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[SOLVED] OPAMP (OPA735) output voltage drop when load is connected

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kingssk

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i designed single supply inverting amplifier using OPA735, when the opamp output is connected to load , the output voltage getting drops upto 1V,
please anyone explain me why it is happening Power.png
Please check the attachment for my schematic

Thanks in Advance
 

Output of transformer is -1260V, by using voltage divider 10M and 11K||100K input to OPA735 Opamp inverting input is -1.26V this will inverted into positive 1.26V and given to the feedback input of LM5001 based on this feed back voltage it will control the transformer output voltage.
in my case am not getting +1.26V there is almost 1V voltage drop in opamp (OPA735) output
 

have you tried to simulate this design?
 

kingssk said:
Output of transformer is -1260V, by using voltage divider 10M and 11K||100K input to OPA735 Opamp inverting input is -1.26V this will inverted into positive 1.26V and given to the feedback input of LM5001 based on this feed back voltage it will control the transformer output voltage.
in my case am not getting +1.26V there is almost 1V voltage drop in opamp (OPA735) output
Click to expand...
This is not correct. The inverting input of the OPA735 is at the same potential that its non-inverting input, it means 0 Volts. So there's no current in R11.
The correct schema is: delete R11. R8 = 0 and R5 = 10 k.
 

Snoopy_94 said:
This is not correct. The inverting input of the OPA735 is at the same potential that its non-inverting input, it means 0 Volts. So there's no current in R11.
The correct schema is: delete R11. R8 = 0 and R5 = 10 k.
Click to expand...

I don't think Snoopy_94 is quite correct... and whilst his/her suggested circuit amendments look logical.. it won’t work… not with this op-amp anyway! It will cause the common mode voltage to be exceeded – by a lot! (I see your logic though!) This is a common mistake often overlooked and I think it could be the reason it’s not working….
1. If you check the data sheet, the common mode voltage is (V-)-0.1V to (V+)-1.5V (this is for Vs=10V close enough to your 12V) giving you an input range of -0.1V to 10.5V.
2. Now, you’re trying to put your error signal in at -1.26V. This is way out of spec! and will give you an unknown operative state.

Also.. Your secondary is grounded on net AGND while everything on the primary is on GND… are these nets physically connected in your application? As this is guaranteed to wreak havoc!

My suggestion would be either:
1. Reference the OPA735 Vin+ to +1.26V this will easily get you back inside the common mode requirement. And still allow unity inverting gain.
2. Provide the OPA735 with a –ve supply (remember max is +-6V)

I’m almost certain this is your problem – learnt the hard way before! Let us know how you go!
 

thunderdantheman said:
I don't think Snoopy_94 is quite correct... and whilst his/her suggested circuit amendments look logical.. it won’t work… not with this op-amp anyway! It will cause the common mode voltage to be exceeded – by a lot! (I see your logic though!) This is a common mistake often overlooked and I think it could be the reason it’s not working….
1. If you check the data sheet, the common mode voltage is (V-)-0.1V to (V+)-1.5V (this is for Vs=10V close enough to your 12V) giving you an input range of -0.1V to 10.5V.
2. Now, you’re trying to put your error signal in at -1.26V. This is way out of spec! and will give you an unknown operative state.

Also.. Your secondary is grounded on net AGND while everything on the primary is on GND… are these nets physically connected in your application? As this is guaranteed to wreak havoc!

My suggestion would be either:
1. Reference the OPA735 Vin+ to +1.26V this will easily get you back inside the common mode requirement. And still allow unity inverting gain.
2. Provide the OPA735 with a –ve supply (remember max is +-6V)

I’m almost certain this is your problem – learnt the hard way before! Let us know how you go!
Click to expand...
Sorry, but my explanation is the good one! (I am retired electronic engineer).
Suppose that the OPA735 is not saturated, so the voltage at the inverting input is equal to that of the non-inverting input, i.e. 0V. The current of resistor R1 is - 1260V / 10M = - 126µA. This current must be balanced by that of R5. If as proposed R5 = 10k, then the output of the OPA735 = 10k X 126µA = 1.26V.
Another problem is the stability of the closed loop. The use of an oscilloscope is mandatory!
A last very important parameter is the HV transformer. Such a transformer is not used as a LV transformer. Look at my posts in this topic:
https://www.edaboard.com/threads/193890/
 

U1
EN Enable / Under Voltage Lock-Out / Shutdown input An external voltage divider can be used to set the line undervoltage lockout threshold. If the EN pin is left unconnected, a 6 µA pull-up current source pulls the EN pin high to enable the regulator.
 

Hi All,
Thanks for your support,

i changed my design with dual(+/-12V) supply opamp, instead of OPA735 now its working fine
 

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