What is a Chopper?

• A chopper is a static device which converts fixed input voltage to variable output voltage.
• A chopper can be thought as a DC equivalent of an AC transformer as it steps-down or steps-up DC voltage.
• The switching device used for a chopper circuit can be power BJT, power MOSFET, GTO or force-commutated thyristors (commutation circuit is needed if thyristors are used).
• A chopper is a high speed on/off semiconductor switch. It connects source to load and disconnects the load from source at a fast speed.
• There are 2 types of choppers
  1. Step-down Chopper (more common)
  2. Step-up Chopper

Step Down Chopper

By turning the switch ON and OFF periodically, the voltage source can be connected and disconnected periodically from the load. By this action,chopped load voltage (V_o) as shown, is obtained from a constant DC supply of magnitude (V_s)

Note: The output voltage remains same for any type of load but the load current waveform will be different for different types of loads as explained below

1) For a purely resistive load

• In a purely resistive load, load current only flows when the switch (sw) is ON. Load current is zero when the switch (sw) is closed. So load current waveform is similar to the voltage waveform.

2) For RL or RLE loads

• In RL or RLE load, when sw is off, the inductor (L) does not allows the load current to become zero.
• The inductor supplies current when switch is OFF and the inductor current flows via the freewheeling diode.
  This causes the voltage across load (V_o = V_diode) to be zero when the switch is OFF.
• Hence the load current is non-zero when the switch (sw) is OFF.
• In RL or RLE loads, a freewheeling diode is purposefully added to maintain the load current value as non-zero.
• Note: A DC motor can be represented as an RLE load
  (R - Armature resistance, L - Field inductance, E - Back emf)

Operating Principle of Step-down Chopper

• The switch (sw) is being operated with a duty cycle D
• For a constant frequency, the duty cycle D is defined as the ratio of the switch on time to the sum of switch on and switch off times.

• T is the time period. T = 1/f (f is the switching frequency)
• The average value of the output voltage is, V_o(avg) = D * V_s
• Since 0 ≤ D ≤ 1, the output voltage (V_avg) is always less than the input voltage (V_s).

Need for a freewheeling diode in chopper circuit

• It is an anti-parallel diode connected across the load to provide a path for a continuous inductor current flow when the switch is in off state
• A freewheeling diode ensures continuous conduction mode if the load current never reaches zero and the load time constant, τ = L/R is much greater than the period T.
• A freewheeling diode is only required when RL or RLE loads are connected. When the load is purely resistive, the current will become zero (when switch is OFF) with or without freewheeling diode.
• You can click on the button below to turn ON/turn OFF the switch and understand the significance of freewheeling diode.

Turn OFF the switch

Advantages of DC Chopper

1. Fast dynamic response: High operating frequencies allow for achieving a faster dynamic response to rapid changes in the load current and/or in input voltage.
2. High conversion efficiency: Choppers have very low power loss.
   When the switch is ON, voltage drop across the switch is almost zero (V_diode = 0), so power loss (P_loss) = V_diode * I_load = 0
   When the switch is OFF, no power is consumed by the load as well as by the chopper.
3. Compact size: Since choppers can operate at high switching frequencies, the smaller and lighter the transformers, filter inductor and capacitors.
4. Simple and Smooth control: Since V_o(avg) = D * V_s and duty cycle of the switch can be adjusted steplessly by PWM pulse generator, the output voltage can be varied smoothly.
5. Less maintainance required
6. It is Cheaper