Reduce transformer noise on mosfet - Page 1
Forum section: electronics / projects

author: evansg
author_rank: Contributor
timestamp: November 07, 2020, 09:59:35 am
content: Hi there,I have implemented the circuit below. (drives a small EL wire)It is a small audio transformer (placed reversed) which transforms a 5V signal to around 90V. I use a logic level N mosfet (through a PWM 500Hz source) to drive this transformer.Circuit works fine but I can see a lot of noise on the mosfet gate pin (oscil1.jpg)Lower noise can be seen on the FREQ pin (oscil2.jpg) (of microcontroller which produces the PWM signal)If I use a small smt ferrite bead (on gate or source pin of the n-fet?)would this reduce the noise? Any other better solution here?Thank you!

author: jonpaul
author_rank: Super Contributor
timestamp: November 07, 2020, 12:17:21 pm
content: Check your probe ground lead, looks like stray noise into the probe.Use differential probe or bayonet ground return.Jon

author: David Hess
author_rank: Super Contributor
timestamp: November 07, 2020, 03:43:03 pm
content: That looks like either stray pickup through the probe's ground lead or common mode noise between the DUT and oscilloscope.Minimize stray pickup by using as short a ground lead as possible, and preferably a coaxial connection to the probe tip.Common mode noise can be reduced or eliminated by using a differential or isolated probe, or two probes to make a differential measurement.To verify that either of the two problems above really exist, make a measurement with the probe tip and ground lead both connected (shorted) to the point where the ground lead would be connected.

author: evansg
author_rank: Contributor
timestamp: November 10, 2020, 01:04:54 pm
content: Thanks for your answers.It does not seem to be stray noise into the probe. When I disable PWM and transformer does not work the noise goes away.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 10, 2020, 01:07:31 pm
content: Well sure, load current goes away. Try again with PWM off and a load resistor in parallel with your circuit?Tim

author: jonpaul
author_rank: Super Contributor
timestamp: November 10, 2020, 01:26:48 pm
content: Use NO probe: Take 1m of RG198/u coax, make direct connection, with VERY SHORT area between coax shield and center conductor.The coax capacitance will affect ckt but at MOSFET gate insignificant.ALL PROBES PICKUP NOISE from even a few inches of ground clip!Jon

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2020, 05:08:04 pm
content: Quote from: jonpaul on November 10, 2020, 01:26:48 pmThe same result can be attained by making a coaxial connection to the probe tip.

author: jonpaul
author_rank: Super Contributor
timestamp: November 10, 2020, 05:23:50 pm
content: David, some TEK probes have a bayonette adapter with spring loaded pinsJon

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2020, 05:37:51 pm
content: Quote from: jonpaul on November 10, 2020, 05:23:50 pmOne way I have done it is to solder in a RG-178 or RG-179 pigtail with a coaxial connector and attach the probe to the coaxial connector with a probe tip to BNC adapter. As you point out, with low impedance circuits the added capacitance of the cable is insignificant.