Electrical – Transformer Outputs Voltage High Above Expected Voltage


I have recently put together this circuit, which works basically as expected.

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Due to my lack of certain parts, however, I had to replace the 68 Ohm resistor with a 52 Ohm resistor, and the MOSFET I'm using is an IRF3205.

I am powering the circuit with a 7.4V LiPo battery.

Despite the changes I've made, the circuit drives the transformer fairly well, and the MOSFET doesn't get too hot, especially with a decent heatsink.

However, after attaching the secondary of my transformer to a high voltage bridge rectifier, the output measured by my multimeter was above 600V! With 10 turns on the primary, and 275 turns on the secondary, I expected about 200V.

Output Voltage = Input Voltage * (Secondary Turns / Primary Turns)

Output Voltage = 7.4 * (275 / 10)

Output Voltage = 7.4 * 27.5

Output Voltage = 203.5

I figured a smoothing capacitor might help with the reading on my multimeter, so I added a 150uF 450V capacitor in parallel to the multimeter.

Still, the voltage quickly rose to the cap's maximum rated voltage (according to my multimeter), so I deactivated the circuit.

Is my multimeter wrong?

Or is my math wrong?

What is the cause behind this?

Best Answer

It's a bit of a misnomer to call the flyback converter magnetics "a transformer" because it is being used as an inductor with a load so the inductance is chosen to support the low DCR conduction losses and same for the secondary.

Normally the inductance ratio is adjusted to give around 50% duty cycle for the desired load or less at minimum losses.

Basic math

The energy storage density has a limit to transfer about 100~200W max before the topology becomes less efficient than a forward converting transformer topology where the transformer no longer has to store the energy being transferred as it is coupled by the mutual coupling with a much lower impedance path.