Transistor for DC-DC converter?

[Wolfreak]

I'm building a 200kHz switching DC-DC converter with an input between 6V and 12V, and output of 5V at 1A. The transistor I've found that should work gets very hot with the full load, even though its saturated when running. Is this normal for any switching supply running at 1A, or is there something better? I'm also trying to run the control circuitry (inherent short protection this way, short the load, shorts out the ocillator, stops the circuit), which means I'd like my 5V PWM output to (eventually) power a transistor at 12V potential.

 

[John Popelish]

Wolfreak wrote:

I'm building a 200kHz switching DC-DC converter with an input between 6V and 12V, and output of 5V at 1A. The transistor I've found that should work gets very hot with the full load, even though its saturated when running. Is this normal for any switching supply running at 1A, or is there something better? I'm also trying to run the control circuitry (inherent short protection this way, short the load, shorts out the ocillator, stops the circuit), which means I'd like my 5V PWM output to (eventually) power a transistor at 12V potential.

Switches have two separate components of loss. There are the DC
losses that are just the switch voltage drop times the switch current,
times the fraction on the cycle that the switch is on. But there is
also a switching loss that occurs every time the switch changes state,
when there is much more voltage across the switch while it still
conducts considerable current. The higher the switching frequency,
the more of these high loss events per second, so the higher the total
switching loss.

Do you have a schematic of your supply that we might criticize?
Graphic files cannot be attached to messages in this group, but they
are allowed on alt.binaries.electronics.schematics. Or a URL to a web
page that shows it would be good.

--
John Popelish

 

[John Popelish]

John Popelish wrote:

Wolfreak wrote:

I'm building a 200kHz switching DC-DC converter with an input between 6V and 12V, and output of 5V at 1A. The transistor I've found that should work gets very hot with the full load, even though its saturated when running. Is this normal for any switching supply running at 1A, or is there something better? I'm also trying to run the control circuitry (inherent short protection this way, short the load, shorts out the ocillator, stops the circuit), which means I'd like my 5V PWM output to (eventually) power a transistor at 12V potential.

Switches have two separate components of loss. There are the DC
losses that are just the switch voltage drop times the switch current,
times the fraction on the cycle that the switch is on. But there is
also a switching loss that occurs every time the switch changes state,
when there is much more voltage across the switch while it still
conducts considerable current. The higher the switching frequency,
the more of these high loss events per second, so the higher the total
switching loss.

Do you have a schematic of your supply that we might criticize?
Graphic files cannot be attached to messages in this group, but they
are allowed on alt.binaries.electronics.schematics. Or a URL to a web
page that shows it would be good.

--
John Popelish

Oops. That group for graphics is alt.binaries.schamatics.electronic.
--
John Popelish

 

[Jamie]

it depends on your components of use ?
assuming that your starting with DC i can see
that you must be using transisters.
problem #1.
if the rise time on the bias is slow you
can get heating due to the trans not being in
saturation as it aprouches its full saturation level.
doing this at 200Khz can cause a problem..
to fix this problem there are a couple of things i can
think of.
one you can tailor an input so that Square wave or a
very fast slew rate is generated with low Z on the Base of the
transistors
or you can use HexFet transistors which have a very low passive
res and requires only a very small voltage input with very High Z .,
also consider bias cut off problems, you shouldn't be allowing the
transistor to go into Bias cut off., that is if your using inductive
passive system.


Wolfreak wrote:

> I'm building a 200kHz switching DC-DC converter with an input between 6V and 12V, and output of 5V at 1A. The transistor I've found that should work gets very hot with the full load, even though its saturated when running. Is this normal for any switching supply running at 1A, or is there something better? I'm also trying to run the control circuitry (inherent short protection this way, short the load, shorts out the ocillator, stops the circuit), which means I'd like my 5V PWM output to (eventually) power a transistor at 12V potential.