Abe Ibraheim, Central Applications Intern, Kenneth Armijo, Central Applications Engineer, and Piyu Dhaker, Staff Engineer
Why is my coil making sound or overheating?
Possibly it is a poor sizing of the coil or a failure to comply with the minimum saturation current supported, which can cause different problems in DCDC converters, such as ringing noise or overheating.
This article is the first in a series that will discuss common switching power supply (SMPS) design errors as well as their rectification. The goal is to address the complications that arise with the power stage design of DC-DC switching regulators, focusing on the coil. Designers opt for coil values outside the recommended range to gain one of the following benefits: smaller output ripples or minimizing the solution footprint. However, selecting components with values that are too large or too small has unintended consequences that can lead to serious damage to the chip as well as decreased efficiency. This article also examines what happens when proper care is not taken to ensure that the charging current does not exceed the maximum saturation of the coil.
What is a switching power supply?
A switched mode power supply (SMPS) is a type of high-efficiency regulator that either reduces an input voltage (converter buck), the input voltage increases (converter boost) or both (buck boost). In Figure 1 you can see the basic topologies of this type of regulators.
Figure 1. Common switched regulator topologies and their output formula.
All SMPS switching power supplies work by storing energy in the coil and use PWM (Pulse Width Modulation) techniques to obtain the desired output. The principle that regulates these converters is given by what is known in English as Volt second Balance Law which dictates that the average current in a coil over a period, when operating in a steady state, must be zero. This means that the coil must discharge all the energy stored during the charging stage, before starting a new period.
Converter operation buck
Only converters are used in this article. buck (reducers) to demonstrate common design errors. Four components make up the power stage of a buck converter: the coil, the output capacitor, the upper FET represented by a switch, and the lower FET, which is represented by a diode (see Figure 2).
Figure 2. Simplified power stage of the buck converter
The voltage across the coil is VL = L diL/dt. This voltage is the difference between the output voltage and the voltage at the switching node. When the upper FET is activated (ON state), VL It is the difference between the input and the output. However, when the upper FET is disabled (OFF state), the difference is 0V minus the output, since the switching node is grounded.
diL/dt (∆iL) is the change in coil current per unit time, commonly known as the coil current. ripple of the coil. When the upper FET is closed (and the lower one open), the coil stores energy in the form of magnetic flux as the current passing through the coil increases. When the upper FET is open and the magnetic field collapses, the lower FET creates a path towards GND allowing current to flow towards the output as it decreases. This can be seen in the coil current waveform shown in Figure 3. The output capacitor is used to smooth out the output ripple and to help maintain the output voltage. The output voltage of a buck converter is obtained as VOUT = D.V.IN, where D is the duty cycle (duty cycle) and is defined as the percentage of time in which the upper FET remains active and is loading the coil.
Figure 3. Waveform of the current in the coil. The current through the coil charges when the upper FET is active and discharges when it is not.
Recommended Coil Sizing
When designing an SMPS source, the correct value of inductance must be selected to ensure an acceptable ripple current (∆iL). For type converters buck It is recommended that the ripple current be between 30% and 40% of the load current. This range is considered optimal since it is large enough to provide a sufficient signal for the feedback system (feedback) of current control, but not too large to prevent the converter from operating in discontinuous conduction mode (DCM). discontinuous driving mode). This mode is a state where the ripple current is very large and needs to cross the 0A line to keep the charging current at the desired value. However, when the current reaches 0A, the internal diodes of the FETs no longer conduct, preventing the current from dropping below 0A.
A general way to select the correct coil can be obtained by the formula:
This formula shows that switching frequency and inductance are inversely proportional, meaning that at higher frequencies, charging time is reduced, allowing proper operation with a smaller coil (saving footprint size and cost). ).
Saturating the coil
One of the most common and catastrophic mistakes in SMPS design is neglecting the saturation current limit when selecting the coil. When the current through the coil exceeds the rated saturation current, the inductor core saturates, meaning that the magnetic field generated will no longer increase proportionally to the current drawn. This disrupts the balance law, leading to a loss of linear characteristics in both the ripple current and the output voltage ripple. When the iron core becomes saturated, it rapidly loses inductance, behaving more like a resistor than an inductance. Since the series resistance (ESR) of the coil increases and the inductance decreases, the change in current is forced to increase to satisfy the balance. The peak observed in the saturated current waveform is due to the exponential increase in current slope and can be seen in Figure 4. This current peak is carried over to the output voltage, resulting in more noise and voltage spikes, as seen in Figure 5. Noise and voltage spikes can potentially damage downstream components if the voltage rises too high and exceeds the maximum voltage rating of a downstream component, as well as degrade the performance of EMI.
Figure 4. Current waveform of a saturated coil. The waveform behaves normally until the current exceeds the rated saturation current.
Figure 5. Output voltage ripple with a saturated coil. The peaks are transferred to the output in the form of voltage spikes and noise.
Additionally, at high current fluctuations, the coil experiences rapid hysteresis loss leading to excessive heat dissipation, as seen in Figure 6, as well as audible noise. This excess heat can damage other nearby components, especially the regulator itself.
Figure 6. Dissipation of a saturated coil – 107.78°C (226ºF).
To avoid this problem, designers should choose coils with a rated saturation current at least twice the expected maximum current. When calculating the maximum current, it is important to take into account the ripple current as well as the load current of the output. Additionally, designers can consult the data sheet of the selected coil to find out at what current the inductance drops between 10% and 30% of its original value, which is where saturation is defined. Choosing a coil with the appropriate rated saturation current will result in normal operation of the system, with a linear current through it as seen in Figure 7; and the voltage peaks at the output will disappear, as seen in Figure 8. Finally, the system will operate at a much lower temperature, as seen in Figure 9, stressing the device less and improving its useful life.
Figure 7. Waveform of a coil within the nominal current range.
Figure 8. Output voltage ripple with a coil within its nominal current range.
Designers should be careful when choosing the coil so that it provides a ripple current of approximately 30% to 40%. This will reduce the magnitude of the ripple current and return the device to its continuous mode (CCM) from DCM, as seen in Figure 12. This will also improve output voltage ripple and eliminate surge spikes. voltage, as seen in Figure 8. If a designer is having trouble calculating the desired coil value and choosing a viable component, they can use LTPowerCAD to support their design and to select power stage components.
Figure 9. Dissipation of a coil within its nominal current range 37.61°C (99.7ºF).
Complications with coils below what is recommended
Designers often prefer smaller coils to save space, as lower value coils typically have smaller physical dimensions due to fewer turns. However, if the inductor is too small, the ripple current will be large and force the converter into DCM, which is undesirable for the SMPS converter because the device will be less efficient and will exhibit worse electromagnetic interference (EMI) performance. ). This degraded EMI performance can be seen in the presence of oscillations (ringing) at the switching node, caused by parasitics, as well as by the LC circuit (creating a resonant circuit), which can be seen in Figure 10. This ringing will carry over to the output voltage, causing higher ripple and more voltage spikes, as seen in Figure 11. Additionally, the power supply would no longer be in continuous conduction mode (CCM) and the output formulas for an SMPS converter no longer apply.
Figure 10. Current waveform of an incorrectly sized coil. The ringing present in the current and in the sense resistance indicates that the converter is in DCM mode.
Figure 11. Output waveform with an incorrectly sized coil. Oscillations can be observed in the switching node.
Figure 12. Coil current waveform within its nominal range.
Complications with coils above what is recommended
The downstream electronic components connected to an SMPS typically have specified supply voltages with an associated tolerance. If the ripple in the voltage is too large, it will drastically affect the performance of the system. For example, if a microcontroller has a power specification of 3.3 V ±50 mV, having ripple greater than ±50 mV can cause the microcontroller to shut down. One way designers often try to mitigate this curl is by increasing the size of their coil. However, if the coil is sized too large, both the ripple current and the output voltage ripple decrease significantly. Although this sounds desirable, it will lead to problems with the feedback system and may also result in a much slower transient response. A little ripple will make it extremely difficult to detect any change by the sense resistor, distorting the usual triangular waveform that is passed to the feedback loop. When the ripple current is small, the signal-to-noise ratio (SNR) also deteriorates causing the feedback loop to register noise as the coil signal, resulting in unwanted instability in the output, manifested as a fluctuation (see Figure 13).
Figure 13. Jitter caused by output instability. The waveform of an oversized coil is displayed with the persistence function to view instability.
Additionally, with higher value coils, the rated saturation current is usually lower. This can cause coil saturation, which is dangerous for the device, as explained in the “Saturating the coil” section. The effect of saturating an extremely oversized coil can be seen in Figure 14.
Figure 14. Waveform of a saturated coil with a coil 22 times the nominal value. The current range does not increase proportionally with the inductance.
To mitigate this problem, designers should keep in mind that output ripple can be controlled by altering the output capacitors. By increasing the value of the output capacitor or decreasing its ESR, the ripple can be reduced without having to increase the value of the inductor. This will allow the coil ripple current to remain at a value between 30% and 40%, allowing the detection architecture to correctly acquire the signal. This can be seen in Figure 15.
Figure 15. Waveform in the sensing resistance with nominal values.
Conclusion
This article serves as a guide to analyze the coil design problems in the case of type converters. buck. Additionally, it aims to provide practical solutions in case designers see any of the unwanted performances shown here. Maintaining the ripple current in the range of 30% to 40% of the output through proper coil sizing is critical to ensuring that the device remains in CCM and does not cause unwanted fluctuations or saturation, which could be fatal to the load or the regulator chip itself.
