MOSFET & It's Application in SMPS :
Jaydeep Shah (06- sep 2019 )
MOSFETs are available in two basic
forms:
- Depletion Type: The
transistor requires the Gate-Source voltage (VGS) to switch the
device “OFF”. The depletion mode MOSFET is equivalent to a “Normally
Closed” switch.
- Enhancement Type: The
transistor requires a Gate-Source voltage(VGS) to switch the
device “ON”. The enhancement mode MOSFET is equivalent to a “Normally
Open” switch.
Depending upon the type
of materials used in the construction, and the type of operation, the MOSFETs
are classified as in the following figure.
The N-channel
MOSFETs are simply called as NMOS. The symbols for
N-channel MOSFET are as given below.
The P-channel MOSFETs are simply called as PMOS.
The symbols for P-channel MOSFET are as given below
A lightly doped P-type substrate
is taken into which two heavily doped N-type regions are diffused, which act as
source and drain. Between these two N+ regions, there occurs diffusion to form
an Nchannel, connecting drain and source. (Diffusion is net movement of anything
from a region of higher concentration to a region of lower concentration.)
A thin layer of Silicon dioxide
(SiO2) is grown over the entire surface and holes are made
to draw ohmic contacts for drain and source terminals.
It is a four-terminal device with
source(S), gate (G), drain (D) and body (B) terminals. The body is frequently
connected to the source terminal, reducing the terminals to three. It works by
varying the width of a channel along which charge carriers flow (electrons or
holes).
The charge carriers enter the channel at
source and exit via the drain. The width of the channel is controlled by the
voltage on an electrode is called gate which is located between source and
drain.
In general, any MOSFET is seen to
exhibit three operating regions
- Cut-Off
Region
Cut-off region is a region in which the MOSFET will be OFF as there will
be no current flow
through it. In this region, MOSFET behaves like an open switch and is thus
used when they are required to function as electronic switches.
- Ohmic
or Linear Region
Ohmic or linear region is a region where in the current IDS increases
with an increase in the value of VDS. When MOSFETs are made to
operate in this region, they can be used as amplifiers.
- Saturation
Region
In saturation region, the MOSFETs have their IDS constant in
spite of an increase in VDS and occurs once VDS exceeds
the value of pinch-off voltage VP.
Under this condition, the device will act like a closed switch through
which a saturated value of IDS flows. As a result, this
operating region is chosen whenever MOSFETs are required to perform
switching operations.
Figure
1a shows the transfer characteristics (drain-to-source current IDS versus gate-to-source voltage VGS) of n-channel
Enhancement-type MOSFETs. From this, it is
evident that the current through the device will be zero until the VGS exceeds the value of threshold voltage VT. This is because under this state, the device will
be void of channel which will be connecting the drain and the source terminals.
Under this condition, even an increase in VDS will result in
no current flow as indicated by the corresponding output characteristics (IDS versus VDS) shown by
Figure 1b. As a result this state represents nothing but the cut-off region of
MOSFET’s operation.
Next,
once VGS crosses VT, the current
through the device increases with an increase in IDS
initially (Ohmic region) and then saturates to a value as determined by the VGS (saturation region of operation) i.e. as VGS increases, even the saturation current flowing
through the device also increases. This is evident by Figure 1b where IDSS2 is greater than IDSS1 as VGS2 > VGS1, IDSS3 is greater than IDSS2 as VGS3 > VGS2, so on and so
forth. Further, Figure 1b also shows the locus of pinch-off voltage (black
discontinuous curve), from which VP is seen to
increase with an increase in VGS.
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·
SELECTION OF THE MOSFET
:
Voltage and Current Ratings
The drain-source voltage VDSS rating is
important in selecting MOSFETs. Application of a voltage exceeding VDSS might
result in the destruction of a MOSFET. It is necessary to choose MOSFETs with a
VDSS sufficiently higher than the voltage at which they will actually be used.
However, MOSFETs with high VDSS ratings tend to have large on-state resistance,
RDS(ON). A downside of using such MOSFETs is increased conduction loss. If you
select MOSFETs according to possible peak surge voltage, you could end up
opting for devices with large on-state resistance. To address this situation. Generally,
on-state resistance determines the upper limit of the drain current ID. Ensure
that not only the loss calculated as ID2×RDS(ON), the permissible power
dissipation but also a temperature rise due to heating does not cause the
device to exceed its operating temperature range.
Switching Speed
When
a power MOSFET switches at a high frequency, its switching loss accounts for a
significant portion of total loss. To reduce total loss for high frequency
switching applications, high speed power MOSFETs should be used.
What is MOSFET Switching Time?
The
MOSFET will turn ON or OFF after the Gate voltage turns ON/OFF. The time in
between turning ON or OFF is called the switching time.
EX:
VGS threshold: VGS(th) :
VGS(th) is
the voltage required between the Gate and Source to turn ON the MOSFET.In other
words, supplying a voltage greater than VGS(th) will turn ON
the MOSFET.
To determine the amount of current that flows through the MOSFET when ON it is
necessary to refer to the specifications and electrical characteristics for
each element.
Table
1 lists the relevant electrical characteristics. In the case of VDS=10V
a threshold voltage of between 1.0V and 2.5V is required for an ID of
1mA.
Parasitic Capacitance
Parasitic capacitance exists in
power MOSFETs as shown in Figure 1.
Sometimes known as stray
capacitance, parasitic capacitance is unavoidable and typically unwanted that
exists between the parts of an electronic component or circuit simply because
of how close they are to one another. Capacitance is the ability of a system to
store an electric charge.
The Gate terminal in a MOSFET is
isolated from the other terminals by an oxide film. The silicon under the gate
has the opposite polarity to the drain and source which results in the
formation of PN junctions (diode) between the Gate, Drain and Source regions.
Cgs and Cgd are the capacitance of the oxide layers, while Cds is determined
by the junction capacitance of the internal diode.
Switching Time and Drive Conditions :
Bipolar transistors need a large base
current to maintain low on-state voltage. In contrast, since power MOSFETs are
voltage-controlled devices, they can be driven just by charging gate capacity,
and are therefore a low in power consumption.
Note, however, that power MOSFETs have a
slightly large input capacitance Ciss. Thus, for high speed switching
applications, it is necessary to quickly charge the input capacitance from a
low-impedance signal source.
Low-impedance drive is required to
reduce turn-on time. However, the use of a high gate voltage results in the
much charging of gate-source capacitance, resulting in an increase in td (off).
Switching time can be controlled via
gate resistance. If you want to change the turn-on and turn-off switching
speeds separately, you can use diodes to change the gate resistance values for
turn-on and turn-off. Figure 2.3 shows some examples.
Gate Drive Resistor
RG is the gate driver resistor for the
power switch, MOSFET. The selection of this resistor value must be done in conjunction with EMI compliance testing
and efficiency testing. Using a larger resistor value for RG slows down the turnon and turnoff of the MOSFET. A
slower switching speed reduces EMI but also increases the switching loss. A trade-off between switching loss
and EMI performance must be carefully performed.
·
LOSSES IN MOSFET :
In the ideal switching regulator shown
in Figure 2, the current is zero when the switch is open and the power loss is
zero, thus VIN is being chopped. When the switch is closed, the voltage across
it is zero and the power loss is also zero. An ideal switch implies zero
losses, thus offering 100% efficiency. An efficient switching regulator results
in less heat dissipation, which reduces system cost and size for elements such
as heat sinks, fans and their assembly. In batteryoperated systems, less power
loss means that these devices can use the same battery for a longer run time
because the device pulls less current from the battery.
MOSFETs have a finite switching time,
therefore, switching losses come from the dynamic voltages and currents the
MOSFETs must handle during the time it takes to turn on or of, Gate-drive
losses are also switching losses because they are required to turn the FETs on
and off.
Conduction loss : RDS(on) * I(sw)-(RMS)
MOSFET switching losses are a function
of load current and the power supply’s switching frequency as shown by below
Equation
Psw = Vin * Iout * Fsw * ((Qgs + Qgd) /
Ig )
where VIN = VDS (drain-to-source
voltage),
IOUT = ID (drain current),
fSW is the switching frequency,
QGS2 and QGD depend on the time the driver
takes to charge the FET
IG is the gate current.
Switch-MOSFET gate losses can be caused by the
energy required to charge the MOSFET gate. That is, the QG(TOT) at the gate
voltage of the circuit. These are both turn-on and turn-off gate losses. Most
of the power is in the MOSFET gate driver. Gatedrive losses are frequency
dependent and are also a function of the gate capacitance of the MOSFETs. When
turning the MOSFET on and off, the higher the switching frequency, the higher
the gate-drive losses. This is another reason why efficiency goes down as the
switching frequency goes up.Larger MOSFETs with lower RDS(on) provide lower
conduction losses at the cost of higher gate capacitances, which results in
higher gate-drive losses.
Power loss in a MOSFET comes from
two sources. Every MOSFET has a resistive element, so it dissipates power as
current is conducted through the device. The resistive parameter is described
as on-resistance, or RDS(ON). These conduction losses are inversely
proportional to the size of the MOSFET; the larger the switching transistor,
the lower its RDS(ON) and, therefore, its conduction loss.
The other source of power loss is
through switching losses. As the MOSFET switches on and off, its intrinsic
parasitic capacitance stores and then dissipates energy during each switching
transition. The losses are proportional to the switching frequency and the
values of the parasitic capacitances. As the physical size of the MOSFET
increases, its capacitance also increases; so, increasing MOSFET size also
increases switching loss.
These sources of power loss create
a significant challenge for power supply designers. While a larger MOSFET will
exhibit less on-resistance – and consequently lower conduction loss – its
larger area drives up parasitic capacitance and switching loss. In many cases,
moving to a larger MOSFET to reduce conduction loss will result in so much
increased switching loss that it outweighs the conduction loss savings.
Designers typically try to achieve a balance between conductive and switching
losses for a particular application.
FAQ :
1) Is it possible to use the Zener diode between the gate
and source for surge absorption?
The Zener diode between gate and source
is intended for electrostatic breakdown protection. Please confirm whether
overvoltage is applied to the gate in the actual state.
2) What does the dv/dt of the MOSFET mean?
The dv/dt of the MOSFET is the changing
rate of the drain-source voltage during the switching transient.
If dv/dt is too large, ringing may
occur, possibly leading to MOSFET damage.
Therefore, dv/dt ruggedness values are
specified for some MOSFETs.
3)
How
does the series gate resistor affect the MOSFET?
When using lower gate resistance,
switching time becomes shorter, and ringing (damped oscillation) may occur.
Ringing can cause oscillation and EMI noise.
When using higher gate resistance, switching time becomes longer.
As a result, switching loss increases and heat is generated.
In the bridge circuit, a short circuit may occur across the upper and lower
MOSFETs by combination of the gate resistances.
Therefore, it is necessary to consider the optimum gate resistance.
4) The MOSFET does not turn off by a turn-off signal. How do
I solve this problem?
Confirm whether the control signal voltage Vdrive is close to
the GND or not.
When the voltage is not close to the GND, the output impedance of the gate
drive circuit may be high. Consider lowering output impedance. (Select
different turn off path for turning off the MOSFET quickly).
Confirm whether the gate-source voltage VGS is the same as the
control signal or not. If it is not, insert a pull-down resistor Rpull between
gate and source to ensure the gate-source voltage VGS to the
GND level.
Please select Rpull accordinglly that does not affect the gate
driving performance.
5) Is the on-state resistance of a MOSFET dependent on
temperature?
The MOSFET has a positive temperature
coefficient. Since values of coefficients depend on the drain-source breakdown
voltages and the process, it is necessary to confirm with the data sheet etc.
When performing thermal design / circuit design, please consider this
temperature fluctuation.
6) Is the MOSFET drive current necessary?
For bipolar transistors, a large base
current is required to maintain low on-voltage. However, since the MOSFET is a
voltage control element, it can drive with small power sufficient to charge the
gate. But because the input capacitance (Ciss) of the power MOSFET is rather
large for high-speed switching, it is necessary to quickly charge the input
capacitance with a low-impedance drive circuit.
For an indication of the gate current
required for charging, it is possible to calculate the drive current simply by
using the Qg characteristic prescribed on the data sheet. The required
switching time is “t (desired)” (for example ton or toff),
which becomes the following expression.
Ig=Qg/t(desired)
7) Are there any special considerations for using MOSFETs?
For MOSFETs, the maximum allowable
current, voltage, power dissipation and other characteristics are specified as
maximum ratings. Maximum ratings are the highest absolute values that must not
be exceeded even instantaneously under any conditions. A device may not be able
to recover from stress that exceeds a specified maximum rating. None of the
absolute maximum ratings may be exceeded. Care should therefore be exercised
regarding bounces in supply voltage, variations in the characteristics of electronic
devices, possible exposure to stress higher than maximum ratings during circuit
adjustment, changes in ambient temperature, fluctuations in the input signal,
and other such factors. Please select the appropriate device according to the
application.
Also, even within the absolute maximum ratings and the operating range,
reliability will change significantly depending on the degree of derating
8) Electrical characteristics of MOSFETs (Dynamic
Characteristics tr/ton/tf/toff)
Switching
characteristics
Since power MOSFETs are majority-carrier devices, they are
faster and capable of switching at higher frequencies than bipolar transistors.
Switching time measurement circuit and input / output waveform are shown below.
1.
td (on):
Turn-on delay time
The time from when the gate-source voltage rises above 10% of VGS until
the drain-source voltage reaches 90% of VDS
2.
tr：Rise time
The time taken for the drain-source voltage to fall from 90% to 10% of VDS
3.
ton：Turn-on time
The turn-on time is equal to td (on)+ tr.
4.
td (off)：Turn-off delay time
The time from when the gate-source voltage drops below 90% of VGS until
the drain-source voltage reaches 10% of VDS
5.
tf：Fall time
The time taken for the drain-source voltage to rise from 10% to 90% of VDS
6.
toff：Turn-off time
The turn-off time is equal to td(off)+ tf.
Characteristics
|
Symbol
|
Min
|
Typ
|
Max
|
Unit
|
Switching time
|
Rise time
|
tr
|
—
|
13
|
—
|
ns
|
Turn-on time
|
ton
|
—
|
26
|
—
|
Fall time
|
tf
|
—
|
14
|
—
|
Turn-off time
|
toff
|
—
|
63
|
—
|
9) Electrical
characteristics of MOSFETs (Charge Characteristic Qg/Qgs1/Qgd/QSW/QOSS)
Gate
charge
Because
the Gate (G) input terminal of a MOSFET is insulated, the amounts of charge Q
seen from the Gate are important characteristics. Figure 1.5 illustrates the
definitions of gate charge characteristics.
Total gate charge Qg
The
amount of charge to apply voltage (from zero to designated voltage) to gate
Gate-source charge 1 Qgs1
The
amount of charge required for a MOSFET to begin to turn on (before dropping drain-source
voltage)
Gate-drain charge Qgd
The
amount of gate charge charged in the Miller plateau
Gate switch charge Qsw
The amount of charge stored in the gate capacitance from when
the gate-source voltage has reached Vth Until the end of the
Miller plateau
Output charge Qoss
Drain-source
charge
The definition of the gate charge amount is shown in the figure below.
Data sheet description
Characteristics
|
Symbol
|
Test
Conditions
|
Min
|
Typ
|
Max
|
Unit
|
Total
gate charge
|
Qg
|
VDD ≈ 20 V, VGS =
10 V, ID = 50 A
|
—
|
103
|
—
|
nC
|
VDD ≈ 20 V, VGS =
4.5 V, ID = 50 A
|
—
|
49
|
—
|
Gate-source
charge 1
|
Qgs1
|
VDD ≈ 20 V, VGS =
5 V, ID = 50 A
|
—
|
25
|
—
|
Gate-drain
charge
|
Qgd
|
—
|
12.4
|
—
|
Gate
switch charge
|
QSW
|
—
|
23
|
—
|
Output
charge
|
QOSS
|
VDS = 20 V, VGS =
0 V
|
—
|
85.4
|
—
|
9)
Electrical characteristics of MOSFETs (Static Characteristics RDS(ON))
Drain-source on-state resistance (RDS(ON)）
The resistance across drain and source
when the MOSFET is in the "on" state
VDS(ON)measurement
The specified constant drain current, ID, is applied until VGSreaches
the specified voltage. At this point, drain-source voltage is measured.
On-state resistance is calculated by dividing it by the value of drain current,
ID.
Note: Keep the open-circuit voltage of the constant-current source below the
drain-source breakdown voltage.
Measurement of forward transfer
admittance
Gate-source voltage, VGS, is increased until drain current, ID,
reaches the specified value. Then, VGSis changed only slightly, and
the resulting change in drain current, ID, is measured.
Data sheet description
Characteristics
|
Symbol
|
Test Conditions
|
Min
|
Typ
|
Max
|
Unit
|
Drain-source on-state resistance
|
RDS(ON)
|
VGS = 4.5 V, ID =
50 A
|
—
|
0.95
|
1.35
|
mΩ
|
VGS = 10 V, ID =
50 A
|
—
|
0.65
|
0.80
|
10)
Electrical characteristics of MOSFETs (Static Characteristics Vth)
Gate threshold voltage (Vth)
Vth stands for
"threshold voltage." Vth is the gate voltage that
appears when the specified current flows between source and drain.
Vth measurement :
Gate-source voltage, VGS, is
increased until drain current, ID, reaches the specified value, at
which point VGSis measured.
Data sheet description
Characteristics
|
Symbol
|
Test Conditions
|
Min
|
Typ
|
Max
|
Unit
|
Gate threshold voltage
|
Vth
|
VDS = 10 V, ID =
1.0 mA
|
1.4
|
—
|
2.4
|
V
|
|
|
|
|
|
|
|
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Author:
JAYDEEP SHAH (E.C )
AHMEDABADN – INDIA
