume you overlooked .
I the Zeta I have made , on turn ON two Irfp250 one from each rail failed
ie ( the drain source are showing short ) , I disconnected these and the
amp is working fine , I do not understand WHY two mosfets failed , I had
used closely matched mosfets two in each rail out of which one from
each rail failed .
How do we decide what is the optimum power requirement of T14 as
per the wattage of the ZETA version we decide to make ,until now you
have said that it is on us if we use Irf9640 or Irfp9240 and as per the
dissipation of these devices we select the drain resistor of 14 anywhere
between 10 ohms and 47ohms .
I also want to make the 900w version of the zeta soon , but I am a little
scared regarding the use of so many parallel mosfets , could anybody
give a feedback if he has made the same . Lars when are YOU going to
make a prototype of the PA version of the ZETA with 6 to 10 pairs of
Irfp250n devices .
With regards
Rajeev
Re[02]: some unanswered questions from my eirlear posts
Rajeev,
I am not an engineer or know that much about amp design as yet. I have been standoffish for a variety of reasons about the Zeta as much as it has encouraging and possible endearing possibilities. In my quest to understand not just the Zeta, but to determine what output devices are best for my needs (low wattage) I have managed to create a set of formulas to do calculations with on driver power ability given watts, operating temperature and impedance. These calculations are based on this thread, and a few others to round out the gaps. It has taken me about a week to sort out which formula, which are valid adn what sequence to use to do calculations that basically say for this impedance, this operating temperature, and a desired amp wattage which devices can be used from a device voltage and power standpoint and what would the rated device power handling be for those operating parameters.
I would be interest Rajeev when you have chance if you can let me know what devices you with to consider, the overall wattage you wish for the amp, what operating temperature you are planning for for the output devices and what is the lowest impedance you are targeting for? I will do some math here and see if it agrees or differs from you expectations.
As to T14, that was to be my next challenge on how to calculate for T14. Some brief mention in this thread of very general nature, but not enough I suspect to make formulas with what I know thus far.
I was actually reading a Application note, even though much of it over my head to understand all the detail concerning paralleled MOSFET devices. I think this application note may offer you some insight to why you had two output drivers go poof.
Check out "Paralleling Of Power MOSFETs For Higher Power Output":
<http://www.irf.com/technical-info/appnotes/para.pdf>
and see if it offers you any insight to your paralleling output driver issues.
Hope some of this information my be of some help to you Rajeev.
Regards,
John L. Males
Willowdale, Ontario
Canada
22 December 2004 07:18
Rajeev,
I am not an engineer or know that much about amp design as yet. I have been standoffish for a variety of reasons about the Zeta as much as it has encouraging and possible endearing possibilities. In my quest to understand not just the Zeta, but to determine what output devices are best for my needs (low wattage) I have managed to create a set of formulas to do calculations with on driver power ability given watts, operating temperature and impedance. These calculations are based on this thread, and a few others to round out the gaps. It has taken me about a week to sort out which formula, which are valid adn what sequence to use to do calculations that basically say for this impedance, this operating temperature, and a desired amp wattage which devices can be used from a device voltage and power standpoint and what would the rated device power handling be for those operating parameters.
I would be interest Rajeev when you have chance if you can let me know what devices you with to consider, the overall wattage you wish for the amp, what operating temperature you are planning for for the output devices and what is the lowest impedance you are targeting for? I will do some math here and see if it agrees or differs from you expectations.
As to T14, that was to be my next challenge on how to calculate for T14. Some brief mention in this thread of very general nature, but not enough I suspect to make formulas with what I know thus far.
I was actually reading a Application note, even though much of it over my head to understand all the detail concerning paralleled MOSFET devices. I think this application note may offer you some insight to why you had two output drivers go poof.
Check out "Paralleling Of Power MOSFETs For Higher Power Output":
<http://www.irf.com/technical-info/appnotes/para.pdf>
and see if it offers you any insight to your paralleling output driver issues.
Hope some of this information my be of some help to you Rajeev.
Regards,
John L. Males
Willowdale, Ontario
Canada
22 December 2004 07:18
Re: Re[02]: some unanswered questions from my eirlear posts
Please notice that this document is about switching applications, not linear.keypunch said:
Re: Re: Re[02]: some unanswered questions from my eirlear posts
Peranders,
Thanks. As I indicated I am not a engineer. I am glad someone can read this application note or is very aware of it. Thanks again for your insight.
There is not even some points that could apply to audio as well?
Regards,
John L. Males
Willowdale, Ontario
Canada
22 December 2004 08:00
peranders said:
Peranders,
Thanks. As I indicated I am not a engineer. I am glad someone can read this application note or is very aware of it. Thanks again for your insight.
There is not even some points that could apply to audio as well?
Regards,
John L. Males
Willowdale, Ontario
Canada
22 December 2004 08:00
Rajeev: There could be a number of reasons why you had the set of MOSFET go 'poof'. Since you ask me, i guess you are assuming there is some systematic explanation why it could happen. Well there isn't. If your MOSFET's are OK, not damaged by hotspots from static charges (the biggest problem when using MOSFET) and firmly mounted on a heatsink, and maximum output power is not exceeded (about 100-200 W per output pair) , they will NEVER blow (at least from a theroretical point of view).
I can not from my side give you a theoretical explanation why your set blew. There are no flaws in the construction. So maybe it's a practical explanation, try one of the above.
About T14 the rule of thumb is this:
1..It's not very critical, but use common sense.
2..The more output devices (or more correctly gate capacitance in the output devices) you have to drive the lower the value of drain resistor. If you are making a 50 W amplifier using 2 IRF540N, you can use 2-300 Ohms, with IRFP250N (180 W power) you should use 47 Ohms, and with 10 parallelled IRFP250N, use 10 Ohms.
The lower value of resistor, the more power you need to dissipate in T14, and thus the bigger die and case size you should use. (As in TO247 / IRFP9240 is bigger than TO220 / IRF9640) In my own 180 W prototype i used an even smaller package the TO262 and 22 Ohms just to be sure there are a lot of safety margin. It worked fine!
I have built a few of these amplifiers by now, and they seem to work every time with none of the problems you guys had.
So i'm guessing there are quite a few readers of this thread who built the amplifier successfully, , if so please post your experiences.
All the best from
Lars
I can not from my side give you a theoretical explanation why your set blew. There are no flaws in the construction. So maybe it's a practical explanation, try one of the above.
About T14 the rule of thumb is this:
1..It's not very critical, but use common sense.
2..The more output devices (or more correctly gate capacitance in the output devices) you have to drive the lower the value of drain resistor. If you are making a 50 W amplifier using 2 IRF540N, you can use 2-300 Ohms, with IRFP250N (180 W power) you should use 47 Ohms, and with 10 parallelled IRFP250N, use 10 Ohms.
The lower value of resistor, the more power you need to dissipate in T14, and thus the bigger die and case size you should use. (As in TO247 / IRFP9240 is bigger than TO220 / IRF9640) In my own 180 W prototype i used an even smaller package the TO262 and 22 Ohms just to be sure there are a lot of safety margin. It worked fine!
I have built a few of these amplifiers by now, and they seem to work every time with none of the problems you guys had.
So i'm guessing there are quite a few readers of this thread who built the amplifier successfully, , if so please post your experiences.
All the best from
Lars
Lars ,
Thanks for the reply ,
Thanks for the project and all the help ,
When are YOU going to make a prototype of the PA version of the ZETA with 6 to 10 pairs of Irfp250 devices .
Regards
Rajeev
Thanks for the reply ,
Thanks for the project and all the help ,
When are YOU going to make a prototype of the PA version of the ZETA with 6 to 10 pairs of Irfp250 devices .
Regards
Rajeev
Member
Joined 2002
Any one want to buy a n-channel board with toroidal tranny with 2 x 15k caps @125v just need's 6 new output fet's.
Hi, Rajeev,
When your amp is cold (not hot yet) and the mosfets are broken, 9 of 10 times it is because excessive gate-source voltage. It can be a very short pulse during turnon, not necessarily continious DC. You can put 12V or 15V zener to protect your gates.
When your amp is cold (not hot yet) and the mosfets are broken, 9 of 10 times it is because excessive gate-source voltage. It can be a very short pulse during turnon, not necessarily continious DC. You can put 12V or 15V zener to protect your gates.
Lumanauw: It's already taken care of in the basic construction of Zeta. If capacinatces in the MOSFET tries to make spikes on the gate voltage above 10V the driver will go in saturation, and block the spike.
If the voltage amplifier want to send out excessive gate voltage the oposing driver will go into it's band-gap mode, and form a Zener diode, of 5-6V on top of the 8 V max. gate voltage, and so 'emergency limit' the gate voltage to 14-15V.
So a Zener diode is really not nescessary.
I am thinking the burnout could be caused by poor thermal coupling between transistor case and heat sink. A mica or silicone isolated case will not dissipate very much power.
🙂
If the voltage amplifier want to send out excessive gate voltage the oposing driver will go into it's band-gap mode, and form a Zener diode, of 5-6V on top of the 8 V max. gate voltage, and so 'emergency limit' the gate voltage to 14-15V.
So a Zener diode is really not nescessary.
I am thinking the burnout could be caused by poor thermal coupling between transistor case and heat sink. A mica or silicone isolated case will not dissipate very much power.
🙂
Hi, LC,
Thanks for the explenation.
I know there are things that is "hidden" in your design. Now you open one of it. 😀
Thanks for the explenation.
I know there are things that is "hidden" in your design. Now you open one of it. 😀
From the begining, LC suggest to directly attach the finals to thick anodized heatsinks.
If you see the photo of my amp again you will see that I have mounted the mosfets directly on solid alumimium chunks 8mm thick and then I have insolated these from the main heatsink hence the problem of heat transfer does not arise , One thing comes in my mind and that is I had kept the quessent current at 200ma and by chance if this was not shared properly by the mosfets this could be the reason of failure on turn on ... please let me have your comments ...
Also note that the amp had worked ok for a few days and after disconecting the defective mosfets the same is still working but now with only two op devices .
Also note that the amp had worked ok for a few days and after disconecting the defective mosfets the same is still working but now with only two op devices .
i have a question regarding the input transistors (both NPN and PNP) differential stage- how well do they have to be matched? Both in respect to each other and the complemetary stage?
of course better matching provides less offset e t c ...
/rickard
of course better matching provides less offset e t c ...
/rickard
Hi, Rajeev,
If you are sure thermal is not a problem, we have to find other causes that makes your FETs to break. We must trace all possibilities one by one. It breaks on turn-on?
Like LC said, Mosfets are quite indestructible, as long as max limits do not exceeded. They are more rugged than bipolars. About steady bias, I think it is not a problem. If you look at Passlabs, they bias mosfets up to 1-2.5A, and they are still working fine. Sharing bias? If you put Rsource or match the mosfets, they should work fine.
Spikes that destroy gates? LC has cover this up. So, what else? Excessive rail voltage during turn-on? Or grounding problem? Static charges? Are you grounding your heatsinks?
John Curl once write that he makes a mosfet amp, and it blows when the output are shorted. But I think this is quite different from your problem, because your's are at turn-on.
If you are sure thermal is not a problem, we have to find other causes that makes your FETs to break. We must trace all possibilities one by one. It breaks on turn-on?
Like LC said, Mosfets are quite indestructible, as long as max limits do not exceeded. They are more rugged than bipolars. About steady bias, I think it is not a problem. If you look at Passlabs, they bias mosfets up to 1-2.5A, and they are still working fine. Sharing bias? If you put Rsource or match the mosfets, they should work fine.
Spikes that destroy gates? LC has cover this up. So, what else? Excessive rail voltage during turn-on? Or grounding problem? Static charges? Are you grounding your heatsinks?
John Curl once write that he makes a mosfet amp, and it blows when the output are shorted. But I think this is quite different from your problem, because your's are at turn-on.
Statement
Hi lars,
Are you sure enough about the "Band Gap mode " formed by opposite Driver.
But still this is not specifically true in nature for the situation where the mosfets are parallelled in numbers therefore a Mismatch can occur and Secondly the Gate Spikes originating during turn -ON are relative to the driver Stage imbalance and ofcourse non-shunting of Gate to Source Miller capacitance. Implementing a Zener will Clamp these Spikes to safe limits and there will be no Breakdown.
Regards,
Kanwar
Hi lars,
Are you sure enough about the "Band Gap mode " formed by opposite Driver.
But still this is not specifically true in nature for the situation where the mosfets are parallelled in numbers therefore a Mismatch can occur and Secondly the Gate Spikes originating during turn -ON are relative to the driver Stage imbalance and ofcourse non-shunting of Gate to Source Miller capacitance. Implementing a Zener will Clamp these Spikes to safe limits and there will be no Breakdown.
Regards,
Kanwar
Work_man: I double checked (just to make sure), with the parts from the schematic to see if they would go in band gap mode as expected.
And sure enough at 7.0 Volts enter bandgap mode, and form a nice Zener diode, that could easily block 1 A.
You can test yourself, by simply adding reverse voltage to a BD135 (+ to E and - to C) or BD136 (opposite).
So i'm thinking yes, it will work as protection like i have anticipated.
🙂
And sure enough at 7.0 Volts enter bandgap mode, and form a nice Zener diode, that could easily block 1 A.
You can test yourself, by simply adding reverse voltage to a BD135 (+ to E and - to C) or BD136 (opposite).
So i'm thinking yes, it will work as protection like i have anticipated.
🙂
Lars Clausen said:
Hi LC_MAN,
Then what about the driver stage imbalance ??. Have you noticed that it causes improper startup of output which inturn causes OFFSET LATCHUP DURING TURN-ON for short interval.
regards,
kanwar
Kanwar: No i have never experienced that kind of behaviour in the Zeta amplifier it starts up and functions well every time. ;-)