AN1660 APPLICATION NOTE
ZVS RESONANT CONVERTER FOR CONSUMER APPLICATION USING L6598 IC
by Helen Ding
In this paper we study the details of the Multi-resonant Zero-current-switching converter which has smaller size and higher efficiency, and low noise operation. It will shown the design equations and demoboard results of a high-end TV power supply using ST L6598. The small size, high efficiency, and low noise operation makes the half bridge resonant topology actrative for converters. It will shown the design equations and demoboard test results of a high-end TV power supply using ST L6598.
1
CIRCUIT DESCRIPTION
The simplified schematic and operating waveforms are shown in Fig.1 and Fig.2. Figure 1. Simplified Schematic
+BUS DRIVER-H Q1 VOUT D1 Time L-1 C1
OUT DRIVER-L Q2
L.2Mag D2 V@CRES
LOAD
C2 Time
D03IN1425
This half-bridge converter consists of switching devices Q1 & Q2, resonant inductor L1, magnetizing inductor of transformer L 2, resonant capacitors C1 & C2, transformer T, rectifier diodes D1 & D 2, output capacitor Cout. CQ1 & CQ2 are the parasitic capacitors of the Q 1 & Q2. DQ1 & DQ2 are the parasitic reverse diodes of Q1 & Q2. Switching devices Q1 & Q2 repeat on and off alternately, and the on- and off-times are the same. 50% duty cycle.
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AN1660 APPLICATION NOTE
This circuit has three operation. 1. The operation of a resonant circuit between L 1 and C1+C2. Which supplies power to the load. 2. The operation of a resonant circuit between L 1+L2 and C1+C 2. Which does not supply power with the load. 3. The operation of a resonant circuit between CQ1 + CQ2, L1+L2 and C1+C2. Which achieves the ZVS of the MOSFET. Figure 2. Operating Waveforms
VDQ1 0V
VDQ2
0V
VQ1
0V
VQ2
0V
IQ1 Current in parasitic diode (IDQ1) IL1pk I1 IL1 -I1 -I2 T Vcmax I2
0A
ICQ1
0A
0A
VC2 Vcmin ID1 t0 t1 t8 t2 t3 t4 t5 t6 t7 t8
0V
0A
D03IN1426
t 0 ~t 1 : In Q1 reverse current flows through parasitic diode D Q1. Q2 is off. The initial value of the resonant current between L1 and C1+C2 at t0 is -I2, which coincide with the current in L2. The current in L2 will increases at the rate of nVout/L2 (n = N1/N2, N2 = N3). At t0 CQ1 is discharged. The voltage becomes zero. ZVS is achieved. The voltage of C2 decreases further. C 2 is discharging. t 1 ~t 2 : Q1 is on and Q2 is off. The resonant current flows through Q1 and in the opposite direction of t0~t1. The resonant current increases sinusoidally and reaches the maximum value then deceases till coincides with the current in L2 at t2. The difference between resonant current and current in L2 flows through the primary winding N1 of the transformer. Power is supplied to the load. t 2 ~t 3 : Q1 is on and Q2 is off. The current I1 in L1 coincides with the current in L2 at t2. No current flows through the secondary winding of the transformer. In this mode L1+L2 and C1+C2 resonate.
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t 3 ~t 4 : Q1 turns off at t3. Both Q1 and Q2 are off. The charge stored in the parasitic capacitor CQ2 of Q2 is discharged by means of the resonant current between L1+L2 and C1+C2. Whereas CQ1 is charged. t 4 ~t 5 : Q1 is off. The resonant current flows through the parasitic diode DQ2 of Q2. At t4 CQ2 is discharged. The voltage becomes zero. ZVS is achieved. The voltage of C2 increases further. t5~t6: Q1 is off and Q2 is on. The resonant current flows through Q2 and in the opposite direction of t4~t5. The resonant current decreases sinusoidally and reaches the minimum value then increases till coincides with the current in L2 at t6. The difference between resonant current and current in L2 flows through the primary winding N1 of the transformer. Power is supplied to the load. t 6 ~t 7 : Q1 is off and Q2 is on. The current -I2 in L1 coincides with the current in L2 at t6. No current flows through the secondary winding of the transformer. In this mode L1+L2 and C1+C2 resonate. t 7 ~t 8 : Q2 turns off. Q1 and Q2 are off. The parasitic capacitor CQ2 of Q2 is charged by means of the resonant current between L1+L2 and C1+C2. Whereas CQ1 is discharged. Here the circuit returns to the first mode and the cycle is repeated. A main advantage of this resonant converter is that there are no turn-on switching losses exist in the FET because its inverse diode carries current and the voltage across the MOSFET is zero before the MOSFET conducts forward current. There are still turn-off switching losses. But it can be erased by placing small snubber capacitors directly across the FET devices. And no discharge resistors are needed. This was because the capacitor is not discharged by turning the FET on but rather is discharged by turning-off the opposite FET. Also the switching losses due to Coss and Crss is eliminated by the same reason mentioned before in the lossless snubbers. The energy stored in any capacitance directly across the device is returned to the DC source by virtue of the opposite FET turning off.
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AN1660 APPLICATION NOTE
2 L6598 DEVICE DESCRIPTION
The L6598 is an integrated circuit realised in BCD OFF-LINE technology. Able to drive POWER MOS or IGBT, in half bridge topology, the L6598 is provided with all the features (such as VCO, SOFT-START, OP-AMP and ENABLES) needed to implement and control properly a resonant SMPS with a minimum components count. Even though the device is able to withstand high voltage (up to 600V), it can operate at low voltage starting from its operative supply. Table 1. Device Pins Description
Pin N 1 Name C SS Function Soft Start Timing Capacitor. The capacitor CSS sets the soft start time, according to the relations: TSS = kSSCSS (typ. kSS = 0.15 s/F). During tSS the capacitor is charged by means of a current which depends on Rfstart value (i.e. on the difference between fstart and fmin). In this way TSS is always set at kSSCSS (i.e. TSS depends only by CSS). In steady state the voltage at pin 1 is 5V. Maximum Oscillation Frequency Setting. The resistance connected between this pin and ground sets the fstart value, fixing the difference between fstar t and fmin (fstar t > fmin). The voltage at this pin is fixed at VREF = 2V, and so Rfstart set the Ifstart current equal to VREF/Rfstar t. The minimum Rfstar t value which can be connected to this pin is 25 kOhm. Oscillator Frequency Setting. The capacitor CF, along with to Rfstar t and Rfmin, sets fstar t and fmin. In normal operation this pin shows a triangular wave. Minimum Oscillation Frequency Setting. The resistance connected between this pin and ground sets the fmin value. The voltage at this pin is fixed at VREF = 2V, and so Rfmin set the Ifmin current equal to VREF/Rfmin. The minimum Rfmin value which can be connected to this pin is 25 kOhm. Out of the operational amplifier. To implement a feedback control loop this pin can be connected to the Rfmin pin by means an appropriate circuitry. Inver ting Input of the operational amplifier. Non Inverting Input of the operational amplifier. Enable 1. This pin (active high), forces the device in a latched shutdown state (like in the under voltage conditions). There are two ways to resume normal operation. The first is to reduce the supply voltage below the undervoltage threshold and then increase it again until the valid supply is recognized. The second is activating EN2 input. The enable 1 is especially designed for strong fault (e.g. in case of short circuit). Enable 2. EN2 input (active high) restarts the start-up procedure (soft-start sequence). Ground Low Side Driver Output. This pin must be connected to the low side power MOSFET gate of the half bridge. A resistor connected between this pin and the power MOS gate can be used to reduce the peak current. Supply Voltage. This pin, connected to the supply filter capacitor, is internally clamped (15.6V typical). Not Connected. It ensures the insulation between the high voltage section and the low voltage one. High Side Driver Floating Reference. This pin must be connected close to the source of the high side power MOS or IGBT. High Side Driver Output. This pin must be connected to the high side power MOSFET gate of the half bridge. A resistor connected between this pin and the power MOS gate can be used to reduce the peak current. Bootstrapped Supply Voltage. Between this pin and VS must be connected the bootstrap capacitor. A patented integrated circuitry replaces the high voltage external diode. This features is achieved by means of a high voltage DMOS, synchronously driven with the low side power MOSFET.
2
Rfstar t
3 4
Cf Rfmin
5 6 7 8
OPout OPonOPon+ EN1
9 10 11
EN2 GND LVG
12 13 14 15
Vs N.C. OUT HVG
16
Vboot
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AN1660 APPLICATION NOTE
3 ANALYSIS AND DESIGN EQUATIONS
Following assumptions is for simplify the analysis. 1) All circuit components are ideal devices. 2) DT (Fig.2) is very short compared to the resonant time. DT = 0, and I1 = I2. Fig.3 is the schematic of L6598 resonant converter demoboard. Figure 3. 180W Half-Bridge resonant with the L6598: electrical schematic
Vin DC370V-420V
R2 160K/2W C8 47F VS EN1 C9 1F R5 5.6K RFSTART R6 27K
R3 10
D6 1N4148
D7 T1 STPR1010CT C19 2200F 50V
24V C20 1000F C24 100nF
R4 110K
C17 20nF HVG VBOOT C12 100nF OUT LVG R15 22 R16 10K OPIN+ R17 R18 1K C15 220pF R11 100K R10 20K R19 0.2 R14 10K L1 90H Q2 STP8NA50 C18 20nF D9 BYT03-400 TRANSFORMER R13 22 Q1 STP8NA50 D8 BYT03-400
115V C21 330F 160V C25 100nF
OPINR7 6.8K OPOUT R8 27K
C10 100nF
L6598
EN2
R21 3.6K U2 4N25
R22 1K
R23 150K
RFMIN GND
C11 1nF
R9 10K CF C13 220pF C14 0.22F CSS
R25 1K C22 C23 0.47F R24 3.3K
R12 10K C26 2200pF/3KV
C16 100nF
U3 TL431
D03IN1427
The spec. of the power supply. Input voltage: DC 370V ~ 420V (output of a PFC stage) Output voltage: 110V/1.2A , 24V/2A Output power : 180W 3.1 Resonant frequency and switching frequency Before the design, we need to decide several operation frequencies of converter. That include the start sw itching frequency - Fstart, minimum switching frequency - Fmin and resonant frequency - Fr. In this application, we choose: Fstart = 250KHz ( by design Rfstart of L6598) Fmin = 68KHz ( by design Rfmin of L6598) Fr = 84KHz ( by design L1, C17, C18) To calculate Rfstart and Rfmin, using below equations: 1. 41 R f m i n = -------------------F m in Cf 1. 41 R f s t a r t = ------------------------------------------------(Fstar t Fm i n)Cf (1) (2)
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Choose Cf = 220pF, then 1. 41 R f m i n = ----------------------------------------------------- = 94.2 k ÷ 100 k 3 12 6 8 10 220 10 1.41 R f s t a r t = -------------------------------------------------------------------------- = 32.5 k 3 12 ( 250 68 ) 10 220 10 Rfstart is 35.2Kohm , this is larger than the minimum value of Rfstart which is 25Kohm. In this application, we use R6, R7 voltage divider to get a fixed voltage to Pin6 (OPin-) as the voltage of Pin2 (Rfstart) is fixed at Vref = 2V. At the same time Pin7(OPin+) connected to primary current sense resister. Voltage of Pin7 (Vopin+) will increase while output power increase. The output of the operational amplifier will be high when Vopin+ tends to be higher than Vopin-. Then another voltage divider R8, R9 will send this high voltage to Pin9 (EN2). L6598 will stop the normal operation and restart as long as EN2 is high. Thus over-current-protection is achieved. If set R6 = 27Kohm , R7 = 6.8Kohm, then the input voltage for Pin6 is 0.4V. Now, Rfstart = R6+R7 = 33.8Kohm Rfmin = R11 = 100Kohm Recalculated Fstart & Fmin. Fmin is 64KHz, Fstart is 254KHz. The design for resonant components will be finished in the followed step. 3.2 Design transformer and resonant components First select core size. 11.1 P i n 1.31 A P = ------------------------k B F For half-bridge converter, k = 0.165 With Pout = 180W, = 94%, B = 0.4T, Fmin = 64KHz, Calculated Ap = 0.402 cm4 Choose core: EC39, with Ae = 1.32cm2 V i n ( m i n ) 10 N p m i n = ----------------------------------------------2 f s m i n B A e 3 70 10 N p m i n = -------------------------------------------------------- = 51.5 3 2 6 8 10 0. 4 1. 32 Vi a x ------n-m--------- Np 2 n = --------- ------------------------Ns1 Vo 1 + VF 1 420 --------2 n ------------------ = 1.9 1 10 + 1 ( Vo 2 + V 2 ) Ns 1 N s 2 = -----------------------F----------------Vo 1 + VF 1
4 4
(3)
(4)
(5)
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AN1660 APPLICATION NOTE
We choose Np = 52T, n = 2 Then Ns110 = 26T. This is 110V winding. The turns of 24V winding is 6. The turns of Vcc supply winding is 3. Now design the resonant components. Set normalize output voltage is M = 0.95 Set normalize output current is J = 0.2 Vi n m a x 2 ------------------ J M 2 Z o = ------------------------------------------Vo Io 420 2 --------- 0.2 0.95 2 Z o = ---------------------------------------------------- = 46.55 ( 110 1. 2 ) + ( 24 2 ) Zo 5---.---5 L r = ---------- = ------------6--5----------- = 88.2 H 3 2 fr 2 84 10 1 1 C r = ---------------------- = --------------------------------------------------- = 40.6 n F 3 2 fr Z o 2 8 4 10 46. 55 Cr = C17+C18, Choose C17 = C18 = 20nF, then Cr = 40nF Recalculate Lr under Fr = 84KHz. Lr = L1 = 90uH Choose primary inductance L2 as 5 ~ 10 times the resonant inductance. L2 = 500uH Now the resonant frequency of total circuit is 1 f o = ----------------------------------------------- = 32.5 k H z 2 (L 2 + L1 ) Cr 3.3 conditions for resonance between L2 and resonant circuit (L1, C17+C18) When the resonant between L2 and the resonant circuit starts, current IL2 varies from -I2 to +I1 during t2t1, and the voltage is nVout and constant. The operating waveforms are shown in Fig. 4 Figure 4. operating waveforms
(6)
(7)
(8)
(9)
IL1 VC2 I I1 IL2 VC min T IL1 L1C1
D03IN1428
IL1p
I
VC max
I2
VC2
IL2
IL1
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AN1660 APPLICATION NOTE
Therefore. I1 + I 2 n V o u t = L2 -------------Tr ----2 T-- = L 1 C r r T' r = --2 When assumed I1 = I2, then n V o u t T' r I 1 = -------------------------2 L2 Calculated value of I1 is 1.32A. 3.4 Maximum voltage Vcmax and minimum voltage Vcmin of resonant capacitor The voltage of the resonant capacitor varies in accordance with the charge and discharge current in both the resonant circuits between L2 and C17+C18, and between L1+L2 and C17+C18. The following equation shows the relation between the maximum value Vcmax and the minimum value Vcmin of the resonant capacitor voltage. Vc max = Vc min + 2(Vin - nVout - Vc min) + Vc (13) (12)
(10)
(11)
nVout: primary voltage of transformer Vc: charge in value of resonant capacitor voltage in the resonant circuit between L1+L2 and C 17+C18 Where Vc max + Vc min = Vin (14)
The Vcmax and Vcmin are derived from the relation with the output current. And the output current Iout is: n I O U T = ------------------T' r + T Where iL1 and iL2 are as follows:
T' r
( i L 1 i L 2 ) dt
0
(15)
Cr t i L 1 = ( V i n n V o u t V c m i n ) ----- sin ---------------- L1 L1 C r I1 + I2 i L 2 = -------------- t I 2 T' r
is the period of t0~t1. While assume T = 0, and I1 = I2
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(16)
(17)
AN1660 APPLICATION NOTE
2 n L1 C r 2 2Cr I o u t = ------------------------ ( V i n n V o u t V c m i n ) ------ I 1 T' r L1 According to this equation, the minimum voltage Vcmin is obtained as follows:
2 T' r 2 I o u t ---------------------------- + I 1 2 n L1 C r V c m i n = V i n n V o u t -------------------------------------------------------------C----r L1
(18)
(19)
The Vcmax can be calculated according to equation (14). The maximum voltage applied to the resonant capacitor tends to increase as the output current increase. Calculated Vcmax in this application is 310V at maximum output power. 3.5 Peak value Il1pk of resonant current Resonant current iL1 is calculated according to equation (16). The peak value of iL1 is Cr I L1 p k = ( V i n n V o u t V c m i n ) ----L1 Calculated IL1pk is 1.9A in this application. 3.6 Over current protection and over voltage protection We set the over current protection point at IL1pk = 2.1A. As the inverting input of operational amplifier is 0.4V, The calculated Rsense will be 0.4V/2.1A = 0.19ohm. Choose 0.2ohm/1W resistor as Rsense (R20). Then the voltage on R20 will be 0.42V at expected OCP point. By choosing the suitable voltage divider R18 and R17, we can get the 0.4V voltage to OPin+. In this application we choose R17 = 39Kohm , R18 = 1Kohm. Over voltage protection is achieved by sense the supply voltage of L6598 --Vs. R4 and R5 is a voltage divider which connected to Pin8 (EN1). When the voltage of Pin8 is higher than the threshold 0.6V, it forces the device in a latched shut down state. By choose R4 = 110Kohm and R5 = 5.6Kohm, the OVP is trigged at Vs = 12.4V. That is 10% higher than the normal operating voltage. 4 EVALUATION RESULT OF DEMOBOARD
(20)
4.1 Operating waveforms See the operating waveform in Fig.5 The measured value : I1 is 1.4A, Vcmax is 300V, IL1pk is 1.9A. That is coincide with the calculated value.
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Figure 5. Operating Waveform
Ch1: Ch3: Ch4:
VQ2 100V/div VC2 100V/div iL1 1A/div
Input voltage Vin = 420V Output 110V/1.2A 24V/2A
Figure 6.
Ch1: VQ2 100V/div Ch3: VC2 100V/div Ch4: iL1 1A/div Input voltage Vin = 380V Output 110V/1.2A 24V/2A
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4.2 Switching frequency characteristics Figure 7. switching frequency vs input voltage 4.3 Efficiency characteristics Figure 9. efficiency vs output power
Figure 8. switching frequency vs output power
Figure 10. efficiency vs input voltage
The maximum efficiency is 95% at maximum output power of 180W.
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AN1660 APPLICATION NOTE
4.4 OCP and OVP The power supply will automatically pass in restart procedure when the output power exceed a certain value. In experiment, set output current 1.8A to 110V & 2A to 24V, the voltage of pin 9 (EN2 of L6598) will be higher than 1V and the IC restarts the start-up sequence. Thus the OCP achieves. The maximum output power is slightly depend on the input voltage. In this demo board the maximum output power is around 240W. The over-current-protection will be active as well when 24V or 110V is under short circuit condition. The operating frequency tends to increase as the output load decrease. The maximum switching frequency is limited by (R10+R12)//R11. Using equation (1) we can calculate the Fmax value. It is 278KHz. When the maximum frequency is reached and the load reduced further, the output voltage will no longer be regulated and tends to increase. So does the Vs voltage of L6598. Once Vs rise to 12.2V, the over-voltageprotection will be triggered. In the demo board, the tested value is Vs = 12.2V and Vo1 = 134V, Vo2 = 29V. So to keep the output voltage regulated, a minimum output power is request. The test result can be find in the following table. Table 2.
Input Voltage (DC) Output minimum power Switching frequency Input power 370V 4W 110V/30mA 24V/30mA 70KHz 7.3W 420V 9.5W 110V/80mA 24V/30mA 240KHz 13W
The maximum switching frequency can be designed to a higher value to reduce the minimum output power by reducing the value of (R10+R12). But the Fmax has a upper limitation given by L6598. It can not exceed 350K Hz . 5 CONCLUSION
We could obtain high efficiency in this circuit. Because both ZVS, ZCS can be achieved of turn-on and ZVS can be achieved of turn-off. The major drawback of this kind of converter is the line regulation is not good. So it is better to use the converter after a PFC circuit.
6
REFERENCE
[1] A High Efficiency 150W DC/DC Converter Yasuhito Furukawa, Kouichi Morite, Taketoshi Yoshikawa [2] A Comparison of Half-Bridge Resonant Converter Topologies ROBERT L. STEIGERWALD, IEEE Power Electronics Specialists Conference Record, 1987,135-144 [3] L6598 datasheet and AN1673. [4] AN628 & AN996
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Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners 2003 STMicroelectronics - All rights reserved STMicroelectronics GROUP OF COMPANIES Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States www.st.com
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