In the first installment of this series, I stated that there are many reasons for switched-mode power supply (SMPS) instability, only one of which is that the control loop has insufficient gain or phase margin. In this installment, I will offer some tips about identifying and curing subharmonic oscillations in peak current mode (PCM)-controlled SMPS systems and talk briefly about input-filter oscillations.
There is a well-known, inherent instability in continuous conduction mode (CCM) PCM control loops when they operate at duty cycles greater than 50%, as shown in Figure 1. Discontinuous conduction mode (DCM), transition mode (TM), average current mode (ACM) and voltage mode-controlled (VMC) systems are not susceptible to this type of instability. But be careful – because DCM, TM, ACM and VMC systems often use PCM control when they operate in current limit.
Figure 1: Subharmonic oscillations
Diagnosis and solution
Subharmonic oscillations appear as large changes of duty cycle from cycle to cycle. They usually persist because the average duty cycle remains greater than 50%, but they can appear transiently if a load change causes the controller to run at a more than 50% duty cycle for a few cycles. It’s also worth noting that without slope compensation, current perturbations take longer and longer to die out as the duty cycle increases towards 50%. Here is a short list of the behaviors you might see.
Most power supplies present a constant power load to their inputs and therefore have a negative incremental input resistance. This means that the input current will decrease as the input voltage increases. In an offline power factor correction (PFC) stage, the current control loop forces the system to emulate a positive resistance at line frequencies so that the input current follows the sinusoidal shape of the input voltage. But the negative input resistance behavior is present at frequencies beyond the control loop crossover.
DC/DC and offline AC/DC converters will normally have some form of input filter like that shown in Figure 2. This filter is necessary to meet conducted electromagnetic interference (EMI) requirements but it can oscillate under some circumstances if not designed correctly. This topic has been widely discussed in literature, but the summary rule is simple enough: the output impedance of the filter must be less than the input impedance of the converter at all frequencies.
Figure 2: Typical AC/DC converter input filter, with the SMPS input impedance in green, the undamped filter output impedance in red and the damped filter output impedance in blue
The simplest way to identify an input-filter oscillation is to remove the input filter by short-circuiting the input-filter inductors. The filter will normally oscillate at one or other of the resonances of the filter. These resonances are normally in the range between 1kHz to 10kHz depending on the filter design. Curing input-filter oscillations requires modifying the input filter to reduce its output impedance while maintaining its effectiveness. Here are the two things to try:
So far, I have discussed classic feedback loop instability, subharmonic oscillations and input-filter oscillations. In the next installment, I’ll look at oscillations caused by remote sensing connections.
All content and materials on this site are provided "as is". TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with regard to these materials, including but not limited to all implied warranties and conditions of merchantability, fitness for a particular purpose, title and non-infringement of any third party intellectual property right. TI and its respective suppliers and providers of content make no representations about the suitability of these materials for any purpose and disclaim all warranties and conditions with respect to these materials. No license, either express or implied, by estoppel or otherwise, is granted by TI. Use of the information on this site may require a license from a third party, or a license from TI.
TI is a global semiconductor design and manufacturing company. Innovate with 100,000+ analog ICs andembedded processors, along with software, tools and the industry’s largest sales/support staff.