FlexDrive - Overheating/Max Current?

[Wolfman29]

Hey everyone.

Got a second FlexDrive (I gotta admit, it's a beautiful little driver...) for my 445 build, and, after setting it to max current and thermally bonding it to the brass pill (I figure that, because so much area is in contact, it should be fine heat sinking), I got some weird results.

First and foremost - it wouldn't even come close to hitting 1.5A of output current. The highest it would go is about 1.36A, and then it would drop to about 500mA (the bottom of that setting) with any more turns of the pot.

Secondly, it dropped in out current as I tested it almost instantly - that is, it started at about 1.36A, then after about 2 seconds, it started to drop about 5mA per second. Is this just a result of overheating? I would have thought that thermally bonding it to the pill would have pretty much solved that issue.

Seems I got a FlexDrive that wasn't top notch - oh well, not a huge deal. Should still give me easily 1W of output power with an Aixiz lens.

 

[DTR]

Does it act normal if you set it to 1A?

Have you tried a different power source to make sure that it is getting the proper amount of current?

 

[Wolfman29]

It should be getting a proper amount of current. I was feeding it through a variable PSU, with a max current of 3A (though I have seen it draw 12A for a split second in my high voltage experiments :evil: ). Anyway, I will try setting it at 1A, see what happens.

 

[DTR]

What do you have the voltage set to and what is the current draw reading when you turn it on?

 

[Wolfman29]

I think it was outputting 2.5A or so? Don't remember though - maybe it was quite a bit lower. I had it set at 4.2V. But I am gonna try it with an actual li-ion now, see what happens.

 

[DTR]

Well with a power supply that is the best source as you can have fixed voltage and observe current draw. 2.5A seems a bit high for 1.3A with the power supply set @ 4.2V.:thinking:

What is the voltage drop of your test load. I normally use a red setting for currents over 1A as test load diodes will drop more voltage at the higher currents giving lower than accurate readings with the blue settings.

 

[Wolfman29]

Like I said, I don't remember - I was paying attention to the output more than anything. It may have been quite a bit lower, maybe around 1.86A? Just throwin' that out there.

EDIT: Just did a test at what it was set at, it was outputting 1.3A with 4.2V, 2.06A in. Lemme check what the output voltage was (may have been outputting higher than 4.5V).

EDIT 2: Yes, it's outputting around 4.75V at 4.2V in, which is a low 71% efficiency. Aren't these supposed to be around 85-90% efficient?

EDIT 3: Tested it at 870mA. Results: 4.2Vin, 1.06Ain, 4.275Vout, 870mA out, 83% efficiency.

 

[Jufran88]

Are you getting the reading straight off the leads or are you using a test load?

If so, how many diodes are you using? You could be using an extra diode, since flex drives can only boost to a limited voltage range thus max current can't be achieved.

How are you calculating for driver efficiency?

 

[Wolfman29]

Of course I am using a test load! I am using 4 diodes and a 1 Ohm resistor. I am calculating driver efficiency by taking the ratio of power out and power in, which is how one calculates efficiency for all things electronic. And the FlexDrive should EASILY be able to boost about .5V.

 

[Jufran88]

Hmm, it seems your doing everything correctly. Maybe you do just have one of those Flexdrives? Try using just 3 diodes and see how that goes.

I did an efficiency calculation on my first build using a flex drive and got about 76% efficiency.

 

[Wolfman29]

That's unfortunate. But meh.