I am trying to drive the MOSFET switches in a cascaded hybrid modular multi-level converter as shown in Fig. 1. Fig. 2 is used here to simplify the circuit and to present the problem (let's focus on Fig.2 instead of Fig.1). The type of the MOSFET used is "IRF530N"https://www.alldatasheet.com/datasheet-pdf/pdf/68169/IRF/IRF530N.html. Each cell is integrated with half-bridge (called a sub-module) while each pack of sub-modules is integrated with full-bridge (called a module). Each module is connected to the next module (cascaded). The type of the Half-bridge driver used is "IR2103S" https://www.alldatasheet.com/datasheet-pdf/pdf/84683/IRF/IR2103S.html. A single IR2103S is used to drive each half-bridge while two of IR2103S are used to drive each full-bridge as shown in Fig. 3 and Fig. 4, respectively. Note that in Proteus software, I have used IR2104S instead of IR2103S (both of them are able to achieve the same function). For each half-bridge (except the last half-bridge used), the wire between the COM pin of "IR2103S" and the source pin of the low-side MOSFET switch is lifted as shown in Fig. 5. Cell voltage=3.4V, Rg=47ohm, a capacitor with 22uf/25V, and DC/DC isolated is used for each half-bridge.
The circuit is operating correctly when I am using only one module (only one full-bridge is used) (i am able to measure the positive and the negative output voltage across the full-bridge). The problem appears when using two modules in a cascade way (see Fig.5). Connecting the load is necessary in this case to complete the circuit. I noted that I can measure the positive output voltage across the two full-bridges correctly (9.7 to 10.1V) while I can't measure the correct negative output voltage (I got -8.42 V).
I have tested the circuit in the lab and by Proteus software and I got the same results.
I have two questions:
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