I am trying to find the best way to take 72VDC and turn it into
60VDC (100W max). At first I thought a simple resistor divider would be ok but then I realized this would be useless if the load changed at all. I went searching for some sort of volage converter but my voltages and power (really about 96Watts peak for start up) were too high.
Vz1 can be 2 ea. 1N4745 and 2 ea. 1N47441 watt zener diodes in series (2 X 16V + 2 X 15V = 62V for a Vo of 60V), with an R1 of 470 ohms. You could use a MJ10016 NPN darlington for Q1. This high gain 120V darlington will ensure you will only need 10mA or so maximum base current (and probably much less) from the 64V zener regulator to give you the 60V at 1.5A to 2.0A you need at the output.
This setup is just about the most basic one, though it will work. In order to deterrmine whether it's the best solution, you'd have to know the answer to a few more questions, like:
Do you need short circuit or overload protection (the above will smoke up quite nicely in case of a short circuit) (by the way, this is called short circuit/overcurrent protection)
How much does the 72VDC vary? And what kind of voltage regulation do you need for that varying of the input power supply? (line regulation)
How much does your load current vary? And how much are you willing to allow the output voltage to vary for that change in load? (load regulation)
What kind of response do you need to rapidly changing loads (transient regulation)
Is there a chance a higher voltage will be applied to the output of this power supply? If that happens, what do you want to have happen? (overvoltage protection)
The above circuit will have poor load regulation if the input voltage varies by more than a volt or so. It may vary a half volt over the load range of 0.5A to 2A. It doesn't have overvoltage protection, and the transient response isn't going to be that great (the 10uF cap in series with the 10 ohm power resistor is a good idea for rolling off response to prevent oscillations, but it also slows things down a bit).
If this circuit doesn't do it for you, feel free to post back with more information. There are a variety of ways to solve your problem, including some one-IC solutions.
It involves using a pass transistor. Page 2 shows the basic circuit - you would need to change the transistor and zener and the 1K load resistor. You could use a TIP33 xsistor with your 60V zener. Change the 1K load resistor to 10K, 1/2 watt.
At 100 watts and 60 volts, current will be about 1.7 amps. With 70 volts in, the TIP33 will need to dissipate ~ 20 watts, so it needs to be on a good heatsink.
If you need better regulation, you could use the circuit shown on page 3. You will need to change the values as before - the circuit shown is for low voltage. The 2N3904 is too low - you could use a 2N5550 for it and the TIP33 for the 2N3055. The zener is once again your 60V zener, and R2 and R3 are changed to 10K 1/2 watt.