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Need a bit of help with a 12V 10A SMPS design using TL494

hrishib

Aug 30, 2013
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I'm designing a 12VDC, 10A SMPS that would drive a brushless motor using the TL494. The motor has an inrush current of about 8A and a nominal current draw of around 5.5A. I was looking at TI's design guide here. The design example (page 24) they've done uses a 5V output (mine however would be 12V). I have two primary questions,

First: To get a 12V output, I'm guessing I would only need to change R8 and R9 divider to get the appropriate value at the error amplifier. Can I use the Vref (instead of dividing it) as one of the references for the error amplifier and then divide the 12V to 5V as the other reference for the error amplifier.

Second: The power switch seems to be a pair of NPN=PNP transistors. Can anyone recommend alternate parts for these? The NTE153 and NTE331 are not available in my part of the world. Also, can I replace this with say a power MOSFET? If so, what modifications would need to be done to power switch circuit?

Any advice would be much appreciated. Thanks!
 

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duke37

Jan 9, 2011
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My old book does not cover the NTE153 and NTE331 but these seem to be nothing special.

My notes of transistors that I have show the following types.

pnp driver
2N1132
BFX29
BFX30

npn power, heat sink needed.
2N6099
2N3055
BDY20

A p-channel fet could be used with a couple of resistors to turn it off and a zener diode (probably 12V) to limit the base voltage. Do a search for fets over 15A and 50V.

There may be some concern about switching speed, depending on the frequency of the oscillator.
 

hrishib

Aug 30, 2013
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Thanks duke!

Whats the concern about the switching? The oscillator frequency is about 25KHz.
 

duke37

Jan 9, 2011
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I have looked up the data sheets of the transistors, they are nothing special.

NTE153 90V -4A 8MHz
NTE331 100V 15A 3MHz

You can google for equivalents.

The frequency of switching is important since the transistor dissipates quite a bit of power during the switching time when there is significant voltage and current. The transitors recommended are quite slow. I doubt if these transistors would work efficiently with a 100kHz switching frequency.
 

hrishib

Aug 30, 2013
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Wonderful! Thanks!

I was thinking of using a complementary pair such as the TIP35/36. Also the switching frequency is 25KHz and not 100KHz right?
 
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