This chapter discusses resonant DC/DC converters. Hard-switching converters experience high switching losses which reduce efficiency. Resonant converters reduce switching losses by forcing zero-current or zero-voltage switching through resonant circuits. This is accomplished by adding inductors and capacitors to shape current and voltage waveforms. While more efficient, resonant converters have more complex circuitry than hard-switching converters.

1. Chapter 3 – Resonant DC/DC Converters
1
Advanced Power Electronics (EE4007A/B/D/4211 )
04-10-2019

2. Outline
2
Chapter 3 – Resonant-mode DC/DC Converters
• Problems of hard-switching converters
• Solutions
 Snubber circuits
 Soft-switching resonant converters
• Purpose
 Reduce switching loss

3. 3
Conduction
loss
Input Power Output Power
Power Losses in Switched Mode Power Converters
Off-leakage
loss
Switching
loss
Other
loss
High power losses mean that:
• Low efficiency
• High temperature
• Large cooling systems
• Higher costs
Chapter 3 – Resonant-mode DC/DC Converters

4. 4
 Conduction loss occurs when the conductors and semiconductor
devices are conducting such as wires, inductors, MOSFETs and diodes.
 Conductors and semiconductor devices have conduction loss mainly
due to two reasons: either they are resistive or they produce forward
voltage when they carry current flow
Type 1: Forward Voltage
• Diodes
• IGBTs
• BJTs
• Thyristors
Conduction Loss
Chapter 3 – Resonant-mode DC/DC Converters

5. 5
Type 2: Resistive
• MOSFETs
• Inductors
• Capacitors
Capacitor
Conduction Loss
Chapter 3 – Resonant-mode DC/DC Converters

6. Switching Loss
6
 Produced when transistor switched ON and OFF
 Overlapping of V & I
 Junction Capacitors of transistor
 Increase when fS increases
 A cause of limitation of switching frequency
 Occurred in hard-switching power electronic circuits
Chapter 3 – Resonant-mode DC/DC Converters

7. Switching Loss
7
 Switching loss from VI overlapping
Every time when the transistor of a power converter is switched on or off,
the voltage or the current cannot drop down to zero immediately. The
overlapping of voltage and current waveforms generates power loss.
voltage and current waveforms
Switching loss = V×I
 Turn-on loss and turn-off loss
Features of this loss:
 increases with switching frequency
Chapter 3 – Resonant-mode DC/DC Converters

8. Switching Loss
8
 Switching loss from Junction Capacitors
As introduced previously, junction capacitors exist between pairs of
junctions of semiconductor devices.
If the voltage of the transistor is VT when the transistor is OFF and the switching frequency is fS,
the switching loss due to the junction capacitors is:
ON: The junction capacitor releases the energy through the transistor.
Coss is equivalent junction capacitor of the switch
Drain
Source
Gate
OFF: The junction capacitor stores energy
As a result, power is dissipated on the transistor during ON state
2
_
1
2

SW C oss T s
P C V f
 
oss gd ds
C C C
Chapter 3 – Resonant-mode DC/DC Converters

9. 9
Off-state leakage loss occurs because of the leakage current of devices when the
devices are off or reverse biased.
Let Ilk is the leakage current, Plk is the off-state leakage loss, D is the duty ratio of
the gate signal and VT_pk is the peak voltage of the device, the off-state leakage loss
of the device is
Leakage current of semiconductor devices are usually very low. So the off-state
leakage loss is very small as well.
leakage current
Chapter 3 – Resonant-mode DC/DC Converters

10. 10
 Additional power for accessories of the converter such as gate drive
circuits and controllers
 Additional power for Fans or Ventilators
 Additional power for Indicators or display
Chapter 3 – Resonant-mode DC/DC Converters

11. 11
Total power loss = Off-state leakage loss + Conduction loss +
Switching loss + Other losses
Chapter 3 – Resonant-mode DC/DC Converters

12. Transient Voltage and Current of Transistors
12
 High transient voltage and current on switching devices in practice
 All conductors containing parasitic inductance, Lp
 Resonance of junction C & parasitic L /
leakage L of transformers
 Difficult to estimate
 Higher V and Ipeak ratings
 Higher EMI
Chapter 3 – Resonant-mode DC/DC Converters

13. Switching Noise of Hard-switching Converters
13
 Rapid change of V or I resulting EMI
 EMI produced while switching
 Switching noise
 Twice in one TS per device
 Transient V and I increasing switching noise
 Affecting output voltage
 Affecting signals
Electromagnetic interference (EMI) is a disturbance generated by an
external source that affects an electrical circuit by electromagnetic induction,
electrostatic coupling, or conduction. The disturbance may degrade the
performance of the circuit or even stop it from functioning.
Chapter 3 – Resonant-mode DC/DC Converters

14. Snubber Circuits
14
 High transient voltage of devices
 Resonance of parasitic inductor and
junction capacitor
 Over-voltage rating
 High transient current of devices
 High EMI
 High conduction loss
 RLD turn-on snubber circuits
 RCD turn-off snubber circuits
 limiting device transient voltage during switch-off to reduce voltage stress
 limiting the rate of change of voltage during switch-off to reduce EMI
 limiting device transient current during switch-on to reduce current stress
 limiting the rate of change of current during switch-on to reduce EMI
RLD turn-on RCD turn-off
Chapter 3 – Resonant-mode DC/DC Converters

15. RCD Turn-off Snubber Circuits
15
 Without turn-off snubber
 Resonance of Lp and Coss
 High transient voltage
 Swinging voltage
 High dv/dt
 High EMI
 High VI overlapping area
Chapter 3 – Resonant-mode DC/DC Converters

16. RCD Turn-off Snubber Circuits
16
 With turn-off snubber
 Lower transient voltage
 Resonance damped by RS
 Lower EMI
 Lower VI over-lapping area
 Switching loss from CS
 Conduction loss from RS
 Very low and negligible
Chapter 3 – Resonant-mode DC/DC Converters

17. Snubber Circuits
17
 Although snubber circuits reduce the area of voltage and current
overlapping of a transistor, they reduce the efficiency of the power
electronic circuits unless the switching frequency is very low
 Actually, turn-off snubber circuits are very commonly used in products
while turn-on snubber is not popular because the inductance of a turn-
on snubber circuit may cause high transient voltage of the switching
devices.
Chapter 3 – Resonant-mode DC/DC Converters

18. Resonant Converters
18
 Features of Resonant Converters
 Using resonance of L and C to force transistor in zero-current
switching and / or zero-voltage switching
 Soft-switching
 Very low switching loss
 Low EMI
 High switching frequency
 Complicated structure
 High cost
Chapter 3 – Resonant-mode DC/DC Converters

19. LC Natural Resonance
19
 Occurring when L and C existing in a system
 Like pulling and pushing between L and C
 
0
0
sin
1 cos




  

in
Lr
cr in
V
i t
Z
v V t



 



 


Lr
r in cr
cr
r Lr
di
L V v
dt
dv
C i
dt
Chapter 3 – Resonant-mode DC/DC Converters

20. LC Natural Resonance
20
 Starting at t0
• Both iLr & vCr are 0
• Lr and Cr charged
 vCr = Vin at t1
• Lr starts discharging
• Cr keeps being charged
Chapter 3 – Resonant-mode DC/DC Converters

21. 21
LC Natural Resonance
 iLr = 0 and vCr at peak at t2
• C stops being charged
• iLr starts going to negative
• C starts discharging
 vCr = Vin at t3
• vLr = 0
• Lr starts being charged
• Cr keeps discharging
 iLr & vCr = 0 at t4
• End of one resonant period
Chapter 3 – Resonant-mode DC/DC Converters

22. 22
LC Natural Resonance
 Angular resonant frequency
 Resonant impedance
0
0
2 1
 
r r
T L C


0  r
r
L
Z
C
0
 
in
r r in
V
I V V
Z
What happen if additional Lr and Cr
are added to the switching devices to
produce natural resonance?
Chapter 3 – Resonant-mode DC/DC Converters
Re-shape the current or voltage
waveforms of the switching devices

23. Resonant Switches
23
 Resonant switch was promoted in mid 1980s by Fred C. Lee to reduce the
switching loss
 Applied in any classical hard-switching converters
• Buck, boost, buck-boost, flyback, forward, etc
• Replacing transistor of hard-switching converters
• Modifying to be basic quasi-resonant converters
• ZCS or ZVS
• Half-wave mode and full-wave mode
• Switching loss and EMI
 Hard-switching is like the operation of a water
tap. When you turn it off, there are still some
water drops (current) coming out. These
water drops (switching loss) are wasted.
Chapter 3 – Resonant-mode DC/DC Converters

24. Zero-current Resonant Switches
24
Reforming the shape of the current of Lr while Lr is in series with the transistor,
T. Transistor can be switched off when the transistor current is zero in a specific
period of time. There is no overlapping of voltage and current of the transistor
when the transistor is switched off. The transistor current increases gradually
when it is switched on because of Lr. Overlapping of voltage and current is very
low when the transistor is switched on.
Chapter 3 – Resonant-mode DC/DC Converters

25. 25
Zero-voltage Resonant Switches
When the switch is turned off, considerable time is required to charge Cr from
zero voltage and hence the voltage across the switch is not increased
instantaneously. The overlapping between the current and voltage of the switch
is therefore very small.
Chapter 3 – Resonant-mode DC/DC Converters

26. Switching trajectory
26
 For observing of power loss of transistors in oscilloscopes
 Area enclosed by the curve and the coordinate axes representing
power loss
Chapter 3 – Resonant-mode DC/DC Converters

27. Types of Resonant Techniques
27
 Quasi-resonant
 Incomplete resonant cycle
 ZCS QR converters
 ZVS QR converters
 Multi-resonant converters
 Load resonant
 Load is part of resonant circuit
 Series resonant converters
 Parallel resonant converters
 Series-parallel resonant converters
 Resonant transition
 Resonate in transition and back to normal
 Phase-shift resonant converters
 Extended-period QR converters
 Resonant transition converters
Chapter 3 – Resonant-mode DC/DC Converters

28. Pros and Cons of Resonant Converters
28
 Advantages
 High efficiency
 Very low switching loss
 Over 90% efficiency
 Able to be light and small
 Able to operate with high fS
 Up to Several hundred kHz
 Size of L and C reduced
 Low EMI
 Rate of change of V and I reduced
 Suitable for noise sensitive systems
 Disadvantages
 More components
o Complicated structure
o Higher costs
 Complicated control
o Frequency control for quasi-resonant
converters
o Usually more than one switch for
PWM resonant converters
 Usually high V rating for ZCS
 Usually high I rating for ZVS
Chapter 3 – Resonant-mode DC/DC Converters

29. 29
ZCS quasi-resonant buck converters
Ensure zero current flowing through the switch during ON and OFF by
using resonance.
 Assumption for Analysis
 Value of main inductor assumed as very high
• Assumed as a current source
 Value of main capacitor assumed as very high
• O/P voltage assumed as constant
Chapter 3 – Resonant-mode DC/DC Converters

30. 30
ZCS quasi-resonant buck converters
 State I (Linear) [t0 – t1]  State II (Resonant) [t1 – t2]
 State III (Recovering) [t2 – t3]  State IV (Free Wheeling) [t3 – t4]
Chapter 3 – Resonant-mode DC/DC Converters

31. 31
ZCS quasi-resonant buck converters
Duration of State I:
Boundary Condition at t1
 State I (Linear) [t0 – t1] ON OFF
When switch is turned on at t0, diode
DF is still conducting the load current
Io through LF in the previous stage.

32. 32
ZCS quasi-resonant buck converters
Angular Resonant Frequency
Resonant Impedance
 State II (Resonant) [t1 – t2] ON OFF
Diode DF stops conducting.
Lr and Cr starts to
resonant.

33. 33
ZCS quasi-resonant buck converters
 State II (Resonant) [t1 – t2]
Boundary Condition at t2
Half-wave
Full-wave
at t2
ON OFF
0 2 1
( )
 
set t t
 
Switch can be
turned off after t2

34. 34
ZCS quasi-resonant buck converters
Boundary Condition at t3
 State III (Recovering) [t2 – t3]
Duration of State III
ON OFF
Resonant stops. Cr begins to be
discharged through LF with a
discharging current equal to Io

35. 35
ZCS quasi-resonant buck converters
Duration of State IV
 State IV (Free Wheeling) [t3 – t4] ON OFF
DF is conducting, output current
freewheels through DF.

36. 36
Zero current switching(ZCS) Vs hard switching
ON OFF
ON OFF
Focus on the switch voltage and
current during ON or OFF instants
Chapter 3 – Resonant-mode DC/DC Converters

37. 37
ZCS quasi-resonant buck converters
Half-wave Full-wave
Chapter 3 – Resonant-mode DC/DC Converters

38. 38
Condition of ZCS quasi-resonant buck converters
The condition for zero-current switching is that iLr must reach zero so that the
switch can be turned off during this time. Therefore the condition for zero-
current switching is
Chapter 3 – Resonant-mode DC/DC Converters

39. 39
Voltage Conversion Ratio of ZCS quasi-
resonant buck converters
 Derived by equating Input and output
energy
ON OFF

40. 40
 Load dependent for half-wave mode
 Load independent for full-wave mode
 Maximum O/P power limited for ZCS
Voltage Conversion Ratio of Zero-current
switching quasi-resonant buck converters
Half-wave Mode Full-wave Mode
Chapter 3 – Resonant-mode DC/DC Converters

41. 41
 0 < Φ < 1
 Function very close to π for full wave
mode
 Implying load independent for full-wave
mode
Voltage Conversion Ratio of ZCS quasi-
resonant buck converters
Chapter 3 – Resonant-mode DC/DC Converters

42. Zero-voltage Switching (ZVS) Quasi-resonant
Converters
42
Chapter 3 – Resonant-mode DC/DC Converters

43. ZVS Quasi-resonant Converters
43
 State I (Linear) [t0 – t1]  State II (Resonant) [t1 – t2]
 State III (Recovering) [t2 – t3]  State IV (Free Wheeling) [t3 – t4]
Chapter 3 – Resonant-mode DC/DC Converters

44. ZVS Quasi-resonant Converters
44
Duration of State I:
Boundary Condition at t1
 State I (Linear) [t0 – t1] ON
OFF
Cr begins to be charged

45. ZVS Quasi-resonant Converters
45
Angular Resonant Frequency
Resonant Impedance
 State II (Resonant) [t1 – t2] ON
OFF
When Cr increases to Vin, voltage
across DF becomes positive
Chapter 3 – Resonant-mode DC/DC Converters

46. ZVS Quasi-resonant Converters
46
 State II (Resonant) [t1 – t2] ON
OFF
Boundary condition at t2
Half-wave
Full-wave
Chapter 3 – Resonant-mode DC/DC Converters

47. ZVS Quasi-resonant Converters
47
Boundary Condition at t3
Duration of State III:
 State III (Recovering) [t2 – t3] ON
OFF
Resonance stops, Lr begins to be charged
by Vin. This state finishes when iLr reaches
Io, then DF on longer conducts
Switch can be
turned on after t2
Chapter 3 – Resonant-mode DC/DC Converters

48. ZVS Quasi-resonant Converters
48
 State IV (Free Wheeling) [t3 – t4] ON
OFF
Output current Io freewheels through Lr and
switch SW.
Chapter 3 – Resonant-mode DC/DC Converters

49. Condition of zero-voltage switching of ZVS
Quasi-resonant Converters
49
The condition for zero-voltage switching is that vCr must reach zero
so that the switch can be turned on during this time. Therefore the
condition for zero-current switching is
Chapter 3 – Resonant-mode DC/DC Converters

50. Voltage conversion ratio of ZVS Quasi-
resonant Converters
50
 Derived by equating Input and output
energy
ON
OFF
Chapter 3 – Resonant-mode DC/DC Converters

51. 51
Voltage conversion ratio of ZVS Quasi-
resonant Converters
Half-wave Mode Full-wave Mode
Chapter 3 – Resonant-mode DC/DC Converters

52. Switching Loss of Full-wave Mode of ZVS
Quasi-resonant Converters
52
 Energy stored in the junction capacitor
NOT released outside
 The energy dissipated through the
transistor
 Switching loss produced
 Not ZVS for the transistor
 NOT recommended
Chapter 3 – Resonant-mode DC/DC Converters

53. Comparison of ZCS and ZVS Quasi-resonant
Converters
53
 It is noted zero-voltage switching resonant technique can help to
reduce the switching losses from both current voltage overlapping and
from junction capacitors. Zero-current switching resonant techniques
can only reduce the switching loss from current voltage overlapping
but not the switching loss from junction capacitors. Both resonant
technique can reduce EMI efficiently
 The zero-current switching is to switch the converter under zero-
current conditions. In fact, the switching device usually has a junction
capacitor. When the device is under off-state, the device behaves as a
capacitor and it will be charged with the off-state voltage. the energy
will then dissipated internally when the device is turned on. The loss
appears in every switching cycle. Therefore, its loss increases as the
switching frequency increases and the operational voltage increases.
Chapter 3 – Resonant-mode DC/DC Converters