simingx
Dec 24, 2006, 01:57 AM
First of all, I would like to state that any project of this type is absolutely NOT recommended to be built by anyone who is not familiar with the extreme danger of direct mains-connected equipment!!
"Advanced" test equipment such as an oscilloscope and (preferably) a signal generator are of absolute necessity because of the need to verify that the proper waveforms are being generated in the circuit! Failure to test the circuit throughly before connection to 230V mains will most assuredly result in (at least) blown semiconductors and (at worst) serious injury.
Practical it is not (buying one off the shelf costs less and is way safer) but it's a very good learning experience.
So here goes...
Inspired by the project here (http://www.qrp4u.de/docs/en/smps_new/index.htm), I decided to get my hands dirty by building my very own 250W SMPS.
However, it is not so easy to find the required power supplies with the correct sized transformers (especially the main power transformer), so I decided to wind my own. It's not very difficult and saves you quite a bit of headaches.
First of all, the main power transformer is wound. I used an ETD39 core (pretty standard size and easily available). Primary is 2x20 turns, wound as 2 layers. Instructions follow:
Wind 20 turns of 0.8mm wire around the core. The ETD39 core is quite substantial, so you can probably use thicker wire too.
Bring out a centertap (it won't be used however). Wrap this first layer with some insulating tape (I used 3M Super 33+) and wind a further 20 turns [b]in the same direction[/i] on top of this.
This completes the primary winding. Wrap this with a further 2-3 layers of insulating tape.
For the secondary, it consists of a main winding of 14 turns centertapped and a further 4 turns at each end to drive the semi-synchronous rectifier. It's wound as follows:
First, use some thin wire (0.5mm is more than sufficient) and wind 4 turns around the (insulated) primary. Attach this to 2 pins on the bobbin. Wrap one layer of insulating or plumber's tape.
Next, take 2 strands of 1.0mm wire (in parallel) and wind 7 turns. Bring out a centertap in the form of a pigtail (make a large loop with the wire).
Wind a further 7 turns, all in the same direction. Anchor the 1.0mm winding. This is the main output winding.
Wrap this with some more plumber's tape.
Use the thin wire again and wind another 4 turns in the same direction.
Now, you should have (from inside out):
20 turns of 0.8mm wire
Insulation
20 turns of 0.8mm wire
Lots of insulation
4 turns of 0.5mm wire
Insulation
7 turns of 2x1.0mm wire
Insulation and a centertap
7 turns of 2x1.0mm wire
Insulation
4 turns of 0.5mm wire
The 2 primary windings are connected in series.
The 4 secondary windings are also connected in series (in this order: 1st aux->first main->centertap->second main->second aux).
Make sure ALL windings (at least for the primary and secondary) are in the same direction!!!
To test this transformer, connect a audio signal generator to the primary side.
Adjust for ~50kHz, sine wave.
The output on the secondary side should also be a sine wave, related by the turns ratio. In this case, ratio is 14/40=0.35 for the main secondary wind, and 22/40=0.55 for the main+auxiliary secondary wind.
Using an oscilloscope, measure from centertap to one end of the main secondary winding, make sure the Vpp is correct.
Measure also from centertap to the end of the auxiliary winding and make sure the voltage is HIGHER than what you measure from the main winding. If it is lower, the phasing of the aux. winding is wrong and you will have to swap connections.
That concludes the main transformer.
"Advanced" test equipment such as an oscilloscope and (preferably) a signal generator are of absolute necessity because of the need to verify that the proper waveforms are being generated in the circuit! Failure to test the circuit throughly before connection to 230V mains will most assuredly result in (at least) blown semiconductors and (at worst) serious injury.
Practical it is not (buying one off the shelf costs less and is way safer) but it's a very good learning experience.
So here goes...
Inspired by the project here (http://www.qrp4u.de/docs/en/smps_new/index.htm), I decided to get my hands dirty by building my very own 250W SMPS.
However, it is not so easy to find the required power supplies with the correct sized transformers (especially the main power transformer), so I decided to wind my own. It's not very difficult and saves you quite a bit of headaches.
First of all, the main power transformer is wound. I used an ETD39 core (pretty standard size and easily available). Primary is 2x20 turns, wound as 2 layers. Instructions follow:
Wind 20 turns of 0.8mm wire around the core. The ETD39 core is quite substantial, so you can probably use thicker wire too.
Bring out a centertap (it won't be used however). Wrap this first layer with some insulating tape (I used 3M Super 33+) and wind a further 20 turns [b]in the same direction[/i] on top of this.
This completes the primary winding. Wrap this with a further 2-3 layers of insulating tape.
For the secondary, it consists of a main winding of 14 turns centertapped and a further 4 turns at each end to drive the semi-synchronous rectifier. It's wound as follows:
First, use some thin wire (0.5mm is more than sufficient) and wind 4 turns around the (insulated) primary. Attach this to 2 pins on the bobbin. Wrap one layer of insulating or plumber's tape.
Next, take 2 strands of 1.0mm wire (in parallel) and wind 7 turns. Bring out a centertap in the form of a pigtail (make a large loop with the wire).
Wind a further 7 turns, all in the same direction. Anchor the 1.0mm winding. This is the main output winding.
Wrap this with some more plumber's tape.
Use the thin wire again and wind another 4 turns in the same direction.
Now, you should have (from inside out):
20 turns of 0.8mm wire
Insulation
20 turns of 0.8mm wire
Lots of insulation
4 turns of 0.5mm wire
Insulation
7 turns of 2x1.0mm wire
Insulation and a centertap
7 turns of 2x1.0mm wire
Insulation
4 turns of 0.5mm wire
The 2 primary windings are connected in series.
The 4 secondary windings are also connected in series (in this order: 1st aux->first main->centertap->second main->second aux).
Make sure ALL windings (at least for the primary and secondary) are in the same direction!!!
To test this transformer, connect a audio signal generator to the primary side.
Adjust for ~50kHz, sine wave.
The output on the secondary side should also be a sine wave, related by the turns ratio. In this case, ratio is 14/40=0.35 for the main secondary wind, and 22/40=0.55 for the main+auxiliary secondary wind.
Using an oscilloscope, measure from centertap to one end of the main secondary winding, make sure the Vpp is correct.
Measure also from centertap to the end of the auxiliary winding and make sure the voltage is HIGHER than what you measure from the main winding. If it is lower, the phasing of the aux. winding is wrong and you will have to swap connections.
That concludes the main transformer.