Obtaining the average current

AN2267

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Obtaining the average current

For linear torque control, it is important that we sample the average phase current as

feedback to the current regulator. It is best to get this information from the DC link current

using only a shunt resistor because of its low cost and simplicity. However, the DC link

current is not continuous and is present only during PWM on time. As a simple model for

current control, assume a simple buck converter feeding an RL load as shown in Figure 3.

Figure 3.

Buck converter feeding an RL load

The switching frequency, PWM on time and load inductance are such that the load current is

continuous. Figure 4 shows the load voltage, load current and DC link current waveforms. A

close look at the load current waveform reveals that its average value is equal to its

instantaneous value during the middle of PWM on time or off time. Since the load current

flows through the DC link during PWM on time, sampling the DC link current during the

middle of PWM on time gives the average load current.

Figure 4.

Buck Converter - Waveforms

PWM

CONTROL

SW1

1

2

D1

BT1

R1

L1

V