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Practical power supplies

Switch mode

The solution to the problem of big inefficient power supplies was to use a more sophisticated design. In a sense the key was for the power supplies to become more like the computers that they were feeding.

The big problem wasn’t really the power that the computer needed it was the power that had to be wasted to ensure that the voltage was kept constant that was the real problem. To regulate the voltage power had to be thrown away.

A transistor was placed in series with the load, i.e. the computer, and it turned on more as the voltage fell and off as the voltage went up. You can think of the transistor as a regulating tap that opens and closes to adjust for changes in water pressure and outflow. Your task is to alter the setting of the tap so that the water level in a tank stays the same.

 

reg1

Just keep the level fixed!

The problem is that when a transistor is only half on it wastes power and gets hot. Think of the situation as if the tap got hot due to the friction as the water flowed though it. The more you close the tap down the more resistance to the flow of water and the more heat produced. As a result this sort of power supply – a linear power supply – gets very hot in use and wastes more than 50% of the power it supplies.

The key to stopping this waste is to notice that a transistor wastes no power when it is 100% off or 100% on. In both of these states it doesn’t get hot at all because no power is being dissipated in it. This is the same for the tap - when it is full on it isn't restricting the water in any way and when it is full off it is restricting the water completely to the point where there isn't any flow and hence no friction. Full on and full off are 100% efficient for entirely opposite reasons!

Full on and full off seem to be of no use when you want to control the output to an intermediate voltage – but that’s where you would be wrong. If the transistor switches on and off fast enough you can alter the average voltage by the ratio of the time on to the time off. Going back to the water analogy in this case you have a tap that is either full on or full off but you can control the amount of water that flows in by pulsing the tap altering the ratio of on to off time.

 

reg2Switching the tap on and off provides the same control.

 

This is the principle of the switch-mode power supply and believe it or not it made the personal computer possible.

There are other advantages to switching the current on and off at high speed – the transformer can be smaller, the capacitor needed to smooth the output can be smaller and so on – but it all means that a switch mode supply is small and efficient.

Notice also that switch mode supplies include a feedback mechanism that uses an operational amplifier, comparator and this effectively makes it a small analog computer so inside every modern digital computer there is still and analog machine not trying to get out!

The first switch mode power supplies were probably used in the Nova Minicomputer but the really surprising thing is how long they took to catch on in the microcomputer world.

The Apple II was a revolutionary wonder compared to the huge Z80 based boxes of the same generation. It was small and portable typewriter size and one of the reasons it was so small and so portable was – yes you’ve guessed it – a switch mode power supply. 

This humdrum component really did make a difference and made the computer that would one day grow up into the Mac something very special.

 

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