L6599

L6599 High-voltage resonant controller Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 50% duty cycle, variable frequency control of resonant half-bridge High-accuracy oscillator Up to 500kHz operating frequency Two-level OCP: frequency-shift and latched shutdown Interface with PFC controller Latched disable input Burst-mode operation at light load Input for power-ON/OFF sequencing or brownout protection Non-linear soft-start for monotonic output voltage rise 600V-rail compatible high-side gate driver with integrated bootstrap diode and high dV/dt immunity -300/800mA high-side and low-side gate drivers with UVLO pull-down DIP-16, SO-16N packages ■ ■ ■ ■ DIP-16 SO-16N Order code Part number L6599D L6599TR L6599N Package SO-16N SO-16N DIP-16 Packaging Tube Tape and reel Tube Applications LCD & PDP TV Desktop PC, entry-level server Telecom SMPS AC-DC adapter, open frame SMPS ■ ■ Block diagram Vcc DIS 8 1.85V STBY 5 1.25V + Ifmin + RFmin 4 ISEN_DIS H.V. 16 15 14 OUT DRIVING LOGIC LEVEL SHIFTER L DRIVER VG Vs 11 10 QS R UVLO DISABLE + UVLO 12 DIS VBOOT HVG SQ R UV 17V DETECTION UVLO SYNCHRONOUS BOOTSTRAP DIODE HVG DRIVER CBOOT STANDBY LC TANK CIRCUIT 2V DEAD TIME L VG + + 1.5V 6 0.8V 9 LINE_OK ISEN_DIS DIS STANDBY GND ISEN Css 1 CONTROL LOGIC 6.3V 1.25V 15 µA 2 DELAY 7 LINE + PFC_STOP CF 3 VCO May 2006 Rev 1 1/36 www.st.com 36 Contents L6599 Contents 1 2 Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Pin Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1 2.2 Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 4 Typical system block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.1 4.2 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 5 6 7 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Typical electrical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Operation at no load or very light load . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Current sense, OCP and OLP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Latched shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Line sensing function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Bootstrap section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 8 9 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 2/36 L6599 Device description 1 Device description The L6599 is a double-ended controller specific for the resonant half-bridge topology. It provides 50% complementary duty cycle: the high-side switch and the low-side switch are driven ON 180° out-of-phase for exactly the same time. Output voltage regulation is obtained by modulating the operating frequency. A fixed deadtime inserted between the turn-OFF of one switch and the turn-ON of the other one guarantees soft-switching and enables high-frequency operation. To drive the high-side switch with the bootstrap approach, the IC incorporates a high-voltage floating structure able to withstand more than 600V with a synchronous-driven high-voltage DMOS that replaces the external fast-recovery bootstrap diode. The IC enables the designer to set the operating frequency range of the converter by means of an externally programmable oscillator. At start-up, to prevent uncontrolled inrush current, the switching frequency starts from a programmable maximum value and progressively decays until it reaches the steady-state value determined by the control loop. This frequency shift is non linear to minimize output voltage overshoots; its duration is programmable as well. The IC can be forced to enter a controlled burst-mode operation at light load, so as to keep converter's input consumption to a minimum. IC's functions include a not-latched active-low disable input with current hysteresis useful for power sequencing or for brownout protection, a current sense input for OCP with frequency shift and delayed shutdown with automatic restart. A higher level OCP latches off the IC if the first-level protection is not sufficient to control the primary current. Their combination offers complete protection against overload and short circuits. An additional latched disable input (DIS) allows easy implementation of OTP and/or OVP. An interface with the PFC controller is provided that enables to switch off the pre-regulator during fault conditions, such as OCP shutdown and DIS high, or during burst-mode operation. 3/36 Pin Settings L6599 2 2.1 Pin Settings Connection Figure 1. Pin Connection (Top view) Css DELAY CF RFmin STBY ISEN LINE DIS 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 VBOOT HVG OUT N.C. Vcc LVG GND PFC_STOP 2.2 Table 1. N. Functions Pin functions Name Function Soft start. This pin connects an external capacitor to GND and a resistor to RFmin (pin 4) that set both the maximum oscillator frequency and the time constant for the frequency shift that occurs as the chip starts up (soft-start). An internal switch discharges this capacitor every time the chip turns OFF (VCC < UVLO, LINE < 1.25V or > 6V, DIS > 1.85V, ISEN > 1.5V, DELAY > 3.5V) to make sure it will be soft-started next, and when the voltage on the current sense pin (ISEN) exceeds 0.8V, as long as it stays above 0.75V. Delayed shutdown upon overcurrent. A capacitor and a resistor are connected from this pin to GND to set both the maximum duration of an overcurrent condition before the IC stops switching and the delay after which the IC restarts switching. Every time the voltage on the ISEN pin exceeds 0.8V the capacitor is charged by an internal 150µA current generator and is slowly discharged by the external resistor. If the voltage on the pin reaches 2V, the soft start capacitor is completely discharged so that the switching frequency is pushed to its maximum value and the 150µA is kept always on. As the voltage on the pin exceeds 3.5V the IC stops switching and the internal generator is turned OFF, so that the voltage on the pin will decay because of the external resistor. The IC will be soft-restarted as the voltage drops below 0.3V. In this way, under short circuit conditions, the converter will work intermittently with very low input average power. Timing capacitor. A capacitor connected from this pin to GND is charged and discharged by internal current generators programmed by the external network connected to pin 4 (RFmin) and determines the switching frequency of the converter. 1 CSS 2 DELAY 3 CF 4/36 L6599 Table 1. Pin functions Pin Settings 4 RFmin Minimum oscillator frequency setting. This pin provides a precise 2V reference and a resistor connected from this pin to GND defines a current that is used to set the minimum oscillator frequency. To close the feedback loop that regulates the converter output voltage by modulating the oscillator frequency, the phototransistor of an optocoupler will be connected to this pin through a resistor. The value of this resistor will set the maximum operating frequency. An R-C series connected from this pin to GND sets frequency shift at start-up to prevent excessive energy inrush (soft-start). Burst-mode operation threshold. The pin senses some voltage related to the feedback control, which is compared to an internal reference (1.25V). If the voltage on the pin is lower than the reference, the IC enters an idle state and its quiescent current is reduced. The chip restarts switching as the voltage exceeds the reference by 50mV. Soft-start is not invoked. This function realizes burst-mode operation when the load falls below a level that can be programmed by properly choosing the resistor connecting the optocoupler to pin RFmin (see block diagram). Tie the pin to RFmin if burst-mode is not used. Current sense input. The pin senses the primary current though a sense resistor or a capacitive divider for lossless sensing. This input is not intended for a cycle-by-cycle control; hence the voltage signal must be filtered to get average current information. As the voltage exceeds a 0.8V threshold (with 50mV hysteresis), the soft-start capacitor connected to pin 1 is internally discharged: the frequency increases hence limiting the power throughput. Under output short circuit, this normally results in a nearly constant peak primary current. This condition is allowed for a maximum time set at pin 2. If the current keeps on building up despite this frequency increase, a second comparator referenced at 1.5V latches the device off and brings its consumption almost to a “before start-up” level. The information is latched and it is necessary to recycle the supply voltage of the IC to enable it to restart: the latch is removed as the voltage on the Vcc pin goes below the UVLO threshold. Tie the pin to GND if the function is not used. Line sensing input. The pin is to be connected to the high-voltage input bus with a resistor divider to perform either AC or DC (in systems with PFC) brownout protection. A voltage below 1.25V shuts down (not latched) the IC, lowers its consumption and discharges the soft-start capacitor. IC’s operation is re-enabled (soft-started) as the voltage exceeds 1.25V. The comparator is provided with current hysteresis: an internal 15µA current generator is ON as long as the voltage applied at the pin is below 1.25V and is OFF if this value is exceeded. Bypass the pin with a capacitor to GND to reduce noise pick-up. The voltage on the pin is top-limited by an internal zener. Activating the zener causes the IC to shut down (not latched). Bias the pin between 1.25 and 6V if the function is not used. Latched device shutdown. Internally the pin connects a comparator that, when the voltage on the pin exceeds 1.85V, shuts the IC down and brings its consumption almost to a “before start-up” level. The information is latched and it is necessary to recycle the supply voltage of the IC to enable it to restart: the latch is removed as the voltage on the VCC pin goes below the UVLO threshold. Tie the pin to GND if the function is not used. 5 STBY 6 ISEN 7 LINE 8 DIS 9 Open-drain ON/OFF control of PFC controller. This pin, normally open, is intended for stopping the PFC controller, for protection purpose or during burst-mode operation. It goes low when the IC is shut down by DIS > 1.85V, ISEN > 1.5V, LINE > 6V and STBY < 1.25V. PFC_STOP The pin is pulled low also when the voltage on pin DELAY exceeds 2V and goes back open as the voltage falls below 0.3V. During UVLO, it is open. Leave the pin unconnected if not used. GND Chip ground. Current return for both the low-side gate-drive current and the bias current of the IC. All of the ground connections of the bias components should be tied to a track going to this pin and kept separate from any pulsed current return. 10 5/36 Typical system block diagram Table 1. 11 L6599 Pin functions LVG Low-side gate-drive output. The driver is capable of 0.3A min. source and 0.8A min. sink peak current to drive the lower MOSFET of the half-bridge leg. The pin is actively pulled to GND during UVLO. Supply Voltage of both the signal part of the IC and the low-side gate driver. Sometimes a small bypass capacitor (0.1µF typ.) to GND might be useful to get a clean bias voltage for the signal part of the IC. High-voltage spacer. The pin is not internally connected to isolate the high-voltage pin and ease compliance with safety regulations (creepage distance) on the PCB. High-side gate-drive floating ground. Current return for the high-side gate-drive current. Layout carefully the connection of this pin to avoid too large spikes below ground. High-side floating gate-drive output. The driver is capable of 0.3A min. source and 0.8A min. sink peak current to drive the upper MOSFET of the half-bridge leg. A resistor internally connected to pin 14 (OUT) ensures that the pin is not floating during UVLO. High-side gate-drive floating supply Voltage. The bootstrap capacitor connected between this pin and pin 14 (OUT) is fed by an internal synchronous bootstrap diode driven in-phase with the low-side gate-drive. This patented structure replaces the normally used external diode. 12 VCC 13 14 N.C. OUT 15 HVG 16 VBOOT 3 Typical system block diagram Figure 2. Typical system block diagram 6/36 L6599 Electrical data 4 4.1 Electrical data Maximum ratings Table 2. Symbol VBOOT VOUT dVOUT /dt VCC VPFC_STOP IPFC_STOP VLINEmax IRFmin Absolute maximum ratings Pin 16 14 14 12 9 9 7 4 Parameter Floating supply voltage Floating ground voltage Floating ground max. slew rate IC Supply voltage (ICC ≤25 mA) Maximum voltage (pin open) Maximum sink current (pin low) Maximum pin voltage (Ipin ≤1mA) Maximum source current Value -1 to 618 -3 to VBOOT -18 50 Self-limited -0.3 to VCC Self-limited Self-limited 2 -0.3 to 5 Unit V V V/ns V V A V mA V 1 to 6, 8 Analog inputs & outputs Note: ESD immunity for pins 14, 15 and 16 is guaranteed up to 900V 4.2 Thermal data Table 3. Symbol RthJA TSTG TJ PTOT Thermal data Description Max. thermal resistance junction to ambient (DIP16) Max. thermal resistance junction to ambient (SO16) Storage temperature range Junction operating temperature range Recommended max. power dissipation @TA = 70°C (DIP16) Recommended max. power dissipation @TA = 50°C (SO16) Value 80 °C/W 120 -55 to 150 -40 to 150 1 W 0.83 °C °C Unit 7/36 Electrical characteristics L6599 5 Electrical characteristics TJ = 0 to 105°C, VCC = 15V, VBOOT = 15V, CHVG = CLVG = 1nF; CF = 470pF; RRFmin = 12kΩ; unless otherwise specified. Table 4. Symbol IC supply voltage VCC VCC(ON) VCC(OFF) Hys VZ Operating range Turn-ON threshold Turn-OFF threshold Hysteresis VCC clamp voltage Iclamp = 10mA 16 After device turn-on Voltage rising Voltage falling 8.85 10 7.45 10.7 8.15 2.55 17 17.9 16 11.4 8.85 V V V V V Electrical characteristics Parameter Test condition Min Typ Max Unit Supply current Istart-up Iq Iop Start-up current Quiescent current Operating current Before device turn-ON VCC = VCC(ON) - 0.2V Device ON, VSTBY = 1V Device ON, VSTBY = VRFmin VDIS > 1.85V or VDELAY > 3.5V or VLINE < 1.25 V or VLINE = Vclamp 200 1.5 3.5 250 2 5 µA mA mA Iq Residual consumption 300 400 µA High-side floating gate-drive supply ILKBOOT ILKOUT rDS(on) VBOOT pin leakage current VBOOT = 580V 5 5 150 µA µA Ω OUT pin leakage current VOUT = 562V Synchronous bootstrap diode ON-resistance VLVG = High Overcurrent comparator IISEN tLEB VISENx Input bias current Leading edge blanking Frequency shift threshold Hysteresis VISENdis td(H-L) Latch OFF threshold Delay to output VISEN = 0 to VISENdis After VHVG and VLVG low-to-high transition Voltage rising (1) Voltage falling Voltage rising (1) 1.44 0.76 250 0.8 50 1.5 300 1.56 400 0.84 -1 µA ns V mV V ns 8/36 L6599 Table 4. Symbol Line sensing Vth IHyst Vclamp DIS function IDIS Vth Oscillator D Output duty cycle Both HVG and LVG Input bias current Disable threshold VDIS = 0 to Vth Voltage rising (1) Threshold voltage Current hysteresis Clamp level Voltage rising or falling (1) Electrical characteristics Electrical characteristics Parameter Test condition Min Typ Max Unit 1.2 12 6 1.25 15 1.3 18 8 V µA V VCC > 5V, VLINE = 0.3V ILINE = 1mA -1 1.77 1.85 1.93 µA V 48 58.2 50 60 250 52 61.8 % kHz fosc Oscillation frequency RRFmin= 2.7 kΩ Maximum recommended 240 260 500 kHz µs V V 2.08 V A/A 100 kΩ TD VCFp VCFv VREF KM RFMIN Dead-time Peak value Valley value Voltage reference at pin 4 Current mirroring ratio Timing resistor range Between HVG and LVG 0.2 0.3 3.9 0.9 0.4 (1) 1.92 2 1 1 PFC_STOP function Ileak High level leakage current Low saturation level VPFC_STOP = VCC, VDIS = 0V IPFC_STOP =1mA, VDIS = 2V 1 µA VL 0.2 V Soft-start function Ileak R Open-state current Discharge resistance V(Css) = 2V VISEN > VISENx 120 0.5 µA Ω Standby function IDIS Vth Hys Input Bias Current Disable threshold Hysteresis VDIS = 0 to Vth Voltage falling (1) Voltage rising 1.2 1.25 50 -1 1.3 µA V mV 9/36 Electrical characteristics Table 4. Symbol L6599 Electrical characteristics Parameter Test condition Min Typ Max Unit Delayed shutdown function Ileak ICHARGE Vth1 Open-state current Charge current Threshold for forced operation at max. frequency Shutdown threshold Restart threshold V(DELAY) = 0 VDELAY = 1V, VISEN = 0.85V Voltage rising (1) 100 1.92 150 2 0.5 200 2.08 µA µA V Vth2 Vth3 Voltage rising (1) Voltage falling (1) 3.3 0.25 3.5 0.3 3.7 0.35 V V Low - side gate driver (voltages referred to GND) VLVGL VLVGH Isourcepk Isinkpk tf tr Output low voltage Output high voltage Peak source current Peak sink current Fall time Rise time UVLO saturation VCC = 0 to VCC(ON), Isink = 2mA Isink = 200mA Isource = 5mA 12.8 -0.3 0.8 30 60 1.1 13.3 1.5 V V A A ns ns V High-side gate driver (voltages referred to OUT) VHVGL VHVGH Isourcepk Isinkpk tf tr Output low voltage Output high voltage Peak source current Peak sink current Fall time Rise time HVG-OUT pull-down 1. Values traking each other Isink = 200 mA Isource = 5 mA 12.8 -0.3 0.8 30 60 25 13.3 1.5 V V A A ns ns kΩ 10/36 L6599 Typical electrical performance 6 Typical electrical performance Figure 3. Device consumption vs supply voltage Figure 4. IC consumption vs junction temperature Figure 5. VCC clamp voltage vs junction temperature Figure 6. UVLO thresholds vs junction temperature 11/36 Typical electrical performance Figure 7. Oscillator frequency vs junction temperature Figure 8. Dead-time vs junction temperature L6599 Figure 9. Oscillator frequency vs timing components Figure 10. Oscillator ramp vs junction temperature 12/36 L6599 Figure 11. Reference voltage vs junction temperature Typical electrical performance Figure 12. Current mirroring ratio vs junction temperature Figure 13. OCP delay source current vs junction temperature Figure 14. OCP delay thresholds vs junction temperature 13/36 Typical electrical performance Figure 15. Standby thresholds vs junction temperature L6599 Figure 16. Current sense thresholds vs junction temperature Figure 17. Line thresholds vs junction temperature Figure 18. Line source current vs junction temperature Figure 19. Latched disable threshold vs junction temperature 14/36 L6599 Application information 7 Application information The L6599 is an advanced double-ended controller specific for resonant half-bridge topology. In these converters the switches (MOSFETs) of the half-bridge leg are alternately switched on and OFF (180° out-of-phase) for exactly the same time. This is commonly referred to as operation at "50% duty cycle", although the real duty cycle, that is the ratio of the ON-time of either switch to the switching period, is actually less than 50%. The reason is that there is an internally fixed dead-time TD, inserted between the turn-OFF of either MOSFET and the turn-ON of the other one, where both MOSFETs are OFF. This dead- time is essential in order for the converter to work correctly: it will ensure soft-switching and enable high-frequency operation with high efficiency and low EMI emissions. To perform converter's output voltage regulation the device is able to operate in different modes (Figure 20), depending on the load conditions: 1. Variable frequency at heavy and medium/light load. A relaxation oscillator (see "Oscillator" section for more details) generates a symmetrical triangular waveform, which MOSFETs' switching is locked to. The frequency of this waveform is related to a current that will be modulated by the feedback circuitry. As a result, the tank circuit driven by the half-bridge will be stimulated at a frequency dictated by the feedback loop to keep the output voltage regulated, thus exploiting its frequency-dependent transfer characteristics. Burst-mode control with no or very light load. When the load falls below a value, the converter will enter a controlled intermittent operation, where a series of a few switching cycles at a nearly fixed frequency are spaced out by long idle periods where both MOSFETs are in OFF-state. A further load decrease will be translated into longer idle periods and then in a reduction of the average switching frequency. When the converter is completely unloaded, the average switching frequency can go down even to few hundred Hz, thus minimizing magnetizing current losses as well as all frequencyrelated losses and making it easier to comply with energy saving recommendations. 2. Figure 20. Multi-mode operation 15/36 Application information L6599 7.1 Oscillator The oscillator is programmed externally by means of a capacitor (CF), connected from pin 3 (CF) to ground, that will be alternately charged and discharged by the current defined with the network connected to pin 4 (RFmin). The pin provides an accurate 2V reference with about 2mA source capability and the higher the current sourced by the pin is, the higher the oscillator frequency will be. The block diagram of Figure 21 shows a simplified internal circuit that explains the operation. The network that loads the RFmin pin generally comprises three branches: 1. 2. A resistor RFmin connected between the pin and ground that determines the minimum operating frequency; A resistor RFmax connected between the pin and the collector of the (emitter-grounded) phototransistor that transfers the feedback signal from the secondary side back to the primary side; while in operation, the phototransistor will modulate the current through this branch - hence modulating the oscillator frequency - to perform output voltage regulation; the value of RFmax determines the maximum frequency the half-bridge will be operated at when the phototransistor is fully saturated; An R-C series circuit (CSS + RSS) connected between the pin and ground that enables to set up a frequency shift at start-up (see Chapter 7.3: Soft-start). Note that the contribution of this branch is zero during steady-state operation. 3. Figure 21. Oscillator's internal block diagram. L6599 2V + RFmin 4 IR 0.9V 1V KM·I R 2·KM·I R KM·I R 3 CF CF S Q R RFmin R SS R Fmax + + - C SS 3.9V 4V The following approximate relationships hold for the minimum and the maximum oscillator frequency respectively: 1 f min = -----------------------------------------3 ⋅ CF ⋅ RF min 1 f max = -------------------------------------------------------------------------3 ⋅ CF ⋅ ( RF min | | RF max ) 16/36 L6599 Application information After fixing CF in the hundred pF or in the nF (consistently with the maximum source capability of the RFmin pin and trading this off against the total consumption of the device), the value of RFmin and RFmax will be selected so that the oscillator frequency is able to cover the entire range needed for regulation, from the minimum value fmin (at minimum input voltage and maximum load) to the maximum value fmax (at maximum input voltage and minimum load): 1 RF min = ----------------------------------3 ⋅ CF ⋅ f min RF min RF max = -------------------f max ---------- – 1 f min A different selection criterion will be given for RFmax in case burst-mode operation at no-load will be used (see "Operation at no load or very light load" section). Figure 22. Oscillator waveforms and their relationship with gate-driving signals CF HVG TD TD t LVG t HB t t In Figure 22 the timing relationship between the oscillator waveform and the gate-drive signals, as well as the swinging node of the half-bridge leg (HB) is shown. Note that the lowside gate-drive is turned on while the oscillator's triangle is ramping up and the high-side gate-drive is turned on while the triangle is ramping down. In this way, at start-up, or as the IC resumes switching during burst-mode operation, the low-side MOSFET will be switched on first to charge the bootstrap capacitor. As a result, the bootstrap capacitor will always be charged and ready to supply the high-side floating driver. 17/36 Application information L6599 7.2 Operation at no load or very light load When the resonant half-bridge is lightly loaded or unloaded at all, its switching frequency will be at its maximum value. To keep the output voltage under control in these conditions and to avoid losing soft-switching, there must be some significant residual current flowing through the transformer's magnetizing inductance. This current, however, produces some associated losses that prevent converter's no-load consumption from achieving very low values. To overcome this issue, the L6599 enables the designer to make the converter operate intermittently (burst-mode operation), with a series of a few switching cycles spaced out by long idle periods where both MOSFETs are in OFF-state, so that the average switching frequency can be substantially reduced. As a result, the average value of the residual magnetizing current and the associated losses will be considerably cut down, thus facilitating the converter to comply with energy saving recommendations. The device can be operated in burst-mode by using pin 5 (STBY): if the voltage applied to this pin falls below 1.25V the IC will enter an idle state where both gate-drive outputs are low, the oscillator is stopped, the soft-start capacitor CSS keeps its charge and only the 2V reference at RFmin pin stays alive to minimize IC's consumption and VCC capacitor's discharge. The IC will resume normal operation as the voltage on the pin exceeds 1.25V by 50mV. To implement burst-mode operation the voltage applied to the STBY pin needs to be related to the feedback loop. Figure 23 shows the simplest implementation, suitable with a narrow input voltage range (e.g. when there is a PFC front-end). Figure 23. Burst-mode implementation: narrow input voltage range. RFmin 4 RFmin RFmax STBY 5 RFmin RFmax L6599 Figure 24. Burst-mode implementation: wide input voltage range. B+ RFmin 4 RFmin RFmax STBY RA RC RD RB RA + RB >> RC 5 L6599 L6599 7 RC RD LINE 18/36 L6599 Application information Essentially, RFmax will define the switching frequency fmax above which the L6599 will enter burst-mode operation. Once fixed fmax, RFmax will be found from the relationship: RF min 3 RF max = -- ⋅ -------------------8 f max ---------- – 1 f min Note that, unlike the fmax considered in the previous section ("Chapter 7.1: Oscillator"), here fmax is associated to some load PoutB greater than the minimum one. PoutB will be such that the transformer's peak currents are low enough not to cause audible noise. Resonant converter's switching frequency, however, depends also on the input voltage; hence, in case there is quite a large input voltage range with the circuit of Figure 23 the value of PoutB would change considerably. In this case it is recommended to use the arrangement shown in Figure 24 where the information on the converter's input voltage is added to the voltage applied to the STBY pin. Due to the strongly non-linear relationship between switching frequency and input voltage, it is more practical to find empirically the right amount of correction RA / (RA + RB) needed to minimize the change of PoutB. Just be careful in choosing the total value RA + RB much greater than RC to minimize the effect on the LINE pin voltage (see Chapter 7.6: Line sensing function). Whichever circuit is in use, its operation can be described as follows. As the load falls below the value PoutB the frequency will try to exceed the maximum programmed value fmax and the voltage on the STBY pin (VSTBY) will go below 1.25V. The IC will then stop with both gate-drive outputs low, so that both MOSFETs of the half-bridge leg are in OFF-state. The voltage VSTBY will now increase as a result of the feedback reaction to the energy delivery stop and, as it exceeds 1.3V, the IC will restart switching. After a while, VSTBY will go down again in response to the energy burst and stop the IC. In this way the converter will work in a burst-mode fashion with a nearly constant switching frequency. A further load decrease will then cause a frequency reduction, which can go down even to few hundred hertz. The timing diagram of Figure 25 illustrates this kind of operation, showing the most significant signals. A small capacitor (typically in the hundred pF) from the STBY pin to ground, placed as close to the IC as possible to reduce switching noise pick-up, will help get clean operation. To help the designer meet energy saving requirements even in power-factor-corrected systems, where a PFC pre-regulator precedes the DC-DC converter, the device allows that the PFC pre-regulator can be turned off during burst-mode operation, hence eliminating the no-load consumption of this stage (0.5 ÷ 1W). There is no compliance issue in that because EMC regulations on low-frequency harmonic emissions refer to nominal load, no limit is envisaged when the converter operates with light or no load. To do so, the device provides pin 9 (PFC_STOP): it is an open collector output, normally open, that is asserted low when the IC is idle during burst-mode operation. This signal will be externally used for switching off the PFC controller and the pre-regulator as shown in Figure 26 When the L6599 is in UVLO the pin is kept open, to let the PFC controller start first. 19/36 Application information Figure 25. Load-dependent operating modes: timing diagram STBY L6599 1.25V 50 mV hyster. fosc t LVG HVG PFC_STOP t t PFC GATE-DRIVE R esonant Mode Burst-mode Resonant Mode Figure 26. How the L6599 can switch OFF a PFC controller at light load Vcc 12 22 kΩ 100 kΩ BC547 9 PFC_STOP ZCD L6599 L6561/2 9 PFC_STOP L6599 BC547 PFC_OK L6563 (AC_OK) 20/36 L6599 Application information 7.3 Soft-start Generally speaking, purpose of soft-start is to progressively increase converter's power capability when it is started up, so as to avoid excessive inrush current. In resonant converters the deliverable power depends inversely on frequency, then soft- start is done by sweeping the operating frequency from an initial high value until the control loop takes over. With the L6599 converter's soft start-up is simply realized with the addition of an R-C series circuit from pin 4 (RFmin) to ground (see Figure 27). Initially, the capacitor CSS is totally discharged, so that the series resistor RSS is effectively in parallel to RFmin and the resulting initial frequency is determined by RSS and RFmin only, since the optocoupler's phototransistor is cut off (as long as the output voltage is not too far away from the regulated value): 1 f start = ----------------------------------------------------------------------3 ⋅ CF ⋅ ( R ( F min | | R SS ) ) The CSS capacitor is progressively charged until its voltage reaches the reference voltage (2V) and, consequently, the current through RSS goes to zero. This conventionally takes 5 time constants RSS·CSS but, before that time, the output voltage will have got close to the regulated value and the feedback loop taken over, so that it will be the optocoupler's phototransistor to determine the operating frequency from that moment onwards. During this frequency sweep phase the operating frequency will decay following the exponential charge of CSS, that is, initially it will change relatively quickly but the rate of change will get slower and slower. This counteracts the non-linear frequency dependence of the tank circuit that makes converter's power capability change little as frequency is away from resonance and change very quickly as frequency approaches resonance frequency (see Figure 28). Figure 27. Soft-start circuit RFmin 4 RFmin RSS Css |Z(f)| L6599 1 CSS 21/36 Application information Figure 28. Power vs frequency curve in an resonant half-bridge RESONANCE FREQUENCY L6599 |Z(f)| -1 f Steady-state frequency Initial frequency As a result, the average input current will smoothly increase, without the peaking that occurs with linear frequency sweep, and the output voltage will reach the regulated value with almost no overshoot. Typically, RSS and CSS will be selected based on the following relationships: RF min R SS = --------------------f start ----------- – 1 f min C ss = 3 ⋅ 10 --------------------R SS –3 where fstart is recommended to be at least 4 times fmin. The proposed criterion for CSS is quite empirical and is a compromise between an effective soft-start action and an effective OCP (see next section). Please refer to the timing diagram of Figure 31 to see some significant signals during the soft-start phase. 22/36 L6599 Application information 7.4 Current sense, OCP and OLP The resonant half-bridge is essentially voltage-mode controlled; hence a current sense input will only serve as an overcurrent protection (OCP). Unlike PWM-controlled converters, where energy flow is controlled by the duty cycle of the primary switch (or switches), in a resonant half-bridge the duty cycle is fixed and energy flow is controlled by its switching frequency. This impacts on the way current limitation can be realized. While in PWM-controlled converters energy flow can be limited simply by terminating switch conduction beforehand when the sensed current exceeds a preset threshold (this is commonly now as cycle-by-cycle limitation), in a resonant half-bridge the switching frequency, that is, its oscillator's frequency must be increased and this cannot be done as quickly as turning off a switch: it takes at least the next oscillator cycle to see the frequency change. This implies that to have an effective increase, able to change the energy flow significantly, the rate of change of the frequency must be slower than the frequency itself. This, in turn, implies that cycle-by-cycle limitation is not feasible and that, therefore, the information on the primary current fed to the current sensing input must be somehow averaged. Of course, the averaging time must not be too long to prevent the primary current from reaching too high values. In Figure 29 and Figure 30 a couple of current sensing methods are illustrated that will be described in the following. The circuit of Figure 29 is simpler but the dissipation on the sense resistor Rs might not be negligible, hurting efficiency; the circuit of Figure 30 is more complex but virtually lossless and recommended when the efficiency target is very high. Figure 29. Current sensing technique with sense resistor L6599 6 ISEN Cr ICr L6599 Vspk 0 6 τ≈ 10 fmin Rs 23/36 Application information Figure 30. Lossless current sensing technique, with capacitive shunt L6599 τ≈ L6599 pk CB 6 ISEN 10 fmin 1N4148 CA RA VCrpk RB 1N4148 ICr Cr The device is equipped with a current sensing input (pin 6, ISEN) and a sophisticated overcurrent management system. The ISEN pin is internally connected to the input of a first comparator, referenced to 0.8V, and to that of a second comparator referenced to 1.5V. If the voltage externally applied to the pin by either circuit in Figure 29 or Figure 30 exceeds 0.8V the first comparator is tripped and this causes an internal switch to be turned on and discharge the soft-start capacitor CSS (see Chapter 7.3: Soft-start). This will quickly increase the oscillator frequency and thereby limit energy transfer. The discharge will go on until the voltage on the ISEN pin has dropped by 50mV; this, with an averaging time in the range of 10/fmin, ensures an effective frequency rise. Under output short circuit, this operation results in a nearly constant peak primary current. It is normal that the voltage on the ISEN pin may overshoot above 0.8V; however, if the voltage on the ISEN pin reaches 1.5V, the second comparator will be triggered, the L6599 will shutdown and latch off with both the gate-drive outputs and the PFC_STOP pin low, hence turning off the entire unit. The supply voltage of the IC must be pulled below the UVLO threshold and then again above the start-up level in order to restart. Such an event may occur if the soft-start capacitor CSS is too large, so that its discharge is not fast enough or in case of transformer's magnetizing inductance saturation or a shorted secondary rectifier. In the circuit shown in Figure 29 where a sense resistor RS in series to the source of the low-side MOSFET is used, note the particular connection of the resonant capacitor. In this way the voltage across RS is related to the current flowing through the high-side MOSFET and is positive most of the switching period, except for the time needed for the resonant current to reverse after the low-side MOSFET has been switched OFF. Assuming that the time constant of the RC filter is at least ten times the minimum switching frequency fmin, the approximate value of RS can be found using the empirical equation: Vs pkx 5 ⋅ 0.8 4R S = -------------- ≈ ------------------ ≈ -------------I Crpkx I Crpkx I Crpkx where ICrpkx is the maximum desired peak current flowing through the resonant capacitor and the primary winding of the transformer, which is related to the maximum load and the minimum input voltage. 24/36 L6599 Application information The circuit shown in Figure 30 can be operated in two different ways. If the resistor RA in series to CA is small (not above some hundred Ω just to limit current spiking) the circuit , operates like a capacitive current divider; CA will be typically selected equal to CR/100 or less and will be a low-loss type, the sense resistor RB will be selected as: Cr 0.8 πR B = -------------- ⎛ 1 + ------ ⎞ I Crpkx ⎝ C A⎠ and CB will be such that RB·CB is in the range of 10 /fmin. If the resistor RA in series to CA is not small (in this case it will be typically selected in the ten kΩ ), the circuit operates like a divider of the ripple voltage across the resonant capacitor Cr, which, in turn, is related to its current through the reactance of Cr. Again, CA will be typically selected equal to CR/100 or less, this time not necessarily a low-loss type, while RB (provided it is