Author Topic: Why NMOS for buck switchers? (other than generally better NMOS specs)  (Read 1115 times)

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Offline 741Topic starter

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Just reading up on switchers... Assuming a basic circuit, with a catch-diode and a pass transistor: The buck circuit examples I have seen all use an NMOS with drain @Vdd, and Source tied to the inductor, hence a possible difficulty driving the NMOS gate fully ON for smaller Vin-Vout.

I wondered - if I invert the sense of the drive signal, can I use a PMOS, with Source at Vin+, drain tied to inductor? Ominously, I cannot find any examples :-\, must be a catch...maybe to do with impedance of D Vs S?

(In my case I'd like a high current, but pretty low voltage difference (say 3v), between Vin and Vout, hence (for the NMOS solution) I'd need to find an NMOS with low Vto)



Online Kleinstein

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Re: Why NMOS for buck switchers? (other than generally better NMOS specs)
« Reply #1 on: August 09, 2019, 01:53:31 pm »
One could use a P_MOSFET, but as already written, PMOS performance is not as good.

One a chip it is often cheaper to use NMOS and bootstrapped gate drive. It usually requites 1-2 external caps.  This also gives better performance.
 
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Offline 741Topic starter

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Re: Why NMOS for buck switchers? (other than generally better NMOS specs)
« Reply #2 on: August 09, 2019, 02:03:43 pm »
Many thanks for your explanation - my mind is "easier" now.

It seems to work OK in LTSpice.

Offline T3sl4co1l

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Re: Why NMOS for buck switchers? (other than generally better NMOS specs)
« Reply #3 on: August 09, 2019, 02:30:07 pm »
They're out there:
https://www.ti.com/lit/ds/symlink/tps40200-q1.pdf

Performance is king.  The difference is not small -- PMOS is about 2.5x worse than NMOS.  More die area = bigger package and more cost (with knock-on costs from lower production quantities -- i.e., PMOS in general are less common, and cost more as a result).  Poorer Qg*Rds(on) figure of merit = generally higher switching losses, and either higher conduction losses or higher drive losses (drive power is Qg * Vgs(on) * Fsw).

You need a driver anyway -- the gate has generous capacitance in either type, so a mere logic-level output is unsuitable for all but the smallest transistors -- and it's not much added die area (if integrated) or cost (if discrete*) to include the bootstrap driver for NMOS.  So it's very practical to use NMOS in high or low side application. :-+

* "RTL" level discrete that is, i.e., using ICs for basic functions.  Low side, logic-input drivers are cheap and plentiful (TC4427, etc.), as are bootstrap drivers (IR2110, etc.).  If you're doing something truly discrete (bare transistors), it's harder -- a high-voltage, noise-immune level shifter is certainly doable, but it will take more transistors than the alternative (compare this (first page, top and left half) to this, for example).

Tim
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Offline 741Topic starter

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Re: Why NMOS for buck switchers? (other than generally better NMOS specs)
« Reply #4 on: August 11, 2019, 01:14:42 pm »
Hi Tim

"Gate_Driver.png" reminded me of something I saw in a textbook ("circuits for electronic instrumentation", T H O'Dell, p103).

For added drive on fast edges, the top-left transistor had an emitter decoupling capacitor, also both bases of left-side transistors were tied via a capacitor.

Offline T3sl4co1l

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Re: Why NMOS for buck switchers? (other than generally better NMOS specs)
« Reply #5 on: August 11, 2019, 01:52:19 pm »
Yup; you would need a resistor in series with the diodes (probably replace one diode with a resistor of equivalent voltage drop) to allow some "emphasis" to be coupled through a capacitor.  A downside of this connection is, significant supply ripple (more than a modest fraction of Vbe, say) is coupled from the high side to the low side, through the capacitor, potentially causing erratic operation and shoot-through.  So you need to use good bypassing, and to separate or shield the supply from induced fields (fields can be quite intense in a big inverter circuit).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 


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