SMT is pretty easy

[Tim Williams]

So I made a simple PWM driver:

http://myweb.msoe.edu/williamstm/PWM_Gen_Schematic.pdf

http://myweb.msoe.edu/williamstm/Images/PWM_Gen1.jpg
http://myweb.msoe.edu/williamstm/Images/PWM_Gen2.jpg

No waveforms, suffice it to say it works as advertised. Some shoot
through, which is perfect as I intend to use it on current limited
applications.

Tim

 

[Tim Williams]

Turn-off, enforced by Q3, will be limited by the 100R mos output
impedance of the comparator.

Err, typo... Multisim doesn't have an acceptable LM393 model, so I found a
random dual comparator with the same pinout. Actual device is LM393, and
saturation is noticably sharp in both directions (~150mV).

Tim

 

[legg]

So I made a simple PWM driver:

http://myweb.msoe.edu/williamstm/PWM_Gen_Schematic.pdf

http://myweb.msoe.edu/williamstm/Images/PWM_Gen1.jpg
http://myweb.msoe.edu/williamstm/Images/PWM_Gen2.jpg

No waveforms, suffice it to say it works as advertised. Some shoot
through, which is perfect as I intend to use it on current limited
applications.

Tim

Turn-off, enforced by Q3, will be limited by the 100R mos output
impedance of the comparator.

Not so the other side, but then the comparator output is clamped at
two diode drops above gound. Not much turn-off voltage for Q3, and
nearly the entire supply across the comparator's 100R mos driver -
2.4W....

Maybe the comparator's output transistor is open circuited.

Not breadboarded as drawn, anyways.

Nice handiwork though. Pitty.

RL

 

[Tim Williams]

What's wrong with Multisim's LM393 model?

LM393 doesn't exist, but there is TLC393.

Last time I tried a comparator, it wasn't a comparator, it was an op-amp.

And for that matter, half the op-amp models suck -- one circuit I tried
with an LM358 on +12/0V rails blissfully generated outputs in the 30V
range. Others get the bandwidth or slew rate wrong.

Multisim 10, BTW.

TL494 / TL598 don't come with it either, and I haven't seen them available
(at least in any obvious fashion; they didn't seem interested enough to
add an automatic "find models here" feature, but rather a painful database
entry process). I've done more than a few schematics with them and just
placed DIP16s as placeholders. At least the footprint is right then.
Good thing I can simulate these chips faster on paper, or test on the
breadboard, than any simulator anyway.

And then there's the mess of "untestable" circuits I've come up with,
which I can only imagine come from terrible models or some bizarre set of
required simulator conditions (none of which is specified, if it even
could be). But that's more of a SPICE thing than Multisim's fault
necessarily.

Tim

 

[legg]

Err, typo... Multisim doesn't have an acceptable LM393 model, so I found a
random dual comparator with the same pinout. Actual device is LM393, and
saturation is noticably sharp in both directions (~150mV).

Tim

What's wrong with Multisim's LM393 model?

RL