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The original Minipulse design did not have a diode in series with the MOSFET and the coil. So what is the purpose of the series diode in the Minipulse Plus?
When the REV-D version was created, there were a number of requests to provide space for this diode. Supposedly, this allows earlier sampling. Up until now I have not questioned the reasoning behind this request, as others have reassured me that it really does make a difference. But recently I decided to test these claims further:
I cannot remember exactly what the theory behind the diode was supposed to be, but I vaguely remember that it was either:
From increasing the TX-on time such that the flyback voltage was above 400V, showed clearly that option 2 is incorrect, as breakdown still occurred at 400V. In fact, this should have been obvious from studying the circuit diagram. At TX-off, the coil current continues to flow in the same direction whereas the voltage across the coil changes polarity and increases dramatically in amplitude. Hence, although the voltage across the coil is inverted, the polarity of the voltage across the MOSFET remains the same. This of course means that the diode is always forward biased.
So what about option 1?
At first I suspected that the diode may be acting as a non-linear series resistance, which would effectively reduce the maximum coil current, allowing it to sample earlier, rather than the capacitance theory being the cause. However, I was somewhat surprised to find that in fact removal of the diode allowed the main sample to be adjusted to a lower setting. Which is completely opposite to what was being claimed.
Basically, my conclusion is that the series diode does absolutely nothing to improve early sampling, and actually does exactly the opposite.
My assumption here is that the person who came up with this theory had a PI circuit that was just reaching the breakdown voltage of the MOSFET at TX-off. When the diode was inserted, the maximum coil current was restricted, and the breakdown voltage was no longer as high as before. The result being that the design could now sample a little earlier than previously. The wrong conclusion was subsequently drawn from the results
Discuss ...
When the REV-D version was created, there were a number of requests to provide space for this diode. Supposedly, this allows earlier sampling. Up until now I have not questioned the reasoning behind this request, as others have reassured me that it really does make a difference. But recently I decided to test these claims further:
I cannot remember exactly what the theory behind the diode was supposed to be, but I vaguely remember that it was either:
- The diode capacitance is in series with the output capacitance of the MOSFET, and hence the overall capacitance is reduced, allowing earlier sampling.
- The breakdown voltage of the MUR460 is 600V, which is above the IRF740's VDS of 400V, thus allowing the coil current to decay more quickly.
From increasing the TX-on time such that the flyback voltage was above 400V, showed clearly that option 2 is incorrect, as breakdown still occurred at 400V. In fact, this should have been obvious from studying the circuit diagram. At TX-off, the coil current continues to flow in the same direction whereas the voltage across the coil changes polarity and increases dramatically in amplitude. Hence, although the voltage across the coil is inverted, the polarity of the voltage across the MOSFET remains the same. This of course means that the diode is always forward biased.
So what about option 1?
At first I suspected that the diode may be acting as a non-linear series resistance, which would effectively reduce the maximum coil current, allowing it to sample earlier, rather than the capacitance theory being the cause. However, I was somewhat surprised to find that in fact removal of the diode allowed the main sample to be adjusted to a lower setting. Which is completely opposite to what was being claimed.
Basically, my conclusion is that the series diode does absolutely nothing to improve early sampling, and actually does exactly the opposite.
My assumption here is that the person who came up with this theory had a PI circuit that was just reaching the breakdown voltage of the MOSFET at TX-off. When the diode was inserted, the maximum coil current was restricted, and the breakdown voltage was no longer as high as before. The result being that the design could now sample a little earlier than previously. The wrong conclusion was subsequently drawn from the results
Discuss ...
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