AN3014 STMICROELECTRONICS | Alldatasheet
Document overview
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- PDF pages: 29
Technical content
Datasheet sections
- 1 Main characteristics and cir cuit description
- 2 Efficiency measurement
- 3 Harmonic content measurement
- 4 Functional check
- 5 Thermal map
- 6 Conducted emission pre-compli ance test
- 7 Bill of material
- 8 PFC coil specification
- 9 Transformer specification
- 10 Revision history
19 V, 90 W resonant converter with synchronous rectification
can be dramatically reduced or even removed. Figure 1. EVL90WADP-LLCSR: 90 W adapter demonstration board
AN3014 Main characteristics and circuit description Doc ID 16064 Rev 2 5/29
1 Main characteristics and circuit description
The main features of the SMPS are:
- Universal input mains range: 90 − 264 Vac, frequency 45 − 65 Hz
- Output voltage: 19 V at 4.75 A continuous operation
- Mains harmonics: according to EN61000-3-2 class-D or JEITA-MITI class-D
- Standby mains consumption: < 0.26 W at 230 Vac
- Efficiency at nominal load: > 92% at 115 Vac
- EMI: according to EN55022-class-B
- Safety: according to EN60950
- Dimensions: 65 x 155 mm, 25 mm maximum component height
- PCB: double-sided, 70 µm, FR-4, mixed PTH/SMT The circuit is composed of two stages: a front-end PFC using the L6563H, and a LLC resonant converter based on the L6599A. The SRK2000 controls the synchronous rectification on the secondary side. The PFC stage works as a preregulator and powers the resonant stage with a constant voltage of 400 V. The downstream converter operates only if the PFC is on and regulating. In this way, the resonant stage can be optimized for a narrow input voltage range. Startup sequence As indicated previously, the PFC acts as master and the resonant stage can operate only if the PFC output is delivering the nominal output voltage. Therefore the circuit is designed so that at startup the PFC starts first, then the downstream converter turns on. Initially, the L6563H is supplied by the integrated high voltage startup circuit, but as soon as the PFC starts switching, a charge pump connected to the PFC inductor supplies both the PFC and resonant controllers. Once both stages have been activated, the controllers are supplied also by the auxiliary winding of the resonant transformer, assuring correct supply voltage even during standby operation. Because the L6563H integrated HV startup circuit is turned off and therefore is not dissipative during normal operation, it significantly contributes to the reduction of power consumption when the power supply operates at light load, in accordance with current world-wide standby consumption standards. Brownout protection Brownout protection prevents the circuit from working with abnormal mains levels. It is easily achieved using the RUN pin (pin12) of the L6563H. This pin is connected through a resistor divider to the VFF pin (pin 5), which provides the mains voltage peak value information. An internal comparator enables the IC operations if the mains level is correct, within the nominal limits. At startup, if the input voltage is below 90 Vac (typ), circuit operations are inhibited. The L6599A has similar protection on the LINE pin (pin 7). It is used to prevent the resonant converter from working with too low an input voltage, which can cause incorrect capacitive mode operation. If the bulk voltage (PFC output) is below 380 V, the resonant startup is not allowed. The L6599A internal comparator has a hysteresis which allows the turn-on and turn-off voltage to be set independently. The turn-off threshold has been set to 300 V in
Main characteristics and circuit description AN3014 6/29 Doc ID 16064 Rev 2 order to avoid capacitive mode operation, but to allow the resonant stage to operate even in case of mains sag and consequent PFC output dip. Fast voltage feed-forward The voltage on the L6563H VFF pin (pin 5) is the peak value of the voltage on the MULT pin (pin 3). The RC network (R15+R26, C12) connected to VFF completes the peak-holding circuit. This signal is necessary to derive RMS input voltage information to compensate the loop gain, which is mains voltage dependent. Generally speaking, if the time constant is too small, the voltage generated is affected by a considerable amount of ripple at twice the mains frequency, thus causing distortion of the current reference (resulting in high THD and poor PF). If the time constant is too large, there is a considerable delay in setting the right amount of feed-forward, resulting in excessive overshoot or undershoot of the preregulator's output voltage in response to large line voltage changes. To overcome this issue, the L6563H implements the new fast voltage feed-forward function. As soon as the voltage on the VFF pin decreases to a set threshold (40 mV typically), a mains dip is assumed and an internal switch rapidly discharges the VFF capacitor via a 10- kΩ resistor. Thanks to this feature, it is possible to set an RC circuit with a long time constant, assuring a low THD and maintaining a fast response to mains dip. Resonant power stage The downstream converter employs ST’s L6599A, which incorporates all the functions necessary to properly control the resonant converter with a 50% fixed duty cycle and works with a variable frequency. The transformer uses the integrated magnetic approach, incorporating a resonant series inductor. Thus, no additional external coil is needed for the resonance. The transformer configuration chosen for the secondary winding is center tap. On the secondary side, the output rectification is controlled by the SRK2000, an SR driver dedicated to LLC resonant topology. A small LC filter has been added on the output, filtering the high-frequency ripple. D15, R56, R62, R65, R66, Q5 and Q6 implement an output voltage “fast discharge” circuit which quickly discharges the output capacitors when the converter is turned off. It has been implemented to quickly decrease the residual output voltage after the converter is turned off at no load. Output voltage feedback loop The feedback loop is implemented by means of a typical circuit using a TL431 to modulate the current in the optocoupler diode. On the primary side, R34 - connecting the RFMIN pin (pin 4) to the optocoupler phototransistor - closes the feedback loop and its value sets the maximum switching frequency at about 130 kHz. This value has been chosen to limit the switching losses at light load operation. R31, which connects the same pin to ground, sets the minimum switching frequency. The R-C series (R44 and C18) sets both the soft-start maximum frequency and duration.
AN3014 Main characteristics and circuit description Doc ID 16064 Rev 2 7/29 L6599A overload and short-circuit protection The current into the primary winding is sensed by the lossless circuit R41, C27, D11, D10, R39, and C25 and is fed to the ISEN pin (pin 6). In case of overcurrent, the voltage on the pin passes an internal threshold (0.8 V), triggering a protection sequence. The capacitor (C45) connected to the DELAY pin (pin 2) is charged by an internal 150-µA current generator and is slowly discharged by the external resistor (R24). If the voltage on the pin reaches 2 V, the soft-start capacitor is completely discharged so that the switching frequency is pushed to its maximum value. As the voltage on the pin exceeds 3.5 V, the IC stops switching and the internal generator is turned off, so that the voltage on the pin decays due to the external resistor. The IC is soft-restarted as the voltage drops below 0.3 V. In this way, under short-circuit conditions, the converter works intermittently with very low input average power. Overvoltage and open loop protection Both the PFC and resonant circuit stages are equipped with their own overvoltage protection. The PFC controller L6563H monitors its output voltage via the resistor divider connected to a dedicated pin (PFC_OK, pin 7), protecting the circuit in case of loop failures, disconnection or deviation from the nominal value of the feedback loop divider. If the voltage on pin #7 exceeds 2.5 V the IC stops switching and restarts as the voltage on the pin falls below 2.4 V, preventing the output voltage becoming excessive in case of transient due to the slow response of the error amplifier. However, if contemporaneously the voltage of the INV pin falls below 1.66 V (typ.), a feedback failure is assumed. In this case the PFC_OK circuitry latches the L6563H operations and, by means of the PWM_LATCH pin (pin 8) it latches the L6599A as well, via the DIS pin (pin 8). The converter is kept latched by the L6563H HV circuit which supplies the IC, charging the V CC capacitor periodically. To resume converter operation, mains restart is necessary. The DIS pin is used to protect also the resonant stage against overvoltage. The Zener diode D8 detects the auxiliary voltage, which is proportional to the output voltage. In case of loop failure it conducts and voltage on the DIS pin exceeds the internal threshold, and latches off the device. L6563H operation is also stopped by the PFC_STOP pin. Secondary-side synchronous rectification with the SRK2000 The SRK2000 core function is to switch on each synchronous rectifier MOSFET whenever the corresponding transformer half-winding starts conducting (i.e. when the MOSFET body diode starts conducting), and then to switch it off when the current flowing through it approaches zero. For this purpose, the IC is equipped with two pins (DVS1 and DVS2) capable of sensing the MOSFET drain voltage level. Standby power saving The board has a burst-mode function implemented, allowing power saving during light load operation. The L6599A STBY pin (pin 5) senses the optocoupler’s collector voltage, which is related to the feedback control. This signal is compared to an internal reference (1.24 V). If the voltage on the pin is lower than the reference, the IC enters an idle state and its quiescent current is reduced. As the voltage exceeds the reference by 50 mV, the controller restarts the switching. The burst-mode operation load threshold can be programmed by properly choosing the resistor connecting the optocoupler to the RFMIN pin (R34). On this board, the controller operates in burst-mode if the load falls below ~10 W.
Main characteristics and circuit description AN3014 8/29 Doc ID 16064 Rev 2 The L6563H implements its own burst-mode function. If the COMP voltage falls below 2.5 V, the IC stops switching, causing an output voltage drop. As a consequence, the COMP voltage rise again and the IC starts switching again. In order to achieve better load transient response, the PFC burst-mode operation is partially forced by the resonant converter: as soon as the L6599A stops switching due to load drops, its PFC_STOP pin pulls down the L6563H's PFC_OK pin, disabling PFC switching. Thanks to this simple circuit, the PFC is forced into an idle state when the resonant stage is not switching, and rapidly wakes up when the downstream converter restarts switching. This solution prevents significant drop of the bulk voltage in case of abrupt load rising.
Figure 2. Electrical diagram
2 Efficiency measurement
mains voltages. At 115 Vac the average efficiency is 91.27%, while at 230 Vac it is 92.22%. with the new synchronous rectification solution is obtained. Table 1. Overall efficiency
230 V-50 Hz 115 V-60 Hz
Table 2. Efficiency comparison
efficiency is better than 68% even for very light loads, such as 1 W. Figure 3. Light load efficiency diagram Table 3. Light load efficiency
230 V - 50 Hz 115 V - 60 Hz
3 Harmonic content measurement
Figure 4. Compliance to EN61000-3-2 at
230 Vac - 50 Hz, full load
Figure 5. Compliance to JEITA-MITI at
100 Vac - 50 Hz, full load
4 Functional check
Figure 6. Resonant stage oscillator at
230 V - 50 Hz - full load
Figure 7. Resonant stage waveforms at
PFC and LLC bursts are synchronized. Figure 8. Secondary waveforms at
230 V - 50 Hz
Figure 9. Startup sequencing at Figure 10. No load operation at Figure 11. No load operation - detail at
on and discharging the soft-start capacitor CSS. Under output short-circuit, this operation results in a nearly constant peak primary current. down, and operation resumes after an off-on cycle. Figure 12. Transition full load to no load at
265 Vac - 50 Hz
Figure 13. Transition no load to full load at
primary current are limited, preventing converter over-heating and consequent failure. Figure 14. Short-circuit at full load and
115 Vac - 60 Hz
Figure 15. Short-circuit detail at full load
5 Thermal map
In order to check design reliability, thermal mapping by means of an IR camera was done. ambient temperature during both measurements was 27 °C. Figure 16. Thermal map at 115 Vac - 60 Hz - full load Figure 17. Thermal map at 230 Vac - 50 Hz - full load side are working without a heatsink, their operating case temperature is lower than 60 °C. Table 4. Thermal map reference points
6 Conducted emission pre-compliance test
diagrams, under all test conditions the measurements are well below the limits. Figure 18. CE average measurement at 115 Vac and full load Figure 19. CE average measurement at 230 Vac and full load
7 Bill of material
Table 5. EVL90WADP-LLCSR demonstration board bill of material
Table 5. EVL90WADP-LLCSR demonstration board bill of material (continued)
PFC coil specification AN3014 24/29 Doc ID 16064 Rev 2
8 PFC coil specification
General description and characteristics
- Application type: consumer, home appliance
- Transformer type: open
- Coil former: vertical type, 6+6 pins
- Max. temperature rise: 45 °C
- Max. operating ambient temperature: 60 °C
- Mains insulation: N.A.
- Unit finishing: varnished
Electrical characteristics
- Converter topology: boost, transition mode
- Core type: PQ26/20-PC44 or equivalent
- Min. operating frequency: 40 kHz
- Typical operating frequency: 120 kHz
- Primary inductance: 520 µH ± 10% at 1 kHz - 0.25 V (see note below)
- Peak primary current: 4.2 Apk Note: Measured between pins 5 and 9. Electrical diagram and winding characteristics
Figure 20. PFC coil electrical Table 6. PFC coil winding data
- Maximum height from PCB: 22 mm
- Coil former type: vertical, 6+6 pins (pins 1, 2, 4, 6, 7, 10, 12 are removed)
- Pin distance: 3.81 mm
- Row distance: 25 mm
- External copper shield: not insulated, wound around the ferrite core and including the coil former. Height is 8 mm. Connected to pin 3 by a soldered solid wire.
Figure 21. PFC coil mechanical aspect
- Magnetica - Italy
- Inductor P/N: 1974.0002 !-V
Transformer specification AN3014 26/29 Doc ID 16064 Rev 2
9 Transformer specification
General description and characteristics
- Application type: consumer, home appliance
- Transformer type: open
- Coil former: horizontal type, 7+7 pins, two slots
- Max. temperature rise: 45 °C
- Max. operating ambient temperature: 60 °C
- Mains insulation: in accordance with EN60950
- Converter topology: half bridge, resonant
- Core type: ETD34-PC44 or equivalent
- Min. operating frequency: 60 kHz
- Typical operating frequency: 100 kHz
- Primary inductance: 1200 µH ± 10% at 1 kHz - 0.25 V (see Note 1)
- Leakage inductance: 200 µH ±10% at 100 kHz - 0.25 V (see Note 2) Note: 1 Measured between pins 2-4
2 Measured between pins 2-4 with only one secondary winding shorted
Electrical diagram and winding characteristics Figure 22. Transforme r electrical diagram Table 7. Transformer winding data
- Secondary windings A and B are in parallel
- Maximum height from PCB: 30 mm
- Coil former type: horizontal, 7+7 pins (pins 1 and 7 are removed)
- Pin distance: 5.08 mm
- Row distance: 25.4 mm
Figure 23. Transformer overall drawing
- Magnetica - Italy
- Transformer P/N: 1860.0025 2. Aux winding is wound on top of primary winding !-V
Table 8. Document revision history 22-Mar-2010 1 Initial release.