NOTE: This question concerns topology - circuit values are omitted intentionally.

I'm strongly considering the use a P-Channel mosfet to serve as a high-side switch within a circuit.

All P-Channel mosfet options with low Rds(on) (10mOhm or less) have a Vgss limit of 20V or less. This circuit will operate to Vdss of -30V. Driving the gate between Vin and Ground will exceed Vgss. I have not found a gate driver IC that will accommodate this factor.

I've included a schematic below for a topology that I'm considering. The topology uses a smaller TTL N-Channel mosfet paired with an bipolar totem-pole driver to levelshift the signal and drive the P-Channel Switch. The Emitter Follower is biased to the regulated output of the buck converter (14V above ground) which puts Vgss between Vin and 14V - this will be within Vgss spec for most P-Channel mosfet options.

My question: Is there a better option or topology for this circuit? A solution that uses a Gate Driver IC will be well received. A better topology which uses discrete components will be appreciated as well.

Some circuit details below.

  1. The circuit must operate when attached to either a 12v system or a 24V system. Fully charged 24V systems can achieve voltages as high as 30V - this is the upper Vin limit.
  2. When the circuit is connected to a 24V system, the mosfet will be switched at 250KHz as part of a buck converter to regulate voltage to 14V.
  3. When the circuit is connected to a 12V system the mosfet will be fully on. This is why the P-Channel mosfet is desirable for this application.

BONUS QUESTION: How would you add reverse polarity protection to the circuit topology below?

enter image description here

  • \$\begingroup\$ You realize there are stability issues with your proposed circuit with turn-on overshoot, damping factor depends on load with a high L / Rdson ratio with no load. Why not a half bridge? There are lots of ways to level shift HO drivers \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 19 '19 at 19:42
  • \$\begingroup\$ There are also many other functional arrays. Nch PNP, NPN Pch, Pch Superdiode etc. diodes.com/products/discrete/functional-arrays with different characteristics for ESD , load dump , Ron , Cout and voltage ratings \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 19 '19 at 19:57
  • \$\begingroup\$ @SunnyskyguyEE75 Given the application, overshoot is not a concern. \$\endgroup\$ – GabeNix Apr 19 '19 at 20:15

You could use a low-side gate driver, give it a GND of -12V or whatever relative to the high rail (eg. LM7912) and then you only have to shift the input control voltage to it.

You could do that easily with a few discrete parts or use a logic-output optoisolator.

Reverse polarity protection is easily done with the classic Vin to series P-channel MOSFET and a resistor (gate to ground), plus a zener diode (gate to source) to protect the gate. You cannot make the other P-channel MOSFET do double-duty because of the body diode.

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  • \$\begingroup\$ I've never used a negative output regulator and information is sparse. Would this still function properly when the circuit is operated on the 12V system with operational voltage as low as 11.5V considering dropout voltage? Best case quiescent current at 1.5mA having to use a TO220 is a bit high, I'd need a way to shut it off completely. \$\endgroup\$ – GabeNix Apr 20 '19 at 11:27
  • \$\begingroup\$ I could separate the 24V and 12V supplies, but in doing so this thread question would be moot as it would negate the reason for a single PFET and a N-Channel could be used on the 24V supply while a PFET on 12V rail with no Vgss issue. \$\endgroup\$ – GabeNix Apr 20 '19 at 12:04
  • \$\begingroup\$ Negative regulators have similar dropout characteristics to positive ones. So if the regulator gets insufficient input voltage it passes the input minus a drop. So there would be plenty of voltage for the gate driver. Consider that part number and voltage a placeholder anyway- you could use a -9V regulator and maintain regulation with 10V in, for an LDO type. And you could get a lower Iq by picking an appropriate part. \$\endgroup\$ – Spehro Pefhany Apr 20 '19 at 12:42

You must protect the gate of the highside P channel .A zener diode of less than 20 Volt is needed .I have used Zeners of between 10V and 18 V over the last 15 years .Some high current fets have slightly less on resistance when the gate drive is above 10 Volts which means that an 18V zener is better than a 10V zener .At lower currents there is nothing to be gained with a 18V zener compared to 10 V .For reasons of BOM consolidation and parts availability I have sometimes used 12V and 15V .If you just put a Zener in your proposed circuit the mosfet wont blow its gate but the zener will blow if it is say a cheap 1 Watt device .Your driver is not current limited ,in fact its a good voltage source .What I do is use a simple two NPN transistor current source to drive the gate .This means that the worse case zener power losses are defined .If you limit the current by whatever means on your circuit then a cheap low power zener can be applied protecting the gate of the P channel mosfet .The current source scheme has been used thousands of times with input voltages from 24 to 160 .

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