pmos switch question

P

panfilero

Guest
I'd like to control a switch using a microcontroller, I was thinking
I'd feed the output of the microcontroller into an npn that would pull
a pmos gate low and let current flow, it seems to be working ok in
LTSpice except I have a negative spike, when I switch.

Is this an ok way to accomplish my goal, and does anybody know what
that negative spike is about? I'm using 2 pmos transistors so that
the body diodes can block current from flowing when they are off

here's a link to my circuit

http://i555.photobucket.com/albums/jj477/panfilero/Capture.jpg

thanks
 
On Wed, 4 Apr 2012 20:58:41 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

I'd like to control a switch using a microcontroller, I was thinking
I'd feed the output of the microcontroller into an npn that would pull
a pmos gate low and let current flow, it seems to be working ok in
LTSpice except I have a negative spike, when I switch.

Is this an ok way to accomplish my goal, and does anybody know what
that negative spike is about? I'm using 2 pmos transistors so that
the body diodes can block current from flowing when they are off

here's a link to my circuit

http://i555.photobucket.com/albums/jj477/panfilero/Capture.jpg

thanks
Click to expand...
---
M1 is mis-wired, you don't need two MOSFETs, R2 should be in series
with the base of Q1, and the output of the microcontroller probably
isn't 1 volt.

I think the spike is being caused by the MOSFET's gate-to-drain
capacitance differentiating the negative edge of Q1's output.

Slowing down the MOSFET a little, with R4, and lowering the value of
R3 to 100 ohms drops the spike's amplitude to about 18 millivolts:

Version 4
SHEET 1 980 680
WIRE 48 48 -96 48
WIRE 160 48 48 48
WIRE 400 48 256 48
WIRE 400 64 400 48
WIRE 48 80 48 48
WIRE 176 128 176 96
WIRE 400 176 400 144
WIRE -96 224 -96 48
WIRE 48 240 48 160
WIRE 176 240 176 208
WIRE 176 240 48 240
WIRE 176 272 176 240
WIRE 256 320 240 320
WIRE 400 320 336 320
WIRE 400 352 400 320
WIRE -96 480 -96 304
WIRE 176 480 176 368
WIRE 176 480 -96 480
WIRE 400 480 400 432
WIRE 400 480 176 480
WIRE -96 560 -96 480
FLAG 400 176 0
FLAG -96 560 0
SYMBOL voltage -96 208 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 10
SYMBOL npn 240 272 M0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL pmos 256 96 M270
SYMATTR InstName M1
SYMATTR Value Si7137DP
SYMBOL res 32 64 R0
SYMATTR InstName R1
SYMATTR Value 1000
SYMBOL res 352 304 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL voltage 400 336 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value PULSE(0 5 0 100u 100u 10ms 20ms)
SYMBOL res 384 48 R0
SYMATTR InstName R3
SYMATTR Value 100
SYMBOL res 160 112 R0
SYMATTR InstName R4
SYMATTR Value 1000
TEXT -66 514 Left 2 !.tran 50ms

--
JF
 
On Apr 5, 5:29 am, John Fields <jfie...@austininstruments.com> wrote:
On Wed, 4 Apr 2012 20:58:41 -0700 (PDT), panfilero

panfil...@gmail.com> wrote:
I'd like to control a switch using a microcontroller, I was thinking
I'd feed the output of the microcontroller into an npn that would pull
a pmos gate low and let current flow, it seems to be working ok in
LTSpice except I have a negative spike, when I switch.

Is this an ok way to accomplish my goal, and does anybody know what
that negative spike is about?  I'm using 2 pmos transistors so that
the body diodes can block current from flowing when they are off

here's a link to my circuit

http://i555.photobucket.com/albums/jj477/panfilero/Capture.jpg

thanks

---
M1 is mis-wired, you don't need two MOSFETs, R2 should be in series
with the base of Q1, and the output of the microcontroller probably
isn't 1 volt.

I think the spike is being caused by the MOSFET's gate-to-drain
capacitance differentiating the negative edge of Q1's output.

Slowing down the MOSFET a little, with R4, and lowering the value of
R3 to 100 ohms drops the spike's amplitude to about 18 millivolts:

Version 4
SHEET 1 980 680
WIRE 48 48 -96 48
WIRE 160 48 48 48
WIRE 400 48 256 48
WIRE 400 64 400 48
WIRE 48 80 48 48
WIRE 176 128 176 96
WIRE 400 176 400 144
WIRE -96 224 -96 48
WIRE 48 240 48 160
WIRE 176 240 176 208
WIRE 176 240 48 240
WIRE 176 272 176 240
WIRE 256 320 240 320
WIRE 400 320 336 320
WIRE 400 352 400 320
WIRE -96 480 -96 304
WIRE 176 480 176 368
WIRE 176 480 -96 480
WIRE 400 480 400 432
WIRE 400 480 176 480
WIRE -96 560 -96 480
FLAG 400 176 0
FLAG -96 560 0
SYMBOL voltage -96 208 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 10
SYMBOL npn 240 272 M0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL pmos 256 96 M270
SYMATTR InstName M1
SYMATTR Value Si7137DP
SYMBOL res 32 64 R0
SYMATTR InstName R1
SYMATTR Value 1000
SYMBOL res 352 304 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL voltage 400 336 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value PULSE(0 5 0 100u 100u 10ms 20ms)
SYMBOL res 384 48 R0
SYMATTR InstName R3
SYMATTR Value 100
SYMBOL res 160 112 R0
SYMATTR InstName R4
SYMATTR Value 1000
TEXT -66 514 Left 2 !.tran 50ms

--
JF
Click to expand...
thank you sir
 
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