Class-D or Type-D Chopper Explained

Class-D or Type-D chopper is a circuit configuration of chopper in which power can flow in either direction i.e. from source to load and load to source. The operation of this chopper is confined in first and fourth quadrant. This type of chopper is also known as Two quadrant Type-B Chopper. The necessary condition for this chopper is that load should be inductive. This article outlines the working principle / operation of Class-D chopper with circuit diagram and relevant waveforms.

Working / Operation of Class-D Chopper:

To understand the operation / working principle of Type-D chopper, let us first have a look at its circuit diagram shown below.

 

In this circuit, CH1 and CH2 represents two choppers which can be made ON / OFF. We shall consider two cases for explaining the operation.

Case-1: CH1 and CH2 both are switched ON simultaneously.

When both the choppers are switched ON, the load is directly connected to source and hence the output voltage V_o will become equal to the source voltage V_s. The current flows from source to load in this case. Thus, both the current and output voltage i.e. i_o and v_o are positive in this case, hence the operation is in first quadrant.

It should be noted that diode D1 and D2 are reversed biased in this case and hence they can be treated as an open switch.

Case-2: CH1 and CH2 both are switched OFF simultaneously.

When both the choppers are made OFF simultaneously, the current through the load doesn’t suddenly drops to zero due to inductive nature of load. However, it decays gradually and hence a huge amount of voltage is induced in the inductor in the reverse direction (opposite to the direction of v_o). This makes diode D1 and D2 forward biased. Thus, D1 and D2 starts conducting and connects the load to source again. But this time, the current flows from load to source (carefully observe the circuit diagram). It shall be noted that, the direction of load current has not changed. The current is still flowing as shown by the direction of i_o in the diagram but the polarity of v_o has changed. Thus, i_o is positive but v_o is negative and hence operation of chopper is in fourth quadrant. The power flows from load to source.

The first and fourth quadrant operation of class-D / Type-D chopper is shown by the hatched area in the figure below.

Various Waveforms:

Now, we will discuss the various waveform of class-D or Type-D chopper. Figure below represents various waveforms during the time both the chopper are ON.

During the ON time of CH1 & CH2, output voltage v_o is positive and equal to v_s (recall Case-1). When both the choppers are made OFF, the output voltage reverses and hence becomes negative (recall Case-2).

It may be noticed that, the average value of output voltage v_o may either be positive or negative. It depends on T_ON and T_OFF time. If T_ON time is more than T_OFF time then positive part of v_o Vs. t graph will be more than the negative part and hence average value will be positive.

If T_OFF is more than the T_ON time, the negative part of v_o Vs. t graph will be more than the positive part and hence average value of output voltage will be negative. This is depicted in the waveform shown below.

Thus, the average output voltage of Class-D chopper may either be positive or negative depending upon whether T_ON is more or T_OFF.

Average output voltage V_o is given as below.

V_o = V_s [(T_ON – T_OFF) / T]

where T = T_ON+T_OFF

From the above formula, following points can be concluded and the same can be checked from quadrant operation of class-D chopper shown earlier in the figure:

• In case T_ON>T_OFF i.e. duty cycle α>0.5, V_o is positive.
• In case T_ON<T_OFF i.e. duty cycle α<0.5, V_o is negative.
• In case of T_ON = T_OFF, α = 0 and hence, V_o = 0.

Now, refer the waveform of load current. It can be seen that; load current is always positive irrespective of CH1 & CH2 are ON or OFF. Thus, the average value of load current I_o will always be positive.

You may also refer waveform of source current is and chopper current i_CH1 or i_CH2. It may be noticed from source current i_s waveform that it is negative when both CH1 and CH2 are OFF. It may also be said that i_s is negative when diode D1 and D2 conducts. Negative source current means, current is flowing into the source as discussed in Case-2. Kindly write in comment box if in doubt or for feedback. Thank you!