We continue the discussion here on my PCB design and device evaluation from several posts in this thread:
http://www.cnczone.com/forums/open_s...ing_motor.html
The 2nd design shown in the picture includes freewheel diodes and has the current sense resisitors on the top side for better cooling.
The chip with a heatsink like in the picture below doesn't get hot at 2Amps, I measured 45°C approx. with freewheel diodes installed.
But the power dissipation rises exponentially with the current: a rough calculation (see datasheet) gives 3.2W for 2A but at 4Amps it becomes 12.8W = times 4!!
A very quick comparison between the first (no diodes) and second design shows that the freewheel diodes reduce the chip's temperature @ 4Amp by 7°C after 5 minutes running, 63 iso 70°C. I used a tempgun to obtain comparable measurements. This needs to be tested in more detail with exactly the same design and environment. The diodes (8 pcs) are big and expensive. I will order "real" PCB's today and compare this when they arrive.
The ones wich are on order are 2 of these in parrallel: 0.40 ohm, 2W, should be a lot better. I've been thinking to reduce the value lower than 0.2 ohm. This would reduce their heat significantly but in the tips it's not recommended, the datasheet doesn't mention it only in the typical application diagram.
The diodes are already installed: BYV28-100 trr= 30nsec. You can see them in the picture behind the heatsink.
I don't fully understand your last sentence, pin23 OSC2 changes the fchop. The settings wich I use are fast decay, I think this is better at higher speeds.
Do you mean that the heat will be reduced by lowering the chop frequency?
Thinking about it: There are less switching cycles and thus less switching losses, makes sense. What are the disadvantages?
Thanks,
Luc.