Introduction:
From the above plot in fig 5 or in the table above we can see
at Vgs= 3 v,
Vds = 0.1 v and our requirement is Vds < 0.5
Hence we can use SI 2302 for our application
In an embedded system, different peripherals are interfaced to uController. Some times controller don't need these peripherals and we want that peripheral to be shut down to save the Power. In this blog, we will see How to select a MOSFET for a 3.3v powered circuit. A 3.3v powered uController will use this MOSFET for switching ON and OFF the peripherals like any WiFi module.
Before selecting the MOSFET, we need to see the battery fully charged and fully discharged voltages.
Li[Po battery voltages are:
---- 4.2V when fully charged
---- 3 V when fully discharged
So, the requirements from MOSFET are
1) Drain Current, Id >80mA (From datasheet of ESP8266)
2) Voltage across Peripheral Vph > 2.5V (From datasheet of ESP8266)
hence when battery fully discharged (i.e 3v) then
Vds < (3.0-2.5)
Vds < 0.5v when voltage at gate(Vgs) =3v
1) 2N7000-D Small Signal MOSFET__200 mA, 60 Volts
2) IRF510 Power MOSFET
3) BS170-D Small Signal MOSFET__500 mA, 60 Volts
4) BSS138-D N-Channel Logic Level Enhancement
5) Si2302DS N-Channel 1.25-W, 2.5-V MOSFET
Circuit used for analysis is as follows:
All these 5 MOSFETS have maximum drain current Id > 80mA. Hence we are going to check only Vds < 0.5v condition at Vgs=3v.
I plotted Vds vs Vgs graph for all 5 MOSFETs and Id is also measured for each value of Vgs.
 |
| ckt 1: circuit for Peripheral power controlled by uController |
Requirements From MOSFET:
Consider, ESP8266 WiFi Module is a peripheral that we want to switch ON/OFF using a ATTiny45 uController.
 |
| ckt 2: circuit for ESP8266 power controlled by ATTiny45 |
Before selecting the MOSFET, we need to see the battery fully charged and fully discharged voltages.
Li[Po battery voltages are:
---- 4.2V when fully charged
---- 3 V when fully discharged
1) Drain Current, Id >80mA (From datasheet of ESP8266)
2) Voltage across Peripheral Vph > 2.5V (From datasheet of ESP8266)
hence when battery fully discharged (i.e 3v) then
Vds < (3.0-2.5)
Vds < 0.5v when voltage at gate(Vgs) =3v
Analysis of MOSFETs:
I had 5 different MOSETs that are1) 2N7000-D Small Signal MOSFET__200 mA, 60 Volts
2) IRF510 Power MOSFET
3) BS170-D Small Signal MOSFET__500 mA, 60 Volts
4) BSS138-D N-Channel Logic Level Enhancement
5) Si2302DS N-Channel 1.25-W, 2.5-V MOSFET
Circuit used for analysis is as follows:
 |
| ckt 3: circuit used for analysis |
All these 5 MOSFETS have maximum drain current Id > 80mA. Hence we are going to check only Vds < 0.5v condition at Vgs=3v.
I plotted Vds vs Vgs graph for all 5 MOSFETs and Id is also measured for each value of Vgs.
1) 2N7000-D
Vgs
|
Id
|
Vds
|
0
|
0
|
3.29
|
0.95
|
0
|
3.29
|
1.12
|
0.0013
|
3.28
|
1.34
|
0.0252
|
3.28
|
1.53
|
0.1
|
3.28
|
1.65
|
0.5
|
3.28
|
1.8
|
1.031
|
3.27
|
1.83
|
2
|
3.24
|
2.06
|
5
|
3.2
|
2.3
|
10
|
3.1
|
2.55
|
25
|
2.83
|
2.71
|
35.6
|
2.65
|
2.83
|
45.2
|
2.49
|
2.94
|
55.2
|
2.35
|
3.05
|
65.1
|
2.22
|
3.11
|
71.8
|
2.13
|
3.18
|
78.8
|
2.11
|
 |
| fig 1:Vds vs Vgs For 2N7000 |
From above plot in fig1 or in the table above we can see
at Vgs= 3.05v,
Vds =2.22v and our requirement is Vds < 0.5
Hence we can't use 2N7000-D for our application
2) IRF510
Vgs
|
id
|
Vds
|
0
|
0
|
3.29
|
1.5
|
0
|
3.29
|
2.27
|
0.0014
|
3.29
|
2.48
|
0.01
|
3.29
|
2.67
|
0.05
|
3.29
|
2.8
|
0.1
|
3.29
|
2.92
|
0.5
|
3.28
|
3.05
|
1
|
3.27
|
3.09
|
2.52
|
3.26
|
3.17
|
4
|
3.24
|
3.19
|
4.38
|
3.24
|
3.21
|
4.78
|
3.24
|
 |
| fig 2: Vds vs Vgs For IRF510 |
From the above plot in fig2 or in the table above we can see
at Vgs= 3.09 v,
Vds = 3.26v and our requirement is Vds < 0.5
Hence we can't use IRF510 for our application
3) BS170-D
Vgs
|
Id
|
Vds
|
0
|
0
|
3.22
|
1.18
|
0
|
3.22
|
1.44
|
1
|
3.21
|
1.65
|
5
|
3.16
|
1.77
|
10
|
3.09
|
1.95
|
21
|
2.94
|
2.2
|
36.3
|
2.71
|
2.38
|
50
|
2.52
|
2.5
|
65
|
2.3
|
2.72
|
80
|
2.05
|
2.88
|
95
|
1.89
|
3.06
|
110
|
1.72
|
3.11
|
110
|
1.72
|
 |
| fig 3: Vds vs Vgs For BS170-D |
From the above plot in fig 3 or in the table above we can see
at Vgs= 3.06 v,
Vds = 1.72 v and our requirement is Vds < 0.5
Hence we can't use BS170-D for our application
4) BSS138
VGS
|
Id(mA)
|
Vds
|
1
|
0
|
3.3
|
1.19
|
0.05
|
3.3
|
1.29
|
0.2
|
3.28
|
1.36
|
0.511
|
3.28
|
1.43
|
1.013
|
3.26
|
1.5
|
2.05
|
3.21
|
1.63
|
5.09
|
3.08
|
1.77
|
10.44
|
2.88
|
1.93
|
21.4
|
2.64
|
2.1
|
35.6
|
2.17
|
2.25
|
50.1
|
1.75
|
2.4
|
65.5
|
1.32
|
2.6
|
81.8
|
0.94
|
2.81
|
90.5
|
0.78
|
3
|
94.6
|
0.7
|
3.15
|
98.2
|
0.68
|
3.22
|
100.3
|
0.67
|
 |
| fig 4: Vds vs Vgs For BSS138 |
From the above plot in fig 4 or in the table above we can see
at Vgs= 3 v,
Vds = 0.7 v and our requirement is Vds < 0.5
Hence we can't use BSS138 for our application
5) SI 2302
Vgs
|
Id(mA)
|
Vds
|
0
|
0
|
3.3
|
0.2
|
0
|
3.3
|
0.5
|
0
|
3.29
|
0.657
|
0.1
|
3.29
|
0.708
|
0.3
|
3.28
|
0.767
|
1
|
3.24
|
0.802
|
2.01
|
3.19
|
0.85
|
5
|
3.05
|
0.893
|
10.26
|
2.78
|
0.955
|
25.4
|
2.27
|
1
|
40.7
|
1.66
|
1.045
|
55
|
1.16
|
1.091
|
70
|
0.78
|
1.148
|
85.5
|
0.24
|
1.205
|
89
|
0.15
|
1.505
|
91.8
|
0.109
|
2.02
|
94.4
|
0.1
|
3
|
94.4
|
0.1
|
3.3
|
94.4
|
0.1
|
 |
| fig 5: Vds vs Vgs For BSS138 |
From the above plot in fig 5 or in the table above we can see
at Vgs= 3 v,
Vds = 0.1 v and our requirement is Vds < 0.5
Hence we can use SI 2302 for our application
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