The C to E blocking capability of bipolar transistors usually increases when B-E junction is reverse biased. 5 volts of reverse biasing may increase the C to E blocking capability by 40% in some devices
When you drive a reasonable load, as the output voltage approaches one rail, the output devices of the other rail stop conducting and get unbiased in case of a CFP outputs and reverse biased in case of EF outputs
When no load is connected, the biasing is maintained for the entire output voltage range and output devices are forced to sustain the entire p-p output swing while conducting. This may cause distortion due to C-E leakage or violate SOA and destroy transistors if C to E blocking capability is not enough
Note that 2N3055 states 60V C to E blocking capability when conducting and 100V blocking when not conducting and/or B-E junction is reverse biased
That phenomena happens also on switching applications : If the B-E junction gets reverse biased during the turn-off transient, inmediately afther the B-E charge is removed, then the reverse bias SOA is dramatically extended and the turn-off process happens much faster [reverse biasing is a must for any reliable SMPS using power biolar transistors]
Obviously, you can only rely on that 100V blocking capability for SMPS applicacionts with reverse-biasing at turn-off. For linear applications it's better to only rely on 60V
When you drive a reasonable load, as the output voltage approaches one rail, the output devices of the other rail stop conducting and get unbiased in case of a CFP outputs and reverse biased in case of EF outputs
When no load is connected, the biasing is maintained for the entire output voltage range and output devices are forced to sustain the entire p-p output swing while conducting. This may cause distortion due to C-E leakage or violate SOA and destroy transistors if C to E blocking capability is not enough
Note that 2N3055 states 60V C to E blocking capability when conducting and 100V blocking when not conducting and/or B-E junction is reverse biased
That phenomena happens also on switching applications : If the B-E junction gets reverse biased during the turn-off transient, inmediately afther the B-E charge is removed, then the reverse bias SOA is dramatically extended and the turn-off process happens much faster [reverse biasing is a must for any reliable SMPS using power biolar transistors]
Obviously, you can only rely on that 100V blocking capability for SMPS applicacionts with reverse-biasing at turn-off. For linear applications it's better to only rely on 60V
Well thats nice on paper, but it dosen't translate to real world. I did a test driving a 2 ohm load under 20~25% max rated soa of the tip3055/2995 pair with a 78v +/- supply and when the output was removed the swing was less then a 1v rms higher.
Maybe this would be a problem when approching the soa with higher ohm load, but these devices aren't much good for higher then 2 ohm loads.
Maybe this would be a problem when approching the soa with higher ohm load, but these devices aren't much good for higher then 2 ohm loads.
Hi Easyamp,
What's 78 - 60? That would be 12V over the peak C-E rating of a real 2N3055. That's assuming you're using a + / - 34V supply. If you are using a 2N3055H (100V) you are okay. The other point to be made is that you normally would not reach clipping with the speakers connected without knowing it. (it's loud). The heatsink is going to get mighty hot as well.
-Chris
What's 78 - 60? That would be 12V over the peak C-E rating of a real 2N3055. That's assuming you're using a + / - 34V supply. If you are using a 2N3055H (100V) you are okay. The other point to be made is that you normally would not reach clipping with the speakers connected without knowing it. (it's loud). The heatsink is going to get mighty hot as well.
-Chris
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.