Implementing High Speed
Isolation Barriers in SMPS with Primary
Side Digital Control
October 2006
Keith Coffey — Silicon Laboratories, Inc.
Agenda
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2
Applications
Regulator approvals
Key terms
Isolated SMPS with Digital Control
Update time for digital SMPS
Isolation technology
Radiated emissions for RF isolator
Opto-coupler limitations
GMR limitations
Technology summary
Applications
♦ Example circuit applications:
¾ Isolation
¾ Level shifting
¾ Ground loop elimination
Level Shifting & Ground Loop Elimination
♦ Example end applications:
¾ AC-DC and DC-DC switching
supplies
¾ Motor control systems
¾ Isolated analog data acquisition
¾ Plasma TVs
¾ Ethernet/CAN networks
¾ Isolated point-to-point
communications
¾ UPS systems
¾ Operator interface controls
3
Isolated Analog Sampling
PRIMARY-SIDE
POWER STAGES,
DRIVERS AND PEAK
CURRENT SENSING
VIN
SECONDARY-SIDE
POWER STAGES,
DRIVERS AND
OUTPUT FILTER
VOUT
PH3
VSENSE
PH4
(ISOLATED)
ICYC
C8051F30x
C8051F30x
I/O
PH1
AIN0
DIGITAL
Isolator
ISOLATOR
I/O
Analog
and digital
primary-side I/O
Rx
Si8250
Analog and digital
secondary-side I/O
I/O
Tx
Tx
I/O
PH2
Rx
ISOLATED AUX SUPPLY
VDD
GND
Digitally Controlled Power
Regulator Approvals
♦ 60950 — System level certification for information technology equipment
♦ 61010 — System level certification for test & measurement instruments
♦ Underwriters Laboratories — USA
¾ UL1577
¾ 2500 VRMS (1 minute)
¾ Site inspections
♦ CSA International — Canada
¾ Component Acceptance Notice #5A
¾ 2500 VRMS (1 minute)
¾ Site inspections
♦ IEC (International Electrotechnical Commission) — International, most stringent
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¾
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IEC 60747-5-2
2500 VRMS (1 minute)
Site inspections
Quality, reliability and documentation inspections
Key Terms
♦ Propagation delay
¾ The time it takes a signal to transition across the
barrier.
♦ Creepage
¾ The distance along the insulating surface an arc
may travel.
♦ Clearance
¾ The shortest path through air that an arc may
travel.
♦ Safety extra-low-voltage (SELV)
¾ Voltage less than 30 VRMS (42.4 V peak) or 60 V
DC.
5
Key Terms (continued)
♦ Hazardous (also called “live”)
¾ Voltage greater than 30 VRMS (42.4 V peak) or 60 V DC.
♦ Rated voltage
¾ The allowable continuous operating voltage that the manufacturer specifies.
♦ Working voltage
¾ The highest possible root-mean square value of the ac and dc voltage
across insulation when the product is operating at its rated voltage.
♦ Isolation voltage
¾ Isolation voltage is based on qualification test voltages that stresses the
insulation of the product. High-pot (dielectric withstand) tests are performed
to determine isolation level for an isolation product.
6
Key Terms (continued)
♦ Functional insulation
¾ Also called “operational insulation.” Does not protect or isolate against electrical
shock but is required for a product to function.
♦ Basic insulation
¾ A single level of insulation that provides basic protection against electrical shock.
♦ Supplementary insulation
¾ An independent insulation that manufacturers apply in addition to basic insulation to
reduce the risk of electrical shock in the event of a failure of basic insulation.
♦ Double insulation
¾ Comprises both basic and supplementary insulation.
♦ Reinforced insulation
¾ Is a single insulation system that provides electrical-shock protection equivalent to
double insulation.
♦
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Note: Double, reinforced, and basic insulation are the most important for safety isolation.
Double or reinforced insulation is required for voltages ranging from SELV to hazardous.
LOAD
Isolated SMPS with Digital Control
Isolator Concerns
• Greater power efficiency
• Increased system speed
• Greater protection
8
Update Time for Digital SMPS
9
Isolation Technology
♦ Opto-couplers
¾ Performance degrades at high temp
¾ Usually only singles per package
¾ Limited data rate (most 10 Mbps or less)
♦ Pulse transformers
¾ Bulky
¾ Expensive in cost and real estate
¾ No dc correctness
♦ Giant magneto-resistive couplers
¾ Senses magnetic fields
¾ Up to 100 Mbps
¾ Random startup states
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Isolation Technology (continued)
Opto
♦ Implements light detection for
communications
RF
♦ Implements RF modulation
demodulation scheme to transfer data
Pulse
♦ Implements pulse code demodulation
across chip scale transformers
Isolation
Transmitter
Receiver
RF Modulator
(2.1 GHz Carrier
Generator)
RF Demodulator
(2.1 GHz Carrier
Detector)
Modulation Scheme
Input Signal
Modulation Signal
Output Signal
11
Radiated Emissions for RF Isolator
12
Frequency
Measured
FCC Spec
(Part 15)
(GHz)
(dBμV/m)
(dBμV/m)
to Spec (dB)
2.09
70
74
-4
2.17
68.3
74
-5.7
4.21
61.9
74
-12.1
4.34
60.7
74
-13.3
6.32
58.3
74
-15.7
6.51
60.7
74
-13.3
8.67
45.6
74
-28.4
Compared
Opto-Coupler Limitations
♦ Integration
¾ 1 package per multiple channel
¾ External components (Rs, Cs, Diodes)
required for solution
♦ Limited throughput performance
¾ Difficulty with increasing data rate/timing
requirements, typically <10 Mbps
Digital Coupler Solution
Optocoupler Solution
R1
C2
D1
♦ Difficult to use/expensive
¾ Opto-couplers are analog in nature
ƒ Numerous board tweaks
ƒ Increases design time/costs
¾ Digital Couplers are plug and play!
13
C2
R5
R2
Digital
Isolator
Data
Converter
25 mm
¾ Inefficient electro-optic conversions
¾ Sensitive to temperature and variations
in supply voltage
¾ System performance degrades over time
¾ Requires temperature compensation
C1
10 mm
♦ Excessive power consumption and
temperature dependence
Opto
D2
R3
D4
Opto
C1
16 mm
R4
R6
R7
C3
Opto
D3
17 mm
R8
Data
Converter
GMR Limitations
Condition
14
GMR
Power-up/initialization
Output may or may not match the input
Input noise susceptibility
Following a noise spike, output may match the input, may
not match the input, or may oscillate
Slow input rise time
Output may transition to correct state, may remain in
incorrect state, or may oscillate
Output ringing/overshoot
Excessive overshoot (~1.8 V measured) with 2-3 oscillations
Technology Summary
Digital Coupler
(RF)
Digital
Coupler
(Pulse)
Digital
Coupler
(GMR)
Up to 150
Up to 100
Up to 100
Up to 50
12 (5 V,100 Mbps)
31 (5 V, 90 Mbps)
7*
25*
Supply Voltage (V)
2.375 – 5.5
2.7 – 5.5
2.7 – 5.5
4.5 – 5.5
Temp. Range (°C)
-40 to +125
-40 to +125
-40 to +125
-40 to +85
<10
20
15
22
Pulse-width Distortion (ns)
2
3
3
2
Transient Immunity
(kV/μs, min.)
25
25
15/20
10
No. of Channels
4
4
4
2
Protection
Basic/Reinforced
Basic/Reinforced
Basic/Reinforced
Basic/Reinforced
Package
16-Lead
Wide Body SOIC
16-Lead
Wide Body SOIC
8-Lead SOIC
8-Lead SOIC
Parameter
Max. Data Rate (Mbps)
Power Consumption/
Channel (mA, typ)
Prop. Delay (ns)
* Specifications according to manufacturers’ datasheets.
15
OptoCoupler
(Best Grade)
Thank You