SPEEDVAL Kit Half-Bridge Motherboard (MOD-MB-HB-0900V-40A)

Ujala_MG

Hi,

I am using the SPEEDVAL Kit Half-Bridge Motherboard (MOD-MB-HB-0900V-40A) along with the MOD-PWR-MM-C3M0060065J power card to build a synchronous buck converter.

I want to measure the drain-source current (I_ds) of the upper MOSFET. To do this, I used a current probe to measure the supply current from the DC source — specifically, the current flowing from the DC source into the +DC bus. Typically, in a buck converter, this method provides the upper MOSFET’s I_ds current.
However, with this setup, the measured waveform appears sinusoidal, as shown below.

Ch1- Upper MOSFET V_ds, Ch2- Upper MOSFET I_ds

Can I know the reason for this sinusoidal current behaviour (ch2)? Do you have any suggestions to measure the current through the upper MOSFET, directly?

Thank you!

Forum_Moderator

Thank you for your post, it has been approved and we will respond as soon as possible.

Julius_R

Greetings:

Thank you for your question. Without knowing your complete circuit, it is hard to say why you are measuring a sinusoidal current at the input terminals; however, one thing to keep in mind is there is a 48uf bulk capacitor on the DC bus. When you measure the current into the input terminals, you will see the combination of upper MOSFET drain current plus any current that the bulk caps are drawing.

The MOSFET daughtercard does not support direct measurement of the upper MOSFET drain current. This measurement would require an isolated current probe capable of operating at the DC bus voltage. This would also require extensive modification of the MOSFET daughtercard to gain access to the upper MOSFET drain to source current.

Julius R.