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I was looking for a way to drive the gate of a power NMOS. Since I've already have a small FPGA available at a independent backup-3V3 rail, I was thinking I could use it's internal oscillator to build a simple charge-pump.

The following should be a charge-pump relative to the NMOS source (load-side). It is stepped-up 3x3.3V from the independent 3V3 supply.

schematic

simulate this circuit – Schematic created using CircuitLab

I've simulated above circuit in LTSpice and got a nice ~13V/ms rise at the load with a final Vgs around 7V. The FPGA outputs are modelled as Push-Pull drivers and switching frequency is at 100kHz.

I can increase the rise-time by increasing C1 or lowering R2-R4.

I noticed that when charging a capacitive load, the current trough M1 is oscillating with the frequency of the charge pump. Does this matter? How can I avoid it?

Please help me understand how a gate driver is usually build, or point me to a suitable integrated circuit.

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    \$\begingroup\$ A P-FET would be a lot easier to drive. Alternatively, MAX232 or similar chip is sometimes (mis-) used to generate extra voltages. \$\endgroup\$ Aug 1 '14 at 10:26
  • \$\begingroup\$ I've used the LTC4210 for a similar function in the past, but it won't handle 30V. It has a logic-level control input and provides surge current limiting and overcurrent protection. The class of ICs called "hot-swap controllers" are generally quite useful for power management. \$\endgroup\$
    – Dave Tweed
    Aug 1 '14 at 10:51
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    \$\begingroup\$ You feed your charge pump from the load (switched) side. Doesn't that put you in a problem (output voltage needed to switch output voltage on...) \$\endgroup\$ Aug 1 '14 at 10:52
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    \$\begingroup\$ As I understand, I need to drive the NMOS Gate relative to the source (load-side). By putting the diode string at the load side, I reference the charge-pump to the source. Note that the FPGA is supplied by a independent backup rail \$\endgroup\$
    – Arne
    Aug 1 '14 at 10:56
  • \$\begingroup\$ I think that will result in very slow switching: while the FET switches on, its source will rise, but the gate voltage is determined by C1, which is relative to the ground... Anywhow, I suggest you forget this circuit, check for the chips Dave suggested, or use a P FET with a suitable driver. \$\endgroup\$ Aug 1 '14 at 12:18
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If you want simple solution, there are lots of ICs named 'high side n-MOS driver' all over the world. Linear Technology, Maxim, Micrel and other manufacturers make them. Just choose what exactly fits your needs.

If you want to konw, how they designed, then... As discrete solution voltage multiplier circuits are commonly used. So you're on right way. Simplest boost circuit i know consists only of capacitor and a diode (shown at left).

schematic

simulate this circuit – Schematic created using CircuitLab

This circuit use Vin voltage when load is off to charge capacitor and then use it to boost gate voltage. Drawback is that capacitor will discharge by leakage currents, so ot need to switch off and on load time-to-time.

So in general boost need independent clock source to pump charge (you're on right way here too). One solution is to use Villard cascade voltage multiplier. Schematic shown at right show charge pump circuit providing gate voltage Vin+V(~f) with single cascade. If higher gate voltage required more cascades can be added.

Note, resistor inserted between pump and controlling circuit, so switching off does not cause complete discharge of capacitors. In industry-made gate drivers push-pull cascade is used, providing very fast switching thus reducing switching power dissipation and reduce quiescent current through pump.

ADD: I found tips in industry made gate drivers:

  1. One end of pump capacitor is feed from MOSFET source, so it is naturally boosted when transistor is switched on. Thus it acts like my left schematic on this thansition.

  2. Another end of pump capacitor is typically fed from LDO. It is need to limit Vgs.

  3. There may be no output capacitor (like C3 on y scheme), thus internal MOSFET capacitance to keep gate voltage while pump capacitor is recharged in steady turned-on state.

  4. Sometimes there's no pushpull output cascade, pump capacitor is discharged when MOSFET is turned off (like on your schematics). This typical to dirvers not designed for PWM applications.

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    \$\begingroup\$ Problem with most of the ICs I found are made for PWM operation. However, I need to statically turn the load on. Still, I will investigate the circuits you posted. \$\endgroup\$
    – Arne
    Aug 1 '14 at 12:56
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    \$\begingroup\$ Note that the left circuit relies on switching to generate the boost voltage. \$\endgroup\$ Aug 1 '14 at 14:17
  • \$\begingroup\$ @Arne, i didn't notice problems finding drivers capable of instant power on. Look for example at MAX1614, it is very similar to that you need. Maybe it would be hard to find find a chip fits exactly to your needs (voltage, consumption etc.) \$\endgroup\$
    – Vovanium
    Aug 1 '14 at 14:44
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    \$\begingroup\$ @Vovanium yes, the MAX1614 would be an option, albeit the 5V min. supply might be problematic. Still, Maxim ICs are rather expensive.. \$\endgroup\$
    – Arne
    Aug 1 '14 at 14:57
  • \$\begingroup\$ @Arne, anyway i advice you to investigate MAX1614's schematics, it is interesting, how industry made gate driver designed. \$\endgroup\$
    – Vovanium
    Aug 1 '14 at 15:25

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