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Wide Input 800mA DC-DC Converter

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asimkumar

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Vin=4.5V to 18V
Vout=6Volt
Iout=800mA
Now i am trying this with MC34063 inverting mode with internal switch. But the output voltage is not constant with change in current.
Should i use a external transistor or mosfet.

or should i use any other controller ?

thank you
 

The MC34063 should be able to fulfill your specification without an external switch, though under high load and low input voltage conditions it will probably get somewhat hot. Are you using a circuit similar to figure 13 in this datasheet? https://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF Notice that in the inverting/buckboost circuits, the MC34063 is referenced to the negative output, not to circuit ground.

If buckboost converters are giving you trouble, you may consider using a SEPIC. More components, but can be easier to control because they use low side switches and don't invert the voltage polarity.
 

i have used the same circuit but it does not give stable output. when i am connecting the load (a 10 ohm resistor) the output goes down to 4.6volts

thanks for response
 

It may just be current limiting. Try decreasing the current sense resistor (Rsc in that schematic). Raising the input voltage should help too.
 

thanks mtwieg

can u give me some parameters for the inductor and the capacitor,
another question
the MC34063 has comparator input range -3v to 40v
then when am regulating for -6v means the comparator pin will get -6+1.25=-4.75v which is out of range, then how will it work ?
 

The comparator has a 1.25V reference so the feedback always scales to the same threshold of 1.25V.

https://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF
https://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00001232.pdf
https://www.ti.com/lit/ds/symlink/mc33063a.pdf

In the inverse mode the 1.25V reference is tied to the negative output, so the Comparator threshold is 1.25V above the V- using R ratio.

1) scope every pin and confirm your assumptions and connections against data sheets.
2) Check your coil and cap for values, & switch for saturation behavior etc.
 
can u give me some parameters for the inductor and the capacitor,
That's a hard question to answer without any other info. If you just want to get a converter working, then I recommend using values near those in the example schematics, and see how that works for you. Based on that you should adjust component values.
another question
the MC34063 has comparator input range -3v to 40v
then when am regulating for -6v means the comparator pin will get -6+1.25=-4.75v which is out of range, then how will it work ?
Again, keep in mind that in the buck boost converter, the MC34063 is referenced to the output, which is negative. So nominally the output will be -6V and the comparator pin will be -4.75V relative to your circuit ground. But the MC34063 will see the comparator input as +1.25V referenced to its own GND pin, and that's what really matters.
 
The inbuilt switch has less current capacity than the required Ipk=4000mA for output 800mA, should i try with an mosfet ?
can i use MC34063 in sepic mode ?
 

Can i replace the transistor in attached inverter.JPGdiagram with a N channel mosfet IRF510
 

The inbuilt switch has less current capacity than the required Ipk=4000mA for output 800mA, should i try with an mosfet ?
A FET can help if used properly.
can i use MC34063 in sepic mode ?
Yes.

Can i replace the transistor in attached View attachment 73656diagram with a N channel mosfet IRF510
An N channel FET probably wouldn't increase efficiency at all, since the MC34063 doesn't have a high side N channel gate driver. However, in this case a P channel FET may be the best simple solution (as is often the case with buckboost converters). Try to get one with a low Rdson (like 100 milliohms or less).
 

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