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## Capacitor Usage
So far, we already encountered capacitors for many different usages:
### Load Capacitors
We have seen load capacitors used with the 2 crystals in the
discussion about [CPU][1].
A quartz crystal always provides both series and parallel resonance,
the series resonance being a few kilohertz lower than the parallel
one.
Crystals below 30 MHz like ours are generally operated between series
and parallel resonance, which means that the crystal appears as an
inductive reactance in operation, this inductance forming a
**parallel resonant circuit** with externally connected parallel
"load" capacitance. Any small additional capacitance added in parallel
with the crystal pulls the frequency lower in the range between the
series and parallel resonance frequencies, insuring crystal startup
and stable operation.
For modern circuits, these load capacitors have a typical small value
< 20 pF.
### Bulk Capacitors
Bulk capacitors are used to prevent a power supply from dropping too
far during the periods when current is not available. At the same
time, they help to reduce the power supply voltage ripples by
smoothing their output voltage.
Many such capacitors are used at both the input and output of the
numerous linear and switched mode power supplies in the [PMIC
discussion][2].
The main bulk capacitor value is generally high (some µF), but there
may be smaller parallel capacitors added for stability.
### Coupling Capacitors
As you probably know, capacitors are made of 2 parallel conductive
electrodes separated by a (thin) isolating dielectric material (even
if these electrodes are rolled or layered to reduce the component
size). Thus by construction, no DC (Direct Current) can flow from one
electrode to the other, but by influence using the electric field, AC
(Alternative Current) still can go through. This is how coupling
capacitors are used to link 2 circuits while removing any DC bias
voltage on one side or the other of the capacitor.
We use such a coupling capacitor in the [Audio schematic
description][3] for feeding the audio power amplifier from the CPU
audio output.
### Filter Capacitors
We have seen many examples where capacitors are used within passive
filter circuits along with resistors or inductors, mainly to remove
unwanted frequencies from a power supply or a signal.
### Decoupling (Bypass) Capacitors
We use some decoupling capacitors in the [buttons circuit][4].
Active components such as transistors and chips are connected to their
power supplies through conductors featuring a (small) common impedance
made up of complex (resistive, capacitive and inductive)
value. Because of these parasitic components, a device that suddenly
draws some current in spikes will generate a drop in its voltage power
supply. If many devices are sharing the same power supply and
impedance, the state of one device will be coupled to the other ones
through the common impedance of the power supply conductors and may
affect their operation.
In order to decouple the devices, capacitors placed as close as
possible to the device power supply input pins are used, which act as
local energy storage. These capacitors are also named "bypass
capacitors" as they shunt transient energy from the power supplies
past the device to be decoupled, right to the GND return path.
There may be different capacitors values placed on the same power
supply pins in order to filter transients at different frequencies:
the bigger the capacitor value, the lower the frequency. A typical
value is 100 nF, and values from 1 µF to 10 µF are used for lower
frequencies and / or higher current draws, while lower values of a few
nF are used for filtering higher frequencies.
In essence, decoupling capacitors are not very different in their
function from bulk capacitors: the only difference is one of scale,
both of current and of transient duration. Bulk capacitors deal with
large currents and periods of 10s of ms, whereas decoupling capacitors
are used for much lower currents and much briefer periods (typically
10s of ns for TTL or CMOS devices) .
## Schematics
The last part of the FunKey schematics merely contains only decoupling
capacitors:
![Decoupling Schematics](/assets/images/Decoupling_Schematics.png){.lightbox}
One exception is the Allwinner V3s CPU HPR/HPL circuit which features
an RC-to-ground circuit between the amplifier and the preamplifier
input with the resistor **R27** and capacitors **C79** and **C81**, as
recommended in the [V3s hardware design guide][5].
The only other remarkable point left in this schematic is the resistor
divider **R25**/**R28** which provides a reference voltage at half the
DRAM power supply voltage level, which is used for the integrated DDR2
DRAM merged drivers and dynamic on-chip termination already discussed
at the end of the previous [CPU schematic description][6].
[1]: /developers/hardware/cpu
[2]: /developers/hardware/power/pmic
[3]: /developers/hardware/audio
[4]: /developers/hardware/buttons
[5]: https://github.com/Squonk42/V3s_Documentation/blob/master/V3s%20hardware%20design%20guide%20V1.0_20150519%20EN%20Non%20Official.pdf
[6]: /developers/hardware/cpu
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A separate [Sylergy SY8088][1] Buck DC/DC SMPS chip is used to provide
the DDR2 +1V8 DDR2 DRAM power.
This is because the AXP20x is originally the PMU (Power Management
Unit) used by most Allwinner SoCs (A10, A13 and A20), which do not
integrate SDRAM, so the board designer has a wide choice of memory
option: DDR2, DDR3, DDR3L, LPDDR3, LPDDR4 with various voltage
requirements.
But no specific PMIC was created for the Allwinner V3s used in the
FunKey device which however integrates a fixed SiP (System In Package)
512Mbit (64MB) DDR2 SDRAM.
We thus have to design a separate SMPS (DC-DC) power supply for
providing the +1.8V 1A required for the DDR2 DRAM power supply.
For this purpose, we followed closely the [Allwinner Reference
Design][2].
Here is the corresponding DRAM Power schematics:
![DRAM Power Schematics](/assets/images/DRAM_Power_Schematics.png){.lightbox}
Nothing very fancy here: the SMPS chip **U4** has its required input
filter capacitor **C37** and output capacitors **C65** and **C73**.
The low-profile ferrite-core power inductor **L6** (rated with a
saturation current of 1.76A and low < 0.1 Ω resistance) provides the
DC-DC energy storage element.
The **R20**/**R23** precision voltage divider provides the required
+0.6V feedback voltage from the +1.8V output voltage by having a 1/3
resistor ratio.
The last component is a pull-up resistor **R19** which ties the SMPS
chip enable input to its active level permanently. The pull-up voltage
is +3.0V (just as in the original reference design), probably as it is
the next higher voltage available, in order to limit the current in it
to its lowest possible value.
[1]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/C79313_SY8088AAC_2017-03-29.PDF
[2]: https://github.com/Squonk42/V3s_Documentation/blob/master/V3S_CDR_STD_V1_0_20150514.pdf
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Looking back at the section on the [CPU schematics][1], the **FunKey
S** device clearly needs a sophisticated power supply in order to
fulfill the CPU power requirements. They are recalled below, along
with the maximum current requirements found in the [Allwinner V3s
reference design][2] (page 3):
- +3.3V / 1.2A for the I/O power supply
- +3.3V_AO / 30 mA for the Always-On power supply (RTC timer)
- +3.0V / 200 mA for the analog power supply
- +1.8V / 1A for the DDR2 DRAM power supply
- +1.25V / 1.6 A for the core power supply
But why in the first place are there so many different power supply
voltages required?
## Power Efficiency
A first answer is: for better power efficiency.
As P = U x I (Electrical power is the product of voltage level by
current intensity), you can reduce power by decreasing the required
current or by reducing the operating voltage. Assuming you already do
your best to reduce the required current, you can still reduce power
by reducing the voltage.
## Reducing Power Supply Voltage
### Voltage Drop
But how far can you go? Over long distance, you have the voltage drop
from the conductor linear resistance, but this effect can be neglected
for small boards.
### Noise Margin
You have inductive and capacitive coupling between conductive wires
and planes too, but within a PCB, these coupling only have a limited
direct effect on voltage. However, these coupling play a role in that
they will pick up external electromagnetic noise from the surroundings
and inject it into the circuit.
And with digital circuits, a critical limit when lowering the
operating voltage is the "noise margin" or difference in absolute
voltage levels between a logical '0' and logical '1', which determines
the maximum amplitude of spurious voltage spikes that a conductor can
pick up that will trigger an erroneous logic level change.
This phenomenon mostly depends on the circuit scale: a long-distance
circuit between boards will require higher voltages (typically +12V or
+24V) to limit this effect, whereas a circuit between boards a few
meters apart or using through-hole chips on the same board wile
require a lower voltage (typically +5V like the old Arduinos). Using
SMT chips will allow even smaller boards and lower voltages (+3.3V is
typical today), and with wires running on the same silicon die, it is
possible to go down to +1.2V, given the current technological limits.
### Voltage Swing
There are other reasons why you should try to minimize voltages: the
core CPU for example needs to run as fast as possible, and lowering
its operating voltage will shorten the signal rise and fall duration
as the voltage swing is reduced.
## Other Power Supply Considerations
Besides reducing the operating voltage, there are other considerations
that may push to multiply the number of power supplies in a design:
### Quiescent Current
As for power supply used for standby operation providing small
currents, a very-low leakage current ("quiescent current") is required
as it can no longer be neglected compared to the current required by
the light load and even more importantly because this current
consumption is permanent.
### Ripple Voltage
For sensitive circuits such as ADCs (Analog to Digital Converters) or
PLLs (Phase-Locked Loops) which rely on comparing very small voltage
differences, a "clean" power supply featuring very low ripple voltage
amplitude is required to achieve a good resolution and/or
accuracy. This characteristic is only possible to obtain using LDOs
and not SMPS, and the figure to pay attention to is then the PSRR
(Power Supply Rejection Ratio) or how much a variation in the input
voltage will affect the output voltage: the higher, the better! A
value > 50 dB is a good starting point.
## Application to the FunKey Design
Based on these considerations, it is now clear that each V3s power
supply voltage has a good reason to exist:
- +3.3V / 1.2A is used for powering the I/Os to connect between chips
on the board. Given the required current, a SMPS is required for
reaching a good efficiency
- +3.3V_AO / 30 mA for the Always-On power supply (RTC timer)
requires a low quiescent-current, so an LDO is used
- +3.0V / 200 mA for the analog power supply also requires an LDO,
this time to minimize the ripple voltage
- +1.8V / 1A for the DDR2 DRAM power supply: this strange voltage
level is typical for DDR2 DRAM memory chips, and is the result of
driving the large memory array inside the chip
- +1.25V / 1.6 A for powering the CPU core to minimize the voltage
swing and increase the possible CPU frequency. Given the required
current, a SMPS is required for reaching a good efficiency, too
[1]: /developers/hardware/cpu#cpu-schematics
[2]: https://github.com/Squonk42/V3s_Documentation/blob/master/V3S_CDR_STD_V1_0_20150514.pdf
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From the previous section, we can summarize the V3s power supply
requirements to:
- SMPS for +3.3V / 1.2A for the I/O power supply
- LDO for +3.3V_AO / 30 mA for the Always-On power supply (RTC timer)
- LDO for +3.0V / 200 mA for the analog power supply
- SMPS for +1.8V / 1A for the DDR2 DRAM power supply
- SMPS for +1.25V / 1.6 A for the core power supply
On the [LicheePi Zero board][1] used in our **[FunKey Zero][2]**
prototype, a triple SMPS [EA3036][3] is used for generating these
+3.3V, +1.8V and +1.2V voltages, with an additional [XC6206][4] LDO
for the +3.0V (the +3.3V Always On is connected directly to
+3.3V). Although compact (the EA3036 is a tiny 3 mm x 3 mm QFN20
package), this solution is not ideal as it does not provide a battery
charger and monitoring capability, which is a requirement for the
**FunKey S** device.
## PMICs
As it is generally the case with such a complex SoC requiring multiple
voltages, high current and proper voltage sequencing, all major
manufacturers provide dedicated companion chips called PMICs (Power
Management Integrated Circuits), in charge of these tasks. Allwinner
is not an exception through its sister company [X-Powers][5].
Their AXP20x products are highly-integrated PMICs that are optimized
for applications requiring single-cell Li-battery (Li-Ion/Polymer),
multiple output DC-DC converters and LDOs. Here is a block diagram:
![PMIC Block Diagram](/assets/images/AXP20x_Block_Diagram.png){.lightbox}
The AXP20x features:
- A wide choice of input power source, the best source is output as
IPSOUT inside the IPS (Intelligent Power Select) block:
- USB VBUS
- Battery BAT
- ACIN wall plug (not used in the **FunKey S**)
- BACKUP battery (not used in the **FunKey S**)
- A 1.8A fast PWM battery charger (also called DC/DC1) with battery
voltage / current sense and programmable charge indication LED
- A soft key power-on/off logic with timer (just as in smartphones!)
- An I2C interface with interrupt signal to communicate with the CPU
- An optional battery temperature monitoring if the battery is
equipped with an NTC resistor (not used in the **FunKey S**)
- A reference voltage
- A built-in 12-channel 12 bit ADC that measures various voltages and
currents data, as well as feeding an internal Coulomb counter and
fuel gauge system (more on this later)
- A "power OK" output used to generate the global RESET signal for the
**FunKey S**
- 5x GPIOs (not used in the **FunKey S**), GPIO0 can be programmed as
LDO5 output
- 2x DC/DC SMPS DC-DC2 and DC-DC3
- 5x LDOs (only 2 are used in the **FunKey S**, LDO5 is optionnaly
output to GPIO0)
Looking at their datasheets, it is difficult to tell the difference
between the [AXP202][6], [AXP203][7] and [AXP209][8] (any hint
welcome!). In the **FunKey S** design, we use an AXP209 because it is
the one that comes along with the V3s when you buy it on AliExpress.
## AXP20x Application Diagram
For complex dedicated chips like this, the best option is to follow as
much as possible the application diagram and reference design given by
the manufacturer, as the internals of the chips are seldom fully
disclosed, so you need to take their word on some of the external
component values to use.
The [Allwinner V3s Reference Design][9] contains on page 6 the
schematics for using an AXP203 to supply the power to a V3s-based
dashboard camera design. It follows closely the application diagram
provided in the AXP20x datasheets:
![AXP20x Application Diagram](/assets/images/AXP20x_Application_Diagram.png){.lightbox}
More hints are provided in our self-translated [V3s Hardware Design
Guide][10] (page 7) too.
## PMIC Schematics
The **FunKey S** device uses all of the **U5** AXP209 integrated SMPS:
- the PWM charger DC-DC1 for the battery
- the DC-DC2 for providing the +1.25 V / 1.6A to the core
- the DC-DC3 for providing the +3.3V / 1.2A to the I/Os
But compared to the sophisticated reference design above, the **FunKey
S** device only uses 2 out of the 5 integrated LDOs:
- LDO1 supplies the +3.3V / 30 mA Always On for the RTC
- LDO2 provides the +3.0V / 200 mA for the analog power supply
- LDO3 / LDO4 / LDO5 are not used in the **FunKey S**
Here are the PMIC schematics:
![PMIC Schematics](/assets/images/PMIC_Schematics.png){.lightbox}
These schematics may look intimidating and complex, but they are in
fact just a collection of simple basic elements, and it is actually
very close to the manufacturer-recommended design.
Here are the details for each PMIC functions, one by one:
### Power Inputs (East side)
A wall-plug AC adapter input is not used in the **FunKey S** device,
so +VIN is just filtered using C75 on pins 32 and 33.
The USB power input +VUSB on pin 31 is filtered using **C70**, and the
best (between +VUSB and +VBAT) available voltage is output to +VOUT on
pins 34 and 35 and filtered using **C78**.
The BACKUP supply on pin 30 is not used and is left unconnected.
### Internal Connections (All sides)
Some AXP20x signals are externally available and should be connected
to external components:
- The BIAS connection on pin 23 is connected to a precision 200k 1%
resistor **R22**, as recommended
- The reference voltage VREF on pin 24 is decoupled with **C64**
- The +2.5V internal logic voltage VINT on pin 26 is filtered using
the recommended value for **C67**
Additionally, the AXP20x is actually made up of separate flexible
blocks that require external interconnections to set their desired
operation:
- All DC/DC inputs (VIN1 on pin 44, VIN2 on pin 7 and VIN3 on pin
14), as well as LDO3IN input on pin 40 are connected to the best
available voltage +VOUT with filter capacitors **C59**, **C23**,
**C30**, and **C69**, respectively
- LDO1SET on pin 27 is used to set the initial voltage of LDO1, and
according to the datasheets, setting it to VINT sets its voltage to
the desired +3.3V for the +3.3V Always On power supply
- OTOH, combined LDO 2 and 4 input LDOIN24 on pin 13 is instead
connected to +3.3V in order to minimize the voltage drop for LDO2
to generate the +3.0V. Here too, there is a filter capacitor
**C34**
- It is not clear what is the exact function of APS on pin 21 (it is
described as "Internal Power Input"), but it must be connected to
+VOUT, too
### DC-DC1 PWM Battery Charger (North East side)
The battery is connected to J5 (a [2-pin JST 1.0 mm pitch
receptacle][11]) and uses **R21** as a precision current sense
resistor, with **C53**/**C56**/**C60** filter capacitors and **L5** (a
low-profile ferrite-core power inductor rated with a saturation
current of 1.2A and low < 0.1 Ω resistance).
!!! Warning
The battery is not protected on the board against reversing
polarity, as the model used already contains a built-in
protection.
**R24** is mounted to simulate a battery NTC resistor for measuring
temperature, as the chosen LiPo battery does not feature this
temperature sensor.
A user-programmable (through the I2C interface) charge [LED][12]
**D30** is provided, with its current-limiting resistor **R26**, as
well as a TVS diode **d31** to prevent ESD, as the LED body will be
indirectly accessible to user.
### DC-DC2 +1.25V / 1.6A (West side)
This SMPS is built around the ferrite core power inductor **L3** and
filter capacitors **C26** and **C29**.
### DC-DC3 +3.3V / 1.2A (South side)
This SMPS is built around the ferrite core power inductor **L4** and
filter capacitors **C39** and **C43**.
### LDO1 +3.3V Always On 30mA (South East side)
The LDO output on pin 28 is filtered with capacitor **C72**.
### LDO2 +3.0V / 200mA (South West side)
The LDO output on pin 12 is filtered with capacitor **C33**.
### LDO3 (North side)
This LDO is not used and its output on pin 41 is nevertheless filtered
with a capacitor **C63**.
### LDO4 (South West side)
This LDO is not used and its output on pin 11 is nevertheless filtered
with a capacitor **C38**.
### Power Key (North West side)
The AXP20x features a soft power key with internal short and
long-press detection with user-programmable time settings, which
enables turning power ON or OFF much like the way it is done in
cellular phones.
Only a few external components are required: the tactile switch
**S13**, its ESD protection TVS **D29**, and a low-pass filter **R18**
and **C42** for debouncing the switch.
### I2C Bus (North West side)
The AXP20x can be externally controlled by the main CPU using the I2C
bus on pins 1 and 2. This bus has pull-up resistors to +3.3V **R14**
and **R16**, and the IRQ/WAKEUP signal on pin 48 enables warning or
waking up the CPU on a selection of AXP20x-generated events, with a
pull-up resistor **R13** to +3.3V.
### GPIOs (South and West sides)
GPIO0-3 on pins 19, 18, 5 and 3 are not used in the **FunKey S** and
are left unconnected.
### PWROK (South West side)
The PWROK signal on pin 25 is used to generate the global RESET signal
for the whole board, with a pull-up resistor **R15** to the +3.3V
Always On power supply and a filter capacitor **C18**.
### Enable Signals (West side)
The global chip enable signal N_OE on pin 4 is activated by default
through a 47kΩ resistor **R17** to GND, but a magnetic Reed switch
**S14** can disable it by forcing its level to +VOUT, with a filter
capacitor **C83**. This circuit will be disscused later in the
[Magnetic Switch section][13].
The USB enable signal N_VBUSEN on pin 6 is directly tied to GND to
always enable power from the USB bus.
### Monitoring
Through the I2C bus and the numerous internal available registers, the
AXP20x provides a very fine control of its operation, including many
threshold and timing settings, but also many voltage and curent
monitoring values.
### Coulomb Counters / Fuel Gauge
It is well known that battery discharge voltage curve over time is
very flat, making it very difficult to estimate the real
charge/discharge state of the battery. Moreover, this state will vary
with temperature, load, and aging.
The only accurate way to monitor the battery status is to actually
count the energy that is stored when charging, and the one that is
consumed. This particularly important feature is achieved in the
AXP20x using a dual Coulomb counter which continuously sums the
current intensity over time for monitoring the battery accurate charge
and discharge status, with user-defined alert thresholds.
This fuel gauge is providing the ability to precisely report the
remaining battery capacity, just like people are used to with cellular
phones.
[1]: https://licheepizero.us/
[2]: https://hackaday.io/project/134065-funkey-zero
[3]: http://club.szlcsc.com/article/downFile_D72C44885C60F9F1.html
[4]: https://www.torexsemi.com/file/xc6206/XC6206.pdf
[5]: http://www.x-powers.com/en.php
[6]: http://www.x-powers.com/en.php/Info/down/id/55
[7]: https://github.com/Squonk42/V3s_Documentation/raw/master/AXP203_Datasheet_V1.0.pdf
[8]: https://github.com/Squonk42/V3s_Documentation/raw/master/AXP209_Datasheet_v1.0en.pdf
[9]: https://github.com/Squonk42/V3s_Documentation/blob/master/V3S_CDR_STD_V1_0_20150514.pdf
[10]: https://github.com/Squonk42/V3s_Documentation/raw/master/V3s%20hardware%20design%20guide%20V1.0_20150519%20EN%20Non%20Official.pdf
[11]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/1811151533_JST-Sales-America-SM02B-SRSS-TB-LF-SN_C160402.pdf
[12]: https://github.com/FunKey-Project/FunKey-S-Hardware/blob/master/Datasheets/C165977_%E8%B4%B4%E7%89%87LED%E8%93%9D%E8%89%B2_2018-01-26.PDF
[13]: /developers/hardware/magnetic_switch
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Simple DC electronic circuits can be powered by directly connecting a
battery. However, more complex circuits usually require a constant
input voltage for proper operation.
This page is a small sidetrack to explain the different regulated DC
power supply topologies, before looking at the **FunKey S** power
supply schematics in details.
If you are already comfortable with this subject, you can skip this
section entirely!
## Linear Regulators
The easiest method to achieve a constant voltage viewed from the load
despite a varying source voltage is to linearly control the resistance
of the regulator in accordance with the load, resulting in a constant
output voltage.
### Shunt Regulator
The simplest voltage regulator is the [shunt regulator][1], built
around a Zener diode which most interesting characteristic is to
maintain a constant voltage across itself when the current through it
is sufficient to take it into the Zener breakdown region. A simple
shunt regulator looks like this:
![Zener Regulator](/assets/images/Zener_Regulator.gif)
### Series Regulator
By adding a emitter-follower transistor to the simple shunt regulator,
the small base current of the transistor forms a very light load on
the Zener, thereby minimizing variation in Zener voltage due to
variation in the load, resulting in a better regulation. Here is a
schematic for this [series regulator][2]:
![Series Regualtor](/assets/images/Series_Regulator.gif)
### Integrated Linear Regulator
In integrated voltage regulators, the discrete Zener diode is replaced
by a more sophisticated (but easier to integrate) circuit built around
a resistor divider feeding an operational amplifier, a voltage
reference, and a transistor driving the emitter-follower pass
transistor:
![Integrated Regulator](/assets/images/Integrated_Regulator.png)
Usually, the pass transistor and its driving transistor are combined
into a single Darlington transistor plus a controllable current source
like this:
![Darlington Transistor](/assets/images/Darlington_Transistor.jpg)
### LDO (Low Drop-Out) Regulator
The above circuit works well, but its drop-out voltage (the difference
between the input and output voltage) is rather high because of this
transistor cascade, around 1.5V to 2.5V.
By replacing the emitter-follower Darlington transistor by a PNP
transistor in an open collector or open drain topology, the drop-out
voltage is reduced to 0.7V or lower:
![PNP Transistor](/assets/images/PNP_Transistor.jpg)
## SMPS (Switched-Mode Power Supply) or DC/DC Converters
A linear regulator provides the desired output voltage by dissipating
excess power as heat in the Zener diode or in the pass
transistor. Hence its maximum power efficiency is VOUT/ VIN since the
voltage difference is wasted to heat the birds.
In contrast, a Switched-Mode Power Supply changes output voltage and
current by switching non-linear storage elements, such as inductors,
transformers and capacitors between different electrical
configurations.
These elements are said to be non-linear because the inductor and
transformer respond to changes in current by inducing its own voltage
to counter the change in current, whereas a capacitor responds to
changes in voltage by inducing its own current to counter the change
in voltage.
Thus, depending on the way the components are arranged, it is possible
to obtain SMPS circuits that either have an output voltage higher than
the input voltage ("Boost Converters"), or lower than the input
voltage ("Buck Converters", as is it subtracts or “Bucks” the supply
voltage).
Because of technology, power inductors are easier to manufacture, take
less space and are more stable over time than their counterpart
capacitors. This is why most power DC/DC converters are built using
inductors. Capacitor-based SMPS are generally used for lower power
applications, such as for generating the +12V and -12V voltages
required by true RS232 from a +3.3V or +5V power supply in the
ubiquitous MAX232 drivers.
### Boost Converter
The most basic circuit for the Boost converter is the following:
![Boost Converter](/assets/images/Boost_Converter.png)
If the switch is driven by a square wave, the peak-to-peak voltage of
the waveform measured across the switch can exceed the input voltage
from the DC source. This is because the non-linear characteristic of
the inductor, and this voltage adds to the source voltage while the
switch is open.
!!! warning
In this converter, the output voltage is not isolated from the
input voltage.
### Buck Converter
The corresponding basic circuit for the Buck converter is the
following:
![Buck Converter](/assets/images/Buck_Converter.gif)
The way this converter works is described in details
[here][3]. Basically, when the switch is closed, the inductor will
produce an opposing voltage across its terminals in response to the
changing current, reducing the output voltage, and meanwhile the
inductor stores this energy in the form of a magnetic field. When the
switch is opened, the current will decrease and will produce a voltage
drop across the inductor, and now the inductor becomes a current
source, where the stored energy in the inductor's magnetic field is
restored and fed to the load.
!!! warning
In this converter too, the output voltage is not isolated from the
input voltage.
### Isolated SMPS
Isolated Switched-Mode Power Supplies use a transformer to isolate the
input voltage from the output voltage, and thus can produce an output
of higher or lower voltage than the input by adjusting the turns
ratio.
## Pros and Cons
Linear regulators are simpler than SMPS, and their linear behavior
produce a very clean output voltage, but their efficiency is directly
proportional to the difference between the input and output voltage,
which is dissipated as heat.
However, for light loads and/or when the voltage drop-out is low, LDOs
are very useful.
OTOH, SMPS are more complex and require more components, but their
efficiency is much better (typically 80-90%), resulting in less heat,
with the drawback of a switching electrical noise pollution of both
the input voltage (that may couple electrical switching noise back
onto the mains power line) and the output voltage (with
electromagnetic interference (EMI) and a ripple voltage at the
switching frequency and all its harmonic frequencies).
SMPS are thus almost exclusively used when heavy loads are used and/or
when the voltage drop-out is important.
[1]: https://en.wikipedia.org/wiki/Linear_regulator#Simple_shunt_regulator
[2]: https://en.wikipedia.org/wiki/Linear_regulator#Simple_series_regulator
[3]: https://en.wikipedia.org/wiki/Buck_converter#Concept
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