You cannot have load when the forthcoming pulse is under charging to the magnetic field of the transformer - a diode is obligatory to prevent current in the secondary when the switch is conducting. You have a growing current ramp in the primary during the magnetic energy charging period. Switching the mosfet off causes as high voltage to occur in all windings as is needed to let the current continue in some of the windings. This is the flyback pulse. You need so low resistance load that the voltage rating of the mosfet nor the transformer windings aren't exceeded.
If your transformer is 1:1, the output peak current is the same as there was built up in the primary when the mosfet was turned off. If your secondary in use has only half of the number of turns in primary, the peak current in the output is the max primary current doubled.
To keep your mosfet in safe, you must
- use so short ON period of the mosfet that the primary current do not exceed the allowed max peak current of the mosfet. I1max=Uin*Ton/L1 where L1=primary inductance
- use so long OFF period of the mosfet that the transformer current surely has been died before a new ON state. Wait at least 10*L2/R where L2 is the inductance of the used secondary winding and R is the load resistance.
- have a mosfet which stands the input DC voltage plus the flyback pulse voltage calculated of the peak current of the secondary, the load resistance and the winding ratio.
- have such high gate drive capablity that the mosfet turns OFF reasonably fast to prevent it heating due the losses
- have a high enough capacity transformer core which doesn't get saturated with the max primary current. Saturated core = a short circuit
- have so low stray inductances that they do not cause serious overvoltages. You can have 2 zener diodes in series head against head in parallel with the primary. One prevents the input voltage short circuit and the other eats the exessive flyback pulse voltage caused by stray inductance (calculate the expected voltage!) Zener diode turns the overvoltage to losses in the diode.
Then measure the input current pulse and the output voltage pulse with the oscilloscope. Calculate the pulse energies to see how much is lost. There's no need to run more than one pulse if you have a proper memory oscilloscope.
If you have not an oscilloscope with proper bandwidth (say 50MHz or more) and high voltage probes, this task can be a little too much.
Lamps are not good test loads because their resistance is highly varying. Cold filament can have only 15% of the resistance of the same filament as hot. Use a resistor and keep pulses so sparse that you do not burn it.