Where is the protection? IGBT? Diodes? Low voltage?
Posted: Wed Jul 31, 2013 7:33 pm
From the electrical point of view I am very disappointed in level of protection used in your design and I am therefore not surprised by the number of IGBT, diode and low voltage side failures.
The control and driving of the IGBTs is the most important part, with catastrophic failure if it goes wrong. Firstly there should be protection of the gates and secondly protection of the DC bus. TVS diodes should have been used on the gate to limit the voltage to +/-20V something like a P6KE20A or similar bidirectional TVS. The toriod L and gate C have the resonance potential to exceed these limits. Likewise a large TVS should have been used on the DC bus to prevent over-voltage; with the large electrolytic capacitors the bus could swing wildly. A 1.5PKE550A TVS or similar should be used, able to sink 1500W to ensure the bus voltage does not exceed the IGBT ratings. Also to reduce the effect of the DC cap inductance a thin film cap should be placed in parallel (a few uF).
To further protect the rectifier Diodes a thermistor PTC or NTC resistor should have been used on the AC side. Most likely a PTC to limit the inrush current and limit the current if a fault occurs. The fuse protection alone will not act fast enough to protect the diodes.
As for the low voltage side zener diodes should have been placed on both voltage lines 5v and 15v to ensure these remain in tolerance of the ICs. If the regulators do fail or were inserted incorrectly the ICs, or opto-receiver take the full voltage and become the mode of failure. Also in the feedback circuit the diodes used to clamp the voltage D1 and D2 are not sufficient, there should be parallel Shockley diodes or TVS diodes to ensure the signal never goes above or below the rails.
Adding these components is challenging to the existing PCB.
Lance
University of Canterbury NZ
The control and driving of the IGBTs is the most important part, with catastrophic failure if it goes wrong. Firstly there should be protection of the gates and secondly protection of the DC bus. TVS diodes should have been used on the gate to limit the voltage to +/-20V something like a P6KE20A or similar bidirectional TVS. The toriod L and gate C have the resonance potential to exceed these limits. Likewise a large TVS should have been used on the DC bus to prevent over-voltage; with the large electrolytic capacitors the bus could swing wildly. A 1.5PKE550A TVS or similar should be used, able to sink 1500W to ensure the bus voltage does not exceed the IGBT ratings. Also to reduce the effect of the DC cap inductance a thin film cap should be placed in parallel (a few uF).
To further protect the rectifier Diodes a thermistor PTC or NTC resistor should have been used on the AC side. Most likely a PTC to limit the inrush current and limit the current if a fault occurs. The fuse protection alone will not act fast enough to protect the diodes.
As for the low voltage side zener diodes should have been placed on both voltage lines 5v and 15v to ensure these remain in tolerance of the ICs. If the regulators do fail or were inserted incorrectly the ICs, or opto-receiver take the full voltage and become the mode of failure. Also in the feedback circuit the diodes used to clamp the voltage D1 and D2 are not sufficient, there should be parallel Shockley diodes or TVS diodes to ensure the signal never goes above or below the rails.
Adding these components is challenging to the existing PCB.
Lance
University of Canterbury NZ