I would love some help with my overvoltage situation.

The problem is that at the input of the circuit, the voltage can vary from between 20 to 80 volts and the output requires two voltages, 1 that powers a motor at 15 to 30 volts with a max current of 700mA and another that powers a second circuit at 5 volts. The motor voltage can have slight variations but the 5Vs needs to be pretty much 5 volts flat.

The problem is to provide a possible solution that would result in the greatest efficiency. I know that if I was to try to use voltage regulators a lot of energy will be lost/dissipated meaning an inefficient circuit.

Do you have any suggestions into how I could get those two volts from the worst case 80 Volts without having to dissipate most of the energy as heat?

 

Maybe a DC/DC converter can help you. This kind of circuit can converter a high DC voltage into a low DC voltage without dissipating much energy.But there also is a problem.The input voltage can vary between 20v to 80v,which is so large. And in my opinion the 5V still need to get from a linear voltage regulator circuit because it is required to be pretty flat.
You can look for some materials about DC/DC converter.

 

DC/DC converters have very good regulation so they will definitely be 'flat'.

The concern is about switching noise which may or may not be an issue. It's rarely an issue, DC/DC converters are pretty much the norm.

It probably doesn't make much sense to get two expensive high input range converters so it's a good idea to get one DC/DC converter to go from 80V to 15V followed by a second one from 15V to 5V. This second one could be linear if you really want.

 

Thanks Sparkysea and Ghar, DC to DC converters seem the way to go. I have previously made linear regulators from 'scratch' but were never convinced with its efficiency.

I have actually decided to use PWM to get the voltage down using an AVR Chip so all I need now is the 5 volts from the 80. The chip only draws like 15mA which makes things a whole lot more manageable.

I am still unsure regarding the buck switching regulator as I assume you need a reference voltage of some sort? And I only have the highly variable input, I can clap it down with a zener but will it mean a great loss in efficiency?
Will it also simplify things down with a comparator?