For R4 and R5:Could you explain how to calculate the resistance or give references.
You are facing problems at very high/low duty cycles (i.e. the maximum and minimum of your sinewave).
Switching times of your optocoupler depend on forward current and also output load (Fig. 3 https://www.fairchildsemi.com/ds/6N/6N137.pdf). The lower the output load resistance, the faster it will switch on. The higher the LED forward current the faster it will switch off.
Note that at 50 kHz 98% duty, the off time is 400 ns. If your optocoupler delays are about 200 ns you may be facing distortion of the applied PWM signal. At very high/low duty cycles this distortion is huge and causes bad system performance.
In addition IR2110 has a maximum duty cycle for proper bootstrap capacitor charge. If you do not leave enough time for the bootstrap capacitor to charge and the voltage falls below UVLO, the driver shuts down temporally.
Ernest
Note that at 50 kHz 98% duty, the off time is 400 ns. If your optocoupler delays are about 200 ns you may be facing distortion of the applied PWM signal. At very high/low duty cycles this distortion is huge and causes bad system performance.
It sounds like the circuit works perfectly if you increase the LED current. What is the remaining problem? Can't you just increase the LED current?However when the current into optocoupler (LED current) is increased,the waveform is perfect and inductor noise is also eliminated.
This isn't necessarily true. If the PWM signals are simply delayed through the optocouplers and the rest of the signal chain, then that simply results in a delay of the sine wave, and no real distortion. That assumes the the delays are equal to each FET (should be the case if they're all driven with optocouplers), and that the delay of rising and falling edges is equal. That last part is probably not true, and may be the cause of the problem. Other than the gate signals, the FETs themselves may also have different rise and fall times. That can cause some distortion in the sine wave, but a much more serious problem is the possibility of cross conduction. I would try just looking at the gate signals and verify that there is adequate dead time (around 100ns) at extreme duty cycles.You are facing problems at very high/low duty cycles (i.e. the maximum and minimum of your sinewave).
Switching times of your optocoupler depend on forward current and also output load (Fig. 3 https://www.fairchildsemi.com/ds/6N/6N137.pdf). The lower the output load resistance, the faster it will switch on. The higher the LED forward current the faster it will switch off.
Note that at 50 kHz 98% duty, the off time is 400 ns. If your optocoupler delays are about 200 ns you may be facing distortion of the applied PWM signal. At very high/low duty cycles this distortion is huge and causes bad system performance.
Hello,
various actions that would help improve the performance of your circuit:
- Reduce maximum duty cycle to a certain limit where your circuit works fine (i.e. use a modulation sine wave with less amplitude)
- Change optocupler to a faster one with logic output (speed will not depend on load) or use a digital isolator instead of optocoupler (ISOPro Series from Silicon Labs or iCoupler series from Analog Devices)
- Use another MOSFET. The one you are using is old and has a too high RDSon which means high power loses and also high gate charge. You can find 600 V rated MOSFETS (Ok with 400 VDC bus) with half the resistance and lower gate charge. These should reduce power loses and heating.
- IR2110 seems ok for your application 2 A peak should be enough. Reduce gate resistors if your commutation is too slow and want to reduce switching loses.
Ernest
iCouplers are quite interesting, EMC-wise they are not so much fun because you get a VHF generator "for free". This leads to the paradox situation that a straightforward design with low coupling capacitance and maximum air and creepage distances is thwarted by necessary EMC measures.
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