Hello,
My name is Hitesh and I am trying to make a basic power supply that can deliver 300mA @ 5.10 volts DC. I am using a basic setup which includes transformer (this is been hand made by me), followed by a full wave rectifier, a smoothing filter and a linear regulator. Everything is working fine accept the Ground on the secondary side of the transformer.
I know the secondary of the transformer is never grounded (untill its center tap which is not in my case) and after the Full wave Rectification and smoothing the AC is converted to DC with some ripple on in which is then supplied to a linear regulator, till this point everything is fine no problems taking readings and stuff but once the linear regulator comes in to the picture some thing seems so get upset. I mean the regulator gets upset with its ground, As this power supply i am designing is ment for wireless power transfer I cannot use the same groung as I did for the primary side of the transformer, by the way it is an air core transfromer with 2:1 turn ration. I have tried so many different things to give a proper ground to the regulator but it seems like it is not working, I know this issue will get solved once everything is on PCB but before that I want to test it before putting ON to PCB. Can anyone advice me abt what I should use for the ground which will be separate than the earth, Scope, Function Generator ground.
Waiting for the replies
Hitesh
The LDO will regulate the output voltage with respect to its own ground. The secondary side of your circuit can float to any voltage but the regulator should still regulate the output to 5V above the ground pin of the IC. There maybe a ground loop problem introduced by your test equipment but I doubt it. I think what you really have is a noise problem. A bench supply typically provides very clean power. However, the transformer and bridge circuit produce a lot of high frequency noise due to the turn on/off of the diodes each AC cycle. I suspect there is high frequency noise being injected into the input of the LDO which is impacting the internal workings. The TPS715 does not particularly like high frequency noise. You should try adding a ferrite bead in series with the input of the LDO (try a 600 Ohm @ 100MHz bead). You can also add a small (0.01uF) ceramic capacitor from VIN to GND of the LDO to help shunt away any high frequency noise. You may also try a different LDO (different part number) and see if the problem is still there.
Can you describe what you mean by the regulator gets upset with its ground. What is the output of the regulator doing? Also, what is the part number of the regulator?
Yaa sure the part number is TPS71501 it is an adjustable linear regulator.
Presently the output value is adjusted to 5V DC with the load of 100 Ohms.
I first tested the above regulator by porviding a DC input from a regular power supply which used power supply ground it worked fine according to the design equations mentioned in the data sheet (for adjustable version a design equation is given). After making sure that the regulator is working fine I used the power supply i made using an air core transformer, followed by Full wave bridge rectifier, followed by smoothing filter. Now the primary side of the transformer is suppled using a Function generator hence it uses the function generator ground now the secondary side of the transformer, is isolated from the primary ground, no this received AC signal is then rectified and converted to DC with some ripple on it, which is then fed to a regulator which does not have a proper ground as the application I am designing is wirless power transfer the primary is isolated from secondary hence i cannot use the same ground, for this reason i am not able to provide a good ground to the regulator which is then getting upset and does not follow the design equation which it was following when tested with a regular power supply. Which is making me think that there must be somthing that regulator wants to see and that some thing might me a proper ground.
I know i can use scope ground while measuring but it is not working.
I am sorry i forgot to give u answer about what is the output of the regulator is doing.
If you look in to the data sheet for the adjustable version there is a design equation given for a particular Vout which needs a fixed value of resistors R1 and R2. By using these values one can adjust the output to 5V DC. Which is verified by using a regular DC supply . But when same used with my power supply does not work properly, for the same values of R1 and R2 the output is 3.9 volts instead of 5 Volts.
Now I also found a temp fix if I increased the value of R2 to about twice the calculated then the output is 5.4V, which tells me that the internal reference of the Regulator is upsed because it cannot find a proper ground. This is just what I think it could be some thing else that I don't know.
Thank You Scot for your help. I will definitely try your suggestion, I should have mentioned it before that the operating frequency i.e. the resonant frequency is 1MHz. in my case, can you please suggest me ferrite bead for this frequency.
If I understood correctly AC to DC conversion is introducing high frequency noice because of the Diodes ON OFF thing but this high frequency noice is on top of DC beacuse DC is nothing but zero frequency signal. Can you suggest me how i can measure this high frequency noice.
Also following are the alternative LDO I am thinking to use
1. tps63030 Buck-Boost Regulator for LI-ion battery
2. tps77801
are they immune to high frequency DC noice.
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