Using a MCP1407 Driver Correctly

Other Hardware Motors, Mechanics, Power and CNC
1

Hello Forum,

I am trying to use an MCP1407 driver to control a mosfet which in turn switches a motor.

I have actually designed and built a working circuit based on information I found online, primarily this page;
MOSFET Drivers - DIYODE Magazine

I had not found this forum at that point.

Below is the working circuit.
When compared to the circuit from the DIYODE Magazine webpage, I found that I had to add a 10k resistor between the gate and ground otherwise it would turn on without being controlled.
The motor I am controlling is a 12v 775 DC motor.

"Original working circuit"

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I should have taken the win and moved on; however I kept reading about mosfets, drivers etc and found articles about the need for gate resistors. So, I redesigned my circuit to incorporate a 100R gate resistor. At the same time, I also changed the input to my circuit from 12v PWM on my control board to the TTL (after learning about that too). Finally, I also removed the 10k resistor thinking that MCP1407 handled the discharge of the mosfet and that I had got something wrong in my original circuit necessitating the 10k resistor to correct it.

The second circuit was a below - "Gate resistor configuration".

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I tested this circuit with a spare 12v CPU fan that I had and it worked fine, so I set about reworking my original working circuit moving it to the new design.

Once complete I connected my 12v 775 motor and powered the circuit on and applied the TTL signal and promptly killed four MCP1407's (I am a slow learner). Before getting through all four I did add back the 10k resistor between the gate and ground but this didnt change the outcome, still killed the MCP1407.

I went hunting again online for information and found this post here, showing (similar to the DIYODE Magazine webpage) no gate resistor and a 10k resistor between the gate and ground.
Suggestions on how to drive a MOSFET - Using Arduino / Motors, Mechanics, Power and CNC - Arduino Forum

So while I wait for more MCP1407s to arrive, the questions are;

  1. Why did the addition of the 100R gate resistor result in my MCP1407's burning out when switching the 775?
  2. Why did the second circuit, containing the 100R gate resistor work fine when switching the CPU fan?
  3. Is the circuit presenting on the forum post, which matches my original design, a safe implementation of the MCP1407 or was I just lucky in my original experiments?

Any insight would be appreciated.

2

You have the correct idea but the resistor should go to the port pin the gate is connected to, not the gate. When connected to the gate it becomes a voltage divider.

You can simplify your system a lot by switching to a logic level avalanche rated MOSFET. Your motor peaks at about 3A when on 12VDC. Place a 30 Ohm resistor in series with the port pin (assuming ground from the source is compatible with the 12V ground), the source to ground and the drain to the motor - and + to +12. With the proper MOSFET no diode is needed. If you add one it will not hurt but it will never conduct.

If they have to be isolated use a transistor output optocoupler to drive the MOSFET.

3

If you connect the two outputs together you will create a short circuit and destroy the driver.

Please look at the data sheet for how this chip works.

4

Thanks for the suggestion, I went back and double checked the datasheet
MCP1406/07 – 6A High-Speed Power MOSFET Drivers (microchip.com)

There is no explicit statement relating to connecting the output pins together, there are a couple of references to duplicate pins needing to both be connected. I think I picked up the idea for connecting them from the linked articles.

How would you suggest I connect the outputs independently? All other circuits I can find online have the output pins connected together. Possibly with a couple of correctly orientated diodes?

5

Thanks gilshultz,

By port pin do you mean a resistor on pin 2, the input?
I am (well I think I am) isolating with the PS2561D, this seems to work well.

Apologies, I dont think I was clear, I only burn out my 1407s when I connect a 12v 775 DC motor. It draws approximately 30A. I will look into logic level avalanche rated MOSFET; I am all for simplifying.

6

Remember data sheets are not beginners guides, they are written for professional engineers who know what they are doing.

No the linked articles said nothing about your specific driver.

This is the diagram you are misunderstanding

Fet driver
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It shows you that joining the two outputs together will cause a short circuit.

Using diodes correctly wired diodes will stop shorting.

For what you want to do you only need one output, not both.
You only need both if to are driving a differential output, which you are not.

7

Thank-you again Grumpy_Mike.

You are right, I dont completely understand the block diagram. I thought connecting the two pins seemed logical as I thought the highlighted section acted like a half-bridge.

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I will disconnect and have a crack with that config.

Again I appreciate your help

8

When you do connect the one output to your FET then don't add any resistors like you did before as that only undos a lot of the good the driver has in the first place. In other words it slows down the switch on time.

9

Will do.

Final question before I test, do I need to connect the other output to ground? I suspect not but the datasheet said unused pins should be connect to ground.

Scrap that, it would just be a short. Apologies

10

Yes the note in the data sheet says:-

Note 1: Unused inputs should be grounded.

Note also what it says about decoupling capacitors:-

4.3 Decoupling Capacitors

Careful layout and decoupling capacitors are highly recommended when using MOSFET drivers. Large currents are required to charge and discharge capacitive loads quickly. For example, 2.25A are needed to charge a 2500 pF load with 18V in 20 ns.

To operate the MOSFET driver over a wide frequency range with low supply impedance, a ceramic and a low ESR film capacitor are recommended to be placed in parallel between the driver VDD and the GND. A 1.0 μF low ESR film capacitor and a 0.1 μF ceramic capacitor placed between pins 1, 8 and 4, 5 should be used. These capacitors should be placed close to the driver to minimize circuit board parasitics and provide a local source for the required current.

11

Placing a 100 ohm resistor in the gate, defeats the whole purpose of using a high current high speed MOSFET driver.

12

The two outputs are supposed to be connected together, otherwise it would never work.

13

You are correct

14

Jim you are at it again, you are wrong this time as well.

Didn't you notice how the OP blew up his driver again and again when connecting the two outputs together.

16

OK Jim why did the drivers blow up?

17

If he connects only the top output, then it will only turn on and never off.
If he connects only the bottom output then it will do nothing.

18

@Grumpy_Mike No comment?

19

Jim - you just do not understand that diagram either. So you are jumping to wrong conclusions.
Like I said

No comment on that?

20

Not because the two outputs were connected together like they are supposed to be.
I would have to see the PCB layout

YES, JIM IS AT IT AGAIN!!!

21

@fisterroboto I see yo are using the DIP package. Is it soldered directly to the PCB or is it in a socket?