Re: Mosfet floating bridge
Let us assume you have an H-bridge, with all MOSFETs being N-channel devices. Let us also assume that the top of the bridge has a positive DC voltage and the bottom of the H-bridge is at ground. To turn on the MOSFETs, the gate-source voltage must be raised higher than the threshold voltage. To fully turn on this usually means a gate-source voltage of about 10V. The bottom MOSFETs of the bridge have their source terminals tied to ground so putting a pulsed voltage with magnitude of 10V with respect to ground will be able to turn the bottom devices on/off. However putting a 10V magnitude pulsed waveform with respect to ground at the gates of the upper devices will not necessarily be able to turn on/off the upper devices. Why? because what is important is the gate-source voltage of the upper MOSFETs and putting a voltage to the upper gates with respect to ground does not guarantee that.
Now if you take a transformer and put the primary at the pulsed signal and the secondary to the gate-source of the upper MOSFETs you will be able to drive the upper MOSFETs. Because of the transformer, the gate drives are electrically floating for the upper MOSFET -- no direct electrical connection to the gate and source terminals.