Q1Electric Drives

Circuit diagram and working of a step-down DC chopper: a step-down (buck) chopper consists of a controllable semiconductor switch (S, historically a thyristor with forced commutation, now typically an IGBT or power MOSFET) connected in series between the DC supply (Vd) and the load, with a freewheeling diode (Df) connected in anti-parallel across the load (for an inductive/RLE load, such as a DC motor armature), and typically a filter inductor and/or capacitor to smooth the output current/voltage.

(a) Regulation of output voltage: during the switch's on-interval (Ton), the switch conducts, connecting the full DC supply voltage Vd directly across the load (and freewheeling diode Df is reverse-biased, non-conducting); during the off-interval (Toff=T-Ton, where T is the full switching period), the switch is turned off, and the load's inductive current (which cannot change instantaneously) continues to flow through the freewheeling diode, which now conducts, maintaining current continuity while the voltage across the load falls to approximately zero (neglecting diode forward drop). The average output voltage delivered to the load over one complete switching period is therefore:

where D=Ton/T is the duty cycle (0≤D≤1). By continuously varying D via the switch's gate/base control signal, the average output voltage can be smoothly regulated anywhere from 0 up to the full supply voltage Vd, providing efficient, continuously variable output voltage control (and hence, when supplying a DC motor armature, continuously variable speed control) without the significant power dissipation that a series resistive voltage-dropping method would incur.

(b) Numerical calculation: given Vd=200V, chopping frequency f=5kHz (so switching period T=1/f=200μs), and desired average output voltage Vo=100V.

Result: the required duty cycle is D=0.5 (50%), meaning the switch must be turned on for exactly half of each 200μs switching period — that is, Ton=0.5×200μs=100μs and Toff=100μs — to obtain the desired average output voltage of 100V from the 200V DC supply.

Choice of switching device and switching frequency: modern step-down choppers almost universally use fully-controlled semiconductor switches (IGBTs for medium/high-power drives, or power MOSFETs for lower-power applications) rather than thyristors, since thyristors require additional forced-commutation circuitry (an auxiliary commutating capacitor/inductor network) to turn off once triggered, adding cost, complexity, and additional losses. IGBTs and MOSFETs, by contrast, can be turned off directly via their gate signal at any instant, considerably simplifying the control circuit and enabling much higher switching frequencies, which in turn reduces the size of the required output filter inductor/capacitor and reduces audible noise, at the cost of increased switching losses that must be managed through proper heat-sinking and, in high-power applications, snubber circuits to limit voltage/current stress during each switching transition.

Effect of duty cycle variation on motor operation: when this step-down chopper supplies a separately-excited DC motor armature, continuously varying the duty cycle D from a low value toward D=1 smoothly increases the average armature voltage, and hence the motor's no-load speed, in direct proportion to D, providing smooth, continuously variable, high-efficiency speed control across the motor's full operating range. Since power is controlled by rapidly switching the device fully on or off, rather than dropping voltage continuously across a series resistance or partially-conducting device, the chopper itself dissipates comparatively little power internally, with the majority of input power delivered usefully to the load, giving choppers a substantial efficiency advantage over older rheostatic (resistance-based) armature voltage control methods, which waste a significant fraction of input power as heat in the control resistors themselves.