invertermultisimpower supplytransformer
I am trying do design a HF power supply/inverter for school. I stumbled across a problem.
When i feed a IRF740 a sine wave on the gate, the MOSFET gives a modified sine wave on the drain. Why is that? Can this be filtered?
Some screenshots of simulation:
The diodes and transformer are ideal.
You say you're not familiar with digital oscillators yet, but I think it's worth looking into. Using a microcontroller with PWM output you can input the signal directly in the amplifier's format. So you won't need an A/D conversion at the amplifier's input stage.
The code for the microcontroller is very simple. Basically it's like this: you need a sine lookup table, and a timer interrupt generating your sample clock. This can be 5kHz, for instance, then you get 100 samples per cycle, which is enough for an inverter application; the transformer will work as a low-pass filter. Every 200\$\mu\$s you take the next value from the table and set the PWM register with this value. The output will be pulse-width modulated following the sine pattern.
This means that your class-D amplifier doesn't need the input stage with the triangle wave generator + comparator (this would otherwise be you A/D converter), so it only consists of the driver stage for driving the output MOSFETs.
You get a crystal precision frequency virtually for free.
PS: a center-aligned PWM signal will give you a cleaner sine approximation, but many microcontrollers only can generate edge-aligned PWM. This Freescale controller is available in an SOIC-8 package and is an example of a microcontroller which can do center-aligned PWM.
image from this document
Assuming that the inverters are CMOS ones, the circuit is equivalent to:
simulate this circuit – Schematic created using CircuitLab
While in the schematics you provided it seems like the circuits are interrelated, in fact they are not. Two completely separate blocks having a common input.
The basic idea behind this circuit is this:
M2 and M3 are functioning as pass-transistors in this case.
The function of M2 and M4 are less important - they pull the voltages down/up when the LEDs are not required to be ON. I believe that the circuit would function without them as well. However, the presence of the full inverter allows for a very steep ON/OFF characteristic for LEDs - there is no wide transition region between ON/OFF states.
Best Answer
This sounds normal. Look carefully at how the FETs are biased. Almost certainly they don't stay in the linear region over the full range of input signal. Since you are getting blips, look closely at what is happening when the gate voltages are low. Probably the FETs are already mostly off with a gate voltage above the minimum, so lowering it doesn't cause much change on the output.
You said this for high frequency. Depending on how high "high" is, you actually want the FETs to turn off for part of each cycle, more like a class C amplifier than a class B. You then reduce the harmonics with a L-C tank on the output.