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OUTPUT TRANSISTOR SATURATES ON INCREASING PWM--BOOST CONVERTER

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DannyTheGhost:
The circuit you've drawn is actually making no sense
This actually doesn't even look like any dc-dc topology we know so far
Firstly, TL494 has 2 error amps connected in OR configuration, so you can only use only one
Secondly, boost dc-dc topology means 'inductor getting shorted to ground by switch and when switch is disconnected inductor adds its back EMF voltage to VCC, but what you have here on your circuit is 'your output is getting connected to input voltage via inductor and diode'

mikerj:

--- Quote from: Achu on May 23, 2019, 02:33:06 am ---Here is the circuit.
I intend the converter to produce 30V  rated output and a maximum of 55V at a current of 2A.
The inductor I used was a rewound torroidal one from the filter of ATX. It had a value of about 200uH.

--- End quote ---

The schematic is clearly incorrect, but what is your input voltage?  This will determine switch current for a given output.   55v and 2Amps is 110 watts, not a trivial amount of power.

Note that a UF4007 diode is only rated for 1 amp.

Achu:
Sorry I drew the circuit in a hurry.And I have modified it.Please check the initial post.
This is my first easy EDA dawing.
Supply voltage is 12V.

T3sl4co1l:
Can barely make out the labels at that scale.

The boost section looks to be wired like a boost section. :)

It still doesn't look like it should do anything but blow transistors, though.  The "E" pin is pull-up only, there is no way to pull down an already-pulled-up transistor gate.  (I forget how much current goes into the pin; it's not exactly an emitter, there is some internal circuitry attached.  I don't think it can pull down this much, though.)

The missing feature would seem to be that you need a gate driver.

You also need a current sense, and compensation for feedback.  The current sense is key, as that prevents the transistors from exploding.  Everything else is secondary. :)

Tim

T3sl4co1l:
You may find this is more practical,



(the top boost section, don't mind the bottom inverter section).  This uses a peak current mode controller,

There are some typos on this exact schematic; the controller should be UC3843 (starts up at 5V supply, not 16), the 20uH inductor needs to be low loss (a "drum" style ferrite choke will do), and some slope compensation would be nice, or a lower value inductor can be used.

To save power, the current is sensed by a current transformer.  An off-the-shelf 1:100 CT can be used, in which case the 2.2 ohm resistor value drops a little for the same ratings.

To get more output voltage, the feedback divider ratio changes of course, and the transistor and diode ratings need to increase (use 80 or 100V parts for a 55V output).

Note the SB540 and 1uF capacitor across the transistor: these must be placed as close as possible, so that at the moment the transistor turns off, its load current is transferred to the capacitor, and stray inductances discharge into it.  This isn't needed if the SBL2040 diode and output capacitors are equally nearby, but that may be hard to arrange, in which case having the smaller diode and capacitor up close are effective at protecting the transistor.

The 3842 family is easier to use than the TL494, I would say.  It's also safer to use -- because of the current mode operation.

Tim

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