Enhancing Power Delivery Designs with CMOS Based Isolated Gate Drivers

Enhancing Power Delivery System
Designs with CMOS-Based
Isolated Gate Drivers
By Don Alfano, Silicon Labs
Fully-integrated isolated gate drivers can significantly increase the efficiency,
performance and reliability of switch-mode power supplies compared to legacy solutions.
Introduction
As emerging green standards challenge designers to deliver more energy-efficient, cost-effective and
reliable power delivery systems in smaller form factors, the need for greater power and isolation device
integration becomes increasingly important. A critical building block within ac-dc and isolated dc-dc power
supplies is the isolated gate driver. While optocoupler-based solutions and gate-drive transformers have
been the mainstay for switch-mode power supply (SMPS) systems for many years, fully-integrated
isolated gate driver products based on RF technology and implemented in mainstream CMOS provide a
more reliable and power-efficient solution.
Rectifier
PFC
PRIMARY
400VDC
VIN
SECONDARY
FULL BRIDGE
TOPOLOGY
Local VDD
Q1 I1
HIGH SIDE
DRIVER
Q3
Q4
AC
LINE IN
XFMR
PRIMARY
LOW SIDE
DRIVER
Q2
LOW SIDE
DRIVER
Q6
OUTPUT
VS
XFMR
SECONDARY
VS
HIGH SIDE
DRIVER
SYNCHRONOUS
RECTIFIERS
I2
ISOLATION BARRIER
SMPS
Controller
Q5
AC CURRENT
SENSOR
AC CURRENT
SENSOR
ISOLATED
DRIVER
ISOLATED
DRIVER
FEEDBACK
ISOLATION
PMBus
ISOLATION
PMBus
INTERFACE
4
Figure 1: AC/DC Converter Based on Full Bridge Topology
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Anatomy of an Isolated Power Converter
Isolated power converters require power stage and signal isolation to comply with safety standards.
Figure 1 shows an example of a typical ac-dc converter for 500 W to 5 kW power systems, such as those
used in central office telecom rectifier supplies. From a high-level perspective, this two-stage system has
a power factor correction circuit (PFC) that forces power system ac line current draw to be sinusoidal and
in-phase with the ac line voltage; thus, it appears to the line as a purely resistive load for greater input
power efficiency.
The high-side switch driver inputs in Figure 1 are referenced to the primary-side ground, and its outputs
are referenced to the high-side MOSFET source pins. The high-side drivers must be able to withstand the
400 VDC common-mode voltage present at the source pin during high-side drive, a need traditionally
served by high voltage drivers (HVIC). The corresponding low-side drivers operate from a low voltage
supply (e.g.18 V) and are referenced to the primary-side ground. The two ac current sensors in the lowside legs of the bridge monitor the current in each leg to facilitate flux balancing when voltage mode
control is used.
The isolation barrier shown in Figure 1 is provided to ensure that there is no current flow between the
primary- and secondary-side grounds; consequently, the drivers for synchronous MOSFETs Q5 and Q6
must be isolated. The secondary-side feedback path must also be isolated for the same reason.
Gate Drive Solution Options
Although optocouplers are commonly used for feedback isolation, they are not fast enough for use in the
synchronous MOSFET gate-drive isolation circuit. While faster optocouplers are available, they tend to be
expensive and exhibit the same performance and reliability issues typical of optocouplers, including
unstable operating characteristics over temperature and device age and marginal CMTI resulting from a
single-ended architecture with high internal coupling capacitance. In addition, Gallium-Arsenide-based
process technologies common in optocouplers create an intrinsic wear-out mechanism (“Light Output” or
LOP) that causes the LED to lose brightness over time.
Given the above considerations, gate drive transformers have become a more popular method of
providing isolated gate drive. Gate drive transformers are miniature torroidal transformers that are
preferred over optocouplers because of their shorter delay times. While faster than optocouplers, gate
drive transformers cannot propagate a dc level or low-frequency ac signal; they can pass only a finite
voltage-time product across the isolation boundary, thereby restricting ON time (tON) and duty cycle
ranges. These transformers must also be reset after each ON cycle to prevent core saturation,
necessitating external circuitry. Finally, transformer-based designs are inefficient, have high EMI and
occupy excessive board space.
An Optimum Isolated Gate Drive Solution
Fortunately, better alternatives to gate drive transformers and optocouplers are now available.
Advancements in CMOS-based RF isolation technology have enabled isolated gate drive solutions that
offer exceptional performance, power efficiency, integration and reliability. These highly-integrated CMOS
devices are well positioned to supersede both optocouplers and gate drive transformers in SMPS
applications.
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Isolated gate drivers, such as Silicon Labs’ ISOdriver family, combine RF-based isolation technology with
gate driver circuits, providing integrated, low-latency isolated driver solutions for MOSFET and IGBT
applications.
ISOdriver products are available in three basic configurations (see Figure 2): high-side and low-side
isolated drivers with either separate control inputs for each output or a single PWM input, and a dual
isolated driver. Each ISOdriver device is available in 0.5 A and 4.0 A peak output current and is available
in 1 kV, 2.5 kV and 5 kV isolation ratings. The high-side/low-side versions have built-in overlap protection
and an adjustable dead time generator (dual ISOdriver versions contain no overlap protection or dead
time generator). As such, the dual ISOdriver can be used as a dual low-side, dual high-side or highside/low-side isolated driver. These devices have a three-die architecture (see Figure 3) that causes each
drive channel to be isolated from the others as well as from the input side. This allows the polarity of the
high-side and low-side channel to reverse without latch-up or other damage.
VDDI
VDDI
VDDA
LPWM
VOA
UVLO
VOA
DT CONTROL
&
OVERLAP
PROTECTION
VDDI
VDDI
VDD1
VDD1
VDDI
VDDB
ISOLATION
UVLO
VOB
UVLO
GNDB
UVLO
VDDB
VDDI
DISABLE
VOB
VDD1
VDDB
DISABLE
UVLO
ISOLATION
DISABLE
GNDA
STEERING
LOGIC &
DT CONTROL
DT
VDDI
UVLO
VOA
UVLO
GNDA
ISOLATION
VDDI
VIA
UVLO
GNDA
DT
VDDA
ISOLATION
ISOLATION
VDDI
PWM
ISOLATION
VDDA
VIA
VOB
UVLO
GNDB
GNDB
VIB
LPWM
GND
VIB
GND
GND
HS/LS Two Wire Input ISOdriver
Two-Wire Input High-Side/Low-Side
HS/LS PWM Input ISOdriver
One-Wire Input High-Side/Low-Side
Dual ISOdriver
Dual ISOdriver
Figure 2: ISOdriver Family
For example, the high-side driver (GNDA) might ride on a common-mode voltage of 100 V while an
adjacent driver (GNDB) might ride on a common-mode voltage of 200 V. These two common-mode
voltages can reverse (i.e. GNDA = 200 V, GNDB = 100 V) without damaging or upsetting the driver. This
feature makes the ISOdriver useful in systems with fast-changing common-mode voltages or when the
input is a bipolar supply.
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Input Die
Output
Die
Output
Die
Figure 3: Decapsulated Three-Die ISOdriver
Maximizing System Efficiency
The switching mode in high-side/low-side drive applications must be “break-before-make” to avoid
efficiency loss from both MOSFETs being on at the same time (i.e. “shoot-through current”). This time
period between switch transitions where both switches are off is referred to as “dead time” (Figure 4).
Must avoid shoot-through
current (Q1, Q2 on
simultaneously)
Q1
A
A
IOUT = 10A
VOUT
B
Q2 power loss when
ON = IOUT2 x RDSON
= 0.5W
Q2
B
Dead
Time
Q2 Power loss
when OFF =
VT x IOUT
= 7W!!!
BODY
DIODE
(VT)
Figure 4: Dead Time
While an optimum amount of dead time can increase system efficiency, excessive amounts of dead time
can reduce efficiency. As shown in Figure 4, the power dissipation of Q2 is only 0.5 W when Q2 is on but
increases to 7 W when the body diode conducts while Q2 is off. Therefore, the amount of dead time
added to the circuit timing must be only large enough to prevent shoot-through current. High-side/low-side
ISOdrivers have an integrated dead time generator that can be adjusted from 4 ns to 950 µs using an
external resistor, allowing the user to optimize dead time.
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OVERLAP
OVERLAP
VOB
VIA
VIA
50%
VIB
VIB
DT
DT
DT
DT
90%
VOA
VOA
10%
DT
DT
90%
VOB
10%
VOB
Normal Dead Time Behavior
Dead Time Behavior during Overlap
Figure 5: ISOdriver Dead Time Behavior
ISOdrivers also contain overlap protection that causes outputs VOA and VOB to unconditionally go low in
the event VIA and VIB simultaneously go high, as shown in Figure 5.
Dual ISOdriver
While dead time optimization can increase efficiency by as much as +4%, additional efficiency gains can
be achieved by arranging MOSFETs in parallel or by increasing gate drivers to a single, larger MOSFET.
In either case, a dual ISOdriver can be useful in providing additional drive capability. Unlike dedicated
high-side/low-side ISOdrivers, the Si8232/5/6 dual ISOdrivers have no built-in overlap protection or dead
time setting.
The state of each driver output unconditionally follows that of its input as long as the device is powered.
The two driver output circuits are isolated from each other and from the input, allowing the common-mode
voltage of one driver to reverse polarity with respect to the other without damage (i.e. latch-up) or output
errors. Figure 6 shows a common-mode voltage inversion where the polarity of the two drivers reverses
without damage or upset, which can be helpful in systems with bipolar input supplies.
5
5V
VDDA
Si8232/5/6
VDDA
GNDI
GNDA
VOA
OUT A
GNDA
VIA
VDDB
From
Controller
VDDB
VIB
VOB Output Signal
VOA Output Signal
Common Mode Voltage (V)
VDDI
VOB
OUT B
VOB
GNDB
DISABLE
Common Mode Voltage V1
VOB Output Signal
VOA Output Signal
Common Mode Voltage V2
Time
Figure 6: Common Mode Voltage Inversion
In many power applications, such as UPS systems and inverters, switches must be designed in parallel to
enable the system to deliver rated power at high operating efficiencies. The combined capacitive loading
of these switches requires either a higher peak current driver or a less desirable method of distributing the
switches over multiple gate driver ICs. The circuit in Figure 7 shows each Si8232/5/6 output driving
several common ground switches in parallel. When connected in this way, the dual ISOdriver can provide
an equivalent peak drive current of 8 A while 50 ns propagation delay time ensures that all switches are
driven off and on simultaneously.
Isolated
24VDC
LOAD
VDDI
5V
Si8232/5/6
VDDI
VDDA
VOA
GNDA
GNDI
From Controller
VIA
VDDB
GNDB
VIB
VOB
DISABLE
Figure 7: Paralleled Outputs for Increased Peak Output Current
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Power circuits in high-voltage systems, such as imaging systems and plasma flat panels, have split
ground systems to isolate higher voltages from lower voltages. In many cases, local supply regulators are
built using a dedicated controller for each regulator. In other cases, the regulators may use a transformercoupled multi-output design (using flyback or other transformer-coupled topology).
Isolated
V1
VDDA
5V
HV
Si8232/5/6
VDDI
VDDA
VOA
VOUT 1
GNDA
Isolated
V2
GNDI
TWO-LOOP
CONTROLLER
OUT1
LV
VIA
VDDB
OUT2
VIB
VOB
VOUT 2
GNDB
I/O
VFB1
DISABLE
ANALOG SIGNAL
ISOLATION
VFB2
Figure 8: Dual Isolated Buck Converter
Figure 8 shows a dual output isolated buck converter using the Si8232/5/6 dual ISOdriver. A single twoloop controller is used with the ISOdriver to generate two stepped-down output voltages. The ISOdriver
operates as an isolated dual high-side driver with each output isolated from both the adjacent output and
the primary side. While this circuit uses a low-cost Shottkey freewheeling diode, a second dual ISOdriver
can be added to control output synchronous rectifiers for higher efficiency.
Conclusion
CMOS-based isolated gate drive technology, exemplified by Silicon Labs’ ISOdriver family, offers
substantial performance, reliability, integration and per-channel cost advantages over legacy isolation
technologies, such as optocouplers and gate drive transformers. The Si823x and Si822x ISOdrivers are
single-chip, isolated gate drivers that feature ultra-fast 50 ns propagation delays for increased timing
margins. They also offer programmable dead-time control for higher system efficiency, stable operation
over temperature and time, lower BOM costs and smaller PCB footprints.
The ISOdriver family is engineered to deliver industry-leading performance, high integration and
exceptional value, providing an optimal isolated gate drive solution for a wide range of power delivery
systems. With up to 5 kV of isolation, the ISOdriver products are well suited to safety-critical applications
requiring high maximum continuous working voltages. Supporting output power supplies up to 24 V and
0.5 or 4.0 A peak output current, ISOdrivers efficiently drive MOSFET and IGBT power stages in highperformance, isolated switch mode power supplies.
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