Impact on system efficiency

Air-conditioning and refrigeration systems often need a very wide variation of speed control to achieve the highest compressor and heating system efficiency. Traditional IPMs with more than 1µs of dead time and more than 500ns of propagation delay limit the maximum and minimum operating PWM duty cycle and reduce the operating speed range. A higher dead time also reduces the available voltage to the motor and increases the amount of motor current for the same power delivery.

The DRV7308 offers an adaptive dead time, with a maximum dead time less than 200ns and a propagation delay lower than 200ns, helping designers enhance the operating PWM duty-cycle range and thus the speed range, while also increasing the available voltage to the motor. For example, the ability to obtain an ultra-low to high speed in air-conditioning systems helps designers set the highest speeds at startup so that the systems cool and heat faster. Then, after achieving the set temperature, designers can use a finer low speed and capacity control, depending on the air-conditioning load changes. This finer, optimum load-point control helps increase system efficiency.

Ultra-low dead time and propagation delay – with low propagation delay mismatch – enable accurate average current sensing and thus increase the accuracy of control, especially for field-oriented control drives. Figure 6 shows the impact of propagation delay on average current-sensing accuracy. During PWM, sampling the current at the middle of the PWM on-time period will obtain the average motor current in every PWM cycle. Figure 6 also illustrates how the propagation delay deviates the current sensing from the mid value. The error in current sensing (ΔI) depends on the propagation delay, applied voltage, PWM switching frequency and motor inductance. For low-inductance motors, the error will be high. The current-sensing error also affects motor position sensing (estimator) accuracy in a sensorless control driver. A motor position estimation error will result in reduced motor efficiency. The DRV7308, with ultra-low propagation delay and propagation delay mismatch, helps achieve accurate average current sensing and improves motor efficiency.