AN-D66

Supertex inc.
AN-D66
Application Note
Depletion-Mode MOSFET: The Forgotten FET
By Bill Chen, Field Application Engineer
I. Introduction
For most practicing electrical engineers, the last time that
they had contact with the Depletion mode MOSFET is in
their undergraduate junior year. It is mentioned in class as
another version of MOSFET that exists, but often there is no
further mention of it. Here is a refresher on its properties and
where it can be uniquely useful.
The Depletion-mode MOSFET is a cousin of the more
familiar Enhancement-mode MOSFET.
The symbols for these devices are shown below:
DRAIN
Note that the Enhancement-mode FET symbol shows a
broken (normally non-conducting) channel at zero gate bias,
while the Depletion-mode FET symbol shows a normally
conducting channel at zero gate bias.
Common properties:
•
•
•
•
Majority carrier devices, therefore fast switching.
Insulated gates, thus no bias current.
Body diode that points from source to drain.
Behave resistive when VDS is near zero volts, with
positive temperature coefficient.
Differentiating Properties:
GATE
SOURCE
Enhancement-Mode FET
DRAIN
•
Enhancement FET does not conduct when VGS = 0.
Conduction starts when VGS = VGS(th). Current increases as VGS is increased beyond VGS(th).
•
Depletion FET is conducting when VGS = 0. The current
decreases when VGS < 0 until VGS = VGS(th), when
conduction stops. Current increases when VGS > 0.
These differences and similarities are visible in Figures 1
and 2.
GATE
SOURCE
Depletion-Mode FET
Output Characteristics
2.0
Saturation Characteristics
1.0
1.6
VGS = 10V
8.0V
6.0V
0.8
VGS = 10V
6.0V
0.8
ID (amperes)
ID (amperes)
8.0V
1.2
5.0V
0.6
0.4
5.0V
4.0V
0.2
0.4
4.0V
0
3.0V
0
10
20
30
VDS (Volts)
40
50
0
3.0V
0
2
4
6
VDS (volts)
8
10
Figure 1. Enhancement FET: TN2540 Output and Saturation Characteristics
Doc.# DSAN-AN-D66
NR031313
Supertex inc.
www.supertex.com
AN-D66
DN2540 Output Curves
VGS = 1.0V
0.5V
0V
1.2
1.0
-0.5V
ID (amperes)
0.8
0.6
-1.0V
0.3
0.2
0
0
10
20
30
40
50
VDS (Volts)
Figure 2. Depletion FET: DN2540 Output Characteristics
Because of the “normally on” property of Depletion FET,
there are some interesting and unique applications for it.
2. It can be used to charge a capacitor at a constant
rate, generating a linear ramp for timing purposes.
II. Unique Applications of Depletion Mode FETs
3. It can be used as a trickle charger to maintain battery
charge state.
4. The constant current circuit can be used as a Current
Limiter. When the current is below the current limit,
the circuit shown in Figure 3 also behaves like a
resistor with value of 1.01kΩ.
D
G
Q1
DN2540
G
G
S
R1
1kΩ
D
S
Q1
DN2540
+
R1
1kΩ
S
D
Q2
DN2540
Figure 3. Constant Current Source
Figure 4. Bi-Directional Current Limiter
Figure 3 shows a constant current source using a Depletion
FET and a resistor.
Figure 4 shows a Bi-directional current limiter. When the
current is flowing from left to right as shown, the voltage
across the R1 renders VGS of Q1 negative. When the VR1
reaches VGS(th), Q1 goes into current limiting mode. In the
meantime, the VGS of Q2 is positive and Q2 behaves as a
resistor of less than 10Ω.
The current in the circuit is approximately |VGS(th)| / R1. The
value of VGS(th) is typically in the range of (-1.5V to -3.5V),
thus the current is in the range of (1.5mA to 3.5mA). A
variable resistor can be used to set the current to a specific
value.
When the current is flowing from right to left, the reverse
of the above situation occurs and Q2 becomes the current
limiting FET. This circuit is commonly used to protect test
sensitive instruments that can be accidentally exposed to
high voltage sources.
Applications:
1. The constant current source is useful to generate
a bias current that is independent of the voltage
across it.
Doc.# DSAN-AN-D66
NR031313
Supertex inc.
2
www.supertex.com
AN-D66
VIN = Up to 400V
VAUX
D
G
S
+
VOUT
-
Q1
DN2540
R1
100kΩ
PWM IC
+
9.1V
Figure 5. Starting Circuit for SMPS
Applications:
1. Portable testers / Volt meters input over-current
protection.
VDD
HV91xx PWM controllers and
some HV99xx/HV98xx LED drivers
2. Bed-of-nail In-circuit testers input over-current
protection.
VIN
Many popular SMPS (switch-mode power supply) controller
ICs require a local power supply of 10V to 16V to get started.
Once the SMPS is up and running, the auxiliary winding will
generate the 16V rail to keep the IC running. The Starting
circuit will then go into dormant state to conserve power.
Depletion Mode
FET
To
internal
circuits
+
REF
Figure 5 shows a starting circuit using a Depletion FET, a
resistor and a Zener diode.
Figure 6. Internal Linear Regulator using Depletion
Mode FET
The Depletion FET and resistor circuit functions as a current
limiter for the current in the Zener diode. The voltage at the
source of the Depletion FET is at VZ + VGS(th), approximately
10.5V. Once the SMPS is up and running, the VAUX will be
at 16V. The voltage at the source of the Depletion FET will
be at 16V, while the voltage at the gate is 9.1V. Thus, the
Depletion FET will be negatively biased by more than the
gate threshold voltage and the FET will stop conducting. The starting circuit goes into the required dormant state,
consuming almost no power. DN2540, as shown in Figure
5, is a 400V Depletion FET. Thus, the starting circuit can
work up to 400V. If the power source is higher than 400V, the
DN2540 can be replaced with a higher voltage rated FET.
Supertex’s HV91xx PWM Controllers and some HV99xx/
HV98xx LED drivers can be powered directly from its VIN pin
and can work from a high voltage input up to 450V (for some
devices) at its VIN pin. When a voltage is applied at the VIN
pin, the HV91xx and some of the HV99xx/HV98xx maintains
a constant voltage at the VDD pin. The VDD voltage is set by
the internal REF voltage. The voltage at the VDD pin is used
to power the IC and any external, low current circuit, needed
to control the IC.
All Supertex’s PWM controllers and some LED drivers use
high voltage depletion mode FET as the pass transistor for
their internal linear regulator as shown on Figure 6.
Doc.# DSAN-AN-D66
NR031313
An external voltage source that is greater than the internally
regulated voltage can also be connected to the VDD pin to
operate the HV91xx and some HV99xx/HV98xx. This will turn
off the internal regulator of the IC and then the IC will operate
directly off of the voltage supplied at the VDD pin. Please note
that this external voltage at the VDD pin should not exceed
the absolute maximum rating of the respective VDD pin.
Supertex inc.
3
www.supertex.com
AN-D66
circuits. The power source is often nominally 14V but can
vary, from 8V to 100V, for a short duration.
Being able to turn off the internal regulator by applying an
external voltage or boot-strap winding that is higher than the
VDD is beneficial, especially for line powered application, as
this minimizes the power dissipation of the IC.
+
ILED
G
Most LDO will function properly up to an upper limit of 30V.
Beyond that, LDO will self-protect by shutting down. In
situations that require the LDO to provide continuous output,
an input protection circuit will be needed.
D
Figure 8 shows a typical voltage clamp circuit. The circuit
consists of a power resistor, a Zener diode and a bipolar
transistor. The bipolar transistor is used as an emitter
follower. When the input is below the clamp Zener voltage,
the resistor biases the transistor on, and the transistor
operates in saturation. Approximately 0.2V is dropped across
the transistor. When the input voltage surges up beyond the
Zener voltage, the Zener clamps the base of the transistor at
Vz, and the emitter of the transistor is clamped at one diode
voltage below the Vz. Thus, the LDO never sees any voltage
beyond the Zener voltage.
S
S
G
-
D
Photovoltaic Diodes
Figure 7. Normally-On Solid State Relay
For the duration of the clamp action, the transistor absorbs
the surge voltage and carries the output current, and power
is dissipated in the transistor. The transistor must be sized
to withstand the power dissipation for the duration of the
voltage surge.
A normally-on solid state relay can be constructed using
Depletion FETs as the contact elements, as shown in Figure
7. The Depletion FETs are connected back-to-back, with
their sources and gates tied together. The gate and source
terminals are driven by the photo voltaic diode string, which
in turn is driven by the LED diode.
VIN = 12V
Surge 200V
When ILED is off, there is no photo voltaic voltage generated.
VGS = 0. The Depletion FETs are in the conducting state and
the relay is in the on state..
D
G
DN2625
When ILED is on, a photo voltaic voltage is generated with
sufficient magnitude to turn the depletion FETs off, rendering
the relay in the off-state.
S
VOUT +1.5V
CIN
VIN = 12V
Surge 200V
LM2931
COUT
VOUT
5.0V
Figure 9. Linear Regulator, Surge Protected to 200V VIN
using Depletion Mode MOSFET
VZ
CIN
LM2931
COUT
A much simpler solution is to use a Depletion-mode
MOSFET, as shown in Figure 9. The gate of the depletion
FET, a DN2625, is connected to VOUT. The source is
connected to the input of the LDO, LM2931. The load current
flows from VIN through the DN2625 and the LDO to the load.
At IOUT = 100mA, the DN2625 is operating at VGS of -1.5V.
Thus, VSOURCE is at VOUT +1.5V, well above the LDO drop
out requirement. Since there is voltage across and current
through the FET, power is being dissipated in the FET. An
appropriate package must be chosen to dissipate the power.
VOUT
5.0V
Figure 8. Linear Regulator, Surge Protected to 200V VIN
using Transistor/Zener/Resistor Clamp
Low Drop-Out (LDO) linear regulators are often used to
provide a stable low voltage, say 5V, to power electronic
Doc.# DSAN-AN-D66
NR031313
Supertex inc.
4
www.supertex.com
III. References:
The applications shown above are example circuits making
use of the unique properties of the Depletion FETs. More
examples can be found in the references below.
Supertex Application Notes:
AN-D10:
Off-Line Compact Universal Linear Regulator.
AN-D11:
+/- 500V Protection Circuit.
AN-D12:
High Voltage Ramp Generator.
AN-D16:
High Voltage Regulators and Linear Circuits using LND1.
AN-D17:
High Voltage Off-Line Linear Regulator.
AN-D18:
High Voltage Regulators and Linear Circuits using DN25.
AN-D25:
Efficient Switch-mode Power Supply Start-
Up Circuit.
AN-D26:
High Voltage Isolated MOSFET driver.
AN-D30:
Off-Line 5.0V Output Non-Isolated Linear Regulator
These documents can be found at the Supertex website:
www.supertex.com
Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives
an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability
to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and
specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com)
Supertex inc.
©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited.
Doc.# DSAN-AN-D66
NR031313
5
1235 Bordeaux Drive, Sunnyvale, CA 94089
Tel: 408-222-8888
www.supertex.com
Similar pages