common mode rejection vs common mode voltage

Hi,

I was looking at the datasheet of the HCPL-2201, and it specifies a minimum CMR value of 10kV/us at a common mode voltage of 300Volts.

When the common mode voltage is less, ie. 50volts, the CMR goes down to something around 1kV/us.

I have a fet driver circuit that requires a minimum of 10kV/us CMR for the opto, for a floating fet gate, would this opto work properly under these conditions? I am not sure how to know what the common mode voltage would be to know if I will get 1kV/us or 10kV/us CMR or something in between.

HCPL-2201 datasheet:

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cheers, Jamie

Reply to
Jamie Morken
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You're misreading the spec.

On page 12:

HCPL2211/2 has the higher CMRR HCPL2201 has the lower CMRR.

For a specific part number, the CMRR varies, at a specific forward current, depending upon whether the CM is applied when output logic is high or low.

In one state, the applied opto current is effectively nulled by CM current. In the other logic state, all apparent opto reciever current is due to dvdt effects alone.

RL

Reply to
legg

I have somewhere an app note that describes how to improve the CMR of optos. basically DONT drive the led open-ended, as when the LED is off, the impedance preventing any current flow is very high, so dV/dt can capacitively couple current thru to the LED side which can then flow thru the LED turning it on..... a pair of gates driving the LED in push-pull is a good solution; the other is a resistor to Vcc to turn it on, and shorting the LED to turn it off. hungrier this way though.

ISTR some of the FOD series optos have fast enough prop. delays Jamie....

Cheers Terry

Reply to
Terry Given

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I think this is the way I have it set up (resistor to VCC and transistor to gnd): "

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the two wires on the left lead to a transformer secondary (floating fet driver supply)

Yes I've seen some of these, they are mostly 5V supply though, and would require a small 5V regulator for each floating fet driver, nicer to have a opto that can take up to 20V :)

cheers, Jamie

Reply to
Jamie Morken

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(woops that is resistor to GND there, too late for electronics..)

Reply to
Jamie Morken

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Here's the recommended LED drive for ultra high CMR:

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cheers, Jamie

Reply to
Jamie Morken

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