W
Winfield Hill
- Jan 1, 1970
- 0
[email protected] wrote...
Sounds good. I'm curious, how big is your coil? What's it like?
You wouldn't want only two of the 26-volt TVS because a 55V flyback
would be slower than you're looking for, but you could use a modest
number of higher-voltage TVS parts. It's also be nice to drop back
to 1.5kW parts, which are more commonly available. For example, you
could use six of them in series, for a nice 40% safety margin.
Well, if you were to use 450-volts worth of TVS in series, then
you'd get a discharge time I*L/V = 2.8ms, likely faster than you
need. If you pick a lower flyback voltage you'll have an easier
time finding low Ron switches, which is important to avoid a big
fan-cooled heatsink. For example, a 175V flyback would give
you a 7.3ms shutoff but allow the use of 200V FETs.
Fairchild's FQA34N20 or ST's STW34NB20 (both in stock at Mouser
for about $3.20 each), have Rds(on) of about 0.06 ohms, so they'd
dissipate only about 4W, which is a big improvement over what you
were facing with 500V parts. I'd even consider using one of these
(wired backwards) to replace D1, because their 0.5V drop at 8A is
better than you can do with a 250V diode. I'd use good heatsinks
on the FETs, you want to keep the junction temperature down to
avoid an increase in Rds(on).
To get the 175V flyback, you could use six series 1.5ke24a (also
called 1p5ke24a, to eliminate the decimal) TVS, which drop 27-30V
at 8A. 24c at Mouser. Six of these are rated to absorb 9J in a
1ms pulse, even more in your 7ms pulse, fine for your 5J energy.
As for C1, it's be wise to limit the flyback risetime to say 0.2ms,
limiting dV/dt to a modest 0.9V/us. That means a capacitance of
C = I*t/V = 9uF. An ordinary 10uF 250V electrolytic would probably
work fine, given your presumably-long interval between events.
The current ramp has a V/L slope as it starts, but the coil's high
series resistance makes it tail off in the classic manner you're
use to seeing with a charging RC. The time constant is L/R, or
0.16/3 = 53ms. But it'll be within 10% of 8A after about 100ms.
Cheeky blighter that I am, I'll accept for both of us.
Have fun, and please do report back.
Hi Win,
Thanks for the generous offer. I did manage to measure my magnet's
inductance, albeit indirectly. I fed 15VAC@60Hz through the magnet
and a decade box, R, in series:
,----////////---/\/\------,
| |
~ |
| R |
'----------/\/\-----------'
I adjusted R such that the voltage across R and the voltage across
the magnet were equal. Then, using R's value as the reactance, X_L,
I solved for L = X_L/(2*pi*60Hz). I get L = 161mH.
Sounds good. I'm curious, how big is your coil? What's it like?
In my application, I definitely can't tolerate field reversal times
of more than 100ms. In fact it'd be highly preferable to be able to
fully switch in about 10ms. It's okay if the field shutoff and
startup times are unequal, but shutoff should be quicker than startup
(say 20ms shutoff and 80ms startup for the worst-case 100ms scenario).
This balance becomes less important as the total field reversal time
becomes shorter.
So, it seems that for the circuit below, I'd be safe with only two
of the 5kp26A TVS zeners since the magnet energy is E = (161mH*8A^2)/2
= 5.15J.
You wouldn't want only two of the 26-volt TVS because a 55V flyback
would be slower than you're looking for, but you could use a modest
number of higher-voltage TVS parts. It's also be nice to drop back
to 1.5kW parts, which are more commonly available. For example, you
could use six of them in series, for a nice 40% safety margin.
. D1 rapidly rises to Vzener
. V+ ----|>|---+------------------+--------+-----,
. | | | |
. |--' '--| | |
. |<-, Q1 Q2 ,->| | |
. |--+ +--| | \_|_
. | | C2 | /_\
. +------//////------+ === | TVS
. | L + Rs | | | units
. |--' '--| | \_|_
. |--, Q3 Q4 ,->| | /_\
. |--+ +--| | |
. | | | |
. 0v-----------+------------------+--------+-----'
This just leaves me with finding an appropriate value for C2 and
figuring out some startup and shutoff times. It seems as though
shutoff should be very quick (<1ms) since it just depends on the
TVS zener ramp, ...
Well, if you were to use 450-volts worth of TVS in series, then
you'd get a discharge time I*L/V = 2.8ms, likely faster than you
need. If you pick a lower flyback voltage you'll have an easier
time finding low Ron switches, which is important to avoid a big
fan-cooled heatsink. For example, a 175V flyback would give
you a 7.3ms shutoff but allow the use of 200V FETs.
Fairchild's FQA34N20 or ST's STW34NB20 (both in stock at Mouser
for about $3.20 each), have Rds(on) of about 0.06 ohms, so they'd
dissipate only about 4W, which is a big improvement over what you
were facing with 500V parts. I'd even consider using one of these
(wired backwards) to replace D1, because their 0.5V drop at 8A is
better than you can do with a 250V diode. I'd use good heatsinks
on the FETs, you want to keep the junction temperature down to
avoid an increase in Rds(on).
To get the 175V flyback, you could use six series 1.5ke24a (also
called 1p5ke24a, to eliminate the decimal) TVS, which drop 27-30V
at 8A. 24c at Mouser. Six of these are rated to absorb 9J in a
1ms pulse, even more in your 7ms pulse, fine for your 5J energy.
As for C1, it's be wise to limit the flyback risetime to say 0.2ms,
limiting dV/dt to a modest 0.9V/us. That means a capacitance of
C = I*t/V = 9uF. An ordinary 10uF 250V electrolytic would probably
work fine, given your presumably-long interval between events.
but I'm not sure how to calculate the startup time.
Is it just t = L (Im/V)?
The current ramp has a V/L slope as it starts, but the coil's high
series resistance makes it tail off in the classic manner you're
use to seeing with a charging RC. The time constant is L/R, or
0.16/3 = 53ms. But it'll be within 10% of 8A after about 100ms.
Again, Win and Tony, thank you both so much for your help.
Cheeky blighter that I am, I'll accept for both of us.
Have fun, and please do report back.