[lucas]

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.

I think you'd better to use 2X0.47ohm 2512 in parallel instead of 1206.

If necessary, you should use schottky diodes or Fast recovery diodes(below 100ns)to reduce the chips power dissipation .


If you want to reduce the temperature, you can add the fast decay proportion or add the decay time .That means you can increase the resistor value which is connected with PIN23.

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.

[H500]

I would expect that the freewheel diodes would not lower the chip temperature if you were using fast decay.

Do this driver produce a hissing sound, or is it quite quiet?

[lucas]

Hello H500,

as far as I understand: there are at least 2 things heating the chip:
- Lost power due to the RDSon of the fet's, standard issue.
- Switching losses due to parasitic diodes beeing too slow.
The diodes should improve the second one, it might be minimal and not economically worthwile but I will try it out.
Please correct me if I'm wrong.

Didn't pay much attention on the sound while testing, was more focussed on smoke, blowing fuses, etc ...
Can't remember hissing sounds, I will look (hear) for it tomorrow.

Regards,
Luc.

[lihaijiang]

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?

Yes,you got it.The heat will be reduced by lowering the chop frequency.But maybe it will cause hiss noise.So you need to find a balance point.

I think your heatsink is not big enough.You 'd beter add a bigger surface heatsink,eg.dentate heat sink.

[lucas]

Should have mentioned this: The heatsink is just a sturdy base, fixed to the PCB. It's done like this for further testing: just screw another one on top, size as needed, evt. with active cooling.
The one in the pic below seems just enough for 4Amp, chip temp is approx 65 °C with ambient at 25.
This style of heatsinks are available in different sizes and with fins, I've adapted the PCB to match their fixation location. Attached is a sample drawing.

Please bear in mind that the thing is running at 40V and 4Amp per phase wich is very close to the limits, there are almost no heat issues at 2 - 3A per phase.

@H500: can't hear any hissing when the motor is idle at 3A, there might be if cheap wirewound sense resistors are used (as you probably know).

I really like it, it's working fine. A price guestimation for all parts, including PCB, chip and heatsink is approx 35 Euro. Wich isn't too bad for a full drive with these spec's.

[vegipete]

Thank you Luc for posting your results so far! Could you post a circuit diagram so we can see the changes you've made from the application example in the datasheet? (I'd like to know how the diodes are connected.) What are the part numbers for the diodes and current sense resistors you've used?

Thanks for the info and great work!

P.

[lucas]

The schematic is not yet finished and is still a mess, some values need to be changed: this will be done when the PCB's arrive and final testing can be done.

The 8 diodes currently installed are: BYV28-100 trr= 30nsec. Smaller ones like BYV27 might be enough. Also 4 pcs iso of 8 could be sufficient, there might even be none: tests, measurements and economic evaluation will tell. Calculations for this is close to impossible.

Definitive current sense resistors are ordered, not yet installed. So can't tell much about their result.

I could clean up the schematic and post a preliminary version tomorrow if time permits, this design has taken too much of my time recently.

[H500]

Lucas, I think you are correct about the switching losses, but I'm not sure how much power is involved. If you have a good scope and probes, the spike should be visible across the current sensing resistor. If the diodes help, the spike should be substantially lower.
One way to reduce heat in the resistors is to use the slow decay mode. That causes the current to circulate through the lower fets without going through the resistor. On my drives, I'm not sure if the fast decay results in better performance or not.

[lucas]

The effect of the diodes and the effective current sense resistor power depends on a lot of factors:
decay mode, speed, motor inductance, etc...
One could "accidently" try a drive in optimal configuration and be very pleased whilst somebody else has all the bad luck because everything is just on the marginal side and is "pissed off".

EDIT: this is exactly what occured with the TB6560, some are really satisfied, for others it's a piece of junk. The TB6564 looks much sturdier build and won't fail as much like the TB6560. ( I hope coz I've never had an issue with the TB6560 even at 35V 2.5A...)

It's real difficult to compare, some changes give minimal result but several together might be worthwhile.

Wich are the worst, most difficult settings for a drive? Hard to say, some settings and configuration will make it easy for the drive but reduce performance, others are the inverse.
It's a give and take situation, like reducing the chop frequency to reduce the heat just a bit but then possibly end up with an unstable drive...

I learned the theory on switching losses from a few posts by Mariss on the CPLD thread.

My aim is to try and develop a reliable drive as good as possible but that takes a (huge) lot of time.

This chip is definitly a good one if used a bit below it's max. ratings. Like everything else, eg a formula one car, it's very hard to drive it to it's limits reliably in all possible circumstances.
Do we have to do this? Or limit the spec's to something like 35VDC and 3Amp? Price/performance it's still a good deal.
The chip can do more, but everything has to be spot on or we slip out off the corner into the barrier.
Right now I'm pushing it to the limit and it still works, even with different motors and settings, tomorrow I might blow it with a marginal change.

My design is finished, some minor details to be sorted. I will use it "as is" and see how far it goes in the real world.

I will publish the schematic, might offer Pcb's and maybe complete component kit's. Need a few nights of sleep on this.

Thoughts, comments, recommendations ... anyone?

[SteveWill]

This chip is definitly a good one if used a bit below it's max. ratings.
Do we have to do this? Or limit the spec's to something like 35VDC and 3Amp? Price/performance it's still a good deal.

I will publish the schematic, might offer Pcb's and maybe complete component kit's. Need a few nights of sleep on this.

Thoughts, comments, recommendations ... anyone?

One: I am a newbie so I don't know much about drive circuits. With 162 views of this thread I would say there is some interest.
I am looking forward to seeing your schematic.

If this board is produced in a kit is there some protection from reverse power connection? I have seen a diode tied to ground so it blows a fuse when the power is reverse and the diode is forward biased.
Spec are always a problem, First off I do not believe them. If I have a motor that needs 3 Amps I will be looking for a driver that say 3.5 to 4 Amps.
So if I wanted to hold the spec to 3 Amp and 35 volts for a board I was supplying. I would say something like the driver is tuned for 3 Amp motors but the chip is rated for 4.5 amp and 42 volts but would need a bigger heat sink and a fan.

What process did you use to make your raw PC board?

Good job.
Steve

[lucas]

A reverse polarity protection is a good idea: I will look if there's still some empty space on the pcb.
It would be optional: install it just to be sure.

All my proto's pcb's are made using photosensitive PCB's, toner transfer doen't work well enough for me.

@vegipete:
Didn't have time yet to clean up the schematic, for me it's even not needed: will do it when the PCB's arrive and the design details are finalised.

For the diode configuration: have a look here:
http://www.sparkfun.com/datasheets/R...8_H_Bridge.pdf Fig 8. It's exactly the same.

Luc.

[vegipete]

Didn't have time yet to clean up the schematic, for me it's even not needed: will do it when the PCB's arrive and the design details are finalised.

For the diode configuration: have a look here:
http://www.sparkfun.com/datasheets/R...8_H_Bridge.pdf Fig 8. It's exactly the same.

Luc.

Don't worry about the schematic too much - Fig. 8 of the L298 data sheet answers my question. That's a lot of diodes! At least they're not overly expensive. How fast do they need to be?

[H500]

The chip is supposedly good for 85c. Perhaps their recommendation is based on high ambient temperatures, such as inside an enclosure.

[lucas]

Inside an enclosure the temperature will rise significantly, some air flow will be needed and a larger heasink with some airflow will also lower the temp.
But why is the effect of the diodes marginal why they recommend it?

At 3Amp the chip has to dissipate 9Watt, quite a bit, no peanuts.
I did the comparison with a decent heatsink, not too big or small. I had to find a size where the difference would show.
Tests were done at 28° ambient. That gives a delta T of 42° without diodes and 38° with diodes.
Not spectacular at all.

Maybe lihaijiang could comment on these results?

[lihaijiang]

Dear Luc.

Could you please tell me Max reverse recovery time for your diodes? It has en effect in heat cooling.

[boldford]

Lucas may I suggest you retain the option on the final PCB to retain the diodes for those of a nervous disposition. Elimination of them doesn't make such a great saving. Assuming someone is building 3 channels for a straight-forward XYZ gantry mill there would be an overall saving of approximately £12.00 since BYV28-100 are available from Farnell for 50p.

[lucas]

@ lihaijiang: the diodes are BYV28 trr max. = 30nsec.
Did you experiment on the effect of the diodes? The results maybe different depending on the drive settings, supply voltage, motor induction... I just don't have time enough to test this all out.

@boldford: agree, they don't cost that much but are in the way for universal heatsink mounting. There is another version where I removed the fuse wich was really in the way and rearranged the diodes.
The PCB's of these are in, just need some time to charge batteries for a pic.

Luc.

[lucas]

Ok, batteries are charged.
Below is a pic of the pcb without fuse and diodes arranged to the center, this will allow a U-style heatsink, a stand-up one or a raiser style to clear the diodes.
See the 3 examples.

The heatsink can be mounted to the chip with full contact to it's metal tab and still leave space to clear the low height components but not the diodes.
A processor heatsink can then be used eventually with a fan on it.

The pic's just show some possibilities with some heatsinks I had, there are others available wich would fit better.
There's one requirement: the heatsink must be mechanically fixed somehow, you can't just leave a heavy one hanging on the chip as I did with a lightweight one.

[lihaijiang]

@ lihaijiang: the diodes are BYV28 trr max. = 30nsec.
Did you experiment on the effect of the diodes? The results maybe different depending on the drive settings, supply voltage, motor induction... I just don't have time enough to test this all out.

Dear Luc.

Three factors have influenced temperature of chip:reverse recovery time of diodes, decay mode and decay time. 30nsec. is too fast,so you can change the decay time in order to reduce the temperature of chip.

[James Newton]

I agree... Luc does fantastic work. Wait until you see the version he did for me with the chip laying down off the edge of the PCB. I will be making those available very soon, and I'm looking for some people to try them out as beta testers.