High-side Load switching

Discussion in 'The Projects Forum' started by picmark, Aug 19, 2010.

  1. picmark

    Thread Starter New Member

    Aug 19, 2010
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    Hi everybody,

    I am looking for an IC that could act as a high-side load switch similar to the circuit attached. I need to switch loads on and off to save power.
    Last resort, I will have to build it out of discrete components which is very real-state costly for 4 channels.

    I’ve been searching for days but I can’t find anything that meets the following criteria;

    Standby current in Micro Amps.
    Turn on voltage = TTL level.
    Vdd from +5 to +24V.
    Output current = 1 Amp Max.

    Everything I’ve seen so far can’t satisfy all the above.
    I am using a SN754410 right now but it’s a power pig even when all 4 ports are off it consumes about 100mA.

    Can anyone point me to the right direction…?

    Thanks,

    Mark
     
  2. Papabravo

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    Feb 24, 2006
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  3. picmark

    Thread Starter New Member

    Aug 19, 2010
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  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    For some odd reason high side switches arent as abundant as they once were. I've got about 100 of something I recall was really good but is now a discontinued part. It works perfectly, don't know why they dropped it and most everything like it. The IC would easily meet most of those specs but I don't recall the idle current. I'll have to dig them out sometime, I think I know where I stashed them.

    Have you tried drilling down through Maxim's website? They're really big in voltage control and high efficiency chips.
     
  5. picmark

    Thread Starter New Member

    Aug 19, 2010
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    I checked Maxim's site. No luck.
    I would be very interested if you find out what part you have.
    Anyways, please let me know if you ever dig them up.
    Thanks again Marshallf3 for taking the time to reply.

    Mark
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    High-side switches are seldom used nowadays because the gate area required for P-ch MOSFETS that are roughly equivalent to their N-channel counterparts is about 2.5 times as large, so the corresponding gate charge is roughly 2.5 times larger. This means that it takes more current to charge/discharge the gate in order to get approximately the same switching times.

    N-ch MOSFETs can be used on the high side, but they require a charge pump or similar to supply the gates, as to turn them on, the gate must be at least Vs+5v for logic level MOSFETs. The charge pump will result in increased idle current.

    It is far easier to simply use logic-level N-ch power MOSFETs, and switch the ground side, as the N-ch logic level MOSFETs can be switched directly by TTL levels at low speeds.
     
    Last edited: Aug 20, 2010
  7. picmark

    Thread Starter New Member

    Aug 19, 2010
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    All true...

    However, the draw back of using a low-side switch in this manner, is that you are basicaly disconnecting the load from ground at only one point. If the load is a complex one, it can find ground through ground loops and other ports that have access to system ground.
    Low side switching works fine with LEDs, Relays,.. etc.
    Case in point, you can run a Pic by powering one of its ports!

    Thanks for your response though.

    Mark
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Operating a PIC via an I/O pin is of course not recommended, but it can certainly happen inadvertently. The ESD diodes won't take much more than ~10mA current before they're fried.

    I have a schematic of a generic discreet high-side P-ch MOSFET driver if you're interested. There is no current draw unless a MOSFET is switched ON. The current consumption of the driver varies with Vdd; up to 20mA when Vdd=24v. However, it'll take a bit of real estate to implement; not a lot, but certainly more than a single IC.
     
  9. picmark

    Thread Starter New Member

    Aug 19, 2010
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    Could you please post your schematic?
    Thanks
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Sure.
    See the attached.

    V1 represents a logic output from a uC or other TTL-compatible device driving the gate of M1, a 2N7000 MOSFET.

    V2 represents a supply that starts at 5v and increases to 24v.

    When M1 is not conducting, R1 pulls the bases of Q1 and Q2 high, which charges M2 (a low threshold P-ch MOSFET, specifically selected for low Rds(on) and low Qg, or gate charge) via R3. R3 keeps the gate from "ringing", or oscillating due the capacitance of the gate and the inductance of the charge/discharge path to +V/ground. Q1/Q2 are connected as voltage followers which multiply the relatively small current available at the bases.

    When M1 is conducting, the bases of Q1/Q2 are pulled low, but the excursion is limited by D1. Generally, Vgs excursions beyond +/-20v will destroy a MOSFET; some are more sensitive than that. Limiting the excursion to ~5.1v also helps to minimize gate charge/discharge time.

    The plots below the schematic represent the gate voltage (green) and power dissipation in the MOSFET with a 1A load current. Note that the power dissipation is of short duration. The average power dissipation in the MOSFET for this simulation over the 100mS time duration was approximately 15.7mW, which is quite low.

    Keep in mind that this is only a simulation. You will need to test the circuit with real-world components to verify proper operation.
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
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    BTS432E - handle everything you need 10x over.

    Standby current is indeed in the uA range.

    For all I know you can still buy them somewhere, if you find some grab more than 1 or 2 if the price is right.
     
    Last edited: Aug 20, 2010
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    My post seems to have disappeared? Now I see it came back. Oh well, I'll just leave the double, there's more info in this one.

    BTS432E High Side Driver

    4.5V - 42V. 0.03 ohms on resistance

    125W, 11A/35A

    Standby is indeed in the uA range.

    For all I know they're still findable somewhere, if you can get a good price I'd buy more than 1 or 2 because they're really handy.
     
  13. picmark

    Thread Starter New Member

    Aug 19, 2010
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    Thanks marshallf3.

    I forgot about infineon. I designed an H-bridge using their BTS7960 about 5-6 years ago and I was never able to destroy it in my testings and it works to this day.
    Thanks for pointing me to the right direction. After searching their site, I decided on the BTS728L2. Its exactly what I was looking for. It's 2 channels so I would need 2 ICs and Digikey sells them for less than $2 each.

    Thanks again.
     
  14. picmark

    Thread Starter New Member

    Aug 19, 2010
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    Thanks SgtWookie for going through so much trouble.
    However I found the part I am looking for thanks to marshallf3.
    You guys are the best...
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Thanks, but that I doubt on my end. He's got a lot of book knowledge in addition to real world experience that extends a lot farther than mine, I've just got a lot of tinkering and scrounging experience.

    High side switches are ideal in automotive applications because your power source areas are limited but your ground level availability is virtually limitless. Thanks for pointing out a source, if I ever design something with one that might go commercial I've got to do it with parts of which a reliable source exists, same thing if I design a circuit for someone on the other side of the country, they need to be able to buy them somewhere instead of relying on me to dip into my supply.
     
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