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Registered Member #977
Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74
I built a tl494 based flyback driver but have had nothing but trouble with it, so I'm going to build this circuit as it looks a lot simpler, and has less ICs to fry! I'm just wondering why does this halfbridge have no diodes from source to drain, whereas this one, does? Also why does that one have gate resistors unlike the other? And would steve's half bridge section run ok with 1uf caps? I've done some searching but I haven't found any answers!
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
MOSFETs have internal diodes so external ones make no sense (except Schottky diodes, or if you completely block the internal diode, which requires 2 diodes per transistor).
You should always use gate resistors, or at least scope the gate signals to make sure they look ok (mostly with no gate resistors, they don't.)
Use caps which have at least 10 times lower reactance than the assumed primary impedance.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Dark Current wrote ...
Use caps which have at least 10 times lower reactance than the assumed primary impedance.
How is he (me too), to figure this value out? or is 0.68 to 1uF close and common enough?
if those caps are important, id like to add them, ive seen them on others' builds. my circuit doesn't give me any grief, and real good results. my thread :
Registered Member #977
Joined: Thu Aug 30 2007, 06:57PM
Location: England
Posts: 74
Patrick wrote ...
Dr. Dark Current wrote ...
Use caps which have at least 10 times lower reactance than the assumed primary impedance.
How is he (me too), to figure this value out? or is 0.68 to 1uF close and common enough?
if those caps are important, id like to add them, ive seen them on others' builds. my circuit doesn't give me any grief, and real good results. my thread :
Primary impedance will just be its inductance I imagine, as its resistance will be negligible, I measured that with a cheap LC meter. And the capactitor's reactance I calculated using 1/2pifc, with f=40khz for the largest value. That's a really nice looking build
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Dr. Dark Current wrote ...
MOSFETs have internal diodes so external ones make no sense (except Schottky diodes, or if you completely block the internal diode, which requires 2 diodes per transistor).
You should always use gate resistors, or at least scope the gate signals to make sure they look ok (mostly with no gate resistors, they don't.)
Use caps which have at least 10 times lower reactance than the assumed primary impedance.
Just be sure to check the specifications of the internal diode before saying a external is not needed! The internal diode is only a byproduct from when you manufactor MOSFETs
I found that adding an external diode to MOSFET half bridges lowers the junction temp of the chip by about 30%. It is totally worth it imo. I didn't notice much improvement when adding the second diode per fet to totally block off the body diode, but I don't think it would hurt as long as you can live with the additional Vf drop.
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Half bridge flyback questions
