audiomosfetoffset
I am working on an amplifier based on 555 triangle generator, a comparator and the IRAUDAMP1 as a guideline.
My problem is that i have high DC offset voltage at the output (-1.6V).
What is the cause of this?
Is the problem right at the beggining, on the comparator circuit?
Is the only way to fix this to use separate inductors for filtering for each mosfet?
LATER EDIT
The problem was the 555 generator. The exponential triangle causes DC offset problems.
Here's a better way to drive a high-side P-channel MOSFET:
simulate this circuit – Schematic created using CircuitLab
Note that the BJT Q1 is a current sink; the current value is the control voltage divided by R2. If your control circuit puts out 12V, the current will be 12 mA.
This current will create a voltage drop across R1 as well, and if you happen to make the two resistors equal, this voltage drop will be essentially equal to the control voltage supply (minus the VBE drop of Q1). Or you can make the resistors different in order to get a particular relationship.
You need to be aware of two things: The current level that you set will have some bearing on how fast the MOSFET M1 switches; higher current means faster switching. However, keep in mind that Q1 has a lot of voltage across it. When off, it has the full high voltage supply across it. And even when switched on, it has the high voltage minus 2× the control voltage across it. This means that it will be dissipating power equal to that voltage drop times the current when on.
After much testing I finally found the solution. This Article States that :
For high current MOSFETs the Gate Channel Capacitance can be very high and a rapidly changing drain voltage can produce milliamps of transient Gate current. This could be enough to overdrive and even damage delicate CMOS driver chips. Having a series resistor is a compromise between speed and protection, with values of 100R to 10K being typical. Even without inductive loads there is dynamic gate current. Also, MOSFETs are extremely susceptible to damage caused by electrostatic discharge and can be damaged irreversibly by a single instance of Gate breakdown. For this reason it is a very good idea to use gate series resistors of 1K to 10K. This is especially important if the Gate signal comes from another circuit board.
After Reading this I changed my Series Gate Resistor to 1k and the problem vanished entirely. It would appear that when your a higher frequency with a capacitive load you need a higher Series Gate Resistor.
Best Answer
You are trying to get zero out for zero in by dead-reckoning.
Any imbalance anywhere will give rise to a DC offset. Although all the components in the two sides are nominally the same, you cannot assume exact matching.
You use seperate comparators for the H and L FETs, which will have different offsets and speeds, level translators with different speeds, the high and low channel in the 2110 driver will be different, two physically different FETs, and a power supply that may or may not be balanced accurately. The elephant in the matching of course is the high/lowside driver, that uses a bootstrap high voltage supply, and two FETs of the same polarity to do a job that should (for balance) use opposite polarity FETS.
It is a wonder that you get as close to zero offset as you do.
Do not change the output topology to filter the FETs separately, keep the single inductor from the FET junction as you have it.
If you need a zero output offset, then it may be worth closing a low frequency filtered DC correction loop around the whole amplifier.