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High frequency MOSFET Driver

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T3sl4co1l:
I'm fond of this (when I'm not using an IC):



"In" obviously needs whatever level shifting is required to drive a BJT base.

Regarding Baker clamping, it reduces storage time at the expense of more Miller effect.  Since the diode isn't for free, it has some capacitance.  A big ol' BAT54H for example (>10pF) isn't going to do you any favors.  BAS71 would be more appropriate, or preferably anything of similar current rating (and capacitance), but lower voltage rating (and lower Rs, Vf).

If the base drive is from a resistor divider, a "speed-up" cap can also be used there, or the same trick (Baker clamp) done on a tap on the divider, allowing PN diodes to be used instead (e.g. 1N4148 is good enough at ~4pF):



(The BJT variant makes use of the fast recovery and potentially matched Vf of a inverted diode-strapped BJT.)

This driver,



uses the above topology, with a differential input stage to give an adjustable input threshold, and with the output driving a "complementary" pair of small MOS which drive a bigger emitter follower driving the final output (gate drive).

Complementary MOS doesn't really exist, in the same sense that complementary BJTs do; in this case 2N7002 and BSS84 were used, which are similar enough in performance.  Because this MOS inverter stage has massive shoot-thru, there's series supply impedance (R || L), which affects the rising slope (a bit slower) and the settling after the rise (it's clamped to VCC by the NPN, then gently rings down).

The final follower is PBSS4540/PBSS5540.

Performance is pretty good, 8/12ns into 10nF+2.2R.

You wouldn't need to go to such lengths for an IGBT of course; doing the above circuit with PBSS303NX/PX, or ZXGDxxxx something or other, would be more than adequate, and with the current source set for probably <= 10mA and the input transistor's drive set appropriately.

Tim

magic:

--- Quote from: T3sl4co1l on July 14, 2019, 09:52:58 pm ---Regarding Baker clamping, it reduces storage time at the expense of more Miller effect.  Since the diode isn't for free, it has some capacitance.  A big ol' BAT54H for example (>10pF) isn't going to do you any favors.  BAS71 would be more appropriate, or preferably anything of similar current rating (and capacitance), but lower voltage rating (and lower Rs, Vf).

--- End quote ---
Right, I have built an integrator and hence the ramp is so nicely linear.
But even with a tiny diode like BAT81 it is still quite slow if base resistance is a few kΩ. It improves if a diode is added to quickly discharge the base.
Conclusion: 2N7000 is the winner :)

David Hess:

--- Quote from: magic on July 15, 2019, 10:30:46 am ---
--- Quote from: T3sl4co1l on July 14, 2019, 09:52:58 pm ---Regarding Baker clamping, it reduces storage time at the expense of more Miller effect.  Since the diode isn't for free, it has some capacitance.  A big ol' BAT54H for example (>10pF) isn't going to do you any favors.  BAS71 would be more appropriate, or preferably anything of similar current rating (and capacitance), but lower voltage rating (and lower Rs, Vf).
--- End quote ---

Right, I have built an integrator and hence the ramp is so nicely linear.
But even with a tiny diode like BAT81 it is still quite slow if base resistance is a few kΩ. It improves if a diode is added to quickly discharge the base.
Conclusion: 2N7000 is the winner :)
--- End quote ---

Bypass the base series resistor with a small value of capacitance to remove all of the charge at once.

Or if low voltage compliance is not a problem, drive the input transistor at its emitter instead of base.

Or if low voltage compliance is a problem, drive the base and emitter but this requires a differential signal; bipolar transistors turn off really fast when their base-emitter junction is reverse biased.  This configuration is commonly used to add a fast high voltage level shifter to a fast low voltage comparator which has differential outputs but I have also done it with CMOS logic that provides Q and -Q outputs.  I have never seen it used with a MOSFET driver but I suspect this is because nobody needs a discrete MOSFET driver which is that fast.

The 2N7000 is the winner for simplicity but I have always gotten better performance out of bipolar parts at the expense of complexity in the same application.  Even in single ended circuits, I have gotten better performance because of the high output capacitance of the 2N7000.

T3sl4co1l:
I've used both before, see the phase inverter here, after IC1B:
https://www.seventransistorlabs.com/Class_D_Amp.pdf
The top right quadrant together is a modest capability gate driver.  One nice feature of this circuit: the current sources only draw current during commutation, allowing them to be set fairly high without overheating.  (Although not in this case due to the limited current draw of the comparator.)

Tim

David Hess:

--- Quote from: T3sl4co1l on July 16, 2019, 10:41:16 am ---I've used both before, see the phase inverter here, after IC1B:
https://www.seventransistorlabs.com/Class_D_Amp.pdf
The top right quadrant together is a modest capability gate driver.  One nice feature of this circuit: the current sources only draw current during commutation, allowing them to be set fairly high without overheating.  (Although not in this case due to the limited current draw of the comparator.)
--- End quote ---

I have seen similar configurations where the drive to the common base level shifter (emitter of Q4) was also capacitively coupled to the current source (emitter of Q6) so the current source was modulated to follow the output state during transitions.

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