Need help driving two IPS1031 Low-Side MOSFETs from a PIC

You are driving these MOSFET's with 5V logic signals, right?

OK. What about the spec sheet is gibberish to you? This part looks like an ordinary MOSFET with a well-controlled avalanche voltage of 34V or more, and a shutoff-protection feature that operates if Id > 18A or Tj > 165 degrees, typical. The FET's protection circuitry considerably slows down the its response time, but it makes it virtually indestructible.

What's your question? Why did you choose this part?

--
 Thanks,
    - Win

Q2 isn't guaranteed to see the 4.5V minimum datasheet "on" voltage with a 5V nominal supply on the PIC.

As Win asked, why did you choose the IPS1031 rather than a more jellybean part?

Best regards, Spehro Pefhany

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Spehro, I was looking for the Voltage required to turn on. The spec sheet said less than 4.5V could be used, but it didnt specify beyond that, or did it? Thats what confused me. The reason I picked this, over a a more "normal" device such as a 2N2222, was because I was told that, "a dozen of these" could be controlled by a PIC output. As you were, I was also concerned that in combination of the voltage drop from the PIC, as it does when increase mA sourced, and the voltage drop from the diode, the MOSFET would not see the required voltages Perhaps you could suggest another MOSFET or transistor?

Use 3 outputs from the PIC. Eliminate the diodes, do the OR function in the PIC.

Run the PIC on 6 volts to make sure you can turn on the FETS. This may violate the PIC specs (just a bit), but I've run them this way with no problems.

Luhan

Actually, most PICs are specified at 6.5 to 7.5 volts. Just don't try to program them at that voltage.

Luhan

Thanks for the follow-up Sphero. However, those 2 diodes are needed. The PIC is pre-progged. I don't know the exact specs of the 2N4401, I will go look them up. However you believe that a single PIC output could run two of them? My entire design started out with 2N2222s, guess my original design would have most likely worked.

The VIH parameter is given as 4.5V. It says to refer to the application note for operation outside of those conditions. When you look at the application note, they say it's not recommended:

But actually it will work okay under normal conditions since you're hardly drawing any current- 2.4mA on that output.

For Q2 and Q3 you can use a couple of 2N4401's. You can use 4.7K series base resistors on Q2 (two pieces, one to each of the two PIC outputs) and Q3. You don't need the diodes or the 100 ohm resistors or the resistors to ground. I don't know what the relay coil current is, so I can't tell you exactly what would be good for that output. For a small relay (15-20mA) another 2N4401 with a 4.7K base resistor will work fine. For up to 50 or 60mA, just reduce the base resistor to 1K.

Your BOM (for the drivers alone) in 100 quantity will drop from about $5 to less than $0.50 and the parts count will drop from 11 to 7. They are also multiple-sourced, small and easily available. OTOH, you lose the short-circuit protection.

Best regards, Spehro Pefhany

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Like this, no diodes: 4K7 ___ S1 o-|___|-+ | | ___ | to 5K load S2 o-|___|-+ | | 4K7 | |/ +-----| 2N4401 |>

| | === GND

Yes. Even with an extremely conservative design you only need 1/20 of the collector current at the base. So you could drive lots of 5K (2.4mA) loads since each one only would need 120uA source (not sink) current from the PIC output. Fewer 50mA loads.

The guaranteed gain is better on the 2N4401, IIRC, but probably it would have worked.

Best regards, Spehro Pefhany

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Thanks for the quick reply again Spehro. Your diagram isnt readable, sorry. How do you use the transistors, Hfe, in order to find the needed base amperage? Sorry I kinda feel out of the loop on the transistory termanology.

Ok I "decypherd" your schematic LOL. But in your schem your mixing the signals, If i were to do this, the relay would be on whenever a signal was enterd. The 2 diodes allow for the relay to go on when only one specific signal is enterd.

Change your viewing font to a fixed-width font such as Courier and it will be readable. You'll also find many other posts that read better that way, so it's recommended.

You don't use it directly, as you want the transistor to be really well saturated. If you have the full data sheet, look at the minimum hFE over temperature, the curves of typical Vce(sat) vs. base current and collector current etc. Usually Vce(sat) is specified at Ic/Ib =

10, but larger numbers are quite practical in most cases.

The base resistors can be calculated using Ohm's law - supply voltage, minus output drop of the PIC when sourcing the desired current minus base-emitter voltage of the transistor, divided by the desired base current. With Q2 you have 10-15% of current bled off by the resistor to the 2nd output, when one is on and the other is off.

Best regards, Spehro Pefhany

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The PIC outputs actively drive low as well as high*, and the 130uA or so that the "low" relay drive output sinks when it is off, and the other output is on, is not high enough to cause problems. The base resistors could be increased from 4.7K to 10K, 20K or 30K without problems.

If it really bothers you, it would be better to use two 2N4401s in parallel to drive the 5K load than to add diodes (one less part etc.), but it's not necessary.

*Unless deliberately tristated, but that's an unlikely situation.

Best regards, Spehro Pefhany

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The pic is not tristated. The pull-down resistors were only for use by the simulator. I don't know if I will even use them on the actual circuit. Could you clerify your use of two extra 2N4401s in place of the two diodes?? Thanks, Jed

One unnecessary 2N4401:

5K Load to 12VDC o | | +--+--+ 30K | | 30K ___ |/ \\| ___ o-|___|---| |---|___|-o S1 |>

Re-arrange it so the active low outputs at the IPS1031's are ORed to activate the IPS1031 driving the 5K load: View in a fixed-width font such as Courier.

. . 12V . .------------+------------+-------------. . | | | | . - | [150] | . |\\| | | | . RLY4 |\\| | | [5K LOAD] . + |\\| .-+-. | | . D1 - | | | | . | |< >| | | . +--[47K]-| 2x3906|-[39K]--+ | . | |\\ /| | | . | | | - | . | '-+-' V D2 | . | | - | . | | | | . |D | D| | . +----+----+ | +----+----+ | . G| | | | |G | . S2>---| ips1031 | | | ips1031 |---< S1 | . | | | | | | . +---------+ | +---------+ | . | | . | | . | +---------+ | . | G | |D | . '-------| ips1031 |--------' . | | . +---------+ . .

Just noticed that thing wants logic level inputs: View in a fixed-width font such as Courier.

12V .-----------------+-------+-------------. . | | | . | | | . | | | . | 5V | | . - | [150] | . |\\| .----+ | | . RLY4 |\\| | | - [5K LOAD] . + |\\| [10K]|< V D2 | . D1 - +--| - | . | | |\\ | | . | [4.7K] | | | . | | | | | . | | | | | . +--||-----+ | . | | | | . | | | | . |D | D| | . +----+----+ | +----+----+ | . G| | | | |G | . S2>---| ips1031 | | | ips1031 |---< S1 | . | | | | | | . +---------+ | +---------+ | . | | . | | . | +---------+ | . | G | |D | . +-------| ips1031 |--------' . | | | . | +---------+ . [4.7K] . | . | . --- . gnd . .