I read this on a website. sounds pretty extreme to me!
1. Heat each MOSFET to 70C and apply 70 volts from drain-to-source and measure how much gate voltage is needed to turn the fet on sufficient to pass 300mA.
2. Write this initial rough-match Vgs number on the part.
3. Mount pairs (for the smaller amps) or quads (for the larger amps) of devices with similar Vgs values to a single heatsink (important -- they must all be the same temperature).
4. The MOSFETs are connected in parallel with no source resistor, to simulate operation in an amplifier. Gate resistors of 1,000 ohm are used to prevent parasitic oscillations in the RF frequency range. 0.1 ohm resistors are in each MOSFET source connection to measure current, then to ground (power supply return) through a common 10-ohm source resistor.
5. Connect drains to a 70VDC power supply.
6. Apply 70 volts drain-source voltage to all the MOSFETs in the group.
7. Apply sufficient Vgs to turn the entire group on to 500mA. A simple DC feedback servo circuit is used to set the overall current by monitoring the voltage across the single 10 ohm shared source resistor. Five volts across the resistor indicates that 500mA is passing through the entire group.
8. Industrial resistive-element heaters and temperature-controls are used to heat the heatsink to 70 degrees C.
9. As the group of devices heats up monitor individual MOSFET source currents by measuring the voltage across their 0.1 ohm resistors. Some devices will be found that will want to pass most of the current ("current hog"), while others will shut down.
10. Replace the current hogs with new MOSFETs until a stable set is found where each device handles its share of the current (within 10%) for several hours.
This is a very difficult procedure and it generally requires a few thousand devices to come up with a few dozen sets.
1. Heat each MOSFET to 70C and apply 70 volts from drain-to-source and measure how much gate voltage is needed to turn the fet on sufficient to pass 300mA.
2. Write this initial rough-match Vgs number on the part.
3. Mount pairs (for the smaller amps) or quads (for the larger amps) of devices with similar Vgs values to a single heatsink (important -- they must all be the same temperature).
4. The MOSFETs are connected in parallel with no source resistor, to simulate operation in an amplifier. Gate resistors of 1,000 ohm are used to prevent parasitic oscillations in the RF frequency range. 0.1 ohm resistors are in each MOSFET source connection to measure current, then to ground (power supply return) through a common 10-ohm source resistor.
5. Connect drains to a 70VDC power supply.
6. Apply 70 volts drain-source voltage to all the MOSFETs in the group.
7. Apply sufficient Vgs to turn the entire group on to 500mA. A simple DC feedback servo circuit is used to set the overall current by monitoring the voltage across the single 10 ohm shared source resistor. Five volts across the resistor indicates that 500mA is passing through the entire group.
8. Industrial resistive-element heaters and temperature-controls are used to heat the heatsink to 70 degrees C.
9. As the group of devices heats up monitor individual MOSFET source currents by measuring the voltage across their 0.1 ohm resistors. Some devices will be found that will want to pass most of the current ("current hog"), while others will shut down.
10. Replace the current hogs with new MOSFETs until a stable set is found where each device handles its share of the current (within 10%) for several hours.
This is a very difficult procedure and it generally requires a few thousand devices to come up with a few dozen sets.
Matching
Halo,
As far as I know using TO247, TO220 and TO03 case, I dont found any difference in drain (compare each other ) current when heatsink temperature arise but I matching my mosfet as close as posible in term of voltage and current I intented to use.
I think if you care to use an extereme way like that there is no bad side. just I cant imagine to handle a pair or more mosfet with temperature 70 degree its can burn my finger I think.
Sorry english is not my mother language
Halo,
As far as I know using TO247, TO220 and TO03 case, I dont found any difference in drain (compare each other ) current when heatsink temperature arise but I matching my mosfet as close as posible in term of voltage and current I intented to use.
I think if you care to use an extereme way like that there is no bad side. just I cant imagine to handle a pair or more mosfet with temperature 70 degree its can burn my finger I think.
Sorry english is not my mother language
I don't know about worthless, but why doesn't the manufacturer have a curve tracer? That manual method is way too much work, and it brings up the question of why their designs are so sensitive to matching in the first place. Good designs should not be highly dependent on device characteristics. I'd never want a product like that, since it can't be economically serviced after the OEM goes out of business, as they almost certainly will sooner or later.
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