Buck driver design advice needed

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stevetto
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Buck driver design advice needed

I have just finished the first version of my custom buck driver design.

Driver

I would like to drive a 6V XHP-70 led from 2s-3s lipo.
It works, but the inductor gets very very hot.
I use a coilcraft XAL 6060-333, it is 33uH and 5A rated.

Is it a normal behaviour that it gats really hot?

I was suggested to replace it. I also mage a simulation ( it might be wrong) which shows ~30W dissipated heat at 12.6V input and 5A led current.

Edited by: stevetto on 12/06/2018 - 13:11
Lexel
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a typical inductor size for 2-3S to 6V at 5A is 4.7uH

33uH is far too big, how did you calculated the needed parts?

with such high inductance the ripple is also very low which is not ghood for the regulation feedback, depending how the output current is sensed

an usual unsynchronized buck driver from 3S to 6V at 5A shoulc dissipate at most 3W heat

Lightbringer
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What’s the resistance of the coil? It might be “rated” 5A, but that might be saturation current, not design current. That’ll at least get you the raw wattage burning up heat in the coil (I²R losses).

One (design) failure mode of inductors is when the core goes into saturation, so then you’re burning up current that’s not adding to magnetic energy, just adding heat.

09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

Lexel
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here my LM3409 calculation based on the formulas in the buck chips datasheet

this is 13mm full sized inductor with 8.4mOhms, a XAL7070 should get about 16mOhms

Lexel
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also from what I see the input caps are not even close to be sufficient, usually at 500kHz you need about 20uF

so add more safety 3× 10uF 1206 25V capacitors, best is X7R

output cap should be 10uF 16V

22mm

23mm single sided Klarus

stevetto
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Lexel wrote:
a typical inductor size for 2-3S to 6V at 5A is 4.7uH

33uH is far too big, how did you calculated the needed parts?

with such high inductance the ripple is also very low which is not ghood for the regulation feedback, depending how the output current is sensed

an usual unsynchronized buck driver from 3S to 6V at 5A shoulc dissipate at most 3W heat

I use an MAX 16820 LED driver.
https://datasheets.maximintegrated.com/en/ds/MAX16819-MAX16820.pdf
Maximintegrated has a tool to calculate the inductor and the sensing resistor.
https://www.maximintegrated.com/en/design/tools/calculators/product-desi...

The result:
MAX 16820 calculation

It gives 4.62 uH, it is technically the same as that you suggest. But I was also suggested to use ~10 times higher inductance than the calculated one.
And if you see the graphs in the datasheet (page 4), 47 uH is used.
It was the way somehow 33uH was finally selected.

I had only a 22uH coilcraft XAL 6060-223 at home, so I used it instead of 33uH. I was also very hot.

I also made a simulation and it shows ~30W heat dissipated by the inductor. It does not matter what inductance I choose. I might be wrong, I do not know. bBut the led current is more or less OK. (pwm 500 Hz 100% duty)
Simulation

stevetto
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Lightbringer wrote:
What’s the resistance of the coil? It might be “rated” 5A, but that might be saturation current, not design current. That’ll at least get you the raw wattage burning up heat in the coil (I²R losses).

One (design) failure mode of inductors is when the core goes into saturation, so then you’re burning up current that’s not adding to magnetic energy, just adding heat.

Coilcraft XAL 6060-223 22uH was finally used
Maximum DC Current: 5 A
Maximum DC Resistance: 60.63 mOhms

stevetto
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This is the current design:
https://imgur.com/a/5Tkzp3E

Lexel
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I build over two hundred Buck drivers so trust me 4.7uH is right there is no take 10 times the inductance
I managed to get up to 96% efficiency on 4S to 12V builds, so about 72W to the LED and 3W on the driver, the parts stay nice an cool

for a good regulation you need about 0.5-1A ripple depending on the use buck chip, this is what 4.7uH gives you
also such an inductor has less DC and far less AC resistance so the heat will be significant lower

Lexel
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usually I use coils that I use are at least 3-4 times higher rated than the output current to keep losses low,
you cant use a 5A max rated coil for 5A output

also noticed on your design the input caps are missing, no wonder the inductor has a hard time

same with the capacitors there is a calculation that defines minimum which with like 10uF at 500kHz for 3S—>6V,
you can simply calculate it fromm the offtime C=I*toff(/1000000000 for ns)/1,44*(1000000 for uH)*(2 minimum safety value, better x4)
I doubled itat least to be on the safe side,
the more input caps the better for efficiency as the voltage stays higher so we get less current losses

you can trust the tool, the results are almost identical to my LM3409 calculation

0.8A ripple current is perfect

also at 200kHz you need input caps

Lexel
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the thing is its dynamic switching frequency like most high side sensing resistor buck controllers
no way to modify it besides Inductor selection, this leads you down more or less to measure switching frequancy and then if its not around 300-500kHz change inductor value

I took your values and calculated the fwitching frequency
I get about 800kHz with 4.7uH, which is a bit on the high side

the thing is that switching frequency should be calculated by the tool but its not, very odd

Lexel
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you could even drop the LDO as the chip has a 5V 10mA LDO integrated you can put the MCU on it

Lexel
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I calculated the switching frequency now and would even drop the inductor to 3.3uH

this gives me 550kHz switching frequency

with 33uH that thing would run on 5.5MHz no wonder it cookes like crazy

stevetto
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Thank you very much.

I will try it with a 3.3uH inductor and add input capacitor 3×10uF.
Shall I also add a 10uF capacitor accross the LED terminals? Or is it only optional? No calculation of it in the datasheet.

I also calculated the switching frequency according to the formula that can be found in the manual of the 16820 chip.
According to this, if you increase the inductivity the frequency gets lower.
And if you change the switching frequency in the calculator of maximintegrated the min. inductivity do not change.

Lexel
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stevetto wrote:
Thank you very much.

I will try it with a 3.3uH inductor and add input capacitor 3×10uF.
Shall I also add a 10uF capacitor accross the LED terminals? Or is it only optional? No calculation of it in the datasheet.

I also calculated the switching frequency according to the formula that can be found in the manual of the 16820 chip.
According to this, if you increase the inductivity the frequency gets lower.
And if you change the switching frequency in the calculator of maximintegrated the min. inductivity do not change.

I am not sure with low side switched buck, likely no caps across the LEds

stevetto
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Lexel wrote:
I build over two hundred Buck drivers so trust me 4.7uH is right there is no take 10 times the inductance
I managed to get up to 96% efficiency on 4S to 12V builds, so about 72W to the LED and 3W on the driver, the parts stay nice an cool

for a good regulation you need about 0.5-1A ripple depending on the use buck chip, this is what 4.7uH gives you
also such an inductor has less DC and far less AC resistance so the heat will be significant lower

Can you have a look at the modified design?
Would you suggest more modifications?

Design3

(R4 or R3 is optional, only for flexibility, different sizes and power)

stevetto
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I have just had time to build the modified driver.
With the modification you suggested the driver got cooler but it is still hot. Especially the FET.
driver

I connected it to an adjustable power supply and measured the followings:

Input 8.4V 4.16A >> 34.9W
LED 6V 5.04A >> 30.24W
So the efficiency is about 86%

I tried out what happens if I use it without cooling (the driver was not attached to any cooling plate, only the led).
The input current increased and when the efficiency decreased to about 83%, the fet got smooking hot and died.
In this case the driver should have dissipated ~6.5W

So I think that my driver less efficient and much hotter than it should be.

And one thing that is also strange, I ajusted the input voltage from 6 to 12 V. i thought that the led current should be constant. But is changed parallel to the input voltage.
I thought that a buck driver provide constant output if the input voltage is bigger than the led voltage.

Lexel
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Which FET do you use, sounds it has way too high switching losses
You need one with low gate charge, usual decent fast N-Mosfets have 4-7mOhms but only about 5-10nC total gate charge

Has this regulator a fixed switsching frequency or do you need to set it with a resistor?

It likely switches way too fast

The only way to see whats going on is using a decent digital memory oscilloscope

Lexel
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Also looked in the datasheet and according to it the buck has no low dropout, so its not allowed to drive 6V LED with 2S battery configuration

Increasing voltage should stay constant current so something is buggy

stevetto
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I use PSMN013-30MLC115 powerfet.
FET

13.6 mOhm RdsOn
8nC Qg

The XAL7070-332ME is only 3.3 uH, this is a mistake on my grawing.
XAL7070-332ME

Lexel
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The buck chip says regulating voltage is 200mV
So 21.5mOhm would get you 9A

stevetto
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How did you calculated this 21.5 mOhm?

stevetto
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I do not understand it, can you explain it?

Cereal_killer
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It’s a simple omhs law calculation

 RIP  SPC Joey Riley, KIA 11/24/14. Now I am become death, the destroyer of worlds.

stevetto
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Cereal_killer wrote:
It’s a simple omhs law calculation

I know that 200mV/21.5mOhm= 9.3A
How did we get the 21.5 mOhm?

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You do the opposite:

0,200V/9A = 22,2mOhms.

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stevetto
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BlueSwordM wrote:
You do the opposite:

0,200V/9A = 22,2mOhms.

OK. There is a misunderstanding.

There are 2 current sensing resistors on my drawing. Both of them are 43 mOhm. But I use only one of them. They have different sizes, I only have the option to use 1206 or 2512.Only for flexibility… And it is also written on the drawing, 1206 or 2512.
Lexel calculated the equivalant resistance of parallel resistors and he got 21.5 mOhm and from this 9A.

But the current should be only 200mV/43mOhm= 4.6A
But it could go higher and was not constant as I changed the input voltage.

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If your output gets higher with more input voltage there is definately something wrong with the switching frequency or current sensing
the buck has to stay stable in output

I also see no 1uF directly connected to the buck input Vcc that is described in the datasheet
with 3.3uH I calculated 200kHz
also the driver design tool points more to 4.7uH

also too low input caps can cause a drop, but not a increase with voltage over the current that the sense resistor should give

also an output cap is possible to reduce noise

stevetto
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Lexel wrote:
If your output gets higher with more input voltage there is definately something wrong with the switching frequency or current sensing
the buck has to stay stable in output

I also see no 1uF directly connected to the buck input Vcc that is described in the datasheet
with 3.3uH I calculated 200kHz
also the driver design tool points more to 4.7uH

also too low input caps can cause a drop, but not a increase with voltage over the current that the sense resistor should give

also an output cap is possible to reduce noise
!{width:40%}https://media.rs-online.com/t_large/R7542138-01.jpg!

I have read the datasheet of the evaluation kit you linked.
Evaluation kit 16820
I can not see any differences between this and my schematics.

My design
16820 EV Kit

According to the datasheet, C1 and C3 are optional.
_“Typically, capacitor C1 is not required if the power supply is relatively close to the EV kit. If long wires are used
to connect the power supply to the EV kit, install up to
10µF of bulk capacitance at the surface-mount 2220
pads provided for C1.”_

_“Typically, the LED ripple current equals the inductor ripple current. To reduce the LED ripple current, install
optional output capacitor C3. The EV kit provides surfacemount 0603 pads for a capacitor nominal value of 0.1µF.”_

The input capacitor you did not see, is there. This is C4 1uF capacitor on my schematics.

I recalculated the design with Maximintegrated’s design tool:
Design tool

Changing the switching frequeny in the excel sheet does not really influence the L value calculated by it .
And you can also change the input voltage in quite a big range without influencing the L value.

The only new info I got from the eval kit datesheet is:

_To enter hysteretic mode, the following input requirements must be met: set VIN above 5V or above VFLED +
4V (whichever is greater), and provide 1.1A of input current to the EV kit. If low-voltage or low-current input conditions fail to meet the input requirements for hysteretic
mode, the MAX16820 controller operates in linear
mode providing DC current to the LED load._

The excel sheet does not have any warnings about it. According to it the design is feasible.

So I think it means that for a 6V led I need at least 6+4=10V input voltage. Othervise the driver will operates in linear mode. Most probably that is why my output current changed as I changed the input voltage. But if I make a simple calculation: 6V*4,8A=28,8W this is the power consumption of my XHP70. If I increase the input voltage to 10V, the input current would be around 2,8A.
If I increased the output voltage of my power supply over 7,4V (simulating a 2 cell lipo) the current was also increasing until my power supply limited it at around 5A.

So I still do not undertand what the problem is.

Lexel
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stevetto wrote:

I recalculated the design with Maximintegrated’s design tool:
Design tool
Changing the switching frequeny in the excel sheet does not really influence the L value calculated by it .
And you can also change the input voltage in quite a big range without influencing the L value.


the tool say you only if the switching frequency is working or not

stevetto wrote:

The only new info I got from the eval kit datesheet is:

_To enter hysteretic mode, the following input requirements must be met: set VIN above 5V or above VFLED +
4V (whichever is greater), and provide 1.1A of input current to the EV kit. If low-voltage or low-current input conditions fail to meet the input requirements for hysteretic
mode, the MAX16820 controller operates in linear
mode providing DC current to the LED load._

The excel sheet does not have any warnings about it. According to it the design is feasible.

So I think it means that for a 6V led I need at least 6+4=10V input voltage. Othervise the driver will operates in linear mode. Most probably that is why my output current changed as I changed the input voltage.

So I still do not undertand what the problem is.

If you look on the switching frequency diagrams the linear mode is not included, this is why I guessed its not supportet, sure any buck will fall out of regulation at a point,
but as I pointed out its not a specified low dropout like the LM3409 which will fall out of regulation when the Input voltage is Output + losses, you said it right its likely 4V
I simply had not the time to read all datasheet but the diagrams tell me wehats going on on normal operation

At this point I would simply go with a lower shunt like 100mOhm to see if its working properly right there and which current you get with it

stevetto wrote:
But if I make a simple calculation: 6V*4,8A=28,8W this is the power consumption of my XHP70. If I increase the input voltage to 10V, the input current would be around 2,8A. If I increased the output voltage of my power supply over 7,4V (simulating a 2 cell lipo) the current was also increasing until my power supply limited it at around 5A.

The XHP70 should use at 4.8A 6.6V, but also shunt resistor, MOSFET, PCB, MCPCB, Digital Multimeter and wires ect. increase the real voltage so this will be more like 31W
Power measurements are only valid if you use a clamp meter and measure the voltage over the driver oputput terminals not the LED board

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Hi,

I still have problem with my design.
I also contacted maxim’s support and got the following answer:

_I tested on the EVKIT and changed 2 serial LEDs about 5V. When VIN is in the range 5V~20V, the LED current is basically stable and won’t go too high.
For the inductance, the formula V=L(di/dt). The inductance must be higher than the calculated value.
And also for the di, 1A LED current has about 0.3A di, 4.8A LED current has about 1.4A~1.5A di. But the EVKIT assembled 56uH inductor, so for sliding scale, 4.8A LED current also need a inductor bigger than 10uH._

I made a new driver with a 10uH inductor that was suggested by the support, but it did not work properly. The current is still not stable:
https://drive.google.com/open?id=1dAtkcULNviTW0ErYAIzLTISyOvWxBC6t

I also made some measuremets at the point indicated on the last picture of the link below:
https://imgur.com/a/hYBgiya

1. measurement at D2 at 25%pwm dimming signal
2. measurement at D2 at 50%pwm dimming signal
3. 4. and 5. measurement at D2 at 100%pwm dimming signal

My measurement (if it is correct) shows 3 MHz switching frequency?
Of course the driver is still to hot.

Shall I change to LM3409? Can it be better for my configuration (xhp 50 , xhp 70; 2 and 3 cell drivers)

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