What are the cons of PWM

PWM (pulse width modulation)

Analog control technology

Note: This article focuses on the aspect of power transmission and does not deal with the transmission of information using modulated PWM.

In analog circuit technology, current is limited with the help of a series resistor (or power transistor). This creates a voltage drop across the resistor. The power loss is calculated with P = U * I. The resulting power loss can be considerable.

If, for example, a 12V, 20W LED consumer is to be limited to 50% of its power, then 10W of power loss must be dissipated at the series resistor using analog control technology. PWM is used to avoid this power loss.

Digital power control with PWM (resistive load)

The digital state control with PWM only knows the switching states ON and OFF. With OFF the current flow is blocked, with ON the line is open. The maximum voltage is always at the PWM output.
Since the current flow is 0 when OFF, there is also no power loss. When ON, there is only a minimal voltage drop (depending on the forward voltage of the power transistor). With OFF, 0% power is delivered, with ON 100% power is delivered. The frequency is constant with PWM. The output power is measured according to the time Ton or Ton / (Ton + Toff). A 50% power output means that half of the time is ON and the other half is OFF.

The output power corresponds to the integral over the time constant.

Comparison to analog control (series regulator)

The advantage of the PWM control is that the current flow is either completely prevented (here there is no power loss on the transistor) or that it is switched through completely (the power loss on the transistor is minimal). As a result, even large loads can be controlled with very little power loss.

In comparison, with a traditional series regulator there is much more power than loss (or heat), which then has to be dissipated by cooling.

Disadvantages of PWM

There are actually only 2 disadvantages of PWM, but they can prohibit the use of PWM in certain situations:

  • The current flow is not uninterrupted because it is repeatedly interrupted by the control to the zero value (unless it is at 100%). Many hardware devices require uninterrupted current and voltage curves
  • The modulation with a rectangle leads to the emission of multiple radio frequencies.

Often one tries to generate a good signal resolution and quasi-uninterrupted voltage level by increasing the frequency (reducing the time constant). The higher the frequency, the higher the losses due to radiation.

The efficiency of a PWM controller is largely determined by the voltage delta emitter-collector or source-drain occurring at the power transistor and the cleanliness of the square function (the smaller, the better). The higher the frequency, the lower the resolution. The more unclean the rectangle (depends on the transistor properties or price), the more power loss due to (waste) heat. The cleaner the rectangle, the more electromagnetic radiation. As a result, every PWM controller is also a compromise between these specifications, which often contradict one another in practice.