PSRR of 2SK1058/2SJ164

[U130421]

These lateral MOSFETs are very commonly used on this forum, has anyone noticed the very poor power supply ripple rejection? Any solution? The feedback is bringing down to -90 db only if the gates are driven by high impedance.

 

[arpagon]

Can you replace the K1058/J164 with 2SC5200/2SA1943 and post the results from simulation please?

 

[U130421]

At your service.

 

[U130421]

This is for IRFP240/IRFP9240.

 

[nigelwright7557]

I don't understand the poor PSRR.
The output voltage is defined by the gate voltage not the power rails.

 

[EUVL]

Early effect and capacitances.


Patrick

 

[nigelwright7557]

Early effect and capacitances.


Patrick

Thanks.
One way around it is to use bigger smoothing capacitors.
Feedback will help too.

 

[EUVL]

Cascoding.


Patrick

 

[U130421]

Early effect and capacitances.


Patrick

About early effect , the 2sj164 sim model has 138 ma for 0v gate when drain is 50v while the k has only 1.8ma . Is this normal or wrong modeled?

 

[DF96]

The output voltage is defined by the gate voltage not the power rails.

Surely the output of any FET is defined by gate and drain voltages? FETs are not high output impedance devices like BJTs.

PS I thought Early effect applies to BJTs alone - it expresses how far short the transistor falls from being a perfect current source.

 

[Elvee]

These lateral MOSFETs are very commonly used on this forum, has anyone noticed the very poor power supply ripple rejection? Any solution? The feedback is bringing down to -90 db only if the gates are driven by high impedance.

Two distinct mechanism play a role here, and they shouldn't be mixed up: one is the equivalent of the Early effect for FET devices, the channel modulation, which happens under all conditions, including DC, and apart from massive feedback or cascoding has no solution, the other is the Miller capacitance effect between D and G, which is frequency-dependent and can easily be cured with a low drive impedance (the cascode also works in this case)

 

[U130421]

Yes , you are right . It is the early voltage remains the main problem with low driven impedance . As you you see it is worse with the P type. People designing no overall feedback must be aware of this .