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Schematics

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:

PCB

  • 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:

Setup

  • 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.

Shorted path on schematics
Shorted path on PCB

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?

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  • \$\begingroup\$ What power source will drive current through the MOSFETs to light the LED? \$\endgroup\$
    – Andy aka
    Commented Mar 1, 2021 at 15:00
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    \$\begingroup\$ 1) Why are Diode Dbs and resistor Rbs not placed, they're essential for the bootstrap function of this gate driver, see the circuit on the first page of the datasheet. 2) this gate driver is NOT for driving an isolated circuit, the ground must still be common. Your "hack" (the red wire) connects those grounds together. The "MOSFET side" can be at +100 V, that is something different than "isolated". TLDR: you're not using this gate driver IC as the manufacturer intended. I suggest using an isolated gate driver. \$\endgroup\$ Commented Mar 1, 2021 at 15:06
  • \$\begingroup\$ You didn't implement the bootstrap circuit (Dbs) properly so the MOSFETs will not be fully on, the LED will still light up as there is enough voltage headroom (because the supply is 12 V and LED needs only a few volts). For a high current load you need the MOSFET to be on properly (large Vgs) and for that you need the bootstrap circuit. \$\endgroup\$ Commented Mar 1, 2021 at 15:12
  • \$\begingroup\$ Or, instead of the bootstrap, you need a bias supply about 10v above V+ to get Vgs=10V. \$\endgroup\$
    – user16324
    Commented Mar 1, 2021 at 15:19
  • \$\begingroup\$ If you refer to the 'Functional Block Diagram' on page 5 of the IRS10752 datasheet, you'll see that the control signals between the low-voltage input block and the high-voltage output block depend on that pair of MOSFETs being able to pull the signals low. Without a common ground that's not going to happen reliably. \$\endgroup\$
    – brhans
    Commented Mar 1, 2021 at 15:20

1 Answer 1

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From the discussion above, in my opinion you should not be using the IRS10752LPBF.

As the IRS10752LPBF:

  • does not provide isolation
  • is intended for use with a bootstrap diode + capacitor, which you do not use and do not want as you want to be able to make this work without periodic switching (DC).

I would use this solution:

schematic

simulate this circuit – Schematic created using CircuitLab

As this provides:

  • isolation (left and right part of the circuit completely separated).

  • no bootstrap but using an external voltage (Vbs).

Note that "high side switching" using an N-channel MOSFET is more complex than "low side switching", if possible I would use:

schematic

simulate this circuit

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  • \$\begingroup\$ This is indeed a good idea if I change the optocoupler with an isolator. A 4 channel isolator like SI8645 will be able to drive four of these circuits. \$\endgroup\$ Commented Mar 2, 2021 at 13:30
  • \$\begingroup\$ Isolator of optocoupler, indeed both can work. I see that isolators are less than $2 for multiple channels so then you have similar cost as when using optocouplers. \$\endgroup\$ Commented Mar 2, 2021 at 13:38

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