I am simulating a boost converter in ltspice.

This is the circuit

vin=3V, vout_bst=12V, ideal inductor, capacitor, diode and voltage source. When I use an ideal switch, raising the frequency doesn't affect the output voltage but when I use MOSFET transistor BSZ036NE2LS the voltage drops when I raise the frequency.

enter image description here

*The highest is @100kHz, the lowest @1MHz (100kHz steps)

I understand the transistor causes it but I didn't find a frequency restriction in it's datasheet and I don't know how to explain this derating.

edit: Rload=17.142 (I wanted for the simulation Iload=0.7A)

  • 1
    \$\begingroup\$ Have you tried adding a load resistor on the output? Basic boost conversion needs a load and feedback system to control duty cycle in order to regulate the output voltage. \$\endgroup\$
    – Andy aka
    Jan 28, 2021 at 10:17
  • 2
    \$\begingroup\$ Yeah it doesn't look like there's any feedback. With a real mosfet, switching losses increase when you increase frequency so that could be a factor. The graph makes me think of a really primitive feedback circuit I use sometimes because of the overshoots, but my circuit would burn out a mosfet if it had to to maintain the same voltage even if you did something stupid with frequency, so there must be a difference in how we decide more or less. I just charge and discharge a cap and the voltage on the cap sets my duty cycle. You don't mention what range your sample frequencies are at either. \$\endgroup\$
    – K H
    Jan 28, 2021 at 10:45
  • 2
    \$\begingroup\$ Real devices take a finite time to switch and have various parasitics like capacitance. This might explain your observation. \$\endgroup\$
    – Kartman
    Jan 28, 2021 at 11:29
  • \$\begingroup\$ Thank you for the responses. About Rload, it's about 17 Ohms (I made and edit) and ok I get that there are switching losses. I would like to calculate somehow and how they are mathematically related to the frequency. I want to show a calculation that shows what is the maximal frequency that I can work with, but I can't find the way... \$\endgroup\$
    – Dennis
    Jan 28, 2021 at 13:32
  • \$\begingroup\$ What is your gate driver circuit? \$\endgroup\$
    – Andy aka
    Jan 28, 2021 at 13:36


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