Going to a full bridge is a more complex and expensive driving arrangement, but it reduces the primary current by 50% because it doubles the effective primary voltage. The relationship between the PWM signal and its equivalent analog signal are exactly the same. With the center-tapped primary you had a 12 Vpp PWM signal creating a 6 Vrms waveform. With full bridge drive you have a 24 Vpp PWM signal creating a 12 Vrms waveform.
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