DC Chopper Control Techniques
Need of Chopper Control schemes
To control the operation of Chopper, we need to control average output voltage of chopper circuit
The average output voltage is varied or controlled by periodic opening and closing of switches used in chopper circuit.
Chopper Control schemes
Time Ratio Control
Constant frequency system/Pulse width modulation control
Variable frequency system/Frequency modulation control
1.By varying on period of switch keeping chopping frequency ,f constant.
Constant frequency system/Pulse width modulation control
Switches are turned on at constant chopping frequency and on time of switch is varied to vary average output voltage.
The total period T of one cycle of output waveform is constant i.e. f=1/T is constant.
The variation of on period results in pulse width control, therefore this method is called as PWM control
Features of Constant frequency system/Pulse width modulation control
Low ripple content as compared to variable frequency method
Small size of filter required
Preferred for faster response
Applications
DC motor speed control
(2) By varying frequency f and keeping on period of switch constant.
Variable frequency system/Frequency modulation control
The on period of switch TON is kept constant and average output voltage is varied by varying chopping frequency f
In PWM scheme low values of output voltage is obtained by reducing TON but in practice TON cannot be reduced below a minimum value Tmin
Therefore, duty ratio, D is reduced by increasing total period T, keeping Ton constant at Tmin.
Duty ratio can be varied by varying T with either TON or TOFF constant to get controlled V0avg
As frequency is varied , this method is called Frequency modulation
Features of Variable frequency system/Frequency modulation control
At low frequencies, the current may become discontinues causing high harmonic content.
As this method gives harmonics at unpredictable frequencies, filter design is difficult
System cannot be optimized due to variable frequency
Control range of frequencies is limited
Operation at high frequencies is accompanied with losses in component
System become sluggish at low frequencies
Current Limit Control
In this technique load current I0 is allowed to vary between predetermined max and min limits.
Since the chopper operates between Imax and Imin with Imin ≠0, the output current is continues
If I0 tends to increase beyond maximum limit, thyristor is turned OFF and if I0 tends to fall below minimum limit the thyristor is turned ON
The switching frequency is determined by difference of Imax and Imin
If Imax - Imin is large ⇒ fS will be less and ripple in output current ∆I = Imax - Imin will be more.
If Imax - Imin is small ⇒ fS will be more and ripple in output current ∆I = Imax - Imin will be less.
In this scheme chopper is turned On when I0 is equal to preset min value Imin and chopper is kept ON till I0 increases to another preset value Imax.
The chopper frequency and pulse width are dependent on load parameters.
Advantages of Current Limit Control
The ripple in load current ∆I = Imax - Imin can be regulated to small preset value for wide range of loads
Applications
DC motor control and SMPS
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