MOSFETs

Components of a MOSFET Datasheet:

1. General description (voltage, on-resistance, current ratings)

2. Table of absolute maximum rating

   1. Exceeding any of these values may lead to failure, since the manufacture cannot guarantee proper operation and state of MOSFET.

3. Table of thermal resistance parameters

4. Table of electrical characteristics, with 3 important pieces of info

   1. a description (symbol and name) of a parameter

   2. a numerical specification, including units

   3. the conditions under which the specification is valid.

5. Figures (Graphs, Diagrams)

6. Package info

Important MOSFET Specs:

1. Operating Junction and Storage Temperature Range.

   1. Most important item in the absolute maximum ratings list.

   2. Most MOSFETs are rated either at a maximum +150°C or +175°C junction temperature.

2. Junction-to-Case thermal resistance R_θJC.

   1. Example: Thermal resistance of 0.50 °C/W means that for each watt of power dissipated in the junction, the junction temperature will rise 0.5 °C above the case temperature.

3. Drain-to-Source Breakdown Voltage.

   1. MOSFET can be controlled below the breakdown voltage.

   2. As long as the voltage across drain-to-source is below the threshold, the drain current will not exceed its threshold (there is uncertainty if this voltage limit is exceeded).

   3. Most MOSFETs aren’t used up to its full rated voltage; instead, derating in the 60-70% range is common (inductive spikes may exceed the breakdown voltage)

4. Static Drain-to-Source On-Resistance R_DS(on)

   1. Used to ensure that the voltage drop across the switch is acceptable given a specific current level.

   2. If you are using a MOSFET as a switch and cannot meet the on-resistance spec, don’t use it. 

   3. Example: If you supply a logic output from 3.3V system to the MOSFET via the gate, you cannot MOSFETs spec’d at 4.5V, since there’s not enough voltage to guarantee a full turn-on. Simply using“5V” logic (a logic 1 (HIGH) from a 3.3 V device will be at least 2.4 V Logic Levels - learn.sparkfun.com) doesn’t mean the actual output high voltage will be 5V.

   4. That’s why logic-level MOSFETs are generally specified at 4.5V V_gs, to ensure your output high level is above this threshold.

   5. Failing to meet the V_gs spec may lead to the drain-to-source resistance being generally higher than the R_DS(on) spec.

5. Gate Threshold Voltage V_GS(th).

   1. Voltage at which the MOSFET just barely starts to conduct.

   2. Example: The IRFP260N lists 2.0-4.0V for a drain current of 250uA. Concluding that a V_GS of 4.5V is sufficient to turn the device on is WRONG. You need the full 10V. 

   3. However, it is useful formaking sure the device is off, where zero voltage between gate and source is ideal to hold the MOSFET off. (This is IDEAL, but no no gate driver is perfect in that regard)

   4. If the driven gate voltage is below the minimum gate threshold of 2.0V, might be acceptable to turn a MOSFET off, and less than threshold current will flow.

   5. Good for power applications, but for signal applications, a quantity such as 250uA is a large number, and most MOSFETs won’t stay completely off (RESEARCH THIS MORE)

6. Gate Charge.

   1. Determines how fast the MOSFET will switch from ON to OFF and back.

   2. Just divide total gate charge by the gate driver current to determine this time.

   3. Example: With a I have a 1A gate driver, 234nC will take up to 234ns to turn completely on or off.

7. Turn-on, Rise, Turn-off, and Fall times.

   1. Useful only for understanding the minimum switching times.

   2. Usually gate drive circuit won’t switch as fast as these numbers, and are helpful only as a rough frame of reference.

8. Input Capacitance (C_iss): The combination of the gate, oxide layer, and body connection of a MOSFET act as a small capacitor that begins charging when voltage is present at the gate. It takes time to charge which results in an ON-state delay. Choose a MOSFET with the lowest input capacitance possible to avoid long delays and minimize in-rush current which can be very high initially but lessens as the capacitor charges.

9. Channel Type (N channel or P channel):

   1. Channel Type refers to the construction of the silicon inside the device. N channel MOSFETs will turn on with a positive voltage on the gate relative to the source while P channel MOSFETs will turn on with a negative voltage on the gate relative to the source.

   2. Depending on where the device is used in the circuit and what voltage will be applied to the gate can help select the part to be used

10. Maximum DC drain current:

    1. This is the max. current a device can withstand indefinitely given adequate cooling.

    2. This rating may be inaccurate since depending on the Vds, the device may only be able to conduct a small fraction of the current before failure.

    3. The only way to be sure that the device can withstand the desired current is to refer to the safe operating area curve in the device’s datasheet (sample shown below)

    4. 

MOSEFT Quadrants of Operation

1. V_DS < 0, the body diode conducts (check image)

   

2. V_DS > 0 but sufficiently low (CHECK THIS), the body diode conducts (check image)

3. V_DS is high but below the breakdown voltage, the MOSFET acts like a constant current sink (current into the circuit which it drains to ground)

4. V_DS is above the breakdown voltage, anything can happen; the MOSFET is likely to conduct.

General Tips

• Verify any information listed in the marketing summary with the specs (for example, “V_DSS = 200V” means that the V_DSS is 200V at a specific current and temp only included in the specs)

• When using a power MOSFET as a switch, provide a gate-to-source voltage V_gs that is at least one of the values mentioned in the R_ds(on) specification. V_gs below these values are not enough to ensure the device works properly as a switch.