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Topic: IR2111 gate half bridge gate driver (Read 3938 times) previous topic - next topic

sreedevk

I have attached the basic connection diagram for ir2111 gate driver ic from the datasheet...i read the app note and still cannot understand the use of Vs pin...i know that the MOSFET on top has its source floating when it is off and since gate voltage is always referenced to the voltage at the source terminal, any voltage applied to gate will have no effect with the source floating....so does the Vs pin serve to provide the source with some sort of voltage. And what happens to this pin after the MOSFET is turned on.....

Thanks

MarkT

#1
Jan 18, 2014, 11:06 pm Last Edit: Jan 18, 2014, 11:11 pm by MarkT Reason: 1
The driver for the top side MOSFET has a floating bootstrapped supply, the diode
and capacitor serve to produce a voltage rail 12V above the source of the top MOSFET.

This relies on the bottom MOSFET being on regularly so the capacitor can recharge through
the diode.  While the bottom MOSFET is off the capacitor provides the top-side gate supply.

Thus such a driver has to either have PWM provided to it or be parked with the bottom
MOSFET on.   If you leave the bottom MOSFET off for more and a few ms the top-side gate
supply will droop and the MOSFET driver will then shutdown.

The MOSFET bridge is suppling current to recharge the capacitor via the Vs pin of the
driver and defining the voltage of the floating top-side driver.

The bootstrap components are crucial, the capacitor should be about 10 to 20 times
the gate capacitance of the MOSFET and ceramic, the diode must be schottky or
ultra-fast rectifier (no significant reverse-recovery delay).

You should run the gate and source connections from each MOSFET to the relevant
pins on the IR2111 close together (if traces on PCB) or as twisted pair (if wires).  Keep
the leads as short as possible.

[ Oh yes, one more thing, the decoupling capacitor on Vcc and Com should be ceramic and
at least 10 times the value of the bootstrap capacitor - that circuit shows it as electrolytic,
which isn't good enough unless a carefully chosen low ESR / low ESL.  The decoupling
capacitor must be right next to the chip and bootstrap components (a few mm is good)
only, its carrying large current pulses ]
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

sreedevk

Can you expain what bootstarp supply is?? I gooogled a bit but couldn't understand much.

MarkT

The gate supply is bootstrapped from the output of the H-bridge - it lifts
itself up by its own bootstraps, as it were since the output from the H-bridge
provides the charge packets (in combination with the 12V supply) to pump
the capacitor of the floating supply upto 12V.  Its not really circular
dependency though, since the bottom MOSFET is doing the work really.

[ Don't worry about it though, just build the circuit as described. ]

[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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