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TPA3255: High idle-power and extreme hot coils

Part Number: TPA3255
Other Parts Discussed in Thread: TPA3251, TPA3251EVM,

Hi everyone,

I'm developing a very small amplifier for selfpowered speakers. My problem now is Heat! The outputfiltercoils are running extreme hot in a short time ( 100°C in less than 30s).

I use the Würth 7443551730 and a 1uF/100V capacitor for the output filter. I already had a conversation with Robert C. The first test I did with the tpa3251evm without any problems, then I tested the tpa3251 with my own circuit. All worked but here running with hot coils. Now in my design the tpa3255 is used because I need 48V of input voltage. Here the really extreme problem with the hot filter coils. 

The heat comes up with or without connected load. If I remove the coils the tpa3255 starts up for only a few seconds and high current (there is no load! outputs of th IC have no connection only the bootstraps are connected) and then the tpa shuts down. By reconnecting the filter coils the tpa is running correctly and if I connect a speaker, audio is played very well, but there is still the extreme heat!

The PWM is normal and I can select any switching frequency I want. I use for sure the correct half bridges in BTL-mode and there is no short in my circuit (all tested with 3 different pcbs)

Powerconsumption of the tpa is at about 250mA at 48V so about 12 watts.

I designed a very simple test board to exclude my amp-design but there is still the same: heat!!

It is better when I use coils with a much higher impedance. By using coils with 33uH and 330nF capacitors the powerconsuption is reduced to ca. 7 watts but the coils are still running hot, not that fast but they become hot (70°C after about 5min.)

Did anyone of you have these issues?

I hope you can help me?

Best Regards