Car amp smps

noorworkshop

New member
Hi all
This is the car amp's smps core
It's 4x70w(280w) and class ab
Smps ferquency is 50khz
Skin depth in 50khz is 66/223.6=0.3mm
Pri current in each rail is:
(280x2(efficiency about 50%)/12)/2=23A
In this amp pri winding is 3 layer of 0.8 wire and we need to give 7.8A from per layer

But How it can give 7.8A in this ferquency and this skin depth?

I attach amp pic
Please help me to give same power from same winding
Thanks
 

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Silvio

Member
Hi noorworkshop, Your argument is totally right. Regarding the skin depth at 50KHz a 0.6mm copper wire will be more appropriate however you must add more wires to come to the right current density. Not all that is done by diyers is perfect but if you wish to make it your way then here are few things to consider.

1) The current density for the primary should be 4A/mm² for 50% duty cycle. You must add also 20% to this as the smps will not be 100% efficient and usually around 80% or so.

I explain better if for example your full load current is 10A then 50% will be 5A then add 20% to this we get 6A so your design current will be for 6A. Find the cross sectional area of one wire and multiply it by 4. This will give you the current rating for one wire. Finally see how many wires you need to get to 6A rating.

How the wires will fit in the core and how many layers you need is according to the core used. It could be you might need a bigger core to accommodate the wires neatly.

I hope that helps Silvio
 

noorworkshop

New member
Thanks for helping.
Can you calculate a smps with this details:
Input voltage 11Vdc
Output voltage +-40Vdc
Power(that need in output): 400W
 

Silvio

Member
Please give me core dimensions and core material. Does your circuit diagram shows if it has voltage regulation to the output?

Regards Silvio
 

noorworkshop

New member
My core diameters:
A: 38mm
B: 15mm
C: 19mm
Core picture attached

I made smps with tl494 and it's open loop(because i can't set gain to tl494 error amp,i made it open loop)

I attached smps picture.





I winded ei33 core for 200w with:
Pri: 3+3 turns (17layer per rail)
Sec: 9+9turns (6 layer per rail) and wire is 0.5mm
Can i give 200w from this winding?
 

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Silvio

Member
My core diameters:
A: 38mm
B: 15mm
C: 19mm
Core picture attached

I made smps with tl494 and it's open loop(because i can't set gain to tl494 error amp,i made it open loop)

I attached smps picture.





I winded ei33 core for 200w with:
Pri: 3+3 turns (17layer per rail)
Sec: 9+9turns (6 layer per rail) and wire is 0.5mm
Can i give 200w from this winding?

EI 33 can give you 200-300w

17wires X 0.5mm get you 3.33mm² of copper wire. This can handle 20A or more for audio purpose

250w / 12v = 20A

Your switching frequency with 0.5mm wire can go up to 70Khz

With 3 turns primary @ 12v you get 4v per turn

so secondary voltage at 9+9 you get a little less than 36v-0-36v This voltage however will drop with load as there is no feedback. If you want to add feedback to your smps you must start at a higher secondary voltage like 45v to 50v this then will be regulated to the wanted 36v and also compensate when there is voltage drop in the battery voltage and load.

For regulated output you will also need an output inductor and also output capacitors with each rail otherwise the feedback will not work.

PDF file below with data of ring core

View attachment Noors ws.pdf

Regards Silvio
 

Silvio

Member
Usually we regulate at around 0,707 of the peak value. So if you have a peak voltage of say 45v then we multiply that by 0.707 and we get around 32v. This holds true for an off line smps because we are rectifying the mains and smoothing it with the capacitors. Loading this will surely have voltage drop depending on the load and amount of input capacitance used.

In your case things are a little different. A car battery is pure DC and the only voltage drop you will experience is in the wiring and a little in the mosfets and transformer and output diodes. This would not be much in the region of 1v or so, However you must consider other things as well as when it comes to regulation of the output there comes the need to fit also an output inductor. This in itself will create a little voltage drop, so headroom has to be left so that there will be enough voltage left at full load so that the PWM has where to adjust its pulse width during full load to maintain regulation of the output.

Do not forget that you are stepping up the voltage around 6 times so every volt lost on the primary is 6v on the secondary. I suggest you try with around 5v head room on each rail. This amount will suffice. To make things clear if you have a regulated double secondary of 30-0-30 then the whole winding should be around 70v
 
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