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Randy Knutson 18th November 2004 02:28 PM

Any Switching Power Supply Experts???
I'm building a SMPS based off the SG3525 [with no feedback]. Without a load it outputs +/- 40 volts without any problems. I did some load testing and even while outputting 250 Watts the MOSFETS remain cool. As soon as I break the 300 Watt barrier the Mosfets start heating up very rapidly. I'm using a 5+5 turns 14ga "6"-FILAR primary in push-pull configuration. [3 mosfets in parallel per side--6 mosfets total]

I was finally able to get an output of 450 watts RMS for about 15 seconds before the mosfets blew. Right before the mosfets blew I heard this strange high pitch sound coming from the torroid.... Could that mean core saturation and if so how can I avoid it?

Here are the critical stats:

Controller Configuration:
Push Pull [3 mosfets per side]
No feedback
80KHZ [40Khz to each mosfet]
40% Max Duty Cycle on each mosfet pair [it has plenty of dead-time]

Mosfet Parameters:
Rds(on) = 0.009OHM
Id = 100A
Pd = 170W
Qgs = 29nC
tr = 114nS
tf = 19nS

Torroid Core:
I obtained the torroid from as a sample a few years back. It is a ferrite type with these physical dimensions:
~2" OD
~1.5" ID
~.5" Thick

I'm assuming the problem is with the torroid when in fact it might not be...

Any suggestions?


Ouroboros 18th November 2004 03:09 PM

Have you any means to view the current waveform in the primary winding (a low-value sense resistor perhaps?) If the core is going into saturation you'll see a distinct increase in di/dt as rthe incremental permeability crashes.

I'm more used to using ETD series ferrite cores rather than toroids, but the size does seem to be a bit on the small side.

Are you aware of the extremely useful site:

which has on-line simulations and transformer/choke winding help for various SMPSU configurations.

Randy Knutson 18th November 2004 03:45 PM

Hey "Ouroboros",
Thanks for the reply! I went to that web site you posted and chose: "half-bridge push-pull converter". It's not exactly the same thing I'm doing but it should give me a good idea as to how big my xformer should be...

Vin(min) = 10V
Vin(max) = 16V
Vout = 100
Iout = 10
Frequency = 40 Khz

It outputs this:
L/H = 128E-6
DeltaIL/A = 4
N1/N2 46.84E-3
Vin/V = 14

It than gives me a suitable core type and here are the "Very good" ones.
UI 93/104/16

Where can I buy these cores?


gmarsh 19th November 2004 02:34 AM

Sounds like saturation... A saturated transformer is effectively a short - which means you're effectively shorting the input voltage supply with your MOSFETs.

Do you have another identical toroid core? stack 2 and wind the same transformer on those, and see if you get more power. You'll know immediately if it's saturation you're up against...

As for where to get magnetics, I've ordered small quantites of parts from Magnetics Inc ( directly. Get in touch with their applications folks and they'll help you out.

Eva 19th November 2004 03:35 AM

Your problem may be caused by the EMI radiation of the own circuit, whose magnitude depends on output current.

At some power levels, EMI may be strong enough to interfere with the control circuit and cause asymetric drive to the transformer [clock or duty cycle interferences]

Improper ground PCB layout of the control IC may also cause ground loops and asymetric duty cycle [I've experienced this sometimes in prototypes]

Any transformer that works with no load should never saturate under load. Core size is not directly related to output power. Output power is only directly related to the windings [resistance and leakage inductance], and the core just limits the maximum amount of copper and the minimum amount of turns before saturation

gmarsh 19th November 2004 01:51 PM


Originally posted by Eva
Any transformer that works with no load should never saturate under load. Core size is not directly related to output power. Output power is only directly related to the windings [resistance and leakage inductance], and the core just limits the maximum amount of copper and the minimum amount of turns before saturation
hang on now... (amps * turns) / (core cross-section area) has to be kept under a certain value. Amps increase with power, so yes, core size (or cross-sectional area, at least) is related to power. :D

But you're sort of right - if a transformer doesn't have enough magnetizing inductance for the circuit it's in (not enough windings) then you'll have trouble regardless of loading.

Another question for the thread starter - are you using output inductors on this supply? If you've got lots of output capacitance and a low-leakage transformer, and your output inductors begin to saturate, the peak current in your primary side MOSFETs will start increasing exponentially. I'd expect this to blow a rectifier first, but it's still a possiblity...

Randy Knutson 19th November 2004 05:55 PM

2 Attachment(s)
Thanks for the replies!

Here is the Schematic:

Randy Knutson 19th November 2004 06:14 PM

2 Attachment(s)
Here is the board layout for the schematic.... there are a few discrepencies between the schematic and the board.... Input capacitors and Feedback network for one... Actually dont pay any attention to the feedback network in the schematic... it is totally wrong... Because of the file size limit I had to crop part of the image down on all 4 corners.... On the top and botton the groundplane continues for about another 3/4 inch... the part of the circuit that got cut off is the rest of the bridge rectifier and output capacitors

Randy Knutson 19th November 2004 06:27 PM

Eva-- you bring up an interesting point... If you check my board layout the output of the torroid is in close proximity to the SG3525 controller IC.... Albeit when I start drawing 300+ watts the field gets pretty intense.... do you think i should lay out another circuit board or try some sort of a metal doghouse? I know in another post of yours you mentioned metal shielding is good for electrostatic fields but not nearly as good for magnetic fields...

Eva and Gmarsh -- Do you think this torroid core would be suitable ?

Al = 2725
OD=2.40 inch
ID=1.400 inch
Length = 0.500 inch
Material Type = 77
U= 2000

What I really want to do is just buy something that I KNOW will work....


gmarsh 19th November 2004 07:02 PM

In the multi-kilowatt designs I've built using ETD cores, i've never run into any big magnetic field problems. As long as your control IC and any other sensitive analog stuff is grounded well and isn't adjacent to high power magnetics and high power traces, you're fine.

You're running the 3525 open loop anyway, so I doubt that's a problem.

That's a big core - I doubt you'll have any trouble winding it :D I'm personally fond of F material (i've yet to use anything else in a toroid supply, and I've never had problems) but 77 material should work just fine.

One comment though - i'd go with a heavier transistor for your gate drive. IIRC, the 2n390x parts can only do 250mA collector current - moving up to 4401's or 5551's will give you a bit more gate drive current. Personally, I tend to go overkill and put UC3710's everywhere...

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