Thanks, John. Up to ~ 65K operations in a holiday (unattended, worst case) weekend, so that one is a no go. Interestingly - I had already started on an SSR replacement board, but haven't found the reliability specs for switching, so I'll have to test it myself. No big rush - the G2R serves well for now. But one of those tiny ones like the Fujitsu sure would be nice. Fortunately, it's not mandatory. :-)
That's interesting - I had not been aware of that.
Well, two batteries can deliver current at one time if they're at the exact same voltage, but yes uninterrupted current flow is necesarry. And yes, it'd be problematic if one battery started charging another, especially at 500A (the batteries I'm using are rated for 20A continuous, 30A for up to 10 seconds, though I'm not sure about how they handle really large transients). It's unfortunate that I have to throw some power away in a Schottky, but as far as I can see my options are very limited, unless I'm missing something. Doing source to source FETs will require incredibly accurate timing along with a large capacitor to make uninterrupted, again unless I'm missing something. Thus it seems like the Schottky + single FET is the best solution?
Here's one other thought that just struck me: what if I had two FETs source to source as discussed. However, what if I put in place a switching scheme so that the top (connected to battery) switch was switched separately from the bottom FET, and that the bottom FETs are driven in such a way that only one or less of them can be on at any given time. Additionally, there would be a Schottky in parallel with each bottom FET. The idea is that by placing Schottkys in parallel with the FETs hot swapping would be enabled, but the second FET could be switched on a moment after the other bottom FETs were switched off, decreasing power loss.
Here are some pics of the behavior of a tiny Omron surface-mount relay, an equivalent to the Fujitsu part.
The first "on" bounce is consistantly about 5 usec wide, 50x or so longer than needed to fully charge a gate. Turnoff is interesting... a brief increase in resistance, then a fall, but no bounce.
Contact risetime is very clean and under 10 ns, probably limited by my setup.
ftp://66.117.156.8/Omron.zip
With different drive, pullin/dropout times could be reduced.
You are missing a great deal, a little something called "working knowledge" of the practice of the art and its components. All you have to do is explain the application with some amount of quantitative detail, which you're not doing very well at all, and we'll decide what is or is not "incredible."
Yes, it makes perfect sense. Schottky diodes to insure a continuous voltage availability (Fred's suggestion of an inductor would do that as well, but would require you to consider the flyback voltages from the stored energy in the inductor during switching -- might be reasonable), and the paralleled MOSFET switched on later, when it's safe, to reduce heating.
The setup would be a bit different than the one in Fred's drawing following your post.
The primary issues are figuring out when to switch the "B" MOSFETs (e.g., turn on a delay-time after the "A" mosfets, but **only** if V_BattN is > than V_battM, and turn off immediately with "A" mosfets), and devising the appropriate gate-drive voltage (the same as for the "A" mosfet). This extra trouble would save you a little more than 10.5 - 6.3 = 4 watts, call it 5 watts, over wiring 4 sections of two 60CTQ045 diodes in parallel. I dunno if it's worth it.
Now he's saying it's 10A continuous. This is nothing for an SCR, maybe
0.1V x 10A=1W additional Pd over an SD, if that, vastly simplified transient triggering, and they shut themselves off. A single MOSFET to the load controls load power as well as SCR commutation when it turns off.
Yes, but John has the waveform correct, with the addition of the gate capacitance, the voltage will be the integral of the charging-current pulses through the gate resistor's relay-contact connected time.
Well, no, not if the gate resistor is large enough to make nice slow turn-on waveforms, e.g. 15V/us as Fred suggested.
Well, the issue is the gate resistor. I doubt we'd like a value small enough to get ns gate-voltage risetime, but if 5us of contact can be guaranteed, then even 15V/us will be fast almost enough, after accounting for all the gate charge. Myself, I'd prefer say 5x faster yet, which means the first relay-contact tick should fully charge the gate.
65k operations is not that many for a relay, and some relays, such as the NAiS NR long-life series, specify rapid operation, 500 operations/sec for the NR types. But, whew, all the noise! All the mechanical mashing!
You might consider optical-coupled MOSFETs, in a nice miniDIP package, for about five bucks. I especially like the Panasonic (formerly Mitsubishi NAiS) parts, which they call PhotoMOS. Clare has their OptoMOS, and Toshiba, omron, CEL and IR make good stuff as well (IR was an early leader but hasn't introduced much in the last few decades). DigiKey shows a nice selection, pp1979-1987 in the Sept-Dec 2007 catalog.
On of the "largest" photoMOS relays is in Panasonic's AQV series: the AQV251 has an ON resistance of 0.25 to 1 ohms (depending on whether you need to switch ac or not), and is rated to about an amp (it uses big internal MOSFETs). The higher-current parts have lower voltage ratings, only 40V in the case of the AQV251, so that might be an issue.
You can also roll your own opto-relay, with MOSFETs of your choice, high-current and high-voltage, using the infra-red-LED to PD-stack parts made by IR, etc., as we discussed earlier in this thread.
Not explode? Well then, you've come to the wrong newsgroup. ;-)
Anyone have a solution that uses an optoisolator? Maybe a bit of an overkill for 55V.
--
Paul Hovnanian paul@hovnanian.com
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Procrastinators: The leaders for tomorrow.
Or he could use a big capacitor to hold up the load like so, giving the turning off sw a 10us headstart on the turning on sw: View in a fixed-width font such as Courier.
That's an interesting circuit, where'd you find it? It solves the problem of making a capacitor-coupled flying V+ gate-drive voltage, for the awkward case of when the destination voltage's V- is floating.
Exactly, the high side reference node can be at any impedance..
Park & Jahns,"A Self-Boost Charge Pump Topology for a Gate Drive High-Side Power Supply," IEEE, 2003. "... features high voltage and current capabilities for use in Integrated Power Electronic Modules (IPEMs)." There is also an adaptation for a bipolar boosted drive.
Nothing in particular, found it on Fairchild's site, and available from Mouser for $0.60 in unit quantity, in stock, and TO-92.
I agree, using a P-channel mosfet would be great - far easier, if a good enough one can be found. Generally one gets better N-channel types and often doesn't even bother looking for an acceptable P-channel part. The parts Fred has found aren't bad, but as he said, since they come in TO-263 (D2PAK)packages, the mounting to obtain optimal heat removal will be non-trivial.
I have ST's stp80pf55 mosfets in my stock drawers. These may be a better choice, because they come in TO-220 packages, easy to clip a heatsink onto, and cost only $2.46 at DigiKey. Its typical rRds(on) is 16*1.2 = 20 milli-ohms at 100 deg-C, so at 10 amps it'd dissipate under 2 watts, assuming a small 3W clip-on heatsink. But if it was meant to switch 30A for more than a second or two, I'd use two or three of these 80pf55 mosfets in parallel. However, even at that rate it would be much more simple than the other approaches we've been discussing here.
stp80pf55 fuse diodes batt --X----+--- S D ----|>|----+--- bus | G | 2.7k | : | | etc +------' | TTL C ------- B E 2n5551 | 2.2k | gnd
Note: paralleled mosfets should have individual gate resistors, say 100 ohms each.
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