A problem with lm2576HVS and 48v

[Sharagim]

Hi,
I am trying to get 5V DC from a 48V source with LM2576HVS.
For test I used a 24V as input and all seems to be ok.
Then I used 48V and in output I see 48V instead of 5V after that when I plug it to 24V I see 24 on output also. I tried with another LM2576HVS and same result!

 

[schmitt trigger]

Are you applying an output load at all??

SMPS require a minimum load to regulate properly, specially at higher voltages where the "on time" becomes very, very narrow.

I suggest as a minimum 10% of your full load current.

 

[Sharagim]

Thx for reply.
Just I have a lm1117 for 3.3V in output

 

[FvM]

I don't see a LM2576HV specification requiring a minimal load, particularly no hint that the device may be damaged without it.

As far as I rmemember, I have used other "simple switcher" ICs with no load.

My first guess would be that you got a faked LM2576HV which is actually standard LM2576. Another explanation is that you have a large series inductance of the supplying voltage source, resonating with an input capacitor and producing a large input overvoltage, or a different problem resulting in VIN > 63V.

 

[Sharagim]

I tried the circuit without input and output Capacitors also and same result!