Buck-boost

[Shadow_warrior]

Hello everyone,
I am making a buck boost converter with TL494. Operating frequency is 250KHz. problem is that whenever I am increasing the duty more than 40% ,switch is getting burned. Note Input voltage is only 18 V and output current is 0.5A. I am using IRF540 . Inductor is 110uH. Can anyone tell the reason?

 

[Dick Cappels]

To begin with the TL494 was designed to drive bipolar transistors, just before MOSFETs capable of any appreciable power began to appear, so your gate drive circuit would be most interesting to see.

250 kHz is very fast for a TL494, from the era in which a 30 kHz switching frequency would have evoked gasps of amazement.

Please post a schematic of your power supply. The verbal description does not provide enough insight for us to help you.

 

[Shadow_warrior]

To begin with the TL494 was designed to drive bipolar transistors, just before MOSFETs capable of any appreciable power began to appear, so your gate drive circuit would be most interesting to see.

250 kHz is very fast for a TL494, from the era in which a 30 kHz switching frequency would have evoked gasps of amazement.

Please post a schematic of your power supply. The verbal description does not provide enough insight for us to help you.

Hello Sir,
I am uploading my circuit along with TL 494 connection. Please look through this. Thanking in anticipation. I have also included LT spice simulation of my closed loop model. But I am not able to implement open loop itself

 

[chemelec]

Hello everyone,
I am making a buck boost converter with TL494. Operating frequency is 250KHz. problem is that whenever I am increasing the duty more than 40% ,switch is getting burned. Note Input voltage is only 18 V and output current is 0.5A. I am using IRF540 . Inductor is 110uH. Can anyone tell the reason?

18 Volts on the Gate is Pretty High.
And a Push-Pull configuration would be More efficient.

 

[Shadow_warrior]

18 Volts on the Gate is Pretty High.
And a Push-Pull configuration would be More efficient.

but i am giving only 15Volts. Anyway, 20volts Vgs maximum can be given (from datasheet)

 

[alec_t]

But I am not able to implement open loop itself

Sim runs ok with open loop.

 

[Dick Cappels]

What are the values of the two fixed resistors connected to TL494 pin 11?

With a switching frequency of 250 kHz you only have 4 microseconds per cycle. I suspect that you cannot use low enough value resistors on pin 11 to charge and discharge the 1500 pf input capacitance fast enough.

You may need to increase the inductance if you do this, but if you don't want to add a MOSFET gate driver to the circuit low the clock down by a factor of 10 and it might start to work.

If you have an oscilloscope you can see what the gate waveform looks like. If you can see it, please post an image or describe it.

I rarely have anything positive to say about these plug-in breadboards, because I used them extensively for a while, but they are particularly problematic in circuits that involve high frequencies and those that have high currents. I think this circuit has both.

Bypass capacitors for the TL494 and the input to the switching circuit are a good idea.

 

[chemelec]

Personally I Only Trust Real Circuits, Not Simulations.
Perfect Parts in Simulations Do Not Exist in real life.

Also your Mosfet NEEDS a HEAT SINK.
The Rated Power on Transistors and Mosfets is ONLY when they have an Appropriate Size of Heat Sink on them.

 

[Shadow_warrior]

Sim runs ok with open loop.

Yes

 

[Shadow_warrior]

Personally I Only Trust Real Circuits, Not Simulations.
Perfect Parts in Simulations Do Not Exist in real life.

Also your Mosfet NEEDS a HEAT SINK.
The Rated Power on Transistors and Mosfets is ONLY when they have an Appropriate Size of Heat Sink on them.

I applied heat sink too. Now I am using different power source in which you can set input current. Beyond that it will turn off. This is what is happening. It is always turning off.

 

[Shadow_warrior]

What are the values of the two fixed resistors connected to TL494 pin 11?

I am using 10 ohms as gate resistance and the other one is 150 ohms. Both are high wattage resistances.

You may need to increase the inductance if you do this, but if you don't want to add a MOSFET gate driver to the circuit low the clock down by a factor of 10 and it might start to work.

For buck converter (250Khz) I used the same pulse generator without any problem.

 

[Dick Cappels]

(Some text removed for clarity) I am using 10 ohms as gate resistance and the other one is 150 ohms. Both are high wattage resistances.

That should being your gate drive rise time down to about half a microsecond.