TOPSWITCH Vpin Problem..

Hello All,

i have designed the circuit as per the PI-EXPERT Software.

The circuit is working at no load giving Clean output of 27V

But when i am trying to Load say about 100mA, output voltage gets fluctuated continuously and it is not stable.



After Various trial and error, When i pulled down vpin to GND iT WORKED..!!

Now but the circuit is starting very small voltage and i want it to turn on at the 90VAC

Schematics is attached with the post.

I have tried the resistors till 3 mOHM From 4 mOHM but it is not working.

Pls guide where is the problem and how to avoid..??

Regards
Marx

It's difficult to make a diagnosis without being 'hands on' but my first guess is the circuit isn't capable of supplying even 100mA and the feedback is 'fighting' to keep it stable.

Did you use exactly those parts, in particular the diode types as they are critical?

Brian.

Hi Brian

When i am connecting Vpin to the gnd it works.

isnt the V pin to do with overvoltage......have you got your bias winding set to too high voltage..........look at datasheet...there is an overvoltage level for the V pin...i am sure V pin is for overvoltage shutdown protection feature.

There must be some problem with your under-voltage and over voltage detection circuit. My guess there is a problem with your over voltage circuit, why there is a 24V zener, how did you designed the zener values, because it will depend upon the bias winding turn ratio to give proper over voltage. And when you are pulling the Vin pin to ground it disables all the functionality of Vin pin, so everything is working without any kind of protection.

Hi
The zener value is suggested by the pi expert.

And for now I have bypassed that zener resistor and diode.
Still the same.. I cross checked the resistor values as well it has been lowered down to 4mE to 2mE but still not working.

By dissabling the vpin function, customer will turn on at any voltage lower than the 90vac which creates the problem for Transformer saturation.
I am troubleshooting the same but not getting exact clues

Thanks
Marx

Are you using a BAV20 diode for D4?

There are specific guidelines for connection and layout to the V pin:

In order to reduce the no-load input power of TOP264-271
designs, the V pin operates at very low currents. This requires
careful layout considerations when designing the PCB to avoid
noise coupling. Traces and components connected to the V pin
should not be adjacent to any traces carrying switching currents.
These include the drain, clamp network, bias winding return or
power traces from other converters. If the line sensing features
are used, then the sense resistors must be placed within 10 mm
of the V pin to minimize the V pin node area. The DC bus
should then be routed to the line-sense resistors. Note that
external capacitance must not be connected to the V pin as this
may cause misoperation of the V pin related functions.

Click to expand...

Hi E-design,

Somewhat i am agree with you.. The traces are coming from the Noisy nodes.

Will be cutting the traces and update the same


Thanks
Marx

- - - Updated - - -

The reason of doing such is that to minimise the width of PCB and it happens from my PCB Designer

Thanks Again

Hello all..

Routing new pcb with the VPIN Corrections.

in the same board, i am connecting Vpin to the Gnd which means that disabling the Voltage startup.
Now with the load, giving supply from the Variac (0-265vac)

When i am monitoring the voltage On the multimeter, after 120vac the Meter shows the AC Voltage of 340vac Which makes noise in the same smps. but raising voltage to 230v Point smps works well (I dont know why after 130vac it shows the above 300vac)

When variac is unloaded and checked the Voltage from 0-265vac it shows correct voltage but after connnecting my boards it behaves like this.
also multimeter is Fluke brand (No problem in the Multimeter)

Confused..?? What is this behavior of SMPS..??

Thanks
Marx

- - - Updated - - -

Hi E-design,
here i have attached my Layout.
Where the previous and new Connections.

By connecting the Resistors (2mOHM+2mOHM) before input capacitors still the same problem we are facing.

Only by connecting vpin to GND is the way
But i want a startup at the 90vac

Pls suggest

Thanks
Marx

can you look at the output of your variac with a sufficiently voltage rated diff probe into a scope and see its ok..it doesnt sound right

I dont have HV Probes.. Will let you know the same.

Marx