Pulse-frequency modulation
A modulation method encoding analog signals by varying pulse frequency.
Pulse-frequency modulation (PFM) is a technique for encoding an analog signal using just two levels (1 and 0). It works similarly to pulse-width modulation (PWM), but with a key difference: while PWM keeps the frequency constant and changes the width of each pulse, PFM keeps the pulse width fixed and changes the frequency instead. The amplitude and width of the pulses remain constant, and the frequency of the pulse train shifts according to the instantaneous amplitude of the modulating signal at each sampling interval.
PFM is used to convert analog signals into trains of square pulses, so it has many potential applications. However, designing electronics with non-fixed frequencies presents practical challenges—such as transmission line effects on circuit boards and difficulties in selecting magnetic components—so PWM is generally preferred. Still, PFM has specific advantages in certain situations.
One common use is in buck converters (step-down DC-DC converters) to improve efficiency when driving light loads. At medium to high loads, the DC resistance of the switching elements largely determines efficiency. But under light loads, DC resistance matters less, and AC losses in the inductor, capacitor, and switches become more significant—especially in discontinuous mode, where the inductor current drops below zero, discharging the output capacitor and increasing losses.
PFM mode addresses this by lowering the switching frequency and using a control method that prevents the inductor current from going below zero. Instead of applying pulses of varying widths to the inductor, it uses fixed 50% duty-cycle pulse trains to charge the inductor to a set current limit, then discharge it to zero (but not below). The frequency of these pulse trains is varied to achieve the desired output voltage, with help from the output filter capacitor.
This approach saves switching losses in several ways. The inductor operates with known peak current levels, which—if chosen carefully with respect to saturation current—can reduce core losses. And because the inductor current never falls below zero, the output capacitor isn’t discharged and doesn’t need to be recharged every cycle to maintain the output voltage. The trade-off is increased output voltage and current ripple, caused by the lower switching frequency and the gaps between pulse trains.
Lore & Background
Pulse-frequency modulation fixes the width of square pulses while varying their frequency, in contrast to pulse-width modulation which varies pulse width at a constant frequency. The amplitude and width of the pulses remain constant, and the frequency of the pulse train changes according to the instantaneous amplitude of the modulating signal at sampling intervals. PFM is a method of encoding analog signals into trains of square pulses and therefore has a wide variety of applications. There are practical difficulties in the design of electronics when working with non-fixed frequencies, such as transmission line effects in board layout and magnetic component selection, so generally PWM mode is preferred. There are, however, select cases in which PFM mode is advantageous.
Reader's Guide
PFM mode is a common technique for increasing the efficiency of switching step-down DC-DC converters (buck converters) when driving light loads. In medium to high loads, the DC resistance of buck converter switching elements tends to dominate overall efficiency. When driving light loads, however, the effects of DC resistances are reduced and AC losses in the inductor, capacitor, and switching elements play a larger role. PFM mode operation allows the switching frequency to be reduced and provides a control method that prevents the inductor current from dropping below zero during light loads. Rather than applying square pulses of varying widths to the inductor, square pulse trains with a fixed 50% duty cycle are used to charge the inductor to a predefined current limit then discharge the inductor current to, but not below, zero. The frequency of these pulse trains is then varied to produce the desired output voltage with the aid of the output filter capacitor. This allows for a number of switching loss savings, including reduced switching losses in the magnetic core and prevention of output filter capacitor discharge. These benefits come at the expense of output voltage and current ripple, which increases as a result of the reduction in switching frequency and the gap between pulse trains.
Did You Know?
- PFM fixes the width of square pulses while varying the frequency, unlike PWM which varies pulse width at a constant frequency.
- PFM is used to increase the efficiency of buck converters when driving light loads.
- PFM prevents the inductor current from dropping below zero during light loads, avoiding discharge of the output filter capacitor.
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