How to match the impedence

[Murugesh_89]

Hi,
I want to match the impedence of my load with that of the smps source.
Can anyone tell me the procedure to do that?

Also how to find the internal resistance or output impedence of the smps?

Thanks in advance.

Regards,
Murugesh

 

[mtwieg]

Why would you want to impedance match...? If you're looking for max power transfer, then impedance is usually irrelevant, and it just comes down to the power handling capability of the converter.

The output impedance of a SMPS depends mostly on the feedback controller, and the impedance will be frequency dependent. Most SMPS use some form of PI or PID control, which results in very low output impedance at low frequencies (like milliohms). The impedance can be measured or modeled, but the method is a bit tricky.

 

[Murugesh_89]

Hi,
Thanks for your reply.

I am using a single smps to power the units as below.


Each board draws 180mA and am using a smps 12V/15A. After each board the input voltage reduces. Eventually it becomes less than 8 volts after 7/8 boards. So i thought if i match the impedence i can get out off this issue.
Will it work? or Any other suggestions?

Even if it is due to resistance of PCB tracks and connectors, how to overcome this?


Thanks,
Murugesh

 

[mtwieg]

Hi,
Thanks for your reply.

I am using a single smps to power the units as below.
View attachment 85082

Each board draws 180mA and am using a smps 12V/15A. After each board the input voltage reduces. Eventually it becomes less than 8 volts after 7/8 boards. So i thought if i match the impedence i can get out off this issue.
Will it work? or Any other suggestions?

Even if it is due to resistance of PCB tracks and connectors, how to overcome this?


Thanks,
Murugesh

I don't see how impedance matching would help. To minimize voltage drop, you want to minimize source impedance, period. The load impedance of the linear regulators on the boards will be a high impedance. Trying to impedance match would result in more voltage drop, not less.

edit: do you want to minimize voltage drop, or make the voltage drop equal for all boards?

 

[Murugesh_89]

Thanks much.

I want to have low voltage drop. Also if it is equal i can able to calculate for how many boards i need a SMPS. I want the last board to have a input voltage of atleast 9.5V. Because i am using a 7808 IC along with 7805 in each board.

Now the voltage drop is almost 0.5V/board and hence i can't go for more than 7 boards.

If i give a power supply to each board in parallel not in looping as showed above, i can able to run. But i should not do that.

I am in need to operate atleast 30 boards using a single SMPS as per the figure. In this regard please give me a solution.

 

[mtwieg]

Then impedance "matching" has nothing to do with the problem. You need to find out why the resistance of your connections is so high, and reduce it. If each board draws 0.18A and causes a 0.5V drop, then that's almost 3 ohms of resistance. That's an exceptionally high resistance, even for PCB traces, and should be easy to measure with a DMM. So you should start probing things and see where the high resistance is.

 

[Murugesh_89]

Hi,
The measured resistance between Vcc is 1 ohm and that of gnd is 1 ohm. I put that in the figure with the actual setup in a single board. Here I have used 8 number of 7808 each will be given to one amplifier circuit and also 7805 taken from 12v only. Will it be the reason for the huge drop?



Also if I will use the setup as shown below, will it be avoided?


Thanks again!!!

 

[mtwieg]

Hi,
The measured resistance between Vcc is 1 ohm and that of gnd is 1 ohm. I put that in the figure with the actual setup in a single board. Here I have used 8 number of 7808 each will be given to one amplifier circuit and also 7805 taken from 12v only. Will it be the reason for the huge drop?

Is it 1 ohm or 10ohm (picture says 10)? Either way, yes that's a significant resistance which will lead to voltage drop.

Also if I will use the setup as shown below, will it be avoided?
View attachment 85119

Thanks again!!!

No, that won't decrease the resistance. The issue is in the board layout itself. You either need to make the traces carrying power between boards wider, shorter, or thicker. There isn't much else to it.

Also I doubt you need separate regulators for each circuit on a single board.

 

[Murugesh_89]

Is it 1 ohm or 10ohm (picture says 10)? Either way, yes that's a significant resistance which will lead to voltage drop.

It is one ohm only not ten ohms..

No, that won't decrease the resistance. The issue is in the board layout itself. You either need to make the traces carrying power between boards wider, shorter, or thicker. There isn't much else to it.

Ok thanks i will increase the layout width in my new design..

Also I doubt you need separate regulators for each circuit on a single board.

So you mean that i need to put separate 7808 for each amplifier circuit as now. What happens if i put a single 7808 with high current rating as a voltage source for all the amplifier circuit??