# Simple step-up transformer not working as expected

I'm doing some pretty basic "experimenting" with some transformers to step up voltage for an embedded project. I've got various 1:N transformers from coilcraft: http://www.coilcraft.com/lpr6235.cfm

I'm driving a FET with a 5V to 0V square wave (not -2.5V to 2.5V) from a function generator, and I'm powering the primary side with a power supply. Unlike the flyback circuit example in the data sheet, I've got a high power resistor in the circuit to limit the current to not exceed the saturation current of the transformer.

I was expecting that with a 1:N transformer, my output voltage would be N times my peak voltage, provided that I reach the saturation current. So, with a 1:10 transformer and a 5V square wave, I expected a peak to peak voltage of 50V, but I'm not getting anywhere near that. simulate this circuit – Schematic created using CircuitLab

The FET I'm using is actually FQP30N06L

Since I'm using a function generator, I vary the frequency until I hit peak voltage on my rms multimeter. I'm using an N-fet to switch a 5V supply, and I am expecting 100V, since the transformer ratio is 1:20, but my multimeter only shows about 19 V.

Here's the output of the scope: The yellow output is measured at Test_Point. The Blue output is measured at V_supply. The gate signal is not shown, but it's very clean by comparison. Also worth noting: I switch out the R1 resistor for various values to try to hit saturation current. When the resistor is 2 Ohms, and the current is about .56 A, I get a higher voltage than when I use a lower resistor and run at saturation current (.7 A). Either way, the output voltage is about 1/5 what I was expecting. • Although I can almost "see" what you're trying to do, it would be better if you included a schematic just to avoid any confusion. There's a circuit editor: edit your question and press the circuit-symbol to draw a schematic. Including a picture is also OK. Jun 13, 2017 at 13:26
• One of the reasons is that you don't have 5Vpp across the transformer... attach a scope and observe that. Jun 13, 2017 at 13:37
• At what frequency are you driving the transformers? You say you are using a high power resistor but power does not matter, resistance does. What is the value of the resistor? Jun 13, 2017 at 14:00
• do you get about half of what you expect? Jun 13, 2017 at 14:53
• Your load also makes a difference, what is the load on the secondary? Jun 13, 2017 at 21:30