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Step up converter schematic - which transistor and inductor type should i use?

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Garyl

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Hey
I have schematic but no description. Please take a look:

My questions are:
- what is the maximum current provided by that power supply?
- does MOSFET really requires heatsink?
- what kind of inductors (there are two on schematics) are required here? Can I use simple axial 10uH THT inductor?
- can I replace BUZ with BUT11AX or IRF640 or tip122?
thanks in advance
 

Hi,

Have you read the TL494 datasheet, or the Fairchild KA7500 datasheet ? - You may find some useful explanations in the TI one, it's quite a detailed document.

A very quick look seems to say ~200mA. Apparently only for the gate of the MOSFET

The MOSFET heatsink question can be ascertained by doing the junction temperature rise calculation, and/or PD. Section 10.2.2.5 has "Transistor Power-Switch Calculations", but for heatsinks, there are online calculators that will do it for you unless you prefer to do it by hand.

Read the datasheets for the transistors you ask about, the electrical specifications will show how interchangeable they are.

Not understanding the circuit too well, and going on snippets in the datasheet, I'm intrigued, why does a 22mA LED need a PWM IC that seems to need a lot more current to operate the MOSFET?!
 

I don't recommend to use switched mode supply for Tuning Voltage of TV Tuners because the picture will be noisy due to high Phase Noise of the VCO.
Even if the signal is digital QPSK, QAM etc. there will be some impacts on the reception.
 
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    Garyl

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A very quick look seems to say ~200mA. Apparently only for the gate of the MOSFET
Is it the Tl494 output current or the current capability of 33V source?

I don't recommend to use switched mode supply for Tuning Voltage of TV Tuners because the picture will be noisy due to high Phase Noise of the VCO.
Even if the signal is digital QPSK, QAM etc. there will be some impacts on the reception.
So what do you suggest? I have disseasembled several TVs with tuners and they all have SMPS power supplies... I haven't seen any TV or computer monitor with 50Hz transformer in a long time...
 

Is it the Tl494 output current or the current capability of 33V source?


So what do you suggest? I have disseasembled several TVs with tuners and they all have SMPS power supplies... I haven't seen any TV or computer monitor with 50Hz transformer in a long time...
Yes you're right, nowadays every equipment uses SMPS but Vtune is very sensitive against supply noises and if it's not very well filtered, it will create a big trouble.If this circuit will be used in a Digital Reception System, noise can be tolerated by modulation scheme but analog systems are more sensitive.My suggestion was just a warning.
 

Hi,

200mA is the maximum output current the datasheet specifies for the gate of the MOSFET, the 33V power supply will have a higher current capability. You'd need to know what draws current from the 33V supply - apart from the 22mA LED - to estimate how much it uses, and go over the datasheet of the 494 to understand the current through the inductor and the MOSFET.
 

I'm puzzled by the complexity and size of this thing. The tuning voltage for a TV tuner typically feeds a PLL and then varicap/varactors for generating the LO. The current drawn is usually less than 1mA so using a circuit with a 13,500mA 95W rated MOSFET and parallelled gate drivers seems like gross overkill.

A 'Joule Thief' small self oscillating stage and Zener would do the trick for almost no cost and be hundreds of times more power efficient.

Brian.
 

You are correct, Brian, about the minuscule current requirements of a varactor tuning system.

Many years ago, I saw a minimalist and effective way: it took a microcontroller line that was toggling something else, and used it to additionally drive a small transistor boost circuit. The duty cycle was not very consistent, but the voltage was post-regulated with a zener. Very clever!

So I would say that a simple joule thief with a zener regulator should suffice.
 

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