Friday, 6 July 2012

Jones chopper

Jones chopper is an example of class D commutation in which a charged capacitor is switched by an auxilary SCR to commutate the main SCR. In this circuit SCR1 is the main switch and SCR2 is the auxilary switch which is of lower capacity than SCR1 and is used to commutate SCR1 by a reverse voltage developed across the capacitor C. "The special feature of the circuit is the tapped autotransformer T through a portion of which the load current flows".
        When T1 is ON, capacitor C discharges resonantly through T1, L1, D1 . This discharge current doesnot flow through L2 and back to the battery because of transformer action of T. The load current is picked up by T1 and the freewheel diode D1 is reverse biased. As the capacitor voltage swings negative, the reverse bias on diode D2 decreases.  This continues upto a time  pi(L1C)^1/2.
            When T2 is on the negative voltage on capacitar C is applied across T1 and it becomes OFF.The load current which is normally constant starts to flow in T2 and capacitor C. The capacitor C charged positively at first upto a voltage equal to supply voltage Vdc.The freewheel diode become forward bias and begins to pickup load current. And capacitor current starts to reduce. After this the energy 1/2LI^2 is the inductance L2 is forced in to the capacitor C.Charging is positively to 1/2CV^2  the capacitor current continues to decrease as a result current through T2 decreases gradually become OFF. The cycle repeat when T1 is again turned ON.
The main advantage of JONES chopper over other  the circuit is that
* It allows the use of higher voltage and lower microfarad commutating capacitor. This is because the trapped energy of inductor L2 can be forced in to the commutating capacitor rather than simply charging the capacitor by supply voltage.
* In this circuit there is no starting problem and anyone of the SCR can be turned on initialy there is great flexibility in condrol also.