Example:

Please post your ltspice file. thanks.

 

hi P,
Using 0.77R load, this is what I see.
Note: mutual inductance coupling of L1 & L2 =1
E

Unfortunately it think that it cannot reach 8.4V

 

hi P,
As a test I have changed the MOSFET for a lower Vgate Vth.
Find a suitable power MOS with a lower Vgs On.

E

 

hi P,
As a test I have changed the MOSFET for a lower Vgate Vth.
Find a suitable power MOS with a lower Vgs On.

EView attachment 246117

Perfect, thank you very much. Tt seems to work reaching 8.4 V for PW of 2.1u. Do you maybe know which component values should i change to reduce voltage ripple on output?

EDIT: Solved by increasing some capacitor values

 

C1 has serious current flow. Pick the right type of capacitor.

 

hi Peter,
I hope you have noted the Gate drive voltage on the sim post #43 has been increased in order to highlight the insufficient Gate drive voltage for the original MOSFET.?

The problem is that the 5V Vgate is not switching the MOSFET hard ON.

If you can find a suitable MOS that will be switched fully On with a 5V Gate voltage, it should help with the Vout problem


As Ron points out, you have high current levels in some components.

E

 

hi Peter,
I hope you have noted the Gate drive voltage on the sim post #43 has been increased in order to highlight the insufficient Gate drive voltage for the original MOSFET.?

The problem is that the 5V Vgate is not switching the MOSFET hard ON.

If you can find a suitable MOS that will be switched fully On with a 5V Gate voltage, it should help with the Vout problem


As Ron points out, you have high current levels in some components.

E
View attachment 246139

View attachment 246141

Hi ericgibbs,
to be honest i didint notice that you increased the gate drive voltage. I thought that replacing transistor irf540n with irf7413 makes the cicrcuit operate even with 5V gate drive voltage. I also simulated with 5V gate drive and it run properly. That's my last simulated circuit .

 

hi Peter,
You must attach a Load on the output, else the Vout result will be meaningless.

Try your 0.77R
E

 

Peter523,
You can use one more, independent, self-oscillating converter
specially to provide 12V for PWM's of all your converters.
for example:
Pololu 12V Step-Up/Step-Down Voltage Regulator S18V20F12
Vin=2.9V to 32V
Vout=12V
Iout=up to 2A
The ENABLE pin can be used to reduce the quiescent current to well under 1 mA.

ADDED:

Hi Danko,
The problem is that the TS wants to lift the source voltage from+3V to ~+8V , with an expected load of 0.77R.

My English is stupid, so trying to say the same, using picture:

 

Hi Danko,
The problem is that the TS wants to lift the source voltage from+3V to ~+8V , with an expected load of 0.77R.

Eric

 

Rds(on)=0.0007Ω
Vgate=12V
P, dissipated by M1=5.1W
P on R of L1 =?
P on R of L2 =?
P on D1 =?
P on C1 =?
P on C2 =?
_____

 

hi Danko.
Nice solution.
The TS will have to level shift his 5V PWM [ MCU derived] to the 12V level required, that should not be a problem.

E

 

Or maybe he can use a logic-level MOSFET.
5.1W seems like a lot of dissipation for a 0.7mΩ MOSFET - is it all switching losses?
If so, probably a MOSFET with a smaller die (higher Rds(on)) would have less loss?

 

hi Peter,
You must attach a Load on the output, else the Vout result will be meaningless.

Try your 0.77R
E

Sorry i may wasnt clear. The SEPIC i posted on post #47 had on its output the three DC-DC converters and it reached the desired voltage on its output (8.4V) with gate drive voltage of 5V. That's why i think problem is solved

 

Sorry i may wasnt clear. The SEPIC i posted on post #47 had on its output the three DC-DC converters and it reached the desired voltage on its output (8.4V) with gate drive voltage of 5V. That's why i think problem is solved

Circuit #47 has 7.4 V on its OUT.
Transistor dissipates 86,7 W of power:

============================================================================================================================
ADDED:
For example, how it can work:

 

Circuit #47 has 7.4V on its OUT.
Transistor dissipates 86,7 W of power:
View attachment 246217View attachment 246218

I didnt place the 0.77 Ohm resistance on circuit #47 but three DC-DC converters (i just didnt show them in the screenshot).

 

Transistor dissipates 86,7 W of power:

I don't have your spice file so I can't tell. Something is really wrong if this in true. The transistor should be under one watt.

 

hi Ron,
This is what my LTSpice run shows for the power.

I have not seen the TS's PWM control method, but the circuits require tight control of the PWM in order to maintain the desired Vout
E

 

I don't have your spice file so I can't tell. Something is really wrong if this in true. The transistor should be under one watt.

I agree. Something seems to be wrong with that amaount of dissipated power. But the output voltage is the desired one. What may be the problem...

 

Hi P,
With that circuit you drawing almost 60A from the 3V supply.
E