Voltage*time product of pulse transformer
The limitation on transformer volt.seconds is the core flux.
A 1 MHz square wave spends 0.5 us in one state, and 0.5 us in the other.
During the first 0.5 us at 15 V, and assuming it starts from zero flux, the core will go to a flux representing 7.5 Vus. During the second phase, the flux will go back to zero. Subsequent repeats will cycle the core through the same flux swing. If operated like this, you therefore need a 7.5 Vus core specification.
If you can contrive to deliver 'half' a pulse first, so 0.25 us to swing the core to 3.75 Vus, then full pulses of 0.5 us to swing it between +/- 3.75 Vus, finally finishing with half a pulse to get it back to zero, you can use a 3.75 Vus transformer.