Savel brain dump in English!

This web page and lots of other small web pages are powered from recycled computers. The hardware is very old… Yesterday, very old Seagate SCSI disk drive failed… This was very sad news, as the backup of the disk is not created. So all stuff in /home directory was almost lost. When drive was removed from the system and cooled down, the data was recovered. Almost all data. There was lots off abandoned stuff in various hidden directories and few semi finished projects. The data was from old pictures for ebay auction to some heavyweight warezes.

Here is the picture of our RIP, Seagate ST32171W Barracuda hard disk.

According to some label attached to it, it was sold in 1997. This disk started carrier as part of RAID system of some local company. When bigger company acquired that smaller company, the hard disks were stored in some warehouse. After some storage years, the disk was recovered, reformatted and placed in my web server.

Disk didn’t managed to stay alive for ten years… Rest in peace… You think so? No. The drive will be disassembled to various interesting parts. Like grinder, few electronic details and quite powerful magnets.

Don’t forget to create backups of your important data!

PWM transformer are wounded like ordinary transformers. But there are some important differences. As the working frequency of the transformer are much higher than ordinary line transformer, the windings contains much less turns. Also, skin effect is introduced. Also there is some effect with magnetic flux. The simplified description is that the flux tends to stick to core. So all the windings must be as close as possible to core. Engineer must select the core that all the windings fills the core window.

There few simple rules when winding the transformer. Keep highest current winding nearer to center core. Keep all winding interleaved across the bobbin width to optimize coupling with the primary winding.

Example transformer: 4 windings = 5V, 5 windings = 7V, so 4+5 = 12V.

Since so few turns are involved in these outputs, it is usual for multiple parallel wire strands to be used on each output winding, and for the 5 V and 7 V outputs to be constructed as separate windings. This means, that sometimes you need to start even 8 wires at once.

To make coupling between primary and secondary (-ies) better, especially when it is impossible to place all secondaries in one layer, it is possible to split primary.

Np-primary, Na-BIAS.

Red lines in the picture indicates wrapped insulation of Polyester Tape, 3 layers. Ordinary tape sold in local stationer’s (office) shop is not good. The tape must be thermal-proof. Original tape used in transformers can withstand even hot iron touch.

And here is some old manual winding machinery. And one half of primary are already done.

Ferrite core transformers are used in high frequency power supply. The main core parameters are core material and area of core cross-section. Area can be calculated using regular geometric formulas. The problem is to determine core material- they all look dark gray… But we can assume, that manufacturer used most common and cheapest core. So when cross section area and other physical dimensions are measured, we can adapt then to some datasheet.

Various ferrite cores

Another important parameter is air gap. The core can be ungapped and with some air gap. The gap size can be from some standard range, but sometimes it is specially made by rasping central core element.
Ungapped core is used in push-pull schematics- the core is force re-magnetized. Gapped core is used to prevent core saturation. The problem with the gap is, that all formulas and design software calculate core gap, ant there is NO software to calculate all other parameters when gap is known.

Gapped and Ungapped core

While making powerful transformers sometimes the wire is very thick. The diameter of either the primary or the secondary winding must not be larger than the recommended maximum (0.4 mm), for Skin Effect. Due to Skin Effect, the cross section of the wire may be under utilized, resulting in excessive heating of the windings and thermal related transformer failure. The thick wire is replaced with few smaller one winded at once, or using special multithread wire. If the bare conductor diameter of the wire is larger than that of the 27 AWG for 132 kHz or 25 AWG for 66 kHz, a parallel winding using multiple strands of thinner wire should be used to minimize skin effect.

In older and very powerful PSU I found copper foil used instead of wire.

Replacement of thick wire

Number of windings in transformer can be calculated using special formulas of special software. Here you can see the output of special transformer design software. It was downloaded form semiconductors manufacturer’s site for free.

Output of transformer design software

The main component of PSU is TOPSwitch-GX chip. There are produced whole line of these chips: TOP242 (weakest), TOP243, TOP244, TOP245 (used in this design), TOP246, TOP247, TOP248, TOP249 and TOP250 (Most powerful- 10A switch, up to 290W output power).

The chip is connected in typical way according to manufacturer’s recommendation. Eagle schematics is a bit skew, but it is useful and working. I highly recommend to read all application sheets for manufacturer’s site.

R2 resistor is already discussed in older post. Other important elements in live side are R1, D1, D2 and C2. C2 must be high voltage one, not less than 400…500V. Interesting diode D2 is P6KE170. D2, C2 and R1 are used to protect chip’s output drain for overvoltage spikes. It is leaking inductance clamping circuit. C2 and R1 are selected such, that D2 dissipated very little power except during overload conditions.
R1 is 2W power resistor. D1 is 1N4937 and is not recommended to use any other cheaper alternative. It is fast recovery rectifier, 600V, 1A (pulse 30A)

R10 sets the device current limit to 80% of typical to limit overload power. So, if your transformer and output diodes are powerful, you can short circuit PSU without any problems.

Bias current for optocoupler in live side is fed from special winding of the transformer. It is very low current, 12V winding, so the wire can be very thin. And diode D5 is very small.

The control pin components (C5, R4, C6) are not very clear for me. C6 is high frequency filter. And the rest is something used during startup. In datasheet there is the line: CONTROL pin capacitor: 47 μF, 10 V, low cost electrolytic (Do not use low ESR capacitor). But the donor PCB from some LCD monitor is with cap which looks like low ESR…

Now let’s look at the transformer.

This is simple, quite powerful and economical power supply. You don’t need to switch it off- when load is off, the psu draws on fraction of Wat of the power. (80 mW @ 110 VAC, 160 mW @ 230 VAC). The main component is TOP242-250 chip.

Special transformer is used in this supply.

Schematics for printing and viewing.

The schematics is universal. It is possible to change output voltage and number of outputs. Also it is possible to change the power of whole power supply. When using most weak chip in closed box without ventilation, it is possible to build 10W power brick. Using most powerful chip and proper cooling- the power increases up to 290W!

The schematics consist of few main modules:

Live side rectifier and filter. It is used to produce high voltage DC and to filter all possible interferences.
Strange resistor near C18 is NTC resistor. It is used to reduce current spike when device is plugged into mains. C18, TR1, CX and C3 are used to filter all high frequency stuff from and to the device. If device is made in Chinese way, all this stuff can be omitted. R12 (the symbol in circuit diagram is bad) is varistor. It is used to protect the device from voltage surge. If voltage is more than 270V it shortens the mains and fuse blows. We can add R2 to the protection circuit. When voltage in DC side is more than some value defined by R2, the top switch chip is switched off.

As you can see from PCB image, some duplicated devices (like F2 and F3, CX1 and CX2) are place on on others top. This makes PCB more universal- it is possible to use different size devices. Additional chokes (L6-L9) are just simple jumpers. It is possible to make PCB without them, but some traces will become much narrower and longer. And this PCB is designed to be with thick tracks. So everybody can make it using all simple PCB making technologies.

Side view of the PSU. That trimmer is used to adjust output voltage. When PSU will be tuned, the trimmer will be replace with simple resistors.

Continued…