I am planning on eventually building a car amp, was wondering if anyone had any tips for larger (1000+ watts) amp power supplies.
Firstly , I presume i'll have to use 2 small toriods rather than the standard 1. Is there a trick to getting the voltages matched so the ground is evenly between them.
Secondly, what sort of constant load is required for stability, like i could use a 1k ohm on a 35 rail and only pull 1 watt, or does it require more.
It appears that the secondary side ground can be totally floating compared to the primary. Is it best to totally separate them, to have a medium resistor linking, or to actually connect them directly?
Any advice would be welcomed, thanks.
Firstly , I presume i'll have to use 2 small toriods rather than the standard 1. Is there a trick to getting the voltages matched so the ground is evenly between them.
Secondly, what sort of constant load is required for stability, like i could use a 1k ohm on a 35 rail and only pull 1 watt, or does it require more.
It appears that the secondary side ground can be totally floating compared to the primary. Is it best to totally separate them, to have a medium resistor linking, or to actually connect them directly?
Any advice would be welcomed, thanks.
check out rod elliots sote
www.sound.au.com
u can find a 350W smps , use couple of those , one 4 each channel , i think that will be enough
www.sound.au.com
u can find a 350W smps , use couple of those , one 4 each channel , i think that will be enough
Immo_G said:
Do you have an oscillscope? A good one? If not, my advice is to get one. Get a good one. You can't fix problems you can't see.
(Tektronix TDS1000, TDS2000 are good. TDS3000 series, are my favorites)
Stablility depends on your feedback network. It's different for every design. The bias current in my amp is enough load on power supply at idle.
Use good fets. Uset 55 to 75 volt rated devices, it'll give you a little room for leakage inductance on your transformer. Watch the transient voltage on your good fets! (Thus the good scope)
Wind both of the secondary windings at the same time (Cut two peices of wire, and wind them exactly in parallel). that will help to ensure that you get the same rail voltages. And, you only really need one transformer. (I get 2Kw from one xfmr)
The biggest thing, is go slow. Test everything at small loads, and work your way up. Even build some small supplies... ...when you burn things up, it'll be cheaper (Ask me how I know).
There are lots of possible gotcha's...
Oh, yeah... ...start collecting parts as soon as you can! Unless you have bottomless pockets, cash can dissappear fast!
Good Luck!
-Dan
1000+ watt smps recommendation
At a power level of 1000+ watts, I'd recommend a full bridge converter. This requires a PWM controller, four MOSFETs (n-channel recommended), two high-side n-channel MOSFET drivers, a power inductor, and a single-primary, center-tapped secondary transformer, two ultrafast or Schottky rectifiers, and input and output filter capacitors.. The PWM controller should preferably be current-mode control. There are a lot of high-powered SMPS schematics floating around that use no filter inductor. Definitely avoid those. Everyone who attempts to build them has chronic trouble. Check out the Texas Instruments and National Semiconductor sites for application notes. I hope this helps.
At a power level of 1000+ watts, I'd recommend a full bridge converter. This requires a PWM controller, four MOSFETs (n-channel recommended), two high-side n-channel MOSFET drivers, a power inductor, and a single-primary, center-tapped secondary transformer, two ultrafast or Schottky rectifiers, and input and output filter capacitors.. The PWM controller should preferably be current-mode control. There are a lot of high-powered SMPS schematics floating around that use no filter inductor. Definitely avoid those. Everyone who attempts to build them has chronic trouble. Check out the Texas Instruments and National Semiconductor sites for application notes. I hope this helps.
Yeah i've heard someone mention a full bridge before, hadn't considered using it, but since i'll make my own transformer, i may as well use it.
Re the transformer, parts store here has a outside diam 75, inside 21, height 30mm, prewould one i would rewind, or I could go a powdered iron one thats a fair bit smaller, though i don't know how the various materials affect it. One of the schematics mentioned only to use a ferrite core.
Thanks for the advice about cutting both wires the same length, that seems a good way to match the rails.
Re the transformer, parts store here has a outside diam 75, inside 21, height 30mm, prewould one i would rewind, or I could go a powdered iron one thats a fair bit smaller, though i don't know how the various materials affect it. One of the schematics mentioned only to use a ferrite core.
Thanks for the advice about cutting both wires the same length, that seems a good way to match the rails.
I have been working of SMPS designs which do not require full bridge. They are half bridge design with capacitor coupling between the mosfets and transformer. They are capable of lots of output power and can do positive and negative rails from the same supply. Using half-bridge provides advantages like stopping DC bias from reaching the transformer, and only needing half the drive circuitry and number of output mosfets. The switching power does not have to pass through two sets of mosfets in series, as would be required for full bridge, so the circuit is simpler. Check out the link below this post.
You would use a ferrite core for the power transformer, not powdered iron for efficiency. You use two windings on the same transformer, both are full-wave rectified to each power rail.
You would use a ferrite core for the power transformer, not powdered iron for efficiency. You use two windings on the same transformer, both are full-wave rectified to each power rail.
full bridge and half bridge pros/cons
Actually, the half bridge does have a problem with dc bias reaching the transformer. Charge imbalance between the two capacitors can produce flux imbalance in the transformer core. The volt-seconds in the positive and negative directions are unequal, and runaway and core saturation will take place. This can happen with either voltage-mode or current-mode control.
The full bridge, and the push-pull topologies which employ current-mode control do not have this problem. A full bridge does use four MOSFETs vs. two for the half bridge, but each FET in a full bridge need only withstand the input supply voltage. In a half bridge, each FET must withstand *twice* the input voltage. The full bridge can use lower voltage MOSFETs, which have lower on resistance and switching losses. Four parts in the full bridge will run cooler than the two parts in the half bridge, and higher efficiency is obtained.
A rough rule of thumb is to use push-pull for power levels around 200 to 500 watts. Above 500 W, the next choice has traditionally been half bridge. Then, full bridge is recommended for the highest levels of power. I personally eschew the half bridge for the full. If the push-pull is not quite enough, I go for the full bridge. Simpler is not always better. Fewer parts generally means more stress on each one. I just want you to know the tradeoffs involved.
As far as the schematics go, I'd avoid any circuit without an inductor. The transformer output should be followed by a power inductor. National Semi, Texas Instruments, and Linear Technology have good app notes on their sites. Best regards.
Actually, the half bridge does have a problem with dc bias reaching the transformer. Charge imbalance between the two capacitors can produce flux imbalance in the transformer core. The volt-seconds in the positive and negative directions are unequal, and runaway and core saturation will take place. This can happen with either voltage-mode or current-mode control.
The full bridge, and the push-pull topologies which employ current-mode control do not have this problem. A full bridge does use four MOSFETs vs. two for the half bridge, but each FET in a full bridge need only withstand the input supply voltage. In a half bridge, each FET must withstand *twice* the input voltage. The full bridge can use lower voltage MOSFETs, which have lower on resistance and switching losses. Four parts in the full bridge will run cooler than the two parts in the half bridge, and higher efficiency is obtained.
A rough rule of thumb is to use push-pull for power levels around 200 to 500 watts. Above 500 W, the next choice has traditionally been half bridge. Then, full bridge is recommended for the highest levels of power. I personally eschew the half bridge for the full. If the push-pull is not quite enough, I go for the full bridge. Simpler is not always better. Fewer parts generally means more stress on each one. I just want you to know the tradeoffs involved.
As far as the schematics go, I'd avoid any circuit without an inductor. The transformer output should be followed by a power inductor. National Semi, Texas Instruments, and Linear Technology have good app notes on their sites. Best regards.
Re: full bridge and half bridge pros/cons
I have not noticed this kind of problem happening maybe because of the capacitor coupling of the transformer. I also place a capacitor between the transformer and the recitfiers for overload protection.
Does having to withstand double the input voltage matters when the voltage is only 12 to begin with? I don't think they run cooler for the same output power, but I guess that the power efficiency may be better due to less current having to pass through the circuit. But in the mosfets themselves, no theoretical efficiency benefit is gotten since the mosfet channel resistance is doubled, which admittedly does not consider that manufacutures of mosfets make the lower voltage ones better.
An output inductor is definitely important for efficiency. Thanks for your thoughts. Best Regards.
Claude Abraham said:
I have not noticed this kind of problem happening maybe because of the capacitor coupling of the transformer. I also place a capacitor between the transformer and the recitfiers for overload protection.
Claude Abraham said:
Does having to withstand double the input voltage matters when the voltage is only 12 to begin with? I don't think they run cooler for the same output power, but I guess that the power efficiency may be better due to less current having to pass through the circuit. But in the mosfets themselves, no theoretical efficiency benefit is gotten since the mosfet channel resistance is doubled, which admittedly does not consider that manufacutures of mosfets make the lower voltage ones better.
Claude Abraham said:
An output inductor is definitely important for efficiency. Thanks for your thoughts. Best Regards.
half vs. full bridge clarification
Yes. Yes. Emphatically yes! Log onto any MOSFET producer's web site. Compare the premium 20 volt part against the premium 40 volt part. Likewise with 30 V vs. 60 V, 100 vs. 200, etc. The lower voltage parts offer less on resistance without increasing the switching losses (rise time, fall time, total gate charge). The full bridge has four FETs, while the push-pull and half bridge have only two, but the four parts in the full bridge are each subjected to lower stress and losses.
However, one mistake I made needs correcting. I shouldn't rely entirely on memory when making comparisons. The FETs in a push-pull are subjected to the same current as in a full bridge, but twice the voltage. I earlier stated that the stress on each FET in a half bridge is twice the voltage, and the same current as a full bridge. Actually, it's twice the current, and same voltage as full bridge. Since there are two FETs in a half bridge, only one conducts at a time, but the conduction loss is four times that of each FET in a full bridge. The loss is equal to current squared times Rdson, or (I^2)*Rdson. Twice the current yields four times the loss, but since only one drop is incurred vs. two in the full bridge, the total loss is half of four, or twice that of a full bridge. So, the half bridge incurs twice the conduction loss as a full bridge. Even worse, the higher power loss is dissipated by only two FETs, vs. four in the full bridge. The temperature rise is four times as great per individual FET. Since the half bridge FETs run a lot hotter, their Rdson will increase even further. It should be very clear to all who read this, why the full bridge is the premium topology for applications which demand the highest power. At 1000+ watts, stated earlier by the person who started this thread, the full bridge would be the best course of action.
As far as the dc bias issue goes, a series coupling capacitor would indeed eliminate the problem. Best regards.
Yes. Yes. Emphatically yes! Log onto any MOSFET producer's web site. Compare the premium 20 volt part against the premium 40 volt part. Likewise with 30 V vs. 60 V, 100 vs. 200, etc. The lower voltage parts offer less on resistance without increasing the switching losses (rise time, fall time, total gate charge). The full bridge has four FETs, while the push-pull and half bridge have only two, but the four parts in the full bridge are each subjected to lower stress and losses.
However, one mistake I made needs correcting. I shouldn't rely entirely on memory when making comparisons. The FETs in a push-pull are subjected to the same current as in a full bridge, but twice the voltage. I earlier stated that the stress on each FET in a half bridge is twice the voltage, and the same current as a full bridge. Actually, it's twice the current, and same voltage as full bridge. Since there are two FETs in a half bridge, only one conducts at a time, but the conduction loss is four times that of each FET in a full bridge. The loss is equal to current squared times Rdson, or (I^2)*Rdson. Twice the current yields four times the loss, but since only one drop is incurred vs. two in the full bridge, the total loss is half of four, or twice that of a full bridge. So, the half bridge incurs twice the conduction loss as a full bridge. Even worse, the higher power loss is dissipated by only two FETs, vs. four in the full bridge. The temperature rise is four times as great per individual FET. Since the half bridge FETs run a lot hotter, their Rdson will increase even further. It should be very clear to all who read this, why the full bridge is the premium topology for applications which demand the highest power. At 1000+ watts, stated earlier by the person who started this thread, the full bridge would be the best course of action.
As far as the dc bias issue goes, a series coupling capacitor would indeed eliminate the problem. Best regards.
transformer primaries in parralel
i have this idea, to get more power out of a pc psu, i am toying with the idea of parallelling the transformer primaries of the smps, i will use identical units, one will connect to the mains, the other one i will hook-up the primary to that psu..i will be installing them in the same casing, i have done dual psu set-ups but one is an ATX, the other an AT all in the same casing..any comments, anybody tried this before?
i have this idea, to get more power out of a pc psu, i am toying with the idea of parallelling the transformer primaries of the smps, i will use identical units, one will connect to the mains, the other one i will hook-up the primary to that psu..i will be installing them in the same casing, i have done dual psu set-ups but one is an ATX, the other an AT all in the same casing..any comments, anybody tried this before?
Claude, I agree now. The power loss through the mosfets is double while the power provided is four times greater. That gives a two times improvement in efficiency. But I would like to just try using higher power mosfets and save on drive circuitry if possible to keep the temperature of the mosfets low enough not to have much additional heating Rds increase.
Joan2, I would give an opinion, but you lost me. Maybe someone else will get your gist.
Joan2, I would give an opinion, but you lost me. Maybe someone else will get your gist.
subwo1
oops, what i mean is that i will have 2 identical ATX psu's, on the one psu, i will take out the pwm ic, mains rectifiers and filters, and switching trasistors, leaving only the chopper transformer and the 3.3,5, and 12v rects, now what i am going to do is connect the primary of that transformer in parrallel with the transformer from other working psu, so that now the other psu is driving two transformer primaries instead of just one, if you followed me, can you comment please? thanks
subwo1,
oops, what i mean is that i will have 2 identical ATX psu's, on the one psu, i will take out the pwm ic, mains rectifiers and filters, and switching trasistors, leaving only the chopper transformer and the 3.3,5, and 12v rects, now what i am going to do is connect the primary of that transformer in parrallel with the transformer from other working psu, so that now the other psu is driving two transformer primaries instead of just one, if you followed me, can you comment please? thanks
subwo1 said:
Another thought is to parallel two mosfets for each switch in the push-pull or half bridge. Losses are split between two devices, RdsOn is smaller (parallel resistance), and drive circuitry won't change (with the exception of gate resistors, if you don't have any). Losses due to Gate charge are increased, but they should be small compared to other losses anyway. Or better yet, parallel 3 devices.
Full bridge looks nice in theory, but building the circuitry to drive the high side mosfets keeps me from using the topology also. (Getting a supply above the input rail and translating drive signals seems like a lot of trouble)
-Dan
full bridge smps issues
**Nice in theory?!** That's a polite way of saying it! "Building the circuitry" has already been done for us. One example is the IR2117 high side FET driver from International Rectifier, and many others along that line. They switch faster than most home-brew efforts and offer up to 600 volts of isolation, come in a small SO-8 package, and cost a little over one dollar US. Don't forget that with a half bridge topology, one of the FETs is a high side device, and will require one high side driver. The full bridge requires two. The full bridge does not require the two caps used in the half bridge, nor the series coupling cap, also needed in the half bridge. Also, the full bridge utilizes all of the copper in the transformer primary (no center tap), whereas the push-pull and half bridge only utilize half of the copper, and require a center-tapped primary, making the transformer a bit more complex than that of the full bridge. There is a reason why every producer of SMPS components and finished assemblies recommends full bridge for the highest power requirements. All of these issues and tradeoffs have been thoroughly examined for about half a century and full bridge has remained the premium topology hands down. The original question stipulated "1000+ watts" explicitly. Several posters seem to be determined to argue the case against the full bridge, which I simply just can't understand. Best regards to all.
**Nice in theory?!** That's a polite way of saying it! "Building the circuitry" has already been done for us. One example is the IR2117 high side FET driver from International Rectifier, and many others along that line. They switch faster than most home-brew efforts and offer up to 600 volts of isolation, come in a small SO-8 package, and cost a little over one dollar US. Don't forget that with a half bridge topology, one of the FETs is a high side device, and will require one high side driver. The full bridge requires two. The full bridge does not require the two caps used in the half bridge, nor the series coupling cap, also needed in the half bridge. Also, the full bridge utilizes all of the copper in the transformer primary (no center tap), whereas the push-pull and half bridge only utilize half of the copper, and require a center-tapped primary, making the transformer a bit more complex than that of the full bridge. There is a reason why every producer of SMPS components and finished assemblies recommends full bridge for the highest power requirements. All of these issues and tradeoffs have been thoroughly examined for about half a century and full bridge has remained the premium topology hands down. The original question stipulated "1000+ watts" explicitly. Several posters seem to be determined to argue the case against the full bridge, which I simply just can't understand. Best regards to all.
dkemppai said:
Dan, I like the idea of paralleling mosfets. I have had trouble doing it with bridge configurations, but my experimenting with it for push-pull was successful because the separate windings buffers one set of mosfets from other ones. In the standard bridge configuration, the parallel mosfets produced destructive transients which exceeded the maximum dv/dt rating of the mosfet channels. But now I have come up with the idea for half-bridge parallel mosfets which uses two windings on the transformer, much like push pull, but which still has capacitive coupling for the return path of the drive current. That coupling capacitor hopefully can eliminate the DC winding bias associated with the standard push-pull method.
Claude, sorry I seem so intransigent about avoiding full bridge, but my experimentations have given results which seem to eliminate the need for the extra IR2113 to drive another totem pole of mosfets. I have found that by buffering the output of the IR2113 with something like an IR7343, there is not much of an upper limit on how much gate charge the output mosfets can have. Yet, I may do more experimenting in the future, and reserve the possibility of using the full-bridge because it does provide all the benefits you have kindly mentioned.
One case where the full-bridge is needed is class D audio which suffers from PSU pumping. Then, the full bridge is needed to keep the power supply rails from rising out of control. But for driving a transformer with an AC signal, PSU pumping does not occur. I have designed a full-bridge class D amp which can be seen at http://www.diyaudio.com/forums/showthread.php?s=&threadid=14847&perpage=15&pagenumber=32
Best Wishes.
subwo1 said:
I meant IRF7343. Another buffer choice is the IRF9952, which may work better in some cases since though its mosfets are rated for less peak current, they also have less gate charge. Since these buffers are connected as source followers, it is better to drive power output mosfets that have a gate turn-on threshhold rated at 3-5 volts (usually about 4 volts nominal). That way, the 1 volt gate threshold of the buffer mosfets does not significantly affect the switching characteristics of the power output mosfets.
Re: full bridge smps issues
So this chip generates the voltage to drive the FET gate from thin air? Seems to me you'd still need 10 to 20 volts above your supply rail to make it work. Which means a second voltage source -> and more parts. (Unless I'm missing something)
-Dan
Claude Abraham said:
So this chip generates the voltage to drive the FET gate from thin air? Seems to me you'd still need 10 to 20 volts above your supply rail to make it work. Which means a second voltage source -> and more parts. (Unless I'm missing something)
-Dan
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