Designing an Inverting Boost-Buck Convertor Issue

Thread Starter


Joined Jul 22, 2023
I'm trying to design a boost buck convertor from a 3D printer power module and I am using Multisim to simulate it before starting but I think I'm having issues with the biasing. (EE student noob here so bear with me!)

My requirements are
Vin 12V-30A
Vout 0-50V (inverted)

For the PWM I used a function generator here but I'm going to replace it with a 555 PWM setup

Screenshot 2023-07-22 at 7.26.43 PM.png

During the time when the mosfet is closed the current flows to the inductor correctly but when the mosfet is open the current from the inductor instead of going through the load it goes back to source since:

Screenshot 2023-07-22 at 7.29.31 PM.png

The green line is the inductor current (IL) which is correct and the duty cycle in this screenshot is around 70% which sounds right for the graph.

The red line is the diode current which is not expected. When the switch opens and IL starts dropping I get a spike in the diode current as if it's trying to go to forward bias but fails and drops back to zero.

Screenshot 2023-07-22 at 7.34.32 PM.png

Notice how the blue line which is current at source is also the same as the inductor current whereas you'd expect the falling current go immediately to zero and that falling current be present in the diode current and the sum of IDiode + ISource = IL graph but nothing is flowing through the load and consequently the diode.

The inductor voltage is also not behaving properly. At 70% duty cycle you'd expect it to be in boost mode but:
Screenshot 2023-07-22 at 7.46.57 PM.png

Interestingly when I short the load (set R1 to zero) the IDiode and ISource behave properly but the VL is still the same as above. This plus the Isource having the transient fall of current was what made me suspect a biasing issue.

Screenshot 2023-07-22 at 7.39.29 PM.png

My PWM parameters:
Screenshot 2023-07-22 at 7.48.21 PM.png

Any ideas what might be causing this? I've burnt a few days staring at the monitor trying to figure this out :D



Joined Aug 7, 2020
You need a P-channel MOSFET.
For an N-channel to work, as the source goes down to V-, the gate must do so as well.
If the gate remains at 0V, then there is a positive voltage between gate and source and the MOSFET turns on.