ISO5851: ISO5851

Hi Yenwu,

Yes, the ISO5851 datasheet application section on page 31 is correct. 

Ron_max= 4 ohms and Roff_max=2.5 ohms.

Is there any specific reason you are looking for min /typical value? 

Thanks

Sasi

1. Thanks. Sasi, do you have min or typical values for ISO5851 ?

 I assume that Ron_typ, Roff_typ values will be less than Ron_max and Roff_max values?

 So does this mean that there will be LESS loss in those Ron_typ and Roff-typ values when using the ISO5851 or not?

 Because based on eq. 7 in P. 31,  those Ron and Roff values will affect the total switching power loss.

2. Another question regarding the total power loss shown in eq. 7,  

Does DESAT pin ( pin 2 )  of ISO5851 will cause some extra power loss when the ISO5851 is driving the related IGBT, ..., etc, which eq 7 does not include?

This is because when ISO5851 is driving the related IGBT, the DESAT pin will be charging and discharging at the about same rate as the output pin ( pin 6 ) of the ISO5851, so will this DESAT pin charge and discharging will cause some extra power loss which eq. 7 is not included?

Thanks

yen

Hi Yenwu,

Let me get back to you by tomorrow.

Thanks

Sasi

Thanks for your reply on yesterday. Saai, Look forward to hearing from you today, 

yenwu

Thanks. Sasi, 

I see, I still have questions on both of the question asked, 

For Eq 7's result, it is clearly that if you use smaller internal Ron/Roff value for example, Ron=2.8ohm instead of Ron=4 ohm, Roff=1.87 ohm instead of 2.5 ohm and with same external gate resistance, you will get smaller power loss in Eq 7 result, so if possible, I hope to know the typical  and the minimum value of Ron and Roff value so that I can know the bias supply will consume how much power in min. typ. and max value of the internal Rron/Roff condition.  

For Desat pin power consumption, when ISO5851 output pin 6 is driving the related IGBT load, when IGBT is running in normal operation conditions, the Desat pin (pin 2) usually will reach how much voltage? if the related IGBT collect to Emitter voltage drop is about 2.5V above GND2 pin of the ISO5851, then this internal 0.5mA charging Desat pin current will be flowing through the Dsat diode and the related series connected 1k ohm Dsate resistor, when IGBT is turned on which, Dsate pin ( pin 2 ) will reach about 3.5V, because 0.5mA X1kohm + Desat diode forward drop =0.5V, total drop = about 1 V,  So Desat  pin voltage will swing between about 0.5V when IGBT is turned off and reach about 3.5V when IGBT is turned on, so the Desat cap charging and discharging power is not much as you said, and the constant internal 0.5mA from the Vdd2 ( usually15V) supply can be considered to flow constantly to the Desat pin no matter whether IGBT is turned on of off, When IGBT is on . this internal 0.5mA will flow through IGBT collector, when IGBT is turned off, this internal 0.5mA will flow through the discharge FET inside Desat pin 2 of ISO5851 device, Not sure if my understanding of this internal 0.5mA operation pattern is correct or not?

Not sure if this internal 0.5mA is considered to be part of the Io2 output supply quiescent current or not? It can be seen in P. 9 of the ISO5851 datasheet, Section 7.9  top table last line indicated that Io2 , "Output Supply Quiescent Current"  typical value is 3.6mA, So I think this internal 0.5mA should be part of this typical 3.6mA quiescent current. please confirm if my understanding is correct or not?

Thanks. yenwu

Hi Yenwu,

Yes having a smaller internal Ron/Roff will have less losses on the gate driver. But for bias supply calculations, you have to consider the max Ron/Roff and max power losses only.

There is no min/typical char data available for this device. 

DESAT pin is in  current sourcing mode only when the INP is high. When INP is low, the DESAT pin will not be in sourcing mode - but will pull DESAT pin to GND2 through the internal FET. So its not fixed 500uA of current to be considered.

Let me get back to you about the quiescent current question.

Thanks

Sasi

Thanks. Sasi, Look forward to hearing from you at your earliest convenience. 

Thanks again

yenwu