The above link shows a circuit design for a motordriver.
And here are the datasheets:
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What is the purpose of the resistance in front of each gate of the MOSFET-transistors? And what about the other components (capacitors, diode)? What are their purpose/function in this context?
And can someone please tell me, what exactly goes on in this circuit?
And what are the advantages of using MOSFET instead of another kind of transistor?
Thanks
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They'd be current limit resistors, i.e, protection.
The cap between Vcc and Com is the bypass capacitor for the controller chip, keeps a smooth voltage supply to the chip. The cap between Vb and Vs is similar, but this one keeps a smooth voltage on the high side mosfet. The diode would be for protection as well, keeping any high voltage transients from the low voltage control side. I would think that the two caps would have a value of 0.1 uF and the resistors maybe 100 R or 1k.
The value of the cap across the fets would depend on what motor and voltage you were using. It's purpose is explained in the link below.
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Transistors require a current to the base to turn them on. In a high current application, say where there is a high current motor, then the current required to drive the transistor is quite high. This can complicate the circuitry. Mosfets, on the other hand, are controlled by voltage and are very easy to turn on and off regardless of the current running between drain and source. The current to the gate is almost negligible in all cases. Another advantage of mosfets is that there resistance is quite low, meaning that current flows more easily through them and they don't get as hot.
Just to be clear, power MOSFET's have a significant gate capacitance. The resistor is used to limit drive current, not load current.
The advantage is greater output drive and less heating. Realistically only three options are available for power drivers, MOSFET, Bipolar, and IGBT transistors.
MOSFET is superior for currents up to maybe 20 amps because they are essentially resistive when on while both bipolar and IGBT have a significant voltage drop.
MOSFET is also superior at high frequencies because they have less capacitance.
MOSFET's are superior to bipolar in that they have a negative temperature coefficient, their resistance increases with head reducing current while both bipolar and IBGT's can encounter 'thermal runaway' as their resistance _decreases_ with increasing temperature aggreviating any thermal problems.
At high currents IGBT and Bipolar are still superior in voltage drop and heating as individual components but MOSFET's can be paralleled without current-hogging as is the case with Bipolar/IGBT's. In other words MOSFETs are even replacing Bipolar in their last bastion.
-- Regards, Albert
---------------------------------------------------------------------- AM Research, Inc. The Embedded Systems Experts
If you want to understand this, read ALL the data (including, especially, the application notes) on the MOSFET driver. This circuit is probably too simple to be reliable with large MOSFETs unless the layout is absolutely ideal (hard to achieve with mechanical heat-sinking requirements. More than the slightest bit of source inductance can be fatal at high currents and fast switching speeds.
Read the data sheets. The diode and capacitors are part of a bootstrap supply circuit to generate MOSFET gate drive voltages exceeding the
+ve rail. This is necessary if you want to make a half-bridge using only the cheaper and better n-chanel devices.
It is well explained in the data sheets and application notes.
IGBTs are better for high voltage at moderate to low speed. There is a trade-off of Rds(on) vs. Vdss on MOSFETs that means that high voltage MOSFETs tend to have a lot of loss for a given chip area (cost). You can get 1200V IGBTs that can carry serious curents. At
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