We are using an LM5046 to provide isolated +/-82V dc rails to power an IRS2092-based 100V output full-bridge PA/Voice alarm class-D amplifier. The amplifier stage works well in providing a direct drive 100V rms audio output up to 220W.
Our problem is with the LM5046 isolated full-bridge dc-dc stage. We have two amplifier channels on our PCB, each channel having two isolated dc-dc stages (a high power +/-82V stage, and a low-power flyback +/-12V stage). We have no problem with the 12V low power flyback stage. The input power is from 21V to 30V dc.
We have no problems with one of the high power dc-dc stages on our 4-layer pcb, but the high-power (250W) dc-dc stage on our second channel does not control the phase-shift of the full-bridge stage correctly at low power levels, causing an asymmetric drive to the primary of our transformer, resulting in incorrect operation and failed MOSFETs.
Given that the first channel works OK, and that the second channel only has slight layout differences compared to the first on our PCB, I suspected that we were getting noise on the board affecting the switching. Note that each channel has its own dc input connectors, so I can power up just one of the channels at a time.
To investigate this I removed our dc-dc transformer on this channel, and replaced it with a 2.7k resistor, so that I could monitor the differential waveform at the bridge output with a 'scope, with only low currents flowing on the PCB tracks and ground plane. To produce an artificial feedback signal I connected a 47k potentiometer across the transistor of the NFB opto-coupler so that I could emulate the NFB signal.
Looking at the effective PWM waveform across the load resistor I saw that at full power the two driving square waves are operating correctly. As I reduced the potentiometer resistance value one of the drive waveforms shifted its phase to produce the expected PWM waveform across the load resistor. However, once the differential signal was showing the equivalent of approx. 30% PWM width, ie the transferred power was getting lower, the phase-shifted drive signal suddenly changed from the correct 50% duty cycle one to a duty cycle of approx. 70%. This is harmless into a resistor load, but is catastrophic into a transformer load as the primary is now being driven asymmetrically, resulting in saturation of the ungapped ETD39 ferrite core.
As the other channel on the PCB works correctly, (and I have tried another LM5046 on the faulty channel to prove it isn't the chip) I can only assume that we have some sort of noise coupling problem. Can you give me any idea of what might cause this. It does not seem to be the over-current trip, as the resistor replacing the transformer primary is only taking a very small current. I have found that the 33k resistor on the RT pin is very sensitive, as if I put the tip of a 'scope probe on the RT net the switching frequency increases considerably. This net trace on the PCB is less than 10mm long.
I have attached a .PDF of the dc-dc converter circuitry. (Please let me know if this isn't attached so that I can email it to you directly)
Regards,
Chris Morriss.
