Pulse-Frequency Modulation for Switching Regulators
What’s the difference between PFM and PWM? We explore pulse-frequency modulation as a technique for controlling the output voltage of a switch-mode voltage regulator.
I’ve written several articles recently about DC-DC converters, also known as switching voltage regulators. These are power-supply circuits that use an inductor, a diode, an electronic switch, and output capacitance to efficiently decrease or increase the magnitude of an input voltage. To achieve robust regulation, these circuits monitor the output voltage and respond to variations by adjusting the waveform that controls the switch.
The adjustment technique that appears most frequently in discussions of switching regulators is pulse-width modulation (PWM), and that’s what I’ve been using in my LTspice simulations thus far. However, PWM isn’t the only means of adjusting output voltage. This article will explore an important alternative: pulse-frequency modulation (PFM).
What Is Pulse-Frequency Modulation?
Figure 1 depicts the operational modes of pulse-width modulation and pulse-frequency modulation.
Figure 1. Top: Pulse-width modulation. Bottom: Pulse-frequency modulation. Image used courtesy of Robert Keim
The logic-high density of both waveforms increases over time. In the PWM waveform, logic-high duration increases and logic-low duration decreases so that the period can remain constant. The duty cycle changes, but the frequency does not. The PFM waveform is visibly different: though the pulses in the diagram all have the same duration, the time between identical pulses varies.
Though the term “pulse-frequency modulation” could be interpreted as “modulating the frequency of a pulse train,” that’s not at all what’s happening here. In pulse-frequency modulation, a pulse occurs periodically, and the frequency at which this pulse occurs is modulated. The result is not “frequency modulation” in the FM radio sense, but rather modulation of the frequency of fixed-width pulses.
The scheme shown above is fixed-on-time PFM: logic-high duration is unchanged, and frequency is adjusted by changing the logic-low duration. In fixed-off-time PFM, it’s the other way around: logic-low duration is unchanged, and frequency is adjusted by changing the logic-high duration.
The Advantage of Pulse-Frequency Modulation
As I explained in my primer on switching regulators, switch-mode voltage conversion achieves superior efficiency by favoring a switching element’s fully on and fully off states—in other words, by avoiding the high-power-dissipation transition region. The switch in a voltage converter needs to change states, however, which makes transitions unavoidable. A 1 MHz PWM waveform will have one million rising-edge transitions and one million falling-edge transitions per second, regardless of the duty cycle.
When the load circuitry requires very little current, the switch doesn’t need to spend much time in the on state. With PWM, these low-load situations require the same number of transitions per second as high-load ones, meaning that energy is wasted due to transitions that a different control scheme would render unnecessary.
PFM is that different control scheme. As required load current decreases, pulse frequency can decrease along with it, to the point where the number of PFM transitions per second is significantly lower than it would be in a corresponding PWM waveform. Fewer transitions means less wasted energy, and less wasted energy means that the regulator circuit is operating more efficiently.
The bottom line: PFM allows for higher efficiency during light-load conditions. Under heavy-load conditions, however, the disadvantages of PFM become apparent.
The Disadvantage of Pulse-Frequency Modulation
Switch-mode voltage conversion produces switching noise that can negatively affect other circuitry via conduction and RF emissions. This noise cannot be completely eliminated—instead, integrated circuit (IC) designers work to mitigate its effect on the system. It’s easier to do this when the switching frequency is stable, for the following reasons:
- Filtering noise is easier if you know the frequency of the noise.
- Sensitive frequencies are more easily avoided if the regulator’s frequency doesn’t change.
- Fixed-frequency operation allows multiple regulators to be synchronized.
PFM throws a wrench into all of these techniques. Unlike PWM, PFM does not maintain a constant or predictable switching frequency, and as a result it exacerbates noise and EMI issues, including output ripple. Higher output ripple is a potential side effect of PFM control.
Two Control Schemes Are Better Than One
When less load current is needed, PFM improves efficiency. The lower switching frequencies that occur under these conditions are less likely to cause problematic interference. When more load current is needed, however, PWM facilitates noise countermeasures without reducing efficiency through excessive switching.
To maximize efficiency under both heavy-load and light-load conditions, IC designers have therefore created regulators that switch back and forth between PWM and PFM in response to changes in load current. Figures 2 and 3 show the operation of one such circuit, the MAX17503, in PWM mode and PFM mode, respectively. Compare the efficiency values for ILOAD = 100 mA in each plot, and you’ll see the beneficial effect of operating in PFM mode.
Figure 2. PWM mode circuit efficiency vs. load current for the MAX17503. Image used courtesy of Analog Devices
Figure 3. PFM mode circuit efficiency vs. load current for the MAX17503. Image used courtesy of Analog Devices
Due to the proliferation of small, battery-powered systems that spend large amounts of time in various types of low-current operational modes, low-load efficiency has become increasingly important. If you design compact electronic devices and need to maximize battery life, pulse-frequency modulation is a valuable tool. In my next article, I’ll show you how to simulate PFM for a switching regulator in LTspice.