What is the purpose of the series diode?

[Carl-NC]

I don't understand, it sounds like you want the lower the current but increase the flyback voltage.
If you lower the turn-on current then that will lower the flyback voltage. You could also add some capacitance to lower the flyback voltage but that will also make it slower.

[Op04]

I don't understand, it sounds like you want the lower the current but increase the flyback voltage.
If you lower the turn-on current then that will lower the flyback voltage. You could also add some capacitance to lower the flyback voltage but that will also make it slower.

Ok, my setup is 12.2V 2.5A dc 5x4700f/63V 290.2uH 3Khz 128us PW 0.89R (100W) series with mosfet R damp was 184R (20W mox its hot) it was good! but I need faster so I put 289R damp but the mosfet is freezed within 1 minute. I was plan to gradually achieve to 3.2khz but I assume I can use 600V fast diode with it to get sample more soon, what happens if diode is lower than the mosfet breakdown?

[Op04]

Um... so this fet is have good voltage drop spec, I still waiting the new one with diode, simple question is: does putting 600V diode series with fet that has 700-900V breakdown will make fet acts like it was mosfet spec with 600V breakdown? I need above setup but it seem the fet couldn't survive

[Carl-NC]

No, using a series diode that has a lower breakdown than the FET won't affect the breakdown of the FET. What will happen is the diode won't turn off until the flyback drops below its breakdown, which will limit its effectiveness.

I don't understand the problem. I don't see anything unusual except that your damping R seems very low, I would expect it to be 500Ω - 1000Ω. Is the FET avalanching?

[Op04]

No, using a series diode that has a lower breakdown than the FET won't affect the breakdown of the FET. What will happen is the diode won't turn off until the flyback drops below its breakdown, which will limit its effectiveness.

I don't understand the problem. I don't see anything unusual except that your damping R seems very low, I would expect it to be 500Ω - 1000Ω. Is the FET avalanching?

Thank you for the answer Sir Carl. Now I have to find better FET. About damping is low It's because I put a rheostat and switched parallel resistor jig to see the best respond to target (some sort mix copper / bronz / brass) from 100R-700R selector jig, turn slowly at184R it does not avalanche, but detection was good, but I trying to see how better respond, until somehere in 333R it start to avalanche near 800V when I put lower than 700V it stay in 289R I was satisfy. And I put real parallel resistor with no jig, it can only stay a minute or two and it freezed

[Op04]

Thank you for the answer Sir Carl. Now I have to find better FET. About damping is low It's because I put a rheostat and switched parallel resistor jig to see the best respond to target (some sort mix copper / bronz / brass) from 100R-700R selector jig, turn slowly at184R it does not avalanche, but detection was good, but I trying to see how better respond, until somehere in 333R it start to avalanche near 800V when I put lower than 700V it stay in 289R I was satisfy. And I put real parallel resistor with no jig, it can only stay a minute or two and it freezed

Sorry. After look at the notes again I'm put wrong value in above statement, also forgot to mention, the R input was 1k then connected to 220R and 4.7uf cap before opamp. I also gradually increase the DC volt from 8V to 12.2V. The right value is DC at 12.2V with R damp at 184V at 540V flyb, at 333R at 950V and 289R avalanche in 833V

[Carl-NC]

the R input was 1k then connected to 220R and 4.7uf cap before opamp.

I don't understand. A drawing would help.

to see the best respond to target (some sort mix copper / bronz / brass)

With such a low damping R it is surely overdamped. It sounds like you do not need an aggressive sample delay, if that's the case then there are other options. You could simply add some capacitance in parallel with the coil to reduce the flyback. You could add a snubber circuit which slows down the rising part of the flyback but not the falling part. You could also use a higher inductance coil which will be slower and probably more sensitive.

[Op04]

I don't understand. A drawing would help.



With such a low damping R it is surely overdamped. It sounds like you do not need an aggressive sample delay, if that's the case then there are other options. You could simply add some capacitance in parallel with the coil to reduce the flyback. You could add a snubber circuit which slows down the rising part of the flyback but not the falling part. You could also use a higher inductance coil which will be slower and probably more sensitive.

That's the word I was looking for thanks! Slow down the rising part of fb,,, but I like this if it can survive (fet) the machine like the target I intend to search. Ok I will try to add capacitance with the coil, what value it should be?

You are right, it should be more turn / inductance but that would need differ fet a new budget for the shield resin etc, I want to keep ATurn above 120

[Carl-NC]

OK, now I understand. I would try adding a snubber. Here is how to calculate C:

The energy in the coil at turn-off is 0.5*L*I² = 0.5*290uH*2.5A² = 906uJ
Suppose you want a maximum flyback of 650V so the FET does not avalanche. The cap energy stored is 0.5*C*V² and this will be equal to the coil energy above. So

0.5*C*650V² = 906uJ or C = 4.3nF

Using the nearest common value of 4.7nF results in a flyback of 620V. Make sure the cap is rated for that much voltage.

[Sean_Goddard]

In the old days of the CRT television, Sony used to put TWO resistors in series in the gun driver stage of the CRT. My electronics Tutor asked the class why we thought they did this. I said "Well, given they are using carbon resistors which have ENDCAPS, a resistor is not a pure resistor. A reistor is essentially an RLC circuit in which the prevailing value is R, HOWEVER the endcaps have C, so in placing two in series, you HALVE the C component (1/C +1/C) so you increase the HF response". I was the ONLY one in the class who spotted this. He told me this and subsequelty told me my understanding of electronics was way ahead of the others on the course.

To answer the topic, a LOW CAPACITANCE diode should be chosen, and the track connecting to the FET as short as possible. The combined capacitance of the D and the FET will reduce the overall C and thus lead to a much faster decay, and thus sampling earlier is possible.

The snubber clamps the coil voltage spike at a level below the FET avalanche voltage. You CAN use a TVS if the recovery time is fast enough, and Dave Emery's "biased diode blocker". I've used all of these and 4us is not impossible.

[Op04]

OK, now I understand. I would try adding a snubber. Here is how to calculate C:

The energy in the coil at turn-off is 0.5*L*I² = 0.5*290uH*2.5A² = 906uJ
Suppose you want a maximum flyback of 650V so the FET does not avalanche. The cap energy stored is 0.5*C*V² and this will be equal to the coil energy above. So

0.5*C*650V² = 906uJ or C = 4.3nF

Using the nearest common value of 4.7nF results in a flyback of 620V. Make sure the cap is rated for that much voltage.

Amazing thank you so much for this. The DC supply for this I also using very clean volt and current, I think you have solve my problem, hope the parts come soon. Again Thank you Chief!

[Op04]

In the old days of the CRT television, Sony used to put TWO resistors in series in the gun driver stage of the CRT. My electronics Tutor asked the class why we thought they did this. I said "Well, given they are using carbon resistors which have ENDCAPS, a resistor is not a pure resistor. A reistor is essentially an RLC circuit in which the prevailing value is R, HOWEVER the endcaps have C, so in placing two in series, you HALVE the C component (1/C +1/C) so you increase the HF response". I was the ONLY one in the class who spotted this. He told me this and subsequelty told me my understanding of electronics was way ahead of the others on the course.

To answer the topic, a LOW CAPACITANCE diode should be chosen, and the track connecting to the FET as short as possible. The combined capacitance of the D and the FET will reduce the overall C and thus lead to a much faster decay, and thus sampling earlier is possible.

The snubber clamps the coil voltage spike at a level below the FET avalanche voltage. You CAN use a TVS if the recovery time is fast enough, and Dave Emery's "biased diode blocker". I've used all of these and 4us is not impossible.

Thank you Sir.... so now there's a lot more parameter to consider.

[Detectorist#1]

Hi all,

I made some tests with different fast diodes connected in series of MOSFET to achieve more short delay. It is interesting - low reverse voltage schottky diodes like STPS2L25U ( 25V 2A) have more short delay than using of MUR460 diode (600V 4A). Maybe diffused capacitance of diode in conducting state haves remarkable influence in this case.
Attached are simple scheme for measuring of diffused capacitance and results for some fast diodes.

[kt315]

no deep sense to use HV diode if your PULSE WIDTH is 100-200us. or you meant Delta Pulse? but it works with big FIRST DELAY, 100+us.

so What schematic of a detector you meant?

[Teleno]

Hi all,

I made some tests with different fast diodes connected in series of MOSFET to achieve more short delay. It is interesting - low reverse voltage schottky diodes like STPS2L25U ( 25V 2A) have more short delay than using of MUR460 diode (600V 4A). Maybe diffused capacitance of diode in conducting state haves remarkable influence in this case.
Attached are simple scheme for measuring of diffused capacitance and results for some fast diodes.

Which MOSFET? The low output cap ones discharge quickly so you may use low reverse voltage schottkys. They also have lower capacitance at low voltage.