|MODIFYING A SEVENTEAM 230 WATT SWITCH MODE POWER SUPPLY FOR 13.8 VOLT 20 AMP OPERATION|
WHY I CHOSE THE SEVENTEAM SMPS TO MODIFY.
Quite simple really - I opened up a whole bunch of SMPS (around 12) from different manufacturers and carefully examined them. The worst power supply was a Chinese made brand which advertised a 250 watt rating together with 5 volts at 25 amps on the cover plate but on examination used 1N5404s in the 5 volt supply line (3 amps max.) and completely deleted all input and output RF filtering components. As the quality moved slowly upward from the this extraordinary low, each of the supplies had various serious shortcomings except for the Seven Team units. Most common was the serious under rating of the 5 volt rectifier diodes and/or the omission of the RF filtering components, and only one supply type (the ST221/230/300W series) had a rectifier diode rating exceeding 15 amps for the 5 volt rail, even though top plate ratings were given as 20 or 25 amps.
These supplies use a 30 amp diode which supplies both the 5 volt rail and the 12 volt rail. They also have more generously proportioned heat sinks for both the switch mode transistors and rectifier diodes, as well as a transformer nearly twice the size of any other similarly rated supply (much easier and safer to re-wind etc). Furthermore the quality of components used and assembly standards are impressive. In short this series of supplies are continuously rated units, unlike any of the other designs examined which are intermittently rated at best. They are AT supplies, but the same comments apply to their supplies of ATX form which can also be modified.
If you must use another supply other than the Seven Team unit, then make sure that the 5 volt rectifier diode pair is rated at 20 amps minimum. The supply should also use 2 switch mode transistors together with an LM339 quad comparator and a TL 494 or equivalent IC as the pulse width modulator driving IC for the switch mode transistors.
If you do discover another high quality, mass produced, conservatively rated switch mode supply, please let me know who made it and its part number.
HOW TO IDENTIFY SEVEN TEAM POWER SUPPLIES
Seven Team power supplies are easily identified amongst a bunch of scrap power supplies by looking for a large, bright canary yellow label which covers most of the power supply top. In the best case the label will have the Seven Team brand and logo prominently printed together with a smaller bright red sticker above the main label with ST-230WHF or similar printed on it.
However, Seven Team also manufactures power supplies for computer makers under their own brand, and typical brands include Ipex and Compaq. In these cases the label is still large and canary yellow but you will have to search for the power supply type number on the label which always commences with ST (guess what - Seven Team).
Power supplies having numbers smaller than ST-220 (i.e. power outputs less than 220 watts) are not worth bothering with because their output is less than 220 watts, and also their internal layout uses a single common heatsink for both switch-mode transistors and rectifiers, making modification extremely difficult.
In the AT group of power supplies, models up to 300 watts exist both with and without cooling fans, and any supply from 220 watt up uses an essentially common circuit in which a few component sizes and values are changed to get the required output. The ST 230 series supply provides an extremely good starting point for a 13.8 volt 20 amp plus amateur radio power supply.
MODIFYING SWICH MODE SUPPLIES- GENERAL PRINCIPLES.
For reasons explained in the pre-amble, the only supplies modified so far are the Seven Team ST-200 series. These SMPS units use the LM339 comparator and TL494 PWM driver chips and their equivalents, which include the HA17339, and the KIA494, KA7500, IR3M02, and MB3759.
There are some important feedback paths (see the circuits below) within these power supplies and these are:
A sample of the output voltage is taken and applied to pin 1 of the TL494 which, when compared with a regulated reference on pin 2, (normally 2.5volt) varies the pulse width at the outputs of the TL494 to adjust the output voltages to the correct value.
A sample of the output voltage is taken and applied to pin 7 of the LM339. This sample is compared with a regulated reference on pin 6 (normally 2.5v) and if the output voltage rises too high, an SCR made up of Q5 and Q6 is fired and shuts down the supply.
A current transformer in series with the 340VDC primary supply monitors the amount of current drawn by the supply (and hence the total wattage supplied to the outputs) and develops an output voltage proportional to the primary supply current. This output voltage is applied to pin 5 on the LM339 and compared with a regulated reference on pin 4 (normally 2.5volts). In the event of over-current, the Q5/Q6 SCR is fired and shuts down the supply.
The re-design process is thus quite simple. All of these paths have to be identified on the PCB. The resistor values in the feedback paths mentioned in parts 1 and 2 above are re-worked to provide a nominal 13.8 volt output and overvoltage shutdown of about 15 volts. The feedback path in part 3 above is left well and truly alone to avoid damage to the primary switching transistors through overcurrent.
There is one other feedback path which must be identified. This path supplies the control circuitry with power, and exists between the +12 volt dc output and pin 12 of the TL494. This auxiliary supply voltage is then further regulated by the TL494 to form a 5 volt rail (on pin 14) for the control circuitry.
The Seven Team design philosophy remains remarkably constant throughout their entire range of AT type switch mode supplies, and once one of their units has been modified, it is a very easy matter to modify another in the series.
REBUILDING THE SUPPLY
The two circuit diagrams above, together with the photos shown, should be carefully studied to discover what components must be removed from the main PCB. (This circuit with very minor modifications is used for other late model Seven Team AT supplies).
After this has been done, the transformer is rewound in accordance with the detailed instructions here and re-installed.
Next, the multi winding filter choke is modified. The mounting pins are clipped off all windings except for the two originally used for the 5V output on the unmodified supply, and the choke is then refitted. Now, a small PCB is fabricated to hold the new output group of 4@2200uF filter capacitors (or equivalent), and is joined to the main PCB using U loops of 1.6mm dia bare copper wire. These loops are fitted through the holes originally used to terminate the 5 volt and earth wires which emerged from the SMPS case, and are heavily soldered to both PCBs. A heavy bridging link is substituted for L5, connecting the 30 amp rectifier output to the positive terminals of the filter capacitors just fitted.
Next, the appropriate resistors are fitted to the rear of the PCB to establish new feedback paths between the supply output, and pin 1 of the TL494 and pin 7 of the LM339. The auxiliary power supply for the control circuitry is re-established by connecting the anode of D11 to the new output using a wire jumper on the rear of the PCB. Finally, the PCB is pre-tested (see instructions following).
The case is now drilled and fitted with a mains switch and a pair of output terminals on an insulated plate (see photos). The bottom IEC mains output connector is removed. The hole that is left is covered with a small aluminium plate fabricated for the purpose.
The PCB is then re-installed in the case and all internal wiring completed. L5 (20 amp RFC) is used to connect the top of the electrolytic PCB to the positive output terminal while heavy braided wire (e.g. RG58 outer) is used for the connection to the negative terminal.
MAKE SURE THAT THE NEGATIVE OUTPUT TERMINAL IS SECURELY EARTHED TO THE STEEL CASE (DO NOT RELY ON THE PCB CONNECTION). THIS CONNECTION WILL SAVE YOUR LIFE IN THE EVENT OF TRANSFORMER PRIMARY TO SECONDARY BREAKDOWN.
PRETESTING THE FINISHED SUPPLY
The trick in getting a modified switch mode power supply running properly is to start with an unmodified supply which is working correctly. Do not try to modify a non-working supply - fix it first.
When the modifications detailed above have been finalized i.e. PCB reworked, transformer re-wound etc., there is a simple and safe way to check out the mods. on the PCB before it is re-installed in the case, using low voltage DC only.
First, break the auxiliary supply voltage feedback path from the DC output to pin 12 of the TL494. Supply pin 12 of the TL494 with 14-16 volts DC from an external regulated power supply. Use an oscilloscope to monitor one of the two TL494 PWM output pins (pin 8 or 11). Now, from a second variable regulated DC supply, apply a slowly increasing DC voltage to the positive DC supply output. (Both of the external DC supplies only have to provide a few milliamps). If the circuitry is working correctly, a rectangular wave should be observed on pin 8/11 of the TL494, until the output voltage exceeds 13.6 volts. At this output voltage the rectangular wave should abruptly vanish as the chip completely turns off the supply of energy to the transformer secondary. Now, slowly increase the output voltage further, in say 0.2 volt increments, backing off regularly to below 13.6 volts to check whether a rectangular wave still appears on pin 8 or 11. When the output voltage finally exceeds 15 volts, backing off and then further increasing the output voltage will no longer cause a rectangular wave to appear on pin 8/11, because the Q5/Q6 SCR has fired and shut down the TL494 permanently. When this happens the only way of re-starting the supply is to remove the auxiliary supply voltage on TL494 pin 12. This process demonstrates quite conclusively that the voltage setting feedback to the TL494 is working correctly and the over-voltage protection is also functional. Because the supply was working before modification started, it is not unreasonable to assume that the primary over-current feedback circuit is also working. However, this can be checked later during full load tests on the finished supply. The auxiliary voltage feedback path to pin 12 from the supply output is then reconnected.
After once again double checking all wiring, the fan is reconnected and the cover screwed into place. Note that a load MUST be provided for the supply, before mains testing starts. The minimum load with which the supply can safely operate is the 12 volt cooling fan. Without some form of a load the supply will probably destroy itself, because the TL494 cannot shut down permanently, meaning that energy is being supplied to the secondary without anywhere to go. This causes large primary voltage spikes that destroy the primary switching transistors.
A voltmeter is connected to the output terminals and mains power then applied briefly. Around 13.6 volts DC should appear at the supply output. If this doesn’t happen, check your wiring, as pre-testing the PCB has removed it from consideration. For full load testing, you will need a bucket of water, enough steel wire of around 1mm dia to make up a resistance of 1 ohm, some small crocodile clips, and a 20 amp shunt for your DVM. A 10milliohm shunt can be easily fabricated from around a 1.5 metre length of 7 X 0.64mm dia. mains flex to cause a voltage drop of 200mV @ 20amps. Proceed cautiously, gradually increasing the output current by moving the clips along the steel wire in the water bucket. Considerable amounts of heat will be liberated…. Good Luck!!