buck converter design in MultiSim Blue

[simba26]

I downloaded the student (free) edition of MultiSim Blue at my university. I am looking to design a 12V to 5V and 20A converter. Is there supposed to be a sample buck converter design to be found in the misc section of the components? It would help a lot if I could get an idea of what the design includes. I've read that some have done it with MultiSim, but not MultiSim Blue.

To be more particular, I am confused on the part where a controller like a 555 timer switches a MOSFET on and off at a specific frequency. This cycle detemines the output voltage of the converter. 

Please excuse my inexperience, but here's what I have so far. I think I have the main components (with random values that don't work), but it's the general idea.

[OStep]

Hello simba26,

Here are some tips on getting started with building and understanding the operation of a buck converter

• Start off with ideal elements to make it easy to calibrate the design and choose key component values such as output capacitor and inductor - use ideal switching elements (for the transistor and diode) and use a basic pulse/clock voltage clock source to drive the transistor. As a starting point you really DO  NOT NEED to use a 555 timer because it adds unncessary complications; you can use this component later in your simulation if you are actually planning on using it to drive the MOSFET gate in your design. 
• In any switching design, it is critical to view the waveforms, so you use a scope instrument and not just a DMM. 
• Your buck converter was missing the load. You should probably load your circuit, using a either a resistor or current source, otherwise you are not simulating the nominal condition of the buck converter. 

I attached a couple of circuits that simulate the operation of a 5V, 1A buck converter.

The ideal circuit uses ideal elements. The transistor switch simply turns ON (low resistance) when its gate voltage is 5V, and turns off (high resistance) when the gate voltage is 0V. Note that the output is exactly 12V * 0.416 duty cycle of the clock source. 

The more realistic circuit uses models of actual orderable MOSFET and Diode parts. Note that I increased the gate drive voltage to 17V, because this is we are now using a realistic MOSFET where the gate must be higher than the source voltage in order for the MOSFET to be turned ON. 

Play around with the component values (L, C, Rload, switching frequency) and note how they affect the waveforms. 

Once you are ready to use a 555 timer, I suggest you first get the 555 timer working in separate simple circuit and only then combine it with the buck converter. I noticed that you are actually not using the 555 timer correctly (you need specific R,C components connected in a specific way in order for the 555 timer to generate a pulse train), so you may want to google-around to see how to connect it up. 

[simba26]

Wow thanks, you really didn't have to go through the trouble to actually make one! I was mostly confused on how I could control the opening/closing of the MOSFET gate to control the pulse. I didn't know that a clock voltage source doesn't just that. And can't believe I forgot to put on a load to complete the circuit.

I'll keep your tips in mind. I'll start with the basic, assuming ideal stuff and see how everything works, then see if I can add more components to get it to spec.

[Ammadk]

Hi Ostep

I am designing a buck conv 20~15v to 15v.After going through different calculation a came up with values of L and C that give values nearly equal to 15v but as i mentioned earlier my Vin varies from 20 to 15 as in order to bring my Vo to 15v i must change PWM but this circuit has no affect when I change PWM e.g When my Vin is 20V my PWM has to be 75% almost but this circuit gives me 14.5 V when PWM is not 75%...even if i vary Vin and PWM Vo is always 14.5V...14.5V is nearly my requirment but varying PWM should have some affect on Vo.

I am attaching circuit file..Please cheack how this problem can be solved

[OStep]

Please remember that in order to turn On a MOSFET, the gate-to-source voltage (Vgs) must be some positive voltage. For power mosfets Vgs is typically somewhere between 3V to 10V. In your circuit, the source would be 20V when the MOSFET is ON and therefore your gate voltage would need to be at least ~23V. But you're only suppling 17V. Increase V1 to 25V and you should see much better results.

[Ammadk]

I've tried with 25Vgs but it now sticks at 19.5v and shows alot of Vo fluctuations when i change load then at 17Vgs.

And furthur more 25Vgs should be kept just for simulation purpose of it should be 25Vgs when building hardware.