Problem with upper two FETs in a H bridge that has 4 N type MOSFETs

[magnetra]

I tried to make H bridge ckt using 4 N type MOSFET. But they don't work. The upper two FET,never switch on despite positive gate voltage. What could be the prob

 

[seyyah]

Re: H bridge

Your gate must be positive against the source. Thinking upper sides, source side of the mosfet is at bus voltage when it is open. So if your bus voltage is say 48V then your gate must be 60V or so according to the ground and 12V (If your gate needs 12V to be driven) according to the source. To accomplish this, you need a bootstrap circuit or an isolated supply.
How did you make the connection or how did you drive the upper mosfets?