Problem Statement: Issues with single phase AC Chopper Circuit (230V / 50Hz)

• I am working on designing a single-phase AC Chopper circuit to chop a 230V @ 50Hz AC signal. For testing and safety, I am currently using 25V AC from a variac with only one MOSFET in use (auxiliary circuitry disconnected). The circuit is being tested on a zero PCB.
• I have attached the circuit diagram and oscilloscope waveforms (observations) for reference.
• Even if after trying with better components the output remains the same, just some slight improvement.
• Figures uploaded:
• Figure 1: Waveform observed when Input is 230V AC and output is measures across a voltage divider.
• Figures 2, 3, 4, 5.1 & 5.2 are tested with 25V AC input
• Figure 2: Waveform across Collector-Emitter
• Figure 3: Waveform across final output with resistive load (5K resistor)
• Figure 4: Waveform across final output with resistive load (100-ohm resistor)
• Figure 5.1: Gate waveform (zoomed in )
• Figure 5.2: Gate waveform (zoomed out)
• Figure 6: Circuit diagram.

• With a resistive load connected:

5kΩ load: The chopped AC waveform shows proper chopping in the negative cycle, but not in the positive cycle. (Figure 3)

100Ω load: The chopped AC waveform appears on both cycles, but the positive half-cycle chopping is still improper (Figure 4)

I suspect the issue might be related to insufficient MOSFET turn-off time during the positive cycle.



Components Tested (Output remains unchanged):

1. Diode Bridge:

• MUR460 with RC snubber (R = 10Ω, C = 1nF)
• S20C70CE Schottky diode

2. Power Devices (MOSFETs/IGBTs):

• H20R1203 IGBT
• IRFP460 MOSFET
• IRFP350 MOSFET
• STW48N60M2 MOSFET

Connected RCD snubber across collector and emitter(R= 10 ohm, D=MUR460, C=1nf )

3. Gate Drivers:

• IR2110 (2.5A)
• TC4420 (4A)
• IXDN602PI (2A)
• UCC27322 (9A)
  (Tried higher current drivers, but no improvement in switching behaviour.)

4. Gate Resistor + Diode:

• MUR460 + 10Ω resistor
• S20C70CE + 10Ω resistor

5. Microcontroller (STM32F401CCU6):

• PWM frequency: 4.4 kHz @ 10% duty cycle
• Future goal: make duty cycle variable

​

Request for Help:

• Why does the positive half-cycle not chop properly, while the negative cycle does?
• Could this be due to MOSFET turn-off delay, snubber design, or something in the gate drive timing?
• Are there design changes or driver configurations that could ensure symmetrical chopping across both half-cycles?

 

hi,
Remove all RC snubbers from the diodes and transistors. Capacitors can distort the voltage from the bridge output.
Place a resistor of about 100 kOhm at the bridge output

 

For the moment remove the rest of the electronics from the power supply once you have a stable DC voltage you can work on the add-ons for troubleshooting.

 

hi,
Remove all RC snubbers from the diodes and transistors. Capacitors can distort the voltage from the bridge output.
Place a resistor of about 100 kOhm at the bridge output

Thank you for your reply, I tried but it is not working. Observation: Positive cycle is not chopping and negative cycle is behaving like the waveform of positive cycle as shown in figure3(curved figure).

 

For the moment remove the rest of the electronics from the power supply once you have a stable DC voltage you can work on the add-ons for troubleshooting.

Thank you for your reply, I tried but it is not working.

 

Check 12V supply for ucc27322 with an oscilloscope

 

I would need to see a final schematic. You appear to have Vdd of the drivers connected to ground and it isn't clear to me what Q2 is doing. Incoming neutral doesn't seem to be referenced to anything and there seems to be nothing preventing the switching waveform from Q1 ending up on the bridge (D2/D3).