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How to drive a low Rdson mosfet for PWM

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sahko123:
I can always lower the frequency I just want to keep it as high as possible so that I dont need as large a capacitor bank for decoupling.

On a seperate note I've read that thermocouples have a low source impedance which akes sense so shouldnt it be easy to extract a reasonably precise differential mode signal from a large common mode voltage? It seems that I can amplify the signal when no common mode voltage is present but once applied the output gets tied to close to 0V (with Vos). The op-amp is driven off of 24V  ad the common mode voltage is also ~24V with a max of 24.5V. Would this common mode maximum be before damage occurs or should the cmmr also work at this voltage?

Zero999:

--- Quote from: DavidAlfa on May 23, 2022, 11:32:27 pm ---Why keep going with this and the silly overcomplicated thing?
Switching a heater at 36KHz is plain stupid, a normal implementation will be 10-100Hz, so the turn-off resistor will do the job fine, neither a zener is required, just a resistor divider.

--- End quote ---
Agreed. Note the schematic I posted doesn't contain a potential divider. Attached is it with a supply voltage of 24V and the base at 5V. VGS = 16.5-24 = -9.5V. The voltage across R2 is 0.7V below the base voltage, giving 4.3V. The collector current is roughly equal to the emitter current and R2 = 1k, giving 4.3mA, which drops 9.5V across R1, a 2k2 resistor.



--- Quote from: sahko123 on May 24, 2022, 09:20:23 am ---I can always lower the frequency I just want to keep it as high as possible so that I dont need as large a capacitor bank for decoupling.
--- End quote ---
The frequency makes no difference to the decoupling capacitance requirement. If anything too higher frequencies pose a risk of interference. It's not so much the frequency, but the switching speed, which can be slow if it's operated at a low frequency.


--- Quote ---On a seperate note I've read that thermocouples have a low source impedance which akes sense so shouldnt it be easy to extract a reasonably precise differential mode signal from a large common mode voltage? It seems that I can amplify the signal when no common mode voltage is present but once applied the output gets tied to close to 0V (with Vos). The op-amp is driven off of 24V  ad the common mode voltage is also ~24V with a max of 24.5V. Would this common mode maximum be before damage occurs or should the cmmr also work at this voltage?

--- End quote ---
A thermocouple has a low impedance, but the voltage is tiny, which makes it more difficult to read it, when there's a large common mode voltage.

The absolute maximum input voltage rating of an op-amp which is safe limit, before damage can occur, is often different to the common mode voltage range, which is the operational range.  For example the LM358's maximum input voltage rating is 32V, but its input voltage range is V+−1.5. This means, when run off 24V, its inputs can tolerate 32V, without being damaged, but it will only work properly with its inputs biased up to 22.5V.
https://www.ti.com/lit/ds/snosbt3j/snosbt3j.pdf?ts=1653328133496&ref_url=https%253A%252F%252Fwww.google.com%252F

No one but you knows how the op-amp is configured. Post a schematic.

By the way, go for the P-MOSFET, high side drive. It's much easier. Refer to the schematic at the start of this post.

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