Suited solution to cut 300A current - Page 1
Forum section: electronics / beginners

author: Yaroooo
author_rank: Regular Contributor
timestamp: August 12, 2021, 03:24:39 pm
content: Hello,I've a 110V DC power supply that power a load at 150A of rated current, with possible peaks up to 300A of fraction of seconds. Actually on system there's voltage meter, current meter, a slow blow fuse and now I'd like to add an additional protection to cut-off power when I detect abnormal current absorption. i.e. I don't exclude possibility of a 300A peak for more than 2 sec, since fuse at this current rate will blow in about 10s (it's good just for short circuits), I'd like to cut-off power supply to avoid to damage it.Devices that I've found are SSRs and HIGH current MOSFETS. But I've found only TRIAC SSRs not suitable and MOSFETS will blow due thermal increase.How should I proceed? Maybe some parallel MOSFETs to reduce load?

author: Terry Bites
author_rank: Super Contributor
timestamp: August 12, 2021, 04:58:29 pm
content: At these power levels its always preferable to use a module eg VMO 500-02f. Paralleling mofets looks easy but its complicated. Drain ballast resistors of the right value are needed, this is not easy to calculate Individual gate resistors are needed to keep the circuit from turning into a mega-power oscillator and the magic smoke will appear! The drop voltage across the ballast resistors must be taken into account when setting Vgs.

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2021, 06:08:45 pm
content: I have seen a few designs which used parallel power MOSFETs as circuit breaker with an SCR to short the gate drive signal and shut them off.Quote from: Terry Bites on August 12, 2021, 04:58:29 pmWhy would drain ballast resistors be used in a saturated switching application?When saturated, the channel resistance has a positive temperature coefficient which is what allows MOSFETs to easily share current within the same device and between multiple devices.QuoteIn saturated switching applications the gate resistors should not be needed, but I usually include them anyway because they make layout easier. They are also a way to limit di/dt which becomes important at high currents where the physical layout constraints raise the source inductance.The issue with source inductance and high current is why I select the parts and figure the layout out first and then design the rest around it.

author: langwadt
author_rank: Super Contributor
timestamp: August 12, 2021, 06:20:34 pm
content: Quote from: David Hess on August 12, 2021, 06:08:45 pmhttps://assets.nexperia.cn/documents/application-note/AN11599.pdf

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2021, 08:19:10 pm
content: Quote from: langwadt on August 12, 2021, 06:20:34 pmIn a practical design I doubt it matters because the margin for worst case current and power sharing will be overwhelmed by the derating for reliability, unknown factors, and overall power dissipation.But if you do want to add ballasting for current sharing, which could almost double peak current handling capability, it is easy to select the value of the ballast resistor based on the saturated channel resistance specifications, but beware that this makes the power dissipation situation so much worse that it is of questionable utility.There are some TO-264 parts with a drain current rating of 300 amps but I would still tend to use at least two in parallel to divide the heat up which also halves the loss, making the cooling solution 1/4 the size, and then ballasting becomes irrelevant.

author: langwadt
author_rank: Super Contributor
timestamp: August 12, 2021, 08:33:20 pm
content: Quote from: David Hess on August 12, 2021, 08:19:10 pmas soon as you parallel devices I'd think the wiring could alone could do of the balancing, even at 300A wire size you quickly get milliOhms and copper is roughly +0.4% per C

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 12, 2021, 08:55:18 pm
content: For an intermittent switching application like this, a few transistors being short-circuited for a fraction of a microsecond makes no difference whatsoever. The worst case condition is one transistor turning on early (due to Vgs(th) variation, and circuit geometry having different resistance/inductance to them), and it only sees full load voltage, and short circuit current more or less, until the rest turn on and finally pull down the load voltage.Or vice-versa on turn-off, and note you want closely interleaved TVSs to clamp below Vds(max). You can't afford the stray inductance (20nH+) of having one off to the side of a gang of transistors.Once all are turned on, the L/R time constant of the stray inductance between them, and their resistances (Rds(on) plus the wiring up to the common terminals), determines the rate at which currents spread out evenly again. Ballpark say 20nH and 3mΩ is ~7µs, plenty of time for a circuit breaker.More to the point, why not stop it at the source (power supply)? Is it not controllable? Is it an off-the-shelf unit? Is it not hackable?Also, how quickly does this need to operate? Microseconds? Milliseconds? Seconds? What kind of response shall it have, instantaneous (peak), some ride-through, time-delay? How much fault current must it withstand during that delay? Should it limit current instead, and if so for how long before faulting out?For most MOSFETs, a current limit duration over 100us or so, will require proportionally more devices in parallel, or aggressive derating, to have enough total die volume, or power dissipation, to handle all that energy.An alternative is using a bulk resistor, or TVS or MOV array, in a switching circuit that regulates load current within some band -- this introduces ripple, but saves cost in active semiconductors, while still offering a long fault duration.Tim

author: Yaroooo
author_rank: Regular Contributor
timestamp: August 13, 2021, 09:06:43 am
content: Ok, let's assume this:- Power supply will be always ON and LOAD will change between low current consumption state (1A) or completely ON (150A);- I'll never use this system as an ON - OFF switch but just as emergency cut-off;- LOAD will never drain at start more than 1A , will drain 150A but only when I'll command to LOAD to go in full drain.In this case I should not have oscillations because it's not a switching device. All MOSFETS will turn ON when load is very low. When MOSFETS will turn OFF they will never turn ON with full load, but just at low current state.My doubts are:- Will all MOSFET conduct at same current at normal ON operation?- When I cut-off power, can there be oscillations that may destroy some mosfets?

author: Jeroen3
author_rank: Super Contributor
timestamp: August 13, 2021, 09:37:16 am
content: You can buy a circuit breaker?Schneider has some, but you may want to ask sales fist.Looking at the website it seems you could be set for around $4k.But maybe you can whip up something solid state for less. Though you will lose about 100 Watts of heat in normal operation that way.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 13, 2021, 11:13:49 am
content: What is the source of the high current in your circuit? How fast must it switch? What is the load impedance?There are ways to quench the arc that destroys relay contacts. Introducing an ac waveform on top of the DC creates a zero crossing point where the relay can be safely opened. Timing is everything of course.There are a number of similar methods seehttps://circuitglobe.com/hvdc-circuit-breaker.htmlYou have to experiment at low currents first. Get it wrong and bang goes your relay contacts. Get it right and you have a low loss switch.A thyristor is going to have to dissipate at least 1W per amp so thermal mangement is essential. All the DC quenching schemes apply to thyristors as well. A simple way of "Commutating" of thyristors can be achieved by connecting smaller auxiliary thyristor and a capacitor across the main thyristor. page 29https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiK3ar17K3yAhWoSEEAHeIdDmYQFnoECAQQAQ&url=https%3A%2F%2Fgetmyuni.azureedge.net%2Fassets%2Fmain%2Fstudy-material%2Fnotes%2Felectrical-engineering_engineering_power-electronics_commutation-techniques_notes.pdf&usg=AOvVaw3U0-_E_dU1aXsn-jK8DSHGA brief pulse of opposite polarity to appears across the main thyristor turning it off.https://www.alibaba.com/showroom/thyristor-400a.html