Re: Power Dissapation
The power rating of a transistor is the amount of average power that may be reliably dissipated *in* the device. It has nothing to do with power output of the circuit where you are going to use that transistor.
I think that you have still some misconceptions to fix before attempting to build a prototype.
Concerning forward versus full bridge discussion, two-transistor forward converters are more fool-proof because there is only one switching phase. Full bridge converters have two phases and four switching cells to synchronize, and if you get distracted and make some mistake when you wire them, you can toast $15 of IGBTs in a fraction of a second like I have just done
Pungie said:
Good morning every one. Busy with calculations on my design. One thing I like to know. (Perhaps a stupid question). I'm selecting IGBT's for the switching. In the data sheets it says 140W Power Dissapation. If the IGBT is rated 200V 30A then V X I isn't = to the power dissapation. What is power dissapation then?
The power rating of a transistor is the amount of average power that may be reliably dissipated *in* the device. It has nothing to do with power output of the circuit where you are going to use that transistor.
I think that you have still some misconceptions to fix before attempting to build a prototype.
Concerning forward versus full bridge discussion, two-transistor forward converters are more fool-proof because there is only one switching phase. Full bridge converters have two phases and four switching cells to synchronize, and if you get distracted and make some mistake when you wire them, you can toast $15 of IGBTs in a fraction of a second like I have just done
Full-or-Nothing
Pungie,
Just to clarify things (or further confuse them ), the one-transistor and two-transistor forward, half- & full-bridge are all part of the Forward Converter family. It's just that the last two are double-ended, meaning their cores are magnetized in both directions (they have flux swings in both directions of the B-H graph). Same thing for +12V-fed center-tap push-pull topologies.
The reason they are all called forward topologies is because energy is transferred from the primary to the secondary windings while the switch is on, or when current is moving forward. For the one-transistor forward topology, a third winding, called the reset winding is necessary to clear the transformer core of any residual magnetic field. This happens during the switch's off-time.
Flybacks, on the other hand, transfer their energy from primary to secondary during the switch's off-time, eliminating the need for the reset winding. However, one would never consider a flyback for your power levels, beacuse of the peak currents involved.
I'm still convinced that for your power levels, even given its relative complexity, the full-bridge is the way to go. Do a topic search for Eva's 1.8kW (yes, that's 1800 Watts) fullbridge switcher. Filled with lots of good info. I think she said it uses only 4 MJE13009s (400V @ 12A, I think). Pretty impressive, considering most AT & ATX SMPS manufacturers that actually used these old Motorola bi-polars for the primary switches had trouble getting anything more than 200-300W through them.
Pungie,
Just to clarify things (or further confuse them
), the one-transistor and two-transistor forward, half- & full-bridge are all part of the Forward Converter family. It's just that the last two are double-ended, meaning their cores are magnetized in both directions (they have flux swings in both directions of the B-H graph). Same thing for +12V-fed center-tap push-pull topologies. The reason they are all called forward topologies is because energy is transferred from the primary to the secondary windings while the switch is on, or when current is moving forward. For the one-transistor forward topology, a third winding, called the reset winding is necessary to clear the transformer core of any residual magnetic field. This happens during the switch's off-time.
Flybacks, on the other hand, transfer their energy from primary to secondary during the switch's off-time, eliminating the need for the reset winding. However, one would never consider a flyback for your power levels, beacuse of the peak currents involved.
I'm still convinced that for your power levels, even given its relative complexity, the full-bridge is the way to go. Do a topic search for Eva's 1.8kW (yes, that's 1800 Watts) fullbridge switcher. Filled with lots of good info. I think she said it uses only 4 MJE13009s (400V @ 12A, I think). Pretty impressive, considering most AT & ATX SMPS manufacturers that actually used these old Motorola bi-polars for the primary switches had trouble getting anything more than 200-300W through them.
Pungie,
Regarding power dissipation of the device, you may not exceed the rated voltage, OR the rated current, OR the rated power. This defines the safe operating area of the device. When switching, the V(on) x I(on) and V(off) x I(off) mustn't exceed Pmax. You shouldn't ever have Maximum voltage across the device AND maximum current flowing through it!
John
Regarding power dissipation of the device, you may not exceed the rated voltage, OR the rated current, OR the rated power. This defines the safe operating area of the device. When switching, the V(on) x I(on) and V(off) x I(off) mustn't exceed Pmax. You shouldn't ever have Maximum voltage across the device AND maximum current flowing through it!
John
SMPS
Thanks all who particapated. I have enough info to guide me. If I get stuck, I will check for some help. By the way, any one who comes accross this thread and is interested in designing, have a look at this guide. http://www-s.ti.com/sc/techlit/slup057.pdf
Thanks all who particapated. I have enough info to guide me. If I get stuck, I will check for some help. By the way, any one who comes accross this thread and is interested in designing, have a look at this guide. http://www-s.ti.com/sc/techlit/slup057.pdf
Steve,
I must be pretty good at keeping a secret. Actually, I guessed who you were, but it could be because my mind is so idle. Actually, I may not have thought of who you were without your signature below your posts. I became quite sure after linking information in your posts to some I have seen on the group site. I am glad you are a moderator of the group, thanks.
I must be pretty good at keeping a secret.
Actually, I guessed who you were, but it could be because my mind is so idle. Actually, I may not have thought of who you were without your signature below your posts. I became quite sure after linking information in your posts to some I have seen on the group site. I am glad you are a moderator of the group, thanks.
Moderation
Subwo1:
Yes, it has been quiet, except for the occasional joker who asks for complete plans for an SMPS, along with SPICE models, graphs, test results, pc board layout and full BOM so he can do his senior paper or Master's Thesis!
Pungie,
I know we've gotten a little off-thread, but do let us know how you will proceed, and with some results.
Steve
Subwo1:
Yes, it has been quiet, except for the occasional joker who asks for complete plans for an SMPS, along with SPICE models, graphs, test results, pc board layout and full BOM so he can do his senior paper or Master's Thesis!
Pungie,
I know we've gotten a little off-thread, but do let us know how you will proceed, and with some results.
Steve
Moderation
Subwo1:
Yes, it has been quiet, except for the occasional joker who asks for complete plans for an SMPS, along with SPICE models, graphs, test results, pc board layout and full BOM so he can do his senior paper or Master's Thesis!
Pungie,
I know we've gotten a little off-thread, but do let us know how you will proceed, and with some results.
Steve
Subwo1:
Yes, it has been quiet, except for the occasional joker who asks for complete plans for an SMPS, along with SPICE models, graphs, test results, pc board layout and full BOM so he can do his senior paper or Master's Thesis!
Pungie,
I know we've gotten a little off-thread, but do let us know how you will proceed, and with some results.
Steve
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