I need to use a mosfet driver in my next project. For this purpose I made a demo board with this schematics. My aim is to light the led by either applying a logic high level voltage from the control input or by pressing the button.
Schematic Notes:
- The components marked with DNP are not fitted.
- I didn't solder the bootstrap circuit elements (i.e.; Dbs and Rbs), because the switching frequency will be too low (1Hz at most). In my opinion, the bootstrap circuit can't keep the capacitor Cbs0 filled up at that frequency.
- You see two mosfets connected in parallel. One is low power, the other is high power. Only one of them will be soldered in this demo board.
- The points where I soldered external wires are marked in the schematics.
- Supply voltage is 12V.
- Control voltage is 0V/12V.
- Isolated supply voltage is 12V.
- The voltage the mosfets turn on or off is 12V.
My final PCB is like this:
- The leftmost cable pair is for the supply.
- The cable pair second from left is for the control signal.
- The third cable pair is for the isolated suplly.
- The rightmost cable pair for the voltage with the mosfet will turn on or off.
I did a setup like this:
- My power supply is HMP4040. Please note that, all of its outputs are isolated.
- Every channel is adjusted to 12V and the channels are connected to "supply", "control", "isolated supply" and "voltage controlled by the mosfet" from left to right.
- I am trying to turn on the led by changing the voltage at the "control" input (the 2nd channel of the supply).
Now, are we okay until here? I am finally coming to the issue.
In this setup, I can toggle the led for a few times as intended (either by by pressing the button or changing the "control" input). But after a few times, the led stays either turned on or off and it doesn't change anymore. After I turn off then turn on the "supply" input, again I can light the led a few more times and it stays turned on or off forever until I reset the supply voltage again.
In this "stuck" or "uncontrollable" state:
- I check the mosfet driver output (the gate and source pins of the mosfet) to see that the IC is trying to turn on and off the mosfets as intended.
- The floating high side voltage (measured between Vb and Vs pins of the IC) is correct (12V) all the times. The IC requires it to be between 10V and 18V.
- The voltage between "Ground" and the floating high side ground (Vs pin of the IC) doesn't stay fixed regardless of the "control" input value. It random walks between -10V and +10V. I expected it to be fixed (0V when "control" is logic low, 12V when logic-high).
While I was trying different things and struggling the find what was wrong, I discovered something. When I short circuit the "Ground" and "Vout-" as seen in the images below along the hand drawn red line, the problem is solved. By "solved" I mean, I can now turn on and off the led as many times as I want by both pressing the button and changing the control signal. This solution is okay for me, because in my final application, the load ground will not be isolated from the circuit ground.
But I want to understand the reason behind this phenomenon. When the mosfet driver applied enough voltage between the gate and source pins of the mosfet, its channel (between drain and source pins) will have low resistance. Regardless of applying an isolated or non-isolated voltage between the drain and source pins, some current must flow through the led. But it doesn't happen like that when the controlled voltage is isolated.
Can you please explain me what is happening here when the controlled voltage is isolated?
