1N5406 Diode suitable?

[MisterD]

Hi,

I'm building a portable bluetooth speaker with some 18650 batteries I had laying around.
My circuit/components so far:

Old 19v/4.7A laptop charger connected to a buck converter. I set the output voltage of the buck converter to 12.6V.
The output of the buck converter is then connected to a "3S" BMS for my three 18650 cells in series.
I connected everything and then limited the current on the buck converter to a max of 2A so the laptop charger could provide the current.

Now when I disconnect the charger I think there is reverse current flowing/leaking to the buck converter: during charging I see a charging current of 1.xAmps, the charger is removed and I see a negative current of 0.2A.
For that I salvaged a 1N5406 from an old powersupply and connected it between the buck converter (+) output and the BMS.
According to the datasheet the Maximum repetitive peak reverse voltage is 600V at 3.0A.

As far as I can see everything is working as it should (0Amps measured) but I noticed that the diode gets quite hot. I measured about 80°C (was drawing about 1.9A during a charging cycle).
Is this normal behaviour? The datasheet lists an operating voltage - 50 to + 150°C. I'm just worried that once everything is in an enclosure the temp of the diode might get even higher?
Perhaps I'm not using the right diode?

Best regards,
Jan

[Manul]

This diode has quite high forward voltage drop, so it dissipates a lot of power in your application. If you want less heat, you should search for a diode with a lower forward voltage drop. Maybe a schottky diode (but some of them have noticable reverse leakage).

P = Vf * I. So if forward voltage is 1V and current is 1.9A, you have 1.9W of diode power dissipation (quite a lot).

[bob91343]

You can use a bigger diode or parallel a few smaller ones to reduce the temperature rise.  Keep the diode leads short to allow heat to flow in them.

[Manul]

Also notice, that a diode voltage drop lowers the voltage on BMS. So if voltage drop on a diode is 1 volt, then BMS gets 12.6 - 1 = 11.6V. And voltage drop depends on current. So you loose voltage precision. This is more like a workaround, better would be to solve the problem by changing buck converter or BMS.

[not1xor1]

Hi,

I'm building a portable bluetooth speaker with some 18650 batteries I had laying around.
My circuit/components so far:

Old 19v/4.7A laptop charger connected to a buck converter. I set the output voltage of the buck converter to 12.6V.
The output of the buck converter is then connected to a "3S" BMS for my three 18650 cells in series.
I connected everything and then limited the current on the buck converter to a max of 2A so the laptop charger could provide the current.

Now when I disconnect the charger I think there is reverse current flowing/leaking to the buck converter: during charging I see a charging current of 1.xAmps, the charger is removed and I see a negative current of 0.2A.
For that I salvaged a 1N5406 from an old powersupply and connected it between the buck converter (+) output and the BMS.
According to the datasheet the Maximum repetitive peak reverse voltage is 600V at 3.0A.

As far as I can see everything is working as it should (0Amps measured) but I noticed that the diode gets quite hot. I measured about 80°C (was drawing about 1.9A during a charging cycle).
Is this normal behaviour? The datasheet lists an operating voltage - 50 to + 150°C. I'm just worried that once everything is in an enclosure the temp of the diode might get even higher?
Perhaps I'm not using the right diode?

Best regards,
Jan

you could replace the diode with a PMOSFET. When the laptop charger is connected the TL431 cathode should be about 2V and bring the MOSFET in conduction.

[not1xor1]

you could replace the diode with a PMOSFET. When the laptop charger is connected the TL431 cathode should be about 2V and bring the MOSFET in conduction.

(Attachment Link)

R3 should be 10k (I've not simulated it - you can easily do that yourself)

[MisterD]

Thank you all for your replies so far.

Before I try to implement the solution provided by not1xor1: would the 1N5822 schottky diode be more suitable?
It has a Vf of 0.52V, compared to 1.2V of the 1N5406. But then I guess the leaking current is 2mA vs 5µA for the 5822 and 5406 respectively. So not sure yet.

What I noticed as well when I plug everything in:
https://youtu.be/ydiBPQf9D90

Any way to avoid the sparks when plugging the device in?
This is the buck converter I use by the way: https://www.aliexpress.com/item/32247251105.html?spm=a2g0s.9042311.0.0.27424c4d6LYWwm

Best regards,
Jan

[klausES]

After a buck converter, the diode 1N5406 could be a very slow diode when it heats up the diode.

[Kleinstein]

The schottkey diode should be better sited, because it is lower Vf. Even there 0.52 V is still relatively high - so better use a larger one or one with lower V_f, e.g. with lower voltage rating.


The MOSFET circuit could also be build with an NMOS FET if the switching is at the ground side.
The diode D1 in the circuit is not really needed as a separate part, there is already a diode insider the FET.

[Manul]

Vf of 0.52V is ok, but 2mA Ir is horrible (it depends a lot on temperature and reverse voltage, check datasheet for that, but still). If you want a diode solution, look for schottky with little reverse leakage.

[edavid]

You can use a bigger diode or parallel a few smaller ones to reduce the temperature rise.

No, you can't shouldn't put diodes in parallel.  They don't match well enough.

Before I try to implement the solution provided by not1xor1: would the 1N5822 schottky diode be more suitable?
It has a Vf of 0.52V, compared to 1.2V of the 1N5406. But then I guess the leaking current is 2mA vs 5µA for the 5822 and 5406 respectively.

The 1N5822 won't leak 2mA because you won't have 40V across it.  Look at the graph in the datasheet.

After a buck converter, the diode 1N5406 could be a very slow diode when it heats up the diode.

The slowness of the diode is not relevant since it only has DC applied to it.

If you want a diode solution, look for schottky with little reverse leakage.

There is no such thing, the reverse leakage is directly related to the forward voltage.

[bob91343]

Yes you can parallel diodes.  The currents won't share equally but they will share, accomplishing the goal.  A look at the forward voltage vs current graph will show the sharing isn't too bad.

[Manul]

There is no such thing, the reverse leakage is directly related to the forward voltage.

Come on. There are differencies, just do some parameter search.

[MisterD]

Thank you for all your replies (again!). Since fast charging is not that critical I might lower the max current to let's say 1A which would lower the temp of the diode as well.

Any ideas how I can avoid the sparks when plugging the powersupply in? See youtube link I provided.

best regards, Jan

[edavid]

There is no such thing, the reverse leakage is directly related to the forward voltage.

Come on. There are differencies, just do some parameter search.

There is no way to do a parameter search for leakage at a specified reverse voltage 

Usually it's only vaguely specified in a low resolution graph.

[Manul]

Well, but parameter search can give you the "feel" (don't know how to say, because of my lacking english) and then datasheet can give more specific info.

So here is 1N5822 (diode 1) and I chose a random diode to compare B340AE (diode 2):

Same max reverse voltage 40V

Diode 1 Vf = 0.525V @ 3A
Diode 1 max Ir = 2mA @ 40V @ 25C

Diode 2 Vf = 0.47V @ 3A
Diode 2 max Ir = 0.2mA @40V @ 25C

It is not easy to compare very precisely, but 10x less leakage must mean something. Also I'm saying this from years of experience, leakage is not simply proportional to Vf it is a complex result of die manufacturing process.

[edavid]

OK, I agree, the B340 looks great.

Of course it's possible that a modern day 1N5822 would be better than the datasheet spec.

[Manul]

OK, I agree, the B340 looks great.

And it can be even greater.

PMEG40T30EP
Vf=450mV @ 3A typical
Ir max 28uA @ 40V @ 25C

I remember I used diodes from this series from Nexperia, they really leak little at low temp. It is surprising, how semiconductor technology advances.

[not1xor1]

The schottkey diode should be better sited, because it is lower Vf. Even there 0.52 V is still relatively high - so better use a larger one or one with lower V_f, e.g. with lower voltage rating.


The MOSFET circuit could also be build with an NMOS FET if the switching is at the ground side.
The diode D1 in the circuit is not really needed as a separate part, there is already a diode insider the FET.

yes... thanks
I forgot to specify that the diode was drawn just to highlight the MOSFET body diode 
I didn't mean any external diode