DC/DC Converter Help

LukeKnepp · Jul 8, 2019

To anyone who can help me, PLEASE!!!!!

I have a circuit board I am working on that is to have 3 outputs...

12V at 1-2A, 6.8V at 1-2A, and 33V at a higher amp rating, about 3-4A...

I have the designs attached below, and I have the board here. I have assembled 2 boards, and neither of them is working...

Can anyone help me or see what is wrong with my design, or is it all in my assembly that it is going wrong???

Any help is greatly appreciated, and the sooner the better, as my customer is selling these as I speak!!!!

Here are the datasheet links - https://www.monolithicpower.com/pub/media/document/MPQ3426_r1.01.pdf

and - https://www.ti.com/lit/ds/symlink/tps54383.pdf

ronsimpson · Jul 8, 2019

LukeKnepp said:
neither of them is working.

Not much information there.
What voltage is Vin? What does 33V measure? Probably Vin-1V

MPQ3426, pin 2 must be more than 1V.
Remove the 33V load and see if the supply can make 33V at no current.
What is happening on pins 4,5,6? Do you have a scope?
---edited---
SSR12260-150 = 15uH, the core saturates at 4.6A which is too little. Probably the inductance has dropped to 10uH at 4.6A. With out knowing the input voltage I can say what the inductor current is but I think you need a 2x increase in current caring.

eTech · Jul 8, 2019

What is 2LGND? Is it connected to ground?

LukeKnepp · Jul 9, 2019

Thank you so much for the quick responses!!!

eTech said:
What is 2LGND? Is it connected to ground?

LGND and 2LGND are low power grounds connected at a single point to the power GND. I had to connect those in the package of the ICs.

ronsimpson said:
What voltage is Vin?

VIN is 20-16V...

ronsimpson said:
What is happening on pins 4,5,6? Do you have a scope?

I do not have an oscilloscope, but I did connect it with no load attached, and the 33V outputs about 19V with 20V IN, and the dual converter output was 12.6V for a while, but then it dropped to 0V and the other output never did anything, meaning stayed at 0V...

ronsimpson said:
MPQ3426, pin 2 must be more than 1V.

What is my pin 2 voltage, or how do I calculate it, or do I just have to test it with a multimeter?

ronsimpson said:
What does 33V measure? Probably Vin-1V

Right On!!!
How did you figure that out?

ronsimpson said:
SSR12260-150 = 15uH, the core saturates at 4.6A which is too little. Probably the inductance has dropped to 10uH at 4.6A. With out knowing the input voltage I can say what the inductor current is but I think you need a 2x increase in current caring.

So...I just need a larger current rated inductor, or do I need to increase to 22 or 33uH inductance?

Thank you again so much for the quick responses!!!

ronsimpson · Jul 9, 2019

LukeKnepp said:
How did you figure that out?

Look at the schematic. Think about if most of the parts are removed. Only "VIN"---L3---D3---"V33" Power will flow from VIN to V33.

LukeKnepp said:
I just have to test it with a multimeter?

Yes. I find that often on the forum the schematics are not how the board is made. Pin-2 is the shut off pin. People often have a computer turn on/off the PWM. The voltage bust be above 1V for the IC to work. Measure.

LukeKnepp said:
I just need a larger current rated inductor

For now just keep the load small and we will fix the inductor later.

LukeKnepp said:
VIN is 20-16V

Vout=33@3-4A, Vin=16@6-8A, boost=2x,
The RMS current at Vin is 8A, I do not know the peak current. (must think) But your inductor current is much higher than you think. The IC current limits at 8.5A so the inductor needs to not saturate at 8.5A. Your inductor saturates at 4.6A.

I am concerned that pin-11, 2Lgnd is not connected to ground. Please test with a ohm meter.
What is the voltage on pin-14? It should be 0.5 volts.

Please send pictures, and PCB layout if you can. I am thinking you might have solder shorts or bad solder joints.

LukeKnepp · Jul 9, 2019

Here is my PCB Layout top and bottom layers in GerbV...

Here are the pictures of the actual bare board...

Here is the Assembled BRD...

Thank you!!!!
I will check the other things you asked for as soon as I can!!!!

LukeKnepp · Jul 9, 2019

ronsimpson said:
I am concerned that pin-11, 2Lgnd is not connected to ground. Please test with a ohm meter.

Pin 11 is connected...

ronsimpson said:
The voltage must be above 1V for the IC to work. Measure.

I tested it, it reads about 10V, and also increses the longer it is powered up... Do I have to connect a small load???

ronsimpson said:
What is the voltage on pin-14? It should be 0.5 volts

My Voltmeter reads -608mV, but the longer it is powered up, the voltage creeps higher...
Is it supposed to be negative?

kubeek · Jul 9, 2019

LukeKnepp said:
I am not sure... My voltmeter lead is too big to get to that pin...

Than measure at the resistor, not the IC. Why do you use such tiny parts, such as R10 which looks to be 0402 or smaller, if you are not used to working with them and doing measurements in tight spaces? You can also solder a thin wire and connect your probe to that.
Next time you spin a board, do yourself a favor an use component sizes that you can cope with, and if possible give yourself adequate testpoints ( a spot of bare copper 1mm in diameter for the probe to touch to is enough), and a 0 ohm reistor in series with input and output, so that you can measure input and output current. One in series with the inductor is also not a bad idea if you have not quite enough experience withe the topology and the whole design.

LukeKnepp · Jul 9, 2019

I figured that out!!!!!!!

This is my first board that I ordered with SMD parts, and I did not realize how small the parts were!!!

Thank you for the advice!!!!

LukeKnepp · Jul 9, 2019

Is it just bypassing the IC, and how do I fix it, or is it somehow only set 20V???

The Feedback pin voltage is about 0.666V...

ronsimpson · Jul 9, 2019

LukeKnepp said:
The Feedback pin voltage is about 0.666V...

The FB pin want 1.25V so the 0.666V said to run at max duty cycle. (but it is not running)

LukeKnepp said:
My Voltmeter reads -608mV, but the longer it is powered up, the voltage creeps higher...
Is it supposed to be negative?

No not negative. Maybe your leads are backwards. Maybe 2Lgnd is not connected. Last time I used that IC package it was very hard to hand solder. The solder looks good but is not. The pins are under the part.
Are you certain the part is not in backwards? (I have done that) Check pin-1.
----edited----
I would go back and re-solder every pin and see if that helps.
I have good luck with solder paste on that type of package.

LukeKnepp · Jul 10, 2019

ronsimpson said:
The FB pin want 1.25V

So is my resistor divider wrong?? According to the datasheet Vout=Vfb x (1+R11/R10)
Vout=1.225 x (1+500,000/17,400)
Vout=1.225 x (1+28.7356)
Vout=1.225 x (29.7356)
Vout=36.4V

This is just fine for my application, but am I calculating this wrong???
If I use the actual Feedback Voltage...

Vout=0.666 x (1+500,000/17,400)
Vout=0.666 x (1+28.7356)
Vout=0.666 x (29.7356)
Vout=19.8V
Which is exactly what it reads...

Am I doing something wrong here???

ronsimpson said:
Yes. I find that often on the forum the schematics are not how the board is made. Pin-2 is the shut off pin. People often have a computer turn on/off the PWM. The voltage bust be above 1V for the IC to work. Measure.
For now just keep the load small and we will fix the inductor later.

Do I have to apply a small load??? Should the 100k resistor be a voltage divider instead???

ronsimpson said:
No not negative. Maybe your leads are backwards. Maybe 2Lgnd is not connected. Last time I used that IC package it was very hard to hand solder. The solder looks good but is not. The pins are under the part.
Are you certain the part is not in backwards? (I have done that) Check pin-1.
----edited----
I would go back and re-solder every pin and see if that helps.
I have good luck with solder paste on that type of package.

I did use solder paste, and under the microscope every pin looks connected except maybe pin 4...
2LGND tests closed circuit to input GND...
Leads are backwards??? You mean like the IC is flipped 180 degrees???
I will try re-soldering everything...

Thanks again for all the help & advice!!!!!!

kubeek · Jul 10, 2019

LukeKnepp said:
2LGND tests open circuit to input GND...

These need to be connected together.

ronsimpson · Jul 10, 2019

LukeKnepp said:
Am I doing something wrong here???

Your "FB" voltage is low because your output is low. We need to get the IC working. When you get 33V out then the FB voltage will be right.

LukeKnepp said:
Do I have to apply a small load?

No

LukeKnepp said:
I will try re-soldering everything...

Because the lead is under the IC it is hard to see. My first try with this IC package I used a solder iron and had too much trouble. I had to move to soldering with a oven. Also I had to increase the size of the pads.

LukeKnepp said:
2LGND tests open circuit to input GND

2Lgnd is the ground for the "smart" part of the IC, not the power parts. (oscillator and error amplifier)

LukeKnepp · Jul 10, 2019

Sorry, LGND tests closed circuit is what I meant, see edit above...

Does it dislodge the other components if I put it into a soldering oven?

Thank You!!!

ronsimpson · Jul 10, 2019

LukeKnepp said:
Does it dislodge the other components if I put it into a soldering oven?

No. The solder will milt and the component might move the smallest amount.
When I hand solder it looks like this: (see black resistor)

When I use the oven the parts will turn and align better.

LukeKnepp · Jul 10, 2019

Okay, so is there a better way to take OFF a QFN???
Hand de-soldering does not work for me, is there a way to do it with the soldering oven below??

It is the T962C Puhui Oven...

I am just building an entire other board for now, but I would love to be able to fix this one...

Thanks to ronsimpson for all the help...
Is there any way I can boost your "score" on this forum, other than follow??

ronsimpson · Jul 10, 2019

LukeKnepp said:
Is there any way I can boost your "score" on this forum, other than follow??

"Like" in the right bottom of each comment. Thanks
My oven: looks like this.

LukeKnepp said:
so is there a better way to take OFF a QFN?

I put the board in the oven and heat until I can see the solder is molten. (silver shinny, not gray) Then reach in and grab the IC, or push it to one side or if I want all the parts off the board, I turn over the board and bang it on the desk.

Do you have the "boost" PWM working? 33 volts out?
----edited----
Before you make more boards, we should talk about your layout.

LukeKnepp · Jul 10, 2019

ronsimpson said:
Do you have the "boost" PWM working? 33 volts out?

No, I do not, and I do not know why...

I put one together very carefully, and all of the pieces and pins are soldered on solidly as far as I can tell, except for R12, on pin 1, did not solder...
I have to get something figured out to solder that better... Do I have to put more paste on to put it in the oven again???

And it still only puts out same voltage as I put in...

On pin 2, the voltage is about 10V...

On pin 14, the voltage is about 66mV, but this may just be voltmeter fault...
Is there a such thing as the pull up 68k resistor not "pulling up?"

ronsimpson said:
Before you make more boards, we should talk about your layout.

Tell on, I need some advice, as I am only getting started, but am expected to have a working board within a week if possible...

Everything else on this board is working flawlessly, it is only this boost part that does not...

LukeKnepp · Jul 10, 2019

Should my design work, if everything is correctly connected???

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