Hello,
I use MP174 to make 12V / 300mA (as like eval board EV174A-J-00A) but I have problem saturation of the power inductor.
My currently inductor is : LPS6235-474 (Isat (10%)= 0.22A, 470uH, Irms = 0.43A)
This inductor don’t work, the supply 12V is not stable with a load.
But with the eval board’s inductor (DR9X12P2M1.2-00), it’s perfect.
Unfurtenely, I don’t find more informations about this inductor DR9X12P2M1.2-00.
Just that : Inductor;1.2mH;1.8;400mA
1.8 it’s the saturation current ? Or it’s 400mA ?
I would like to use this inductor : LPS8045B-474 (Isat (10%)= 0.43A, 470uH, Irms = 0.53A)
But I’m don’t sure if it’s good. Could you confirm that ?
Thanks in advance
Hello,
I would like to recommend switching to the MP174A regulator since the MP174 is no longer recommended for new designs.
Regarding your design with MP174:
The 400mA DR9X12P2M1.2-00 would be the saturation current or the rated current. Generally, you want to size your inductor so that saturation current is well above the inductor’s peak current.
The RMS and the saturation current on the LPS8045B-474 look good, but the inductance is too high for this design and may cause some stability problems. Please refer to the MP174(A) data sheet’s application section on how to size the inductor for your design.
I would also recommend looking through MPS’ selection of inductors to find the right size inductor.
I hope this was helpful!
Thank you,
Eliza
Hello Eliza,
thank you for your help.
Sorry for my fault, it’s not MP174 but it’s good MP174A that I use.
Ok for saturation current. I saw the calculation on datasheet and I found the good result.
Before, I didn’t undustand the calculation with “Ipeak”. But I think that “Ipeak” == “Isat” ?
What is the risk to put a big inductor ? On the eval board, there is a 1.2mH inductor and it’s seem good. It’s same on example schematic.
But in fact, in the datasheet, it’s also recommanded to use the minimum inductor depending on peak power. There’s a contradiction.
So for 12V / 300mA (=3.6W), it’s recommanded to use 360uH inductor.
I’m a bit lost.
What’s the good solution ? I think to use 470-680uH inductor with an Isat >400mA.
Thanks in advance
Hello,
“Ipeak” == “Isat” ?
Ipeak has to do with the current ripple, the amount by which the current changes during a switching cycle. It is the peak DC current through your inductor and is usually designed to be within 30% to 40% of the IRMS or rated current.
Iripple = 0.3A * 40% = 0.12 A
Ipk = 0.3A + 0.12A/2 = 0.36 A
Isat is an inductor characteristic, it is the amount of DC current the inductor can support before the nominal inductance drops by a certain percentage. For this reason, we would want the Isat to be well above the Ipk so that it operates with the inductance we expect.
This application note goes into more detail regarding these parameters.
What is the risk to put a big inductor ?
The risk of an inductor that is too large is that is can overload the load.
Sorry for the confusion, I made a mistake when I initially looked at your inductor values units.
The minimum inductance value is 360uH. During your testing, you were likely gettting saturation issues because you were running it at 300mA when the saturation was at 220mA. This caused the effective inductance to decrease below the 360uH that’s required for the system to be stable.
The 470-680uH inductor should work just fine since the new saturation is 430mA.
Just be mindful that inductance value can also change based on rated current and the temperature of operation.
I hope this helps.
Thanks,
Eliza
Hello Eliza,
-
Okay, thank you for this information.
So, the Isat must to be higher than Ipeak. With Isat >0.4A, I won’t have problem if my Ipeak is just 0.3A.
-
I undestand that the inductor’s value can decrease if the current rise up at Isat.
But I think it’s dangerous to play with that because the decrease is exponentiel.
If my Isat is higher than my Ipeak, we can considere the inductor’s value is stable ?
So, could you give more information about the risk of overload the load ?
During my tests with the inductor DR9X12P2M1.2-00 (1.2mH // 0.4A), it’s seemed good even a small load (40mA // 120mA // 240mA == 0.48W // 1.44W // 2.88W).
For my application, I need to have 0.48W in 95% part time but I need to have 3W during short moment.
Thanks in advance
Hello,
If my Isat is higher than my Ipeak, we can considere the inductor’s value is stable ?
Both the Isat and the rated current should be greater than the Ipk. Surpassing the rated current will cause the inductor to heat up which will also decrease the inductance value.
So, could you give more information about the risk of overload the load ?
Generally, a large inductance is good since it decreases the ripple of the system. However, a large inductance will also make the transients slower and the system less efficient. The reason it could overload the load has to do with how the OLP feature works.
Please refer to the operation sections “Start-Up and Under-Voltage Lockout” and “Over-Load Protection” in the MP174A data sheet.
Before the OLP fault is triggered, the feedback voltage needs to drop to 1.7V. A larger inductor would mean it would take longer for the feedback voltage to drop, causing more high current to go through the load and risk overloading.
Thanks,
Eliza
The other point to worry about is the voltage capability of the inductor. Can the insulation of the wire stand the voltage that you are applying to it? Many of these inductors are designed for 12 to low conversion so the insulation sees 12V during operation. This inductor will see hundreds of volts, can the insulation stand it? Most data sheets won’t say, so good luck everybody!
Coilcraft has a inductors rated for high voltage operation for instance the RFC0807BV series is rated for 800V.
Hello Eliza,
okay, thank you very much for this informations.
I don’t quite understand the risk of overloading the load in despite to read the datasheet, but I think that the good solution for me is to use 570-680uH Inductor (Minimal = 360uH) with Isat >0.4A (maybe I will use a inductor with Isat 0.5A to be confortable and limited the heat).
The ripple supply is also important for me to pass certification. But system stability is my priority.
jshannon : Thank you for your reminder and your inductor reference in 800V.
I already use a High Voltage inductor, the serie LPS8045B but it’s 400V max. It’s enought for me.
