The picture below is an excerpt of Figure 2 of a DC/DC boost converter (see PDF here). It is part of a capacitance multiplier. What is the purpose of this (anti-parallel) diode between emitter and base of the transistor? I couldn't find this in any literature for capacitance multiplying.
The diode protects the BJT (inverted BE voltage) assuming the input voltage disappears.
Output capacitors can then discharge through the diode ... and the 500 Ohm safely.