guys just asking why is my simulation result not the same as my calculation I am not sure why.

My design has the specification of Vin (power supply) = 1.5V , Vout = 5Volt with a Pout(power output of) = 1 watt , running on 10kHz frequency with 0.1V voltage ripple.

enter image description here My calculation ^

enter image description here

My Schematic ^

enter image description here The output voltage ^

Using the MOSFET Si7866DP enter image description here

I am pretty sure that my calculation is correct but I am not sure why is my output not as my calculation. I did a few tests and I am pretty sure it has nothing to do with the mosfet(not sure if I am right). But can anyone tell me what's wrong and how do I fix it so that the output will be 5V instead of what I got?

  • 2
    \$\begingroup\$ "pulse 0 1.5V" won't turn the MOSFET on very hard. Try 4.5V pulses at which Rds is specified 2.6 mOhm. \$\endgroup\$ – Brian Drummond Jun 18 at 19:31
  • \$\begingroup\$ Your formulas are wrong - it operates in DCM with an on time of 45 us, a discharge time of about 16 us and a dwell time of about 38 us. That's with my diode; yours isn't specified. \$\endgroup\$ – Andy aka Jun 18 at 21:03
  • \$\begingroup\$ "But due to forward voltage drop of the diode we have to see Vs as Vin - 0.7V" - this is wrong. Diode voltage drop subtracts from the output, not the input. Where did you get this information from? \$\endgroup\$ – Bruce Abbott Jun 18 at 23:17
  • \$\begingroup\$ @BruceAbbott Oh I just that that is how I am supposed to calculate it, due to the fact that Vin to the load will ultimately be the Vsupply - 0.7V, So just to clarify I should subtract 0.7V or add 0.7V from the Vout in my calculation? \$\endgroup\$ – tw.C4444 Jun 19 at 0:29
  • \$\begingroup\$ The text says Vs is 1.5V - 0.7V = 0.8V. This is wrong. 0.7V should be added to the output voltage, ie. D = 1 - 1.5V/5.7V = 74%. In simulation (with corrected Gate drive) this ratio gives 5V output to the load when frequency is increased to 50kHz get continuous conduction. \$\endgroup\$ – Bruce Abbott Jun 19 at 5:28

One issue I see with your design is that the gate of the mosfet is not being driven to a high enough voltage. You are driving the gate to 1.5 volts is not enough to turn it completely on so it will have a significant resistance which interferes with the charging of the inductor. Setting the mosfet gate to a higher voltage should fix this problem

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  • \$\begingroup\$ Also add that using zero valued trise/tfall doesn't mean that there is no rise and fall time, they just default to 0.1*min(T, T-Ton). So in this case it's 0.1*(100u-84u)=0.1*16u=1.6u, which not only it might not be what OP wants (large rise/fall transitions), but also increases Ton by 1.6u, or (trise+tfall)/2. \$\endgroup\$ – a concerned citizen Jun 18 at 20:22

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