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Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
is there a minimum switching frequency required for a buck converter? I've got a 555 PWM generator that gives me 17kHz at 50% duty cycle. can I just connect the gate of an IRFP250N straight to the output of the 555 IC or do I need something else so that it fully turns on/of
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
There is no lower limit to the switching frequency BUT as frequency decreases larger values of inductance and capacitance are required, higher frequency = smaller/cheaper. Also if there is electronic feedback the time constants become longer and the psu loses it's ability to respond to rapid changes in the load.
For a simple buck-converter I'd use a p-channel device as the switch, you could use an n-channel device as a synchronous rectifier though. If you use an n-channel transistor you will 'loose' voltage due to the gate-source voltage required.
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
ok I followed this schematic
and the mosfet get rather hot with a 4 amp load, and yes i did use a heatsink. so, am I not turning on the gates on properly resulting high on-resistance?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
an IRFP250N starts to turn on with about 4V gate-source voltage, so if the transistor is connected to the same supply as the 555 the maximum ON voltage at point 'Q' is about Vin - 4v(irfp250) - 2v(555) i.e. you 'loose' about 6V, so what you expect to be 12V is only 6V etc.
If you put a p-channel fet in there you would get almost full voltage swing.
Registered Member #2063
Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352
is there a p-channel version of the IRFP250N?
I used this schematic for the PWM generator, but instead of R2 and the NPN BJT I connected the gate directly to pin 3 fo the 555 IC.
when I used it to dim a 12v 50W halogen lamp, the mosfet stayed at room temp, even after an hour sine it didn't heat up I must be driving the gates correctly. but when I make it a buck converter with the schematic on my previous reply, the mosfet heats up like mad within 2-3 minutes. whats up with that?
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help needed for a buck converter
