BroMikey's Capacitor Dump Circuit

[TinselKoala]

Faugh. I looked at that post and I can see that BroMikey is terribly confused and is clearly doing something wrong.

The IRFP460 has Rds 0.27 ohms (when switched fully ON; higher if not), Id max 20 A reducing to 13 A at 100 degrees C, and 500 Vdss. Its max _power dissipation_ is 280 Watts when it's on a good heatsink.

He is thinking that 3 amps at 80 volts is getting close to the mosfet's limit! Because 3 x 80 is 240 Watts and the data sheet says 280 Watts at 25 C!!!
But this is derated 2.2 w/degree C, so at 110 C the power dissipation limit is actually 280 - (2.2 x 85) = only 93 Watts. Hence the need for cooling at high power levels.

But really, if the  mosfet is carrying 3 amps it is dissipating I²R or 3x3x0.27 = about 2 and a half Watts !!!! 6 amps boosts it up to nearly 10 Watts. The mosfet can easily handle this even if not on a heatsink or fancooled. You'd think he'd notice the room getting a bit warm, if his bank of mosfets was really dissipating nearly 3 kW.

Let's say the mosfet is carrying 20 amps.  How much power is it dissipating? 20 x 20 x 0.27 = 108 Watts, exceeding the max power limit at higher temperatures. How about that. It must be kept cool in order to operate at that power level. So again we begin to understand the relationship between mosfet power dissipation, the maximum rated limits of Id and Pd, and the need for proper cooling at high power levels.

But he's pulsing at 3 Hz with 250 ms ON time for a duty cycle of 75 percent, so that lowers the actual average power dissipation accordingly.

So if his mosfets are running hot and failing it most probably means he isn't switching them properly _and_ he is getting huge dissipation from avalanching on switch-off, not failing from carrying too much current per se. They are nowhere near their actual power handling capacity when properly switched.

When reconsidering I would have to say that each fet might be capable of 3 amps of surging high voltage. This fet is rated at 280 watt max burnout.

I have not put my thinking cap on much till things burnout.

I am passing 35 amp pulses using 6 parallel IRFP460 FETS.

This divided up evenly = 6 amps per FET and since each Fet is also passing voltage with it we must multiple 80vdc X  6 Amps = 480 watts in 250 mS. That is 480 watts EACH so X 6 =2800 watts.

Insert facepalm here.



ETA: That "also passing voltage with it" phrase indicates to me that BroMikey's mental model of electricity is wrong, and he is misleading himself because of it.

Cure #1: put an ultrafast, high current diode reverse biased across each mosfet. MUR1560 for example, anode to Source and cathode to Drain. These diodes may also need to be heatsunk.

[Farmhand]

Maybe if he is dumping enough capacitance charged to a high enough voltage into a battery he might be exceeding the maximum pulsed current rating for the mosfet, and with several in parallel one mosfet might be taking most of the abuse, as a cap discharge can produce a very high initial peak current.  That is why it's done, isn't it.

And coupled with poor switching and high "on"resistance then "Pop goes the mosfet".

He also mentioned at one stage he w is using quite long cables, like meters in length.

I've made a drawing for him with a store bought transformer run from the wall to give a 12 + 12 volt supply so he can rectify 12 volts for switching supply and 24 volts for charging the caps with a Large inductor between smoothing caps and dump caps to partially isolate the dump cap from the supply when dumping.  Maybe a MOT primary would work there, (just remove the secondary for safety XXX).

..

[TinselKoala]

Some reading material.

[TinselKoala]

Maybe if he is dumping enough capacitance charged to a high enough voltage into a battery he might be exceeding the maximum pulsed current rating for the mosfet, and with several in parallel one mosfet might be taking most of the abuse, as a cap discharge can produce a very high initial peak current.  That is why it's done, isn't it.

That's true. A capacitor discharge can produce very high currents, the more capacitance the more current ... into low impedance loads. The max pulsed current for the P460 is 80 amps and that's under pretty narrow pulse parameters. So if he's charging to 80 volts, then dumping the caps through a single mosfet due to poor paralleling, he needs at least one whole ohm of impedance in the circuit after the mosfet, to keep the max current below 80 amps.  R = V / I  so 80/80 = 1 ohm. Oh, wait, the mosfet itself has 0.27 ohms minimum resistance, so the rest of the circuit needs less than three quarters of an ohm impedance (resistance) to keep the maximum surge current below 80 amps. No matter the capacitance.

I'm still having my first coffee of the day, so please check my math and reasoning.

Dumping a cap through a mosfet into a load is a lot less problematic than charging an empty cap through a mosfet switch or crowbarring a cap bank with a mosfet. The empty cap looks like a dead short, so if the supply voltage is there, the surge current in the mosfet can be very high. But dumping a cap into a resistive or inductive load with a mosfet is much less problematic because the load impedance limits the surge current. I think.

And coupled with poor switching and high "on"resistance then "Pop goes the mosfet".

He also mentioned at one stage he w is using quite long cables, like meters in length.

Then he should put the diode I mentioned above, at the load end of the cables, and supplement with an additional similar diode right at the mosfet pins.

I've made a drawing for him with a store bought transformer run from the wall to give a 12 + 12 volt supply so he can rectify 12 volts for switching supply and 24 volts for charging the caps with a Large inductor between smoothing caps and dump caps to partially isolate the dump cap from the supply when dumping.  Maybe a MOT primary would work there, (just remove the secondary for safety XXX).

..

Is the complete actual circuit he is actually presently using right now, available for study? I'd like to take a look but I can't be arsed to sift through page after page of nonsense at EF.

[Kator01]

Hello,

this subject has been covered by diifferent physicists starting with Heinrich´s first publication 1985, so I wonder why this topic is brought up anew again and again.

Here are some publications, starting with Heinrich´s work:

http://freenrg.info/CAPTRET/Entropy_Change_when_charging_a_cap.pdf

http://xputers.informatik.uni-kl.de/conferences/patmos/patmos98/desoete.pdf

http://arxiv.org/pdf/1201.3890.pdf

Regards

Kator01