The worldwide supply
of multilayer ceramic
capacitors (MLCCs) is
not keeping up with demand. This is due in no small part to increased electronic complexity
of cell phones, increased sales of electric cars, and a worldwide
expansion of electronic content across industries. Some smartphones have
doubled MLCC usage over a few years; an electric vehicle can quadruple usage
over a typical modern internal combustion engine (Figure 1). The supply shortage of MLCCs,
appearing near the end of 2016, has made it especially difficult to obtain
large-capacity products (several tens of µF or more) necessary for the operation of prolific
power supplies used in the latest electronics.
Manufacturers looking to reduce their MLCC requirements inevitably look
to the capacitor requirements of power supplies—in particular, switching
regulators. This places power supply designers on the front lines of mitigating
the cap shortage.
Figure 1.
Increases in worldwide MLCC use in electric automobiles (a) and cell phones
(b), without commensurate increases in production, have led to shortages.1
Power Circuits Use Capacitors,
A Lot of Capacitors
电源电路使用电容——大量电容
A typical dc-to-dc
buck converter uses the following capacitors (see Figure
2):
典型的直流-直流降压变换器使用下列电容（参见图2）：
Output capacitor: Smooths out both output voltage
ripple and supply load current during load transients. Generally, a large
capacitor measuring several tens of μF to 100 μF is used.
Input
capacitor: In addition to stabilizing
the input voltage, it plays the role of instantaneously supplying the input current.
In general, several μF to several tens of μF are used.
输入电容：除了稳定输入电压之外，它还被用于输入电流的即时供应。一般在几μF到几十μF之间。
Bypass capacitor: Absorbs noise generated by
switching operation and noise from other circuits. 0.01 μF to 0.1 μF are generally used.
旁路电容：吸收开关操作产生的噪声和来自其他电路的噪声。一般在0.01 μF到0.1 μF之间。
Compensation capacitor: It secures the phase margin in the feedback loop and
prevents oscillation. Several hundreds of pF or several tens of nF are often
used. Some switching regulator ICs incorporate the compensation
capacitor.
The best way to reduce capacitance is to focus on
minimizing the output capacitors. A strategy for reducing output capacitance is
explored next, followed by solutions to reducing bypass capacitor requirements
and, to some extent, input capacitors.
Figure 2.
Capacitors used in a typical buck regulator.
图2.典型降压稳压器使用的电容。
Increase Switching
Frequency to Reduce Output
Capacitance
增加开关频率，以降低输出电容
Figure 3a shows a typical current-mode buck converter
block diagram, with the shaded area denoting the feedback loop and the
compensation circuit.
图3a显示的是典型的电流模式降压变换器的框图，下部电路区域表示反馈回路和补偿电路。
The characteristic of the feedback
loop is 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 for a regulator supporting a rapid load current
increase from 1 A to 5 A. The results are shown for crossover
frequencies of 20 kHz and 50 kHz, resulting
in 60 mV and 32 mV dropouts, respectively.
Figure 3. Block diagram of a typical
buck regulator (a) and typical feedback characteristic (b).
图3.典型降压稳压器(a)的框图和典型的反馈特性(b)。
Figure 4.
Comparing the load step responses of a buck regulator at two crossover
frequencies.
图4.比较采用两种交越频率时，降压稳压器的负载阶跃响应。
On the surface, increasing the crossover frequency looks like an easy choice: load step response is improved by minimizing the output voltage
drop, so the output
capacitor can be reduced. Raising the crossover frequency, though, brings up two issues. First, it is necessary
to secure a sufficient phase margin of the feedback loop to prevent
oscillation. Generally, a phase margin
of 45° or more (preferably 60° or more) is required
at the crossover frequency.
The other issue is the relationship between switching frequency (fSW) and fc. If they are
similar in magnitude, negative feedback can respond to the output voltage
ripple, threatening stable operation. As a guideline, set the crossover
frequency to one-fifth (or less) of the switching frequency, as shown in Figure 5.
Figure 5.
If the switching frequency and control loop crossover frequency are too close,
the negative feedback may respond to output voltage ripple. It is best to keep
the crossover frequency below one-fifth of the switching frequency.
To increase the crossover frequency, you must also raise the switching
frequency, which in turn results in higher switching losses via the top and
bottom FETs, reducing conversion
efficiency and generating additional heat. Any savings in capacitance is offset
by the complexity of additional heat mitigation components: fins, fans, or
additional board space.
Is it possible to maintain high efficiency at high frequency operation?
The answer is yes. A number of Power by Linear™ regulator ICs from Analog Devices do
just that by incorporating a unique FET control that keeps efficiency high even
at higher switching frequencies (Figure 6).
是否能够在高频率下保持高效率？答案是肯定的。使用ADI公司提供的Power
by Linear™稳压器IC就可以达到这种效果，这些稳压器IC采用独特的FET控制功能，在更高开关频率下也能保持高效率（图6）。
For example, the LT8640S 6 A output buck regulator maintains greater than 90% efficiency over its full load
range (0.5
A to 6 A) while
operating at a frequency of 2 MHz (12 V input and 5 V output).
This regulator also lowers the capacitance requirements by reducing inductor current ripple (ΔIL),
which in turn reduces the output ripple voltage (ΔVOUT)
as shown in Figure 7. Likewise,
a much smaller inductor can be used.
With a higher
switching frequency, the crossover frequency
can be increased, improving load step response and load regulation, as shown in Figure 8.
开关频率更高时，可以增加交越频率，以改善负载阶跃响应和负载调整，如图8所示。
Figure 6. Power
by Linear regulators vs. competition. In a typical regulator, when the switching frequency goes up, efficiency
goes down. ADI Power by Linear regulators can maintain high efficiency at very
high operating frequencies, enabling the use of smaller value output
capacitors.
图6.Power by Linear稳压器与竞争产品。对于典型的稳压器，开关频率增高时，效率会下降。ADI的Power by Linear稳压器可以在非常高的操作频率下保持高效率，因而支持使用值更小的输出电容。
Figure 7. Increase switching frequencies to reduce
capacitor and inductor size.
图7.通过增加开关频率来减小电容和电感的尺寸。
Figure 8. Increased switching frequency results in
improved load step response.
How about reducing bypass capacitance?
The main role of the bypass capacitor is to absorb the noise
generated by switching operation itself. If switching noise is reduced in other
ways, the number of bypass capacitors can be reduced. A particularly easy way
to achieve this is through the use of a Silent Switcher® regulator.
How does a Silent
Switcher regulator reduce switching noise? A switching regulator has two current loops: when the top FET is on and the bottom
FET is off (red loop) and when the top FET is off and the bottom
FET is on (blue loop) as shown in Figure 9. The hot loop carries
a fully switched ac current—that is, switched from zero to IPEAK and back to zero.
It has the highest ac and EMI energy, as it produces the strongest
changing magnetic field.
Figure 9. The hot loop in a switching regulator produces
the bulk of the radiated noise because of the alternating magnetic field it
generates.
图9.开关稳压器中的热回路会因为本身产生的交变磁场而导致大量辐射噪声。
Slew-rate control can be used to suppress
switching noise by slowing
the rate of change of the gate signals (lowering di/dt). While effective in suppressing the noise,
this increases switching losses, producing additional heat, especially at
high switching frequencies as previously described. Slew-rate control is effective under certain conditions and Analog Devices
also offers solutions with this feature.
Silent Switcher regulators suppress
electromagnetic noise generated from the hot loop without slew-rate control. Rather it splits the VIN pin in two,
allowing the hot loop to be split into two symmetrical hot loops. The resulting magnetic
field is confined
to the area near the IC, and significantly
reduced elsewhere, thus minimizing radiated
switching noise (Figure 10).
The LT8640S, the second generation of this technology—Silent Switcher 2 (Figure 11)—incorporates the input
capacitors in the IC. This ensures maximum noise suppression,
eliminating the need to carefully position the input caps in the layout. This feature, of course, also reduces the
MLCC requirements. Another feature, spread spectrum frequency
modulation, lowers noise peaks
by dynamically changing
the switching frequency. The combination of these
features enables the LT8640S to
clear CISPR 25 Class 5 EMC standards for automobiles with ease (Figure 12).
Figure 12. The combination of
noise suppression features in a Silent Switcher 2 device, such as the LT8640S,
enables easy clearance
of CISPR 25 Class 5 peak limits even while reducing input and
bypass capacitance.
Power by Linear devices from ADI can help reduce MLCC requirements, helping
designers ride through
the MLCC shortage.
Output capacitance requirements
are reduced by using high frequency
operation while maintaining uncommonly high efficiency. Devices
that feature Silent Switcher architecture significantly suppress EMI noise,
reducing bypass capacitor requirements. Silent Switcher 2 devices further reduce MLCC needs.
ADI公司提供的Power by Linear器件有助于降低MLCC要求，从而帮助设计人员解决MLCC短缺问题。可以通过使用高频率操作来降低输出电容要求，同时保持出色的高效率。采用Silent
Switcher架构的器件可以大幅抑制EMI噪声，从而降低旁路电容要求。Silent Switcher 2器件进一步降低了对MLCC的需求。
Atsuhiko
Furukawa joined Linear Technology (now part of Analog
Devices) in 2006. He has provided technical support for various
applications to small and mid-size
customers for over 10 years. He transitioned
to the automotive segment in 2017 and is now designing huge (several kW) as well
as small safety automotive applications. Atsuhiko is a marathon runner with his
best record being 3 hours and 3 minutes. He can be reached at atsuhiko.furukawa@analog.com.
