H bridge producing sounds and bad wave

Thread Starter

Affan Ahmed

Joined Jan 7, 2019
6
I am trying to produce an inverting wave(square wave) within the range 5V to 25V and frequecy range is from 1Hz to 15Hz. My input is lipo battery with battery range 3.6V to 4.2V and have boosted that to control and produce the required voltage.I am using IXDF604 (low side driver u201) with inverted outputs to drive N channel mosfets FDD8447 in h bridge configuration. My application is on human skin which is not inductive but capacitive load.

Using only the low side which i am using throughout this discussion, unfortunately I get the following wave at load with sound from gate driver when capacitor c202 in parallel with the load resistor R203. If I remove the capacitor the sound goes away but wave is more bulging that the given wave. How can I get this wave bulge solved without sound?




I have been suggested to use IR2110 Full bridge driver as I lack a high side bootstrap circuit but unfortunately I cannot use it as most drivers are designed on motor applications requiring high power. To be specific most high side drivers have to get +10v on the floating offset to give floating absolute voltage. I have already fried many ir2110s. Considering I give the boosted voltage suppose 8V to get something similar but less than 8V at the output How will I be able to give the +10V required to the bootstrap circuit?
 

Alec_t

Joined Sep 17, 2013
10,359
Welcome to AAC!
In view of the low currents and voltages involved, can't you just omit the power FETs and simply use the IC outputs directly?
 
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Thread Starter

Affan Ahmed

Joined Jan 7, 2019
6
Welcome to AAC!
In view of the low currents and voltages involved, can't you just omit the power FETs and simply use the IC outputs directly?
Can you explain a little because I need to have wave with both negative and positive side?
 

Alec_t

Joined Sep 17, 2013
10,359
If you look at Fig 3 of the IXDF604 datasheet you will see that the A and B outputs are each a FET half-bridge, so you could connect a suitable load directly between A and B in a full bridge configuration.
 

Thread Starter

Affan Ahmed

Joined Jan 7, 2019
6
If you look at Fig 3 of the IXDF604 datasheet you will see that the A and B outputs are each a FET half-bridge, so you could connect a suitable load directly between A and B in a full bridge configuration.
How will I be able to get the negative wave?
 

Thread Starter

Affan Ahmed

Joined Jan 7, 2019
6
The load you are using is not a good simulation of skin impedance. There should be some series resistance. This will limit the capacitor charging current that is causing your problem.
Here is some relevant information: http://www.scielo.org.mx/pdf/rmib/v32n2/v32n2a2.pdf
Thank you I will look into it but till now I am in testing phase though I have tested it on skin several times but currently I am just using about 1uf capacitor and about 2.2k resistor in parallel with the expected skin(not currently there).
 

Alec_t

Joined Sep 17, 2013
10,359
How will I be able to get the negative wave?
If the A and B inputs are strapped together then one output will be high and the other will be low. Let's call the wave 'positive' if A out is high and B out is low. If the input signal changes state then the outputs will too, so you get a 'negative' wave when A out is low and B out is high.
 

KeithWalker

Joined Jul 10, 2017
413
Thank you I will look into it but till now I am in testing phase though I have tested it on skin several times but currently I am just using about 1uf capacitor and about 2.2k resistor in parallel with the expected skin(not currently there).
If you have a 1uF capacitor connected directly across the load, when the signal changes polarity, the capacitor inrush current will be very high, only limited only by the resistance of the FETs. This could damage the FETs. It will not change the phase angle but as you see on your scope, it distorts (integrates) the input signal. You need to connect the capacitor and resistor in series to limit the charging current of the capacitor.
The electrical characteristics of skin are usually measured using a sinusoidal signal at different frequencies, not a square wave. The phase angle of a signal in an R/C circuit is frequency dependent. A square wave is composed of a fundamental signal and all of its odd harmonics. Therefore when you apply a square wave to an R/C low pass filter circuit, the phase angle of each component frequency is changed by a different amount. The result on the square wave is not a single phase change but differentiation. The following is an exert from a tutorial by Samita in "Electronics Post" :
Intigration.jpg
 
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Thread Starter

Affan Ahmed

Joined Jan 7, 2019
6
If the A and B inputs are strapped together then one output will be high and the other will be low. Let's call the wave 'positive' if A out is high and B out is low. If the input signal changes state then the outputs will too, so you get a 'negative' wave when A out is low and B out is high.
I am giving it a try lets see
 

Thread Starter

Affan Ahmed

Joined Jan 7, 2019
6
I It will not change the phase angle but as you see on your scope, it distorts (integrates) the input signal. You need to connect the capacitor and resistor in series to limit the charging current of the capacitor.
The electrical characteristics of skin are usually measured using a sinusoidal signal at different frequencies, not a square wave.
View attachment 167383
I agree with you they are however few things to clear, I put the capacitor which made my signal less distorted(it was more distorted or bulging more with spikes at the edges) but when I put it the spikes went away signal still but less bulging. However another problem started is the sound produced from the low side driver. on a side note long ago using smaller mosfets like sot23 packages I even fried the gate driver and it even produced a fire using tc4428 as my gate driver low side.

For the skin part I read few papers determining that square wave penetrates the skin best. I am using electrodes like in tens
 

KeithWalker

Joined Jul 10, 2017
413
I agree with you they are however few things to clear, I put the capacitor which made my signal less distorted(it was more distorted or bulging more with spikes at the edges) but when I put it the spikes went away signal still but less bulging. However another problem started is the sound produced from the low side driver. on a side note long ago using smaller mosfets like sot23 packages I even fried the gate driver and it even produced a fire using tc4428 as my gate driver low side.

The noise and damage For the skin part I read few papers determining that square wave penetrates the skin best. I am using electrodes like in tens
What are you using to display the waveform and is it AC or DC coupled to the output of your circuit?
The RDS of the MOS FETs is about one ohm so the instantaneous charging current of the capacitor on polarity change will be quite high - Supply Voltage/2 ohms.
How is this affecting the supply from your booster? Connect your scope across it and take a look. I think you will be a little surprised.
THE PROBLEMS YOU ARE HAVING ARE BEING CAUSED BY CONNECTING THE CAPACITOR DIRECTLY ACROSS THE OUTPUT.
 
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