Hi,

I am designing a Buck-Boost converter and am new to Power Electronics (mostly have a background in Digital and some analog signals). I was using a BJT in my Buck-Boost and it was working fine in my SPICE model but discovered that they have a large on-resistance implying a large power disappation. FETs I hear don't have that issue.

When I put a FET in and modeled the circuit, it seems that I need 1.5MV to fully turn the FET on. As you can see by the screen-shots I posted of my circuit/simulation. With 15V applied to the Gate, I get an output in the FEMTO volts--yes, femto.

When I apply 1,500,00V, I get the desired result. I think I have the NMOS biased wrong but can't seem to figure out how to bias it correctly.

Any suggestions?


Thank You,
George

 

What's the voltage at the NMOS source? What's the gate-to-source voltage difference?

 

You are using the N-MOSFET as a source follower which is not normally used for this type of circuit. Use a P-MOSFET instead with the source as the input and the drain as the output.

 

Awwww... I was trying to lead him there.

 

Awwww... I was trying to lead him there.

Sorry I jumped the gun.

 

I attached a screenshot of my new circuit with the pmos. I think my problem is how I have the gate biased. I think I need to put a resistor in and bias it towards the drain or source but I have tried several combinations with no success.

Thank you both for your help. This is racking my brain.

 

To be clear, I noticed if I put a large value series resistor between the pmos and ground (or just set L1 to have a large series resistance), I can get the circuit to behave properly. However, this would not work in practicality because it will have 10's of watts flowing through said resistor.

 

Post your .asc file for the circuit.

 

You really need to select a model of a MOSFET. The default NMOS and PMOS leave much to be desired.

Right-click on your NMOS or PMOS to get the "MOSFET Properties" dialog box open, then click the "Pick New MOSFET" button. You will then get a scrollable list of the available MOSFETs. Try to select a MOSFET that has a Vdss rating that's 20% to 50% more than your supply voltage, and then make a trade-off selection between Rds(on) and nC (total gate charge).

Then right-click on your diode and pick a model for it.

Tip: always reference your gate voltage to the source terminal of the MOSFET; as that is what matters to the MOSFET. It does not "care" about ground. If the difference in the voltage between the gate and source terminals exceeds ±20v (±15v for some) even for an instant, it will be destroyed. Your simulation model won't blow up because of such abuse, which is just one item that will conflict with what you would experience in a lab with real parts.