AC-DC Converter Design Using High-Voltage Resonant Mode Controllers
L. Prasad Paruchuri & CC Cheung
怎么改时间STMicroelectronics Ltd.
Abstract
Due to higher efficiency, lower electromagnetic interference and utilization of parasitic inductance and capacitance of power stage components, Resonant Converter topologies are popular in re-arch with small real world applications.
In this paper, we will discuss LCL resonant topology for 70w notebook adapter application with power correction. It us High-voltage resonant mode controller IC, which can achieve 90% efficiency and is smaller in size. Step by step design pro-cedure for Transformer, resonant tank circuit and controller cir-cuit with experimental results will be discusd. This is to help design engineers implement their concepts in real applications with ea, to optimize cost and achieve good performance. It will also discuss ways to extend this topology to power sup-plies for CTV and MONITOR applications.
Introduction
The most popular Load Resonant converters are the Series Reso-nant, Parallel Resonant and Series-Parallel Resonant convert-ers. Series Resonant Converter is the simplest and most under-stood topology with higher part load efficiency. It has a voltage regulation problem at light loads, and requires wide frequency operation with respect to O\P load and I\P Line variations.
To solve the problems, Modified Series resonant converter is implemented by adding the additional inductor across the transformer. This Converter is referred as the LCL type Series Resonant Converter. In practical designs, we can u the opti-mum primary inductance of the transformer as the additional inductor. This can be achieved by introducing the air gap in the transformer. Leakage inductance of the transformer can be ud as Resonant inductance. This topology had Load independent characteristics for the above Resonance mode operation.
It is shown in this paper that LCL type Resonant converter requires a very narrow range of frequency control; from full load to no load, and had higher efficiency at minimum load similar to that of Series Resonant converter. Control circuit for this converter is implemented by dedicated control IC such as L6598 and L6598 using Off-line BCD technology. High side gate drive circuit was integrated, the protection features include Over voltage, Over current, Soft start features and adjustable minimum and maximum frequency limits. Secondary control circuit is implemented using TSM101 with Consta
nt voltage and Current Characteristics which is required for Note book Adapters.
Detailed calculations will also be given for power stage com-ponents like Transformer, Resonant circuit components and Control circuit components. AC-DC Converter is developed without put ratings of 18v \4.0A with PFC front end. Resonant DC-DC Power stage achieved efficiency higher than 92%. Test results are noted down.
Efficiency of the converter can be further improved by uti-lizing Synchronous Rectifier circuit on the condary circuit. This topology can be extended for Power supplies in TV and Monitor applications perforance in the applications. Power Stage Operation
Figure 1: LCL Series Resonant Converter
Figure 2: Operating Wave Forms
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Fig.1 shows the basic circuit diagram of the LCL type Series Resonant converter. The operation of this converter in resonance mode can be explained in a simple way. In Q1, rever current flows through parasitic diode and Q2 is OFF. During this pe-riod, Q1 is on and this results in negligible turn
on loss be-cau of zero voltage across Q1. When Resonant current flows through Q1, Resonant inductor Lr, Transformer primary induc-tance Lp and Resonant Capacitor Cr, will at the same time Trans-ferred to the condary. Diode D3 is conducting during this period, charging the filter capacitor C and supplying the energy to load. After some time, current through diode D reaches zero and will be turned off with less loss. At this time, Lr +Lp will resonate with Cr and no current will flow through condary.Q1 will be off, voltage across Q1 will build up by charging its output capacitance and voltage across Q2 is reducing by dis-charging its output capacitance. If we allow sufficient dead time,Q1 will turn off with less loss.
Adding extra snubber capacitors across the switches will
achieve optimum performance. When the voltage across Q1reaches to Vi, parasitic diode Q2 will turn on and the rever current will flow through Q2. Q2 will turn on and this results in negligible turn on loss. Operation during the condh a l f cycle is the same as the first half cycle.
Control Circuit Operation
The converter output voltage is regulated by varying the frequency operation of power stage. In the continuous mode, the slope of the resonant impedance curve is ud to control the output. Work-ing
with a switching frequency clo to the resonance one (Fig.3),
small variation of the frequency allows the output regulation. The current shape is clod to a sinusoidal. On the contrary, when the operation is distant from the resonance, the current moves away from sinusoidal shape. This reduces its peak and wide frequency range is necessary to control the output. Cloly Control circuit can be implemented by the dedicated control IC L6598. Q2 and Q3 will be driven by L6598 directly.长沙哪里好玩
Floating drive of Q2 is achieved by Bootstrap drive circuit of IC, which can withstand up to 600v. Fig 4 will explain the details of the dedicated control circuit. This IC has V oltage Controlled Oscillator circuit block with adjustable maximum and minimum frequency limits. By choosing the proper values of Rfmin and Rfstart, we can fix the minimum and maximum frequency limits. Minimum frequency has to be chon above the resonance frequency of power stage circuit. Soft start func-tion can be achieved by connecting a capacitor at Css pin. Start frequency is Fmax. and will gradually change to normal oper-ating frequency with specified soft start time.
Low side and High side gate drive circuits provide driving the external Power MOS or IGBT. A high sink \ source drive currents (450mA\ 250mA) ensure fast switching times. The in-
ternal logic ensures a minimum dead time to avoid cross con-
duction of the power devices. Integrated Bootstrap function c-tion replaces the external bootstrap diode, and together with a bootstrap capacitor, is ud to make available the Bootstrapped voltage to drive the high side Power MOS. This function is achieved using an HV DMOS, driven synchronous with the low side external Power MOS. For safe operation, Current flow into the Vboot pin is inhibited, even when a ZVS operation is not ensured.
Two CMOS comparators are available to perform protec-tion functions. Short puls (>200nc.) On comparators are recognized. EN1 input (active high) has a threshold of 0.6v (typi-cal) forces the IC in a latched shut down state. Nor-mal operating conditions are resumed after a power-off power-on quence. Latched Over voltage pro-tection function can be achieved using this compara-tor. EN2 input (active high) with a threshold of 1.0V (typical) restarts a soft start quence. In addition, the EN2 comparator, when activated remove a latched shutdown caud by EN1. Over power protection can be achieved using this comparator. The integrated OP AMP is designed to offer low output impedance, wide bandwidth, high input impedance and wide common mode range. It can be ud to implement a clod loop control or protection function. In this applica-tion, we u this opamp as current amplifier. This IC has Under voltage lock out ction with a start up current of 250uA and operating current of 2-4mA.Secondary control circuit is implemented using dedicated control IC. By u
sing this device, we can get Constant voltage and constant current character-istics on output that is required for Adapter applica-tions. This IC integrates precision reference of 1.25v,and two OP AMPS, and current generator. We are
using one OP AMP for voltage feedback, and cond OPAMP for current feed back.
Compensation circuit is implemented by using Two Pole Two Zero network around voltage feed back amplifier. Regula-tion of the O\P is achieved by clod feedback loop. Secondary control IC ns the O\P voltage and drives the optocoupler and control the current drawn from Rfmin. Pin. This in turn adjusts the operating frequency of the power stage and keeps the O\P voltage in regulation.
We are able to achieve very narrow operating frequency (50-60KHz) range from no load to full load with fixed input volt-Figure 3: Resonance Impedance vs. Frequency
Figure 4: HV Resonant Mode Control Circuit
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age of 380V DC.
三国演义空城计Calculation of Power Stage & Control Circuit
Input V oltage = 200V—380V DC.Output V oltage = 18V DC Output Current
= 4.0A
We choo an EE30 CORE with split bobbin for trans-former design. Area of cross ction is 0.6 Sq. cm. Maximum flux density is 0.6 Tesla.
Primary number of turns for half bridge transformer can be calculated as
Npmin = (Vin min. x 10**4)\ 2 x Fs min.x Ae x Bmax
= (200 x 10**4)\ 2 x 50x 10**3 x0.6 x0.6 = 55.5To meet the no load operation, we need to ensure the trans-former turns ratio
n =Np\Ns > (Vinmax\2) \ (V o+Vf) >(380\2) \ (18.0+0.5) >10.27
Choo Np\Ns as 13Ns = Np\ n= 55.5\13 = 4.26
Choo Ns as 4T
No.of primary turns Np= Ns x n= 4x13= 52 T.
Assume Series Resonant frequency as Fsr=45 KHZ.Assume normalid out put voltage as M=0.9.Assume normalid operating current as J=0.15
Resonant impedance Zo can be calculated as
Zo = (Vin max.\2) (Vin max.\2) x J x M \(V0 xIo)
= (380\2)(380\2)x0.15x0.9 \ (18.0 x 4.0)= 67.7 Ohm
Resonant Inductor Lr = Zo \Wo
= 67.7 \2 x3.14 x 45x 10**3= 0.239 mH
Resonant Capacitor Cr = 1\Zo x Wo
怎么美容
= 1\ 67.7 x 2 x3.14x 45 x 10**3= 52.2 nF Equivalent AC resistance refereed to primary side of transformer Ri as Ri = 8 x n x n x Rl \ (3.14 x 3.14)
= 8 x 13 x13 x 4.5 \ (3.14 x 3.14)= 617 Ohm.In LCL Resonant converter the cond resonant inductor is achieved by utilizing the proper value of Primary inductance of Transformer. To get proper no load operation and higher ef-ficiency entire load range we can choo Lp \Lr as 5So Primary inductance Lp= 1.195 mH Primary peak current through switches Ip
= 2 x Vmin \ (3.14 x Ri)= 2 x 380 \ (3.14 x 617 ) =
0.39A
Peak current of Secondary Diode Id = 3.14 x Io \ 2
= 3.14x 4.0 \2 = 6.28A
Rever peak voltage of Diode = 2 x V o=2x18=36v
components can be calculated as follows Choo Oscillator capacitor Coss = 330pF Fmin = 50KHZ
Rf min.= 1.41\ (Fmin.x Coss)
= 1.41\(50x10**3 x 330pF)=85.4 KOhm
Fmax.= 250KHZ
Rf max.= 1.41\(Fmax x Coss) =1.41\(250x 10**3 x330pF)=
17.0KOhm
Choo Css as 220 nF..
Practical Implementation
Prototype is developed for fixed I\P voltage of 380V . Trans-former was built with Np=52T Ns = 2x 4T, Naux =3T.Lp=1.1mH. Leakage inductance obtained was 220uH. So leak-age inductance is considered as Resonant inductance Lr=220uH.No additional inductance is added. STP6NB50 was chon as switches, and STPS20L45 chon as Rectifier. Two 560uF\25V ,low esr capacitors are chon as filter capacitors.
Efficiency of the converter with 72.0 w Out power is higher than 92%. Operating frequency at this condition is 53.5KHz. In put power at min. load (0.01A) is 2.9w and operating frequency
Figure 5a: Wave Forms of Iq1, Ir and Vq1 (0.01A)
Figure 5b: Waveforms Iqq1, 1r and Vq1 at min
Figure 6: 72W Adapter Schematic for Notebook Application
is 59.0 KHz. Work is under progress to improve the input power at No Load condition.
To get real application on wide range input Adapter, we in-corporated PFC stage with O\P voltage of 380V DC. The Table 1. Give the results of Prototype with wide input range. Fig 5,
will give some of the practical Waveforms.
Application schematic for Adapter is shown in Fig
6. Application for TV application with I\P voltage of
180v-264v AC and180w power can be implemented.
C onclusion
Simple design procedure for LCL type Series Reso-
nant Converter for Notebook Adapter Application is
prented. Detailed explanation of Dedicated control-
ler IC for Resonant Converter Application is also be-
ing outlined. Experimental prototype was built and
results are prented. This shows that new topology is
suitable for real world applications. No Heat sink is
林徽因与梁思成provided for Power devices which helps in reducing
the weight of the converter.Further work is going on
to improve no load Efficiency and full load efficiency by utilizing Synchronous Rectifier on the condary side. We are investigating the concept for TV Power supply application, with I\P voltage from 180-264V AC.
References
风油精的妙用
为什么低血糖1) L6598 Data sheet from STMicroelectronics.
2) Application note of L6598—U Moriconi.
3) Analysis and Design of LCL-type Series
Resonant Converter- AKS Bhat. IEEE Trans.
Indusrial Electronics - Feb.1994.
4) Class D. V oltage -Switching inverter with tapped resonant
inductor. -M.K.Kazimierczuk. IEE Proc.1993
5) A High Efficiency 150W DC\DC Converter-
Y.Furukawa,K.Morita,T.Yoshikawa -IEEE 1994.
Table 1: Test Results of Prototype Board
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