# pulsed 10A current source

Discussion in 'Electronic Design' started by uiu, Mar 7, 2006.

1. ### uiuGuest

I need to make a 10A pulsed current source with variable frequency and
on-time. On-time needs to be adjustable between 25 us and 200 us. I
am planning to use a PIC to do the timing with a switching MOSFET on
the output. I am driving a resistive load.

I would like to use a 1 or 2 A power supply, so I am charging a
capacitor during the output off-time. I've simulated the circuit using
SwCAD and find that as expected,when the output pulse widens, the
output current decreases due to the power supply cap voltage sagging.
I'd like to maintain a more or less constant current (maybe +-5%)
regardless of the output pulse width.

Is there some easy, active technique with fast response time I can use
to regulate the current pulses?

Thanks!

2. ### John_HGuest

Define "fast"

Consider using a supply that's a little higher than you need for 10A to your
resistive load. You can then regulate the output (turning the regulator on
and off under PIC control) with a high current linear regulator. The sag
for your longest pulse has to accommodate the droput voltage for your
regulator at the end of that pulse.

4. ### Spehro PefhanyGuest

Of course you are going to have to increase the supply voltage,
assuming you're going directly into a resistive load.

One really simple way is to use a voltage regulator following the
capacitor and switch the output voltage of the voltage regulator.
Provided the input voltage to the capacitor does not drop below the
minimum required for regulation you'll get constant voltage output,

You could also replace the MOSFET with a switched current source
circuit (again with increased input voltage), but that is going to be
more complex, and possibly more difficult depending on your
specifications for overshoot and speed.

Best regards,
Spehro Pefhany

5. ### uiuGuest

About 2 Ohms and the min/max off times are 3ms/1 sec.

Thanks!

6. ### uiuGuest

Fast enough to slam the current up to 10A and back down to zero again
in under 25us. Rise and fall times are not critical beyond that loose
definition.

Thanks!

7. ### Winfield HillGuest

uiu wrote...
This is a trivial project if you have a 10A regulated supply,
which is also a trivial project, or a \$75 ebay purchase.
Inexpensive power MOSFETs have no trouble switching 10A in
less than 25us, even with wimpy gate drive. We're used to
designs where we switch 25A in 10ns, which is a bit more of
a challenge. Perhaps you can tell us a bit more detail

BTW, if you want a linear mode 10A MOSFET current source,
that's possible, but it's a bit more complicated issue.

8. ### uiuGuest

I realize it's trivial if I want to use a 10A supply. My thinking was
that I'm only averaging a few watts into the load and it's sort of
over-kill to use a 300W power supply (not to mention the size and
weight of a 300W regulated supply). I'm on a student budget so using
a low current supply such as a 2A 24V switcher I already have is the
preferred route.

My simulations indicate that there's no problem with the mosfet
switching speed (though I have no experience with the accuracy of the
model in this regard). I will breadboard the thing and make some
measurements when the mosfets arrive, hopefully during spring break. I
am going to use Si4420DY mosfet(s). Probably overkill for this app,
but I will be using them for other projects down the road...

If there's no easy way to do it I can live with it. I'll add a bunch
more capacitance and maybe use multiple, parallel outputs from the
PIC, selected as a function of the on-time or off-time, with output
each driving a mosfet with different current limiting resistors.

Thanks!

9. ### Fred BloggsGuest

Yeah- it's called using junior high school arithemtic and 10th grade
physics to size the capacitor. Then 5% of 20V=1V and this is 1V*C
coulombs lost to 10A * 200us= 2mC at longest pulse width, making C=2mF=
2000uF. So a 2000 microfarad at 25V WVDC or more should do it, low ESR,
and ripple current rating of approximately 2x power supply max DC
current capacity.

Thank you.