I am trying to build a buckboost converter for converting 5-40v input to 5-35V output at 3A max to use it for many purposes by usign TL494. I am using a SEPIC topology as i want a non inverting output. I simulated the circuit in proteus. The circuit works fine when i connected a 500 Ohm load, I was able to get output of 5 to 35 volts at 100kHz frequency.

But when I used a 5 Ohm load, the voltage drops from 35v to 2v, because it can't able to give enough current, The maximum i get is somewhat around 400mA

I think the problem is in buck-boost stage, because I tested the buck-boost stage seperatly by giving a PWM signal, it has the same result.

I used inductors with 82uH and capacitors with 22uF, Shottkey diode and IRF540 MOSFET, I also change the shotkey diode with a high current one, but no results.

Here the simulation results,

with 500 Ohm load 500 Ohm load

with 5 Ohm load 5 Ohm load

waveform of L1 current, C1 voltage and PWM at 5 Ohm load graph

But, when i changed the capacitor values to 1000uF, inductor values to 220uH and frequency to 5kHz i get 2A for 10v, but still not 3A

  1. So, what i need to do to get desired output, what are the parameters needed to tweak?
  2. Do i have to operate the buck-boost stage at resonant frequency to get max output?
  3. What limits it form providing high current?
  • 1
    \$\begingroup\$ What's the rating of your inductors? If they start saturating, there's no way you're getting more current than that out. \$\endgroup\$
    – Hearth
    Commented Dec 22, 2023 at 14:06
  • \$\begingroup\$ Also, you're using the less-stable voltage-mode control here; if you base your SEPIC around a current-mode controller like an LTC1871 you might get improved stability--though stability probably isn't your main problem here; I just tend to prefer current-mode converters on principle. \$\endgroup\$
    – Hearth
    Commented Dec 22, 2023 at 14:21
  • \$\begingroup\$ @Hearth I am using a generic indcutor for simulation, I also simulated using a high current one and results doesn't change \$\endgroup\$ Commented Dec 22, 2023 at 14:28
  • \$\begingroup\$ Give it a run with 22 µH and 22 µF at 100 kHz for a faster current rise during the on time. \$\endgroup\$
    – Jens
    Commented Dec 22, 2023 at 14:46
  • \$\begingroup\$ Yeah i did it, I got 1A for 5v, but no more than that \$\endgroup\$ Commented Dec 22, 2023 at 15:07

1 Answer 1


Some considerations

  1. The FB output can only sink 0.7 mA, so the value of R4 with 500 Ω is far too low. You just have 350 mV regulation head room. There is no need to use such a low impedance divider R4/R3. This is most probably the reason why the needed duty cycle of around 76 % is not reached.
  2. The value of the gate discharge resistor R2 is too high for a switching frequency of 100 kHz. The gate capacitance to be discharged is around 1.5 nF.
  3. IRF540 needs at least 7 V to turn fully on, so you need a TL494 supply of around 9 V minimum, with safety margin, to reach this (VCE = 1.5V). If you need a minimum input voltage of 5 V, you need another MOSFET with lower threshold voltage.
  4. The 5 Ω load at 35 V gives 7 A output current, which is far above your specification.
  5. The unconnected input pin 15 may be a problem, connect it to the 2.5 V reference.
  6. Using 82 uH creates a relative low ripple current and needs physically large inductors at the required current of around 16A. Lower values have some advantages in this circuit.

With a matching duty cycle and proper gate drive the converter works in this simulation:


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Thankyou, very much for your advice. I'll definitly consider it. But 5/500 Ohm for voltage divider is due to the simulation shows error when i used a higher value. \$\endgroup\$ Commented Dec 30, 2023 at 10:54
  • \$\begingroup\$ Also when I used a 1000uF capacitor and 220uF inductor with switching frequency of 10kHz I was able to get 2A max, but not more than that. So I want to know what limits the current here. Any suggestions @Jens \$\endgroup\$ Commented Dec 30, 2023 at 10:58
  • \$\begingroup\$ @Logesh-0304 As we see in your waveform diagram, the needed duty cycle is not reached. Depending on the operating mode, the TL494 is limited to 45 % duty cycle, but this should not be the problem as long as you connect OCTRL to GND. So my assumption is, that the FB pin cannot reach the required low voltage level with this value of R4. I cannot say, why the simulator produces errors if you do the right thing (e.g. 50 kohm / 500 ohm). \$\endgroup\$
    – Jens
    Commented Dec 30, 2023 at 17:52

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