Hi @ all
first of all, please excuse my English.
I have a pretty big car amp, it´s an older 2channel Amp called Altai B0005V. It´s the same as Caliber CA500 and is capable of handling 500W RMS in bridged mode. Well I got it from a friend of mine when it wasn´t working anymore.
So I checked the amp and was kind of depressed when I saw the mess in there. The switching power supply is divided into two parts. It uses a TL494, 8MOSFET (IRFZ44) per transformer. There are 2 transformers totalling the MOSFET to 16. Each transformer has 2 seperate input coils so that makes 4 MOSFET per transformer-coil.
One of the transformers was busted real bad and broken apart. All MOSFET were dead. So I used a transformer-core that I got from another amp that has the same size and used the same type wiring for it. So the transformers are both the same again and working.
For testing I used only 4x IRFZ34 , one per transformer coil and the amp was back to live, the musik played without distortion on both channels and also in bridge mode. The switching power supply output voltage was nice running +/-52V. The capacitors on the secondary side of the transformers are all ok, so are the diodes.
So I checked on the MOSFET and couldn´t get the IRFZ44 , only IRFZ44N and that one doesn´t have the same specs (IRFZ44=150W, IRFZ44N=110W and lower current capability). So I used IRFZ48N, they come very close to the IRFZ 44. On the spec sheet the IRFZ44 has 28mOhms@ 31A and the IRFZ48N has 16mOhms@32A.
Installed 16 brand new of those and put everything back together.
Now I have the problem that the amp starts running but after 10 minutes it kills the Capacitors on the input 12V side and then the TL494 shuts down. I thought it could also be a problem with the TL494 and replaced it with a new one, but the same problem persits. The stock capacitors on the input side were 2x 1000µF/25V and 2x 470µF/25V . I replaced those with the same 1000µF/40V and 470µF/40V but they also blow after 10 minutes....
It´s definetly not my 12V power supply, I also tried it with a car battery.
Is this the MOSFET causing the problem? I know now that the IRFZ48N has (a bit) less resistance than the IRFZ44, could it be that the transformers suck too much current when the FETs are turned on? I have never had any problems on the TL494 replacing a broken IRFZ44 with an IRFZ48N so far.
Could it help if I take one of the IRFZ48N out per transformer coil, I will still be on the safe side concerning the MOSFET Ptot if I use 3 of them per transformer coil.
Thx
Sdoom
first of all, please excuse my English.
I have a pretty big car amp, it´s an older 2channel Amp called Altai B0005V. It´s the same as Caliber CA500 and is capable of handling 500W RMS in bridged mode. Well I got it from a friend of mine when it wasn´t working anymore.
So I checked the amp and was kind of depressed when I saw the mess in there. The switching power supply is divided into two parts. It uses a TL494, 8MOSFET (IRFZ44) per transformer. There are 2 transformers totalling the MOSFET to 16. Each transformer has 2 seperate input coils so that makes 4 MOSFET per transformer-coil.
One of the transformers was busted real bad and broken apart. All MOSFET were dead. So I used a transformer-core that I got from another amp that has the same size and used the same type wiring for it. So the transformers are both the same again and working.
For testing I used only 4x IRFZ34 , one per transformer coil and the amp was back to live, the musik played without distortion on both channels and also in bridge mode. The switching power supply output voltage was nice running +/-52V. The capacitors on the secondary side of the transformers are all ok, so are the diodes.
So I checked on the MOSFET and couldn´t get the IRFZ44 , only IRFZ44N and that one doesn´t have the same specs (IRFZ44=150W, IRFZ44N=110W and lower current capability). So I used IRFZ48N, they come very close to the IRFZ 44. On the spec sheet the IRFZ44 has 28mOhms@ 31A and the IRFZ48N has 16mOhms@32A.
Installed 16 brand new of those and put everything back together.
Now I have the problem that the amp starts running but after 10 minutes it kills the Capacitors on the input 12V side and then the TL494 shuts down. I thought it could also be a problem with the TL494 and replaced it with a new one, but the same problem persits. The stock capacitors on the input side were 2x 1000µF/25V and 2x 470µF/25V . I replaced those with the same 1000µF/40V and 470µF/40V but they also blow after 10 minutes....
It´s definetly not my 12V power supply, I also tried it with a car battery.
Is this the MOSFET causing the problem? I know now that the IRFZ48N has (a bit) less resistance than the IRFZ44, could it be that the transformers suck too much current when the FETs are turned on? I have never had any problems on the TL494 replacing a broken IRFZ44 with an IRFZ48N so far.
Could it help if I take one of the IRFZ48N out per transformer coil, I will still be on the safe side concerning the MOSFET Ptot if I use 3 of them per transformer coil.
An externally hosted image should be here but it was not working when we last tested it.
Thx
Sdoom
http://www.digikey.com/ may have your original Mosfets and I think they will ship to you.
You do have a strange problem. Sounds like too much ripple current for these capacitors. With the small inductors in series with the input and other capacitors you could have an oscillation problem, but I think the probability of this is low. If you have an Oscope, monitor the voltage across the capacitors that fail, to see if an oscillation occurs. Don't let them run for 10 minutes and have another failure, though.
Using Mosfets with lower RDS will increase the ripple current a bit. The only other thing I can think of that may put extra stress on these caps, is cross current conduction in the Mosfets. If something is wrong causing the PWM to run too high of a duty cycle without enough dead time, this could cause much higher ripple current on the caps. I'm pulling at straws here and guessing some. Let us know your progress with this problem.
Good luck!
You do have a strange problem. Sounds like too much ripple current for these capacitors. With the small inductors in series with the input and other capacitors you could have an oscillation problem, but I think the probability of this is low. If you have an Oscope, monitor the voltage across the capacitors that fail, to see if an oscillation occurs. Don't let them run for 10 minutes and have another failure, though.
Using Mosfets with lower RDS will increase the ripple current a bit. The only other thing I can think of that may put extra stress on these caps, is cross current conduction in the Mosfets. If something is wrong causing the PWM to run too high of a duty cycle without enough dead time, this could cause much higher ripple current on the caps. I'm pulling at straws here and guessing some. Let us know your progress with this problem.
Good luck!
Hi
thx for your reply.
Well I don´t have access to a scope, that´s definetly the next thing I will get !
I also think that the resistance of the FET is too low. I ordered the IRFZ44N and new caps now, hopefully with those it will work.
But it´s strange because I have replaced the FET in some other amps already and never had any problem. I doublechecked it on a standard 200W AMP half year ago when I had a scope and there was really no difference between the 2 MOSFET types. But I assume now with 16 of those the TL494 cant shorten the ON-time of the FET anymore but wouldn´t I have a higher idle current ?
I checked the input voltage with a digital multimeter (good one) and it always reads 13V DC . On the AC -setting I read a few millivolts, if I connect another amp to the DC-power I get the same readings.
I will try again when the new MOSFET arrive and let you know.
Thx again
thx for your reply.
Well I don´t have access to a scope, that´s definetly the next thing I will get !
I also think that the resistance of the FET is too low. I ordered the IRFZ44N and new caps now, hopefully with those it will work.
But it´s strange because I have replaced the FET in some other amps already and never had any problem. I doublechecked it on a standard 200W AMP half year ago when I had a scope and there was really no difference between the 2 MOSFET types. But I assume now with 16 of those the TL494 cant shorten the ON-time of the FET anymore but wouldn´t I have a higher idle current ?
I checked the input voltage with a digital multimeter (good one) and it always reads 13V DC . On the AC -setting I read a few millivolts, if I connect another amp to the DC-power I get the same readings.
I will try again when the new MOSFET arrive and let you know.
Thx again
If you have access to an oscilloscope, you should check power supply waveforms [at MOSFET gates and drains, and at output diodes]
Capacitors may be blowing due to overheating caused by a too high ripple current being applied. You should check for temperature increase on the capacitors
The power supply may be operating at the wrong frequency [some cores could break when operated at certain frequencies], or it may be suffering cross-conduction or transformer saturation. All these problems may cause very high AC ripple currents across the input capacitors
Also, don't asume two toroid cores have the same volts*second/turn characteristic just because they are the same size. This parameter determines the number of primary turns needed for a given supply voltage and a given core. When core specifications are not available, it's a good idea to measure the saturation characteristics of the core
When choosing new mosfets, the most important parameter is the Cgs capacitance since the amplifier has its gate and control circuit resistors sized for a certain capacitance value. If you use MOSFETs with higher Cgs or Cdg capacitances you will usually experience cross-conduction or parasitisctic turn-on and increased switching losses. MOSFETs with equal or lower capacitances will work fine. The other important parameter is Rds-on but it only affects conduction losses
I like IRFZ48V for repairing car amplifier power supplies. These devices have low Cgs capacitance and are the cheapest I can get in my country
If your devices have somewhat higher Cgs capacitance than expected, then you may have to reduce the value of gate or turn-off circuit resistors to speed-up turn-off process
Capacitors may be blowing due to overheating caused by a too high ripple current being applied. You should check for temperature increase on the capacitors
The power supply may be operating at the wrong frequency [some cores could break when operated at certain frequencies], or it may be suffering cross-conduction or transformer saturation. All these problems may cause very high AC ripple currents across the input capacitors
Also, don't asume two toroid cores have the same volts*second/turn characteristic just because they are the same size. This parameter determines the number of primary turns needed for a given supply voltage and a given core. When core specifications are not available, it's a good idea to measure the saturation characteristics of the core
When choosing new mosfets, the most important parameter is the Cgs capacitance since the amplifier has its gate and control circuit resistors sized for a certain capacitance value. If you use MOSFETs with higher Cgs or Cdg capacitances you will usually experience cross-conduction or parasitisctic turn-on and increased switching losses. MOSFETs with equal or lower capacitances will work fine. The other important parameter is Rds-on but it only affects conduction losses
I like IRFZ48V for repairing car amplifier power supplies. These devices have low Cgs capacitance and are the cheapest I can get in my country
If your devices have somewhat higher Cgs capacitance than expected, then you may have to reduce the value of gate or turn-off circuit resistors to speed-up turn-off process
Eva brought up some very good points for potential problems!
The TL494 may have trouble driving the Mosfets with higher input capacitance. The cgs and cgd can really impact the rise and fall times, and turn on/off delays. You don't have much miller effect at 13 VDC with cgd, but enough to be a potential problem. The cgd miller capacitance charge causes turn on/off delays. Without an Oscope you are pretty much in the dark on some things.
Is there a current sensing resistor from the Mosfet sources to ground? If so, you can measure the DC voltage across it and knowing the resistor value, calculate the average current. You may have a current transformer considering that this is a high powered amp. It will have a current sensing resistor somewhere on the secondary side.
You could also have a stability problem. With these DC to DC converters, you can usually hear a stability problem. If you hear an un-even bad sound, you may have a stability problem and this can certainly cause problems. If you hear a steady clear high frequency, you probably don't have a stability problem. Usually these converters run at a frequency of 25 KHz or higher so that they are silent to the human.
Another potential problem could be a leaking output (secondary) filter cap (not likely). The audio amp portion of the unit could have a problem causing excessive current draw from the converter, also. You can do some good troubleshooting with just your DVM, but having an Oscope would be much better.
If you can isolate the power amp stage from the DC to DC converter this could help. If you have a high wattage power resistor for placing a load on the converter without the power amp section connected, this may help in case the power amp is causing the problem.
Hope you find and fix the problem before burning out more new parts!
Good Luck!
The TL494 may have trouble driving the Mosfets with higher input capacitance. The cgs and cgd can really impact the rise and fall times, and turn on/off delays. You don't have much miller effect at 13 VDC with cgd, but enough to be a potential problem. The cgd miller capacitance charge causes turn on/off delays. Without an Oscope you are pretty much in the dark on some things.
Is there a current sensing resistor from the Mosfet sources to ground? If so, you can measure the DC voltage across it and knowing the resistor value, calculate the average current. You may have a current transformer considering that this is a high powered amp. It will have a current sensing resistor somewhere on the secondary side.
You could also have a stability problem. With these DC to DC converters, you can usually hear a stability problem. If you hear an un-even bad sound, you may have a stability problem and this can certainly cause problems. If you hear a steady clear high frequency, you probably don't have a stability problem. Usually these converters run at a frequency of 25 KHz or higher so that they are silent to the human.
Another potential problem could be a leaking output (secondary) filter cap (not likely). The audio amp portion of the unit could have a problem causing excessive current draw from the converter, also. You can do some good troubleshooting with just your DVM, but having an Oscope would be much better.
If you can isolate the power amp stage from the DC to DC converter this could help. If you have a high wattage power resistor for placing a load on the converter without the power amp section connected, this may help in case the power amp is causing the problem.
Hope you find and fix the problem before burning out more new parts!
Good Luck!
me again 
)
I took the whole assembly apart today because I got new parts.
The audio part seems to work fine . I connected +/- 50V and let the amp play musik over night, no excessive hheat or destortion so that shouldn´be the probelm. If I had a fault in there I assume the DC-current would be much higher. There´s no bad sound with this DC-DC converter, I know what sound you mean , had this once on a Kenwood amp where UA7815 and UA7915 were broken and caused a short , that sounds real ugly.)
I will install the new MOSFET and I replaced the caps and, maybe that was the problem, there are two small capacitors parallel to the caps that burn, they should have 100nF each. I took them out and checked them with a good capacity-meter and they had 240 and 310nF . So that´s out of the tolreance limit. I replaced those with 250V types , so they wont fail !
The caps I have now are new and the guy who sold them said they can be used in DC-DC converters w/o any problem.
The resistors connected to the FET have 150Ohms, I have never seen that resistor value in a DC-DC converter before, usually 47-100 Ohms is what I have seen.
If that value was too high the FET is not as quick as needed in turning off , right?
OK, I will install the FET/Caps I have now and check for the resistor current as you said .
thx guys, I hope this baby gets back to live soon)
)I took the whole assembly apart today because I got new parts.
The audio part seems to work fine . I connected +/- 50V and let the amp play musik over night, no excessive hheat or destortion so that shouldn´be the probelm. If I had a fault in there I assume the DC-current would be much higher. There´s no bad sound with this DC-DC converter, I know what sound you mean , had this once on a Kenwood amp where UA7815 and UA7915 were broken and caused a short , that sounds real ugly.
)I will install the new MOSFET and I replaced the caps and, maybe that was the problem, there are two small capacitors parallel to the caps that burn, they should have 100nF each. I took them out and checked them with a good capacity-meter and they had 240 and 310nF . So that´s out of the tolreance limit. I replaced those with 250V types , so they wont fail !
The caps I have now are new and the guy who sold them said they can be used in DC-DC converters w/o any problem.
The resistors connected to the FET have 150Ohms, I have never seen that resistor value in a DC-DC converter before, usually 47-100 Ohms is what I have seen.
If that value was too high the FET is not as quick as needed in turning off , right?
OK, I will install the FET/Caps I have now and check for the resistor current as you said .
thx guys, I hope this baby gets back to live soon
)sdoom said:
In my experience, a resistor value of 150 ohms for the gate drive is found for medium powered Mosfets or in applications other than DC to DC converters. In my designs for commercial and military avionics, the typical value for gate drive resistors was more like 10 ohms. Generally, a higher value gate drive resistor means slower switching speeds, lower efficiency, less EMI, and less power dissipated in snubbers.
The value of gate drive resistance effects both turn on and off speed. Running high duty cycle as many of these DC to DC Converters are designed, means the chance of cross current conduction (Mosfets on the push and pull side being on simultaneously) is possible, unless the dead time (set with the timing capacitor of the PWM, usually) is selected properly to prevent this condition. I've not used the TL494 so I would need to look at the data sheet to be sure how the dead time is selected. Since this is not a design problem, hopefully, then there must be a bad or marginally bad component somewhere in this converter. However, as Eva mentioned, using Mosfets with higher input capacitance can be your problem! Without an Oscope you really need to replace the Mosfets with the same part numbers as the originals. Using Mosfets with less input capacitance is better than using Mosfets with more input capacitance than orginals, I think. You may have noise related problems, however.
Good luck and let us know what happens when you fire it up!
As Eva also mentioned, you need to make sure that transformer core you used as a replacement is the same as original. Size is not a good enough indication of the correct core. If it is a ferrite core, then the core material is very important as well. Also, the winding method (how windings are wound on the core) can be critical in some applications.
Repairing a SMPS without an Oscope is not easy, unless the problem is minor. You have done some major repair work.
Repairing a SMPS without an Oscope is not easy, unless the problem is minor. You have done some major repair work.
Hi guys, right now this thing is starting to p. me off,
OK, I got the MOSFET , the guy said I can replace a IRFZ44 with a IRFZ44N, they are the same.
I replaced all of them and also the caps.
Turned on the amp and now I have this ugly noise . Can´t determine from which transformer it comes, I assume both as I plugged out one fuse to have one side run seperately and the noise was still there. And also now I have a hige idle current on the 12V side which I didn´t have with the IRFZ48N. It starts with about 2A , then the noise gets louder and the current draw goes up to 5A and by that time I shut the AMP off.
I tried to measure some frequencies. The TL494 has to outputs and a refernece voltage output. The ref-voltage is 5V and it´s stable.
those are the frequencies I could get from the TL494-pins:
Pin 8 (Collector1) : running from 10kHz to 99 kHz and back
Pin 9Emitter1) 29 -30 kHz no big change
Pin 10(Emitter2): 25-30 kHz no big change
Pin 11(Collector2): 80kHz at start , when noise gets loud it goes to a 120kHz
Pin 14: 5V stable ref out voltage
The AC-voltage across the capacitors is about 1V , but with 120kHz no wonder they blow in smoke.
There are 4 transistors (2SA1266) that drive the 4 Mosfet-packs. Each MOSFET is connected with a 100Ohm (sorry I mentioned 150 earlier but that was wrong) and there is also a gnd-resistor that has 1kOhm.
Sorry guys for beeing annoying. But if I get a scope where would I have to look first. I checked all parts in the power supply and they are all ok. If it would help I could make a curcuit diagramm of this thing ?
How can I check on the transformer coils, I assume that Eva is right and I cant use the transformer . But the company that made the Amp is out off business and I cant order the exact one. As far as windings concern they are absolut identical, so it must be the core.
OK, I got the MOSFET , the guy said I can replace a IRFZ44 with a IRFZ44N, they are the same.
I replaced all of them and also the caps.
Turned on the amp and now I have this ugly noise . Can´t determine from which transformer it comes, I assume both as I plugged out one fuse to have one side run seperately and the noise was still there. And also now I have a hige idle current on the 12V side which I didn´t have with the IRFZ48N. It starts with about 2A , then the noise gets louder and the current draw goes up to 5A and by that time I shut the AMP off.
I tried to measure some frequencies. The TL494 has to outputs and a refernece voltage output. The ref-voltage is 5V and it´s stable.
those are the frequencies I could get from the TL494-pins:
Pin 8 (Collector1) : running from 10kHz to 99 kHz and back
Pin 9
Emitter1) 29 -30 kHz no big changePin 10(Emitter2): 25-30 kHz no big change
Pin 11(Collector2): 80kHz at start , when noise gets loud it goes to a 120kHz
Pin 14: 5V stable ref out voltage
The AC-voltage across the capacitors is about 1V , but with 120kHz no wonder they blow in smoke.
There are 4 transistors (2SA1266) that drive the 4 Mosfet-packs. Each MOSFET is connected with a 100Ohm (sorry I mentioned 150 earlier but that was wrong) and there is also a gnd-resistor that has 1kOhm.
Sorry guys for beeing annoying. But if I get a scope where would I have to look first. I checked all parts in the power supply and they are all ok. If it would help I could make a curcuit diagramm of this thing ?
How can I check on the transformer coils, I assume that Eva is right and I cant use the transformer . But the company that made the Amp is out off business and I cant order the exact one. As far as windings concern they are absolut identical, so it must be the core.
sdoom said:Hi guys, right now this thing is starting to p. me off,
OK, I got the MOSFET , the guy said I can replace a IRFZ44 with a IRFZ44N, they are the same.
I replaced all of them and also the caps.
Turned on the amp and now I have this ugly noise . Can´t determine from which transformer it comes, I assume both as I plugged out one fuse to have one side run seperately and the noise was still there. And also now I have a hige idle current on the 12V side which I didn´t have with the IRFZ48N. It starts with about 2A , then the noise gets louder and the current draw goes up to 5A and by that time I shut the AMP off.
I tried to measure some frequencies. The TL494 has to outputs and a refernece voltage output. The ref-voltage is 5V and it´s stable.
those are the frequencies I could get from the TL494-pins:
Pin 8 (Collector1) : running from 10kHz to 99 kHz and back
Pin 9
Pin 10(Emitter2): 25-30 kHz no big change
Pin 11(Collector2): 80kHz at start , when noise gets loud it goes to a 120kHz
Pin 14: 5V stable ref out voltage
The AC-voltage across the capacitors is about 1V , but with 120kHz no wonder they blow in smoke.
There are 4 transistors (2SA1266) that drive the 4 Mosfet-packs. Each MOSFET is connected with a 100Ohm (sorry I mentioned 150 earlier but that was wrong) and there is also a gnd-resistor that has 1kOhm.
Sorry guys for beeing annoying. But if I get a scope where would I have to look first. I checked all parts in the power supply and they are all ok. If it would help I could make a curcuit diagramm of this thing ?
How can I check on the transformer coils, I assume that Eva is right and I cant use the transformer . But the company that made the Amp is out off business and I cant order the exact one. As far as windings concern they are absolut identical, so it must be the core.
Grim!
The IRF44 and IRF44N are similar, but have differences. Download the PDFs on each and compare them to each other. Not much difference, but some.
Download the data sheet and an application note for the TL494. The dead time control has a separate input from the RC oscillator inputs. I've not used this IC before, but you should see a fixed frequency on either the R or C input pin. Without your schematic for the DC to DC converter it's hard to speculate, but I would think that either pin 8 or 9 and either pin 10 or 11 should have a fixed frequency. Something is definitely wrong with the converter section.
I think you need to do some troubleshooting with just the TL494 powered, without the Mosfets or Transformers in the circuit. Need to make sure everything is alright with the PWM first. If you can scan or draw us a schematic of just the converter or even a partial schematic it may help us to see your problem.
One method to get an idea if the transformer core has similar permeability of your other original transformer is to do a resonant frequency test and calculation. You will need a variable frequency sinewave generator for this method. With your transformer disconnected and the secondary windings open circuit. Connect your signal generator (Red lead) to one lead of the primary and connect a .1uf to 1uf capacitor to the other lead. Then connect a 1K ohm resistor to the other lead of the capacitor. Connect the other resistor lead to the return or ground connection of the signal generator (Black lead). Connect your Oscope or a DVM (one that can measure up to 30 KHz or so) across the resistor. Adjust the signal generator and look for a peak in the voltage across the resistor. You should find a resonant peak where the voltage is maximum. Either side of this resonant point the voltage across the resistor should drop. You want to start at a low frequency and adjust upwards to avoid finding a resonance on a harmonic of the fundamental. You may have to try different values of capacitance to find a resonance within the range of your signal generator and DVM. Once and if you find the lowest resonant point. Use this frequency to calculate the inductance of the primary:
Lprimary = 1/((2*pi*f)^2)*C
pi = 3.414
f = resonant frequency
C = capacitor value
Lprimary = primary inductance of transformer
Now repeat the experiment with your other transformer(s). See if the inductances are within 10 percent of each other. If the inductance is far different then the replacement core probably has a different permeability (made from a different material) and probably will not work in this application.
Again, try to isolate the problem further without having the Mosfets or transformer in the circuit. We need to make sure the TL494 is working correctly first. Try to get us a schematic.
I can imagine your frustration, but you stand a chance of learning something important. Try to be patient.
Hi
I assumed you would say that and I really appreciate your help.
It will take me about a week to get the scope and I hope I can have the schematics ready by the mid of next week.
What I find strange about this whole thing is when I first started to do some work on the Amp was that I only used 1 FET per coil and there was no noise and I had the whole thing running for at least 30 minutes without a heatsink and this thing was working. I do have some understanding of electronics but I totally agree I can learn a lot from you guys when it comes to PWM-power supplies.
I assume the manufacturer already had some problem with this thing as they had connected an additional small 10µF capacitor across a switching transistor.
I will also make pictures and post them. Concerning the reason for the amp to malfunction I think one of the windings of transformer No.2 had an isolation problem and caused a short , that was how it looked like. The core wasn´t broken completely but there was a big edge broken out.
thx again
stephan
I assumed you would say that
and I really appreciate your help.It will take me about a week to get the scope and I hope I can have the schematics ready by the mid of next week.
What I find strange about this whole thing is when I first started to do some work on the Amp was that I only used 1 FET per coil and there was no noise and I had the whole thing running for at least 30 minutes without a heatsink and this thing was working. I do have some understanding of electronics but I totally agree I can learn a lot from you guys when it comes to PWM-power supplies.
I assume the manufacturer already had some problem with this thing as they had connected an additional small 10µF capacitor across a switching transistor.
I will also make pictures and post them. Concerning the reason for the amp to malfunction I think one of the windings of transformer No.2 had an isolation problem and caused a short , that was how it looked like. The core wasn´t broken completely but there was a big edge broken out.
thx again
stephan
sdoom said:
I apologize for treating you like someone with little experience. Some people can have a lot of experience and education, and still have trouble understanding magnetics and switching power supplies. This is a subject most colleges and technical schools don't teach and must be learned the hard way, at least in my experience. I have many years of experience with power conversion products and still get stumped sometimes. This is not trivial stuff.
From what you are saying, it sounds like this product had or maybe still has some design problems. Maybe the design is so close to the edge, any little component change causes instability. A marginal design could have caused the original failure. I've had my patience tested many times by SMPS stability and noise problems.
Since the manufacturer connected that 10 uf cap across the switching transistor, they may have had some noise induced stability problem(s). To me, these are the most difficult ones to identify and solve. I suspect that I will learn from this experience.
I have an old scope and need a newer one. Ebay has some good deals or at least they did a few months ago. Those used Tektronix Scopes such as the 465 and 475 are great old scopes. I've seen some refurbished ones with warranty go for around $200.00 or a little more.
See if you can find whether or not this converter has feedback for regulation. Usually they just use a push-pull topology with a transformer and no output inductors. This one of yours sounds more complicated than that, however. If it does not have feedback for regulation, it may be easier to figure out. If you are reverse engineering it to make a schematic, that is difficult.
Since you will probably have to re-design this converter and make more modifications, you will likely need a scope, unfortunately.
Keep us posted and good luck!
Mark
@ mwh-eng
please don`t apologize , I am really really thankful for your help. There are not many people around who really have knowledge about the stuff like you do, and I wanna learn,too ! And your description on how to test the cores is really good.
See, I know in basics how a PWM-supply is working and how I can test a MOSFET if it is working or not, but I am not into the stuff like you are, so far, I fixed about 30-40 car amps (and they still work ..
..) A lot of work (not only in electronics) can be done without the exact knowledge of how it works, but as in this case in German you ´d say
"jetzt geht es an das Eingemachte" meaning now you have to take out the last preserves"
And yes I had an electronics job training (lasted 3 years) . But that is a long time ago and we did not even had MOSFET in that job training. Also now I attend evening school again. I passed almost 3 years now and there is one to come, but it all goes into data transfer and not into power electronics.
Today I was helping my brother who was moving. Besides boxes and furniture I haven´t seen anything today and I ´ll go to bed pretty soon . Maybe tomorrow I´ll find time to get the schematics for you. As far as I have overlooked it, there aren´t too many parts.
I´m not sure if the whole assembly is really running on the upper limit, at least what I have seen on car Amp power supplies, in most standard car amps they use 4 IRFZ44 with a 40 Amp fuse.
And as far as I understand the TL494 it uses a fixed frequency for switching and only changes the ON/OFF-time depending on the load. So in my understanding there must be a feedback . Don´t they use the reference voltage generator for that and compare the output voltage to it ? There are a few Z-diodes where they probably "waste " the voltage on and take "what´s left off it" and compare it to the refernece voltage ? Or am I totally wrong on understanding this thing? Because, what I have overlooked, not all Z-diodes are for generating the +/-15V supply voltage for the OP-Amps.
thx again
stephan
please don`t apologize , I am really really thankful for your help. There are not many people around who really have knowledge about the stuff like you do, and I wanna learn,too ! And your description on how to test the cores is really good.
See, I know in basics how a PWM-supply is working and how I can test a MOSFET if it is working or not, but I am not into the stuff like you are, so far, I fixed about 30-40 car amps (and they still work ..
..) A lot of work (not only in electronics) can be done without the exact knowledge of how it works, but as in this case in German you ´d say "jetzt geht es an das Eingemachte" meaning now you have to take out the last preserves"
And yes I had an electronics job training (lasted 3 years) . But that is a long time ago and we did not even had MOSFET in that job training. Also now I attend evening school again. I passed almost 3 years now and there is one to come, but it all goes into data transfer and not into power electronics.
Today I was helping my brother who was moving. Besides boxes and furniture I haven´t seen anything today and I ´ll go to bed pretty soon . Maybe tomorrow I´ll find time to get the schematics for you. As far as I have overlooked it, there aren´t too many parts.
I´m not sure if the whole assembly is really running on the upper limit, at least what I have seen on car Amp power supplies, in most standard car amps they use 4 IRFZ44 with a 40 Amp fuse.
And as far as I understand the TL494 it uses a fixed frequency for switching and only changes the ON/OFF-time depending on the load. So in my understanding there must be a feedback . Don´t they use the reference voltage generator for that and compare the output voltage to it ? There are a few Z-diodes where they probably "waste " the voltage on and take "what´s left off it" and compare it to the refernece voltage ? Or am I totally wrong on understanding this thing? Because, what I have overlooked, not all Z-diodes are for generating the +/-15V supply voltage for the OP-Amps.
thx again
stephan
Hi,
I didn't read the whole thread so maybe this has been covered and in that case forget it. But, there could be higher current spikes from the primary caps through the input chokes, which means when this higher (or faster) curent switches off, there is (much) higher voltage across the caps, causing them to overheat and fail. Try higher voltage caps, even with a bit lower capacitance, I bet that solves it.
Jan Didden
I didn't read the whole thread so maybe this has been covered and in that case forget it. But, there could be higher current spikes from the primary caps through the input chokes, which means when this higher (or faster) curent switches off, there is (much) higher voltage across the caps, causing them to overheat and fail. Try higher voltage caps, even with a bit lower capacitance, I bet that solves it.
Jan Didden
@ janneman
well right now I have the MOSFET installed that are supposed to be working in there and now the whole thing is totally instable. The switching frequency changes rapidly from 15kHz up to 120kHz causing a 5A (12V side) idle current. It´s definetily not he secondary side of the supply curcuit. The diodes are ok and the caps on the secondary side ,too. With no load the idle current should be around 500mA to 1A and there should not be any noise. Problem is I don´t have a scope yet so I cannot really see what is goinf on in there. With the IRFZ48N I had at least 10 min. of clear signals and the output voltage was +/-52V stable. When the caps on the input side have died the current draw went way up and I had the bad noise. The manuf. already had a small cap soldered across a switching transistor so I assume the stock layout is vulnerable for stress/instability. But well, the amp was fine for at least 6 years working time.
The original Caps had 25V , I already installed 40V. With a 63V capacitor I am kind of running out of space here)
well right now I have the MOSFET installed that are supposed to be working in there and now the whole thing is totally instable. The switching frequency changes rapidly from 15kHz up to 120kHz causing a 5A (12V side) idle current. It´s definetily not he secondary side of the supply curcuit. The diodes are ok and the caps on the secondary side ,too. With no load the idle current should be around 500mA to 1A and there should not be any noise. Problem is I don´t have a scope yet so I cannot really see what is goinf on in there. With the IRFZ48N I had at least 10 min. of clear signals and the output voltage was +/-52V stable. When the caps on the input side have died the current draw went way up and I had the bad noise. The manuf. already had a small cap soldered across a switching transistor so I assume the stock layout is vulnerable for stress/instability. But well, the amp was fine for at least 6 years working time.
The original Caps had 25V , I already installed 40V. With a 63V capacitor I am kind of running out of space here
)
sdoom said:
Most engineers/technicians don't have the opportunity to attend a college that offers courses in power electronics. As time passes more and more American universities are offering courses in power electronics, however. Most of us, like you and I must learn this difficult subject the hard way through experience. I was fortunate enough to work with others that had good power electronics experience and I owe much to them.
Some consumer products are designed such that they are pushed too close to the limits, causing them to have high failure rates. An automobile can be a hostle environment. Think about the inside temperature after sitting in the hot sun for a few hours. Depending on the lattitude, it can be close to 150 F. Jump in the car and turn the audio power amp on full blast and you could have a heat related failure. It's best to turn off all audio and other unnecessary electronics until the inside of an automobile has cooled off to the ambient outside temperature. Not much cool off in a place like Phoenix, Arizona at 122 F summer time temperature. The heat sink of a power amp has some thermal mass and an associated thermal time constant. Of course, don't tell your customers about this because you won't have as many amps to repair. :^)
Most auto audio power amps have buck derived DC/DC converters, or straight DC/DC squarewave converters without an output inductor. The DC/DC converter with output inductor of the proper size is usually a buck derived converter. This converter may or may not have feedback. On a buck converter with feedback the duty cycle changes with load only to make up for rectifier voltage drops, I*R drops in the transformer and output inductor, RDSon*I drop, and other I*R drops. The transfer function of a buck regulator is Vout = Vin*Dutycycle. Ideally it's not load dependent; however, it is not ideal and has losses that must be made up for if a regulated output is maintained.
The transformer in a DC/DC converter cannot have a 100 percent power transfer/coupling. Coupling is related to leakage inductances. Voltage drops across leakage inductances = Lleak*(di/dt).
Since Vout = Vin*D.C.; Vout varies in direct proportion to Vin. The battery voltage in an auto typically varies only with temperature (engine running), because a lead acid battery voltage is somewhat temperature dependent. If the DC/DC converter has a 1:4 turns ratio, then a 1 volt change on the battery yields a 4 volt change at the converter's transformer secondary.
Adding the transformer turns ratio to the voltage transfer function of the buck:
Vout = Turns_Ratio*Vin*D.C.
In the case of a DC/DC squarewave converter without an output inductor, there is usually no feedback for regulation. They work best running at high duty cycle. The secondary of the transformer is rectified right into capacitors without an inductor. The output voltage will vary due to rectifier voltage drop variations, I*R drops, and input voltage variations. These converters tend to be simple but noisy.
Note: Since I've not been active in SMPS design or development work in nearly 15 years, there could be errors in my explanations. I need to find a good practical textbook to suggest. There are many books on switch-mode power supplies, but most are not complete, or are out of date with modern techniques. A lot can be learned from application notes. Unitrode Corporation made some of the best ICs for power conversion. Their seminars were great and their application notes are some of the best. Texas Instruments bought them out a few years ago. Check out all these power conversion products starting at www.ti.com and follow the Power Management Link.
The following is a very good textbook on power electronics, but expensive.
FUNDAMENTALS OF POWER ELECTRONICS, 2nd Ed.
by Erickson, Robert & Maksimovic, Dragan
Contents: Principles of Steady State Converter Analysis; Steady-State Equivalent Circuit Modeling, Losses, & Efficiency; Switch Realization; Discontinuous Conduction Mode; Converter Circuits; AC Equivalent Circuit Modeling; Converter Transfer Functions; Controller Design; Input Filter Design; AC & DC Equivalent Circuit Modeling of the Discontinuous Conduction Mode; Current Programmed Control; Basic Magnetics Theory; Inductor Design; Transformer Design; Power & Harmonics in Nonsinusoidal Systems; Line-Commutated Rectifiers; Pulse-Width Modulated Rectifiers; Resonant Conversion; Soft Switching.
hi folks
Sorry for not writing sooner. I´m doing the schematics right now and hope to be done with it tonight. And I will get a scope tomorrow, too.
If the only problem was the transformer core #2 could I use the old one and rewire it? There is a "big" edge missing but this core is still complete
I will make pictures of everything by the time the schematics are done so you will have a complete package.
Thx again
Sorry for not writing sooner. I´m doing the schematics right now and hope to be done with it tonight. And I will get a scope
tomorrow, too. If the only problem was the transformer core #2 could I use the old one and rewire it? There is a "big" edge missing but this core is still complete
I will make pictures of everything by the time the schematics are done so you will have a complete package.
Thx again
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