Mosfet replacement

[Petvanbev]

Hi

I have a board with a MDF7N50 and a P2610ATFG

http://www.datasheet4u.com/datasheet-pdf/MagnaChip/MDF7N50/pdf.php?id=829092

http://www.datasheet4u.com/datasheet-pdf/UNIKC/P2610ATFG/pdf.php?id=1095139

the MDF7N50 does not turn on unless 10V to the gate.

I bread boarded both mosfets and used 12 VDC
in a lamp circuit, the fets turn on when the gate
is momentarily charged with12VDC, and then stay on.
They both turn off with finger touch to G and S

The diode multi-meter test does not show any
turn on of the MDF7N50 mosfet when gate is charged by the positive probe,
using the "quick and dirty multi-meter test"

The P2610ATFG does respond, but not to a very low value.
.335 on diode mode test.
Reverse negative D and positive S
shows .530 on diode mode

Wondering if this is normal for these mosfet? Or is the
capacitance compromised on these mosfets?

 

[Harald Kapp]

the fets turn on when the gate
is momentarily charged with12VDC, and then stay on.

Not a good idea. As the gate will discharge over time, the MOSFET will come into the transistion regioen between on and off. It will dissipate very much energy as waste heat in that state and can be destroyed. Always ensure that the gate is driven to a voltage either much higher or much lower than the threshold voltage. This ensures a defined on/off state of the MOSFET.

The diode multi-meter test does not show any
turn on of the MDF7N50 mosfet when gate is charged by the positive probe,

No wonder. the diode test voltage is < 1 V, not enough to turn on the MOSFET.

Reverse negative D and positive S
shows .530 on diode mode

This is normal behavior. The diode you measure is the so called body diode.

Here's some basic information on the operation of MOSFETs to read up.

 

[Petvanbev]

Not a good idea. As the gate will discharge over time, the MOSFET will come into the transistion regioen between on and off. It will dissipate very much energy as waste heat in that state and can be destroyed. Always ensure that the gate is driven to a voltage either much higher or much lower than the threshold voltage. This ensures a defined on/off state of the MOSFET.

thanks...

yes i only did a test circuit on a bread board, so I had no resistors in between 12 VDC and the Gate, and then ground, just to see if it would turn on and stay on , then turn off by discharging with just finger touch.
I assume then because of high VGS = 10V, then this mosfet is likely OK?


No wonder. the diode test voltage is < 1 V, not enough to turn on the MOSFET.

Makes sense..

This is normal behavior. The diode you measure is the so called body diode.

OK...

Here's some basic information on the operation of MOSFETs to read up.

Thanks, I will assume these Mosfets are OK. So far I have tested every resistor, transformer, capacitor and diode on the board, but there are also several SMD IC's, which have NO number on them and a Photocoupler, not checked, which perhaps is the culprit for this control board not turning on and functioning. This board is a control board for a secondary board which has mechanical relays, (also checked) which control motors and lights.

120AC in to a diode bridge, for DC, so I assume a small pulsed power supply and control for the relays. No schematic.

Thanks