Why does Mosfet bridge has to be floating ?

Mosfet floating bridge

Could someone explain a bit of detail about Mosfet drivers and why they need to be 'floating' in a bridge circuit and more precisley exactly what 'floating' means, perhaps with an example circuit? I'm a laymen/hobbyest but I have a good general grasp of electronics, just a little confused about the above terms. Some simple reading links would be helpfull as well. I ask primarily because I would like to use a 5 volt cmos output (micro controller) to switch an IRF510 (at low frequencys) but I need to use at least 7-10 volts in order to fully drive the gate.

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.