Still rather new to the world of SEPIC, I have a problem with a design.
I have made a circuit based on the datasheet for the LM3478. It is intended to run with Vin from 10V to 32V and deliver 24V out with a maximum of 4Amps.
My testsetup runs at 32V input. The circuit is as can be seen here:
formatting link
A colleague of mine recently built something quite similar but with the LTC1871 controller instead. But I'd rather use the national part, since we already has this part in stock.
When I run with no load, output is 24V and everything is nice. But when I load the darn thing, even only by one amp, everything gets so hot and eventually the sense resistors crack.
I plan to change R79 so that I run at 200kHz, because according to my calculations 100kHz is a little to slow for my 22uH/13A inductors.
This heat issue has me puzzled somewhat but before i dive into measuring, I wonder if anyone would have a comment on the schematic. Especially regarding the values for R103/C16 and R145 which I have been struggling to determine.But also if I need anything or need to change anything. Not much is to be found on google regarding SEPIC with this chip, unfortunately.
I am eager to learn, but still a beginner in this field, so be gentle ;-)
C73/C74 should not be electrolytics but ceramics instead. Also, you don't really need this much capacitance. There are no ceramic caps on input and output, there need to be some substantial ones. At least several uF.
Two other things to check:
a. Does L4 saturate?
b. What current max are the sense resistors rated for?
BTW, your FQP50 is rated rather marginally for your app, only 60V breakdown. Could it be avalanching?
Just as a hint, if the inductors in a SEPIC are not on a common core then a whole lot of ripple goes out into the input rail. It needs to stomach that and all I see there is one lone 1000uF electrolytic.
I am not too familiar with European schematics but it seems your electrolytics symbol is a bit, ahem, unorthodox. Shouldn't plus be the boxy electrode?
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
"Joerg" skrev i en meddelelse news:nZvyl.15321$ snipped-for-privacy@flpi146.ffdc.sbc.com...
Dear Joerg,
Thank you for your reply.
About C73/C74, I always thought that using electrolytics was more safe. I though I read that somewhere. But I could easily be wrong and cofusing two unrelated subjects. But the psysical size of ceramics should be quite large to handle the job, shouldn't they? Do they handle currents better than electrolytics?
I'll check L4, I use these
formatting link
the TDH2420 series, last columns in the table. Will my 1000uF suffice for this?
My sense resistors should be able to handle 12Amps. But something made them break with a one amp load tonight.
I'll check if my MOSFET does avalanche. I think I have a few 100V lying around if need be.
Time to pull out the oscilloscope and get this beast running ;-)
Hehehe... actually we have been discussing that electrolyte schematics symbol ourselves. There is indeed a rather large difference between european and american symbols, some use a solid box to designate negative and an unfilled box to designate positive. Some use a straight line for positive and a curve for negative and the some use the symbol we use. I admit it is rather unclever and I would like much if there was just one standard for this. No need to complicate matters with a "european" and an "american" standard. In this day and age, with everything bein so global and "one village"-like, we should just settle on one system that satisfies all and avoid the confusion.
I just use whatever the client wants or what's on their schematics.
4K7 does have an advantage when print quality is poor and you could mistake it for 47ohms. Heck, because of international clients I even changed my office clock from AM/PM to 24h display :-)
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
I have not had experience with a SEPIC, but I have worked with several single inductor boost converters. Looking at a similar circuit in the LTSpice library, I see that the capacitors are 20 times smaller for a 1 amp output, so I think you might be able to use something like 100 uF for C73-74 and 200 uF for the output filters. Also, all the electrolytics must be lew ESR high frequency types, and should be bypassed with about 200 nF ceramics. If the capacitors get hot, they have too high ESR.
Using the scope you should be able to see if saturation occurs, which will cause very high power in the sense resistors. Use a 10 amp fuse or circuit breaker in the supply, and monitor the input current. Slowly increase the load, and watch for a sharp increase in current. Check the duty cycle at this point and see if it makes sense.
I also recommend using LTSpice to simulate your circuit. They have something similar to the LM3478. Try LT1241, LT1247, or LT1317 as a start.
Keep in mind that both SEPIC and Cuk converters have their switches seeing the sum of the input and output currents and voltages. Thus, at 24V-to-24V and 4A loading, the stresses on the switches are easily above 8A and 48V.
At 32V input, MBR2545 would see at least 56V. It will certainly avalanche this Schottky diode which is only rated at 45V.
At least, look at the voltages at the gate and at the source of the power MOSFET.
On Wed, 25 Mar 2009 22:02:30 +0100, Henrik [7182] wrote:
Ceramics have lower ESR than electrolytics. And they aren't polarised.
About the only factor that electrolytics have in their favour is that large values are available and affordable. Looking at Farnell's site, a
100uF@16V ceramic costs £0.70, versus £0.02p for an electrolytic. And that's the largest ceramic they list. So if you actually wanted 1000uF of ceramics, that's £7.
But as Joerg says, 1000uF seems excessive, although admittedly I only have the vaguest clue about power conversion circuits.
4A through 22uH = (1/2)*L*I^2 = 0.5*22e-6*4^2 = 176uJ. 24V on 1000uF = (1/2)*C*V^2 = 0.5*1e-3*24^2 = 288mJ.
That's a >1600:1 energy storage ratio.
Doing a back-of-the-envelope simulation in LTSpice (with a fixed duty cycle and parasitic values pulled out of ... thin air) doesn't show a great deal of difference between 1uF and 1000uF for the pass caps.
LTSpice model appended in case anyone wants to point out flaws in my reasoning.
Version 4 SHEET 1 1192 680 WIRE 224 16 64 16 WIRE 320 16 224 16 WIRE 464 16 400 16 WIRE 528 16 464 16 WIRE 656 16 592 16 WIRE 768 16 656 16 WIRE 928 16 832 16 WIRE 1072 16 928 16 WIRE 1136 16 1072 16 WIRE 64 80 64 16 WIRE 224 80 224 16 WIRE 464 80 464 16 WIRE 656 80 656 16 WIRE 928 80 928 16 WIRE 1136 80 1136 16 WIRE 416 96 384 96 WIRE 384 112 384 96 WIRE 336 144 320 144 WIRE 416 144 336 144 WIRE 320 176 320 144 WIRE 64 288 64 160 WIRE 224 288 224 144 WIRE 224 288 64 288 WIRE 320 288 320 256 WIRE 320 288 224 288 WIRE 464 288 464 160 WIRE 464 288 320 288 WIRE 656 288 656 160 WIRE 656 288 464 288 WIRE 928 288 928 144 WIRE 928 288 656 288 WIRE 1136 288 1136 160 WIRE 1136 288 928 288 WIRE 64 320 64 288 FLAG 64 320 0 FLAG 384 112 0 FLAG 1072 16 out FLAG 464 16 P1 FLAG 656 16 P2 FLAG 336 144 clk SYMBOL voltage 64 64 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 132 Left 0 SYMATTR SpiceLine Rser=50m SYMATTR InstName V1 SYMATTR Value 24V SYMBOL cap 208 80 R0 SYMATTR InstName Cin SYMATTR Value 1000µF SYMATTR SpiceLine Rser=50m SYMBOL ind 304 32 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L1 SYMATTR Value 22µH SYMATTR SpiceLine Rser=50m SYMBOL cap 528 32 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C1 SYMATTR Value {C} SYMATTR SpiceLine Rser=50m SYMBOL ind 640 64 R0 SYMATTR InstName L2 SYMATTR Value 22µH SYMATTR SpiceLine Rser=50m SYMBOL cap 912 80 R0 SYMATTR InstName C3 SYMATTR Value 3000µF SYMATTR SpiceLine Rser=50m SYMBOL res 1120 64 R0 SYMATTR InstName R1 SYMATTR Value 6R SYMBOL sw 464 64 R0 SYMATTR InstName S1 SYMBOL voltage 320 160 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(-1 1 5us 50ns 50ns 2.6us 5us) SYMATTR InstName V2 SYMBOL schottky 768 32 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D1 SYMATTR Value 1N5817 TEXT 320 328 Left 0 !.model SW SW(Ron=50m,Roff=1Meg) TEXT 32 352 Left 0 !.tran 0 10ms 9.95ms TEXT 784 328 Left 0 !.ic V(out)=24V TEXT 704 384 Left 0 !.step dec param C 1uF 1000uF 2
What is the point of R97? It does not do anything significant , that I can see anyway. Maybe I need to get new glasses :0) Unfortunately this does not point to your problem though :0(
I personally grew up with zig-zag lines, but I can cope with either.
When you're reading a 2nd generation photocopy of a schematic (or a crappy, scanned PDF), you're likely to prefer 4K7 to 4.7K.
As for polarised capacitors, yes, it's pretty weird to see the solid bar being the positive terminal.
--
W
. | ,. w , "Some people are alive only because
\\|/ \\|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
Looks like it's "burping", start-stop-start-stop and so on. But the average current looks like around 10 amps. Something must be seriously shorted or an inductor is instantly saturating.
Looks like 2MHz, the LM3478 can't go that high. Is something oscillating out of control? If the wave form is really correct (the downslope cannot be explained and this node can't really go much negative ...) then there is no ramp but the current flies up to the top right away. Like when the inductor core instantly saturates. Have a datasheet and P/N for the inductors?
Here you can see why the 60V rating of the FET is not sufficient and it might already be damaged: Goes well past 60V. That can cause anything between avalanching and a loud bang with stuff flying around. Even if it only avalanches then the switcher will never properly function. You need to switch to a part with 100V or higher rating.
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
Yes, entirely possible. But the layout also matters a lot. The LM3478 is a bit picky about this. Can you post that, too?
That would be under 200kHz, then your circuit is choking.
13.5A Imax is cutting it close. The LM3478 has a max Vsense trip point of 190mV which would equate to 19A with you resistor values. IIRC mine was switching around 160mV but that would still be 16A in your case. I'd back that off a bit.
Those open core inductors spew EMI around, a lot. Besides EMC issues this can cause spikes to be induced in longer traces and get into the chip. Shielded inductors would be a lot better here. They are never 100% shielded but easily an order of magnitude better than drum cores.
Hard to say without seeing the layout.
Well, we've all started at some point. Switchers take a lot of learning but once you get the hang of it you'll be a sought expert in the industry ;-)
--
Regards, Joerg
http://www.analogconsultants.com/
"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.