dc to dc

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Hello everybody

I have a Load his VCC=5V , I=1.5A
But my power supply is battery his volte=24V dc
I use ka34063 dc to dc converter in step down mode, I use also External NPN Switch (bc337) because IC Peak Greater than 1.5 A.

but the circuit don't work (the power disconnet after few second with the temperature of transistor is very hot)

what the typical value of External NPN Switch transistor I must use.

what is my erorr

Help me Please"¦.
Waseem

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The data sheet indicates the internal switches can handle 1.5 Amps but there is no reference design shown. You did not upload a schematic so no one here can offer advice until you do that. There are lots of SMPS controllers that do provide design examples and also free spice software. National Semi offers online design tools and Linear Tech has downloadable free tools SWCAD

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hi

this My Schematic

waseem

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I am not an expert on this layout but I suspect you may need some resistance between the SWE pin and the base of the external resistor. The only thing limiting base to emitter current now is the resistance of the inductor. Did you find this schematic somewhere as an example?

EDIT
The BC337 transistor appears to be a 1 A max transistor so it is not as good as the internal switches. It appears to be a small plastic transistor. Is that the one you are using? I did find a Motorola data sheet that has example of external transistor but it does not give any details of the recommended switch or inductor.
I think you need a better transistor.

Last Edited: Tue. Mar 2, 2010 - 07:55 PM
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The BC337 has a maximum Ic of 800 mA. Therefore, using a BC337 to increase the 34063's maximum inductor driving current of 1.5 A doesn't make sense.

Further, the internal output stage of the 34063 makes it difficult to drive an NPN transistor as a switch, because it is used as an emitter follower in that case. Not turning the BC337 fully on or off is likely the reason for it getting to hot and then getting destroyed.

Switch to a PNP power transistor (a real one, not a 800 mA toy), use the internal power stage in emitter darlington configuration to drive the PNP, and carefully calculate the PNP's driving and turn-off resistors.

Stealing Proteus doesn't make you an engineer.

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alwelch wrote:
The only thing limiting base to emitter current now is the resistance of the inductor. Did you find this schematic somewhere as an example?
A 34063 does current limiting via the IPK sense input. If the difference between IPK and VCC is 330 mV or higher, the IC limits the inductor current.

Stealing Proteus doesn't make you an engineer.

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Emitter follower does not saturate except under unusual circumstances. This results in more forward drop and higher power dissipation when used as a switch. This is exactly what is happening in your circuit.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Just thought i'd wack up a step down schematic with external pnp.

oddbudman

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Hi

I know how use PNP Transistor in my circuit because I have full information about the 34063 chip by its datasheet.

Now I want to know what the typical value of External Switch transistor I must use (when I use NPN transistor and When I use PNP transistor) in my circuit.

thanx to everyone
waseem

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Since you have the complete data sheet you would need to look at what the base drive voltage is and make sure the transistor you select can handle that as well as the collector voltage and the expected load current. Also the switching speed. I still think you would have an easier time with a newer smps IC with complete schematic suggestions from the vendor. Most use FET transistors now and high frequency to improve efficiency. On the National Semi web you can input your load current, VCC, etc and it will build you a design and BOM

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Try this, it might work.

oddbudman

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You would use the NPN in the boost configuration. In the buck configuration, you would not be able to put enough current into the transistor to turn it on fully - pin 1 would have to rise above the input voltage for it to work properly. Therefore in buck config, you must use a pnp. A BD140 might be a good candidate. Note that your schotty diode is only a 1A device (1n5817). Basically you want oddbudman's circuit.