Where are MOSFETs used

MOSFETs and logic levels


At the digital outputs of a controller, MOSFETs are used as switches. An ideal switch has the following properties:

  • There are exactly two states: off and on.
  • In the off-state the resistance is infinite, no current flows. The "off" characteristic is identical to the voltage axis (current = 0)
  • In the on state, the resistor is 0. There is no voltage drop. The "on" characteristic is identical to the current axis (voltage = 0).
  • The control of the switch does not require any energy.
The "resistance" between source and drain of a MOSFET can be varied with a signal at the gate. The conditions under which it is suitable as a switch are examined below. A MOS-FET can be used as a switch if the following requirements are met:
  • A MOSFET can only switch DC voltage with the correct polarity.
  • The permissible voltage on the MOSFET is not exceeded.
  • The current permissible for the MOSFET is not exceeded.
  • The control voltages on the GAte must be selected appropriately.
MOSFETS must be well cooled at high power. The channel resistance increases as the temperature rises, and so does the power loss. Thermal overload can occur without cooling. The determining parameters are the permissible drain current (I.D.) and the resistance of the MOSFET when it is fully turned on (RDSon). Even with a small RDSon a power loss occurs at the MOSFET which is calculated according to Ohm's law as follows: P = I * I * RDSon For example, if you take the type BUZ11 mentioned below, the maximum current is 50 A and an R.DSon of 0.04 Ω a power of. They have to be "cooled down" on everyone. As the junction temperature rises due to the power loss, the sheet resistance increases. The path resistance, which increases with increasing temperature when several MOSFETs are connected in parallel, ensures that the current is evenly distributed to parallel-connected Power MOSFETs. This is why MOSFETs can easily be connected in parallel to increase performance.

MOSFETs do not require a series resistor at the gate input because they are voltage-controlled and extremely high-resistance on the input side. Nevertheless, one often sees a series resistor in front of the gates in circuit diagrams, which is referred to here as "RG". This resistor on the MOSFET gate has a completely different job. At the moment of switching, the rather steep characteristic is passed through. Without RG, some MOSFETs tend to briefly oscillate at high frequencies during the switch-on and switch-off edges. This is prevented by RG. Resistance values ​​of a few 10 ohms are often sufficient. You are usually right with 100 Ω. Despite the high input resistance, you should not use higher resistance values, because the capacitances between gate and source and also between gate and drain have to be recharged when switching, which causes a significant reduction in the edge steepness.

Driving the gate with logic levels

MOSFETs can be used as power drivers at the output of controller circuits with an operating voltage between 3.3 V and 5 V if they are suitably designed. Standard MOSFETs only switch when there is a source-gate voltage (UGS) of 10 V or more fully through (saturation) and only then does the advantage of a very low resistance between source and drain (RDSon) come to light. If the drain current is less than 100 mA, the all-rounder MOSFET BS170 can be used. The problem with standard MOSFETs is that at low UGS work in the linear range of the characteristic curve and therefore a higher power loss occurs at the MOSFET. Look at the characteristic of a BUZ11, which according to data sheet can switch 30 A at 50 V and an RDSon of 0.04 Ω (sounds good at first):

The characteristics show that the BUZ11 only allows 5 A current at a gate-source level of 4.5 V, while it is specified for 50 A continuous current. This is because it needs a gate voltage of around 10V for the maximum current. At 3.3 V it is almost completely blocked. It is therefore unsuitable for logic control.

There are logic level MOSFETs for use with microcontrollers. These are MOSFETs with very small gate-source voltages for switching. You are looking for an N-channel MOSFET that is saturated with a gate-source voltage of 3.3 V so that the drain-source resistance is in the lower mΩ range, so that the power loss remains as low as possible. You have to be very careful when evaluating. In many MOSFET summary tables, the values ​​of RDSon often referred to a gate-source voltage of 10 V. A quite comprehensive overview list with MOSFET types is available from Mikrocontroller.net. In any case, you have to look for the relevant values ​​in the data sheet in the diagrams. The following characteristics come from the IR3803. You can see that here at 3 V UGS a current of 11 A is possible.

With a gate-source voltage of 4.5 V, the IRLB3034PbF has a maximum drain-source resistance of only 2 mΩ and a drain current of 170 A. The input capacitance, however, is approx. 10 nF. The following table shows a small selection of suitable types, although P-MOSFETs suitable for many applications are also listed here, although these cannot necessarily be referred to as "logic level types":

N-channel MOSFET
IRL100440 V75 A9 mΩ
IRL3705N55 V75 A18 mΩ
IRL380330 V75 A9 mΩ
IRL540100 V28 A80 mΩ
IRLB3034PbF40 V195 A1.7 mΩ
IRLL270555 V3.8 A40 mΩ
IRLML250220 V4 A45 mΩ
IRLZ34N55 V30 A35 mΩ
SMP60N03-10L30 V60 A10 mΩ
STN4NF03L30 V6.5 A39 mΩ
STP36NF06L60 V30 A40 mΩ
2N700260 V0.3 A2.8Ω
P-channel MOSFET
IRF4905-55 V-74 A20 mΩ
IRF9540-100 V-19 A200 mΩ
IRF9540N-100 V-23 A117 mΩ
IRLML6402-20 V-3.7 A65 mΩ
FDV304P-25 V-0.46 A1500 mΩ
Si2333CDS-12 V-4.5 A45 mΩ
NTS2101PT1G-8 V-1.4 A78 mΩ

The data sheet shows right at the beginning whether it is a standard MOSFET or a "logic level" type. Will the RDSon specified for a voltage of 10 V or higher, it is usually not a logic level MOSFET. A logic level MOSFET gives the R.DSon for VGS 5 V or 4.5 V. A second creiterium is the threshold voltage (Vthresh). This is the gate voltage at which the MOSFET completely blocks (less than a few microamps). Will Vthresh specified in the range of 2 V to 4 V, it cannot be a logic level MOSFET. V.thresh for these MOSFETs is usually 0.5 V to 1 V.

Direct connection or not?

Does it make sense to connect a power MOSFET directly to the output of a controller chip? It should not be forgotten that the capacitances between gate and source and between gate and drain, especially with MOSFETs for high currents, can amount to several nF. This may lead to high switching current pulses at the output pin of the controller, which can lead to the destruction of the controller output. The gate resistance mentioned above could be increased to dampen these current pulses, but this reduces the steepness of the switching edges. This means that the MOSFET stays longer in the linear range of the characteristic when switching and the power loss of the MOSFET increases for a short time. Please also note the "Maximum Safe Operating Area" diagram for the MOSFET used.

The controller output can be protected with an additional small bipolar transistor and its output current is reduced to less than 1 mA. The preamplifier with the 2N2219 has a collector circuit resistance of 4.7 kΩ, which ensures fast charging times for the gate capacitance.

Even better, however, is push-pull control with complementary transistors, in which both switching through and blocking takes place dynamically.

When designing a MOSFET circuit, you should be aware that instead of a collector-emitter voltage of around 0.7 V to 1.5 V for a bipolar transistor, a fully saturated MOSFET acts as a low-ohmic, linear resistor. For example, if you want to switch 8 A at 12 V, a bipolar power transistor with, for example, 1.1 V UCEsat a power of 8 * 1.1 = 8.8 W consumed in the semiconductor. The MOSFET IRLB3034PbF has an R.DSon of 1.7 mΩ, here the power loss is 8 * 8 * 0.0017 = 0.11 W. With an IRL540 it would be 8 * 8 * 0.08 = 5.1 W. The IRL540 would therefore need an appropriate heat sink.

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