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Oscillations with LM358 electronic load

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Kleinstein:
One could lower R3 a little, but should not go much below some 100 Ohms.
The capacitor C3 would be the one to make a little lager. Alternatively R4 could be a little larger (e.g. 10 K or 22 K). This slow down the loop - with such a high capacitance MOSFET the loop needs to be slower.

The FET given is not a good choice: it has a lot of gate capacitance and likely a poor DC SOA. So it would be save for low voltage (e.g. < 10 V) only.
There is a SOA curve with DC in the Datasheet, but it's not credibly - it's probably just calculated from P_tot and the transient heat capacity. This type of SOA curve ignores thermal instability and thus does not help.
A more suitable MOSFET would be IRFP250 or IRF840. More like much higher voltage rating and careful with more modern types (with some exceptions for super junction Fets).

vk6zgo:
358s do rather like to oscillate, especially if they have a reactive load like C3.
Try putting a 220 \$\Omega\$ resistor in series between the device's output pin & C3.
We had a similar circuit in a piece of RF gear in which the 358 would oscillate at around 80kHz at some settings.

salami738:

--- Quote from: Kleinstein on July 14, 2019, 07:18:53 pm ---One could lower R3 a little, but should not go much below some 100 Ohms.
The capacitor C3 would be the one to make a little lager. Alternatively R4 could be a little larger (e.g. 10 K or 22 K). This slow down the loop - with such a high capacitance MOSFET the loop needs to be slower.

The FET given is not a good choice: it has a lot of gate capacitance and likely a poor DC SOA. So it would be save for low voltage (e.g. < 10 V) only.
There is a SOA curve with DC in the Datasheet, but it's not credibly - it's probably just calculated from P_tot and the transient heat capacity. This type of SOA curve ignores thermal instability and thus does not help.
A more suitable MOSFET would be IRFP250 or IRF840. More like much higher voltage rating and careful with more modern types (with some exceptions for super junction Fets).

--- End quote ---

Hi, thanks for this detailled information!
I didn't know, that the DC SOA can be tricky, I only saw that there was a DC SOA and ordered some MOSFETs  :palm:
At the moment i do not have a IRFP250 or IRF840 lying around, but will order those in the future.
I will try the following mosfets I have in my stock pile:
- STW13NK60Z
- IRFP250M

Will post an update later on!

Kleinstein:
The  STW13NK60Z looks good, at least for no too high voltages (e.g. < 50 V), though the SAO curve may also be the same useless theoretical one ignoring the important part.  The trans-conductance is much lower and thus less stability problem expected.
Gate capacitance is also smaller, which helps.

salami738:

--- Quote from: Kleinstein on July 14, 2019, 07:18:53 pm ---...
The FET given is not a good choice: it has a lot of gate capacitance and likely a poor DC SOA. So it would be save for low voltage (e.g. < 10 V) only.
There is a SOA curve with DC in the Datasheet, but it's not credibly - it's probably just calculated from P_tot and the transient heat capacity. This type of SOA curve ignores thermal instability and thus does not help.
A more suitable MOSFET would be IRFP250 or IRF840. More like much higher voltage rating and careful with more modern types (with some exceptions for super junction Fets).

--- End quote ---

Hi, I tested a IRFP250M (TO247) and it works like a charm! Thanks for the recommendations.
The problem is, my pcb is suited for TO220 only. Will the IRF530 be suited (https://www.reichelt.de/index.html?ACTION=7&LA=3&OPEN=0&INDEX=0&FILENAME=A100%2FCKIRF530_DATA_EN.pdf)?
Maybe I will redisign the pcb because the TO247 can dissipate more power, which is good.

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