Serial stacking Linear regulars to increase Vin range

Question

So I can buy a Positive Linear Regulator rated for -60V input (LM317HV). But the Negative Linear Regulators are -35v (LM7909) range. Yes there are a few odd parts that are -40v or -50v, but none I've found are -60v.

My question is, can Linear Regulators be serial connected to increase the operating range? or is there are simpler way to increase the Input rang I've missed?

My thought was having the first regulator inline with a Zenor Diode (25V) to get it to handle the raw 60V and feed its output (under -35v) to a second to produce -9v. It would have to scale or the first NLR would drop out as the voltage sags to -12V so that the circuit and still produce the -9v.

Specifically I'm looking for -9v from an input of range -12 to -60. And in the 10mA range, which would dissipate 500mW

Answer 1

You could use a transistor zener regulator to drop most of the voltage, and put it in series with a linear regulator. A basic transistor zener regulator looks like this:

^{simulate this circuit – Schematic created using CircuitLab}

The output voltage is set by the zener. Q1 must be rated for the full Vin voltage, and for dissipating a power of Vin * Imax, so it may need a heatsink. R1 is chosen by trading off the current and heating at high Vin against having sufficient current for effective regulation at low Vin. For a negative regulator, turn the zener around and swap the transistor for a PNP.

The design can be improved in several ways, such as by adding a capacitor in parallel with the diode to give better regulation at higher frequencies. Or using a MOSFET for larger currents and/or lower dropout voltage. Ultimately, this kind of circuit is not going to be as good at regulating as a linear regulator, but that doesn't matter if you have a regulator too. And it's pretty cheap.