Cascode Design using TOP242G

Just uploading original design file.

Hi,

As such the design should work. I am unable to view your schematic correctly and read component values though so you may need to provide me a better copy.

Please verify the voltage at the control terminal of the TOPSwitch. Is it reaching 5.8V?

Regards

PI-Sarek

re-uploading the image file.
One question, is the cascode configuration is suitable to be done using TOP242G chips ?
Thanks.

I've measured the control pin voltage & found it zero !
i think that the chip cannot work form the beginning & the problem is not in the feedback loop as the design was working properly in the non-cascode version.
Thanks.

any solution for this issue?
Thanks.

Hello, thanks for your reply.
I've attached a better copy for my design.
Kindly recommend any trouble shooting point to can follow.
Thanks in advance.

Hi,

If your circuit worked fine otherwise but stopped working only after you made it a cascode, then there is something wrong with the cascode arrangement assembly.

The circuit should work as a cascode. I suggest you verify the connections of all the components in the circuits including the pin assignments on the MOSFET package used.

Note that the minimum input voltage required to start the circuit with the cascode arrangement will be slightly higher than the voltage needed to start the circuit without the cascode so if you have not increasd the input voltage while testing since the circuit will not start, I suggest you verify operation again with the nominal input voltage for the circuit.

You have indicated that you do not get any voltage at the control pin which means the MOSFET connected above is not conducting.

Generally a resistor of 6.8 ohm should be connected in series with the capacitor connected to the C-pin however that is required for compensating the control loop for stability. Absence of this resistor in your circuit should have nothing to do with the problem though I suggest you add it once your problem source is found.

Regards

PI-Sarek

Hello Sarek,

I've worked in this problem yesterday & found an assembly mistake of D13, it was assembled in the wrong direction; exactly as you guessed about the cascode arrangement.

Now, there's a minor problem, the MOSFET becomes warm, but not hot; is there any idea to reduce its power loss?

Thanks in advance,
Moataz.

Hi,

The Cascode circuit tends to be less efficienct as compared to one without the cascode.

How hot is the MOSFET? Can you provide some test data for both low line and high line conditions?

The efficiency will be typically lower at highest input voltage.

Simplest solution will be to add a heatsink and for a low power circuit this can be a really small heatsink.

Regards

PI-Sarek