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I'm trying to drive a RFP30N06LE N-Channel MOSFET with TC4420 MOSFET Driver. The MOSFET is used to switch a 1W LED. But my circuit is not working: it seems that the VDD somehow leaks to the input of the MOSFET driver, causing it to switch on constantly.

The VDD in the circuit is 16V, and VCC is 5V coming from an Arduino Nano. The VCC goes into the MOSFET driver's input to turn it on.

I find my LED operates best at a current draw of 291 mA. With a VDD of 16 V, I put the LED in series with a 56Ω resistor.

Here is my circuit:

(removed picture, please see the updates)

And photo of the breadboard implementation (LED and resistor are off board and not shown here). Power rail runs VDD.

(removed picture, please see the updates)

I'm providing the VDD with a bench power supply. I have my scope hook to 1) yellow-MOSFET driver input 2) Blue-MOSFET driver output 3) Purple-Between drain and source of the MOSFET.

enter image description here You can see in this picture when I first turn on the VDD power supply at 0.5V. The yellow signal in the scope raises above the others by a little bit. I measured ~0.4V on the input pin of the driver. This does not really make sense to me because the input should be isolated from the VDD. The LED (in the foreground of the photo on the left) does not turn on.

Now when I set power supply to 4.1V. The LED turns on and it starts to draw about 0.47 A. This should not happen since I'm not giving anything to the input of the MOSFET driver (the switch between Arduino 5V and the MOSFET input is open). Yet the scope shows the voltage on the MOSFET input is as high as on its output, turning the MOSFET on. See photo below. enter image description here

I suspect there are some shorting between the MOSFET input and VDD so I check with my DMM but they are open.

I'm really at a lose as to what to do. Please let me know I didn't make something clear or if you want more information. Any insight would be greatly appreciated!

EDIT1: update the breadboard image with annotations

UPDATE1: I modified the circuit with:

  • added a 10KΩ pull-down resistor, thanks for the suggestion!

  • using two bypass capacitors for each VDD pin of the MOSFET driver

  • reduce the length of the power loop

Here're the new schematics and breadboard connection enter image description here enter image description here

Here are the signals I'm getting now @ronsimpson; when I set the bench power supply to minimal at 0.05V, I'm able to get a trigger on the driver's output with a rise time of 20.5 µs. Please see the following scope photo. The yellow (ch1) measures the driver's input, and the blue (ch2) is the driver's output. The purple (ch3) is the drain-to-source voltage of the MOSFET. enter image description here

However, when the input still rises and falls with VDD despite adding a pull-down resistor at the input. The behavior has not changed. Here's a video showing it not working.

You can see when I turn up the VDD from the power supply. Both yellow (driver input) and blue (driver output) rise, and the LED turns on. Pressing the button will give the driver input a small rise, which I don't quite understand. But the button does not affect the switching. The MOSFET is always on. Any suggestion on what might be going on?

UPDATE2: I put together this circuit to test the driver with a supply of 9 volts.

enter image description here

Given that my connection is correct, I think the driver might be defective. This is what happened when I connect the input to 5V:

enter image description here

Again, purple is the driver input (between the pull-down resistor and GND), and blue is the driver output. When the input is connected to 5V, the voltage is pulled low to give to the input. However, the blue output is not affected! I got my TC4420s in a package of 4. All of them have more or less the same behavior. The blue never moves a bit. Additionally, I noticed the driver gets hot just with its GND and VDD connected. It gets worse when there's input.

I think this means my package of TC4420s are bad?

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    \$\begingroup\$ You need a pull down resistor after your switch, at the input of the gate driver. the input is probably floating high, or worse yet toggling on noise. Try 10k to ground. \$\endgroup\$
    – Bryan
    Commented Jul 2, 2022 at 19:09
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    \$\begingroup\$ treat gates like a floating cap and never leave them open as they are susceptble to stray feedback and can be damaged by ESD. \$\endgroup\$ Commented Jul 2, 2022 at 19:42
  • \$\begingroup\$ If the Arduino is 5V then the gate driver is redundant. \$\endgroup\$ Commented Jul 2, 2022 at 19:43
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    \$\begingroup\$ Thanks y'all for bringing up the pulldown resistor and @Tony Stewart EE75 for the explanation. I'm using a driver + 16V VDD because my project goal is to switch a vibrating motor rated at 16V (I use the LED for now to test the switching circuit). The RFP30N06LE MOSFET doesn't fully turn on at 5V to move the motor. With 16V at the gate, the MOSFET turns on the motor. That's why I decided to use VDD on the MOSFET gate with a driver. \$\endgroup\$
    – Leo Li
    Commented Jul 2, 2022 at 20:04
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    \$\begingroup\$ User Roofman01 reddit.com/user/Roofman01 on reddit suggested replacing the bypass capacitor on the driver with ceramic ones. And add a 10Ω resistor in series with them. Can anyone explain why this helps get clean input to the driver IC? \$\endgroup\$
    – Leo Li
    Commented Jul 2, 2022 at 21:35

1 Answer 1

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Just want everybody know that the problem is solved, quite undramatically.

My new TC4420 arrived today, and I swapped my old one out with the new one. It switches like a chime. With the old ones, when I close the gate, the Vout doesn't have any change (see my previous post). And here is the switch event of the working new drivers. The gate is in purple, where I connect it to Arduino 5V to turn the driver on. The rising edge of Vout (blue) from the driver follows closely. VDD is at 10 V. enter image description here

I got the old one from Amazon. Now look at them closely. They don't have the little dot indent at the lower left side. enter image description here

I see this dot in both my new drivers and in the datasheet. Maybe the old ones I have is a different packaging, and I may have messed up the pin layout. I'm not sure.

enter image description here

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  • \$\begingroup\$ There are quite a few fake and defective components being sold on eBay and probably Amazon as well. I usually order my parts from Mouser or DigiKey, but sometimes I have tried to take advantage of the free and fast shipping from other suppliers. Usually I have not had any problem getting refunds, but it was frustrating figuring out the problem, and extra time getting good parts and unsoldering the defective components. \$\endgroup\$
    – PStechPaul
    Commented Jul 9, 2022 at 3:39

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