Even switch mode power supplies with fixed switching frequency may not always exhibit a continuous pulse pattern. Sometimes pulses may appear to be missing for various reasons. This is important with regard to output ripple voltage and EMI.

Switching regulators for voltage conversion operate using variable or fixed switching frequencies. This value is listed on the first page of the switching regulator IC data sheet. The choice of switching frequency for a power supply circuit has important implications for several reasons. It affects the size and cost of external passive components. The switching frequency also affects the achievable conversion efficiency. In addition, the choice of switching frequency can affect not only the power converter but also other circuit parts of the entire system. Therefore, the switching frequency should be chosen so as to minimize disturbance to the entire system. This is because the switching frequency of the power supply can cause capacitive and inductive coupling to other parts of the system due to parasitic components on the PCB.

Circuit designers are sometimes surprised when they evaluate an actual circuit after selecting a switching frequency that they think is appropriate. This is because the designed circuit may appear to behave differently than expected at the selected switching frequency. There are two reasons for this.

Many applications require high conversion efficiency even at very low output loads. When the required output power is only a few milliwatts, the supply current from the switching regulator may be too high for the required output power. This is especially important when efficiency is expressed as a percentage. In such cases, the switching regulator IC may have a special burst mode to improve efficiency. Figure 1 shows the voltage over time of a switching regulator in Burst Mode®. The switching node switches once, followed by a long quiescent phase. During this quiescent phase, many functions of the switching regulator IC go into sleep mode and require very little energy. Figure 1 shows the switching node voltage, inductor current, and output voltage.





When operating in burst mode, the voltage ripple of the output voltage increases. The frequency is much lower than the normal operating frequency. Depending on the voltage converter IC and circuit conditions, burst operation can be implemented using a very small number of pulses, such as one pulse, or a large number of pulses. Typically, as many pulses as necessary are generated until the output voltage reaches a predetermined upper threshold. Then, a stationary phase follows, where the output voltage drops until it reaches a minimum threshold. During this time, switching still occurs at the selected switching frequency, but at a much lower frequency in the spectrum, depending on the burst phase and the stationary phase.

Another case is pulse skip mode. Pulse skip mode can be seen in various power converters. In many topology designs, whenever a pulse is generated at the switching node, a certain amount of energy is transferred from the input side to the output side of the power converter according to the minimum on time. If the load does not need energy or only needs a small amount, the output voltage increases. In this case, some pulses can be skipped to prevent the output voltage from increasing excessively. In this case, the voltage ripple of the output voltage also increases. Pulse skip mode is activated by the overvoltage comparator at the feedback node. For example, if every second pulse is skipped, the frequency shown on the spectrum (FFT representation) will be half of the set switching frequency.





Unlike burst mode, pulse skip mode does not save much energy as it only maintains the output voltage within a certain range, and therefore improves conversion efficiency only slightly.

As discussed earlier, if a switching regulator appears to be switching at a frequency different from the set switching frequency, this could be due to burst mode or pulse skip mode.

There may be other reasons why a switching node may show discontinuous pulses, such as control loop instability, reaching a set current limit, or becoming hotter than its thermal shutdown limit.

Switch mode power supplies can exhibit pulses at frequencies other than the set switching frequency. This is usually the case under light load conditions. When evaluating switch mode power supply circuits, it is important to understand the mechanism by which this behavior occurs. This allows design engineers to design power supplies with confidence that they will operate reliably.

※ About the author
Frederik Dostal, FAE, Analog Devices, Inc., holds a degree in microelectronics from the University of Erlangen, Nuremberg, Germany. He began his career in 2001 in the Power Management business unit and held various application positions, including four years in Phoenix, Arizona, where he worked on switch mode power supplies. He joined Analog Devices in 2009 and is currently a Field Application Engineer (FAE) for Power Management at Analog Devices in Munich.