"MTX Plus⁺" Power Supply

Production Version

The prototype MTXPlus⁺ backplane had connections for external power using two VCC planes and two ground planes. The VCC planes were reserved for +5VDC and +3.3VDC (for future use) and backplane pin 17C was reserved for +12VDC.

Version 1 of the power board was designed to allow an ATX power supply to feed power to the backplane.

Version 2 of the power board was designed to allow "wall wart" power adapters to feed power to the backplane, after suitable conditioning on the power board.

When I started building and testing the prototype boards for the system, I was fortunate enough to pick up a cheap Eurocard backplane that I used for testing, but I knew that anyone else that might be interested in assembling their own MTXPlus⁺ may not be so lucky.

When I got to the point of designing PCBs for MTXPlus⁺, I wanted to start with a backplane that others could use to build the system. Although I wanted to keep the footprint small to keep the manufacturing costs down, I realised that I could put the power conditioning components on the backplane PCB and do away with a separate power board and the first production PCB was therefore a combined backplane and power board.

I was able to further simplify the design by dispensing with the 12VDC supply and regulation circuit. MTXPlus⁺ only uses 12VDC for some optional features, including automatic SCART aspect ratio switching, and perhaps a disk drive motor if a physical disk drive were ever to be connected. Rather than adding components to provide a regulated 12VDC supply from another "wall wart", I have designed the board to accommodate a cheap step up voltage module that will generate +12VDC from a lower input voltage. You can see the small PCB installed close to the power input connector. These modules are available for under £1 on eBay.

At Martin's suggestions, the backplane now also incorporates the MAX705 supervisory IC that controls system startup and reset. The MAX705 will be removed from the CPU board when I get around to making a CPU PCB, having reset on the backplane will save having to build the same circuit on every CPU board that could be used in the system.

Backplane & Power Supply PCB
Schematic diagram for the combined backplane and power board. More details on the power conditioning components can be found on the Version 2 power board prototype design page.
First version of the PCB (V1.11), it has some minor clashes around the TIP 2955 that will be corrected in the next version, but otherwise, it works fine. This PCB has been fully populated with the components required to clean up the output from an unregulated AC/DC adapter and includes the TIP2955 that might be needed to satisfy a high (>2A) power demand from MTXPlus+.
This PCB has been populated with just the minimum components required to clean up the output from an unregulated AC/DC adapter to provide 5VDC. The TIP2955 and the 3.3VDC regulator have been omitted. The 5VDC regulator fitted here is a L78S05CV, which is capable of delivering up to 2A, which is more then adequate for the CPU, Video and I/O boards.

Notes

MTXPLus⁺ Power Consumption

During the design phase, I "guesstimated" the power requirements for MTXPlus⁺ based on the datasheets for the major components, but once I had the prototype boards made, I could empirically determine a more accurate power consumption figure by removing the supply fuse from the power board and measuring the current in series with the load.

Configuration	Current drawn (mA)
All bus slots empty	17 mA (backplane LEDs)
I/O Board	170 mA (CF connected)
Video Board	505 mA
Z80 CPU Board	235 mA
Z180 CPU Board	165 mA
6502 Co-Processor Board	172 mA
Diagnostic Board	128 mA (switched off)
Diagnostic Board	200 mA (switched on)
Typical Configurations
Video & I/O Board	635 mA
Video, I/O & Z80 CPU Boards	800 mA
Video, I/O, Diagnostic & Z80 CPU Boards	845 mA (Diag board off)
Video, I/O, Diagnostic & Z80 CPU Boards	1000 mA (Diag board on)

The figures above indicate that an MTXPlus⁺, configured with CPU, I/O and video boards would require significantly less than 1A of power, even allowing for some variation in component tolerances and some fluctuations in the load.

Even allowing for some additional power for the serial ports and other expansions, it should be possible to use a high power 7805 regulator (e.g., an L78S05CV is rated for 2A) on its own and dispense with the TIP 2955. This can easily be done by replacing the sense resistor with a wire link and omitting the TIP 2955.

Temperature Considerations

The combined backplane and power board includes a linear voltage regulator and a power transistor that are used to regulate the raw DC voltage from the "wall wart" power adapter. These components are cheap and provide good power regulation, the downside is that they can generate a lot of heat. The efficiency of a linear regulator depends on the difference between the input and output voltages and how much current is being drawn by your circuit. The greater the difference between input and output voltage or the greater the current, the more heat will be dissipated by the regulator. ¹

The LM78xx series need a minimum of around 2.5VDC differential  between the supply and output voltages to provide stable regulation and the voltage dropped over the regulator is dissipated as heat, the greater the voltage difference, the greater the heat produced. 9VDC and 12VDC power adapters are the most common, but 7.5VDC adapters are also readily available and is the suggested device to provide the power input to the PCB.

Even using a 7.5VDC supply, heat sinks will probably be needed for the LM7805 and, if fitted, the TIP2955. With a 7.5VDC supply, these heat sinks can be quite small, but if the user chooses to use a larger voltage supply, larger heatsinks will almost certainly be required.

The maximum operating junction temperature for a TIP2955 is typically around 150 degrees C and for an LM7805, typically 125 degrees C. Although the devices will operate satisfactorily at these temperatures, your fingers accidentally coming into contact with them would get a nasty surprise! For personnel protection, it is advisable to keep the surface temperature below ~70 degrees C.

I knocked up a rough 'n' ready calculation in Excel to calculate the size of heatsink required for a given current consumption, there is a link to an on-line calculator at the bottom of the page², but you can find other calculators on the web if you want to look. I plan to use the cheap plate type heatsinks that rely on natural cooling (convection) but forced cooling (i.e., using a fan) would reduce the size of the heatsinks needed should you want to use a higher voltage power adapter.

Heatsink thermal resistance is usually given in Degrees C/Watt and gives a measure of the temperature rise that the heatsink will undergo when cooled by natural convection for a given power load that it is trying to dissipate.

References

¹ sparkfun.com, Power and Thermal Dissipation

sparkfun.com, Heat sinks and thermal dissipation

electronics-cooling.com, How to Select a Heat Sink

² daycounter.com, Heat Sink Temperature Calculator

designworldonline.com, How to Select a Suitable Heat Sink

giangrandi.ch, Calculating heat sinks

Texas Instruments, Understanding Thermal Dissipation and Design of a Heatsink