design and implementation of a low-cost and compact …€¦ · design and implementation of a...
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Design and Implementation of a Low-cost and Compact Floating Gate Drive Power Circuit for
GaN-based Flying Capacitor Multi-level Converters
1
Zichao Ye
Advisor: Robert Pilawa-Podgurski
University of Illinois at Urbana-Champaign
March 13th, 2017
Outline
Motivation Flying capacitor multi-level converter Challenge in gate drive circuit
Proposed methods Cascaded bootstrap Double charge pump Gate-driven charge pump
Experimental results
Conclusion
2
Challenge in providing floating power
Source terminal of switch is floating
Need floating power supply to power the gate driver
Isolated DC/DC converter is commonly used as floating power supply Low efficiency Large footprint High cost
3
Goal Alternative ways to power the
floating switches Inexpensive, simple, efficient and
compact
A 2 kW seven-level FCML inverter prototype[1]
Vin
S3A
S3B
L
+Vout
_
Co
S2A
S1A
S2B
S1B
Vx
Cfly1 Cfly2
[1] Y. Lei, C. Barth, S. Qin, W.-C. Liu, I. Moon, A. Stillwell, D. Chou, T. Foulkes, Z. Ye, Z. Liao, and R. C. N. Pilawa-Podgurski, “A 2 kW, single-phase, 7-level, GaN inverter with an active energy buffer achieving 216 W/in3 power density and 97.6% peak efficiency,” in 2016 IEEE Applied Power Electronics Conference and Exposition (APEC), 2016, pp. 1512–1519.
Power density: 216 W/in3 (13.2 W/cm3)
Proposed methods
Cascaded bootstrap + Double charge pump To power low-side switches
Gate-driven charge pump To power high-side switches
4
Half-bridge gate driver
5
Half Bridge
Powered by isolated DC/DC converter
Half Bridge
Half Bridge
Texas Instruments LM5113w/ internal bootstrap function
Reduce the number of gate drivers, isolated power supplies and digital isolators by half
Q2
Q1C1
C2
VpwrGD1
GD2
Q3C3GD3
Q4C4GD4
Q5C5GD5
Q6C6GD6
Vin
D2
D3
D4
D5
D6
L
+Vout
_
Co
Cascaded bootstrap
6
Can we just do this?
-Yes and No.
GaN FET has large voltage drop in reverse conduction
Can utilize a special overcharge mechanism of bootstrap
× Bootstrap diode has forward voltage drop
× Narrow allowed gate voltage for GaN (4.5 to 6 V)
Overcharge mechanism of bootstrap
Over-charge C2
1. Current might flow into inductor through body diode
2. Body diode has forward voltage drop
3. Switching node Vx would be negative
4. Bootstrap operation has a different path
5. C2 will be over charged to
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Vin
Q2
Q1
C1
C2
Vpwr
Dboot
GD1
GD2
Vx < GND
L +Vout
_
Co
Vin
Q2
Q1
C1
C2
Vpwr
Dboot
GD1
GD2
Vx < GND
L +Vout
_
Co
Example:Vpwr = 5 V, VDboot = 0.7 V, VDbody = 2 V
Vx = -2 V, VC2 = 6.3 V
𝑉𝐶2= 𝑉𝑝𝑤𝑟 − 𝑉𝐷
𝑏𝑜𝑜𝑡+ 𝑉𝐷
𝑏𝑜𝑑𝑦
Dbody
Dbody
𝐼𝐿 = 𝐼𝐷𝑏𝑜𝑜𝑡 + 𝐼𝐷𝑏𝑜𝑑𝑦
Cascaded bootstrap
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The key idea is to use switch’s body diode voltage drop in reverse conduction to compensate the bootstrap diode’s forward drop, so that bootstrap can cascade
Constraints
Direction of inductor current
Amplitude of load current
Amount of gate drive power
Length of deadtime
Direction of inductor current
Only the body diode of low-side switches conduct during deadtimes
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Q2
Q1C1
C2
VpwrGD1
GD2
Q3C3GD3
Q4C4GD4
Q5C5GD5
Q6C6GD6
Vin
D2
D3
D4
D5
D6
L
+Vout
_
Co
Vin
Q2
Q1
C1
C2
Vpwr
Dboot
GD1
GD2
Vx < GND
L +Vout
_
Co
Direction of inductor current
Only the body diode of low-side switches conduct during deadtimes
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Q1
Q2
Q3
Q4
Q5
Q6
Q2
Q1C1
C2
VpwrGD1
GD2
Q3C3GD3
Q4C4GD4
Q5C5GD5
Q6C6GD6
Vin
D2
D3
D4
D5
D6
L
+Vout
_
Co
Vin
Q2
Q1
C1
C2
Vpwr
Dboot
GD1
GD2
Vx < GND
L +Vout
_
Co
Amplitude of load current
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Switches may have insufficient gate voltage during start-up and light load condition
Effect of deadtime
Longer deadtime helps charge to higher voltage
Need a balance between gate voltage and efficiency
Not suitable for ultra-high frequency converters
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33 ns 66 ns 100 ns
C1 6 V 6 V 6 V
C2 5.62 V 5.82 V 6.52 V
C3 5.28 V 5.86 V 7.26 V
C4 5.95 V 6.96 V 8.44 V
5
5.5
6
6.5
7
7.5
8
8.5
9
33 66 100
Vo
ltag
e (
V)
Deadtime (ns)
Measured bootstrap voltages at 1 A load current
C1
C2
C3
C4
Double charge pump
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Existing problem w/ cascaded bootstrap:• Not enough gate voltage on certain switches
Solution:• Use the gate terminal of gate driver to implement a voltage doubler• Then bootstrap the doubled gate voltage (~10 V) to the next switch• Use LDO to regulate the gate voltage to 5-6 V again (efficiency penalty!)
Q2
Q1
Q3C3GD3
Q4
Q6
Vin
Q5C5GD5
Cfly
Cpump
D1
D2
Gate-driven charge pump
Pump power from low-side to high-side switch A method to charge C5 in this example
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State 1 State 2Connect capacitor Cpump to the gate terminal of Q3
Q2
Q1
Q3C3
Q4
Q6
Vin
Q5C5GD5
Cfly
Cpump
D1
D2
GD3
Q2
Q1
Q3C3GD3
Q4
Q6
Vin
Q5C5GD5
Cfly
Cpump
D1
D2
Q2
Q1
Q3C3GD3
Q4
Q6
Vin
Q5C5GD5
Cfly
Cpump
D1
D2
Q2
Q1
Q3C3
Q4
Q6
Vin
Q5C5GD5
Cfly
Cpump
D1
D2
GD3
Gate-driven charge pump
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State 1 State 2
25 V State 1:Cfly = 20 V Cpump = 20 V
State 2:C3 = 5 V C5 = 5 V +
20 V- +
5 V-
+20 V
-
+20 V
- +20 V
-
+5 V
-
Need two low-voltage diodes and one high-voltage capacitor Little voltage loss from low-side to high-side High efficiency No start-up circuit required Applicable to other switched-capacitor topologies
Gate-driven charge pump
Potential effects to GaN switch
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Connecting to the gate driver side of the gate resistor has negligible effect to the turn-on behavior of GaN switch
Complete solution
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Modular switching cell design
CascadedBootstrap
Double Charge Pump
Gate-drivenCharge Pump
Parts count 1 diode 2 cap + 3 diodes 1 cap + 2 diodes
Switch to power Low-side Any High-side
Efficiency High Medium High
Constraints High Medium Low
Proposed methods comparison
Experimental Results
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Experimental results
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Hardware prototype
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50 % overall
size reduction!
Isolated DC/DC This work Improvement
Partcounts
6 ICs, 36 capsand 12 res
11 diodes, 11 cap 5 LDOs and 2 res
Size > 112 mm2 x 6 ~ 130 mm2 5X
Efficiency ~ 20% ~ 36% 2X
Price ~ $ 19.00 $ 3.35 6X
Conclusion
Proposed three combined methods to power the floating gate drivers of FCML converters Correct functionality Higher efficiency Lower cost Smaller footprint
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Conclusion
Proposed three combined methods to power the floating gate drivers of FCML converters Correct functionality Higher efficiency Lower cost Smaller footprint
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Thank You!
Pure cascaded bootstrap
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EPC Synchronous Bootstrap
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From EPC2108 data sheet
Gate-driven charge pump
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