New Bipolar Boost TX and Front End

[KingJL]

Just finished some test plots for the FQS4901...

Capture of FQS4901 transfer plot:

[KingJL]

The M6 (M7) with the 2n2 may not be a problem, but as I will address below, the 4n7 definitely is a problem. The M6 (M7) gate waveform may appear to be low on alternate TX cycles, but cannot be verified without seeing the source also. The reason that alternate TX cycles appear low is that the other main MOSFET (M2 or M4) brings the reference to GND and depending which (M6 or M7) you are veiwing, the signal is alternatel going through the damp resistance. It would appear that the droop on the 2n2 is acceptable (it appears that the drive is at least 12V). The key is the delta between gate and source at MOSFET off time.Actually, I am afraid that the 4n7 used with M6 and M7 is a major problem. It appaears that the difference between gate and source during MOSFET off time is ~4V. With a Vth range for the FQS4901 of 2 - 4V... the damp will never be turned off.

Just finished some test plots for the FQS4901... For a typical FQS4901, the value of Vth seems to be about 3.5V... With a typical MQS4901, at 2V the RDS is about 6.5Meg, 3V is about 26.5k, 3.5V is about 410 ohm, 4V is about 23 ohm, 5V is about 3.6 ohm, 6V is about 3.27 ohm, 10V is about 3.23 ohm.

For M6 and M7 the key is a difference during off of less that 3V with a drive of anything above 5V being satisfactory. At 3V during "OFF" total resistance added to the damp parth is over 100k (which is more than sufficient). With an "ON" voltage of 5V total resistance added to the damp path is ~14 ohm (6V is ~.13).

[KingJL]

Back to testing with 10Meg /2n2F for R25,R30,R35,R41 / C29,C32,C35,C38. It did reduce the difference between G-S but I noticed that it also had quite an effect on M6/M7 waveform, which could be a problem.

with 3n3 [ATTACH]49006[/ATTACH] with 2n2 [ATTACH]49007[/ATTACH]
Waveform on M6 G with 2n2 [ATTACH]49008[/ATTACH] with 4n7 [ATTACH]49009[/ATTACH]

The M6 (M7) with the 2n2 may not be a problem, but as I will address below, the 4n7 definitely is a problem. The M6 (M7) gate waveform may appear to be low on alternate TX cycles, but cannot be verified without seeing the source also. The reason that alternate TX cycles appear low is that the other main MOSFET (M2 or M4) brings the reference to GND and depending which (M6 or M7) you are veiwing, the signal is alternatel going through the damp resistance. It would appear that the droop on the 2n2 is acceptable (it appears that the drive is at least 12V). The key is the delta between gate and source at MOSFET off time.

... After further experimenting I found that leaving the 2n2F/10Meg combination for the Boost drive (C29/R25, C32/R31) and increasing the value of C35/C38 to 4n7F and leaving R35/R40 at 10Meg gave the best results but there is a slight discrepancy between M6/M7 drive

M6 with 10meg/4n7f combination [ATTACH]49010[/ATTACH] same with M7 [ATTACH]49011[/ATTACH] Yellow trace= Gate, Blue trace=Source

Droop on M6/M7 G [ATTACH]49012[/ATTACH] Droop M7 G [ATTACH]49013[/ATTACH]

Need to sit back and have another look but it appears that the peak gate drive volts for M1 /M3 has increased using 2n2F/10Meg combination

Actually, I am afraid that the 4n7 used with M6 and M7 is a major problem. It appaears that the difference between gate and source during MOSFET off time is ~4V. With a Vth range for the FQS4901 of 2 - 4V... the damp will never be turned off.

[Mdtoday]

Found the reel of 0805 caps, right in front of me on the bench, looked everywhere but the bench, can?t believe it...
have loaded them and will test in the morning and post results.

Back to testing with 10Meg /2n2F for R25,R30,R35,R41 / C29,C32,C35,C38. It did reduce the difference between G-S but I noticed that it also had quite an effect on M6/M7 waveform, which could be a problem.

with 3n3 with 2n2
Waveform on M6 G with 2n2 with 4n7



Swapping, the toroids around between the different drive circuits had minimal impact but changing the value of capacitors did.



After further experimenting I found that leaving the 2n2F/10Meg combination for the Boost drive (C29/R25, C32/R31) and increasing the value of C35/C38 to 4n7F and leaving R35/R40 at 10Meg gave the best results but there is a slight discrepancy between M6/M7 drive

M6 with 10meg/4n7f combination same with M7 Yellow trace= Gate, Blue trace=Source

Droop on M6/M7 G Droop M7 G

Need to sit back and have another look but it appears that the peak gate drive volts for M1 /M3 has increased using 2n2F/10Meg combination

[KingJL]

Found the reel of 0805 caps, right in front of me on the bench, looked everywhere but the bench, can?t believe it...
have loaded them and will test in the morning and post results.

I've done similar... still have not found my 50 BAT54S's, that I had to re-order.

I seem to have an issue with J9 or the mating connector on the I/F board... I will be sorting that out.

[Mdtoday]

If the 2n2 improves the delta, all the better. But, the levels that you have with the 3n3 I think are good enough to proceed. A delta of 1.8V with a drive of ~13V. A check of M6,M7 to check the droop. At 520 usec prt, I think the 3n3 should produce ~160mv droop and the 2.2 ~250mv.

Found the reel of 0805 caps, right in front of me on the bench, looked everywhere but the bench, can’t believe it...
have loaded them and will test in the morning and post results.

[Mdtoday]

[QUOTE=KingJL;265066]Much better... at least we are now safely under minimum threshold. if you don't mind, try a 2n2... it should bring g-s closer together yet.[/QUOTE
Ill load 2n2F and try again, I’ve got a reel of them ....somewhere... will post results

[KingJL]

Much better... at least we are now safely under minimum threshold. if you don't mind, try a 2n2... it should bring g-s closer together yet.

If the 2n2 improves the delta, all the better. But, the levels that you have with the 3n3 I think are good enough to proceed. A delta of 1.8V with a drive of ~13V. A check of M6,M7 to check the droop. At 520 usec prt, I think the 3n3 should produce ~160mv droop and the 2.2 ~250mv.

[KingJL]

I changed replaced the 5n6F with original value of 3n3F left the 10Meg in place.

[ATTACH]48996[/ATTACH] [ATTACH]48997[/ATTACH]

Pulse transformer indexed as 1P in the below attached is M1 drive
Pulse transformer indexed as 4P is M3 drive

[ATTACH]48998[/ATTACH]

Much better... at least we are now safely under minimum threshold. if you don't mind, try a 2n2... it should bring g-s closer together yet.

[Mdtoday]

5n6 should make the difference between source and gate larger.

I changed replaced the 5n6F with original value of 3n3F left the 10Meg in place.



Pulse transformer indexed as 1P in the below attached is M1 drive
Pulse transformer indexed as 4P is M3 drive

[KingJL]

Just measured R&C values, I have 10Meg and 5n6F loaded on the test board, the originals were 2.2Meg / 3n3F

5n6 should make the difference between source and gate larger.

[Mdtoday]

If I am reading correctly source is at 4v (during boost time) and gate is at 17 which give a gate drive of ~13 volts. What really concerns me is the difference of ~2.5 volts between gate and source during the off time... that is very close to the min threshold voltage of 3V.

Yes, that is correct, I will poke around, do some more measurements and see if I have missed something.

Just measured R&C values, I have 10Meg and 5n6F loaded on the test board, the originals were 2.2Meg / 3n3F

[KingJL]

Here is the result of test above, Yellow trace = gate, blue trace= source. Baseline 0v is centre at blue flag 2

[ATTACH]48994[/ATTACH] [ATTACH]48995[/ATTACH]

I will now change the 2.2Meg resistors to 10Meg and re-test

If I am reading correctly source is at 4v (during boost time) and gate is at 17 which give a gate drive of ~13 volts. What really concerns me is the difference of ~2.5 volts between gate and source during the off time... that is very close to the min threshold voltage of 3V.

[Mdtoday]

Actually, I think the amplitude may be fine. Remember, the amplitude we are concerned about is between gate and source not gate and GND. To get the true measure put the gate on channel A and the source on channel B and you may see something like the following attachment:
[ATTACH=CONFIG]48990[/ATTACH]
Here green trace is gate and blue trace is source for M1 (coil disconnected). As you see the amplitude of the green trace seems to only increase by 11V, but when compared with the blue trace the increase at the leading edge is 15.7V until the Vth of the MOSFET is reached and then is 13.5V.

I would guess that the output from your pulse transformer gate drive is satisfactory.

What values are you currently using for R25 (R30, R35,R41 ) and C29 (C32,C35,C3 )?

Here is the result of test above, Yellow trace = gate, blue trace= source. Baseline 0v is centre at blue flag 2



I will now change the 2.2Meg resistors to 10Meg and re-test

[Mdtoday]

Actually, I think the amplitude may be fine. Remember, the amplitude we are concerned about is between gate and source not gate and GND. To get the true measure put the gate on channel A and the source on channel B and you may see something like the following attachment:
[ATTACH=CONFIG]48990[/ATTACH]
Here green trace is gate and blue trace is source for M1 (coil disconnected). As you see the amplitude of the green trace seems to only increase by 11V, but when compared with the blue trace the increase at the leading edge is 15.7V until the Vth of the MOSFET is reached and then is 13.5V.

I would guess that the output from your pulse transformer gate drive is satisfactory.

What values are you currently using for R25 (R30, R35,R41 ) and C29 (C32,C35,C3 )?

Thanks JL, all original values I swapped them back after those earlier experiments.
I will setup again this morning and do some more measurements and post results