# Buck-Boost Converter Simulation

Discussion in 'The Projects Forum' started by everibarra, Nov 14, 2013.

1. ### everibarra Thread Starter New Member

Nov 8, 2013
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Hello everyone,
I am currently working on a buck-boost converter for a solar cell while trying to vary the output voltage by switching the input signals' duty-ratio.

I uploaded a picture of the simulated schematic using ICircuits.

The offset of the pulse input is set in order to keep the transistor in saturation mode (ON) . Also I understand that the voltage across the inductor (VL) should equal (Vin), and as you guys can see, the voltage decreases all the way down to milivolts for some reason.

Why is this happening? Should the transistor be switching functionality ? If so, would a simple pulse between -5V and 5V work?

NOTE: Solar Panel MAX open circuit voltage : 38V , MAX open circuit current: 9A

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2. ### Alec_t AAC Fanatic!

Sep 17, 2013
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What output voltage are you expecting with the component values shown?
What gate voltage are you applying?
What is the zener voltage?

3. ### everibarra Thread Starter New Member

Nov 8, 2013
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It's an NMOSFET.
I'm simulating with a solar cell with a MAX open circuit voltage of 38 V , so I am expecting something around that value.
Gate Voltage applied is a signal input voltage, more specifically an square voltage signal from 1V to 5V (Threshold voltage of transistor is 1V)
Zener Voltage at 5mA is 5.6V

4. ### crutschow Expert

Mar 14, 2008
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The transistor is used as a source-follower so the inductor voltage will never be more than 5V minus the transistor gate threshold voltage.

Your circuit is unlikely to do what you want without significant design changes.

What is the purpose of the zener diode?

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5. ### everibarra Thread Starter New Member

Nov 8, 2013
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I got this Buck-Boost Converter design from a book called "Power Electronics" by Ned Mohan.

From what I read, the zener diode will allow the inductors' current to flow when the transistor is turned off.

6. ### crutschow Expert

Mar 14, 2008
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Is that the exact circuit Ned showed in the book? It doesn't appear to work properly as shown.

For best efficiency you either want a PMOSFET for the switch or use a bootstrap driver for the NMOSFET so that the full solar cell voltage appears across the inductor when the MOSFET is ON. Any voltage drop across the MOSFET reduces efficiency.

And I realize the diode is not a zener, it's a Schottky diode.

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7. ### everibarra Thread Starter New Member

Nov 8, 2013
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Can you talk more about the bootstrap driver? Can you and would you please provide a schematic of the driver ? I would really appreciate

8. ### crutschow Expert

Mar 14, 2008
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There are some IC types shown on this page. This is a application note for bootstrap drivers.

Edit: Here's a thread on that subject.

Last edited: Nov 15, 2013
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9. ### everibarra Thread Starter New Member

Nov 8, 2013
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So I have to rely on ICs ... I have no way of simulating the circuit with the program I originally used? The only way to simulate is to get the actual bootstrap driver and test the actual circuit?

10. ### Alec_t AAC Fanatic!

Sep 17, 2013
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Try simulating this:

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11. ### crutschow Expert

Mar 14, 2008
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Code ( (Unknown Language)):
1.
That will work as a boost circuit but not a buck.

12. ### Alec_t AAC Fanatic!

Sep 17, 2013
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True, but the original OP circuit didn't work as either buck or boost

13. ### crutschow Expert

Mar 14, 2008
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Below is the simulation of a discrete bootstrap circuit in LTspice.

Due to the high supply voltage D3 and D4 are 10V zeners added to keep the peak MOSFET Vgs ON voltage below 20V. It could be one 20V zener but I didn't have that in my model library. Those aren't needed if the supply voltage is below 20V.

The transistors are just representative ones in my library. They can be any similar devices that meet the circuit voltage and current requirements.

14. ### crutschow Expert

Mar 14, 2008
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So half a loaf is better than none?

15. ### everibarra Thread Starter New Member

Nov 8, 2013
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So I tried your circuit Alec_t, it is pretty much working as a follower for me... Not boosting nor bucking. Did it work for you? Please let me know if it did!
Either way thank you very much for your help.

16. ### Alec_t AAC Fanatic!

Sep 17, 2013
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It worked much as you found. All I did was vary your circuit by changing the FET round. A proper buck/boost circuit would be more complex.