HD PSU by JSaily
A repairable heavy duty power supply for
Designed by Jussi Saily
Don't start this project if you're not trained in electrical safety or are unsure how to handle the mains voltage! It can (and will) kill you if you are not careful, It's better to be safe than sorry.
Mains transformer specifications:
Two mains transformers are typicallyneeded to get the desired voltages, although it is possible to use a multiple secondary winding transformer. The isolated 9VAC is generated with a smaller transformer, and the 16-24VAC with the larger one. The 9VAC is fused on the switcher board and goes to the output connector as such. The 16-24VAC is fused, rectified with a power diode bridge, and filtered with large capacitors. The DC output voltage after rectification (in an unloaded case) is 1.41 times (sqr(2)) the AC voltage, i.e., 23.35 VDC so take this into account when selecting the capacitor voltage ratings! As the load current increases, the voltage can drop a few volts.
The schematic diagram is based on the output from National Semiconductors Simple Switcher program and uses the integrated step-down buck converter LM2679T-5.0. The design is simple and has few components. Estimated efficiency is 85% over the given input DC voltage range of 16.36 volts.
The most critical components are the power inductor and the Schottky diode. Also the input and output capacitors should be low-ESR types meaning they have low serial resistance and high current capability. Luckily, the design program suggests the proper components directly. Use the given components unless you can't obtain them easily. You can also substitute multiple capacitors of lower value and use them in parallel to reduce the total ESR (just like paralleling same-value resistors reduces the effective resistance). The available PCB for this project contains places for multiple parallel capacitors.
The buck converter operates by rapidly switching the voltage to the load between the input voltage (16-36V) and below ground through the Schottky diode. The voltage pulses are fed to a LC circuit consisting of a series inductor and a tank capacitor which maintain the output voltage at a DC level (+5V in our case). A feedback wire from the output capacitor goes to the buck converter to maintain the correct DC level. Check the datasheet of LM2679 for more information. The output of the switcher is not exactly pure DC but contains also AC components at the switching frequency 260 kHz and all its harmonics (see the provided FFT diagram, Fig. X). The switching remains are difficult to filter out, as they present themselves also as common mode currents (in both 5V and ground lines). Ferrite beads in the power lines are the most effective means for filtering these.
The voltage difference between the desired output voltage and the input voltage determines the duty cycle of the switcher. For example, a 20V input voltage to a desired 5V output should result in a duty cycle of 25%. This is in an ideal case, in real life the voltage drops e.g. in the switching transistor and the load inductor increase the duty cycle. The efficiency of the switcher is typically higher with higher input voltages because the duty cycle is reduced (the switching transistor does not conduct so often). The maximum permissible duty cycle (91% for LM2679) limits the lowest acceptable input voltage for a certain output voltage.
- Assemble the switcher board. Use large heat sinks for the Schottky and LM2679T-5.0. Put a bit of thermal grease to the component backside before attaching. If you have a suitable AC wall-wart in the range of 12VAC to 24VAC, you can test the switcher board with a light load attached (like 10 ohm / 10W).
- Put all the stuff inside your case, and plan in advance how to mount them. See my pictures for an example. Drill the transformer mounting, mains connector, fuse, and mains switch holes. Also mount the output connector (DIN-7 socket) if you plan to use one. Mark and drill the switcher board mounting post holes to the case. Make the connections between mains voltage parts using a thick wire. I used a 1.5mm^2 copper wire.
Apply heat-shrink tubing on all mains voltage connections, so you (kid/wife/girlfriend) won't accidentally touch them!Use the same thick wire also for the 9VAC and 16-24VAC lines going to the switcher board.
- Mount the mains fuse (400 mA for 220V and 800 mA for 110V). Turn mains on and measure the AC voltages from 9VAC and 16-24VAC lines before connecting to the switcher board! If the fuse breaks, you have a problem. Correct it, replace the fuse and try again.
- Connect 9VAC and 16-24VAC lines to switcher, and test with a light load on both (10 ohm / 10W for 5V output and 20 ohm / 10W for 9VAC). Measure the voltages. Use a ferrite ring on the 16-24VAC input lines to reduce the switching noise propagation (260 kHz and it's harmonics). Multiple cable windings on the ferrite increase the effective line impedance at the switching frequency, thus reducing the EMI emission from you PSU.
- If everything looks okay, turn PSU off and remove mains cable. Solder the 5V, ground and both 9VAC lines to the appropriate terminals of the DIN-7 socket
Check Out Lemon64 for more information Lemon64 Forum Link To Repairable Commodore Power Supply.