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ISL6753
Data Sheet April 4, 2006 FN9182.2
ZVS Full-Bridge PWM Controller
The ISL6753 is a high-performance, low-pin-count alternative, zero-voltage switching (ZVS) full-bridge PWM controller. Like the ISL6551, it achieves ZVS operation by driving the upper bridge FETs at a fixed 50% duty cycle while the lower bridge FETS are trailing-edge modulated with adjustable resonant switching delays. Compared to the more familiar phase-shifted control method, this algorithm offers equivalent efficiency and improved overcurrent and lightload performance with less complexity in a lower pin count package. This advanced BiCMOS design features low operating current, adjustable oscillator frequency up to 2MHz, adjustable soft-start, internal over temperature protection, precision deadtime and resonant delay control, and short propagation delays. Additionally, Multi-Pulse Suppression ensures alternating output pulses at low duty cycles where pulse skipping may occur.
Features
* Adjustable Resonant Delay for ZVS Operation * Voltage- or Current-Mode Operation * 3% Current Limit Threshold * 175A Startup Current * Supply UVLO * Adjustable Deadtime Control * Adjustable Soft-Start * Adjustable Oscillator Frequency Up to 2MHz * Tight Tolerance Error Amplifier Reference Over Line, Load, and Temperature * 5MHz GBWP Error Amplifier * Adjustable Cycle-by-Cycle Peak Current Limit * Fast Current Sense to Output Delay * 70ns Leading Edge Blanking
PKG. DWG. #
Ordering Information
PART NUMBER PART MARKING TEMP. RANGE (C) PACKAGE -40 to 105
* Multi-Pulse Suppression * Buffered Oscillator Sawtooth Output * Internal Over Temperature Protection * Pb-Free Plus Anneal Available and ELV, WEEE, RoHS Compliant
ISL6753AAZA ISL6753AAZ (See Note)
16 Ld QSOP M16.15A (Pb-free)
Add -T suffix to part number for tape and reel packaging NOTE: Intersil Pb-free plus anneal products employ special Pb-free material sets; molding compounds/die attach materials and 100% matte tin plate termination finish, which are RoHS compliant and compatible with both SnPb and Pb-free soldering operations. Intersil Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020.
Applications
* ZVS Full-Bridge Converters * Telecom and Datacom Power * Wireless Base Station Power * File Server Power
Pinout
ISL6753 (QSOP) TOP VIEW
VERR 1 CTBUF 2 RTD 3 RESDEL 4 CT 5 FB 6 RAMP 7 CS 8 16 VREF 15 SS 14 VDD 13 OUTLL 12 OUTLR 11 OUTUL 10 OUTUR 9 GND
* Industrial Power Systems
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2005, 2006. All Rights Reserved All other trademarks mentioned are the property of their respective owners.
Functional Block Diagram
VDD VDD VREF UVLO 50% OUTUR PWM STEERING LOGIC OUTLL PWM OUTLR OUTUL
OVERTEMPERATURE PROTECTION
2
GND + VREF OVER CURRENT COMPARATOR 1.00V 70 nS LEADING EDGE BLANKING 80mV + CT OSCILLATOR PWM COMPARATOR 0.33 VREF 1 mA RAMP CS RESDEL
ISL6753
RTD
VREF
CTBUF + 0.6V
VERR
SS
SOFTSTART CONTROL
FB
FN9182.2 April 4, 2006
Typical Application - High Voltage Input ZVS Full-Bridge Converter
P1 VIN+
Q1 FQB6N50 4 3 R13 10.0k 1 2 R14 4.99 0805 CR3 SS12
T3 P0544 6 Q5 BSS138LT1 3 1 2 8 CR4 SS12 R15 4.99 0805 R16 10.0k
Q2 FQB6N50 C17 100pF 250V COG R18 10 5% 2512
Q6 BSS138LT1
5
3
+
+ C1-C4 33uF 450V + + R1 4.7k 5% 2512
T1 2, 3
15, 16 2 Ns 1,C
1
300 - 400 VDC
7,8 1
C13 0.1uF 2 4 2 4
7,8 1 Q9 ZXTDB2M832
Np
13, 14 11, 12 Ns
CR5 CSD10060G CR6 CSD10060G L1 PB2020.103
3
FN9182.2 April 4, 2006
P4 RETURN
R2 4.7k 5% 2512 R3 4.7k 5% 2512
Q10 ZXTDB2M832
6, 7 9, 10 2 1,C C18 1uF 100V 1210 + C19 1uF 100V 1210 C20 470uF 63V + C21 470uF 63V R28 10.0k 2512 P3 + Vout (48V@10A)
3 5,6 Q4 FQB6N50 7,8 1
3 5,6
7,8 1 C12 1.0uF 3 Q8 ZXTDB2M832 R17 10.0 C16 100pF 250V COG C23 4700pF 250VAC SAFETY R19 10 2512
2 4
2 4
Q3 FQB6N50
3 5,6
C22 4700pF 250VAC SAFETY
P2 VIN8 7
5,6
Q7 ZXTDB2M832
ISL6753
2
3
3 T2 P8205
1 R11 3.65k VERR CTBUF RTD R5 100k 1206 R8 45.3k
U1 ISL6753 VREF SS VDD OUTLL OUTLR OUTUL OUTUR GND 4 U2 PS2701-1P 1 R20 499 2 C14 4.7nF R12 20.0k 3 2 R24 100k R22 3.74k 1206 CR7 BAT54 3 1 R21 3.74k 1206 R25 37.4k 0805 C15 220pF
R27 10.0k 0805 R26 10.0k 0805
1 CR1 BAV70
RESDEL CT FB1 RAMP CS
R4 4.7k 5% 2512 R6 5.11k 2 Q11 MJD50 3 BIAS 1 1 CR2 BAT54C R9 499
3 R29 20.0k R30 20.0k VR2 BZX84-C6V8 1 U3
1 2 3 R23 1.10k
2 C5 0.1uF
3 3
R7A, B 18.7 0805
C6 180pF 5% COG C7 47pF C8 1.0nF C9 0.47uF C10 0.1uF
VR1 BZX84-C12
1
R10 10.0k
C11 0.1uF
ISL6753
Absolute Maximum Ratings
Supply Voltage, VDD . . . . . . . . . . . . . . . . . . . GND - 0.3V to +20.0V OUTxxx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GND - 0.3V to VDD Signal Pins . . . . . . . . . . . . . . . . . . . . . . . GND - 0.3V to VREF + 0.3V VREF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . GND - 0.3V to 6.0V Peak GATE Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1A ESD Classification Human Body Model (Per MIL-STD-883 Method 3015.7) . . .3000V Charged Device Model (Per EOS/ESD DS5.3, 4/14/93) . . .1000V
Thermal Information
Thermal Resistance (Typical) JA (C/W) 16 Lead QSOP (Note 1). . . . . . . . . . . . . . . . . . . . . . 95 Maximum Junction Temperature . . . . . . . . . . . . . . . . -55C to 150C Maximum Storage Temperature Range . . . . . . . . . . . -65C to 150C Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . . 300C (QSOP- Lead Tips Only)
Operating Conditions
Temperature Range ISL6753AAxx . . . . . . . . . . . . . . . . . . . . . . . . . . .-40C to 105C Supply Voltage Range (Typical). . . . . . . . . . . . . . . . . . . . 9-16 VDC
CAUTION: Stresses above those listed in "Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
NOTES: 1. JA is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details. 2. All voltages are with respect to GND.
Electrical Specifications
Recommended operating conditions unless otherwise noted. Refer to Block Diagram and Typical Application schematic. 9V < VDD < 20V, RTD = 10.0k, CT = 470pF, TA = -40C to 105C (Note 3), Typical values are at TA = 25C TEST CONDITIONS MIN TYP MAX UNITS
PARAMETER SUPPLY VOLTAGE Supply Voltage Start-Up Current, IDD Operating Current, IDD UVLO START Threshold UVLO STOP Threshold Hysteresis REFERENCE VOLTAGE Overall Accuracy Long Term Stability Operational Current (source) Operational Current (sink) Current Limit CURRENT SENSE Current Limit Threshold CS to OUT Delay Leading Edge Blanking (LEB) Duration CS to OUT Delay + LEB CS Sink Current Device Impedance Input Bias Current RAMP RAMP Sink Current Device Impedance RAMP to PWM Comparator Offset
VDD = 5.0V RLOAD, COUT = 0 8.00 6.50 -
175 11.0 8.75 7.00 1.75
20 400 15.5 9.00 7.50 -
A mA V V V
IVREF = 0 - -10mA TA = 125C, 1000 hours (Note 4)
4.850 -10 5
5.000 3 -
5.150 -100
V mV mA mA mA
VREF = 4.85V
-15
VERR = VREF Excl. LEB (Note 4) (Note 4) TA = 25C VCS = 1.1V VCS = 0.3V
0.97 50 -1.0
1.00 35 70 -
1.03 50 100 130 20 1.0
V ns ns ns A
VRAMP = 1.1V TA = 25C
65
80
20 95
mV
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ISL6753
Electrical Specifications
Recommended operating conditions unless otherwise noted. Refer to Block Diagram and Typical Application schematic. 9V < VDD < 20V, RTD = 10.0k, CT = 470pF, TA = -40C to 105C (Note 3), Typical values are at TA = 25C (Continued) TEST CONDITIONS VRAMP = 0.3V (Note 4) MIN -5.0 6.5 TYP MAX -2.0 8.0 UNITS A V
PARAMETER Bias Current Clamp Voltage PULSE WIDTH MODULATOR Minimum Duty Cycle Maximum Duty Cycle (per half-cycle)
VERR < 0.6V VERR = 4.20V, VRAMP = 0V, VCS = 0V (Note 5) RTD = 2.00k, CT = 220pF RTD = 2.00k, CT = 470pF
0.85
94 97 99 0.8 0.33 -
0 1.20 0.9 0.35 VSS
% % % % V V V/V V
Zero Duty Cycle VERR Voltage VERR to PWM Comparator Input Offset VERR to PWM Comparator Input Gain Common Mode (CM) Input Range ERROR AMPLIFIER Input Common Mode (CM) Range GBWP VERR VOL VERR VOH VERR Pull-Up Current Source EA Reference EA Reference + EA Input Offset Voltage OSCILLATOR Frequency Accuracy, Overall (Note 4) (Note 4) (Note 4) ILOAD = 2mA ILOAD = 0mA VERR = 2.5V TA = 25C (Note 4) TA = 25C
0.7 0.31 0
0 5 4.20 0.8 0.594 0.590
1.0 0.600 0.600
VREF 0.4 1.3 0.606 0.612
V MHz V V mA V V
165 -10
183 0.3 4.5 1.5 -200 20 0.80 2.80 2.00 2.00 2.0 0.40 -
201 +10 1.7 -207 23 0.88 2.88 2.05 2.03 2 2.05 0.44 0.10
kHz % % % % A A/A V V V V V V/V V V
Frequency Variation with VDD Temperature Stability
TA = 25C, (F20V- - F10V)/F10V VDD = 10V, |F-40C - F0C|/F0C |F0C - F105C|/F25C (Note 4)
-193 19
Charge Current Discharge Current Gain CT Valley Voltage CT Peak Voltage CT Pk-Pk Voltage RTD Voltage RESDEL Voltage Range CTBUF Gain (VCTBUFp-p/VCTp-p) CTBUF Offset from GND CTBUF VOH
TA = 25C
Static Threshold Static Threshold Static Value
0.75 2.75 1.92 1.97 0
VCT = 0.8V, 2.6V VCT = 0.8V V(ILOAD = 0mA, ILOAD = -2mA), VCT = 2.6V
1.95 0.34 -
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FN9182.2 April 4, 2006
ISL6753
Electrical Specifications
Recommended operating conditions unless otherwise noted. Refer to Block Diagram and Typical Application schematic. 9V < VDD < 20V, RTD = 10.0k, CT = 470pF, TA = -40C to 105C (Note 3), Typical values are at TA = 25C (Continued) TEST CONDITIONS V(ILOAD = 2mA, ILOAD = 0mA), VCT = 0.8V MIN TYP MAX 0.10 UNITS V
PARAMETER CTBUF VOL SOFT-START Charging Current SS Clamp Voltage SS Discharge Current Reset Threshold Voltage OUTPUTS High Level Output Voltage (VOH) Low Level Output Voltage (VOL) Rise Time Fall Time UVLO Output Voltage Clamp THERMAL PROTECTION Thermal Shutdown Thermal Shutdown Clear Hysteresis, Internal Protection NOTES:
SS = 3V
-60 4.410
-70 4.500 0.27
-80 4.590 0.33
A V mA V
SS = 2V TA = 25C
10 0.23
IOUT = -10mA, VDD - VOH IOUT = 10mA, VOL - GND COUT = 220pF, VDD = 15V(Note 4) COUT = 220pF, VDD = 15V(Note 4) VDD = 7V, ILOAD = 1mA (Note 6)
-
0.5 0.5 110 90 -
1.0 1.0 200 150 1.25
V V ns ns V
(Note 4) (Note 4) (Note 4)
130 115 -
140 125 15
150 135 -
C C C
3. Specifications at -40C and 105C are guaranteed by 25C test with margin limits. 4. Guaranteed by design, not 100% tested in production. 5. This is the maximum duty cycle achievable using the specified values of RTD and CT. Larger or smaller maximum duty cycles may be obtained using other values for these components. See Equations 1 - 5. 6. Adjust VDD below the UVLO stop threshold prior to setting at 7V.
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FN9182.2 April 4, 2006
ISL6753 Typical Performance Curves
1.02 Normalized Vref 1.01 1 0.99 0.98 25 CT Discharge Current Gain 40 25 10 5 20 35 50 65 80 95 110 Temperature (C) 24 23 22 21 20 19 18 0 200 400 600 800 RTD Current (uA) 1000
FIGURE 1. REFERENCE VOLTAGE vs TEMPERATURE
CT = 1000pF 680pF 470pF 330pF 220pF 100pF
FIGURE 2. CT DISCHARGE CURRENT GAIN vs RTD CURRENT
1 10
4
1 10
3
Deadtime - TD (nS)
1 10
Frequency (kHz)
3
100
100
10
RTD= 10k 50k 100k
0 10 20 30 40 50 60 RTD (kohms) 70 80 90 100
10 0.1
1 CT (nF)
10
FIGURE 3. DEADTIME (DT) vs CAPACITANCE
FIGURE 4. CAPACITANCE vs FREQUENCY
Pin Descriptions
VDD - VDD is the power connection for the IC. To optimize noise immunity, bypass VDD to GND with a ceramic capacitor as close to the VDD and GND pins as possible. Supply voltage under-voltage lock-out (UVLO) start and stop thresholds track each other resulting in relatively constant hysteresis. GND - Signal and power ground connections for this device. Due to high peak currents and high frequency operation, a low impedance layout is necessary. Ground planes and short traces are highly recommended. VREF - The 5.00V reference voltage output having 3% tolerance over line, load and operating temperature. Bypass to GND with a 0.1F to 2.2F low ESR capacitor. CT - The oscillator timing capacitor is connected between this pin and GND. It is charged through an internal 200A current source and discharged with a user adjustable current source controlled by RTD.
RTD - This is the oscillator timing capacitor discharge current control pin. The current flowing in a resistor connected between this pin and GND determines the magnitude of the current that discharges CT. The CT discharge current is nominally 20x the resistor current. The PWM deadtime is determined by the timing capacitor discharge duration. The voltage at RTD is nominally 2.00V. CS - This is the input to the overcurrent comparator. The overcurrent comparator threshold is set at 1.00V nominal. The CS pin is shorted to GND at the termination of either PWM output. Depending on the current sensing source impedance, a series input resistor may be required due to the delay between the internal clock and the external power switch. This delay may result in CS being discharged prior to the power switching device being turned off. RAMP - This is the input for the sawtooth waveform for the PWM comparator. The RAMP pin is shorted to GND at the termination of the PWM signal. A sawtooth voltage
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FN9182.2 April 4, 2006
ISL6753
waveform is required at this input. For current-mode control this pin is connected to CS and the current loop feedback signal is applied to both inputs. For voltage-mode control, the oscillator sawtooth waveform may be buffered and used to generate an appropriate signal, RAMP may be connected to the input voltage through a RC network for voltage feed forward control, or RAMP may be connected to VREF through a RC network to produce the desired sawtooth waveform. OUTUL and OUTUR - These outputs control the upper bridge FETs and operate at a fixed 50% duty cycle in alternate sequence. OUTUL controls the upper left FET and OUTUR controls the upper right FET. The left and right designation may be switched as long as they are switched in conjunction with the lower FET outputs, OUTLL and OUTLR. RESDEL - Sets the resonant delay period between the toggle of the upper FETs and the turn on of either of the lower FETs. The voltage applied to RESDEL determines when the upper FETs switch relative to a lower FET turning on. Varying the control voltage from 0 to 2.00V increases the resonant delay duration from 0 to 100% of the deadtime. The control voltage divided by 2 represents the percent of the deadtime equal to the resonant delay. In practice the maximum resonant delay must be set lower than 2.00V to ensure that the lower FETs, at maximum duty cycle, are OFF prior to the switching of the upper FETs. OUTLL and OUTLR - These outputs control the lower bridge FETs, are pulse width modulated, and operate in alternate sequence. OUTLL controls the lower left FET and OUTLR controls the lower right FET. The left and right designation may be switched as long as they are switched in conjunction with the upper FET outputs, OUTUL and OUTUR. VERR - The control voltage input to the inverting input of the PWM comparator. The output of an external error amplifier (EA) is applied to this input for closed loop regulation. VERR has a nominal 1mA pull-up current source. FB - FB is the inverting input to the error amplifier (EA). SS - Connect the soft-start timing capacitor between this pin and GND to control the duration of soft-start. The value of the capacitor determines the rate of increase of the duty cycle during start-up. SS may also be used to inhibit the outputs by grounding through a small transistor in an open collector/drain configuration. CTBUF - CTBUF is the buffered output of the sawtooth oscillator waveform present on CT and is capable of sourcing 2mA. It is offset from ground by 0.40V and has a nominal valley-to-peak gain of 2. It may be used for slope compensation.
Functional Description
Features
The ISL6753 PWM is an excellent choice for low cost ZVS full-bridge applications employing conventional output rectification. If synchronous rectification is required, please consider the ISL6752 or ISL6551 products. With the ISL6753's many protection and control features, a highly flexible design with minimal external components is possible. Among its many features are support for both current- and voltage-mode control, a very accurate overcurrent limit threshold, thermal protection, a buffered sawtooth oscillator output suitable for slope compensation, voltage controlled resonant delay, and adjustable frequency with precise deadtime control.
Oscillator
The ISL6753 has an oscillator with a programmable frequency range to 2MHz, and can be programmed with an external resistor and capacitor. The switching period is the sum of the timing capacitor charge and discharge durations. The charge duration is determined by CT and a fixed 200A internal current source. The discharge duration is determined by RTD and CT.
T C 11.5 10 CT
3
S
(EQ. 1)
T D ( 0.06 RTD CT ) + 50 10
-9
S
(EQ. 2)
1 T SW = T C + T D = ----------F SW
S
(EQ. 3)
where TC and TD are the charge and discharge times, respectively, TSW is the oscillator period, and FSW is the oscillator frequency. One output switching cycle requires two oscillator cycles. The actual times will be slightly longer than calculated due to internal propagation delays of approximately 10ns/transition. This delay adds directly to the switching duration, but also causes overshoot of the timing capacitor peak and valley voltage thresholds, effectively increasing the peak-to-peak voltage on the timing capacitor. Additionally, if very small discharge currents are used, there will be increased error due to the input impedance at the CT pin. The maximum duty cycle, D, and percent deadtime, DT, can be calculated from:
TC D = ----------T SW DT = 1 - D (EQ. 4)
(EQ. 5)
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ISL6753
Soft-Start Operation
The ISL6753 features a soft-start using an external capacitor in conjunction with an internal current source. Soft-start reduces component stresses and surge currents during start-up. Upon start-up, the soft-start circuitry limits the error voltage input (VERR) to a value equal to the soft-start voltage. The output pulse width increases as the soft-start capacitor voltage increases. This has the effect of increasing the duty cycle from zero to the regulation pulse width during the softstart period. When the soft-start voltage exceeds the error voltage, soft-start is completed. Soft-start occurs during start-up and after recovery from a fault condition. The softstart charging period may be calculated as follows:
t = 64.3 C ms (EQ. 6)
Voltage Feed Forward Operation
Voltage feed forward is a technique used to regulate the output voltage for changes in input voltage without the intervention of the control loop. Voltage feed forward is often implemented in voltage-mode control loops, but is redundant and unnecessary in peak current-mode control loops. Voltage feed forward operates by modulating the sawtooth ramp in direct proportion to the input voltage. Figure 5 demonstrates the concept.
VIN
ERROR VOLTAGE RAMP
where t is the charging period in ms and C is the value of the soft-start capacitor in F. The soft-start voltage is clamped to 4.50V with a tolerance of 2%. It is suitable for use as a "soft-started" reference provided the current draw is kept well below the 70A charging current. The outputs may be inhibited by using the SS pin as a disable input. Pulling SS below 0.25V forces all outputs low. An open collector/drain configuration may be used to couple the disable signal into the SS pin.
CT
OUTLL, LR
FIGURE 5. VOLTAGE FEED FORWARD BEHAVIOR
Gate Drive
The ISL6753 outputs are capable of sourcing and sinking 10mA (at rated VOH, VOL) and are intended to be used in conjunction with integrated FET drivers or discrete bipolar totem pole drivers. The typical on resistance of the outputs is 50.
Overcurrent Operation
The cycle-by-cycle peak current limit results in pulse-by-pulse duty cycle reduction when the current feedback signal exceeds 1.0V. When the peak current exceeds the threshold, the active output pulse is immediately terminated. This results in a decrease in output voltage as the load current increases beyond the current limit threshold. The ISL6753 operates continuously in an overcurrent condition without shutdown. If voltage-mode control is used in a bridge topology, it should be noted that peak current limit results in inherently unstable operation. The DC blocking capacitors used in voltage-mode bridge topologies become unbalanced, as does the flux in the transformer core. A latching overcurrent shutdown method using external components is recommended. The propagation delay from CS exceeding the current limit threshold to the termination of the output pulse is increased by the leading edge blanking (LEB) interval. The effective delay is the sum of the two delays and is nominally 105ns.
Input voltage feed forward may be implemented using the RAMP input. An RC network connected between the input voltage and ground, as shown in Figure 7, generates a voltage ramp whose charging rate varies with the amplitude of the source voltage. At the termination of the active output pulse RAMP is discharged to ground so that a repetitive sawtooth waveform is created. The RAMP waveform is compared to the VERR voltage to determine duty cycle. The selection of the RC components depends upon the desired input voltage operating range and the frequency of the oscillator. In typical applications the RC components are selected so that the ramp amplitude reaches 1.0V at minimum input voltage within the duration of one half-cycle.
1 16 15 14
VIN
2 3
R3
4 5 6
ISL6753
13 12 11 10
7 RAMP 8
GND 9
C7
FIGURE 6. VOLTAGE FEED FORWARD CONTROL
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FN9182.2 April 4, 2006
ISL6753
The charging time of the ramp capacitor is
V RAMP ( PEAK ) t = - R3 C7 ln 1 - --------------------------------------- V IN ( MIN ) S
(EQ. 7)
For optimum performance, the maximum value of the capacitor should be limited to 10nF. The maximum DC current through the resistor should be limited to 2mA maximum. For example, if the oscillator frequency is 400kHz, the minimum input voltage is 300V, and a 4.7nF ramp capacitor is selected, the value of the resistor can be determined by rearranging Equation 7.
- 2.5 10 -t R3 = ------------------------------------------------------------------------- = ------------------------------------------------------------9 1 V RAMP ( PEAK ) 4.7 10 ln 1 - --------- C7 ln 1 - --------------------------------------- 300 V IN ( MIN ) ) = 159 k
-6
The criteria for determining the correct amount of external ramp can be determined by appropriately setting the damping factor of the double-pole located at half the oscillator frequency. The double-pole will be critically damped if the Q-factor is set to 1, and over-damped for Q > 1, and under-damped for Q < 1. An under-damped condition can result in current loop instability.
1 Q = ------------------------------------------------ ( m c ( 1 - D ) - 0.5 )
(EQ. 12)
where D is the percent of on time during a half cycle. Setting Q = 1 and solving for Se yields:
1 1 S e = S n -- + 0.5 ------------ - 1 1-D
(EQ. 13)
(EQ. 8)
where t is equal to the oscillator period minus the deadtime. If the deadtime is short relative to the oscillator period, it can be ignored for this calculation. If feed forward operation is not desired, the RC network may be connected to VREF rather than the input voltage. Alternatively, a resistor divider from CTBUF may be used as the sawtooth signal. Regardless, a sawtooth waveform must be generated on RAMP as it is required for proper PWM operation.
Since Sn and Se are the on time slopes of the current ramp and the external ramp, respectively, they can be multiplied by Ton to obtain the voltage change that occurs during Ton.
1 1 V e = V n -- + 0.5 ------------ - 1 1-D
(EQ. 14)
where Vn is the change in the current feedback signal during the on time and Ve is the voltage that must be added by the external ramp. Vn can be solved for in terms of input voltage, current transducer components, and output inductance yielding:
T SW V R CS N O S1 -V e = ----------------------------------------- ------- -- + D - 0.5 N CT L O NP V
Slope Compensation
Peak current-mode control requires slope compensation to improve noise immunity, particularly at lighter loads, and to prevent current loop instability, particularly for duty cycles greater than 50%. Slope compensation may be accomplished by summing an external ramp with the current feedback signal or by subtracting the external ramp from the voltage feedback error signal. Adding the external ramp to the current feedback signal is the more popular method. From the small signal current-mode model [1] it can be shown that the naturally-sampled modulator gain, Fm, without slope compensation, is
1 Fm = ------------------SnTsw
(EQ. 15)
where RCS is the current sense burden resistor, NCT is the current transformer turns ratio, LO is the output inductance, VO is the output voltage, and Ns and Np are the secondary and primary turns, respectively. The inductor current, when reflected through the isolation transformer and the current sense transformer to obtain the current feedback signal at the sense resistor yields:
N S R CS D T SW NS V CS = ------------------------ I O + -------------------- V IN ------- - V O N P N CT 2L O NP V
(EQ. 16)
(EQ. 9)
where Sn is the slope of the sawtooth signal and Tsw is the duration of the half-cycle. When an external ramp is added, the modulator gain becomes
1 1 Fm = -------------------------------------- = --------------------------( Sn + Se )Tsw m c SnTsw
where VCS is the voltage across the current sense resistor and IO is the output current at current limit. Since the peak current limit threshold is 1.00V, the total current feedback signal plus the external ramp voltage must sum to this value.
V e + V CS = 1
(EQ. 10)
(EQ. 17)
where Se is slope of the external ramp and
m c = 1 + Se ------Sn
Substituting Equations 15 and 16 into Equation 17 and solving for RCS yields
(EQ. 11)
N P N CT 1 R CS = ----------------------- ----------------------------------------------------VO NS ------- T SW 1 + D IO + - -- --- L 2
O
(EQ. 18)
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ISL6753
For simplicity, idealized components have been used for this discussion, but the effect of magnetizing inductance must be considered when determining the amount of external ramp to add. Magnetizing inductance provides a degree of slope compensation to the current feedback signal and reduces the amount of external ramp required. The magnetizing inductance adds primary current in excess of what is reflected from the inductor current in the secondary.
V IN DT SW I P = ------------------------------Lm A
Assuming the designer has selected values for the RC filter placed on the CS pin, the value of R9 required to add the appropriate external ramp can be found by superposition.
( D ( V CTBUF - 0.4 ) + 0.4 ) R6 V e - V CS = -----------------------------------------------------------------------------R6 + R9 V
(EQ. 22)
Rearranging to solve for R9 yields
( D ( V CTBUF - 0.4 ) - V e + V CS + 0.4 ) R6 R9 = -----------------------------------------------------------------------------------------------------------------V e - V CS
(EQ. 19)
(EQ. 23)
where VIN is the input voltage that corresponds to the duty cycle D and Lm is the primary magnetizing inductance. The effect of the magnetizing current at the current sense resistor, RCS, is
I P R CS V CS = ------------------------N CT V
The value of RCS determined in Equation 18 must be rescaled so that the current sense signal presented at the CS pin is that predicted by Equation 16. The divider created by R6 and R9 makes this necessary.
R6 + R9 R CS = --------------------- R CS R9
(EQ. 20)
(EQ. 24)
If VCS is greater than or equal to Ve, then no additional slope compensation is needed and RCS becomes
N CT R CS = ------------------------------------------------------------------------------------------------------------------------------------NS DT SW NS V IN DT SW ------- I O + ---------------- V IN ------- - V O + ------------------------------NP 2L O NP Lm
Example: VIN = 280V VO = 12V LO = 2.0H Np/Ns = 20 Lm = 2mH IO = 55A Oscillator Frequency, Fsw = 400kHz Duty Cycle, D = 85.7% NCT = 50 R6 = 499 Solve for the current sense resistor, RCS, using Equation 18. RCS = 15.1. Determine the amount of voltage, Ve, that must be added to the current feedback signal using Equation 15. Ve = 153mV
(EQ. 21)
If VCS is less than Ve, then Equation 18 is still valid for the value of RCS, but the amount of slope compensation added by the external ramp must be reduced by VCS. Adding slope compensation is accomplished in the ISL6753 using the CTBUF signal. CTBUF is an amplified representation of the sawtooth signal that appears on the CT pin. It is offset from ground by 0.4V and is 2x the peak-topeak amplitude of CT (0.4 - 4.4V). A typical application sums this signal with the current sense feedback and applies the result to the CS pin as shown in Figure 7.
1 2 CTBUF 3 4 R9 ISL6753 5 6 7 8 CS R6 RCS C4
Next, determine the effect of the magnetizing current from Equation 20. VCS = 91mV Using Equation 23, solve for the summing resistor, R9, from CTBUF to CS. R9 = 30.1k Determine the new value of RCS, R'CS, using Equation 24. R'CS = 15.4 The above discussion determines the minimum external ramp that is required. Additional slope compensation may be considered for design margin.
FN9182.2 April 4, 2006
FIGURE 7. ADDING SLOPE COMPENSATION
11
ISL6753
If the application requires deadtime less than about 500ns, the CTBUF signal may not perform adequately for slope compensation. CTBUF lags the CT sawtooth waveform by 300-400ns. This behavior results in a non-zero value of CTBUF when the next half-cycle begins when the deadtime is short. Under these situations, slope compensation may be added by externally buffering the CT signal as shown below.
CT
DEADTIME PWM PWM
OUTLL
OUTLR
PWM
PWM
1 2 3 4 5 CT R9 6 7 8 CS R6 ISL6753
VREF 16 15 14 13 12 11 10 9
OUTUR
RESONANT DELAY
OUTUL
RESDEL WINDOW
FIGURE 9. BRIDGE DRIVE SIGNAL TIMING
To understand how the ZVS method operates one must include the parasitic elements of the circuit and examine a full switching cycle.
CT
VIN+
RCS
C4
UL
UR LL
D1
VOUT+
FIGURE 8. ADDING SLOPE COMPENSATION USING CT
RTN
Using CT to provide slope compensation instead of CTBUF requires the same calculations, except that Equations 21 and 22 require modification. Equation 21 becomes:
2D R6 V e - V CS = --------------------R6 + R9 V
LL
LR D2
VIN-
FIGURE 10. IDEALIZED FULL-BRIDGE
(EQ. 25)
and Equation 22 becomes:
( 2D - V e + V CS ) R6 R9 = -----------------------------------------------------------V e - V CS
(EQ. 26)
The buffer transistor used to create the external ramp from CT should have a sufficiently high gain so as to minimize the required base current. Whatever base current is required reduces the charging current into CT and will reduce the oscillator frequency.
ZVS Full-Bridge Operation
The ISL6753 is a full-bridge zero-voltage switching (ZVS) PWM controller that behaves much like a traditional hardswitched topology controller. Rather than drive the diagonal bridge switches simultaneously, the upper switches (OUTUL, OUTUR) are driven at a fixed 50% duty cycle and the lower switches (OUTLL, OUTLR) are pulse width modulated on the trailing edge.
In Figure 10, the power semiconductor switches have been replaced by ideal switch elements with parallel diodes and capacitance, the output rectifiers are ideal, and the transformer leakage inductance has been included as a discrete element. The parasitic capacitance has been lumped together as switch capacitance, but represents all parasitic capacitance in the circuit including winding capacitance. Each switch is designated by its position, upper left (UL), upper right (UR), lower left (LL), and lower right (LR). The beginning of the cycle, shown in Figure 11, is arbitrarily set as having switches UL and LR on and UR and LL off. The direction of the primary and secondary currents are indicated by IP and IS, respectively.
12
FN9182.2 April 4, 2006
ISL6753
resonant delay. The voltage applied to RESDEL determines how far in advance the toggle occurs prior to a lower switch turning on. The ZVS transition occurs after the upper switches toggle and before the diagonal lower switch turns on. The required resonant delay is 1/4 of the period of the LC resonant frequency of the circuit formed by the leakage inductance and the parasitic capacitance. The resonant transition may be estimated from Equation 27.
1 = -- ----------------------------------2 2 R 1 -------------- - --------2 LL CP 4L L
VIN+
UL
UR D1 LL
IS
VOUT+
IP
RTN
LL
LR D2
VIN-
(EQ. 27)
FIGURE 11. UL - LR POWER TRANSFER CYCLE
The UL - LR power transfer period terminates when switch LR turns off as determined by the PWM. The current flowing in the primary cannot be interrupted instantaneously, so it must find an alternate path. The current flows into the parasitic switch capacitance of LR and UR which charges the node to VIN and then forward biases the body diode of upper switch UR.
where is the resonant transition time, LL is the leakage inductance, CP is the parasitic capacitance, and R is the equivalent resistance in series with LL and CP. The resonant delay is always less than or equal to the deadtime and may be calculated using the following equation.
V resdel resdel = ------------------- DT 2 S
VIN+
(EQ. 28)
UL
UR D1 LL
IS
VOUT+
IP
RTN
where resdel is the desired resonant delay, Vresdel is a voltage between 0 and 2V applied to the RESDEL pin, and DT is the deadtime (see Equations 1 - 5). When the upper switches toggle, the primary current that was flowing through UL must find an alternate path. It charges/discharges the parasitic capacitance of switches UL and LL until the body diode of LL is forward biased. If RESDEL is set properly, switch LL will be turned on at this time.The output inductor does not assist this transition. It is
VIN+
LL
LR D2
VIN-
FIGURE 12. UL - UR FREE-WHEELING PERIOD
The primary leakage inductance, LL, maintains the current which now circulates around the path of switch UL, the transformer primary, and switch UR. When switch LR opens, the output inductor current free-wheels through both output diodes, D1 and D2. During the switch transition, the output inductor current assists the leakage inductance in charging the upper and lower bridge FET capacitance. The current flow from the previous power transfer cycle tends to be maintained during the free-wheeling period because the transformer primary winding is essentially shorted. Diode D1 may conduct very little or none of the free-wheeling current, depending on circuit parasitics. This behavior is quite different than occurs in a conventional hard-switched full-bridge topology where the free-wheeling current splits nearly evenly between the output diodes, and flows not at all in the primary. This condition persists through the remainder of the halfcycle. During the period when CT discharges, also referred to as the deadtime, the upper switches toggle. Switch UL turns off and switch UR turns on. The actual timing of the upper switch toggle is dependent on RESDEL which sets the 13
UL
UR LL
D1
IS
VOUT+
IP
RTN
LL
LR D2
VIN-
FIGURE 13. UPPER SWITCH TOGGLE AND RESONANT TRANSITION
purely a resonant transition driven by the leakage inductance.
FN9182.2 April 4, 2006
ISL6753
The second power transfer period commences when switch LL closes. With switches UR and LL on, the primary and secondary currents flow as indicated below.
VIN+
RESDEL is set properly, switch LR will be turned on at this time.
VIN+
UL UL UR LL D1
VOUT+
UR D1 LL
IS
VOUT+
IP
RTN
RTN
LL
LR D2
LL
LR D2
VIN-
VIN-
FIGURE 14. UR - LL POWER TRANSFER
FIGURE 16. UPPER SWITCH TOGGLE AND RESONANT TRANSITION
The UR - LL power transfer period terminates when switch LL turns off as determined by the PWM. The current flowing in the primary must find an alternate path. The current flows into the parasitic switch capacitance which charges the node to VIN and then forward biases the body diode of upper switch UL. As before, the output inductor current assists in this transition. The primary leakage inductance, LL, maintains the current, which now circulates around the path of switch UR, the transformer primary, and switch UL. When switch LL opens, the output inductor current free-wheels predominantly through diode D1. Diode D2 may actually conduct very little or none of the free-wheeling current, depending on circuit parasitics. This condition persists through the remainder of the half-cycle.
The first power transfer period commences when switch LR closes and the cycle repeats. The ZVS transition requires that the leakage inductance has sufficient energy stored to fully charge the parasitic capacitances. Since the energy stored is proportional to the square of the current (1/2 LLIP2), the ZVS resonant transition is load dependent. If the leakage inductance is not able to store sufficient energy for ZVS, a discrete inductor may be added in series with the transformer primary.
Fault Conditions
A fault condition occurs if VREF or VDD fall below their undervoltage lockout (UVLO) thresholds or if the thermal protection is triggered. When a fault is detected, the softstart capacitor is quickly discharged, and the outputs are disabled low. When the fault condition clears and the softstart voltage is below the reset threshold, a soft-start cycle begins. An overcurrent condition is not considered a fault and does not result in a shutdown.
VIN+
UL
UR D1 LL
IS
VOUT+
IP
RTN
Thermal Protection
Internal die over temperature protection is provided. An integrated temperature sensor protects the device should the junction temperature exceed 140C. There is approximately 15C of hysteresis.
LL
LR D2
VIN-
FIGURE 15. UR - UL FREE-WHEELING PERIOD
Ground Plane Requirements
Careful layout is essential for satisfactory operation of the device. A good ground plane must be employed. VDD and VREF should be bypassed directly to GND with good high frequency capacitance.
When the upper switches toggle, the primary current that was flowing through UR must find an alternate path. It charges/discharges the parasitic capacitance of switches UR and LR until the body diode of LR is forward biased. If
References
[1] Ridley, R., "A New Continuous-Time Model for Current Mode Control", IEEE Transactions on Power Electronics, Vol. 6, No. 2, April 1991.
14
FN9182.2 April 4, 2006
ISL6753 Shrink Small Outline Plastic Packages (SSOP) Quarter Size Outline Plastic Packages (QSOP)
N INDEX AREA H E -B1 2 3 SEATING PLANE -AD -CA 0.25 0.010 h x 45 L GAUGE PLANE 0.25(0.010) M BM
M16.15A
16 LEAD SHRINK SMALL OUTLINE PLASTIC PACKAGE (0.150" WIDE BODY) INCHES SYMBOL MIN MAX MILLIMETERS MIN MAX NOTES
A A1 A2 B C D E
A2 C 0.10(0.004) C AM BS
0.061 0.004 0.055 0.008 0.0075 0.189 0.150 0.230 0.010 0.016 16 0
0.068 0.0098 0.061 0.012 0.0098 0.196 0.157 0.244 0.016 0.035 8
1.55 0.102 1.40 0.20 0.191 4.80 3.81 5.84 0.25 0.41 16 0
1.73 0.249 1.55 0.31 0.249 4.98 3.99 6.20 0.41 0.89 8
9 3 4 5 6 7 Rev. 2 6/04
e
B 0.17(0.007) M
A1
e H h L N
0.025 BSC
0.635 BSC
NOTES: 1. Symbols are defined in the "MO Series Symbol List" in Section 2.2 of Publication Number 95. 2. Dimensioning and tolerancing per ANSI Y14.5M-1982. 3. Dimension "D" does not include mold flash, protrusions or gate burrs. Mold flash, protrusion and gate burrs shall not exceed 0.15mm (0.006 inch) per side. 4. Dimension "E" does not include interlead flash or protrusions. Interlead flash and protrusions shall not exceed 0.25mm (0.010 inch) per side. 5. The chamfer on the body is optional. If it is not present, a visual index feature must be located within the crosshatched area. 6. "L" is the length of terminal for soldering to a substrate. 7. "N" is the number of terminal positions. 8. Terminal numbers are shown for reference only. 9. Dimension "B" does not include dambar protrusion. Allowable dambar protrusion shall be 0.10mm (0.004 inch) total in excess of "B" dimension at maximum material condition. 10. Controlling dimension: INCHES. Converted millimeter dimensions are not necessarily exact.
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation's quality certifications can be viewed at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements 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 Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com 15
FN9182.2 April 4, 2006

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