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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Shrad wrote ...
no ground plane ?
I'm going to try to deal with the 'ground plane' issue once I've got the component layout sorted. I imagine loads of de-coupling capacitors sprinkled all over the place will help here, but my current thoughts are to have two 'ground planes' connected by a thin trace. One for the 556 and associated components, and one for the MOSFET and associated components.
The routing of the trace from the 'output' of the 556 to the MOSFET is the bit I'm currently working on, and the layout of the heatsinks.
Is it worth putting a resistor in the trace that connects the two ground planes? This would allow them to both remain at the same potential, but might improve stability at the 556?
I will be adding plenty more capacitors around the MOSFET, etc. A bag of 100 SMD ceramic capacitors costs next to nothing
Before you go into adding bells and whistles of layout, I'd work on getting to a stable prototype. Simply working on a grounded metal surface is usually good enough up until the 500KHz level from what I've seen first hand, and even then some things will do ok even up to ~3MHz. It depends on a lot of factors. Still, there's no reason to break your back on layout until you know whether the schematic you're using is even working.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Sigurthr wrote ...
Before you go into adding bells and whistles of layout, I'd work on getting to a stable prototype. Simply working on a grounded metal surface is usually good enough up until the 500KHz level from what I've seen first hand, and even then some things will do ok even up to ~3MHz. It depends on a lot of factors. Still, there's no reason to break your back on layout until you know whether the schematic you're using is even working.
It's just the layout of the heatsinks for the MOSFET and three associated Schottky diodes that I'm working on at the moment.
I need at least three heatsinks, unless I use insulating pads, which I don't really want to do.
The bigger the heatsinks, the further apart the components are. Using smaller heatsinks presumably means using fans as well.
I was referring to the ground plane discussion going on =).
Yes, component layout is of course important to consider for the process of getting a working prototype. I suggest a single large heatsink and using either 1mil kapton tape (ala Steve Ward) or even high grade sil-pads (as I tend to do). Yes, there is quite a difference in thermal performance, but if the electronics and design are solid then getting a an acceptable stable thermal state shouldn't be too much of an issue.
To reference an appropriate build; my 1.5kW CW SSTC is an entirely hard switched half bridge with all bridge components on a single 4" x 4" x 1/4" aluminium heatsink that has 3/4" x 1/8" fins on about 60% of the radiating surface. It runs continuous duty with only a ~25CFM fan blowing over it. I used relatively inexpensive Sil-Pads for it too.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Sigurthr wrote ...
I was referring to the ground plane discussion going on =).
Yes, component layout is of course important to consider for the process of getting a working prototype. I suggest a single large heatsink and using either 1mil kapton tape (ala Steve Ward) or even high grade sil-pads (as I tend to do). Yes, there is quite a difference in thermal performance, but if the electronics and design are solid then getting a an acceptable stable thermal state shouldn't be too much of an issue.
To reference an appropriate build; my 1.5kW CW SSTC is an entirely hard switched half bridge with all bridge components on a single 4" x 4" x 1/4" aluminium heatsink that has 3/4" x 1/8" fins on about 60% of the radiating surface. It runs continuous duty with only a ~25CFM fan blowing over it. I used relatively inexpensive Sil-Pads for it too.
Well, I've given this some thought, and the optimum setup, with the four components requiring heatsinks to be closest together on the board, and for the maximum amount of heatsinking, is to use four heatsinks, two on each side of the board.
I'm trying to keep this as simple as possible, so I may want to compromise here, and either use three heatsinks mounted reasonably close together on one side, or one heatsink, with insulating pads for the Schottky diodes, and mount the MOSFET directly onto the heatsink, as this will probably run hotter than the Schottkys.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I've given this quite a bit of thought, and decided the best way forward from here is to not put everything on one board, but go for a 'modular' setup, with the driver on one board, and the power components on a separate, stacked, board.
Apart frm aiding swapping bits in and out, it also provides the shortest distance from the 'output' pin and gate resistor to the gate of the MOSFET, assuming these are directly above and below each other once the boards are stacked.
The sketch below, showing two heatsinks and fans, is what convinced me that a modular, stacked approach makes a lot more sense than just using one board for everything.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
You will need a low pass filter/smoother/snubber between the power components and the driver. I made that mistake last year - had a load of SMPS boards made; ended up having to cut the power trace to the driver and then bridge it with a 100uH inductor. I also put a little electrolytic on the driver side to form an LC low pass filter. Works now: I was getting basically random noise coming from the H-Bridge before!
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
hen918 wrote ...
You will need a low pass filter/smoother/snubber between the power components and the driver. I made that mistake last year - had a load of SMPS boards made; ended up having to cut the power trace to the driver and then bridge it with a 100uH inductor. I also put a little electrolytic on the driver side to form an LC low pass filter. Works now: I was getting basically random noise coming from the H-Bridge before!
So the capacitor is in series with the inductor?
What value capacitor should I start with, assuming I use a 100uF inductor?
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