Application Note AN850

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Power MOSFETs
Application Note AN850
Power MOSFET Basics: Understanding the Turn-On Process
by Sanjay Havanur
The question of how to turn on a MOSFET might sound trivial, since ease of switching is a major advantage of field-effect
transistors. Unlike bipolar junction transistors, these are majority carrier devices. One does not have to worry about current gain,
tailoring the base current to match the extremes of hfe and variable collector currents, or providing negative drives. Since
MOSFETs are voltage driven, many users assume that they will turn on when a voltage, equal to or greater than the threshold,
is applied to the gate. However, the question of how to turn on a MOSFET or, at a more basic level, what is the minimum voltage
that should be applied to the gate, needs reappraisal with more and more converters being controlled digitally. While digital
control offers the next level of flexibility and functionality, the DSPs, FPGAs, and other programmable devices with which it is
implemented are designed to operate with low supply voltages. It is necessary to boost the final PWM signal to the level required
by the MOSFET gate. This is where things begin to go wrong, because of the misconceptions about what really turns on a
MOSFET. Many digital designers look at the gate threshold voltage and jump to the conclusion that, just as with their digital
logic, the MOSFET will change state as soon as the threshold is crossed.
TABLE 1 - GATE THRESHOLD SPECIFICATION FOR SIR826ADP (TJ = 25 °C, unless otherwise noted)
PARAMETER
SYMBOL
TEST CONDITION
MIN.
TYP.
MAX.
UNIT
VDS
VGS = 0 V, ID = 250 μA
80
-
-
V
-
47
-
-
-5.7
-
1.2
-
2.8
Static
Gate-Source Breakdown Voltage
VDS Temperature Coefficient
ΔVDS/TJ
VGS(th) Temperature Coefficient
ΔVGS(th)/TJ
Gate-Source Threshold Voltage
VGS(th)
ID = 250 μA
VDS = VGS, ID = 250 μA
mV/°C
V
Another curve given on the datasheet refers to the MOSFET turning on with increasing gate voltage - the transfer characteristics.
This is illustrated for the same SiR826ADP device in figure 1. However, the transfer characteristics are more a measure of current
variation with respect to temperature and applied gate voltage. The VDS is maintained at a constant but high value, sometimes
as much as 15 V, and not always disclosed in the datasheet. In the curve shown below, at a current of 20 A it is not enough to
apply 3.2 V to the gate. The combination would maintain a VDS of 10 V typical and a continuous dissipation of 200 W. The
transfer curve is quite important when the MOSFET is operated in the linear mode, but has little relevance for switching
operations.
Revision: 23-Jun-15
Document Number: 68214
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APPLICATION NOTE
First, the threshold voltage VGS(th) is not intended for system designers. It is the gate voltage at which the drain current crosses
the threshold of 250 μA. It is also measured under conditions that do not occur in real-world applications. In some cases a fixed
VDS of 5 V or higher may be used as the test condition, but is usually measured with gate and drain shorted together as stated.
This does not require searching for fine print, it is clearly stated in the datasheet. Table 1 shows the VGS(th) specification and test
conditions for the SiR826ADP. In many applications there are concerns about the so called “induced” gate voltage, such as in
the low-side MOSFET of a synchronous buck. Again, taking the gate voltage above the threshold does not automatically drive
the device into a shoot-through-induced failure. VGS(th) is a MOSFET designer’s parameter and defines the point where the
device is at the threshold of turning on. It is an indication of the beginning, nowhere near the end. Gate voltage should be held
below the threshold in the off state to minimize the leakage. But during turn-on, system designers can, and should, ignore it
altogether.
Application Note AN850
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100
100
80
80
VGS = 10 V thru 4 V
ID - Drain Current (A)
ID - Drain Current (A)
Power MOSFET Basics: Understanding the Turn-On Process
60
TC = 25 °C
40
20
60
40
20
TC = 125 °C
VGS = 3 V
TC = - 55 °C
0
0
1
2
3
4
VGS = 2 V
0
5
0
1
2
3
4
5
VDS - Drain-to-Source Voltage (V)
VGS - Gate-to-Source Voltage (V)
Fig. 1 - Transfer Characteristics
Fig. 2 - Output Characteristics
The curve that has data with the MOSFET fully on is called the output characteristics, as shown in figure 2. Here, the MOSFET
forward drop is measured as a function of current for different values of VGS. Designers may refer to this curve to ensure that
the gate voltage is sufficient. For each gate voltage where RDS(on) is guaranteed, there is a range where the VDS drop maintains
strict linearity with current, beginning from zero. For lower values of gate voltage, as the current is increased the curve loses
linearity, goes through a knee, and flattens out. Figure 3 shows the detailed output characteristics for gate voltages between
2.5 V to 3.6 V. MOSFET users usually think of this as the linear mode. However, device designers refer to the gray area as the
current saturation region - for the given gate voltage, the current that can be delivered has reached its saturation limit. Any
increase in applied VDS will be sustained with only a slight increase in the current, whereas even a slight change in current can
lead to a relatively large increase in VDS. For higher gate voltages, when the MOSFET has been fully turned on, any operating
point will be located in the area shaded in green to the left, marked as the resistive (or ohmic) region. Note that all curves are
typical, with no minimum or maximum limits, and derived at 25 °C. At lower temperatures the gate voltage required to keep the
device in the resistive region will be higher, increasing at the rate of 0.3 %/°C.
Resistive Region
10
25.0
20.0
VGS = 3.4 V
17.5
VGS = 3.3 V
VGS = 3.1 V
12.5
VGS = 3.0 V
10.0
VGS = 2.9 V
7.5
VGS = 2.8 V
5.0
VGS = 2.7 V
2.5
VGS = 2.5 V
VDS = 30 V
8
6
VDS = 40 V
VDS = 50 V
4
2
0
0.0
0
1
2
3
4
5
6
7
8
9
10
VDS (V)
Fig. 3 - Expanded Output Characteristics
Revision: 23-Jun-15
0
12
24
36
48
Qg - Total Gate Charge (nC)
60
Fig. 4 - Gate Charge Characteristics
Document Number: 68214
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APPLICATION NOTE
VGS = 3.2 V
Current Saturation Region
VGS - Gate-to-Source Voltage (V)
VGS = 3.5 V
15.0
IDS (A)
ID = 20 A
VGS = 3.6 V
22.5
Application Note AN850
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Power MOSFET Basics: Understanding the Turn-On Process
When confronted with the output characteristics, designers invariably demand to know the RDS(on) at their particular operating
conditions. Typically it will be at a combination of VGS and IDS, where the curve has strayed from the straight and narrow line
into the gray area. For example, in the case above it could be a gate voltage VGS = 3.1 V and a startup current of 10 A. They
understand RDS(on) will be higher than specified, but can the MOSFET manufacturer provide an approximate indication? Since
both VDS and IDS are available on the curve, there is a temptation, often succumbed to, to divide the two and arrive at the
“effective” RDS(on). Unfortunately there is no RDS(on) to calculate here. It does not exist under the given conditions. Any segment
of the load line that represents a resistance must pass through the origin in a linear fashion. One can of course model the load
line in its entirety as a non-linear resistance. If nothing else it will ensure that any understanding of real-world behavior is
maintained at the origin (0, 0).
The real clue to turning on the MOSFET is provided by the gate charge curve shown in figure 4. While the curve is routinely
offered for every MOSFET, its implications are not always understood by designers. In addition, recent developments in
MOSFET technology, like trench and shielded gates and charge-compensating superjunction structures, demand a fresh
appraisal of this information. To start with, the term “gate charge” itself is somewhat misleading. The linearized and segmented
curve does not look like the charging voltage of any capacitor, no matter how non-linear its value. In reality the gate charge
curve represents a superposition of two capacitors which are not in parallel, have different values, and carry different voltages.
In the literature, the effective capacitance as seen from gate terminal is defined as
Ciss = Cgs + Cgd.
While this is a convenient entity to measure and specify in the datasheet, it is worth noting that Ciss is not a physical capacitance.
It would be a misconception to imagine that the MOSFET is turned on by simply applying a voltage to “the gate capacitance
Ciss.” As shown in figure 5, prior to turn-on the gate source capacitance Cgs is uncharged, but the gate drain capacitance Cgd
has a negative voltage / charge which needs to be removed. Both capacitors are non-linear, whose values can vary widely with
respect to applied voltage. The switching characteristics, therefore, are dependent more on their stored charges rather than the
capacitance value at any given voltage.
Drain
ILOAD
Up to
rated VDS
+ Cgd
_
Gate
VGS
Cds
IDS
+
VIN
Rg
Cgs
Gate
0V
Source
Source
Fig. 5 - Gate Capacitances with Initial Voltages
Fig. 6 - Simplified Inductive Turn-On Circuit
T0 - T1: Cgs is charged from zero to VGS(th). There is no change in VDS or IDS.
T1 - T2: Current begins to rise in the device as the gate voltage rises from VGS(th) to the plateau voltage Vgp. IDS rises from 0 A
to the full load current, but there is no change in VDS. The charge associated is the integral of Cgs from 0 V to Vgp and specified
in the datasheets as Qgs.
T2 - T3: The flat region between T2 and T3 is also known as the Miller plateau. Prior to turn-on, Cgd is charged to supply voltage
VIN and holds it till IDS has peaked to ILOAD at T2. Between T2 and T3, the negative charge of (VIN - Vgp) is converted to the
positive charge corresponding to the plateau voltage Vgp. This is also seen as the fall of the drain voltage from VIN to near zero.
The charge associated is approximately the integral of Cgd from zero to VIN and specified in the datasheets as Qgd.
T3 - T4: As the gate voltage rises from Vgp to VGS, there is very little change in VDS or IDS. However, the effective RDS(on) reduces
marginally with rising gate voltage. At some voltage above Vgp, the manufacturers feel confident enough to guarantee an upper
limit on the effective RDS(on).
Revision: 23-Jun-15
Document Number: 68214
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APPLICATION NOTE
Since the two component capacitances that make up Ciss are physically different and are charged to different voltages, the
turn-on process also has two stages. The exact sequence is different for inductive and resistive loads; however, in most
practical applications the load is heavily inductive and can be described using the circuit model shown in fig 6. With reference
to the timing diagram in figure 7,
Application Note AN850
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Vishay Siliconix
Power MOSFET Basics: Understanding the Turn-On Process
When the load is inductive, the rise of the current in the MOSFET channel must be completed before the voltage begins to fall.
At the beginning of the plateau, the device is off with simultaneous high current and voltage across drain and source. Between
T2 and T3, a charge of Qgd is delivered to the gate, and at the end of it the MOSFET characteristic has changed from constant
current to constant resistance mode. During this entire transition there is no significant change in the gate voltage Vgp, which is
why it is not meaningful to associate the turning on of a MOSFET with any specific gate voltage.
Similar analysis can be made for turn-off, where the same two charges must be removed from the gate in reverse order. While
the sum of Qgs and Qgd guarantees that the MOSFET will be fully on, it does not guarantee how fast. The rate of change in
voltage or current is determined by the rate at which the gate charge components are applied or removed, which is nothing but
the gate drive current. While fast rise and fall times are necessary to reduce switching losses, they also introduce system-level
problems of high peak voltages, ringing, and EMI, especially during inductive turn-off.
10 000
ILOAD
VIN
VGS
Projected
VDS fall
Ciss
1000
VGS(th)
CXSS (pF)
V gp
Actual
VDS fall
(for HV )
100
Coss
10
T0
T1
T2
T3
Qsw
Qgs
Qgd
Crss
T4
Turn ON
sequence
1
0
100
200
300
400
500
600
Qg
VDS (V)
Fig. 7 - Gate Charge Components and Timings
Fig. 8 - Capacitance Variations w.r.t. Applied VDS
The simplified linear voltage fall shown in figure 7 assumes a constant value of Cgd, which is rarely true for real MOSFETs. In
particular, the Cgd of a high-voltage superjunction MOSFET shows extreme non-linear behavior, as illustrated in figure 8 for the
SiHF35N60E. There is more than a 200:1 variation in Crss value within the first 100 V. As a result, the actual fall time of voltage
against the gate charge curve may look more like the red dashed line in figure 7. The rise and fall times, and their corresponding
dV/dt values, depend more on the Crss values at higher voltages rather than the integral of the entire curve specified as Qgd.
While comparing devices across different design platforms, users need to be aware that a MOSFET with half the Qgd will not
necessarily switch twice as fast or have half the switching losses. Depending on the shape of the Cgd curve and its value at
higher voltages, it is quite possible to have a device that has low Qgd in the datasheet but shows no increase in switching speed.
CONCLUSIONS
Revision: 23-Jun-15
Document Number: 68214
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For technical questions, contact: [email protected]
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
APPLICATION NOTE
In the real world, turning on a MOSFET is not an event but a process. Designers have to stop thinking of applying a VGS(th), or
some other voltage, as an input at the gate which will toggle the output from high to low RDS(on). Questions like what is the RDS(on)
at a gate voltage below some value or the other, are not valid because it is not the gate voltage per se that turns on a MOSFET.
It is the two charges Qgs and Qgd, injected into the device through the gate pin, that do the job. The gate voltage will rise above
VGS(th) and Vgp in the process, but that is secondary. The speed with which a modern power MOSFET turns on or off is also not
a simple function of Qgs or Qgd. A detailed study of both the gate charge curve and capacitance characteristics is necessary to
compare switching speeds, especially for superjunction MOSFETs.
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