Using MOSFETs as Switches

7/30/2019 Using MOSFETs as Switches

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USING MOSFETs AS SWITCHES

A Comprehensive note on safe operation of MOSFETS

By Athar Rasul

14th

March, 2012


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MOSFETs are Transconductance devices

- which is a fancy way of saying that a

voltage on the input (Gate-Source)

causes a current to flow on the output

(Drain-Source). They have very highinput impedance in the 10s of mega

ohms which is very desirable. The high

input impedance means that there is

very little power required to turn a

MOSFET on.

General Notes

Enhanced MOSFETs when ON, allow current in either direction with an essentially identical

RDS(ON). When OFF, they block current in one direction. MOSFETs have a threshold voltage thatmust be reached in order to turn them on. It is called VGS(TH).

Gate Resistor

Unfortunately, the high input impedance causes static problems and other handling problems.

If the Gate is left open (no connection), the MOSFET can self-destruct. The Gate is a very high

impedance device and noise can trigger the MOSFET. A pull-down resistor connecting Gate to

Source should be added (100K to 1M is generally ok) to stop the MOSFET from self-destructing.

It is very important to install this resistor BEFORE installing the MOSFETs. You will find that after

this resistor is installed, the MOSFETs are quite stable devices. The resistor pulls-down the Gateand turns off the MOSFETs, not to mention, adds some static protection.

Using a low value resistor between the MOSFET driver and the MOSFET gate terminal dampens

down any ringing oscillations caused by the lead inductance and gate capacitance which can

otherwise exceed the maximum voltage allowed on the gate terminal. It also slows down the

rate at which the MOSFET turns on and off. If you are driving a MOSFET from a bouncy, possibly

noisy, line (for instance relay contacts), you should use a small series gate resistor close to the

MOSFET, to suppress VHF oscillation. 22 ohms is plenty, you can use less.

For high current MOSFETs, the Gate Channel Capacitance can be very high and a rapidlychanging drain voltage can produce milliamps of transient Gate current. This could be enough

to overdrive and even damage delicate CMOS driver chips. Having a series resistor is a

compromise between speed and protection, with values of 100R to 10K being typical. It is a

good idea to use gate series resistors of 1K to 10K. This is especially important if the Gate signal

comes from another circuit board.


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Gate Noise Rejection

If MOSFETs are used as switch for motors, you may want to add a capacitor in parallel with the

pull-down resistor to minimize the effect of noise and slow the turn ON time of MOSFET. The

problem with doing so is that if MOSFET takes too much time to turn on, it may burn. So

instead of slowing gate rise time to mute noise, run the output of MOSFET through some

inductors in the 40uH to 100uH range (depending on load). Place the inductors in series with

the motor, as close to the FETs as possible. This is quite useful, and it keeps the FETs cool.

Basic MOSFET Selection Rules / Checks

The Drain to Source max voltage rating (VDS,max) determines the maximum voltage you

can switch.

The Gate threshold voltage (VGS) determines the voltage difference you need to apply to

the gate to make the MOSFETs conduct.

The Gate to Source max voltage (VGS,max) is a critical factor that must not be exceeded

(even for a few nS) or the MOSFET will be destroyed. Will the power rails spike? If so

provide protection of some sort (e.g. transient suppressor) or select a device with a

higher rating. When switching high voltage rails (e.g. 24V from low voltage logic you can

often meet this requirement using a potential divider to provide the MOSFET with a

gate voltage above 0V).

Do you need to use a MOSFET driver IC? If the MOSFET has a high Gate switching

current (e.g. high current MOSFETs) or will be switched fast (to ensure that the MOSFET

operates efficiently with minimal power dissipation) then this may be necessary. Often

MOSFETs require a 12A drive to achieve switching efficiently at frequencies of

hundreds of kHz. This drive is required on a pulsed basis to quickly charge and discharge

the MOSFET gate capacitances.

Paralleling MOSFETs

MOSFETs may be placed in parallel to improve the current handling capability. Simply join the

Gate, Source and Drain terminals together. Any number of MOSFETs can be paralleled up, but

note that the gate capacitance adds up as you parallel more MOSFETs, and eventually the

MOSFET driver will not be able to drive them.

Why MOSFETs Fail?

1. Insufficient gate drive

MOSFET devices are only capable of switching large amounts of power because they are

designed to dissipate minimal power when they are turned on. You must ensure that the


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MOSFET is turned hard ON. If the device is not fully turned on and far from saturation then the

device will have a high conduction resistance and will dissipate considerable power as heat.

2. Over-Voltage

Exceed a MOSFETs voltage rating for just a few nS and you can destroy it. Select MOSFETdevices conservatively for the anticipated voltage levels and ensure you allow for or deal with

suppressing any voltage spikes or ringing.

3. Peek current overload

Overload currents for a short duration can cause progressive damage to a MOSFET often with

little noticeable temperature rise prior to failure. MOSFETS often quote high peek current

rating but these are typically only for peek currents of a few 100 S. If switching inductive load

ensure you overrate the MOSFET to handle peek currents.

4. Prolonged current overload

If a MOSFET is passing a high current then its on state resistance will cause it to heat up. If the

heat sinking is poor then the MOSFET can be destroyed by excessive temperature. A solution to

this can be to parallel multiple MOSFETs to share high load currents between them.

5. No free-wheel current path

When switching inductive loads, there

must be a path for back EMF to free-

wheel when the MOSFET switches off.

Free-wheeling path must be provided

across the load as well as the MOSFET.

Enhancement mode MOSFETs

incorporate a diode that provides this

protection. A similar diode and/or

Snubber circuit can be provided across

the load to have a better voltage

suppression in back EMF condition.

6. H or Full Bridge Configuration Shoot-through / Cross conduction

When using P and N MOSFETS between voltage rails to provide an H or L output voltage, if the

control signals to the MOSFETs overlap then they will effectively short circuit the supply and

this is known as a shoot-through condition. When this happens, any supply decoupling


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capacitors are discharged rapidly through both devices every time a switching transition occurs

resulting in very short but large current pulses.

To avoid this a dead time must be allowed between switching transitions, during which neither

MOSFET is ON.

7. Slow reverse recovery of MOSFET body diode

High Q resonant circuits are capable of storing considerable energy in their inductance and self-

capacitance. Under certain tuning conditions, this causes the current to free-wheel through

the internal body diodes of the MOSFET devices as one MOSFET turns off and the other device

turns on. A problem rises due to the slow turn-off (or reverse recovery) of the internal body

diode when the opposing MOSFET tries to turn on. MOSFET body diodes generally have a long

reverse recovery time compared to the performance of the MOSFET itself. If the body diode of

one MOSFET is conducting when the opposing device is switched on, then a short circuit

occurs similar to the shoot-through condition described above. You can solve his problem by

adding a Schottky diode connected in series with the MOSFET source (prevents the MOSFET

body diode from ever being forward biased by the free-wheeling current) and a high speed (fast

recovery) diode connected in parallel to the MOSFET/Schottky pair so that the free-wheeling

current bypasses the MOSFET and Schottky completely. This ensures that the MOSFET body

diode is never driven into conduction. The free-wheel current is handled by the fast recovery

diodes which present less of a shoot through problem.

8. Excessive gate drive

If the MOSFET gate is driven with too high a voltage the gate oxide insulation can be punctured

effectively destroying the MOSFET. Ensure that the gate drive signal is free from any narrow

voltage spikes that could exceed the maximum allowable gate voltage.

9. Slow switching transitions

Little energy is dissipated during the steady on and off states, but considerable energy is

dissipated during the times of a transition. Therefore it is desirable to switch between states as

quickly as possible to minimize power dissipation during switching. Since the MOSFET gate

appears capacitive, it requires considerable current pulses in order to charge and discharge the

gate in a few tens of nano-seconds. Peak gate currents can be as high as an amp.

10. Spurious oscillation

MOSFET inputs are 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. (Frequencies usually in the low


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MHz.) A low impedance Gate-drive circuit should also be used to prevent stray signals from

coupling to the gate of the device.

11. Conducted interference with controller

Rapid switching of large currents can cause voltage dips and transient spikes on the powersupply rails which may interference with the control circuitry. Good decoupling and star-point

grounding techniques should be used.

High Side and Low Side Switching

All the circuits are feasible when correctly driven, but 2 & 3 are far more common, far easier to

drive well and far safer.


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Topology # 3 is the most practical N-channel scheme. The source is at a fixed voltage with

respect to ground, which means you can provide a fixed gate-source voltage to control it. The

MOSFET will be ON anywhere from +2.5 to +12V above ground, depending on the device.

When would you ever not use this topology? The only major reason to do so is if you have a

load that needs to have one terminal tied to circuit ground, for electrical safety or to minimize

electromagnetic radiation/susceptibility. Some motors/fans/pumps/heaters/etc. must do this,

in which case you are forced to use the high-side topology # 1 or # 2.

Topology # 1 is tricky. When the MOSFET is off, the source is somewhat of a floating node

(imagine a resistor divider with the top resistor enormous) sitting somewhere close to zero.

When the MOSFET is on, the source will be very close to 400V assuming saturation. A moving

source means that the gate-to-ground control voltage would have to move as well to keep the

MOSFET on.

An N-channel high-side switch has better performance than a comparably sized/priced P-

channel high-side switch, but the gate drive is more complicated, and has to be relative to the

N-channel MOSFET source terminal, which varies as the circuit switches, but there are

specialized gate drive ICs which are meant to drive high-side N-channel MOSFETS. High-voltage

or high-power applications generally use this topology.

Topology # 2 is straightforward like topology # 3. If the control voltage is referenced to ground,

proving 397.5V to 388V from gate to ground (-2.5 to -12V from gate to source) will turn the

MOSFET ON. The source is fixed (always at +400V) so controlling the gate means a fixed voltage

is all you need. (Unless your 400V bus collapses, but that's another issue).

A P-channel high-side switch has worse performance than a comparably sized/priced N-channel

high-side switch (mainly because of its higher RDS(ON)), but the gate drive is simple: connect gate

to the positive rail ("+400V" in above fig) to turn it off, and connect gate to a voltage that is 5-

10V below the positive rail to turn it on. At low supply voltages (5-15V), you can essentially just

connect the gate to ground to turn the MOSFET on. At higher voltages (15-50V), you can often

create a bias supply with a resistor and a zener diode. Above 50V, or if the switch has to switch

ON fast, this gets impractical and this topology is less often used.

Topology # 4, like topology # 1, is tricky. When the MOSFET is off, the source sits near 400V.When it is on, it will fall to near zero. A variable source means a variable gate supply with

respect to ground, which is again a messy proposition.

The low-side P-channel switch has the worst of all worlds (worse device performance, complex

gate drive circuit) and is essentially never used.

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Using MOSFETs as Switches