DC-DC converter

Claim 1

A DC-DC converter for supplying DC power to a load, comprising: (a) a pair of DC supply terminals; (b) a field effect transistor connected between the pair of DC supply terminals and having drain, source and gate; (c) a transformer having a primary winding connected in series with the field effect transistor, a secondary winding, and a tertiary winding having a first extremity connected to the gate of the field effect transistor and a second extremity connected to the source of the field effect transistor; (d) a rectifying and smoothing circuit connected between the secondary winding and the load; (e) a capacitor connected in series to the tertiary winding; (f) means for charging the capacitor; (g) a current detecting resistor having a first extremity connected to one of the DC supply terminals and a second extemity connected to both the source of the field effect transistor and the second extremity of the tertiary winding of the transformer, said current detecting resistor providing a current detecting voltage; (h) a source of DC voltage; (i) a variable voltage dividing circuit connected across the DC voltage source, said variable voltage dividing circuit having a pair of output lines for providing a variable fraction of the DC voltage, one of the output lines being connected to the second extremity of the current detecting resistor; and (j) a switching element having a first main electrode connected to the gate of the field effect transistor, a second main electrode connected to the first extremity of the current detecting resistor, and a control electrode connected to the other of the output lines of the variable voltage dividing circuit, the switching element being turned ON when the sum of the current detection voltage and the variable fraction of the DC voltage becomes higher than a prescribed value.