Ground Bounce, Noise, and Inductive Load - Page 1
Forum section: electronics / projects

author: Charles Creations
author_rank: Contributor
timestamp: March 24, 2015, 11:32:10 pm
content: I wanted to get another set of eyes on how I could solve my noise problem with the circuit attached.This circuit is driving a ultrasonic transducer with a 5v supply at 40kHz. It resonates the primary winding of a transformer with the LC setup, so it can produce high voltage on the secondary. This all works delightfully. The problem is the noise it is spilling into the rest of the circuit. The entire 5V rail sees almost 500mV of AC ripple when this circuit is activated. This noise is causing trouble in some analog amplifiers. I should note that this is running off a long USB cable. The high transient current causes the supply voltage to drop momentarily.I have tried to fix it with the pi filter, a large 100uF electrolytic cap, a diode, and adding series resistance to the gate of the FET. I am open to some other solutions.Thanks,Charlie

author: Marco
author_rank: Super Contributor
timestamp: March 24, 2015, 11:54:28 pm
content: Quote from: Charles Creations on March 24, 2015, 11:32:10 pmNot really ... if the drawing is accurate you're driving it with a square wave while getting a very hot 4 uF capacitor (and a DC current in the transformer, so that's probably not happy either).

author: dom0
author_rank: Super Contributor
timestamp: March 25, 2015, 12:19:34 am
content: Resonant <anythings> work usually with a second control winding to have a closed loop which then oscillates at the resonant frequency of said loop (which is then very near the resonant frequency of the transformer). And I don't see that in your circuit anywhere, in fact the external fed signal without any feedback pretty much dictates a non-resonant mode of operation.

author: Charles Creations
author_rank: Contributor
timestamp: March 25, 2015, 02:33:27 am
content: The 40kHz square wave signal is very intermittent (less then 1% duty cycle, 250us pulse lengths), so heat doesn't build up.As far as resonance, I was referring to a parallel LC tank circuit. I sized the capacitors (4uF) so the combination of the transformer and capacitor has a natural frequency equal to the drive frequency (in this case 40kHz). This produces a nice sine wave from the square wave input.I am still open to solutions on the noise problem because the rest of the circuit works perfectly. The real challenge is just controlling the noise this circuit produces.

author: Marco
author_rank: Super Contributor
timestamp: March 25, 2015, 02:59:55 am
content: The problem is the 4 uF capacitor ... when the MOSFET turns on you are trying to instantaneously change it's voltage, which causes a huge current spike.Your circuit is fundamentally flawed.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2015, 03:30:08 am
content: The MOSFET direct driving a capacitor, and the diode preventing resonant free-wheeling, are pretty notable problems. What's your layout look like?Tim

author: Charles Creations
author_rank: Contributor
timestamp: March 25, 2015, 03:50:15 am
content: Thank you for the feedback.I have attached a screenshot of the area of interest. On the board, I use four parallel 1uF capacitors instead of a single 4uF. Those capacitors are in the top left quadrant. The 100uF capacitor is C36 in the bottom left. The large circle is the is the ultrasonic transducer, and the transformer sits right between the pins of the transducer centered on the right side. The FET is in the top center and labeled Q2 with its Sot-23 package.As far as direct driving the capacitor with the FET, my attempts to add series resistance always reduce output power considerably. I need all the power I can get, so I have been looking for a solution that better stores the required energy locally without impacting the rest of the circuit. The pi filter was my attempt at that approach. My original thinking (perhaps flawed) was that the parasitic inductance of the transformer would reduce the instantaneous change in voltage as the FET switched on and charged up the capacitor.I have ran this circuit with and without the diode. There isn't any difference from a noise prospective.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 25, 2015, 05:57:01 am
content: With careful timing of the turn-on pulse to the FET you can turn on losslessly when the voltage across the resonant cap is at maximum, like in a resonant converter. Then there will be no leading edge current spike. This is probably easiest done by varying the switching frequency rather than changing tank values.Also, remove the freewheel diode so the circuit can resonate for a full cycle. Also, the size of the load on the secondary will determine the loaded Q of the circuit. If it is above 1 then you'll be storing energy from cycle to cycle.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2015, 01:56:44 pm
content: I don't suppose you have an RLC equivalent circuit for the transducer handy..? And the transformer.This needs to be a Class C or E style output stage, with an inductive load for the transistor and enough inductance and capacitance to carry it around a full cycle.To do it class D, you would need a transformer right at the transistor, with a CT'd primary, +V to CT, transistor to one side, diode from GND to the other side. This allows a single transistor to create a square wave ("1/4 bridge" configuration). (Such a configuration is on the top left here:http://seventransistorlabs.com/Images/Mag_Amp2.png) The transformer secondary (or just couple it off one of the primary ends) goes through an inductor (filters harmonics, transforms impedance) to the load, which may still need a transformer for changing the ratio, and which has the right combination of L and C to resonate at the transducer frequency.Tim

author: Marco
author_rank: Super Contributor
timestamp: March 25, 2015, 02:27:13 pm
content: Quote from: Charles Creations on March 25, 2015, 03:50:15 amSeries resistance is not the problem ... the whole topology simply doesn't sense. Just drive the transformer with a full bridge, or a half bridge with a DC blocking capacitor in series (don't absolutely need a resonant tank unless you need a really clean waveform, the crystal is high'ish impedance at frequencies beyond it's resonance frequency any way).

author: Charles Creations
author_rank: Contributor
timestamp: March 25, 2015, 03:37:16 pm
content: Quote from: Richard Head on March 25, 2015, 05:57:01 amA great idea. Thank you.Quote from: T3sl4co1l on March 25, 2015, 01:56:44 pmI do not. I measure the transducer as having about 2.25nF capacitance, but I don't have the means to really characterize it and the datasheet doesn't offer any values.Quote from: T3sl4co1l on March 25, 2015, 01:56:44 pmI will look into this topology. Thank you for the link.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 26, 2015, 08:39:09 am
content: If you have a function or audio generator and scope you could do a frequency sweep of the transducer looking for any self resonant points. You may be lucky and find a resonant point that will make impedance matching between the driver and transducer a little easier. There may even be an intentional operating frequency that yields a predominantly resistive load.

author: SArepairman
author_rank: Frequent Contributor
timestamp: March 26, 2015, 07:35:36 pm
content: speaking of ultrasonic transducers, using a power op amp (like 100-200mA) to drive a step up transformer with a sine wave works to drive a piezo-fan according to a LT app note.what piezo device requires such an odd stimulation signal?

author: Rog520
author_rank: Regular Contributor
timestamp: March 26, 2015, 07:46:08 pm
content: Have you tried powering it with a 5v bench supply to see if that makes a difference? Or using a much shorter USB cable?

author: Charles Creations
author_rank: Contributor
timestamp: March 28, 2015, 10:30:54 pm
content: Quote from: Rog520 on March 26, 2015, 07:46:08 pmYes, both. They don't make a noticeable difference. I recently switched the choke in the Pi filter out for a 47 ohm resistor. The result was a RC time constant of several milliseconds which combined with my low duty cycle effectively isolated the power consumption of the transmitter from the rest of the circuit. This significantly reduced the noise. The 100uF cap. looses about 1V as it is drawn from as the transmitter is active. Once the transmitter stops, the 100uF cap. rises back to nominal voltage within 10 milliseconds which leaves plenty of time until the next transmit wave.Quote from: SArepairman on March 26, 2015, 07:35:36 pmThe piezo device I am using is very sensitive to the distortion of the wave form. A sine wave at 40kHz is the ideal signal, and since I am driving it from an atmega328, the LC tank circuit was the easiest way I could think of of getting a sine wave from a square wave with the necessary power and settle time. The power op amp is a good idea.

author: Circlotron
author_rank: Super Contributor
timestamp: March 28, 2015, 10:45:27 pm
content: If the diode is removed so that the circuit actually resonates then the MOSFET drain voltage will try to swing below earth but will be clipped by the MOSFET body diode. Put a fast diode in series with the drain so that it can attain full amplitude.

author: Marco
author_rank: Super Contributor
timestamp: March 28, 2015, 10:58:49 pm
content: Quote from: Charles Creations on March 28, 2015, 10:30:54 pmHave you actually scoped the signal on the primary of the transformer?

author: Charles Creations
author_rank: Contributor
timestamp: March 31, 2015, 02:51:19 am
content: Quote from: Marco on March 28, 2015, 10:58:49 pmI attached the waveform and a FFT of the secondary and a waveform of the primary. Its not perfect by any means, but it produces much greater output power then a comparable square wave.Quote from: Circlotron on March 28, 2015, 10:45:27 pmI completely agree. I had this in some of the earlier versions.