I am designing a 84 W battery charger half-bridge converter from an AC source of 220 - 240 VAC. I am using an SG3525A for the design. From a simulation I did in LTspice, there are some oscillations at the primary side switching node (where both MOSFETs are connected together) at the time when both transistors should be OFF. The oscillations are there even if I add a dead-time resistor. I have attached the image of the schematic of the circuit I am using below:
The drain-to-source voltage waveforms of the highside MOSFET (green) and lowside MOSFET (blue):
I have placed an RC snubber circuit (200 pF capacitor and 9.5 kΩ resistor across the primary transformer winding) but the oscillations are only slightly damped. If I increase the capacitor and reduce the resistor, the oscillations are damped further, but at the expense of higher power dissipation in the resistor.
How do I efficiently damp these oscillations without affecting efficiency too much? (The leakage inductance of the primary side with the secondary windings shorted is 20 μH. The primary inductance is 11.26 mH and the secondary inductance is 470 μH).
Can these oscillations cause shoot through in the MOSFETs?
The drain current waveforms of the highside MOSFET (green) and lowside (blue):
From my calculations, the peak current of the primary side switch should be about 1.1 A.
- The high-side MOSFET's gate drive voltage also shows some spikes when in the low state and this spikes reach up to 4.5 V. How can I damp these spikes? Since they can lead to false turn-on of the MOSFET.
The gate to source voltage of the highside MOSFET is:
- Also, the gate drive voltage has been reduced from 12 V to slightly less than 10 V. I assume it's because the turn off spike goes to below ground. Kindly explain to me how I can avoid the ringing and prevent the voltage from being clamped?
The gate voltage is in blue, the source voltage is in red and the gate to source voltage is in green: