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A Winding Switching Mechanism is a circuit that selects one out of many transformer secondary taps (2 in this case) and feeds it to the linear regulator. Such a mechanism is required to keep the power dissipation of the pass element of the linear regulator low.
The mechanism works as follows. A relay is used to switch between the upper and lower taps of the secondary. The selected voltage is fed to the full bridge rectifier. If the coil is not powered, the higher voltage tap is selected, but if the coil is powered, the lower voltage tap is selected. This can be seen as a multiplexer.
A comparator is used to compare the output voltage of the linear regulator with a 12V reference, so when the output voltage of the regulator goes below 12V, the comparator output goes high and the coil is excited. This causes the relay contacts to change over and the lower voltage tap is selected. Similarly, when the output voltage of the regulator goes above 12V, the higher voltage tap is selected.
Why is such a mechanism necessary? Won't a linear regular work just fine even if it has a constant 30V input?
Yes, a linear regulator will work even with a constant 30V input, but let us recall how a linear regulator works. A linear regulator works by dropping excess input voltage to ensure that only the required voltage is made available at the output. This obviously results in a high power loss. The power dissipation is given by
Pdiss = (Vin - Vout)*Iout
For Vin = 34V, Vout = 20V and Iout = 500mA, then the power dissipated is 7W.
But when Vout is, say, 5V, then the power dissipated is 14.5W!
By reducing the input voltage to the regulator, the power dissipated can be reduced. When the output is 5V, the lower voltage winding is selected and the input to the regulator is around 17V, which reduces the power dissipation to a more manageable 6W.
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