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TEA1751T
HV start-up DCM/QR flyback controller with integrated DCM/QR PFC controller
1. General descriptionThe TEA1751T is the third generation of green Switched Mode Power Supply (SMPS)
controller ICs. The TEA1751T combines a controller for Power Factor Correction (PFC)
and a flyback controller. Its high level of integration allows the design of a cost-effective
power supply with a very low number of external components.
The special built-in green functions provide high efficiency at all power levels. This
efficiency applies to quasi-resonant operation at high-power levels, quasi-resonant
operation with valley skipping, as well as reduced frequency operation at lower power
levels. At low-power levels, the PFC switches off to maintain high efficiency.
During low-power conditions, the flyback controller switches to frequency reduction mode
and limits the peak current to 25 % of its maximum value. This mode ensures high
efficiency at low-power and good standby power performance while minimizing audible
noise from the transformer.
The TEA1751T is a Multi-Chip Module, (MCM), containing two chips. The proprietary
high-voltage BCD800 process which makes direct start-up possible from the rectified
universal mains voltage in an effective and green way. The second low voltage
Silicon On Insulator (SOI) is used for accurate, high speed protection functions and
control.
The TEA1751T enables the design of highly efficient and reliable supplies with power
requirements of up to 250 W using the minimum number of external components.
Remark: All values provided throughout this data sheet are typical values unless otherwise stated.
TEA1751T
HV start-up DCM/QR flyback controller with integrated
DCM/QR PFC controller
Rev. 3 — 10 January 2013 Product data sheet
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
2. Features and benefits
2.1 Distinctive features Integrated PFC and flyback controller Universal mains supply operation (70V (AC) to 276V (AC)) Dual-boost PFC with accurate maximum output voltage (NXP patented) High level of integration, resulting in a very low external component count and a
cost-effective design
2.2 Green features On-chip start-up current source
2.3 PFC green features Valley/Zero Voltage Switching (ZVS) for minimum switching losses (NXP patented) Frequency limitation to reduce switching losses PFC is switched off when a low load is detected at the flyback output
2.4 Flyback green features Valley switching for minimum switching losses (NXP patented) Frequency reduction with fixed minimum peak current at low-power operation to
maintain high efficiency at low output power levels
2.5 Protection features Safe restart mode for system fault conditions Continuous mode protection with demagnetization detection for both converters (NXP
patented) UnderVoltage Protection (UVP) (foldback during overload) Accurate OverVoltage Protection (OVP) for both converters (adjustable for flyback
converter) Mains voltage independent OverPower Protection (OPP) Open control loop protection for both converters. The open-loop protection on the
flyback converter is safe restart OverTemperature Protection (OTP) Low and adjustable OverCurrent Protection (OCP) trip level for both converters General-purpose input for latched protection to provide system OverTemperature
Protection (OTP) for example
3. Applications The device is used in all applications requiring an efficient and cost-effective power
supply solutions up to 250 W. Notebook adapters in particular can benefit from the
high level of integration
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
4. Ordering informationTable 1. Ordering informationTEA1751T SO16 plastic small outline package; 16 leads; body width 3.9 mm SOT109-1
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
5. Block diagramNXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
6. Pinning information
6.1 Pinning
6.2 Pin description
Table 2. Pin descriptionVCC 1 supply voltage
GND 2 ground
FBCTRL 3 flyback control input
FBAUX 4 auxiliary winding input for demagnetization timing and flyback OVP
LATCH 5 general-purpose protection input
PFCCOMP 6 frequency compensation pin for PFC
VINSENSE 7 mains voltage sense input
PFCAUX 8 auxiliary winding input for demagnetization timing for PFC
VOSENSE 9 sense input for PFC output voltage
FBSENSE 10 flyback current sense input
PFCSENSE 11 PFC current sense input
PFCDRIVER 12 PFC gate-driver output
FBDRIVER 13 flyback gate-driver output
HVS 14 and15 high-voltage safety spacer, not connected 16 high-voltage start-up / flyback valley sensing
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7. Functional description
7.1 General controlThe TEA1751T contains a controller for a power factor correction circuit as well as a
controller for a flyback circuit. The typical configuration is shown in Figure3.
7.1.1 Start-up and UnderVoltage LockOut (UVLO)Initially, the capacitor on the VCC pin is charged from the high-voltage mains using the HV
pin.
When VCC is less than Vtrip, the charge current is low. This low current protects the IC if
the VCC pin is shorted to ground. To ensure a short start-up time, the charge current above
Vtrip is increased until VCC reaches Vth(UVLO). When VCC is between Vth(UVLO) and Vstartup,
the charge current goes low again to ensure a low, safe restart duty cycle during fault
conditions.
The control logic activates the internal circuitry and switches off the HV charge current
when the voltage on pin VCC passes the Vstartup level. First, the LATCH pin current source
is activated and the soft-start capacitors on the PFCSENSE and FBSENSE pins are
charged.
When the LATCH pin voltage exceeds the Ven(LATCH) voltage, and the soft start capacitor
on the PFCSENSE pin is charged, the PFC circuit is activated.
If the soft-start capacitor on the FBSENSE pin is charged, the flyback converter is also
activated. The flyback converter output voltage is then regulated to its nominal output
voltage. The auxiliary winding of the flyback converter takes over the IC supply. See
Figure4.
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controllerIf during start-up the LATCH pin does not reach the Ven(LATCH) level before VCC reaches
Vth(UVLO), it is deactivated. The charge current is then switched on again.
When the flyback converter starts, VFBCTRL is monitored. If this output voltage does not
reach its intended regulation level within a specified time, the voltage on the FBCTRL pin
reaches the Vto(FBCTRL) level. An error is then assumed and a safe restart is initiated.
When one of the protection functions is activated, both converters stop switching and the
VCC voltage drops to Vth(UVLO). A latched protection recharges capacitor CVCC using the
HV pin, but does not restart the converters. To provide safe restart protection, the
capacitor is recharged using the HV pin and the device restarts (see block diagram,
Figure1).
If OVP of the PFC circuit (VVOSENSE >VOVP(VOSENSE)) occurs, the PFC controller stops
switching until the VOSENSE pin voltage drops to less than VOVP(VOSENSE). If a mains
undervoltage is detected, VVINSENSE
until VVINSENSE >Vstart(VINSENSE) again.
When the voltage on pin VCC drops below the undervoltage lockout level, both controllers
stop switching and re-enter the safe restart mode. In the safe restart mode, the driver
outputs are disabled and the VCC pin voltage is recharged using the HV pin.
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller80 A. Switching off is stopped as soon as the V.V
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7.1.4 Fast latch reset
In a typical application, the mains can be interrupted briefly to reset the latched protection.
The PFC bus capacitor, Cbus, does not have to discharge for this latched protection to
reset.
When the VINSENSE voltage drops below 750 mV and is then raised to 870 mV, the
latched protection is reset.
The latched protection is also reset by removing the voltage from the VCC and HV pins.
7.1.5 Overtemperature protection
An accurate internal temperature protection is provided in the circuit. When the junction
temperature exceeds the thermal shut-down temperature, the IC stops switching. As long
as OTP is active, the capacitor CVCC is not recharged from the HV mains. If the VCC
supply voltage is not sufficient, the OTP circuit is supplied from the HV pin.
OTP is a latched protection. It is reset by removing the voltage from the VCC and HV pins
or by the fast latch reset function (see Section 7.1.4).
7.2 Power factor correction circuit
The power factor correction circuit operates in quasi-resonant or Discontinuous
Conduction Mode (DCM) with valley switching. The next primary stroke is only started
when the previous secondary stroke has ended and the voltage across the PFC MOSFET
has reached a minimum value. VPFCAUX is used to detect transformer demagnetization
and the minimum voltage across the external PFC MOSFET switch.
7.2.1 ton control
The power factor correction circuit is operated in ton control. The resulting mains harmonic
reduction is well within the class-D requirements.
7.2.2 Valley switching and demagnetization (PFCAUX pin)
The PFC MOSFET is switched on after the transformer is demagnetized. Internal circuitry
connected to the PFCAUX pin detects the end of the secondary stroke. It also detects the
voltage across the PFC MOSFET. To reduce switching losses and electromagnetic
Interference (EMI) (valley switching), the next stroke is started if the voltage across the
PFC MOSFET is at its minimum.
If a demagnetization signal is not detected on the PFCAUX pin, the controller generates a
Zero-current Signal (ZCS), 50 s after the last PFCGATE signal.
If a valley signal is not detected on the PFCAUX pin, the controller generates a valley
signal 4 s after demagnetization is detected.
To protect the internal circuitry during lightning events, for example, add a 5 k series
resistor to PFCAUX. To prevent incorrect switching due to external disturbance, place the
resistor close to the IC on the printed-circuit board.
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7.2.3 Frequency limitation
To optimize the transformer and minimize switching losses, the switching frequency is
limited to fsw(PFC)max. If the frequency for quasi-resonant operation is above the fsw(PFC)max
limit, the system switches over to DCM. The PFC MOSFET is only switched on at a
minimum voltage across the switch (valley switching).
7.2.4 Mains voltage compensation (VINSENSE pin)
The equation for the transfer function of a power factor corrector contains the square of
the mains input voltage. In a typical application, this results in a low bandwidth for low
mains input voltages and a high bandwidth for high mains input voltages.
To compensate for the mains input voltage influence, the TEA1751T contains a correction
circuit. The average input voltage is measured using the VINSENSE pin and the
information is fed to an internal compensation circuit. Using this compensation, it is
possible to keep the regulation loop bandwidth constant over the mains input range. This
feature yields a fast transient response on load steps, while still complying with class-D
MHR requirements.
In a typical application, a resistor and two capacitors connected to the PFCCOMP pin set
the bandwidth of the regulation loop.
7.2.5 Soft-start-up (pin PFCSENSE)
To prevent audible transformer noise at start-up or during hiccup, the soft-start function
slowly increases the transformer peak current. This increase is achieved by inserting RSS1
and CSS1 between the PFCSENSE pin and the current sense resistor RSENSE1. internal current source charges the capacitor to:
(1)
The voltage is limited to Vstart(soft)PFC.
The start level and the time constant of the increasing primary current level are adjusted
externally by changing the values of RSS1 and CSS1.
(2)
The charging current Istart(soft)PFC flows as long as VPFCSENSE is below 0.5 V. If VPFCSENSE
exceeds 0.5 V, the soft-start current source starts limiting current Istart(soft)PFC. When the
PFC starts switching, the Istart(soft)PFC current source is switched off; see Figure5.
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7.2.6 Low-power mode
When the output power of the flyback converter (see Section 7.3) is low, the flyback
converter switches over to frequency reduction mode. The power factor correction circuit
is then switched off to maintain high efficiency.
During low-power mode operation, the PFCCOMP pin is clamped to a minimum voltage of
2.7 V and a maximum voltage of 3.9 V. The lower clamp voltage limits the maximum
power that is delivered when the PFC is switched on again. The upper clamp voltage
ensures that the PFC returns to its normal regulation point in a limited time when returning
from low-power mode.
When the flyback converter leaves the frequency reduction mode, the power factor
correction circuit restores normal operation. To prevent continuous switching of the PFC
circuit, a small hysteresis is built in (75 mV on the FBCTRL pin).
7.2.7 Dual-boost PFC
The mains input voltage modulates the PFC output voltage. The mains input voltage is
measured using the VINSENSE pin. If the voltage on the VINSENSE pin drops below
2.2 V, the current is sourced from the VOSENSE pin. To ensure the stable switch-over, a
200 mV transition region is inserted around the 2.2 V, see Figure6.
At low VINSENSE input voltages, the output current is 15 A. This output current, in
combination with the resistors on the VOSENSE pin, sets the lower PFC output voltage
level at low mains voltages. At high mains input voltages, the current is switched to zero.
The PFC output voltage is then at its maximum. As this current is zero in this situation, it
does not affect the accuracy of the PFC output voltage.
To ensure proper switch-off, the VOSENSE current switches to its maximum value of A when the voltage on pin VOSENSE drops below 2.1V.
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7.2.8 Overcurrent protection (PFCSENSE pin)
The maximum peak current is limited cycle-by-cycle by sensing the voltage across an
external sense resistor, RSENSE1, on the source of the external MOSFET . The voltage is
measured via the PFCSENSE pin.
7.2.9 Mains undervoltage lockout/brownout protection (VINSENSE pin)
To prevent the PFC from operating at very low mains input voltages, the voltage on the
VINSENSE pin is continuously sensed. When the voltage on this pin drops below the
Vstop(VINSENSE) level, switching of the PFC is stopped.
The voltage on the VINSENSE pin is clamped to a minimum value, Vstart(VINSENSE) Vpu(VINSENSE). This voltage clamping provides for a fast restart when the mains input
voltage is restored after a mains dropout.
7.2.10 Overvoltage protection (VOSENSE pin)
To prevent output overvoltage during load steps and mains transients, an overvoltage
protection circuit is built in.
When the voltage on the VOSENSE pin exceeds the VOVP(VOSENSE) level, switching of the
power factor correction circuit is stopped. Switching of the PFC recommences when the
VOSENSE pin voltage drops to less than VOVP(VOSENSE) again.
When the resistor between the VOSENSE pin and ground is open, the overvoltage
protection is also triggered.
7.2.11 PFC open-loop protection (VOSENSE pin)
The power factor correction circuit does not start switching until the voltage on the
VOSENSE pin exceeds the Vth(ol)(VOSENSE) level. This feature protects the circuit from
open-loop and VOSENSE short-circuit.
7.2.12 Driver (PFCDRIVER pin)
The driver circuit to the gate of the power MOSFET has a current sourcing capability of
500 mA and a current sink capability of 1.2 A. These capabilities permit fast turn-on and
turn-off of the power MOSFET for efficient operation.
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7.3 Flyback controller
The TEA1751T includes a controller for a flyback converter. The flyback converter
operates in quasi-resonant or DCM with valley switching. The auxiliary winding of the
flyback transformer provides demagnetization detection and powers the IC after start-up.
7.3.1 Multimode operation
The TEA1751T flyback controller operates in several modes; see Figure7.
At high output power, the converter switches to quasi-resonant mode. The next converter
stroke starts after demagnetization of the transformer and detection of the valley. In
quasi-resonant mode switching losses are minimized. This minimization is achieved by
the converter only switching on when the voltage across the external MOSFET is at its
minimum (see also Section 7.3.2).
To prevent high frequency operation at low loads, the maximum switching frequency is
limited to 125 kHz. When the frequency limit is reached, the quasi-resonant operation
changes to DCM with valley skipping. The frequency limit reduces the MOSFET switch-on
losses and conducted EMI.
A Voltage Controlled Oscillator (VCO) controls the frequency at very low power and
standby levels. The minimum frequency is reduced to zero. During frequency reduction
mode, the primary peak current is kept at 25 % of its maximum level to maintain a high
efficiency. As the primary peak current is low in frequency reduction operation, no audible
noise is noticeable at switching frequencies in the audible range. Valley switching is also 8).
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7.3.2 Valley switching (HV pin)
A new cycle starts when the external MOSFET is switched on. VFBSENSE and VFBCTRL
determine the on-time. The MOSFET is then switched off and the secondary stroke starts.
After the secondary stroke, the drain voltage shows an oscillation with a frequency of
approximately:
(3)
where Lp is the primary self-inductance of the flyback transformer and Cd is the
capacitance on the drain node.
When the internal oscillator voltage is high and the secondary stroke ended, the circuit
waits for the lowest drain voltage before starting a new primary stroke.
Figure 9 shows the drain voltage, valley signal, secondary stroke signal and the internal
oscillator signal.
Valley switching allows high frequency operation as capacitive switching losses are
reduced, see Equation 4. High frequency operation makes small and cost-effective
magnetic components possible.
(4)
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7.3.3 Current mode control (FBSENSE pin)
Current mode control is used for the flyback converter because of its good line regulation.
The FBSENSE pin senses the primary current across an external resistor and compares it
with an internal control voltage. The internal control voltage is proportional to the FBCTRL
pin voltage, see Figure 10.
NXP Semiconductors TEA1751T
HV start-up flyback controller with integrated PFC controller
7.3.4 Demagnetization (FBAUX pin)
The system is always in QR or DCM mode. The internal oscillator does not start a new
primary stroke until the previous secondary stroke has ended.
Demagnetization features a cycle-by-cycle output short-circuit protection by immediately
lowering the frequency (longer off-time), thus reducing the power level.
Demagnetization recognition is suppressed during the first tsup(xfmr_ring) time of 2 s. This
suppression can be necessary at low output voltages and at start-up. It can also be
required in applications where the transformer has a large leakage inductance.
If the FBAUX pin is open-circuit or not connected, a fault condition is assumed and the
converter immediately stops. Operation restarts as soon as the fault condition is removed.
7.3.5 Flyback control/time-out (FBCTRL pin)
The FBCTRL pin is connected to an internal voltage source of 3.5 V using an internal
resistor of 3 k. When the voltage on this pin exceeds 2.5 V, the connection is disabled
and the pin is biased with a small current. If the voltage on this pin exceeds 4.5 V, a fault is
assumed, switching is inhibited and a restart is made.
If a capacitor and a resistor are connected in series to this pin, a time-out function is
created to protect against an open control loop. See Figure 11 and Figure 12. The
time-out function is disabled by connecting a resistor (100 k) to ground on the FBCTRL
pin.
If the pin is short-circuited to ground, switching of the flyback controller is prevented.
Under normal operating conditions, the converter regulates the output voltage. The voltage on the FBCTRL pin then varies between 1.4 V for the minimum output power and
2 V for the maximum output power.
