DC/DC Converter Help

[LukeKnepp]

Does the same IPP072N10N3 MOSFET not work for the small MOSFET as well?

I tried just removing the transistor, and this is what I got...

And in my board, I am using the IPP072N10N3, because I had nothing else...

It seems to work perfectly in the simulator, so I guess it is something else that is incorrect??

I measured across the small ohm resistor and got about 30mV...
So I changed it to a 2.9 foot long 22 gauge wire that is supposedly 0.047 ohm, according to this website...

And it still only measures about 30mV across it???

Do I have to apply a load so that the wire heats up and starts acting as a resistor???

Thank you!!!

 

[ronsimpson]

Remove the connection from OUTPUT to Rt/Ct.

 

[ronsimpson]

So I changed it to a 2.9 foot long 22 gauge wire that is supposedly 0.047 ohm, according to this website...

2.9 foot long wire = inductor.

 

[LukeKnepp]

What do I do??

Do I have to order a resistor???

Remove the connection from OUTPUT to Rt/Ct.

What will this do??

 

[ronsimpson]

What will this do??

I do not see how it will work the way you wired it.
Normally the Rt/Ct pin only has a RC on it. This sets the frequency and the max duty cycle. This is normal for PWM. Set frequency and variable duty cycle.
When I add a small MOSFET the supply changes to constant off time. The frequency will change with load. I like constant off time style. Most people do not understand how they work. I think they are stable.

 

[LukeKnepp]

Watts through small ohm resistor!!!!!

How could I do a soft-start??

 

[ronsimpson]

How could I do a soft-start??

From data sheet. I have not done figure 24. I used figure 25 with R1=open. You can try this with spice.

 

[ronsimpson]

The current you are seeing at startup is not a function of the PMW. Soft start will not effect this.
Remove most of the parts. Only have C4 input cap, L1 coil, C5 output filter.
In spice the input voltage goes from 0 to 16V in 0nS. (zero time)
C4 charges up.
C5 charges up through L1. The current is a 1/2 sign wave at the resonant frequency of L1 * C5.
This happens with the IC removed. So soft start will not effect this.

I changed V1 to ramp up to 16V in 700uS and the current spike is much smaller. This is more like real world conditions.

 

[LukeKnepp]

So I do not even need soft start??

 

[LukeKnepp]

All I need is to find the correct low ohm resistor, and it should work???

 

[LukeKnepp]

Anything wrong Here?????

 

[ronsimpson]

Please share your spice file. I lost mine. Thanks

 

[LukeKnepp]

Never mind!!!!!

I built one in real life and it is working great so far!!!!

 

[kubeek]

I really don´t like that overshoot at the start, 5 volts above the target voltage seems a little too much. It looks like your compensation network is not correct.

 

[ronsimpson]

overshoot at the start

The loop response is probably not right. The inductor (inductance) is large. There is no soft start. In SPICE the input voltage goes from 0V to 20V in zero uS. (will not happen in real life) The LC of V2-L1-C3,4 will ring when pounded by the sharp start up voltage in SPICE. This ring might go to a over voltage condition with out the PWM working.

 

[LukeKnepp]

I am wanting to protect this circuit from low voltage...
I need a universal solution for low voltage protection that would shut off between 16.1V and 15.5V...

Are there any integrated circuits I would not have to program that would work for a low drain shutoff??

 

[ronsimpson]

16.1V and 15.5V

This circuit uses a TL431 voltage reference. But is uses it as a voltage comparitor and 2.5V referance.
R1 & R2 divide down the VIN voltage to 2.5 volts. Play with the resistor to get the switching point where you want.
The TL431 decides if the voltage is above or below 2.5V.
Q1 pulls "COMP" pin of the PWM to ground. That will force a 0 duty cycle.

 

[LukeKnepp]

So, I then leave everything else off of the comp pin?

Also how to calculate drain after shutoff????

Thank you!

 

[ronsimpson]

So, I then leave everything else off of the comp pin?

NO
Do not make other changes.

Also how to calculate drain after shutoff????

R1,2 are now set to 1mA but could be 10x in resistance to reduce the current to 100uA and maybe 100x to get 10uA.
The other parts do not matter.

 

[LukeKnepp]

What am I doing wrong????