The Catch
3. The Price of Speed
While PWM minimizes resistive losses, it introduces another type of loss: switching losses. Remember all that rapid on-off switching? Well, each time a transistor switches between its on and off states, it takes a tiny amount of time to complete the transition. During this brief transition period, the transistor is neither fully on nor fully off, and it dissipates some power as heat. It’s like a car that’s quickly accelerating or braking — it uses more fuel during those transitions than when cruising at a constant speed.
The faster the switching frequency (how often the PWM switches on and off per second), the more significant these switching losses become. So, there’s a trade-off. A higher switching frequency can provide finer control and reduce ripple in the output (making it smoother), but it also increases switching losses. Finding the optimal switching frequency is a delicate balancing act.
Furthermore, other components in the circuit, like inductors and capacitors, can also contribute to losses. Inductors can have internal resistance, and capacitors can experience dielectric losses. These losses, though often smaller than resistive or switching losses, can still impact the overall efficiency.
Therefore, claiming that PWM is always more efficient isn’t entirely accurate. It depends on the specific application, the switching frequency, and the quality of the components used. It’s a nuanced picture, not a simple black-and-white answer.