Here is a power supply design that has a
no loss Negative dropout voltage.
The spice circuit show above has useful features that are uncommon.
1) First it is fully floating with no real limit on the voltage it can regulate with only one active component subject the full power supply high voltage. This allows the design to be scaled up and down in output voltage capability easy from 10s of volts to 1,000s of volts. Only the reference resistance R17 , C8 and the pass MOSFET M1 need to be adjusted to cover this range of voltages. This is great circuit for Screen or Plate voltage regulators in Tube amplifiers. The lack of a any voltage drop makes adding this regulated power supply easier as no voltage drop from the regulator gets added to the amplifier. This can allow the same transformers to be used as were in the unregulated version while holding the same plate and screen voltages.
2) R17 approximately sets to the desired output voltage in K ohms per volt output. For the shown example that is set to 500 volts output R17 is set to 497K ohms. The small error in the R17s value is adjusted out in the finial circuit with a trim POT. This makes it very easy to select any output voltage. By using a number of 1/4 watt resistors is series very high voltages can be regulated with only low cost 1/4 resistors. Use one 1/4 resistor in the series string for every 200 volts or so output
3) C6 is optional but greatly improves the noise reduction in the power supply. It should have a voltage rating of 120% or more of the output voltage.
4) M1 can be a MOSFET or IBGT. It must have a voltage rating at or above the INPUT voltage to the regulator plus 16 volts to withstand the startup voltage. The extra 16 volts is needed to allow for the regulators internal floating power supply.
5) The power rating for M1 to withstand the power loss during normal operation is important. I suggest lots of margin as having M1 fail shorted with the full input voltage to arriving on the power supply output terminals can cause down stream damage. The power loss in M1 is
((Input voltage+16) - Output voltage) * output current = power loss across M1
So in this example if we assume a 525V input voltage, a 16V internal power supply, a output voltage of 500 volts and a power supply current of 500mA the loss in m1 becomes.
((525V + 16V) - 500V) *0.5A = 20.50 watts power loss in M1. A manageable power level with good heat sink.
6) The 16 volt power supply needed to run the regulator can be created from a voltage doubler using 5 to 7V AC transformer winding. A good way to go is switch to a no loss solid state rectifier from a tube type and then repurpose the 5V filament winding to power the regulator. This results in good input voltage to the regulator and the lower current draw on the 5 volt filament winding will help to keep the power transformer temperatures down.
This design was inspired by a old but brilliant analog IC the MC1466L designed by Motorola in what I saw as the "golden days" of analogue IC design in the USA.
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