“Globally, multilayer ceramic capacitors (MLCCs) are in short supply. Much of this is due to the increased Electronic complexity of cell phones, increased sales of electric vehicles, and the expansion of electronic content across industries across the globe. Some smartphones have doubled their MLCC usage compared to a few years ago; EVs have at least a 4x increase in MLCC usage compared to cars using a typical modern internal combustion engine (Figure 1). MLCCs have been out of stock since late 2016, making it especially difficult to produce products with large capacitance values (tens of µF or more), which are required for high-energy power supplies used in the latest electronics to operate.
Globally, multilayer ceramic capacitors (MLCCs) are in short supply. Much of this is due to the increased electronic complexity of cell phones, increased sales of electric vehicles, and the expansion of electronic content across industries across the globe. Some smartphones have doubled their MLCC usage compared to a few years ago; EVs have at least a 4x increase in MLCC usage compared to cars using a typical modern internal combustion engine (Figure 1). MLCCs have been out of stock since late 2016, making it especially difficult to produce products with large capacitance values (tens of µF or more), which are required for high-energy power supplies used in the latest electronics to operate. Manufacturers wanting to reduce MLCC requirements inevitably want to start with the capacitance requirements of the power supply, especially the capacitance of switching regulators. Therefore, the power supply designer becomes the key to solving the problem of capacitor shortage.
Figure 1. Global use of MLCCs for electric vehicles (a) and mobile phones (b) has increased without a corresponding increase in production, resulting in a shortage of MLCCs. 1
Capacitors used in power circuits – a large number of capacitors
A typical DC-DC buck converter uses the following capacitors (see Figure 2):
Output Capacitor: Smooths output voltage ripple and power supply load current during load transient response. Generally, large capacitors ranging from tens of μF to 100 μF are used.
Input Capacitance: In addition to stabilizing the input voltage, it is also used for the immediate supply of input current. Generally between a few μF to tens of μF.
Bypass Capacitor: Absorbs noise generated by switching operations and noise from other circuits. Typically between 0.01 μF and 0.1 μF.
Compensation Capacitor: Guarantees phase margin in the feedback loop and prevents oscillation. Usually several hundred pF or tens of nF. Compensation capacitors are used in some switching regulator ICs.
The best way to reduce capacitance is to find a way to minimize the amount of output capacitance. This article will next describe strategies for reducing input capacitance, then introduce solutions to reduce bypass capacitance requirements and, to some extent, reduce input capacitance.
Figure 2. Capacitors used in a typical buck regulator.
Increase the switching frequency to reduce the output capacitance
Figure 3a shows a block diagram of a typical current-mode buck converter, with the lower circuit area representing the feedback loop and compensation circuit.
The characteristics of the feedback loop are shown in Figure 3b. The frequency at which the loop gain is 0 dB (gain = 1) is called the crossover frequency (fC). The higher the crossover frequency, the better the load step response of the regulator. For example, Figure 4 shows the load step response of a regulator that supports a rapid increase in load current from 1A to 5A. The results shown correspond to crossover frequencies of 20 kHz and 50 kHz, resulting in voltage drops of 60 mV and 32 mV, respectively.
Figure 3. Block diagram of a typical buck regulator (a) and typical feedback characteristics (b).
Figure 4. Compare the load step response of a buck regulator with two crossover frequencies.
On the face of it, increasing the crossover frequency seems like an easy solution: you can improve the load step response by minimizing the output voltage drop, thereby reducing the amount of output capacitance. However, increasing the crossover frequency causes two problems. First, it is necessary to ensure that the feedback loop has sufficient phase margin to prevent oscillation. In general, a phase margin of 45° or more (preferably 60° or more) is required with this crossover frequency.
Second, the relationship between switching frequency (fSW) and fc needs to be noted. If they are of equal magnitude, negative feedback will respond to output voltage ripple, affecting stable operation. As a guide, the crossover frequency can be set to 1/5 (or lower) of the switching frequency, as shown in Figure 5.
Figure 5. Negative feedback may respond to output voltage ripple if the switching frequency and the control loop crossover frequency are too close. It is best to keep the crossover frequency lower than 1/5 of the switching frequency.
Increasing the crossover frequency requires increasing the switching frequency at the same time, however, this results in increased switching losses in the top and bottom FETs, which reduces conversion efficiency and generates more heat. The savings achieved on capacitors are offset by the complexity of adding heat sink components: such as finned heat sinks, fans, or extra board space.
Is it possible to maintain high efficiency at high frequencies? The answer is yes. This can be achieved using Power by Linear™ regulator ICs from Analog Devices, which employ a unique FET control feature that maintains high efficiency at higher switching frequencies (Figure 6).
For example, the LT8640S 6 A output buck regulator maintains greater than 90% efficiency over the entire load range (0.5 A to 6 A) when operating at 2 MHz (12V input and 5V output).
This regulator also reduces capacitance requirements by reducing current ripple (ΔIL), which in turn reduces output ripple voltage (ΔVOUT), as shown in Figure 7. Alternatively, use a smaller Inductor.
At higher switching frequencies, the crossover frequency can be increased to improve load step response and load regulation, as shown in Figure 8.
Figure 6. Power by Linear regulators versus competing products. For a typical regulator, efficiency decreases as the switching frequency increases. ADI’s Power by Linear regulators maintain high efficiency at very high operating frequencies, allowing the use of smaller value output capacitors.
Figure 7. Capacitor and inductor size reduction by increasing switching frequency.
Figure 8. Increasing the switching frequency improves load step response.
Silent Switcher Regulators Can Dramatically Reduce Bypass Capacitors
What if I reduce the number of bypass capacitors? Bypass capacitors are mainly used to absorb noise generated by switching operations. If switching noise can be reduced in other ways, the number of bypass capacitors can be reduced. There is a particularly simple way to achieve this effect, using a Silent Switcher® regulator.
How do voltage regulators reduce switching noise? A switching regulator has two current loops: the top FET is on and the bottom FET is off (red loop); the top FET is off and the bottom FET is on (blue loop), as shown in Figure 9. The hot loop delivers fully switched AC current, that is, switches from 0 to IPEAK and back to 0. It has the highest AC and EMI energy and produces the strongest changing magnetic field.
Figure 9. Hot loops in switching regulators can cause a lot of radiated noise due to their own alternating magnetic fields.
Slew rate control can be used to reduce the frequency of gate signal changes (reduce di/dt) in order to suppress switching noise. This approach, while suppressing noise, increases switching losses and results in more heat, especially at the high switching frequencies described earlier. Slew rate control is effective under certain conditions, and Analog Devices also offers solutions that include this control.
Silent Switcher regulators suppress electromagnetic noise generated in hot loops, but instead of using slew rate control. Instead, the VIN pin is split in two, allowing the hot loop to be split into two symmetrical hot loops. The resulting magnetic field is confined to areas close to the IC and substantially reduced elsewhere, minimizing radiated switching noise (Figure 10).
Figure 10. Patented Silent Switcher technology.
The LT8640S is the second generation of Silent Switcher technology, namely Silent Switcher 2 (Figure 11), which integrates high-frequency input capacitors inside the IC. This ensures maximum noise rejection and eliminates the need to place the input capacitors in the layout as carefully as before. Undoubtedly, this will also lower the requirements for MLCCs. Another feature, spread spectrum, reduces noise peaks by dynamically changing the switching frequency. Combining these features, the LT8640S easily meets the CISPR 25 Level 5 EMC automotive standard (Figure 12).
Figure 11. Silent Switcher 2 technology from Analog Devices integrates input capacitors in the IC, thereby simplifying layout and improving noise rejection.
Figure 12. The use of these noise reduction features in Silent Switcher 2 devices such as the LT8640S allows the product to easily meet CISPR 25 Class 5 peak limit standards, even with reduced input and bypass capacitance.
Power by Linear devices from Analog Devices help reduce MLCC requirements, helping designers address MLCC shortages. Output capacitance requirements can be reduced while maintaining excellent high efficiency by using high frequency operation. Devices using the Silent Switcher architecture can significantly suppress EMI noise, thereby reducing bypass capacitor requirements. Silent Switcher 2 devices further reduce the need for MLCCs.