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Transistor Gate Characteristics

A quick intro to MOSFETs from the point of view of using them as a switching device in SSTC applcations. IGBTs share similar gate characteristics so the info in this page applies to those devices too.

 

DC Voltage Characteristics

Let's take a quick look at MOSFETs, particularly and how they work. In basic terms, the MOSFET is a voltage controlled current source, with a voltage difference from gate to source causing a current to flow from drain to source.

N-channel MOSFET schematic symbol

Varying the gate voltage will vary the current in the devices drain. For our immediate concerns, we'll assume that an N-channel device has the characterstics in the below table. We won't bother with P-channel devices here as they are not available in sufficiently high current ratings.

Gate-Source Voltage Operating Mode
+Vgs-max and above Gate breaks down, device damaged
+8V to Vgs-max On
+8V to +4V Linear
+4V to -Vgs-max Off
-Vgs-max and below Gate breaks down, device damaged

Note: These voltages are approximate and the datasheet should be referred to for your specific device. Some devices are designed with lower threshold voltages, some as low as 1.8V, but that is outside the scope of this document.

When on, the MOSFET appears as a low value resistor (typically less than 1 ohm) shown in the datasheet as the RDS-ON value (on resistance from drain to source), which is also dependant on temperature as well as gate voltage. In this low resistance state a large amount of current can flow through the device.

When off, the device appears as a high value resistor and very little current flows. We ideally want to keep the device operating in either the on or the off states.

When in the linear region, the MOSFET acts like a resistor and can dissipate large amounts of power. For switching circuits, we want to avoid operating in this region as it causes heating of the device.

Gate voltages are given limits in the device datasheets (Vgs-max), usually +/- 15V relative to the source. Exceeding this voltage can damage the device, causing a short circuit between gate and drain (or source).

Some devices (like the STE70NM60s I've used in my Stuby Tesla coil) have Zener diodes built into the gate to clamp the gate voltage to +/- 30V to prevent damage. Often in SSTC designs, back-to-back 15V Zener diodes can be seen across gate-source terminals to protect against over voltage on the gate. If you have a good GDT design, these should not be necessary, although some people include them to be safe.

 

Switching Speed

The gate of a MOSFET appears as a capacitor, so large peak currents are required to change the voltage across it. This is according to the equations

dv/dt = I / C

where:
C = capacitance (Farads)
I = current (Amps)
dv/dt = rate of change of voltage (Volts / second)

As can be seen, to increase the dv/dt, or switching speed, for a given capacitance value, the current needs to increase. These large peak currents are usually provided by some kind of MOSFET driver, like those contained in devices like SMPS controllers etc. MOSFET driver ICs are available from various manufacturers such as Texas Instruments and Ixys.

For all intents and purposes (unless you are considering building an OLTC) IGBTs can be considered to be the same as MOSFETs in terms of operating conditions. They do however have a significant input capacitance (approaching 120nF in some cases) which combined with slow turn off characteristics makes some of them unsuitable for high speed work.

Certain types of IGBT are designed as drop in replacements for FETs and these are usually designated by some kind of title like "Warp Speed". A detailed guide to IGBTs is out of the remit of this design guide.

 

Further Reading

Power MOSFET Basics from International Rectifier