I'm trying to make a circuit to drive a transformer from a PWM signal produced by an AVR MCU. Here's the schematic:
The MCU output in fact has 2 complimentary PWMs feeding two transistor pairs. For this scenario, let's suppose I can guarantee both signals coming from the AVR are always complimentary, and are never on at the same time.
The first transistor on each pair is low power, and is there to avoid direct connection between the AVR and the higher power transistor TIP41.
I did some measurements, and initially, all seemed fine. Please note I still didn't add the transformer, it's on the schematics just to show where it's supposed to be connected, but all measurements below were made without it.
The first one shows both TIP41 emitters (in yellow and blue), and their difference, using scope math (in red). Both probe grounds are on circuit ground. This is what I expected: each half of the primary would be driven by one TIP41 at a time, with alternating voltages, so I have half above zero, and half below zero:
Please note that both signals don't invert immediately, there is a delay introduced by the transistors and I don't know how to fix that. Measuring the signals directly on the AVR, I can see they switch immediately, there's no delay there.
Now comes the trouble: when I measure from one emitter to the other emitter of the TIP41s (probe ground on one emitter, and tip on the other), this is what happens:
Same signal zoomed out:
Does anyone have a clue on what may be happening? I expected to see something roughly similar to what I saw in red on the previous picture.
Perhaps I'm just using the scope wrongly, I don't know. I repeat: this all happens with no transformer, the transistor emitters are floating, there's no load.