Advanced Industrial Electronics
Resonant Power Converters
References
[1] Kazimierczuk M., Czarkowski D., Resonant power converters, John Wiley and Sons, Inc. 1995
[2] Kazimierczuk M., Czarkowski D., Solutions manual for - Resonant power converters, John
Wiley and Sons, Inc. 1995
[3] Brown M., Power supply cookbook, Newnes, 2001
[4] Luo F. L., Ye H. Synchronous and resonant DC/DC conversion technology, energy factor, and
mathematical modeling, Taylor and Francis Group, 2006
[5] Hagerman J., Calculating optimum snubbers, Hagerman Technology, 1995
[6] International Rectifier, AN-978 HV floating MOS-Gate driver ICs, International Rectifier
Application Note, (www.irf.com)
[7] Hang-Seok Choi, AN-4151 Half bridge LLC resonant converter design using FSFR-series
Fairchild Power Switch, Fairchild Semiconductor Corporation Application Note, 2007
[8] STMicroelectronics, AN2450 LLC resonant halfbridge converter design
guidline,STMicroelectronics Application Note, 2007, (www.st.com)
References
[9] Bosso C., AND8311/D Understanding the LLC structure in resonant applications, ON
Semiconductor, 2008, (www.onsemi.com)
[10] Cree Inc., C2D05120-Silicon Carbide Schottky Diode, Cree Data Sheet, 2006,
(www.cree.com)
[11] IXYS Corporation, IXDN430 30 amp low-side ultrafast MOSFET/IGBT driver, IXYS
Corporation Data Sheet, 2004, (www.ixys.com)
[12] IXYS Corporation, EVDD 430S/ EVDD 430CY 30A Ultra Fast MOSFET/IGBT driver
evaluation board, IXYS Corporation, 2003, (www.ixys.com)
[13] IXYS Corporation, IXFL32N120P Polar Power MOSFET HiperFET, IXYS Corporation Data
Sheet, 2008, (www.ixys.com)
[14] IXYS Corporation, IXFN60N80P PolarHV Power HiperFET MOSFET, IXYS Corporation Data
Sheet, 2006, (www.ixyys.com)
[15] STMicroelectronics, L6599 High-Voltage resonant controller,STMicroelectronics Data Sheet,
2006, (www.st.com)
[16] Infineon Technologies AG, SKW25N120 fast IGBT in NPT technology, Infineon Data Sheet,
2006
1
Introduction
PWM and resonant power converting ideas
Introduction
DC-DC converter
MAINS AC
230V/400V
50 Hz
Low frequency
rectifier, filter
with PFC
correction
PFC
Controller
DC
320V/560V
Vin
PWM/Resonant
inverter
AC
High frequency
rectifier
and filter
DC
Vout
Load
Converter
Controller
Block diagram of a typical PWM/resonant switching power supply
Introduction
The function of DC-DC converter are as follows:
- to convert a DC input voltage (Vin) into a DC output voltage (Vout)
- to control the DC output voltage (Vout) against load and mains
variations
- to reduce the AC ripple on the DC output voltage (Vout) below the
required level
- to provide isolation between the input source and the load
- to protect the supplied system from electromagnetic interference
(EMI)
- to satisfy various international and national safety standards
2
Introduction
Voltage-switching half-bridge inverters with various resonant circuits
Introduction
Main features of the resonant circuits:
- circuits a), f) and g) supply a sinusoidal output current and
are compatible with current-driven high frequency rectifiers
- inverters (b)-(e) produce a sinusoidal output voltage and
are compatible with voltage-driven rectifier
- for the circuits (b)-(g) resonant frequency depends on the
load
LLC inverter basics
3
LLC inverter basics
A=
- the ratio of the inductance:
- the equivalent inductance:
L1
L2
 1
L = L1 + L2 = L2 ( A + 1) = L2 1 + 
 A
1
=
LC
- the undamped natural frequency: ω0 =
1
(L1 + L2 )C
- the characteristic impedance: Z 0 = ω0 L = 1 = L
ω 0C
C
LLC inverter basics
- the loaded quality factor at f0: QL = ω0CRL =
RL
ω0 L
=
RL
Z0
- the equivalent inductance of the damped circuits:
Leq = L1 + Ls
where
Ls =
- the resonant frequency: ωr =
L2
1+
ω 2 L22
RL2
1
=
LeqC
1
(L1 + Ls )C
- the quality factor at the resonant frequency:
RL
ω (L + Ls ) where R =
1
= r 1
Qr =
s
1 + RL2 / (ωr2 L22 )
ωr CRs
Rs
Gain
LLC inverter basics
capacitive region
ZCS
Mmax
peak gain
inductive region
ZVS
f
f0
4
LLC inverter basics
Capacitive region – current leads the voltage, bridge
MOSFETs operate in zero current switching (ZCS). It means
that power MOSFETs are turned-off (Vds decreases from Vin
to 0) at zero current. Switching-off losses can be neglected.
Inductive region – current lags the voltage. Power switches are
turned-on (Is is increasing from 0 to Ismax) at zero volts (ZVS).
Switching-on losses can be neglected.
For frequency fsw = f0 the MOSFETs turn on and turn off at
zero currents, resulting in zero switching losses and high
efficiency.
LLC inverter basics
*Taken from „Resonant power
converters”,
KazimierczukM.,Czarkowski D.[1]
LLC inverter basics
Operating below resonant frequency (ZCS):
a) conductive sequence is Q1, D1, Q2, D2
b) there are a few detrimental effects of switching-on MOSFET:
- reverse recovery of the antiparallel diode of the opposite switch
- second breakdown of the MOSFET parasitic bipolar transistor
- discharging of transistor output capacitance (additional losses)
- Miller’s effect
5
LLC inverter basics
Operating below resonant frequency:
d) IGBT transistors or thyristors with antiparallel diode should be used instead of
MOSFETs
*Taken from „ SKW25N120 fast
IGBT in NPT technology, Infineon
Data Sheet, 2006 [16]
LLC inverter basics
Operating at frequency fsw=f0:
- transistors turn on and turn off at zero currents
- efficiency is high because of lack the conducting losses
- antiparallel diodes never conduct
- output power or output voltage of the converters can not be
controlled
LLC inverter basics
Operating at frequency fsw > f0:
- the conduction sequence of the semiconductor devices is
D1-Q1-D2-Q2
Vgs1
Td
- MOSFETs
Vgs2
operates at ZVS
Vds2
Vin
i
ZVS
t
6
LLC inverter basics
The LLC resonant converter with a transformer center-tapped rectifier
LLC inverter basics
S2 in ON, D4 is conducting
LLC inverter basics
S2 is ON, D1 – D4 are blocked
7
LLC inverter basics
S1, S2 are OFF, Coss1 id discharging, Coss2 is charging
LLC inverter basics
VCoss2=Vin+Vf, D1 conducts; S1, S2 are OFF; D3, D4 are blocked
LLC inverter basics
S1 is ON, D3 is conducting
8
LLC inverter basics
S1 is ON; D3, D4 are blocked
LLC inverter basics
S1, S2 are OFF, Coss1 is charging, Coss2 id discharging
LLC inverter basics
S1, S2 are OFF, VCoss2 = -Uf, D2 is conducting
9
LLC inverter basics
*Taken from „AND8311/D
Understanding the LLC
structure in resonant
applications”, Bosso C.
ON Semiconductor, 2008
[9]
LLC full-bridge converter
High frequency rectifiers
10
High frequency rectifiers
The features of current driven diode rectifiers:
- have to be driven by current source
- the DC output current is directly proportional to the
amplitude of the input current
- the diode threshold voltage Uf, the diode forward
resistance Rf and filter capacitor ESR reduce efficiency of
the rectifiers
- the center-tapped rectifier has the highest efficiency, while
the half-wave has the lowest
High frequency rectifiers
The features of current driven diode rectifiers:
- half-wave and bridge rectifier are suitable high voltage
applications because the diode peak reverse voltage is Vdm
= -V0
- for the half-wave rectifier both the source and the load
can be connected to the same ground
- the RMS current of capacitor is very high and therefore
the capacitor must be rated accordingly
- the ESL of the filter capacitor may destroy the filtering
effect at very high frequency
High frequency rectifiers
Features of the rectifier:
- it has the highest
efficiency
- its efficiency is low at
light loads
- its not suitable for high
frequency because of
increasing the gatedriver power
11
High frequency rectifiers
High frequency rectifiers
The features of voltage driven diode rectifiers:
- have to be driven by voltage source
- have a second-order LC output filter
- the DC output voltage is directly proportional to the
amplitude of the input voltage
- the peak-to-peak and RMS through the filter capacitor is
relatively low
- the conduction loss in the ESR of the filter capacitor is low
High frequency integrated
transformer
12
High frequency integrated
transformer
The transformer turn ratio:
nt =
The real transformer turn ratio:
N1
N2
n=k
L primary
Lsec ondary
‘k’ is the transformer coupling ratio.
LL 2 =
LL1
2
nt
nt = n
Lm + Lr
= n 1+ λ
Lm
Equivalent load resistance
Transformation the load
resistance to the primary
side of transformer
N
Rac = RL n 2 = RL  p
 Ns



2
Equivalent load resistance
The half-wave rectifier:
Rac =
2n 2 RL
π2
The center-tapped
transformer and the
bridge rectifier:
Rac =
8n 2 RL
π2
13
LLC design procedure
The design procedure of LLC converter was taken from STMicroelectronics,
„AN2450 - LLC resonant half-bridge converter design
guideline”,STMicroelectronics Application Note, 2007 [8].
Design specification:
– Input voltage range: Vdc.min - Vdc.max
– Nominal input voltage: Vdc.nom
– Regulated output voltage: Vout
– Maximum output power: Pout
– Resonant frequency: fr
– Maximum operating frequency: fmax
LLC design procedure
Additional info:
– Parasitic capacitance of the MOSFETs half-bridge: Czvs
– Dead time of driving circuit: TD
General criteria for the design:
– The converter will be designed to work at resonance at nominal input voltage.
– The converter must be able to regulate down to zero load at maximum input
voltage.
– The converter will always work in ZVS in the whole operating range.
LLC design procedure
The converter circuit
14
LLC design procedure
Step 1 - to fulfil the first criterion, impose that the required gain at nominal input
voltage equals unity and calculate the transformer turn ratio:
M nom = 2n
Vout
1 VDC , nom
=1⇒ n =
VDC ,nom
2 Vout
Step 2 - calculate the max. and min. required gain at the extreme
values of the input voltage range:
M max = 2n
Vout
VDC ,min
M min = 2n
Vout
VDC , max
LLC design procedure
*Taken from „AN2450 - LLC
resonant halfbridge converter
design
guidline”,STMicroelectronics
Application Note, 2007 [8].
LLC design procedure
Step 3 - calculate the maximum normalized operating frequency
(according to the definition):
f n ,max =
f max
fr
Step 4 - calculate the effective load resistance reflected at transformer primary
side:
2
Rac =
8
π2
n 2 RL =
8
π2
n2
Vout
Pout
Step 5 - impose that the converter operates at maximum frequency at zero
load and maximum input voltage, calculating the inductance ratio
λ=
1 − M min f n, max
M min f n ,max 2 −1
2
15
LLC design procedure
Step 6 - calculate the max Q value to work in the ZVS operating region at
minimum input voltage and full load condition
QZVS 1 = 95% ⋅ Qmax = 0.95
λ
1
M max
λ
2
+
M max
2
M max − 1
Step 7 - calculate the max Q value to work in the ZVS operating region at
no-load condition and maximum input voltage
QZVS 2 =
λf n, max
2
TD
π (λ + 1) f n ,max 2 − λ Rac CZVS
C ZVS = 2COSS + C stray
Step 8 - choose the max quality factor for ZVS in the whole operating range,
such that:
QZVS ≤ min{QZVS 1 , QZVS 2 }
LLC design procedure
Step 9 - calculate the minimum operating frequency at full load and minimum
input voltage, according to the following approximate formula:
f min = f r
1


1
1
1 + 1 −
4
Q

λ
1+  ZVS 
 M max  Qmax 





Step 10 - calculate the characteristic impedance of the resonant tank and all
component values
Z 0 = QZVS Rac Cr =
1
2πf r Z 0
Lr =
Z0
2πf r
Lm =
Lr
λ
LLC design procedure
Step 11 - calculate the transformer parameters
L p (SO ) = Lr + Lm
primary inductance (with secondary windings open)
L p (SS ) = Lr
primary inductance (with secondary windings shorted)
nt = n 1 + λ
transformer turn ratio
Next, choose a core with an appropriate AL value.
Np =
L p ( SO )
AL
Ns =
Np
nt
Find experimentally the core gap (with secondary winding shorted) to satisfy
appropriate Lr value.
16
MOSFETs protection
RC Snubbers
MOSFETs protection
MOSFETs protection
RC snubber designing
Step 1 – you have to know parasitic L or parasitic C of the
MOSFET half bridge. Calculate characteristic impedance of
resonant circuits:
If we know L
Z = 2πfL
If we know C
Z=
1
2πfC
f is the ringing frequency
We assume that the initial value of the snubber resistor R = Z.
17
MOSFETs protection
Then we can calculate value of the snubber capacitor C:
C=
1
πfR
Power dissipation of the resistor is given by expression:
P = CV 2 f sw
Where V is the voltage across MOSFET when it is OFF, fsw
is the converter switching frequency.
MOSFET drivers
The MOSFET drivers have following features:
- driving high capacitive load
- supply MOSFET gate with high current
- low propagation delay
- low rise and fall times
- low output impedance
MOSFET drivers
*Taken from „AN-978 HV floating MOS-Gate driver Ics”, International Rectifier Application Note, [6].
18
MOSFET drivers
Supplying the high-side driver by bootstrap capacitor.
Resonant converters controllers
The resonant converter controllers features:
- variable frequency control of resonant half or fullbridge
- high accuracy oscillator
- converter protection functions: frequency shift and
latched shutdown
- Interface with PFC controller
- Latched disable input
- Burst-mode operation at light load
- Non-linear soft-start for monotonic output voltage rise
Resonant converters controllers
*Taken from „L6599 High-Voltage resonant controller,STMicroelectronics Data Sheet, 2006, [15].
19
Resonant converters controllers
*Taken from „L6599 High-Voltage resonant controller,STMicroelectronics Data Sheet, 2006, [15].
Resonant converters controllers
*Taken from „L6599 High-Voltage resonant controller,STMicroelectronics Data Sheet, 2006, [15].
Resonant converters controllers
*Taken from „L6599 High-Voltage resonant controller,STMicroelectronics Data Sheet, 2006, [15].
20
Resonant converters controllers
Burst mode
*Taken from „L6599 High-Voltage resonant controller,STMicroelectronics Data Sheet, 2006, [15].
Resonant converters controllers
Soft start
*Taken from „L6599 High-Voltage resonant controller,STMicroelectronics Data Sheet, 2006, [15].
High Power MOSFETs
*Taken from „ IXFN60N80P
PolarHV Power HiperFET
MOSFET, IXYS Corporation Data
Sheet, 2006, [14].
21
High Power, Fast Switching
Schottky Diodes
*Taken from „C2D05120-Silicon
Carbide Schottky Diode, Cree
Data Sheet, 2006, [10].
Summary
22