N87 core for a project - Page 1
Forum section: electronics / projects

author: Neukyhm
author_rank: Contributor
timestamp: July 21, 2017, 07:31:06 pm
content: Hi forum, I need advice with this. I have to choose a ferrite core for the following transformer:Air gap: 1.5mm (this means an Ae=381nH for ETD59 N87)Primary: 3+3 turnsSecondary: 250 turns (for ~4kv output)Desirable power: 150-200wFreq: 320kHz (I know, it's high, but it's the only way I know to reduce primary current below core saturation)Magnetic flux density caused by primary: ~220mTI think that I could use a ETD59 N87 core from Epcos, but I'm not sure how to read the part of the datasheet that talks about core losses.Here you have the core datasheet and N87 material datasheet:http://docs-europe.electrocomponents.com/webdocs/13c0/0900766b813c0f43.pdfhttps://en.tdk.eu/download/528882/325211356745925075927e79bdc2b163/pdf-n87.pdfThank you in advance.Edit: I forgot to say: this machine is intended to run for a few seconds every few minutes only.

author: orolo
author_rank: Frequent Contributor
timestamp: July 21, 2017, 08:39:57 pm
content: I'm not very experienced, so take this with a grain of salt until someone really knowledgeable comes around.First, a sanity check:Starting with the core datasheet. The datasheet claims less than 5.2 Watts of core loss at 100mT, 100kHz, 100ºC.Turning to the material datasheet, relative core losses vs frequency graph (pag. 5/17), it is claimed that for 100kHz, 100mT, 100ºC, the material has core loss equal to about 100kW/m^3. Since the core has a volume of 51200mm^3 = 5.12e-5 m^3, we would expect 100e3*5.12e-5 = 5.12 Watts core loss. It indeed falls within tolerance. Let's call this a pass in the sanity check.Now, for your operating conditions:Now, turning again to the relative core losses vs frequency graph. For about 300kHz, 200mT, 100ºC, it is near 2300kW/m3, and at the limit of the plotted data. So the losses expected are 2300e3 * 5.12e-5 = 117.76 Watts. At 25ºC the relative power is more like 3000kW/m3, so the power would be around 150W.It looks like a lot of power to me.

author: orolo
author_rank: Frequent Contributor
timestamp: July 22, 2017, 09:50:11 am
content: Inthis blog entrythere is a project similar to yours, but with less power. If you use air cores, the size of the inductors will increase, the coupling will be worse, and the magnetic field won't be as confined as with a core. On the other hand, it won't saturate, you can use whatever wire gauge, and you can compute the inductance and coupling from first principles.Hereyou have a very good calculator for air core coils. To get about 13uH for your primary, you would need a coil of 10cm diameter with 10 turns, and a length of 2.5cm; not that big, after all. The series resistance is 0.12 ohms. Then you'd need about 420 turns for your secondary.

author: orolo
author_rank: Frequent Contributor
timestamp: July 22, 2017, 12:18:49 pm
content: Quote from: Neukyhm on July 22, 2017, 11:59:14 amWhere do the datasheets disagree?

author: orolo
author_rank: Frequent Contributor
timestamp: July 22, 2017, 05:55:38 pm
content: The core losses agree for both datasheets: about 5.2W for 100mT, 100kHz. But the core datasheet says nothing about the material at 200mT, 300kHz, so there you need to use the material information. First check both datasheets agree for a known datapoint, then extrapolate to your working conditions.Inthis third datasheetyou have the formulae for inductance and magnetizing current. Let us work at 25ºC, and define k1= 508.0, k2=-0.708, k3=812.0, k4=-0.796.For a coil of N=100 turns:The \$A_L\$ value for a gap \$s\$ is: \$\displaystyle A_L\ = \ k_1\,s^{k2}\$And the DC current \$I_{DC}\$ at which \$A_{L}\$ drops by 10%, marking the onset of saturation, is given by: \$I_{DC} \ = \ \displaystyle\left(\frac{0.9\cdot A_{L}}{k_3}\right)^{1/k_4}\$So, let us assume you have a gap of s=1.5mm. Then, AL = (1.5**-0.708)*508.0 = 381, which exactly agrees with the datasheet.Then, the magnetizing current for saturation is: Idc = (0.9*381.0/812.0)**(1.0/-0.796) = 2.95 A, near 3A.Now, these formulas are for N=100 turns. If you want 3.45uH in each winding, since \$L = A_{L}\cdot N^2\$, then \$N =\displaystyle \sqrt{L/A_{L}}\$ (in nanohenries!), so N = sqrt(3450.0/381) = 3, ok.I think that in your circuit the 3+3=6 turns work as a single inductor, so the DC current for 10% saturation at 25ºC should be 2.95*100/6 = 49 A. I think you are safe enough at 5A max.Be warned, though, that I'm neither an EE, but a mathematician.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 22, 2017, 08:45:00 pm
content: I doubt you'll get that high frequency, at that number of turns. A resonant or quasi-resonant controller may be very helpful.Tim

author: orolo
author_rank: Frequent Contributor
timestamp: July 22, 2017, 09:25:15 pm
content: About your simulation: I think the choke L4 isnotmeant to be coupled. Also, shouldn't the inductors be 3.45uH each, instead of 6.858u? Also, the diodes going from the gates are ideal/generic? If that is the case, perhaps you should use an ultrafast model, like UF4001. I've simulated the circuit in LTSpice, and with 12 Ohm resistors it wouldn't start oscillating. Replacing them with 470 ohms ones did the trick.After replacing the resistors (not the inductances), the oscillation frequency was about 40kHz and the peak current 20Amp, as in your case.

author: orolo
author_rank: Frequent Contributor
timestamp: July 23, 2017, 04:02:07 am
content: Quote from: Neukyhm on July 22, 2017, 10:51:43 pmMy guess is that, if the resistance is too low, the low impedance to 12V will not allow the gate to discharge through the diode reliably: the diode will be instead discharging the 12V supply. Check in your simulation if the mosfets switch at the correct time and don't miss cycles. Also, the waveform for I1 you posted doesn't look right: with the correct resistance, it should look like a smooth sinusoid. Maybe a resistance of about 100 ohms would be a good compromise?Another thing: if you are going to wind the secondary in a the pot core, you might want to use isolating tape from primary to secondary, and over each layer of the secondary. I wouldn't trust magnet wire enamel to isolate 4kV.Good luck with the project!