the comparator 42 the result is that an output signal is produced from the comparator 42 the occurrence of which is determined by the selection of the reference voltage applied at terminal 48 which is synchronized with the periods of current conduction by the mosfet 39 s 10 and 11 that is for example comparator 42 may be set to produce a high level output coincident with the conduction or on times of mosfet 39 s 10 and 11 the result is a synchronizing signal which is applied to a reference generation source the reference generation source includes biasing resistors 51 and 52 transistor 53 zener diodes 54 and 55 fixed resistors 56 60 and capacitors 61 and 62 transistor 53 along with threshhold establishing zener diode 54 may be viewed as normally in a conducting state thus during this normal state a relatively high level of voltage is established at the reference junction a of resistors 57 and 58 the synchronizing signal from comparator 42 is applied to the base terminal of transistor 53 through resistor 51 since the output of the comparator 42 is high coincidentally with the conduction of mosfet 39 s 10 and 11 transistor 53 is turned off also coincidentally with conduction times of mosfet 39 s 10 and 11 with transistor 53 turned off the reference voltage at junction a begins to decay at a rate determined by capacitor 61 and resistors 57 and 58 the reference voltage continues.