I try to design a buck-converter in PSIM simulation program. Let me write the circuit's requirements:

Frequency = 80kHz

Ripple < 0.5%

\$V_{input}\$ = 30V

\$V_{out}\$ = 12V

\$R_{load}\$ = 10 \$\Omega\$

Here are my calculations:

\$ D = \frac{V_{input}}{V_s} = \frac{12}{30} = 0.375 \equiv 67.5 \% \$

\$ L_{min} = \frac{(1-D)R}{2f} = \frac{(1-0.375)*10}{2\cdot80\cdot10^{3}} = 39.0625 \mu H \$

\$ C = \frac{1-D}{8\cdot \left(\frac{\Delta V_{o}}{V_o} \right)\cdot f^2} = \frac{1-0.375}{8\times 48.828 \times 10^{-6} \times \left( 80 \cdot 10^3 \right)^2} = 50 \mu F \$

as shown in the figure.


simulate this circuit – Schematic created using CircuitLab

Here is the schematic that I draw in PSIM with the calculated values. I float some of the values.

Psim, buck converter circuit design

Simulation results for 2ms run:

Input-Output Voltage Diagram:

simulation result 1 - input-output voltage diagram

Output voltage diagram between 1.2ms and 1.6ms: output-voltage-diagram

What is the problem?

As I calculated above, the output voltage value should be 12V but, when analyse the circuit, it returns about 6 volts. I test this circuit for another values and calculations but result always same. It returns half of the calculated-output voltage value. I couldn't figure it out. Why output voltages are always half of the calculated value?

  • \$\begingroup\$ What is your gate signal voltage level? It needs to be higher than the desired output voltage \$\endgroup\$
    – Big6
    Commented Jun 11, 2023 at 15:19
  • \$\begingroup\$ @Big6 I did not use a voltage supply for gate lead. I used Gating Block and the parameters are Frequency: 80000, No. of Points: 2, *Switching Points: 0 68. *. \$\endgroup\$
    – kangret
    Commented Jun 11, 2023 at 15:42
  • \$\begingroup\$ Look at your switch node. Is it going from ground all the way up to 30 V? Are you using the ideal FET model? The Gating Block doesn't work with anything but an ideal FET model. \$\endgroup\$
    – John D
    Commented Jun 11, 2023 at 18:18
  • \$\begingroup\$ The drive generator of the MOSFET should be referenced to its source, unless you swing 10-15 V above the output voltage. Otherwise, try my ready-made templates, they work for most of them with the free demo of SIMPLIS and there are several buck converters in the ZIP file. \$\endgroup\$ Commented Jun 11, 2023 at 18:40

1 Answer 1


Duty cycle on Gating Block is defined by Switching Points Parameter, from 0 until 360 degree. so if based on your specification, you need to defined the switching point to 0 and 360*0.375 to get desired output voltage.

PSIM Simulation using same parameter, within ideal condition (not include stray component


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