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 Posted: Aug 22, 2006 - 03:52 AM |
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Joined: Nov 14, 2005
Posts: 163
Location: Philadelphia, USA
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Hi,
Hoping someone can point out why this circuit seems to keep frying the 4427 driver chips. I thought I was killing the MOSFET, but after a closer look they seem to be okay, but the 4427 gets hot and stops working. the circuit seems to work if I measure the voltage at the motor connectors (motor disconnected) with an analog meter - reads 0 to 12 VDC as PWM duty cycle goes up. But if I attach the motor the 4427 burns up.
Any help would be greatly appreciated
-Tim |
Last edited by timwhunt on Aug 18, 2007 - 08:33 PM; edited 1 time in total
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Posted: Aug 22, 2006 - 04:17 AM |
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Joined: Mar 28, 2001
Posts: 13030
Location: Sydney, Australia (Gum trees, Koalas and Kangaroos, No Edelweiss)
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Never used that chip so....is it supposed to run from 12V? Do you need some small resistance in series with the Gate?
edit: so I have a Micrel data book for the chip  and it works up to 18V. One thing it says is to ground the unused driver, the input only I presume. |
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John Samperi
Ampertronics Pty. Ltd.
www.ampertronics.com.au
* Electronic Design * Custom Products * Contract Assembly
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Posted: Aug 22, 2006 - 04:28 AM |
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Joined: Jun 21, 2006
Posts: 239
Location: Seattle, WA
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This is a complete guess, based upon the posting by js, but on page 9 of the Data Sheet found at http://ww1.microchip.com/downloads/en/DeviceDoc/21422D.pdf it says:
Quote:
3.4 Supply Input (VDD)
The VDD input is the bias supply for the MOSFET driver
and is rated for 4.5V to 18V with respect to the ground
pin. The VDD input should be bypassed with local
ceramic capacitors. The value of these capacitors
should be chosen based on the capacitive load that is
being driven. A value of 1.0 μF is suggested.
(Highlights mine own)
Regards,
Steve
(I really am clueless and guessing...) |
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Posted: Aug 22, 2006 - 04:43 AM |
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Joined: Mar 28, 2001
Posts: 13030
Location: Sydney, Australia (Gum trees, Koalas and Kangaroos, No Edelweiss)
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| By the way where did you get the IRF5303? I have a IRF book but can't find it there. It seems to be the P type from google however have you wired it the right way aroud? |
_________________
John Samperi
Ampertronics Pty. Ltd.
www.ampertronics.com.au
* Electronic Design * Custom Products * Contract Assembly
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Posted: Aug 22, 2006 - 02:25 PM |
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Joined: Nov 14, 2005
Posts: 163
Location: Philadelphia, USA
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Thanks for the help!
I got the MOSFET from digikey, but I just checked my order summary and it says IRF5305 (note last digit is 5). Sorry for the confusion. I read the number from the part when I made the schematic above. I assume I read it wrong, but I'll check when I get home and double check the wiring. I believe the same ciruit except for the diode has worked fine on a breadboard with smaller motors. This motor's "datasheet" is here The IRF5305 datasheet is here It's P type, 31A, 55V
RE resistance at the gate: I got the basic (not exact) idea for this circuit from a robotics book that used gate resistors with MOSFETS in parallel and I think when driving the gate with a transistor, but not with the 4427. If you think it's worth a try (or can't hurt) I'll try it.
RE capacitors bypasing VDD: Yes, those were included in the circuit I based this on, but the explaination made it sound like they were optimizations, so out of laziness, I left them out. Think that could be the problem? I'll kick myself if it is!
I'll also ground the unused driver (input) to cover my bases.
I bought 4 of the 4427's, and I'm down to my last one. I'll order more ASAP. But in the meantime, does any of this seem very likely to solve the problem? I hesistate to burn the last one quickly with the wrong test.
Thanks again! |
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Posted: Aug 22, 2006 - 02:36 PM |
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Joined: Nov 14, 2005
Posts: 163
Location: Philadelphia, USA
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| BTW, how many amps can one usually run through a solderless breadboard? This motor draws 2-20 Amps. It's under fairly light load (spinning a flywheel), so except for startup, it's probably on the lower end of that range. I've been afraid to build the circuit on a solderless breadboard, but soldering these experimental circuits together has been slow and painful. |
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Posted: Aug 22, 2006 - 07:13 PM |
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Joined: Sep 04, 2002
Posts: 13548
Location: Orlando Florida
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| Maybe you need to run the driver chip from 5V and use a pullup on the gate to 12V?? Does it need 12V on the in to turn it on?? (AVR wont give it 12V!) |
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Posted: Aug 22, 2006 - 08:05 PM |
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Joined: Oct 30, 2002
Posts: 3922
Location: The Netherlands
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What frequency is the PWM running at? You really need to look with a scope at the various signals to see whats going on. Parasitic capacitances are injecting charges back into the gate, together with the stray inductances of the board spikes might be generated that kill the driver (there are two parasitic capacitances from the drain to the gate and from the source to the gate; when the FET turns on charge is injected back to the gate, delaying the turn on).
The gate of a MOSFET is high-impedance but that's only when the voltage applied is steady. A large MOSFET has significant capacitance, so large currents are needed to swing the gate around quickly. At high frequencies this can consume quite some power. If not enough current is available the gate voltage will not rise/fall quickly enough and the FET spends too much time in its linear region, dissipating power.
I would try a resistor of 10 to 100 ohms in series with the gate. |
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Posted: Aug 22, 2006 - 09:45 PM |
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Joined: Sep 25, 2003
Posts: 2189
Location: Los Angeles, USA
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Without a really beefy capacitance on the 12V, there is all kinds of junk as a result of the motor.
In addition to 1uF to assure proper operation of the driver, I would add at least 100uF (and maybe even 1500uF since this is a 20A motor) on the 12V, to protect this circuit as well as anything else on the same 12V. |
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Posted: Aug 22, 2006 - 09:55 PM |
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Joined: Nov 14, 2005
Posts: 163
Location: Philadelphia, USA
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Thanks for the additional help!
Bob, sorry if I misunderstood, but the MOSFET does need 12V (at least more than 5V) at the gate to turn off. I experimented with applying 5V at the gate, and it did not turn off much if at all. That's why I'm using the driver chip, to deliver the higher voltage to the MOSFET gate. Sorry if I'm stating the obvious - this is all new to me.
JayJay: It's an 8bit phase correct PWM signal on a chip running at 1MHz. So if my math is correct, the frequency is 1,000,000 / 512 = 1,953Hz. Your explaination makes me think I need to add the capacitor as Steve also suggested above. My limited knowledge doesn't make it obvious why a resistor in series with the gate would help, but I'll try that too. Do you think these things would burn up the 4427 driver chip or just make it more efficient? I ask because if it's a likely fix, I'll try it on my last 4427 chip (before more arrive), but I'd hate to burn that one up if there is something else I should try first or at the same time. Unfortunately, I don't have a scope.
PS: I just saw mneary's comment added as I was Previewing this post. Another vote for adding capacitors. Okay, I'll cross my fingers and try it out. (may not have time tonight  )
Thanks again!! |
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Posted: Aug 22, 2006 - 10:24 PM |
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Joined: Mar 06, 2001
Posts: 527
Location: Moscow
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You need high-side driver or N-chanel Mosfet.
Alexander. |
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Posted: Aug 22, 2006 - 10:37 PM |
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Joined: Nov 14, 2005
Posts: 163
Location: Philadelphia, USA
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| Thanks Alexander, but can you give me a few more words to explain? What's a "high-side driver"? I do have N-channel MOSFETs, so I could set up the analygous circuit, but shouldn't this work with P-channel? |
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Posted: Aug 22, 2006 - 11:06 PM |
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Joined: Mar 28, 2001
Posts: 13030
Location: Sydney, Australia (Gum trees, Koalas and Kangaroos, No Edelweiss)
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The circuit is basically correct IF the gate and drain (pins 1 and 2) are wired up correctly. If they are reversed then the motor will turn the mosfet fully on, putting a full 12V onto the drain which would be connected to the driver chip which in turn would try to pulse it low and.....................
edit: try it out without the motor to start off with, put say a 100R, 5W, in place of it and perhaps a led in parallel (with suitable resistor in series) so you can see the thing working. |
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John Samperi
Ampertronics Pty. Ltd.
www.ampertronics.com.au
* Electronic Design * Custom Products * Contract Assembly
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Posted: Aug 22, 2006 - 11:14 PM |
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Joined: Sep 04, 2002
Posts: 13548
Location: Orlando Florida
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| The p channel fets are like pnp transistors... you pull the gate up to vcc to turn it off, you pull it low to turn it on. The part that confuses me is the avr is giving 0-5v to the driver chip input which is running from 12v... does the output look like its switching full 12v ok? |
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Posted: Aug 22, 2006 - 11:20 PM |
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Joined: Mar 28, 2001
Posts: 13030
Location: Sydney, Australia (Gum trees, Koalas and Kangaroos, No Edelweiss)
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| Bob that's what the driver does, it converts 5v from the input to whatever the chip is running at (up to 18V). That particular chip has also the advantage of being able to deliver 1.5A peak to the load. |
_________________
John Samperi
Ampertronics Pty. Ltd.
www.ampertronics.com.au
* Electronic Design * Custom Products * Contract Assembly
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Posted: Aug 23, 2006 - 03:45 AM |
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Joined: Nov 14, 2005
Posts: 163
Location: Philadelphia, USA
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I checked (and double/triple/etc. checked) the circuit. It is an IRF5305, and it looks to be wired correctly. So far suggestions are a few capacitors, grounding the unused input to the 4427 and a resistor in series with the gate. Just to make sure I have it correct, I've placed the capacitors in the revised schematic below - look correct? The rest is straight forward enough. I still don't understand the series resistor on the gate, but unless anyone thinks it's a bad idea, I'll try it too.
Any other suggestions?
Thanks,
Tim |
Last edited by timwhunt on Aug 18, 2007 - 08:35 PM; edited 1 time in total
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Posted: Aug 23, 2006 - 04:14 AM |
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Joined: Mar 28, 2001
Posts: 13030
Location: Sydney, Australia (Gum trees, Koalas and Kangaroos, No Edelweiss)
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If you put 100R in series with the gate then the current will be limited to about 120mA which should keep your driver safe. I always start with a current limited power supply when I do something like that.
Quote:
It is an IRF5305, and it looks to be wired correctly
So your output to the motor is wired up to pin 2 (centre pin or the tab), the driver chip's output is going to the gate (pin 1 or l/h side) and 12V goes to pin 3 (r/h side this is looking at the front, plastic part of the fet) |
_________________
John Samperi
Ampertronics Pty. Ltd.
www.ampertronics.com.au
* Electronic Design * Custom Products * Contract Assembly
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Posted: Aug 23, 2006 - 04:55 AM |
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Joined: Nov 19, 2001
Posts: 582
Location: San Luis Obispo
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Rehashing comments by jayjay1974 and js.
The 4427 driver current paths are:
(1) Between Vcc and GND of the driver - transition dissipation.
(2) Between OUTA and alternating Vcc/GND - MOSFET gate capacitance charging and discharging.
Adding a resistor between driver output and MOSFET gate will move some of the power dissipation from the driver to the resistor when charging and discharging the MOSFET gate capacitance of 1200pF. The downside is increased power dissipation at the MOSFET, it's designed for that.
The PWM rate or frequency is directly related to how often the MOSFET gate capacitance is charged and discharged by the driver. If 1 kHz works, don't use 100 kHz.
Stan |
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Posted: Aug 23, 2006 - 05:11 AM |
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Joined: Aug 04, 2002
Posts: 952
Location: Brisbane, Australia
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If I may...
I wonder whether the OP realises the fact that with the combination of driver and FET as specified the PWM output needs to be INVERTED since a m inimum outptu on the PWM will cause a MAXIMUM MOTOR CURRENT and vice versa...
Unless the driver chip incorporates an inverter stage ( have not checked the data sheet.. not in the spirit of giving advice...) |
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Posted: Aug 23, 2006 - 05:20 AM |
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Joined: Apr 16, 2001
Posts: 1831
Location: Phoenix, Arizona
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| Don't forget grounding the unused input. |
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