Jone's Chopper

Jone's Chopper

• Jone's chopper is a thyristorised chopper. That means an SCR is used as a chopper switch.
• When an SCR is operated on the DC power supply it cannot turn off naturally. So we have to include a circuit called commutation circuit to turn off the conducting SCR.

Circuit diagram :

• The circuit diagram for jone's chopper is as shown in Figure A.
• This chopper circuit uses a class D type commutation.
• SCR_{1 is main SCR and} SCR_{2 is an auxiliary SCR.}
• _{The special features of this circuit is the tapped autotransformer T. The coils} L_{1 and} L_{2 are closely coupled coils and they ensure that the capacitor C always gets sufficient energy to turn off efficiency.}
• The main thyristor SCR_{1 is the load carrying thyristor whereas the job of the auxiliary thyristor} SCR_{2 is only to turn off the main thyristor.} SCR_{2 is a small SCR because it is not expected to carry the load current.}
• _{The commutation circuit consist of the capacitor C,} SCR_2, D_{1 and the auto transformer.}

Figure A

Operation :

• _{The operation of this circuit can be divided into six intervals as follows :}
• _{Let the initial voltage on the capacitor be V volts with its upper plate positive with respect to the lower plate as shown in figure A.}
• _{The load is RL type of load and we assume that the load current is continuous and ripple-free current.Such a load is called as level load.}
• _Diode D_{2 is connected across the load and it operates as a freewheeling diode.}

_{Interval I (}t₀ – t_{1) :}

• _{Refer waveform of figure B to understand the operation.}
• SCR_{1 is triggered at} t = t_{0. The current through it is the sum of load current} I_{o and the sinusoidal discharge current of capacitor C.}
• _{Capacitor discharge through} SCR_1, L_{1 and} D_{1. At the end of this mode  voltage on C reverses its polarities,} I_c = 0 and the voltage on C is held by hold off diode D_{1. That means} D_{1 is reverse biased and does not allow the capacitor to discharge in the opposite direction.}
• _{The equivalent circuit for interval I is shown in figure B. The} SCR_{1 current is} I_s1 = I_o + I_c.

Figure B

Figure C

_{Interval II (}t₁ = t_{2) :}

• During this interval, the capacitor holds its charge, since there is no path for this capacitor to discharge.
• Hence voltage on C is equal to - V volts and remains constant.
• SCR_{1 continues to conduct and the current through it is equal to the load current} I_{o. The load voltage continues to be equal to + Volts.}
• _{The equivalent circuit for this interval is shown in figure C.}

_{Interval III (}t₂ = t_{4) :}

• SCR_{2 is triggered at} t = t_{2, this will connected the commutating capacitor C across} SCR_1.
• SCR_{1 is turned OFF due to reverse voltage across it. The load current is taken up by} SCR_{2 , and the capacitor C starts charging through} SCR₂ L_{2 and load,towards a positive voltage.}
• _{The current flowing through} L_{2 will store energy 1/2} L_{2  in it. The charging current of C gradually reduces, but as the load is inductive, the freewheeling diode will came into conduction and will keep load current constant.}
• _{At the end of this interval, the voltage on C is higher than V and positive,} I_c = 0 and the freewheeling diode D_{2 starts conducting the entire loas current} I_{o and} SCR_{2 is turned OFF due to natural commutation.}
• _{The equivalent circuit for this interval is shown in figure D.}

Figure D

Figure E

Interval IV (t₄ = t₅) :

• _{Due to the higher voltage on C, it discharges through the input DC supply V,}D_2,L_2,L_{1 and} D_{1 back to the negative plate of the capacitor.}
• _{The energy stored by} L_{2 is returned to the capacitor C.}
• _{As soon as the voltage on the capacitor equals the input voltage V, the discharge current reduces to zero. The freewheeling continues through} D_2.
• _{If the} D_{1 is replaced by an SCR then capacitor dischharge through the input supply will not take place.}
• _{The equivalent circuit for this interval is shown in figure E.}

_{Interval V (}t₅ = t_{6) :}

• During this interval, all the devices except the freewheeling diode D_{2 are in their off state.}
• _{So capacitor remains charged to +V volts and only activity  taking place is the freewheeling.}

_{Waveforms :}

_{The waveforms of the jone's chopper are as shoen in figure F.}

Figure F

How to vary average output voltage?

• _{The average output voltage is changed by changing the ON time of SCR1, keeping the total time i.e. frequency constant.}
• _{Thus the type of control used is PWM or TRC control.}