98% efficient 4 watt 12V DC-DC converter EE13/7/3 N87 ferrite

P E Schoen · 2016-10-04T10:32:16+00:00

I just received some EPCOS EE13/7/3 ferrite cores in type N87 and a matching bobbin for dual isolated windings. I was able to fit 12 turns of #24AWG magnet wire for primary and secondary. I used some heat-shrink tape to hold the core halves together, and the open circuit inductance when hot measured 156 uH, but then settled to 118 uH as expected at room temperature. Leakage inductance is 4.7 uH. Here it is in my simple test jig: The output waveform with 13VDC input at about 10 mA: The asymmetry is probably because of the ugly wiring. The IRS2453D has 1 uSec dead time, which you can see in the 90 kHz waveform. I added a 15 ohm (actually 17 ohm) 2W resistor to see how much the output dropped under load, and I was surprised it didn't drop very much, and looks like about 10.5 VRMS, or 618 mA and 6.5 watts. Input was 13 VDC at 560 mA, or 7.28 watts, and 89% efficiency. Then I connected the output to a FWB of 1N5818 Schottkys, a 20 uF 25V CM capacitor, and a 33 ohm resistor. Here are the results at various input voltages: 13V 0.34A 4.42W 11.77V 0.357A 4.20W 98% 15V 0.40A 6.00W 13.56V 0.411A 5.57W 93% 17V 0.52A 8.84W 15.08V 0.457A 6.89W 78% 18V 0.65A 11.7W 15.90V 0.482A 7.66W 65% I am pleasantly surprised at the performance of this little transformer. The EPCOS catalog rates this size core at 5W for N27 at 25 kHz, but 28W for N87 at 100 kHz. I would have figured on 4x, but 5.6x is surprising. Probably because of greater surface area per watt for the smaller core. Paul

Oct 04, 2016 23 Replies

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Phil Hobbs 9 years ago

Hmm, perhaps. If it's starting to saturate at 15V, though, I'd expect it to pretty well quit entirely by 18V.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net

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P E Schoen 9 years ago

I found that my power supply's current meter is inaccurate, and reads about

0.03A low. So I have updated the figures above. Obviously, my other power readings were similarly skewed. Note to self: always check calibration of instruments!

Thanks,

Paul

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P E Schoen 9 years ago

I found my power supply meter reads about 0.03A low, so revised figure is

87%. Still not bad.

Thanks,

Paul

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Rob 9 years ago

Dave Jones showed a multimeter which indicates -5A when only a lead carrying a 1 MHz square wave is connected to the +A input or even when held near to the case... and it wasn't a cheap meter.

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