indulis said:
How does the TIP31A introduce a LF pole? It has a 3MHz gain bandwidth product and if the supply is running a max Iout of 3A, and the 2N3055 is at it's min beta of 5 (On-semi datasheet and that's at a Ic of 10A not 3A), the TIP31A isn't working "that hard" and your no where near it's bandwidth limit. If the high frequency zero due to the cap ESR is an issue, add a high frequency pole a decade earlier.
Let's analyze the TIP31A from a S-parameter view as measured on a network analyzer using the actual components .
We will look at the response of it operating as a simple emitter follower alone with only resistive terminations. This is normally considered to be a very stable configuration. Surprise!
Good paper here for IEEE members
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?tp=&arnumber=1082247&isnumber=23381
Looking at the first plot of a sweep between 0-10 MHz we see that this transistor is unstable with a K factor < 1 for operation above 50 kHz all the way up to 10 MHz.
This transistor will oscillate without compensation. We normally aim for a K factor of at least >= 1.2
We can see a very good comparison between the measured S-param data and the model used by the simulator. We can thus assume the model is quite accurate.
Next we add a common 10uF capacitor with an ESR of 7 ohm on the output. Now we see that this setup is unstable for anything over 70 kHz
Next we take a look at the BD139 in the same configuration. This will be stable under 1.9 MHz (big difference from 50 kHz).
Adding the same output cap we can see stability is not much affected with ample stability at 1 MHz (K = 1.9). Looking at the plot of the original 2N2219 we see a similar trend.
The original designer may have had good reason for selecting the driver he did. Maybe he was aware of the problems like I am. The importance to select a fast driver is just as important in PSU design as in audio amplifiers.
I can't make it any clearer than this.
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