Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

[Moved]: LTSpice simple transformer simulation problem

Status
Not open for further replies.

faisal349

Junior Member level 1
Joined
Feb 22, 2012
Messages
17
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
1,446
Hi,

I am trying to simulate a simple transformer in LTSpice and want to measure the current and voltage. I started with a simple cirucit to measure the current in the primary and secondary circuit as shown in the attached fig. When I simulated this, I found quite strange result. On the secondary side, when the voltage is 2V, the current is 1mA according to Ohm's law which is fine. But on the primary side the current exceeds more than 500mA which is strange. The current in the primary should also be 1mA at 2V but this is not the case.

Do you have any idea what I might be missing in the simualtion?

Thank you.
 

Attachments

  • LTspice transformer.PNG
    LTspice transformer.PNG
    10.6 KB · Views: 175

Re: LTSpice simple transformer simulation problem

Can you attach the resulted waveform of the simulation ?
It is not clear if you are distinguishing peak and average current on measurement.
 

Re: LTSpice simple transformer simulation problem

I have attached the primary and secondary side voltage and current waveforms.

Thank you.
 

Attachments

  • voltage and current primary.PNG
    voltage and current primary.PNG
    47.6 KB · Views: 149
  • voltage and current secondary.PNG
    voltage and current secondary.PNG
    33.2 KB · Views: 159

Re: LTSpice simple transformer simulation problem

You can answer this yourself by calculating XL and the resulting current at the operating frequency.
 

Re: LTSpice simple transformer simulation problem

Just to make things clear, I have attached another simulation.

We know that:

Voltage across primary * Current through primary = Voltage across secondary* Current through secondary.

If you now consider secondary figure, we see that at 1V, 1mA of current is flowing which is perfectly fine. So, at secondary side we have 1V*1mA = 1mW (which is max. power on secondary side).

Now at primary side, if we take the intersection of current and voltage graph, we see that at around 0.85V, 8A of current is flowing which then equals to 0.85V*8A = 6.8W.

So, in other terms, it violates the above transformer equation. Quite strange! So I may be missing something?

Thank you.
 

Attachments

  • transformer_ltspice.PNG
    transformer_ltspice.PNG
    20 KB · Views: 155
  • voltage_current_primary.PNG
    voltage_current_primary.PNG
    36.6 KB · Views: 145
  • voltage_current_secondary.PNG
    voltage_current_secondary.PNG
    31.3 KB · Views: 131

You are missing transformer main inductance and magnetizing current. Your "transformer equation" is describing an ideal transformer with infinite inductance, but you choosed a rather small inductance of 16 µH.
 
Thank you for the answer. I simulated again with 1000H inductance value and it is working fine.

Actually, I have to amplify -3V to 3V analog signal from a 3W differential amplifier to +-12V signal for the ultrasonic transducer to increase the range. The frequency is 60kHz. In real life, we don't have transformers in thousands of Henry. So, I guess I can't use the ideal transformers equations then.

What I am thinking of is to use a transformer which is small and its inductance in maybe uH or mH range. So what would you suggest would be the best way to go?

Thanks.
 

Normally, in a real transformer, the primary inductance is selected so that the magnetizing current does not saturate the core at the minimum operating frequency and the maximum operating voltage.
So you select a transformer that is rated for the maximum voltage, frequency, and power that you need.

For simulation purposes you can use an inductance that keeps the simulated magnetizing current well below the signal load current.
 

Thank you for the answer. I simulated again with 1000H inductance value and it is working fine.

Actually, I have to amplify -3V to 3V analog signal from a 3W differential amplifier to +-12V signal for the ultrasonic transducer to increase the range. The frequency is 60kHz. In real life, we don't have transformers in thousands of Henry. So, I guess I can't use the ideal transformers equations then.

What I am thinking of is to use a transformer which is small and its inductance in maybe uH or mH range. So what would you suggest would be the best way to go?

Thanks.

In fact your solution is pretty simple.
First, you should consider the load which should be present for your amplifier.In other words, at which load impedance( preferably optimum one) should be at the output of the amplifier so this -3V to +3V amplitude is maintained? You define this impedance then primary coil's value.( you should take the losses and parasitic capacitances as well ). After that, you can find easily secondary coil's value.
Some optimization tweaks will give you a good result.
Let say
 

Thanks for your answers.

I have selected a transformer for my application and it is designed for 40kHz frequency. But I want to use it from 65kHz-70kHz. The input voltage is -3V-3V. So I think I'll be able to use it as my frequency is higher and the voltage is more or less the same. What do you guys think?
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top