Background Information  （背景信息）

With the push for higher efficiency converters, power supply designers are looking
for different topologies to improve their power supply efficiency.
One such topology that has been gaining interest is the resonant converter. Until
recently, resonant converters have been overlooked due to the complexity of
control. The dsPIC digital signal controller with its advanced peripherals and highspeed
DSP engine provides the capability to easily control complex systems such
as Resonant Converters.

High Efficiency

Resonant converters are becoming more and more popular these days due to their
ability to achieve high efficiency through softly commutating the switching devices.

Soft-Switching Converters

Soft-switching techniques, such as Zero-Voltage Switching and Zero-Current
Switching can be implemented which provides better EMI performance along with
higher efficiency

Higher Power Density

Resonant Converters are capable of achieving higher power density meaning the
overall converter size can be reduced as the converter can operate at higher
switching frequencies due to soft-switching described above

High Power Applications

Resonant Converters are well suited for high power applications

Wide Input Voltage Range

They work well over a wide input voltage range

Lower Cost

Lower cost – Due to their Higher switching frequencies the size of passive
components can be reduced

Concept of Resonant Converters

A resonant converter can be divided into four main block sets: the full/half Bridge
Converter, a Resonant Tank, a Rectifier, and a low-pass filter.
Starting on the input side the full/half bridge converter is typically configured in
complementary mode with a fixed duty cycle (~50%) and with some dead-time. The
bridge converter is typically operated by adjusting the duty cycle but in the case of
the resonant converter the bridge converter is frequency controlled. This means that
by changing the frequency of the converter we change the impedance of the
resonant tank.

The output of the bridge converter is a square wave with fixed duty cycle, with an
amplitude equal to Vdc and a DC offset of Vdc/2.

The resonant tank is made up of reactive components (capacitors and inductors)
and can have several different configurations. Depending on the tank configuration,
the output will have either a sinusoidal current or voltage. The resonant tank will
introduce a phase shift between the voltage and current and because of this we are
able to achieve soft-switching.

Combining the bridge converter and the resonant tank we create a resonant inverter
and by adding a rectifier and low pass filter we create a resonant converter.

看不懂哇

電電

很多朋友都說看著費勁。。。

我試著翻譯下，但不一定成功~~

What is Resonance

Resonance occurs in a circuit at a particular frequency
where the impedance between the input and output of
the circuit is at its minimum.

Before we dive any deeper into resonant converters we should first understand a
couple terms that are commonly used in describing resonant converters.
So first let us look at the meaning of resonance.
In a simple series LC circuit (shown in bottom right) there exist a point where the
reactance of the capacitor and the reactance of the inductor are equal in magnitude
but opposite in sign. At this particular frequency the reactance is zero and the
impedance between the input and the output of the circuit is at it’s minimum.
If we look at the impedance vs. frequency plot we will see that the summation of the
capacitor and inductor reactance is at it’s minimum at the resonant point.

Quality Factor

●Quality Factor (Q), of a resonant circuit is a
dimensionless parameter that describes how
damped a resonant circuit is.
●The higher the Quality Factor the more narrow the
bandwidth.
●Q is the ratio between the power stored and the
power dissipated in the circuit.

General Definition:
Q = Pstored / Pdissipated = I2X / I2R

Another term used to describe resonant converters is Quality Factor.
Quality Factor (Q) is a dimensionless parameter that describes how damped a
resonant circuit is.
The higher the Quality Factor the narrower the bandwidth of the system.
As a general definition Quality Factor can be defined as the ratio between the power
stored and the power dissipated in the circuit.
It is important to note that the Quality Factor changes with load, (i.e. it is not a fixed
parameter).

Soft Switching

Zero Voltage Switching

●At transition period from one state to another state of the
MOSFET, the voltage is zero, hence no losses
●ZVS demonstrated only at Switch turn-ON

The following animation demonstrates Zero Voltage Switching.

First the drain-to-source capacitance of the MOSFET is discharged.

Next the PWM enables the MOSFET allowing the current through the MOSFET to
begin to rise

When the PWM is disabled the drain-to-source voltage begins to rise but the current
still flows through the MOSFET.

When the PWM is disabled the drain-to-source voltage begins to rise but the current
still flows through the MOSFET.

At switch turn-on we are eliminating theMOSFET switching losses as the voltage across the drain-to-source is zero (ZVS).At switch turn-off we see that there will be a significant amount of current andvoltage at the transition state which translates into switching losses.

Zero Voltage switching is preferred in high-voltage, high-power applications.

Zero Current Switching

●At transition period from one state to another state of the
MOSFET, current is zero, hence no losses
●ZCS demonstrated only at Switch Turn-OFF

The following animation demonstrates Zero Current Switching.

First the PWM is enabled and the current through the MOSFET begins to rise as the
MOSFETs drain-to-source voltage drops.

Once the current through the MOSFET becomes zero, we disable the MOSFET.
Now the drain-to-source voltage begins to rise.

This is the point at which zero current switching occurs
From the animation we can see that at switch turn-off we are eliminating the
MOSFET switching losses as the current is zero and the drain-to-source voltage is
also zero. At switch turn-on we see that there will be a significant amount of current
and voltage at the transition state which translates into switching losses.
Zero Current switching can be implemented at switch turn-on as well as turn-off.
With soft-switching we can reduce the noise in the system, therefore improving EMI
performance.

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Background Information（背景信息）這一大章傳完了。

翻譯了一點，個人感覺翻譯時比較難，下一個帖可能會上傳一小部分翻譯。

期待更新

 Resonant Converter Topologies(諧振變換器的拓撲結構)

Next we will take a closer look at three different resonant converter topologies.

Series Resonant Converter

? Resonant tank (LR & CR) is in series with the output load
? Resonant tank and the load act as a voltage divider (M <= 1)
? SRC can work at no load but output voltage can not be regulated
? For ZVS, need to operate above resonance (negative slope)
? At low line SRC operates closer to resonant frequency

For a series resonant converter the resonant tank is composed of inductor Lr and
capacitor Cr in a series configuration (shown in schematic). It is called a series
resonant converter because the resonant tank is in series with the output load. Here
the resonant tank and the output load act as a voltage divider and therefore limiting
the gain of the tank to be less then or equal to one. Looking at the Voltage gain
curves (bottom right diagram) we see that all Q-curves intersect at the resonant
frequency point (Fn = 1, and Gain = 1). From the voltage gain plot we can also see
that the converter operates closer to resonance when the input voltage is at low line.
For Zero Voltage Switching the converter must work at or above resonance.

Parallel Resonant Converter

? Load is in parallel with resonant capacitor
? PRC can operate at no load
? For ZVS, need to operate above resonance (negative slope)
? At low line PRC operates closer to resonant frequency
? High circulating currents
? Inherently short circuit protected

For a parallel resonant converter, the resonant tank is similar to that of the series
resonant converter but now the output load is connected in parallel with the
resonant capacitor Cr. Similar to the series resonant converter, the parallel resonant
converter will operate closer to resonant frequency at low-line, and for Zero-Voltage
switching the converter should work at or above resonance. Unlike the series
resonant converter, the parallel resonant converter can operate at no load. One
disadvantage of the parallel resonant converter is that is will operate with high
circulating currents, even at no load conditions.

Series-Parallel Resonant
Converter - LLC

? Can operate at resonance at nominal input voltage
? The LLC converter is able to operate at no load
? Can be designed to operate over a wide input voltage
? Zero voltage switching is achievable over the entire operating range
? Zero current switching is achievable over the entire operating range

Series-parallel converters are a combination of the series resonant converter and
the parallel resonant converter. Here we are showing an LLC resonant converter
which is composed of two inductors and a single resonant capacitor. Another
common Series-Parallel resonant converter is the LCC converter which replaces the
inductor Lpr with another capacitor as in the parallel resonant configuration.

LLC resonant converters have many advantages:
?They can operate at resonance at nominal input voltage
?They can operate at no load conditions
?Have lower circulating currents then the parallel resonant converter

The advantages of resonant converters are their ability to operate over a wide input
voltage range and the fact that ZVS/ZCS is achievable over the entire operating
condition.

LLC Resonant Converter Operating Modes (諧振變換器的操作模式)

Next we will examine the different operating modes of an LLC Resonant Converter.

Operation @ Resonance

First we will start out by examining, in detail, the operational waveforms when the
system operates at resonant frequency. For simplicity we are showing a half-bridge
converter on the primary side and a full-wave rectifier on the secondary. Diodes
D1,D2 and Capacitors C1, C2 have been explicitly drawn here but are essentially
part of the MOSFETs parasitics.
At time t biased. The voltage drop across capacitor C1 is equal to the input voltage (Vdc).

At time t0 secondary diodes D3, D4 are reversed biased. At this time the output capacitor is
supplying the required load current.
On the primary side we see that the magnetizing current is flowing and will begin to
discharge capacitor C1 and charge capacitor C2. The magnetizing current must be
large enough to charge/discharge the capacitors before the dead-time interval ends.

At the time interval t1 on-state. Initially the magnetizing current is negative and the internal body diode of
the MOSFET will be forward biased. Diode D3 is also transitioning to forward
biased.
When MOSFET (Q1) is enabled, the drain to source voltage is zero, therefore Zero
Voltage Switching is obtained.

At time t2 interval in which power transfer takes place. The tank current is supplied by the
input voltage source and the magnetizing current is generated by the secondary
voltage reflected back to the primary by the transformer turns ratio. The magnetizing
inductor is clamped at this voltage, hence the linear rise of magnetizing current.
The sinusoidal tank current flowing in the transformer will generate a quasisinusoidal
current on the secondary side related by the transformer turns ratio. At
the end of the switching cycle the current flowing through diode D3 will be equal to
zero, hence zero current switching is achieved on the secondary.

At the time interval t3 circuit operation is the same as the previous dead-time interval except now,
capacitor C1 is being charged while capacitor C2 is discharged.
The circuit operation for the remaining time intervals will be the reverse of the first
half cycle.

Operation Below Resonance

Now that we have reviewed in detail the circuit operation at resonance, we will
review the differences in circuit behavior when the converter operates above and
below resonance.
When operating below resonance the tank fundamental sine wave will have a
shorter period then that of the switching frequency (decreasing the switching
frequency = an increase in the switching period). From the figure we can see that
the tank current will equal the magnetizing current before the half period ends (tx –
t3). From this point on the current flowing in the primary is that of the magnetizing
current.

Operation Above Resonance

The circuit behavior is somehow reversed compared to the operation below
resonance.
Since the resonant period is longer then that of the switching period, at the end of
the switching half period, the tank current is higher than the magnetizing current.
During the dead time interval the tank current falls rapidly to the value of the
magnetizing current, so that a new half cycle can start.

200W LLC Resonant Converter Reference Design (200 W諧振變換器參考設計)

Overview

Now that we have some background information on resonant converters we can
proceed with looking at Microchip’s 200W LLC Resonant Converter Reference
Design.

LLC Resonant Converter Reference
Design Specifications

●Input Range:
- 350Vdc to 420Vdc
- 400Vdc nominal
● Output Voltage:
- 12Vdc
●Output Power:
- 200W
● High Efficiency: 95%
●Resonant Frequency:
- 200KHz
● Modulated Frequency:
- 150KHz – 220KHz

The LLC Resonant Converter Reference Design is rated for 200W with a nominal
input voltage of 400V DC. This LLC reference design is intended to be the second
stage in a AC-DC application so the nominal input voltage is the typical output
voltage of a PFC converter (385 - 400V). The ability of the LLC converter to operate
over a wide input range allows the bulk capacitors on the PFC converter (which are
required for hold-up time) to be reduced.
As this design is 200W the half-bridge converter was selected for driving the
resonant tank. As for the rectifier, the traditional full-wave rectifier was replaced with
a synchronous rectifier circuit, where MOSFETs replace diodes, to improve
efficiency. High-voltage isolation (~4kV) separating the primary and secondary is
obtained through proper component selection and magnetic structures.
Use of the reference design is Royalty Free, and complete documentation,
software, and hardware design information is available on the Microchip web site.
Demonstration units are also available from worldwide Microchip sales offices.

LLC Resonant Converter Block
Diagram

This picture shows a system-level block diagram of the LLC Resonant Converter. A
single dsPIC33F “GS” series digital signal controller, shown in the center of the
block diagram, drives the power stages (Half-bridge converter and Synchronous
Rectifier), performs control loop operations, power management communication
and fault management routines.

LLC Resonant Converter
Board Layout:

Now that we have seen a functional overview of the LLC Resonant Converter, we
can physically locate all sections on a picture of the Reference Design itself.
This is a top view of the LLC Resonant Converter Reference Design. Positions of
each block of the system are highlighted as follows:

The input terminal can be seen on the bottom left and the output terminal on the top right

The Half-bridge Converter can be found towards the upper left side.

The Synchronous Rectifier is located on the top right

Flyback Auxiliary Power on the bottom Left

And lastly the dsPIC DSC is located on the bottom right of the board.

200W LLC Resonant Converter Reference Design (200 W諧振變換器參考設計)

Half-Bridge Converter / Resonant tank

Next we will look at the operation of the half-bridge converter and the resonant tank.

Half-Bridge Converter / Resonant Tank

? Half-Bridge Converter generates a square wave with amplitude = VDC
and dc offset of VDC/2
? Resonant capacitor CR blocks dc component
? Resonant tank filters higher harmonics, essentially sinusoidal current is
allowed to flow

A circuit diagram of the half-bridge converter and the resonant tank is as
shown.
Two MOSFETs are connected in a bridge configuration and the resonant
tank is connected at the Half-Bridge point. The half-bridge converter is
configured in complementary mode with a fixed duty cycle (~50%) and with
some dead-time The dead-time serves two purposes: first, it prevents shootthrough
(both MOSFETs on at the same time), secondly, it is the time
interval used to charge/discharge the MOSFETs drain-to-source capacitance
used for zero voltage switching (as seen earlier in the presentation).
Because of the high switching frequencies MOSFETs are preferred over
IGBT’s.
The resonant capacitor blocks the DC component of the square wave,
producing a signal that is centered around 0v.
The resonant tank will filter the higher harmonics essentially only allowing
sinusoidal current to flow.

Synchronous Rectifier

Next lets look at the rectifier block found on the secondary side.

Types of Rectifiers

● Three topologies to consider:
- Half-wave rectifier
- Full-wave rectifier (center-tapped)
- Bridge rectifier – High output voltage low output
current

There are three different rectifier topologies to consider: Half-wave, Full-wave, and
Bridge Rectifier. As this application has a low output voltage (12V) and high output
current the bridge rectifier is not a suitable solution.
For this reference design we have used a Full-Wave rectifier but we have replaced
the Diodes with MOSFETs. This is more commonly known as synchronous
rectification. The MOSFETs switching losses and conduction losses are less then
that of the Diodes losses, which helps improve overall efficiency. The MOSFETs
have been placed on the low-side (ground reference) to reduce component count
and complexity.
One thing to note is that now that we have added MOSFETs to the rectifier special
care must be taken to maintain Zero-Current Switching.

Flyback Auxiliary Power

Lets now look at the auxiliary flyback circuit.

Auxiliary Power Block Diagram

Here is a high-level block diagram of the auxiliary power section.
In this design we were targeting high efficiency and very low power at no/light load
operating conditions. To do this the auxiliary circuit has been designed with an auto
shut-off feature providing low stand-by power. The circuit also provides the ability to
restart the auxiliary circuit in the event of a fault condition.
Upon system start-up, the flyback converter provides power to the dsPIC. When the
dsPIC is up and running the output from the LLC converter (12V) will provide the
necessary power for the dsPIC, essentially powering itself.

Summary (概述)

Next, let us recap what we discussed on resonant converters.

● Resonant Converter Background
Information
● Different Resonant Converter Topologies
● LLC Resonant Converter Operation Modes
●  Microchip’s 200W LLC Resonant Converter

We discussed the different resonant converter topologies,
operational waveforms of a LLC resonant converter, and Microchip’s 200W LLC
Resonant Converter Reference Design.
From our discussions we saw that the LLC resonant converter is a suitable DC-DC
converter for high-power applications with it’s high efficiency, high power density,
and its ability to operate over a wide input voltage range.

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原始資料:LLC Resonant Converter Reference Design using the dsPIC? DSC 

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