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# [FAQ] TINA/Spice: Calculating Efficiency for a Power Supply with TINA-TI

Tool/software: TINA-TI or Spice Models

How do I calculate the Efficiency of a Power Converter with TINA-TI?

Nikhil Gupta | Product Folder Modeling

All TI Spice Models can be found here | New to TINA-TI? Its a free spice simulator! Try out this short Video Training Series

• Make sure the probes are hooked up correctly. The thick line is for the current path. The thin line is for the differential voltage.
• Make sure that the polarities are correct. The direction of current is from the positive terminal to the negative terminal. Similarly, make sure your higher voltage is connected to the positive terminal of the voltage line.
• If after simulation you get negative power, check your connections.

Step 2: Run the Transient simulation until the output reaches Steady State. Pin will show drastic chnages for a Buck. This is expected since we are plotting the instantaneous power.

Step 3: Zoom into the region of Steady State since this is where Efficiency is calculated. Make sure you have a good number of cycles in Steady State so that when we perform the next step (averaging), the effect of partial cycles is negligible. If you can not take too many cycles, then you will have to manually ensure that the zoomed in area has integral multiples of switching cycles. For most conditions though, this can be avoided by simply takes a good number of switching cycles.

Step 4: Since we are in Steady State, the output power is almost flat and you can measure this using a probe. For the input power though, we need to average this out over the switching cycle. This can be done by selecting the Pin waveform >> Clicking on "Process" >> "Averages ...". The absolute average value will give the average Pin. Now that have Pout and Pin, Efficiency can be calculated using Eff = Pout / Pin.

TINA-TI Schematic file used for this FAQ: TPS54623_Efficiency.TSC

A note of caution: Although Efficiency can be calculated using this method, the value obtained should be considered on the optimistic side (higher than bench). The reason is that the spice models model the output stage as simple switches in most cases and hence losses from the output stage would be less than those observed in bench.

Nikhil Gupta | Product Folder Modeling

All TI Spice Models can be found here | New to TINA-TI? Its a free spice simulator! Try out this short Video Training Series