Electronic – Is it safe to “top up” bootstrap capacitors using a charge pump

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).

^{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.

^{simulate this circuit – Schematic created using CircuitLab}

Note 1: The input voltages are only \$V_{cc}\$ and \$V_\text{High Voltage}\$. You don't apply anything at the \$V_{BS}\$ node. It is only for representation.
Note 2: Notice that there are two different type of grounds. Those grounds must not be directly connected to each other.

You must drive the MOSFET between its gate and source terminals. Since the source terminal voltage of a high side MOSFET will be floating, you need a separate voltage supply (VBS: \$V_\text{Boot Strap}\$) for the gate drive circuit.

In the schematic below, VCC is the voltage source of the rest of the circuit. When the MOSFET is off, ground of the boot strap circuit is connected to the circuit ground, thus C1 and C2 charge up to the level of Vcc. When the input signal arrives to turn the MOSFET on, ground of the gate drive circuit rises up to the drain voltage of the MOSFET. The D1 diode will block this high voltage, so the C1 and C2 will supply the driving circuit during the on-time. Once the MOSFET is off again, C1 and C2 replenish their lost charges from VCC.

Design criteria:

• RB must be chosen as low as possible that will not damage D1.
• Capacity of C2 must be chosen enough to supply the driving circuit during the longest on-time.
• Reverse voltage rating of D1 must be above \$V_\text{High Voltage} - V_\text{CC}\$.

The input signal must be isolated from the boot-strap circuit. Some possible isolaters are:

Optocoupler

Optocoupler is the most basic method for isolation. They are very cheap compared to other methods. The cheap ones have propagation delay times down to 3\$\mu\$s. The ones with less than 1\$\mu\$s propagation delay are as expensive as isolated gate drivers though.

Pulse Transformer

Pulse transformer is a spacial type of transformer for transferring rectangular pulses. They have less number of turns in order to avoid parasitic capacitance and inductance and larger cores for compensating loss of inductance due to reduced number of turns. They are much faster than optocouplers. Delay times are less than 100ns in general. The image above is for illustration only. In practice, the current they can provide is not enough for driving a MOSFET fast; so they need additional circuitry in practice.

Isolated Gate Driver

Isolated gate driving is a relatively new technology. All the complexity of gate driving is encapsulated in one single chip. They are as fast as pulse transformers, yet they can provide a few amperes of peak gate current. Some products also contain on-chip isolated DC-DC converters, so they don't even need boot-strapping. However, all these super features come with a cost.