Author Topic: adding more mosfets  (Read 1206 times)

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Offline welshman

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adding more mosfets
« on: September 26, 2020, 07:52:06 am »
when adding more mosfets to a power board how do i calculate the resistor values.

on the pj there is 6 mosfets with a 47ohm to the gate from the pwm and there is a 20k from the gate to the source on each of the 4 power boards.

if i increase the mosfets to 8, do i simply add more 47r and 20k's?

that doesnt seem right to me, i imagine i would need to reduce the 47 ohm by a division of something 1.3x to accommodate the correct voltage delivered to the 8 instead of 6. am i overcomplicating this or just talking nonsense?

 is there anyone here with experience of this?

Offline welshman

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Re: adding more mosfets
« Reply #1 on: October 04, 2020, 10:29:41 am »
reading here

Importance of Gate Resistance:

"To get rid of the above issue it is recommended to use low value resistor in series with the logic input and the mosfet gate. With relatively lower frequencies(50 Hz to 1kHz), the value could be anywhere between 100 and 470 ohms, while for frequencies above this the value could be within 100 ohms, for much higher frequencies (10kHz and above) this must not exceed 50 ohms.

The above consideration allows exponential charging or gradual charging of the internal capacitors reducing or blunting of the chances of negative spikes across the drain/gate pins."

"Get Rid of Stray Inductance:
The most common and prime bug in the que is the stray inductance that may be hidden within the circuit tracks. When the switching frequency and current are high, even a slightest unnecessary increase in the connecting path that is the PCB track may result in inter-linked inductance which in turn may affect the mosfet behavior drastically due to inefficient conduction, transients and spikes.

In order to get rid of this issue it's strongly recommended to keep the tracks wider and to keep the devices AS CLOSE AS POSSIBLE to each other and to the driver IC which are being used to drive the respective mosfets. That's why SMD is preferred and is the best way of eliminating cross inductance across the components, also the use of double sided PCB helps controlling the issue due to its short "printed-through-hole" connections across the components. Even the standing height of the mosfets must be brought to minimum by inserting the lead as deep down as possible into the PCB, using SMD is probably the best option."


Using a Resistor Between Gate and Source
Although we have not indicated this inclusion in the previous images, this is strongly recommended to safeguard the mosfet from blowing of under all circumstances.

So how does a resistor across gate/source provide a guaranteed protection?

Well, normally mosfets have the tendency to latch up whenever a switching voltage is applied, this latching effect can sometimes be hard to revert, and by the time an opposite switching current is applied it is already too late.

The mentioned resistor ensures that as soon as the switching signal is removed the mosfet is able to quickly turn OFF, and prevent a possible damage.

This resistor value could be anywhere between 1K and 10K, however lower values would provide better and more effective results."

... So it seems i just continue repeating the same for 8 as i would for 6... making sure the gate voltage is 10v higher than source..