Application Note Applications for Depletion MOSFETs

Appli ca ti o ns for D ep l eti on MO S F ET
Pradeep Kumar Tamma
Application Note
About this document
Scope and purpose
Depletion MOSFETs, unlike Enhancement MOSFETs, are in an On-state even at 0 V of gate to source voltage
(VGS). This feature makes them suitable for using as a constant current source as well as in other ways. This
application note explains how Depletion MOSFETs can be used in different applications.
Intended audience
This document is intended for SMPS or LED driver designers, as an introduction to Depletion MOSFET and
highlighting the advantages.
Table of Contents
1
What is a Depletion MOSFET?......................................................................................................... 2
2
Linear mode operation of a Depletion MOSFET ............................................................................. 3
3
3.1
3.2
3.2.1
3.3
Application Examples..................................................................................................................... 5
Start-up circuit for SMPS ..................................................................................................................... 5
Linear regulators ................................................................................................................................. 8
Examples ....................................................................................................................................... 9
Other examples ................................................................................................................................. 10
4
Depletion MOSFETs from Infineon ............................................................................................... 11
1
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Applications for Depletion MOSFET
What is a Depletion MOSFET?
1
What is a Depletion MOSFET?
As Depletion MOSFETs are in the on-state, they operate as an ON switch even when the gate to source
voltage (VGS) is zero. This can be best shown by comparing the transfer characteristics of both Enhancement
and Depletion MOSFETs.
Enhancement MOSFET
Figure 1
Depletion MOSFET
Transfer Charateristics of Enhancement and Depletion MOSFETs
Figure 1 above illustrates an example of the transfer characteristics of both devices. For a MOSFET, the gate
to source voltage (VGS) should be greater than the gate to source threshold voltage (VGS(th)) in order to
conduct current through it. For an N-channel Enhancement MOSFET VGS(th) is greater than 0 V. Therefore,
even at VGS of 0 V, a Depletion MOSFET conducts current. To turn off a Depletion MOSFET the VGS should be
less than the VGS(th) which is a negative. This is clearly shown in schematic symbols of both. Figure 2 below
shows the schematic symbols for Enhancement and Depletion MOSFETs respectively.
(a) Enhancemnet MOSFET
Figure 2
(b) Depletion MOSFET
Schematic symbols
In the symbols there is a clear difference in the second vertical line from the left. For the Enhancement
MOSFET this line is discontinuous which mean that the MOSFET is in an off state at zero gate voltage. With
the Depletion MOSFET, as it is in an on-state at zero gate voltage, the second vertical line is a solid
continuous one.
Application Note
2
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Applications for Depletion MOSFET
Linear mode operation of a Depletion MOSFET
2
Linear mode operation of a Depletion MOSFET
In the previous section, how a Depletion MOSFET conducts at zero gate to source voltage and how it can be
turned off was described. Now it will be discussed how a Depletion MOSFET can be used in a circuit.
Figure 3 below illustrates a simple Depletion MOSFET circuit. Initially the voltage across the resistor R1 is 0 V,
therefore, the gate to source voltage of Q1 is 0 V. As stated in the previous section, at zero gate to source
voltage there will be current flowing through the MOSFET. Thus, when VIN is applied there will be a current
(ID) flowing in the circuit. Due to this current, there will be a voltage drop across the resistor R1 which in turn
reduces the gate to source voltage of Q1. The value is given by equation 1 below:
GS = −D ∗ 1
(1)
Q1
ID
D
R1
R2
VIN
Figure 3
S
G
Simple circuit using a Depletion MOSFET and a resistor
As the gate to source voltage (VGS) is reduced to a negative, the current through Q1 decreases (from Figure 1).
The current through Q1 is given by equation 2 below:
D = DSS (1 −

GS
GS(th)
2
)
(2)
With:

IDSS = on-state drain current at VGS = 0 V

VGS(th) = threshold voltage of the MOSFET
The drain to source voltage is:
 =  − ( ∗ (1 + 2))
(3)
At lower VDS, Q1 will be in the resistive mode of operation so the drain current ID of Q1 depends upon the
RDS(on) and VDS of Q1. Thus the equation (2) will no longer be valid. But at higher VDS the MOSFET will be in
linear mode region. In this mode the drain current gets saturated and does not depend on VDS which means
equation (2) is valid here. For a MOSFET to be in linear mode VDS should be greater than or equal to
2*ID*RDS(on).
Therefore in the circuit (shown in Figure 3), when Q1 is in linear mode the current ID is independent of the VDS.
Thus, a Depletion MOSFET in series with a resistor can be used as a constant current source whose current
value is independent of the drain to source voltage. As mentioned previously, the VGS of Q1 in the circuit is
Application Note
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Applications for Depletion MOSFET
Linear mode operation of a Depletion MOSFET
less than 0 V. For example, Figure 4 below shows the typical output charateristics of BSP179 and the red
dotted line is the constant current operating region for a respective VDS.
The current through MOSFET Q1 depends on the gate to source voltage and this in turn depends upon the
series resistance R1. Thus for a required current through Q1, R1 can be calculated using the formula below.


1 = GS(th) (√ D − 1)
D
DSS
(4)
VGS(th) is the gate threshold voltage of the MOSFET, IDSS is the on current at VGS = 0 V and ID is the required
current.
Figure 4
Output charateristics of a Depletion MOSFET
Application Note
4
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Applications for Depletion MOSFET
Application Examples
3
Application Examples
3.1
Start-up circuit for SMPS
A major application where a Depletion MOSFET can be used is the start-up circuit for switch mode power
supplies (SMPS).
Vin
ID
VAUX
Vout
Q1
R1
VZ
Figure 5
Z
IIC
VCC
PWM IC
C1
Start-up circuit for SMPS
Commonly used low voltage bipolar or CMOS PWM ICs usually operate from supply voltages of up to 18 V.
When the input power for the converter is available at voltages higher than the maximum voltage rating of
the IC, the voltage has to be reduced with a start-up circuit. A frequent requirement is for operation directly
from a rectified 120 VAC or 230 V AC line without the use of tap changing switches for the selection of different
voltages. Figure 5 shows a starting circuit using a Depletion MOSFET, a resistor and a zener diode.
Depletion MOSFET Q1 is configured as a source follower. Being a Depletion MOSFET, Q1 is in the on-state
when there is 0 V VGS. When power is available at the input, Q1 supplies current to charge up the capacitor C1
through the external source resistor R1. Therefore, the voltage across the capacitor (VCC) starts increasing.
Figure 6 shows the start-up current (Ist) and voltage wave forms of a typical PWM IC. As VCC reaches VCC,ON, it
allows the PWM IC to start PWM pulses to primary MOSFET Q2. As a result the auxiliary voltage (VAUX) from the
auxiliary winding starts building up. During this time the power required by the IC is supplied by the
capacitor C1. Once VAUX is up, it turns off Q1 and the IC’s power is delivered only by the auxiliary winding. The
condition for VAUX to turn off Q1 is
AUX ≥ (ℎ), + Z + 0.7 
(5)
The time taken to reach VCC,ON across the capacitor is given by equation (6) below.
 = C1 ∗
,
 − 
(6)
ID is the required current though Q1. From section 2 it is clear that when the Depletion MOSFET has high VDS
and the VGS is ≤ 0V, the MOSFET operates in linear mode. Here current ID does not depend on VDS and remains
constant, therefore tstc and the start-up time for the SMPS are always constant.
Application Note
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Applications for Depletion MOSFET
Application Examples
IIC
IOP
Ist
VCC
VCC,ON
VCC,OFF




VAUX
tstc
Figure 6
VCC,ON  PWM IC turn on threshold voltage
Ist
start-up current (drawn by IC before Vcc,ON reached)
VCC,OFFPWM IC turn off threshold voltage
IOP
 Operating current (drawn by IC after Vcc,ON reached
tsto
PWM IC start-up waveforms
For example, consider a PWM IC with electrical parameters as stated in Table 1 below:
Table 1
PWM IC electrical parameters
PWM IC
Ist
[µA]
Iop
[µA]
VIN,on
[V]
VIN,off
[V]
300
7000
15
8
Assume that C1 is equal to 10 µF, VIN = 340 V. A reasonable start-up time tstc is 500 ms, so by reconfiguring the
equation (5) the drain current ID is
 = C1 ∗
,
15 
+  = 10µ ∗
+ 300 µ = 600 µ

500 
(7)
This current is constant irrespective of VIN. Therefore, the start-up time tstc is constant even for a wide range
of input (90 to 240 VAC). This is not the case with resistive solutions. For the above same example with a startup time of 500 ms the current in the resistor will be the same as the drain current 600 µA. So at 340 V of VIN
the start-up resistor value can be calculated.
− =

340 
=
≈ 570 Ω

600 µ
(8)
But for a wide input range the start-up time will no longer be a constant in the resistor solution. Figure 7
shows the variations of start-up time with input voltage.
Application Note
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Applications for Depletion MOSFET
Application Examples
Figure 7
Start-up time vs input voltage in the resistor solution
Therefore, using a Depletion MOSFET as a start-up circuit has the greater benefit of keeping the start-up
time constant irrespective of the input voltage. Table 2 shows the basic electrical parameters of the Infineon
Depletion MOSFET BSS126.
Table 2
BSS126 Electical parameters
Part Number
BVDSS
[V]
VGS(th),min
[V]
VGS(th),typ
[V]
VGS(th),max
IDSS,min @VGS = 0 V
[mA]
BSS126
600
-2.7
-2.0
-1.6
7
After the PWM IC is up and running, the only current drawn by Q1 is a small amount of drain to source
leakage current which is typically less than 100µA.
The power dissipation of Q1 during start-up time is given by:
 = ( − , ) ∗ ( )
(9)
Since Q1 conducts only for a short duration, the maximum chip temperature rise is minimal. The power
dissipation in BSS126 is:
 = ( − , ) ∗ ( ) = (340  − 15 ) ∗ 600µ = 0.195 
Thus the chip temperature rise is given by:
∆ =  ∗ ℎ@ ,@
(10)
ZthJA is the maximum transient thermal impedance of BSS126. Since the MOSFET conducts only at the
start-up time of the SMPS, it is considered that the power dissipation is a single pulse event.
Figure 8 shows ZthJA for a single pulse of 500ms duration as in our example is equal to 130 K/W on a standard
PCB. Therefore, the maximum temperature rise of BSS126 in the application example is:
Application Note
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Applications for Depletion MOSFET
Application Examples
∆ = 0.195  ∗ 130
Figure 8
BSS126 max. transient thermal impedance
3.2
Linear regulators

≈ 25 

A Depletion MOSFET can also be used as a pass transistor for a linear regulator. Figure 9 shows a linear
regulator circuit using a Depletion MOSFET and a zener diode.
In the circuit, Q1 acts as a source follower where the source voltage (Vcc) follows the gate voltage (VG) minus
the gate to source voltage. VGS increases with increasing drain current. Thus, with a fixed gate voltage, the
source voltage will drop with increasing load current. For design purposes, VGS under saturation and cut-off
conditions (0 V and VGS(th), respectively) can be used. These values can be readily obtained from the data
sheet, for example Figure 10 shows the cut-off and saturation conditions of the Infineon BSS169. Bias
current for the zener diode is determined by VGS/R1.The output capacitor C1 is to compensate for the ripple
at the input side. The zener diode (Z) sets the gate voltage and should be selected to provide a source
voltage within the range determined, taking into account the variances of VGS with load. The source VCC
generated is approximately equal to:
cc = Z + 
(11)
Substituting VGS value (from equation (2)) in the above equation we get:
cc = Z − (GS(th) (1 − √
Application Note
8
D
DSS
(12)
))
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Applications for Depletion MOSFET
Application Examples
Q1
ID
ILoad VCC
S
D
R1
Vin
G
Load
VGS
IBIAS
C1
Z
VG
Figure 9
Using a Depletion MOSFET as a voltage regulator
The power dissipation for Q1 can be calculated from the voltage drop across it times the current through it:
(7)
1 = ( −  )( +  )
The point to be noted here, is that the MOSFET’s saturation current (IDSS) must be greater than the load and
bias currents otherwise the device can be stressed due to over current which can destroy it.
Figure 10
Cutt-off conditions of BSS169
3.2.1
Examples
The above application can be explained more clearly with an example. Consider a regulator with 5 V output
at 5 mA of output current from 24 V input using Infineon’s Depletion MOSFET BSS169. The important
features of BSS169 are shown in the table below:
Table 3
BSS169 Depletion MOSFET parameters
Part Number
BVDSS
[V]
VGS(th),min
[V]
VGS(th),typ
[V]
VGS(th),max
IDSS,min @VGS = 0 V
[mA]
BSS169
100
-2.9
-2.2
-1.8
90
In the circuit, output current is equal to the current through the MOSFET denoted here by ID. The equation to
calculate the zener voltage can be calculated from equation (12).
Application Note
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Applications for Depletion MOSFET
Application Examples
z = cc,typ + (GS(th),typ (1 − √
z = 5  + (−2.2  ∗ (1 − √
5∗10−3 
90∗10−3 

))

(8)
)) ≅ 3.3 
Due to VGS(th) spreading, the output voltage of the regulator varies from 4.7 V to 5.5 V. The power dissipation
in BSS169 can be calculated from equation (12).
(95)
1 = ( −  )( +  )
1 = (24 − 4.7) ∗ (5 ∗ 10−3 + 30 ∗ 10−6 ) ≈ 0.1 
This is well below the specified maximum power dissipation of BSS169.
3.3
Other examples
A constant current source can be used in many different applications. A few examples are listed below:
− A constant current source is useful to generate a bias current that is independent of the voltage across
it. It can be used to charge a capacitor at a constant rate, generating a linear ramp for timing purposes.
− A constant current source can be used as a trickle charger to maintain battery charge state.
− A constant current source can be used as a current limiter when the current is below the limit.
− A constant current source can also be used as a linear LED driver for driving LED strings typically up to
20mA. Figure 12 below shows a linear LED driver circuit using a Depletion MOSFET and a series
resistor.
Q1
R1
~20 mA
VLED
LED string
Vin
Figure 11
Linear LED driver
Application Note
10
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Applications for Depletion MOSFET
Depletion MOSFETs from Infineon
4
Depletion MOSFETs from Infineon
Infineon offers Depletion MOSFETs in SOT-223 and SOT-23 packages with breakdown voltages ranging from
60 V to 600 V. Table 4 shows Infineon’s Depletion MOSFET portfolio:
Table 4
Depletion MOSFET portfolio
Part
Number
Package
BVDSS
[V]
RDS(on)
[Ω]
VGS(th),max
[V]
VGS(th),min
[V]
IDSS,min
@VGS = 0V
[mA]
BSP149
SOT-223
200
1.8
-1
-2.1
140
BSP129
SOT-223
240
6
-1
-2.1
55
BSP179
SOT-223
400
24
-1
-2.1
20
BSP135
SOT-223
600
45
-1
-2.1
20
BSS159N
SOT-23
60
3.5
-2.4
-3.5
130
BSS169
SOT-23
100
6
-1.8
-2.9
90
BSS139
SOT-23
250
14
-1
-2.1
30
BSS126
SOT-23
600
7
-1.6
-2.7
7
For Infineon’s Depletion MOSFETs VGS(th) is spread across 5 bands. Each band has a distribution of 0.2 V and is
assigned with an identification letter. Limits for each device vary from VGSth,min to VGSth,max. Figure 12 shows an
example of VGS(th) bands of BSP149.
Figure 12
Threshold voltage bands
Please note - a single specific band cannot be ordered separately. However, it can be requested that each
reel contains only one band of VGS(th).
Application Note
11
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Applications for Depletion MOSFET
Depletion MOSFETs from Infineon
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