MOSFET Selection for Switching at High Power

MOSFETs can be used as switches for either DC or AC conduction, assuming the component can handle the power and frequency demands. MOSFETs selected for DC rely on a different set of considerations compared to MOSFETs selected for AC. An AC circuit that uses a MOSFET can either be switching or driven with a harmonic signal, thus providing AC power to a load. MOSFETs used in this way are selected based on their ability to both handle power and switch within the required time frame.

One of the common reasons a MOSFET fails in an AC circuit is because it is not properly matched with the driver. As a result, the gate driver does not properly toggle the MOSFET’s channel conduction, leading to excessive loss and destruction of the component. To overcome this, pay attention to the specifications below as these will determine how the MOSFET is run and whether the circuit will be reliable.

Specifications for Switching MOSFETs

When looking for switching MOSFETs for a power system, the selection of a MOSFET doesn't rely on a single specification. The group of specifications that matter most are those that work with your particular gate driver. If the gate driver is unable to adequately drive the mosfet, you may experience insufficient conduction and excessive loss, leading to failure. This requires looking at the switching time, total switching charge, and the resulting conductance in the channel in the on state.

First, look at the following specifications for the gate driver:

• Driver rise time

• Driver voltage

• Driver supply current

These specifications will determine how deeply the MOSFET channel can be modulated, and the turn-on pulse edge rate. This then determines the power loss due to switching and conduction. Now let's look at the corresponding MOSFET specifications that match a particular driver.

Turn-On and Turn-Off Times

The turn-on and turn-off times for a MOSFET are the lower limits of the rate at which a MOSFET can be switched between conducting and non-conducting states. Datasheets provide these values as a result from a specific test condition which you can attempt to replicate. No matter how fast a driver is, it will never modulate a MOSFET faster than the MOSFET’s minimum turn on/turnoff time. This value depends on the gate drive conditions due to nonlinearity of the conduction channel.

Rise time and fall time specifications given for specific test conditions.

The actual turn-on and turn-off times depend on the amount of charge needed in the gate and the amount of charge the driver can source (i.e., the driver current). Having a slow turn-on time in itself is not necessarily a problem until you start switching the component. During switching, the current in the channel is being modulated and incurs switching plus conduction losses. Slow switching time with fast switching frequency can lead to incomplete modulation and thus high switching losses.

Gates Charge Values

As mentioned above, the role of the driver is to charge up a gate so that it can fully modulate from non-conducting to conducting. There are actually three gate charge values which contribute to loss in a MOSFET:

• Gate to source charge (Qgs)

• Gate to drain charge (Qds)

• Total gate charge (Qg)

The simplest way to estimate the actual turn-on/turn-off times is to use the source and sink currents of the gate driver, respectively. Take the ratio of charge/current; if this is faster than the gate driver’s output edge rate, then the gate will be driver-limited. Typically in advanced SMD MOSFETs, the turn-on time will be gate-limited unless the driver’s output current is very high.

The gate charge is a function of the gate voltage, and this causes the system capacitances to also be functions of gate voltage.

Drain-Source Resistance (Rds) Curve

Finally, the important specification that determines conduction losses when the MOSFET is modulated on is the Rds value. This varies with gate voltage and temperature, and it is related to the physical design of the MOSFET.

When looking at MOSFET datasheets, the Rds value when fully on tends to be larger when the gate charge is smaller. This means a MOSFET might be easier to turn on for a given gate drive current, but it will have higher conduction loss. If we were operating at low current, we might be able to tolerate higher Rds values when there are switching losses as conduction losses would be low. However, in the high power case, there needs to be a balance between the drain-source resistance and the turn-on time to reach minimal loss.

Example Rds curves versus drain current and gate-source voltage (Vgs). From the FDS9945 datasheet.

Modeling any of the above-mentioned switching behavior accurately requires accurate circuit models describing the MOSFET’s gate, source, and drain characteristics. Not all vendors provide pre-made models that can be used in SPICE simulations, but datasheet information can often be sufficient to create an accurate model describing the MOSFET’s switching behavior.

Anytime you need to simulate switching regulator circuits with MOSFETs, use the complete set of circuit simulation features in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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