AN-500: Depletion-Mode Power MOSFETs and Applications

Application Note AN-500
INTEGRATED CIRCUITS DIVISION
Depletion-Mode Power MOSFETs
and Applications
AN-500-R03
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INTEGRATED CIRCUITS DIVISION
1
Introduction
Applications like constant current sources, solid state relays, and high voltage DC lines in power systems require
N-channel depletion-mode power MOSFETs that operate as normally-on switches when the gate-to-source voltage
is zero (VGS=0V). This paper will describe IXYS IC Division’s latest N-channel, depletion-mode, power MOSFETs
and their application advantages to help designers to select these devices in many industrial applications.
Figure 1
N-Channel Depletion-Mode MOSFET
D
ID
+
G
VDS
+
IG
VGS
IS
-
-
S
A circuit symbol for an N-channel depletion-mode power MOSFET is given in Figure 1. The terminals are labeled
as G (gate), S (source) and D (drain). IXYS IC Division depletion-mode power MOSFETs are built with a structure
called vertical double-diffused MOSFET, or DMOSFET, and have better performance characteristics when
compared to other depletion-mode power MOSFETs on the market such as high VDSX, high current, and high
forward biased safe operating area (FBSOA).
Figure 2 shows a typical drain current characteristic, ID, versus the drain-to-source voltage, VDS, which is called the
output characteristic. It’s a similar plot to that of an N-channel enhancement mode power MOSFET except that it
has current lines at VGS equal to -2V, -1.5V, -1V, and 0V.
CPC3710 - MOSFET Output Characteristics
ID (mA)
Figure 2
Output Characteristics
(TA=25ºC)
300
270
240
210
180
150
120
90
60
30
0
VGS=0.0V
VGS=-1.0V
VGS=-1.5V
VGS=-2.0V
0
1
2
3
VDS (V)
4
5
6
The on-state drain current, IDSS, a parameter defined in the datasheet, is the current that flows between the drain
and the source at a particular drain-to-source voltage (VDS), when the gate-to-source voltage (VGS) is zero (or
short-circuited). By applying positive gate-to-source (VGS) voltage, the device increases the current conduction
level. On the other hand, negative gate-to-source (VGS) voltage reduces the drain current. The CPC3710 stops
conducting drain current at VGS= -3.9V. This -3.9V is called the gate-to-source cutoff voltage or threshold voltage
(VGS(off)) of the device. In order to ensure proper turn-on, the applied gate-to-source (VGS) voltage should be close
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to 0V, and to properly turn off, a more negative VGS voltage than the cutoff voltage (VGS(off)) should be applied.
Theoretically, the on-state drain current, ID(on), can be defined as:
V GS  2
I D = I DSS   1 – -----------------V GS  off 
Equation (1)
Note that Equation (1) is a theoretical formula that, in most cases, would not yield an accurate value of the drain
current. VGS(off) has a range of -3.9V to -0.8V and ID(on) depends both on VGS(off) and the temperature.
A list of IXYS IC Division N-channel discrete depletion-mode power MOSFETs is given in Table 1. The table shows
the device’s four main parameters: the drain-to-source breakdown voltage (BVDSX), the on-state resistance
(RDS(on)), the minimum and maximum gate-to-source cutoff voltage (VGS(off)), and the on-state drain current (IDSS)
along with standard discrete package options such as SOT-89 and SOT-223.
Table 1:
2
IXYS IC Division N-Channel Depletion-Mode MOSFETs
Part No.
BVDSX
(V)
RDS(on)
( )
VGS(off)
Min V
VGS(off)
Max V
IDSS
Min mA
Package
CPC3701
CPC3703
CPC3708
CPC3710
CPC3714
CPC3720
CPC3730
CPC3902
CPC3909
CPC3960
CPC3980
CPC3982
CPC5602
CPC5603
60
250
350
250
350
350
350
250
400
600
800
800
350
415
1
4
14
10
14
22
30
2.5
6
44
45
380
14
14
-0.8
-1.6
-2
-1.6
-1.6
-1.6
-1.6
-1.4
-1.4
-1.4
-1.4
-1.4
-2
-2
-2.9
-3.9
-3.6
-3.9
-3.9
-3.9
-3.9
-3.1
-3.1
-3.1
-3.1
-3.1
-3.6
-3.6
600
300
130
220
240
130
140
400
300
100
100
20
130
130
SOT-89
SOT-89
SOT-89, SOT-223
SOT-89
SOT-89
SOT-89
SOT-89
SOT-223
SOT-223
SOT-223
SOT-223
SOT-23
SOT-223
SOT-223
Selecting a Depletion-Mode MOSFET
Depletion-mode power MOSFETs will function in those applications requiring a normally-on switch. The main
selection criteria for a depletion-mode MOSFET, based on the application, are as follows:
1.
Select the breakdown voltage meeting the margin for reliable operation ~ BVDSX, the drain-to-source
breakdown voltage.
The application voltage must be lower than the drain-to-source breakdown voltage of the device. BVDSX needs to
be selected to accommodate the voltage swing between the positive bus and the negative bus as well as any
voltage peaks caused by voltage ringing due to transients.
2.
Identify the current requirement, and pick a package capable of handling that current ~ IDSS, the on-state
drain current.
The application current must be lower than the on-state drain current (IDSS)of the device. It is the maximum current
that can flow between the drain and source, which occurs at a particular drain-to-source voltage (VDS) and when
the gate-to-source voltage (VGS) is zero.
3.
VGS(off), the gate-to-source cutoff voltage
N-channel depletion-mode MOSFETs have a negative channel cutoff voltage, which is designated as VGS(off). A
designer has to know the magnitude of the negative cutoff voltage (or threshold voltage). A negative
gate-to-source voltage (VGS) will reduce the drain current until the device’s cutoff voltage level is reached and
conduction ceases.
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3
Applications
3.1
Current Source #1
Figure 3 shows a very precise current source to the load, RL1. TL431 is a programmable voltage reference IC. The
feedback voltage from the sense resistor RS is controlled to be 2.5V. The circuit will operate as a current source at
any current level below the CPC3710’s rated current rating, IDSS. Note that at 200V power dissipation will be 1W.
Figure 3
Depletion-Mode MOSFET Current Source and the Current Waveform
M1
CPC3710
6
V1
5
+
RL1
500Ω
4
200
ID (mA)
RB
10kΩ
3
2
U1
TL431
1
RS
500Ω
0
0
20
40
60
80 100 120 140 160 180 200
VDS (V)
The theoretical sense resistor value is given by:
V REF
RS  ------------ID
Equation (2)
Where:
• VREF = 2.5V (TL431)
• ID = 5mA (Desired Current)
Note that Equation (2) is a theoretical formula that would probably not estimate the practical values of RS. In most
cases, it’s convenient to use a potentiometer to set the desired current level.
3.2
Current Source #2
Figure 4 shows a current source example with a voltage reference IC and a depletion-mode MOSFET, Q1, which
compensates for supply voltage fluctuations. The current source provides a total current to the load comprising the
set current through the resistor, RS, and the IC quiescent current, IQ. This circuit provides precision current and
ultra-high output impedance.
Figure 4
N-Channel Depletion-Mode MOSFET with a Voltage Reference to provide a Precise Current Source
+V
Q1
+
VGS1
Depletion-Mode
MOSFET
-
ISET =
VIN
VOUT
RSET
10kΩ
0.1%
10ppm/ºC
C+
VREF
0.01μF
GND
ISET
VOUT
RSET
IOUT = ISET+IQ
Output Impedance > 100MΩ
IQ
IOUT
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3.3
NMOS Inverter Circuit
Figure 5 shows an NMOS inverter circuit that uses a depletion-mode MOSFET as a load. The depletion-mode
MOSFET, Q1, acts as a load for the enhancement-mode MOSFET, Q2, which acts as a switch.
Figure 5
NMOS Inverter with Depletion-Mode Device used as a Load
+V
Q1
Depletion-Mode
MOSFET
VGS=0V
VOUT
Q2
Enhancement-Mode
MOSFET
VIN
RG1
3.4
Off-Line Switch-Mode Power Supply
Many applications in industrial and consumer electronics require off-line switch-mode power supplies that operate
from wide voltage variations of 110VAC to 260VAC. Figure 6 shows such a power supply that uses a
depletion-mode MOSFET, Q1, to kick-start the off-line operation by providing initial power to the IC (U1) through
the source of Q1.
Figure 6
Power Supply Start-Up Circuit with Depletion-Mode MOSFET
Depletion-Mode
MOSFET
L1
D1
VAC In
Input
Filter
Rectifier
D3
+VO
C3
D2
VCC
R1
+
PFC GATE
IC
GND
C1
R2
Q1
D4
C2
RG1
Q2
R3
+15V
+
C4
DZ1
Q3
R4
Q1 provides initial power from the output, +VO. R3 and R4 set up a working point to obtain the minimum required
current from Q1. The Zener diode, DZ1, limits the voltage across the IC (U1) to +15V. After the start-up, the
secondary winding of boost inductor L1 generates the supply voltage for the IC through D1, D2 and C3, and
enough current through D3 and R1 for the base of Q3 that turns on and clamps the gate of Q1 to ground.
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3.5
Voltage Ramp Generator
Applications such as high voltage sweep circuits and automatic test equipment require high voltage ramps with a
linear relationship between output voltage and time. The circuit shown in Figure 7 utilizes one depletion-mode
MOSFET to design a voltage-ramp generator circuit.
Figure 7
High Voltage Ramp Generator with Depletion-Mode and Enhancement-Mode N-Channel MOSFETs
+VDD
600
Q1
CPC3980
500
VOUT (V)
G
ID
S
VOUT
C1
1nF
R1
17kΩ
60
VOUT
400
40
300
30
200
R2
25kΩ
50
20
VIN
100
Q2
IXTP02N120P
10
0
VIN
VIN (V)
D
0
0
1
2
3
4
5
6
Time (ms)
7
8
9
10
Q1 is configured as a constant current source charging a capacitor, C1; R1 provides negative feedback to regulate
and set the desired current value. The constant current source charges the capacitor C1, and generates a voltage
ramp, VOUT, across the capacitor. Q2 can be turned on with a TTL or a CMOS control signal to reset the ramp
voltage by discharging the capacitor to ground through R2. Resistor R2 is used to limit the discharge current for Q2
to operate within its SOA rating.
Assume the ramp voltage:
dV
------- = 0.1V/s
dt
The value of capacitor C1 should be small enough to reduce excessive charging and discharging of energy, but
large enough that output loads and stray capacitances will not introduce significant errors. C1 is chosen to be 1nF.
The charging current is defined as:
dV
I = C1  ------dt
I = 1nF  0.1V/s = 100A
Equation (3)
The value of R1 for a 100A current source can be approximated:
V GS  I D

R1  ---------   ---------– 1
I D  I DSS 
Where:
• VGS = Pinch-off voltage = -1.75V @ desired IDS(on)
• IDSS = Saturation current = 100mA, typical
• ID = 100A
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– 1.75V
1.75V100A- – 1 = –---------------R1 = -----------------   ---------------  0.03162 – 1  = 1.695V
----------------- = 16.9k
100A
100A  100mA 
100A
Assume the switching frequency for Q2 is swf=200 Hz and the discharge time is:
t Dischg = 100s
Power loss in the output capacitor, C1:
2
1
P = ---  C1  V  f sw
2
Equation (4)
Using equation (4),
2
1
P = ---  1nF  500  200Hz = 125J  200Hz = 25mJ/s = 25mW
2
Discharging time:
t Dischg = 4  R2C1
Equation (5)
Using equation (5):
100s- = 25k
R2 = -----------------4  1nF
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3.6
Linear Voltage Regulator
Many applications require a linear voltage regulator that operates from high input voltage that is sourced from a
wide voltage range of 120 VAC to 240 VAC with a maximum peak voltage of +/- 340V. Applications like CMOS ICs
and small analog circuits require a 5V to 15V DC power supply that provides protection from very fast high voltage
transients, and that has low quiescent current requirements. Figure 8 shows a high voltage off-line linear voltage
regulator using a Depletion-mode MOSFET that can meet the above requirement of low transient voltage and low
quiescent current.
High Voltage Off-line Linear Voltage Regulator
1.2
10
+HVIN
IOUT
LT3009-5
VIN
C1
1μF
IN
SHDN
VOUT (V) and VIN (V)
Q1
CPC5603
VOUT
OUT
GND
C2
1μF
RL
5kΩ
8
VIN
6
VOUT
4
1.0
0.8
0.6
IOUT (mA)
Figure 8
0.4
2
0.2
0
0
30
60
0
90 120 150 180 210 240 270 300
HVIN (V)
High voltage transients are generated in telecommunication circuits because of lightning and spurious radiation,
and in automotive and avionics circuits because of inductive loads. Low quiescent current is required to minimize
power dissipation in these linear regulators.
HVIN Calculation:
V GS  2
I D = I DSS   1 – -----------------V GS  off 
Solving for VGS:

ID 
V GS = V GS  off    1 – ---------
I DSS

Where:
• V GS = V OUT – V IN

ID 
V IN = V OUT – V GS  off    1 – ---------
I DSS

1mA- = 5 + 2   1 – 0.3162  = 6.38V
V IN = 5 + 2   1 – ------------10mA
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3.7
Current-Monitor Circuit
A simple current monitor circuit using an op-amp and a depletion-mode MOSFET is shown in Figure 9. R1 monitors
the current to the load and the MOSFET, Q1, provides an output voltage proportional to the current being
monitored.
RS  R2
V OUT = I LOAD   ------------------
 R1 
Equation (7)
Resistor, R1, should have a tolerance of 0.1% with an appropriate wattage rating.
Figure 9
Current Monitor using Depletion-Mode MOSFET and a Single-Supply Op-Amp
ILOAD
RS
+VIN
RLOAD
C1
C2
R1
+
D
Q1
VOUT
G
S
R2
For example:
• RS=0.1
• R1=100
• R2=1k
Using equation (7):
RS  R2
V OUT
 1000- = 1V/A
-------------- = ----------------- = 0.1
-----------------------R1
I LOAD
100
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3.8
Normally Closed Solid State Relay
Depletion-mode FETs can be used to create normally closed solid state relays using IXYS IC Division’s optical
driver, FDA217. Figure 10 shows a typical connection of two external CPC3980 depletion FETs arranged in
back-to-back configuration to make an AC/DC switch. FDA217 has internal turn-off circuitry so that no external
bleed resistors are required.
Figure 10
FDA217 used with CPC3980 FETs to create Normally Closed Solid State Relay
FDA217
R2
+
8
+
6
PV
2
3
PV
R1
V1
5V
CPC3980
V2
7
+
1
220VAC
5
4
CPC3980
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IXYS Integrated Circuits Division makes no representations or warranties with respect to the accuracy or completeness of the contents of this publication and reserves the right to make
changes to specifications and product descriptions at any time without notice. Neither circuit patent licenses nor indemnity are expressed or implied. Except as set forth in IXYS Integrated
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products including, but not limited to, the implied warranty of merchantability, fitness for a particular purpose, or infringement of any intellectual property right.
The products described in this document are not designed, intended, authorized or warranted for use as components in systems intended for surgical implant into the body, or in other
applications intended to support or sustain life, or where malfunction of IXYS Integrated Circuits Division’s product may result in direct physical harm, injury, or death to a person or severe
property or environmental damage. IXYS Integrated Circuits Division reserves the right to discontinue or make changes to its products at any time without notice.
Specification: AN-500-R03
©Copyright 2014, IXYS Integrated Circuits Division
All rights reserved. Printed in USA.
10/3/2014
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