n-FET Half-Bridge, bootstrap circuit

[Andreas]

Hello,

I have to build a half-bridge of n-channel-FETs (2x BSS138) to switch
between ground and Vcc. But I have problems to get the "highside" FET
switched (I know usually one would take a p-FET). I found something
about "boostrap circuits" at google. What is it and does it help me with
my problem? How can I make it work? I'm more an embedded software
engineer than a hardware specialist ;-)

Thanks!

 

[Tilmann Reh]

I have to build a half-bridge of n-channel-FETs (2x BSS138) to switch
between ground and Vcc. But I have problems to get the "highside" FET
switched (I know usually one would take a p-FET). I found something
about "boostrap circuits" at google. What is it and does it help me with
my problem? How can I make it work? I'm more an embedded software
engineer than a hardware specialist ;-)

Look at the data sheet of the IR 2111 to see how it works.

Basically, you need a supply above the positive rail during
the time the highside FET must be turned on. Take care that
bootstrapping works only for a short time, so if you need
a static output (say beyond a few seconds), you're better
off with a P-FET.