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Old 01-30-2006, 06:28 PM   #1
john banks
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Default High current PWM circuit - MOSFETS & diodes

I have a nice microcontroller setup that produces 5V PWM.

I previously used this output and put it through a 1K resistor to the base of a TIP122 Darlington NPN power transistor with a modest heatsink to drive the boost control solenoid with success - I switched the ground of the solenoid by connecting it to the collector and the emitter to ground, relying on the internal diode for back EMF protection.

Now I wish to take this sort of output to 14V at 4A to drive a DCCD coil. So about ten times the current and I'm into the realm of power electronics, MOSFETS and hefty diodes that I've not been into before. I've got a bit bogged down reading about switched mode power supplies and need a minimal component solution. Can I just replace my trusty old TIP122 with something more beefy, or do I need to start worrying about the gate current requirement, huge EMF shocks, all sorts of diodes doing a merry dance?

Can someone put it simply for me - what transistor/FET, and where to put the diodes and any resistors between the uC and the gate etc.

I had the option of using a K8004 Velleman kit as others on here, but it is only the driver stage I need, I prefer to use my own control electronics I already have perfected.
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Old 01-30-2006, 09:02 PM   #2
colby
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* Use a MOSFET. The gate drive current will not be that significant because there's no real need to PWM this very quickly. Use a resistor in series with the gate to ease the drive requirements.
* Use a good high speed diode, such as a HEXFRED.
* If you PWM very slow, RF won't be that much of an issue.
* If you want to PWM faster, you should have a LC filter on the output which is local to the switching device and have good bypass capacitance near the switch. This way all of the higher frequency PWM switching currents are localized to a small loop area. The signal going to the DCCD is close to DC. This keeps everything from bouncing all over the place and creating lots of interference.

Colby

Quote:
Originally Posted by john banks
I have a nice microcontroller setup that produces 5V PWM.

I previously used this output and put it through a 1K resistor to the base of a TIP122 Darlington NPN power transistor with a modest heatsink to drive the boost control solenoid with success - I switched the ground of the solenoid by connecting it to the collector and the emitter to ground, relying on the internal diode for back EMF protection.

Now I wish to take this sort of output to 14V at 4A to drive a DCCD coil. So about ten times the current and I'm into the realm of power electronics, MOSFETS and hefty diodes that I've not been into before. I've got a bit bogged down reading about switched mode power supplies and need a minimal component solution. Can I just replace my trusty old TIP122 with something more beefy, or do I need to start worrying about the gate current requirement, huge EMF shocks, all sorts of diodes doing a merry dance?

Can someone put it simply for me - what transistor/FET, and where to put the diodes and any resistors between the uC and the gate etc.

I had the option of using a K8004 Velleman kit as others on here, but it is only the driver stage I need, I prefer to use my own control electronics I already have perfected.
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Old 01-30-2006, 10:17 PM   #3
Kha0S
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colby's got the right idea...

Any decent power MOSFET should do the job... I've driven some pretty serious outputs with an IRF510 and a beefy flyback diode to keep out the back-EMF.

/Andrew
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Old 01-31-2006, 04:58 AM   #4
john banks
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Thanks. Freq is about 100 Hz. So a 1K resistor to the gate and a single beefy diode reversed across the load, otherwise just wire like an NPN? Not played with MOSFETs before...
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Old 01-31-2006, 07:04 AM   #5
Lost_Pete
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Oddly enough MOSFETs/FETs seem a bit foreign if you are used to transistors but once you play with them you'll be fine.

While not trying to be technically perfect just remember the following;

1. MOSFET/FET use a voltage rather than current to control them.
2. Their input impedance is high but also includes capacitance which does require an initial amount of current (series gate resistor controls this).
3. Gate threshold voltage is important particularly if you want to drive it from logic levels. There are logic level versions. But watch the maximum gate voltage allowed.
4. High voltage versions are available but generally at the cost of increased on resistance.
5. Some include flyback zeners but external ones can still be used. A zener will give you better response on your solenoid application.
6. The majority of heat is generated during the switching phase.
7. In any case they are pretty tough and should run cool for your application.

So with the IRF510 I'd try;

1k series resistor to gate
10k resistor from gate to ground (so that the IRF510 is held off while the micro starts up)
flyback zener and diode combination of say 60V

regards
Pete

Last edited by Lost_Pete; 01-31-2006 at 07:57 AM.
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Old 01-31-2006, 07:46 AM   #6
john banks
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Thanks Pete, helpful pointers there
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Old 01-31-2006, 08:06 AM   #7
john banks
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Thanks for the later edit as well Pete. So I don't really need the external diode if there is an internal one in the IRF510?
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Old 01-31-2006, 08:27 AM   #8
Lost_Pete
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Some people use the internal one but I normally put an external one (zener) on. It can be useful to improve the response of the solenoid.

regards
Pete
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Old 01-31-2006, 10:00 AM   #9
ride5000
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also john, part of the tradeoff with mosfets is the extremely delicate gate insulating layer, which can be easily toasted with ESD. they are the most fragile while out of circuit.

even when strapped up, watch that max Vgs like a hawk! i like to put a series string (cathode to cathode) of a couple of zeners from gate to source for that reason. the 510 datasheet marks this as +/-20v.

on the datasheet the irf510 is good for 5.6a of drain current, with an excellent heatsink and 25c temps, but that drops to 4a if the temp is 100c. you might want to consider a beefier part if you're pulling 4a through the DCCD coil on a regular basis. even though mosfets are free from secondary breakdown (thermal runaway) i prefer to run things conservatively. they're cheap enough!

hth
ken
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Old 01-31-2006, 10:19 AM   #10
john banks
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Thanks Ken. You mean connect the anode of one zener to the gate, the cathode of this to the cathode of the next zener, and the anode of this second one to the source (ie ground)? This ESD will come from handling of the board or the automotive environment rather than EMF from the coil?
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Old 01-31-2006, 10:25 AM   #11
ride5000
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Quote:
Originally Posted by john banks
Thanks Ken. You mean connect the anode of one zener to the gate, the cathode of this to the cathode of the next zener, and the anode of this second one to the source (ie ground)?
yup. that will clamp the max differential voltage between source and gate to the zener breakdown + the forward biased 0.7v or so of the other diode.

Quote:
This ESD will come from handling of the board or the automotive environment rather than EMF from the coil?
yeah... just practise safe anti-static procedures when you're taking them out of the packaging materials, etc. the coil EMF will be taken care of by the external flyback diode (and internal if so equipped).
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Old 01-31-2006, 11:04 AM   #12
john banks
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Thanks, slowly getting there I feel ready to draw something out now and get ordering the bits. I think I'll leave the microcontroller bit in a separate box (already have this all built), and stash away the driver circuit somewhere else.
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Old 01-31-2006, 03:04 PM   #13
colby
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Yes - the gate, drain, and source of a N-channel MOSFET will work similarly to the base, collector, and emitter of a NPN bipolar transistor. If you are driving the MOSFET from 5V logic, you should consider using a special MOSFET that is designed with a low threshold voltage (Vt) so that it will turn fully on with a 5V Vgs. For example, in the International Rectifier series, these have an L suffix, e.g. IRF510L.

Colby
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