Advertisement

Article

Driving Large-Die and Paralleled Power MOSFETs Is Easy with the Right Techniques

Today's electronics systems require lower
voltages, higher currents, higher power density, and higher
efficiency than their predecessors needed. These demands
necessitate the use of paralleled power MOSFETs or large-die
MOSFETs. Having to use these high capacitance devices has given
some designers an uneasy feeling about which approach to
use.

The successful design of robust gate-drive circuitry for high
voltage, very high current MOSFETs is no different than the driver
circuitry designed for smaller devices. It depends on an
understanding of the drive requirements, a good model to predict
the results, careful attention to the physical layout of the driver
board, and the selection of the proper circuit topology.

Figure 1 represents a generic gate driver circuit where the
switching speed is controlled by the total gate impedance
(RG ), the total source inductance (LS ), the
load resistance, and the device's characteristics. Once the load is
defined and the power-switching device is selected, the fast
switching speed is achieved by minimizing the two remaining
parameters, RG and LS .


Figure 1: A generic gate driver circuit where the
switching speed is controlled by the total gate impedance
(RG ), the total source inductance (LS ), the
load resistance, and the device's characteristics.

Although this article discusses a single MOSFET device, the
equations can be extended to paralleled devices by adding together
the parameters of the paralleled combination and treating the
paralleled devices as a single lumped device.

Turn-On Breaks Down into Three Intervals

The resistive load turn-on waveforms of a power MOSFET (Figure
2) can be divided into three intervals. In the first interval, the
gate voltage rises to a value where the device just begins to
conduct. This interval is controlled by the device's input
capacitance and input impedance. The second interval is the time it
takes the drain voltage to reach its minimum and the drain current
to reach its maximum. The input impedance, gate-drain and
gate-source capacitances, source inductance, GM of the
device, and load current, control this interval. This period is the
most complex and represents the turn-on power losses. The third
interval is the period it takes the gate voltage to complete its
rise to the gate drive supply voltage and is controlled by the
input impedance and the input capacitance.


Figure 2: The resistive load turn-on waveforms of a power
MOSFET

Interval One (t0 – t1 )

This period begins with the transition of the input voltage
(VIN ) from the low value of the gate drive voltage
(–VGG ) to the high value of the gate drive voltage
(VGG ) and ends when the gate voltage reaches a value
where drain current begins to flow (VGS(ON) ). For this
analysis, the VIN transition is defined as a step
function that occurs in zero time. Because no drain current flows
during this period, the drain voltage remains at the supply voltage
(VDD ).

The equivalent circuit for this period (shown in Figure 3) is a
simple RC network made up of the input impedance (RG )
and the input capacitance (CISS ) and is the sum of the
driver impedance, the gate resistor, and the internal gate
impedance.


Figure 3: The equivalent circuit for Interval One

The time of the period and the waveform can be calculated from
the RC time constant formula:


(1)

Solving Equation 1 for t:


(2)

Where delta VGG is the total voltage swing of
VIN .

From Equation 2, the device in Figure 1 would have a first
interval of:

Where CISS is the typical value from the data sheet
and VGS(ON) is from the transfer characteristic curve on
the data sheet.

Interval Two (t1 – t2 )

This period begins when drain current starts to flow at the end
of interval one and ends when the drain current (ID )
reaches its maximum value. A gate-voltage plateau marks this
period. The gate drain capacitance CGD in conjunction
with the falling drain voltage, known as the Miller Effect, cause
this plateau.

Reading some papers on MOSFET gate-drive and switching
performance could lead you to believe that the rise time of the
drain current lasts for the same time as the gate voltage is
exhibiting the Miller Effect. The following analysis will show this
is untrue.

Using Kirchoff's Law, an approximate equation for the gate node
(when the rate of change of VGS is much less than the
rate of change of VDS ) is:


(3)

Solving Equation 3 for dt:


(4)

The change in gate voltage is related to the change in drain
voltage by:


(5)

Combining Equations 4 and 5:


(6)

Also:


(7)

And the drain current's rate of change is related to the drain
voltage's rate of change by:


(8)

Combining Equations 6, 7, and 8:


(9)

Solving Equation 9 for dt:


(10)

The solution of Equation 10 is not straightforward because
CGD is voltage dependent and varies as a function of the
drain-source voltage and GM is a function of the drain
current. An incremental solution for Equation 10 can be
accomplished using a spreadsheet and 25V incremental values for
dVDS . An example of this technique can be found in the
downloadable spreadsheet.

Interval Three (t2 – t3 )

This period begins when ID reaches its maximum value
and ends when CISS is fully charged. Like the first
period, the equivalent circuit is an RC network made up of
RG and CISS . The time of the period and the
waveform can again be calculated from the RC time constant formula
in Equation 2. However, the value for CISS is much
larger than in the first interval because the drain-source voltage
is much lower. This interval is not of major concern because all
activity with the drain voltage and current has been completed
before this interval starts.

Turn-Off

The resistive load turn-off waveforms of a power MOSFET (Figure
4) can also be divided into three intervals. The equations for
calculating the periods and waveforms are simply the reverse of the
turn-on where interval one corresponds to interval three, interval
two still corresponds to interval two, and interval three
corresponds to interval one.


Figure 4: The resistive load turn-off waveforms of a
power MOSFET

Figures 5 and 6 show actual switching waveforms from an
APT5O2OBN power MOSFET switching a 10.5ohm load at 250V with 51ohm
gate resistors. RD is 4ohms and R1 is 3ohms.
The calculated switching waveforms are overlaid with dots. The
correlation between calculated and measured values is very good,
proving the validity of Equations 2 and 10.



Figures 5 and 6: Actual switching waveforms from an
APT5O2OBN power MOSFET

Layout Considerations

As demonstrated, switching a MOSFET on and off requires only a
simple circuit to charge and discharge the device's input
capacitance. However, this seemingly simple circuit is not without
problems and pitfalls. It is simple only when compared to bipolar
drives.

Problems most often encountered with the gate drive circuit
include:

  • Voltage spikes large enough to rupture the gate oxide
  • Oscillation
  • Ringing or false turn-on.

Usually, these problems are the fault of the layout and not the
driver circuit's electrical design. To minimize these problems, the
following design rules and precautions should be followed when
designing and laying out driver circuits.

As illustrated in the previous section, the source inductance
plays a significant role in the switching speed by acting as a
negative feedback to the gate drive. Engineers cannot reduce the
source inductance in the device's package (LS ), but the
inductance relating to the connecting circuitry (LC ) can
be mitigated. Where the gate signal and the load current share the
same conduction paths (Figure 1) is the problem section. Therefore,
the load current should be diverted from the gate signal's path as
soon as possible — the closer to the source terminal of the
device the better.

Connecting the gate driver return to ground, instead of Point A
in Figure 1, adds the inductor LC to the LS
term of the switching speed (Equation 10) causing the switching
time to increase. Each additional inch of circuitry will add as
much as 20nH. Adding 20nH to the switching speed calculation
changes the results from a 20ns rise time to a 70ns rise time, a
350% increase.

A ground loop is an often-overlooked mistake in the gate driver
circuitry layout. A ground loop occurs when the gate driver
circuitry is tied to the power ground in more than one place,
resulting in load current flowing in the gate driver ground (Figure
7). This current not only results in slower switching speeds, but
also can cause excessive ringing on the gate, false triggering of
the power device, and oscillations.


Figure 7: Example of a ground loop

Minimizing the area of the gate driver circuitry loop, as Figure
8 shows, reduces the inductance in the loop and lowers the driver
impedance.


Figure 8: Minimized area of the gate driver circuitry
loop

A ground plane under the gate driver circuitry is helpful in
reducing noise injection into the drive circuitry. However, the
ground plane should be tied only to the power ground at Point A in
Figure 1. Great care should be taken not to create ground loops by
multiple tie points to the power ground. For a high side driver,
the ground plane should tie to the source of the high side device,
not to the power ground.

Do not intermix gate driver circuitry and high current carrying
load circuitry. Noise can be coupled into the gate driver circuitry
through stray capacitance or induced by radiated fields. The
results of the injected noise could cause excessive ringing on the
gate, false triggering of the power device, or oscillations.

The gate driver power supply should be bypassed with good
quality, high frequency capacitors because the power supply's
impedance is a part of the driver impedance (RG ). The
capacitors should be connected as close as possible to the driver
to minimize the inductance.

Gate Drivers

To this point, the type of driver being used has not been
indicated; only that it represented 4ohms of the total gate drive
impedance (RG ). Once the source inductance is under
control through proper layout, it is clear from the model in Figure
1 that the next way to improve switching speed is to reduce the
driver's resistance. Selection of the proper gate-driver circuit
topology accomplishes this task.

Gate drivers can be divided into two categories: discrete and
IC. In the past, discrete drivers have dominated, but IC types are
increasingly used because of their improved performance and lower
prices. I favor the IC driver over the discrete driver because it
uses fewer components, making the optimal layout easier to
achieve.

Discrete Drivers

Two types of discrete drivers are in common use: the
complimentary pair, bipolar NPN-PNP emitter-follower (Figure 9a)
and the complimentary pair MOSFET P-channel N-channel (Figure 9b).
Both types are referred to as totem poles.


Figure 9: (a) The complimentary pair, bipolar NPN-PNP
emitter-follower discrete driver.
(b) The complimentary pair MOSFET P-channel N-channel discrete
driver.

The bipolar totem pole is non-inverting and offers no voltage
gain to improve the pre-driver rise or fall times. It does provide
current gain to reduce the driver impedance to speed the charge and
discharge of the device capacitances. Once the input capacitances
are charged and the power device has been switched, the driver does
not require holding current. It offers medium speed and does not
perform well at higher conversion frequencies.

To facilitate higher frequency operation and faster switching,
the P- and N-channel complimentary pair MOSFET driver is used.
Unlike the bipolar design, the MOSFET totem pole is inverting and
offers voltage gain to improve on the pre-driver rise and fall
times. This driver suffers from shoot-through current caused by the
threshold voltage overlap during on and off transitions resulting
in increased drive power requirements. Because of the inverting
nature of the driver, it may cause false turn-on of the power
device during power up and power down, requiring under voltage
detection and hold-off circuitry.

IC Drivers

In the past, several companies introduced IC drivers with
reasonable performance and a relatively high cost. Recently these
and other companies have continued to improve the performance and
reduce the cost of the IC driver, making them more cost competitive
with the discrete driver. The IC driver is a better solution when
the fact it requires fewer total components is considered, making
it easier to meet layout design rules.

Driver Comparison

Table 1 shows a comparison between the two discrete drivers and
several IC drivers driving an APT5O2OBN device (0.2ohms, 28A, 500V)
switching a 10.5ohm load, 250V with no gate resistor RG. Shown in
the table, several of the IC drivers equal the MOSFET totem pole
and the remainder range between the MOSFET and bipolar totem pole
drivers.

Table 1

Driver Rise Time (ns) Fall Time (ns)
MOS 20 20
UC3710 20 20
M1C4451 20 20
M1C4429 30 30
UC3708 30 30
UC3711 30 40
Bipolar 60 70

Driving Large Die

To this point we have been examining the APT5O2OBN, a relatively
large device. However, there are even larger devices in use such as
the APT5OM6OJN (0.06ohm, 71A, 500V). This device was switched with
a 3.5ohm load, 250V with 0, 1.5, and 3ohms gate resistor
RG . The gate resistors were added to control excess
ringing caused by inductance in the load on the three faster
devices. The APT5OMGOJN is four times as large as the APT5O2OBN and
it would seem to present a much more difficult gate drive problem.
However, Table 2 shows that if the layout design rules are adhered
to as previously discussed, driving this device is not much more
difficult than driving the smaller device.

Table 2

Driver RG (ohm) Rise Time (ns) Fall Time (ns)
MOS 0 20 20
4451 0 20 20
3710 0 30 40
3711 0 40 50
4451 1.5 40 70
4451 3.0 60 100

The successful design of a MOSFET gate driver not only depends
on the selection of a low-impedance driver circuit topology but
also strongly depends on the layout of the circuit board containing
the driver circuit.

If a low-impedance driver circuit is employed and proper
attention is given to the layout, fast switching of large die
MOSFETs is not more difficult than driving smaller devices.

0 comments on “Driving Large-Die and Paralleled Power MOSFETs Is Easy with the Right Techniques

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.