Hello guys,

I build a powersupply for my relais on my PCB for 12V/3A.
I want to simulate it how good it works but don't know how, can someone help me simulate it?

The powersupply:


Greets,

Rens

 

Your Input-Voltage is stated as being "12V to 24V".
At 12-Volts of Input, you are not likely to get a full ~12-Volts at the Output,
there will be a certain amount of Voltage-Loss in the Circuit.

Your Input-Power-Supply must be capable of supplying more than 3-Amps at 24-Volts.

There will be an unpredictable amount of Electrical-Noise generated by the Buck-Converter,
this may, or may not, create various problems for the Circuit being supplied.
Additional Filtering may be required on the Output of the Buck-Regulator.

A Schematic-Diagram of the Voltage-Input-Source, and the Circuit being powered,
may bring-up many more questions that need to be answered.

We are only seeing a small part of a "System" that must all work together to provide what You expect.
.
.
.

 

Hi R223,
This is what LTSpice shows.
Note that the Q3 PMOS, WST4041 has an internal forward biassed diode [D3] in that configuration.
The circuit requires a redesign.
E
Update: 5v version corrected to 12v version.

 

The Buck-Converter in the original Schematic seems to be rated for a 12-Volt Output,
but the above Schematic from Eric shows a 5-Volt rated part.

A quick search for a Spec-Sheet only turned-up Asian-Language versions.
.
.
.

 

Note that the Q3 PMOS, WST4041 has an internal forward biassed diode [D3] in that configuration.

Yes.
It acts as a (near) ideal diode for reverse input protection.

 

Hi R223,
This is what LTSpice shows.
Note that the Q3 PMOS, WST4041 has an internal forward biassed diode [D3] in that configuration.
The circuit requires a redesign.

Thanks E,

What for type of redesign does my circuit require? Can you be a bit more specific?
To give some more background information:
This circuit will power the coils for switching 12V relais on my PCB, it will also power a small coolingfan.
The relais will be controlled with input from a raspberry PI 5, also the powersupply are boat batteries who operate on 12V or 24V.


Greets,

Rens

 

Hi Rens,
Relook at the PMOS, what are you expecting the circuit to do?
I suspect with the 12v Zener you were trying to drop the 24v down to 12V, before the regulator
But the internal diode shorts out the PMOS

Will the LM2596 give a 12 Vout with a 12V input ???

E

Update: Note the LM has a24V input on my simulation, due to forward biassed internal diode

 

Hi Rens,
Relook at the PMOS, what are you expecting the circuit to do?
I suspect with the 12v Zener you were trying to drop the 24v down to 12V, before the regulator
But the internal diode shorts out the PMOS

Will the LM2596 give a 12 Vout with a 12V input ???

E

Update: Note the LM has a24V input on my simulation, due to forward biassed internal diode

Hello E,

I think you mean that the Mosfet won't work with this setupt because of the 12V voltage enables current to always to flow to the powersupply through the foward biased diode. Is a different P-Mosfet the option or should i go N-Mosfet?

Yes the zener was meant to limit the voltage to the pmos for it not to burn out.
What do you suggest how to redesign the circuit?

Greets,

Rens

 

Hi Rens,
A typical application of a MOS FET in series with a voltage regulator is to reduce the voltage applied to the voltage regulator [ LM2596] so that it operates within its specified power rating.
The problem is that the PMOS you are using has an internal bypass diode, which effectively shorts the PMOS, so that you will get the full 24V input on the LM2596 input.
So check the LM2596 datasheet and ensure that you keep within specification..

I would only use a 24V to 12V step down regulator, when the boat's supply is 24V and have a direct connection to the boats' battery for a boat supply of 12V.... always fused near the battery for safety.

A point to consider is if a boat 24V battery system is being recharged with an onboard generator, the actual; battery voltage could be as high as 28V

E
Added a plot for 24V Input,, 12Vout at 3amps, note the LM2596 Watts [ lower plot]

 

Hi Rens,
A typical application of a MOS FET in series with a voltage regulator is to reduce the voltage applied to the voltage regulator [ LM2596] so that it operates within its specified power rating.
The problem is that the PMOS you are using has an internal bypass diode, which effectively shorts the PMOS, so that you will get the full 24V input on the LM2596 input.
So check the LM2596 datasheet and ensure that you keep within specification..

I would only use a 24V to 12V step down regulator, when the boat's supply is 24V and have a direct connection to the boats' battery for a boat supply of 12V.... always fused near the battery for safety.

A point to consider is if a boat 24V battery system is being recharged with an onboard generator, the actual; battery voltage could be as high as 28V

Hello E,

Yes you're completely right, I have changed my setup now to N-channel mosfet so this can't happen anymore, does it look okay now?

The input for the XL2596S-12E1 is 4.5 to 40V. So the 28V spike when charging shouldn't be a problem.
Also the relais already turn on at 10V it says in the datasheet, so if the voltage is little below 12V it not a big deal for this setup, but your setup is better. Can I make a second 12V input circuit with also the N-channel mosfet as reverse polarity protection and connect to the output of the 24V-12V powersupply shown above here?

Are there any mistakes in this design now which i haven't noticed?

Greets,

Rens

 

Hi Rens,
On your diagram, if the left side Gnd is connected to the right side Gnd, the MOS is shorted out!.
E

 

Hi Rens,
On your diagram, if the left side Gnd is connected to the right side Gnd, the MOS is shorted out!.
E

Hello E,

Ofcourse, stupid of me, now better?


Greets,

Rens

 

Hi Rens,
I would recommend not using a floating ground on your project, it would mean all the equipment connected to 12V would require the ground to be floating.

E

 

Hi Rens,
I would recommend not using a floating ground on your project, it would mean all the equipment connected to 12V would require the ground to be floating.

E

Hello E,

Can i fix this to pull a track with 10K resistor from the system GND to PGND to ensure good grounding?
Or do you have a better suggestion?

Greets,

Rens

 

Hi R,
Did you understand, post #9, you are exceeding the power rating of the LM regulator.

E

Update:
What is the specification of the 12V relays coils you need to drive???

 

Hi R,
Did you understand, post #9, you are exceeding the power rating of the LM regulator.

E

Update:
What is the specification of the 12V relays coils you need to drive???

Hello E,

No, i don't understand, I'm using the XL2596S as shown in the schematic who can handle up to 40V input.
I redesigned the schematic as shown below, the leakage current is very low so don't mind that, and extra reverse polarity protection with the SS54.
What do you think:


Greets

Rens

 

Hi R,
The MOS FET is not required if all you are asking is reverse voltage protection, a simple power diode is what is usually used..
E

 

Hi R,
The MOS FET is not required if all you are asking is reverse voltage protection, a simple power diode is what is usually used..
E
View attachment 345012

Your're correct, I use a Mosfet because of the lower heat dissapation.
Does my setup have right now reverse protection with the extra SS54 diode in the circuit, I think so?

Greets,

Rens

 

Morning Rens,
The MOS as wired in your circuit will not reduce dissipation.

The extra diode wired in parallel with the MOS internal diode has a minor effect, the LM2596 still has close to the full 24V on its input.

A simple diode is all that is required for reverse voltage protection.
E

 

Morning Rens,
The MOS as wired in your circuit will not reduce dissipation.

The extra diode wired in parallel with the MOS internal diode has a minor effect, the LM2596 still has close to the full 24V on its input.

A simple diode is all that is required for reverse voltage protection.
E

He E,

Okay, thanks, but the diode in the Mosfet has already a function for blocking the Reverse voltage right? So the extra diode in parallell is not useful?

I see many high power circuits where a mosfet is used instead of a diode for reverse voltage protection, that's what the Mosfet in my circuit was supposed to do.

Greets,

Rens