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Simulated transistors

Among the transistors that have been simulated:

• DMG3415U
• DMP2035U
• BSS84

In the following, we will see the results of the simulation of the first transistor, DMG3415U. This transistor has an ON resistance of 42.5mΩ and a leakage current of 1nA (for a temperature 25C).

A deeper look inside Mosfet Transistor

Let’s consider the following simplified model of a mosfet transistor:

RDS(on) – On Resistance T

This is the total resistance between the drain and source in a mosfet transistor. RDS(on) is the basis for a maximum current rating of the MOSFET and is also associated with current loss. So RDS(on) is not zero, therefore, it must be taken into consideration because a big resistance causes huge problems with respect to the lowest shunt resistor (~mOhm). Thus, the lower RDS(on), the better.

Intrinsic capacitances

Cgs, Cgd and Cds are the intrinsic capacitances. They are unwanted capacitances, but still are part of the transistor. Together with the resistance in the circuit, they put an upper limit to the speed of the transistor.

Ciss, Input Capacitance

This is the input capacitance measured between the gate and source terminals with the drain shorted to the source for AC signals.

Ciss = C gs + Cgd

The input capacitance must be charged to the threshold voltage before the device begins to turn on, and discharged to the plateau voltage before the device turns off. Therefore, the impedance of the drive circuitry and Ciss have a direct effect on the turn on and turn off delays.

Coss – Output Capacitance

This is the output capacitance measured between the drain and source terminals with the gate shorted to the source for AC voltages.

Coss = Cds +Cgd.

For soft switching applications, Coss is important because it can affect the resonance of the circuit.

Crss – Reverse Transfer Capacitance

This is the reverse transfer capacitance measured between the drain and gate terminals with the source connected to ground.

Crss = Cgd.

The reverse transfer capacitance, often referred to as the Miller capacitance, is one of the major parameters affecting voltage rise and fall times during switching. It also affects the turn-off delay time.

Miller effect in a nutshell

The Miller effect is a special case of the Miller theorem when the impedance element (connected between the amp’s input and output) is a capacitor.

This effect leads to an increase in the input capacitance.The consequence is slowing down the transition and increasing the propagation delay time.

Spurious oscillation

MOSFETs are capable of switching large amounts of current in incredibly short times. Their inputs are also relatively high impedance, which can lead to stability problems. Under certain conditions high voltage MOSFET devices can oscillate at very high frequencies due to stray inductance and capacitance in the surrounding circuit.

Inductances

The connections with the circuit exhibit a parasitic inductance, which is in no way specific to the MOSFET technology, but has important effects because of the high commutation speeds. Parasitic inductances tend to maintain their current constant and generate overvoltage during the transistor turn off, resulting in increasing commutation losses.

A parasitic inductance can be associated with each terminal of the MOSFET. They have different effects:

• the gate inductance and the input capacitance of the transistor can constitute an oscillator. This must be avoided as it results in very high commutation losses (up to the destruction of the device)
• the drain inductance tends to reduce the drain voltage when the MOSFET turns on, so it reduces turn on losses. However, as it creates an overvoltage during turn-off, it increases turn-off losses.
• the source parasitic inductance has the same behaviour as the drain inductance, plus a feedback effect that makes commutation last longer, thus increasing commutation losses.

References

• https://www.microsemi.com/document-portal/doc_view/14692-mosfet-tutorial
• https://www.digikey.com/en/articles/techzone/2014/aug/resistor-capacitor-rc-snubber-design-for-power-switches
• https://en.wikipedia.org/wiki/Power_MOSFET

In conclusion, it is the intrinsic parasitic capacitances that come into play when measuring low current.

Switches – solutions

We saw that several parameters come into play when we want to select a mosfet transistor, this choice of course, depends on the type of application. The manufacturer provides in its datasheet, the values ​​of the various parameters:

• RD (on)
• Ciss
• Coss
• Crss
• I leakage
• 6 …

We  want a transistor with weak resistance, current leakage and capacitance. However, it was impossible to find such a transistor which met all these parameters.

Here is a compiled list of the most interesting transistors we’ve found:

• DMG3415U
• DMP2035U
• BSS84
• RRQ030P03
• NextPowerS3
• CSD25501F3
• …

The solution to this problem is not to find the transistor having a minimum parasitic capacitance (although it would be very useful) because it is insufficient.

Several solutions were considered:

• Using Load Switches (ADG801)
• Configuration Change (Shunt resistors in series and the transistor will be in parallel)

The most realistic solution is therefore to try to manipulate the parameter R (in τ = RC) and not only the parameter C.

Current range

In the previous posts, it has already been established that there will be 4 current ranges, here is a quick recap:

• 739 µA    →    303 mA
• 7.39 µA   →    3,03 mA
• 73.9 nA   →    30,3 µA
• 1 nA        →    303 nA

Bilal ADDAM