What is the purpose of the series diode?

[moodz]

If your detector is "sensitive" enough to be improved by a series diode then you might also consider that the diode junction does not instantaneously cool to ambient temperature at MOSFET off time. In fact the reverse leakage will be quite high for some microseconds as the junction thermal energy dissapates and though the preamp indicates a faster switching time ... its really reacting to the reverse leakage current.

The preamp in light blue ( series diode in place ) appears to be faster than the purple ( no diode ) but the coil currents tell a different story ... leakage current in red ( diode current ).

The answer is to use SIC mosfets ( much lower output capacitance ) and NO diode. Also less voltage drop saves battery.

Besides SIC mosfets have a much lower temperature coefficient at switch off ... that is another story.

[Detectorist#1]

Hi kt315,
pulse width is 25us

Hi Teleno,
MOSFET is IPD95R450P7

Hi Moodz,
The MOSFET is absolutely cool - the pulse is 25us and period is 20ms

Attached is the schematic of these PI detector optimized to work as pinpointer for grain size objects.

[Detectorist#1]

Hi Moodz,

Using of SIC transistor with low Coss is good idea but these transistors needs Ugs more than 15V for Id =8A as my project of fast PI MD. Those detector use popular 9V battery. Peek drain voltage is 900V and SIC type C3M0280090J have 900V max drain voltage. According the datasheet page3 Fig.2 - for Id=8A, min Uds= 8A x 0.3R=2.4V and Ugs min =15V. The Coss at Uds=10V is 180pF. My solution with 950V NMOS + 25V schottky diode have also 180pF effective capacitance in parallel to coil and no problem with Ugs.

[Teleno]

Hi kt315,
pulse width is 25us

Hi Teleno,
MOSFET is IPD95R450P7

Hi Moodz,
The MOSFET is absolutely cool - the pulse is 25us and period is 20ms

Attached is the schematic of these PI detector optimized to work as pinpointer for grain size objects.

Coss is 16pF. There's probably no benefit in using a series diode.

[Detectorist#1]

Hi Teleno,

According datasheet page 5, Coss is 16pF at Uds=400V !!! See please diagram 14 on page 10. Coss is 2000pF at Uds=9V (power is 9V). The real result of min delay without diode is catastrophic !!!

[kt315]

The real result of min delay without diode is catastrophic !!!

add a resistor in MOS-FET SOURCE 1 Ohm (you have 0.01) and a capacitor polar 2200uF, according a classic PI schematic with ultralow FIRST DELAY.

[kt315]

Coss is 16pF. There's probably no benefit in using a series diode.

that is. while his research is sure interesting, for future GT1 reseachers.

[Detectorist#1]

Hi kt315,
Average current consumption of MOSFET switch is only 6mA and adding 1Ohm resistor will reduce power voltage with 6mA x 1Ohm=6mV. This is OK. Using of 2200uF polar capacitor in this project is not practical.
The place for this capacitor in 25mm diameter tube is very limited. Some data for 2200uF polar extremely low impedance capacitors:
2200uF 10V - 10x20mm ESR 38mOhm
2200uF 16V - 16x20mm ESR 33mOhm
2200uF 25V - 16x25mm ESR 28mOhm
2200uF 35V - 16x35mm ESR 20mOhm
I use two polymer polar capacitors 6.3x9mm and common ESR=6mOhm. The ripple voltage on these capacitors is only 180mV as is mentioned on the schematic.
Using of 2200uF 35V polar capacitor will increase the size of the pinpointer and the value of ripple voltage on power line. This will have not affect on first delay absolutely!

[kt315]

This will have not affect on first delay absolutely!

tell this to Carl. LOL.

[Detectorist#1]

Hi kt315,

Adding of 1Ohm resistor and 1000uF or 2200uF capacitor have no influence on first delay. This RC group only absorb the big pulse current in TX time and allow to use batteries with higher internal resistance!
In my project, two capacitors by 470uF have the same role. The resistor 0.01Ohm is current sensor for measuring the peek TX current.
The diode D6 in HH-II project type 1N4004 is too slow for fast coil with 18uH inductance, but in HH-II project coil inductance is 300-400uH and this diode will help for some reducing of the first delay.

[Carl-NC]

This RC group only absorb the big pulse current in TX time and allow to use batteries with higher internal resistance!

True, but when the cap is placed very close to the TX FET it also reduces transient currents in the power supply, which can help with noise. Whether or not you really need it depends on the design. In the Tekpoint pinpointer I used 47uF but the pulse current is only ~50mA.

[Detectorist#1]

Hi Carl,
You are right. It is important to place the cap very close to TX FET. In this project, the pulse TX current is 8A and using of cap with very low ESR is important especially with simple 9V battery.
Attached is the file with PCB assembly information in more clear variant by Ivconic.

[moodz]

Hi Moodz,

Using of SIC transistor with low Coss is good idea but these transistors needs Ugs more than 15V for Id =8A as my project of fast PI MD. Those detector use popular 9V battery. Peek drain voltage is 900V and SIC type C3M0280090J have 900V max drain voltage. According the datasheet page3 Fig.2 - for Id=8A, min Uds= 8A x 0.3R=2.4V and Ugs min =15V. The Coss at Uds=10V is 180pF. My solution with 950V NMOS + 25V schottky diode have also 180pF effective capacitance in parallel to coil and no problem with Ugs.

Well I hope you are not building a state of the art pinpointer because for 18 uH I get it damped in 192 nanoseconds with a SIC mosfet and no blocking diode whereas your example is still thinking about coming down to 0 zero volts at the 1 microsecond mark. So if you are trying to detect very small pieces of metal ( in food etc ) you have a way to go.

I used your circuit ...I used a snubber to limit the flyback because of the fast switching times.

BTW driving a SIC off a 9 volt battery etc is easy ... a simple pulse pump capacitor scheme does the job with no extra power consumption.

[Detectorist#1]

Hi Moodz,

I used MOSFET in TO-252 package. The reason is that most little package for SIC transistors is TO-263-7. The area in 25mm tube is very limited.
BTW show me please this simple pulse pump capacitor scheme that you propose.

[kt315]

BTW driving a SIC off a 9 volt battery etc is easy

driving a SIC off a 9 volt battery is heavy. you forgot that the type of batt is serial circuit of 1.5V cells inside. 6 pcs. 'own batt resistance', we say, is very huge.