Ideal Diodes - here and now, the theory works

Hi All,

A couple of times we have discussed Ideal Diodes on here. Eg: http://endless-sphere.com/forums/viewtopic.php?f=14&t=4607&p=84876&hilit=Ideal+Diodes#p84876

An ideal diode is one that does not have a forward voltage drop and therefore doesn't get hot and waste power when carrying a high current. For instance, we commonly recommend people use Schottky diodes when combining battery packs in parallel, but a Schottky will have 0.5 to 1 V forward drop at high current, so at anything above a few amps it needs a heat sink.

Anyway, I just built some:


This is a little surface mount board with a FET with a control circuit. When the voltages are the right way round the FET turns on. Otherwise the FET is off.

I've set it up so the connections can be made by various ways, such as soldering, or by bolting on terminals


The spacing is also right for Anderson PP's




They can also be bolted together in stacks for various functions. Here is an assembly I used to combine 3 36 V battery packs to run a bike. There are two output cables, one for the controller and one for auxiliaries.


These are designed to operate up to 80 V. I've put 100 V surge suppressors on each end in case of inrush sparks when connecting and disconnecting. They've been used in anger on one of my bikes and survived. So far I've only tested them in the lab up to 20 A, but at 20 A they don't need heatsinks.

Anyway, guys, I'd be interested in your views/comments, etc. As I said, I'm still testing them, but they're looking good so far and I may tweak the design and make another batch.

Nick

Nicely done!
Would you object to posting a schematic?
I know that there are probably several people on this forum who'd also love to use FETs as diodes.

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Schematic is fairly simple


(Look at that, I finally worked out how to post schematics without going to pdf.)

The circuit needs a minimum of about 12 V for the IC to operate. Below that the body diode of the FET still operates, so it still works, just not as well.

In theory it will survive 100 V and then the protection diodes will kick in. Useful operating range should be 15 to 80 V.

Not sure how much use unpopulated boards would be - its not the average home assembly job to put either the IC or the FET down . I did one by hand and then put the others through the machine.

Nick

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I'll buy three of them populated or bare boards if need be.

Any idea how much $$ they would be each?

Cool.
What is the standby current drain?

fechter said:

Cool.
What is the standby current drain?

Click to expand...

Just measured it. Depends a bit on voltage, but at 30 V, I get

0.65 mA if you power the "battery" end - ie., forward biassed but with no load.
0.45 mA if you power the other end - ie,. reverse biassed.

Nick

EVNewbie: Let me do some further testing first and I'll try to work that out.

Gah! So simple! Why didn't I think of it...:?

Might need some if I ever get more than four lipo packs on any of my bikes and need to work out a charging solution other than one-charger-per-battery.

Nick,

i was looking at these last night. just a coincidence. i also examined the fancier LTC4355 dual controller. my intentions were a little different than yours though. i was looking for a way to implement a pack on/off using fets and a small switch of possibly a logic gate.

I actually ordered a pair of the LTC4355 and a pair of the LTC4357's should be here by monday or tuesday depending on how memorial day affects deliveries from the US to Canada.

anyways, a couple of questions.

1. Linears literature suggests that the device can be turned off if the GND pin is disconnected from ground. have you tried this? this could be used as a remote on/off switch. (the function i was looking for)

2. what diodes did yoou use for transient suppression? SMAJ100A?

3. which FET did you choose?

4. did you consider adding a common anode signal diode pair for reverse polarity protection? i realise that you may not need this for yourself, but if you do a commercial thing could save some accidents. after all we need to conserve that magic smoke. if too much escapes.....

5. am i being a pain in the a$$? should i go away? this is more than a couple of questions. i'm rambling

6. do you have a couple of boards to spare? could a purchase be arranged? save me the breadboard time.

rick

oops, my bad. i shoould have led of with this but i was so excited when i saw that someone had already tried these....

boards look really good. i love that you thought of making them the right size to mount anderson connectors directly. do the holes accept the Anderson board mount pins? be a minor rework if they do not.

i'm envious. really i am.

rick

Hi Rick,

No you're not being a pain; they're good questions and ones I've been working on myself.

I haven't tried the on/off switch via the GND pin, but I don't think its what we need. I was thinking of incorporating a master on/off switch, but the FET is the wrong way round - current would still flow through the body diode. So rather than build a board with 2 series FETs I decided I will do the on/off switch separately. What I want to do is to make it a soft start too - I am working on fiendish design that incorporates an automatic inrush limiter.

The transient suppressors are 100 V high power ones, pretty much as you quote, or similar, I'd have to check. They are unidirectional ones, so a reverse polarity one either end would see a short. That's probably as much accident proofness that can be put in without increasing insertion loss or complexity too much.

Nick

rkosiorek said:

oops, my bad. i shoould have led of with this but i was so excited when i saw that someone had already tried these....

boards look really good. i love that you thought of making them the right size to mount anderson connectors directly. do the holes accept the Anderson board mount pins? be a minor rework if they do not.

i'm envious. really i am.

rick

Click to expand...

Our posts crossed there. Do you mean the APP long prongs for soldering to PCBs? That's what's in one of the pictures.

The holes are M3 clearance, and I found some blue crimp eyelets with 6 mm OD to fit the spacing. I haven't found any yellow ones that size yet.

Nick

right, i forgot about those pesky body diodes. i feel so silly.

rick

So explain why I need these to run a parrallel pack?? Or a series pack? Am I crazy running my Li ion packs in a 14s 3p setup?

.

Very nice! Would it be possible to beef it up for a higher rating that 70V ? A 72V LiFePO4 pack (24 cells) can run over 90V fresh off the charger - which would be pushing the 80V 'useful operating range."

Hi Michael,

Fear not, I am a very powerful wizard of the magic smoke. After the holiday break I will be testing it on a 24 cell LiFePo system. If it doesn't behave then I could always make a high voltage version.

That leads to a good question. What voltages and currents would people want to run things like this at? Sensible answers, please.

Nick

Well, just for myself, I would have a need for three 72V packs, five 60V packs, and three 48V packs (all with the direct mounted Andersons).

The 72V and 48V packs would be used with standard Xlite 40amp controllers. The 60V packs will be for a motorcycle conversion with BMI batts, so if it's at all possible, have each capable of handling 100amps......

Very cool diode! I would love to buy a stack of unpopulated boards from you to use in various projects.

Good job Tiberius. You beat me to it. This is a synchronous rectifier (http://en.wikipedia.org/wiki/Synchronous_rectification) and is used in power supplies and for high power rectification in tight places or when heat is an issue. Perfect for high current ebikes. I'm sure there will be high demand for this, I would like some also. Other than cost, I am not sure if there is a down side, I'll check into it. The mosfet(s) has to be rated for the total current of course.

i'ld like some for my 48v 60amp set-up

Its school holidays this week, so electronics work is slower. But I will be doing a new batch - I need to run some more tests and modify the PCB slightly first.

It looks like the voltage handling is enough for most people. I will check it on 90 V (for 24 cell systems) and see how it behaves.

Current handling depends on the heat dissipation without a heat sink. Even at about 6 milliOhms resistance , 30 A is getting on for 5 W and it will get warm. I only have PSUs going up to 25 A, so I will have to combine two or work out a duty cycling system to test to 50 A or beyond.

I'm using a IRFS4310. But its not just a question of the type of FET, or doubling up FETs, at those currents the PCB tracks and the connectors are important too. There is, however, a simple way of getting any current you want, and that is to exploit the configuration and bolt modules together in parallel. Which brings me to the question for those of you running high currents - what cables and connectors would you want to use.

Nick

I like the Anderson SB series of connectors.



i have used the 50A and 120A sizes. although not a written standard, common practice is to use Yellow for 12V, Red for 24V, Blue for 48V. the 50A series readily accepts 6AWG. the 120's will take 2AWG.

the current ratings are at 600V AC or DC so at our lower voltages they can handle a lot more current.

they are readily available and copied by many other manufacturers such as AMP/TYCO. plus they do not cost an arm and a leg.

rick

At the battery pack level, my preference is the Anderson Powerpole series: either the 45amp for e-bikes, or 75amp for larger setups (I always run multiple packs to spread the load). I only use the SB series on the main 'trunk' line.