It's not well known, but there is a circuit called a Jones Chopper that uses two SCRs in a DC PWM type circuit. It was originally described in the old GE SCR Manual. It requires an autotransformer and capacitor as part of the circuit. The autotransformer charges the capacitor which is discharged by the second SCR to momentarily reverse bias the first SCR and turn it off. The complexity of this design has limited its application, but it is an interesting circuit.