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Old 19th May 2015, 01:50 AM   #1
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Default Waveform deformation and voltage drop in transformer

Hi! I have a EE42 N87 core working at 200KHz, 200mT on a Half-Bridge power supply. Everything works great expect that I experience a voltage drop when applying load. The reason seems to be in the transformer, here are the waveforms:

With a 16 Ohm load:
Click the image to open in full size.

With a 4 Ohm load:
Click the image to open in full size.

The primary side has 9 turns with 16 wires of 0.3mm of diameter. The secondary is 4 turns with a 0.1mm copper foil, however, I couldn't find a way to easily connect the foil to the terminals so I used a high temperature cable I had soldered in the copper foil to the terminals. This is the cable used:

Click the image to open in full size.

Connected on this way (replace the copper wires with the cable):

Click the image to open in full size.

Is this termination on the secondary side responsible for this voltage drop?
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Old 19th May 2015, 06:38 AM   #2
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by dragondgold View Post
Hi! I have a EE42 N87 core working at 200KHz, 200mT on a Half-Bridge power supply. Everything works great expect that I experience a voltage drop when applying load. The reason seems to be in the transformer, here are the waveforms:

With a 16 Ohm load:
Click the image to open in full size.

With a 4 Ohm load:
Click the image to open in full size.

The primary side has 9 turns with 16 wires of 0.3mm of diameter. The secondary is 4 turns with a 0.1mm copper foil, however, I couldn't find a way to easily connect the foil to the terminals so I used a high temperature cable I had soldered in the copper foil to the terminals. This is the cable used:

Click the image to open in full size.

Connected on this way (replace the copper wires with the cable):

Click the image to open in full size.

Is this termination on the secondary side responsible for this voltage drop?
Click the image to open in full size.Click the image to open in full size.
What's your schematic exactly? This looks more like a too small series capacitor
Attached Images
File Type: png XbDBkbJ.png (4.5 KB, 16 views)
File Type: png bq7OWYZ.png (4.8 KB, 14 views)
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Old 19th May 2015, 09:28 AM   #3
AndrewT is offline AndrewT  Scotland
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Thanks for attaching the post 1 pics.

The foil is a cheap and very easy to assemble alternative to using 30 (or whatever) parallel enameled 0.3mm diam wires. Your connections should therefore be done using 0.3mm diameter enameled wires of sufficient number to carry the current efficiently.

stranded cable is not the same as insulated strands.
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Last edited by AndrewT; 19th May 2015 at 09:30 AM.
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Old 19th May 2015, 02:27 PM   #4
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@Elvee The DC Blocking capacitor is 100nF. It is too small right? What would be a better value?

@AndrewT yes, that's right, I will rewind the secondary side using enameled wires!
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Old 19th May 2015, 02:35 PM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by dragondgold View Post
@Elvee The DC Blocking capacitor is 100nF. It is too small right? What would be a better value?
That's probably too small at your power level: it is equivalent to ~8 ohm.

Ideally, its reactance should equal that of the leakage inductance of the transformer.

A foil winding is OK provided it is not too thick, and the connecting wires have a sufficient section
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Old 19th May 2015, 02:40 PM   #6
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Quote:
Originally Posted by Elvee View Post
That's probably too small at your power level: it is equivalent to ~8 ohm.

Ideally, its reactance should equal that of the leakage inductance of the transformer.

A foil winding is OK provided it is not too thick, and the connecting wires have a sufficient section
Ok thank you Elvee! I will change that capacitor with a 1uF one and replace the cable I used in the secondary. Thank you!
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Old 20th May 2015, 06:56 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
..................The foil is a cheap and very easy to assemble alternative to using 30 (or whatever) parallel enameled 0.3mm diam wires. .......................
yes, keep that.
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Old 20th May 2015, 08:11 PM   #8
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So, it worked! I changed the blocking cap with a 1uF capacitor and the voltage drop is almost nothing! However I still have a 50Hz ripple at the output of about 7-10V. The capacitors at the input that forms the divider are 680uF 200V caps. Is the capacity small or is there another problem going on?
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Old 20th May 2015, 08:38 PM   #9
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by dragondgold View Post
So, it worked! I changed the blocking cap with a 1uF capacitor and the voltage drop is almost nothing! However I still have a 50Hz ripple at the output of about 7-10V. The capacitors at the input that forms the divider are 680uF 200V caps. Is the capacity small or is there another problem going on?
Output voltage, power, schematic, etc...? Note that if your ripple is actually 50Hz, you have a problem with your rectifier (unless your local mains is 25Hz)
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Old 20th May 2015, 08:51 PM   #10
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Originally Posted by Elvee View Post
Output voltage, power, schematic, etc...? Note that if your ripple is actually 50Hz, you have a problem with your rectifier (unless your local mains is 25Hz)
Yeah sorry, it's 100Hz, my mistake there! Here is the output voltage with a 4Ohm load:

Click the image to open in full size.

The schematic is attached in pdf! C10 and C6 caps are 680uF instead of 470uF. I even tried adding 470uF in parallel to each existing capacitor but the resulta is almost the same.
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