Saturday, January 7, 2017

Modding a cheap 60W SMPS (CR6853T + TL431) from 12V to 24V


Before reading on
 
Warning! This project uses dangerous / lethal high voltage! If you're not experienced with power electronics, do not attempt anything you read in this article.

Capacitors can remain charged to lethal voltage even after the circuits are disconnected from mains! Everything you do on your own risk and responsibility!


The original power supply

A few years ago I picked up a small SMPS power supply (Melarre M-13.8V 5A) for use as a charger for 12V lead-acid batteries. It was very cheap at the time and it worked quite well. The output is 13.8V at a maximum of 5A, although I am a bit skeptical about it being able to continuously pump 60W of power. The construction looks somewhat like a laptop power brick.

Original case

Since I didn't need it anymore as a charger, I decided to modify it to output 24V and use it as a base for a lab power supply. The build quality is not that good, especially the soldering which had lots of splattering, risking short circuits (but nothing that can't be fixed).


The internals

The unit is based on the Chip-Rail CR6853T PWM controller, Toshiba 2SK3067 primary switching transistor, FMB26L Schottky diode for secondary rectification and a transformer with a size of 20 x 20 x 30 mm and 10 mm core diameter.

Transformer core, 1 cm diameter

Unfortunately I couldn't find a schematic for this SMPS, so I had no info on the number of turns inside the transformer. The only way to find out was to remove it from the board then count each winding.


Unwinding the transformer

The transformer had the typical SMPS layout (primary split in 2 parts to minimize leakage):
  • The first half of the primary was wound closest to the core, with bifilar wire, total thickness 0.7 mm, 30 turns
  • The secondary, made from hexafilar wire, total thickness 1.2 mm, 7 turns
  • The second half of the primary, with bifilar wire, total thickness 0.7 mm, 15 turns
  • The auxiliary / feedback winding, thickness 0.4 mm, with 8 turns
So, in total there are 45 turns for the primary, 7 for the secondary and 8 for the feedback.


The modifications

To make it output 24V, I rewound the transformer completely, keeping the same number of turns for both the primary and auxiliary and doubling the number of turns for the secondary (to a total of 15 turns). Also, instead of the hexafilar wire, I used normal, 0.8 mm magnet wire. It's thinner than the original one, but it should hold just fine since it will be used at maximum 1 amp. For the insulation between the windings I used Kapton tape in addition to the original yellow polyester tape.

Transformer after rewinding

Next up, I have drawn the schematic for the secondary part of the SMPS in order to see how the voltage stabilization is done.

Original secondary of the SMPS


The isolated feedback loop is provided with the help of a TL431 adjustable shunt regulator, an optocoupler (PC817) and a few resistors. The filtering is made with the help of 2 electrolytic capacitors (in parallel) of 1000 uF each, which insufficient for a flyback SMPS (the ripple is quite high in this kind of design).

The output voltage is given by the voltage divider created by R1 and R2 and is given by the formula:

VOut = VRef * (1 + R1 / R2)
, where VRef is the TL431 reference voltage (2.5V)

In the original circuit, R2 is made from 3 other resistors in parallel (probably hand picked by the manufacturer in order to get close to the desired voltage). The resistance of R2 is thus somewhere around 3.5 KOhm. Solving the above equation for VOut, we get 2.5V * (1 + 14.7 KOhm / 3.5 KOhm) which is 13V, pretty close to the intended value.

Now, to get 24V, I decided to make R1 10 KOhm. R2 needs then to be 1.16 KOhm so I put in 1K + 100R in series. The modified secondary part of the SMPS looks like this:

Modified secondary

I will also add an inductor and another filter capacitor (on the wires, there's no space on the PCB) to form a PI filter and get rid of some of the ripple.

To test the SMPS, I first ran it at mains (220v) in series with a 100W light bulb, just to be on the safe side. It worked from the first try, so I call this a successful experiment.

Reassembled power supply

Conclusion

To summarize, here are the general steps for modifying a flyback-style SMPS to another voltage:

  • Take out the unsuitable components from the secondary (the capacitors which might be too low voltage, zener diodes) and make sure the secondary rectifiers are able to cope with the current you're willing to draw
  • If a higher voltage is needed rewind the transformer accordingly. Rewinding is usually not necessary if the desired voltage is lower than the one the supply originally provided unless you want to draw bigger current than what was originally provided.
  • Find the 2 resistors that set the voltage of TL431 and change them. Keep the top resistor (R1 in the above schematic) bigger than 5 KOhm so that the TL431 doesn't see excess current.
  • You can choose an arbitrary value for R2, something close to 1 KOhm then solve the above equation for R1 like this: R1 = R2 * (Vo / 2.5 - 1). Or solve for R2 = R1 / (Vo / 2.5 - 1)
  • If you want to have a potentiometer for R1, make sure to put a resistor (at least 470R) in series with it otherwise the TL431 will get damaged when the pot is turned all the way down
  • Put back filtering capacitors with suitable voltage ratings. Derate them as much as possible (I derate them by 50% usually) because the ripple  and noise is usually high in SMPS designs.
  • Make sure the switching transistor and the rectifiers in the secondary have adequate heatsinks

That's about it, thanks for reading!