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02-02-2024, 01:22 PM
Hi Guys
Mosfets have a very wide frequency response and are prone to oscillation if not treated carefully. They must have a gate-stop resistor right at the gate, just as at ube should have its grid-stop right at the grid and a BJT should have a base-stop right at its base.
Some mosfets have built-in gate voltage clamps in the form of zener diodes, but not all do. Those zeners are typocally 20-30V and if they fail due to an excessive input current at and excessive voltage, then the entire device is toast - burnt toast. This is why you see a zener diode across the G to S terminals, or including the gate-stop.
The London Power kits that use mosfets have the gate-stop and gate protection zener right at the mosfet mounting pads. If you choose not to mount the mosfets on the board these few other parts must be relocated with the mosfets to keep the mosfet from oscillating.
Wiring to the "protected mosfet" should be twisted - do not use coax.
The obvious alternative is to leave the mosfets on the PCB and move the entire unit to where it is cool. This means running twisted pairs of wires to the various wire connection blocks. We offered this as the PS-BOX many years ago, which was during the Classic-PS era and had a few issues. Many of those issues are eliminated with the current kits, but it is still preferred to have all the Power Scale circuitry inside the amp chassis.
Note that the external box must be tied to chassis ground to assure that it does its task as an EMI shield.
Mosfets have a very wide frequency response and are prone to oscillation if not treated carefully. They must have a gate-stop resistor right at the gate, just as at ube should have its grid-stop right at the grid and a BJT should have a base-stop right at its base.
Some mosfets have built-in gate voltage clamps in the form of zener diodes, but not all do. Those zeners are typocally 20-30V and if they fail due to an excessive input current at and excessive voltage, then the entire device is toast - burnt toast. This is why you see a zener diode across the G to S terminals, or including the gate-stop.
The London Power kits that use mosfets have the gate-stop and gate protection zener right at the mosfet mounting pads. If you choose not to mount the mosfets on the board these few other parts must be relocated with the mosfets to keep the mosfet from oscillating.
Wiring to the "protected mosfet" should be twisted - do not use coax.
The obvious alternative is to leave the mosfets on the PCB and move the entire unit to where it is cool. This means running twisted pairs of wires to the various wire connection blocks. We offered this as the PS-BOX many years ago, which was during the Classic-PS era and had a few issues. Many of those issues are eliminated with the current kits, but it is still preferred to have all the Power Scale circuitry inside the amp chassis.
Note that the external box must be tied to chassis ground to assure that it does its task as an EMI shield.
