L6599
High-voltage resonant controller
Not for new design
Features
■
50 % duty cycle, variable frequency control of
resonant half-bridge
■
High-accuracy oscillator
■
Up to 500 kHz 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
DIP-16
■
Non-linear soft-start for monotonic output
voltage rise
■
600 V-rail compatible high-side gate driver with
integrated bootstrap diode and high dV/dt
immunity
■
■
Table 1.
SO-16N
Order code
Order codes
Package
Packaging
L6599D
SO-16N
Tube
L6599DTR
SO-16N
Tape and reel
L6599N
DIP-16
Tube
Applications
-300/800 mA high-side and low-side gate
drivers with UVLO pull-down
■
LCD and PDP TV
■
Desktop PC, entry-level server
■
Telecom SMPS
■
AC-DC adapter, open frame SMPS
DIP-16, SO-16N packages
Figure 1.
Block diagram
H.V.
Vcc
DIS
8
12
DISABLE
+
1.85V
STBY
UVLO
5
1.25V
S Q
-
+
DIS
UV
17V DETECTION
R
Ifmin
RFmin
DRIVING
LOGIC
DEAD
TIME
2V
LEVEL
SHIFTER
-
LC TANK
CIRCUIT
LVG
1.25V
15
µA
VCO
+
GND
1.5V
UVLO
+
6.3V
6
ISEN
0.8V
9
PFC_STOP
LINE_OK
ISEN_DIS
DIS
STANDBY
7
DELAY
February 2009
11
+
Q S
CONTROL
LOGIC
2
CBOOT
10
1
3
Vs
LVG DRIVER
R
CF
HVG
OUT
ISEN_DIS
Css
15
14
-
4
VBOOT
UVLO
STANDBY
+
HVG
DRIVER
SYNCHRONOUS
BOOTSTRAP DIODE
16
LINE
Rev 3
This is information on a product still in production but not recommended for new designs.
1/36
www.st.com
36
Contents
L6599
Contents
1
Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1
Connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2
Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3
Typical system block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6
Typical electrical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1
Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2
Operation at no load or very light load . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.3
Soft-start . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7.4
Current sense, OCP and OLP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.5
Latched shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.6
Line sensing function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.7
Bootstrap section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.8
Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
8
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2/36
L6599
1
Device description
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\OFF 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 600 V 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
Pin settings
2.1
Connection
Figure 2.
Pin connection (top view)
2.2
Functions
Table 2.
Pin functions
N.
Css
1
16
VBOOT
DELAY
2
15
HVG
CF
3
14
OUT
RFmin
4
13
N.C.
STBY
5
12
Vcc
ISEN
6
11
LVG
LINE
7
10
GND
DIS
8
9
PFC_STOP
Name
Function
CSS
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 < 1.25 V or > 6 V, DIS > 1.85 V, ISEN >
1.5 V, DELAY > 3.5 V) 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 0.75 V.
2
DELAY
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 0.8 V 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 2 V, the soft
start capacitor is completely discharged so that the switching frequency is pushed to its
maximum value and the 150 µA is kept always on. 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 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.
3
CF
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
4/36
L6599
Table 2.
N.
4
5
6
7
8
9
10
Pin settings
Pin functions (continued)
Name
Function
RFmin
Minimum oscillator frequency setting. This pin provides a precise 2 V 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).
STBY
Burst-mode operation threshold. The pin senses some voltage related to the feedback
control, which is compared to an internal reference (1.25 V). 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 50 mV. 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.
ISEN
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 0.8 V threshold (with 50 mV 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 1.5 V 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
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 1.25 V 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 1.25 V.
The comparator is provided with current hysteresis: an internal 15 µA current generator is
ON as long as the voltage applied at the pin is below 1.25 V 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 6 V if the function is not used.
DIS
Latched device shutdown. Internally the pin connects a comparator that, when the voltage
on the pin exceeds 1.85 V, 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.
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 > 1.85 V, ISEN > 1.5 V, LINE > 6 V and STBY < 1.25 V.
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.
5/36
Typical system block diagram
Table 2.
L6599
Pin functions (continued)
N.
Name
11
LVG
Low-side gate-drive output. The driver is capable of 0.3 A min. source and 0.8 A min. sink
peak current to drive the lower MOSFET of the half-bridge leg. The pin is actively pulled to
GND during UVLO.
12
VCC
Supply Voltage of both the signal part of the IC and the low-side gate driver. Sometimes a
small bypass capacitor (0.1 µF typ.) to GND might be useful to get a clean bias voltage for
the signal part of the IC.
13
N.C.
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.
14
OUT
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.
15
HVG
High-side floating gate-drive output. The driver is capable of 0.3 A 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.
VBOOT
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.
16
3
Function
Typical system block diagram
Figure 3.
6/36
Typical system block diagram
L6599
Electrical data
4
Electrical data
4.1
Maximum ratings
Table 3.
Absolute maximum ratings
Symbol
Pin
VBOOT
16
VOUT
Parameter
Value
Unit
Floating supply voltage
-1 to 618
V
14
Floating ground voltage
-3 to VBOOT -18
V
dVOUT /dt
14
Floating ground max. slew rate
50
V/ns
VCC
12
IC Supply voltage (ICC ≤ 25 mA)
Self-limited
V
VPFC_STOP
9
Maximum voltage (pin open)
-0.3 to VCC
V
IPFC_STOP
9
Maximum sink current (pin low)
Self-limited
A
VLINEmax
7
Maximum pin voltage (Ipin ≤ 1 mA)
Self-limited
V
IRFmin
4
Maximum source current
2
mA
-0.3 to 5
V
1 to 6, 8 Analog inputs and outputs
Note:
ESD immunity for pins 14, 15 and 16 is guaranteed up to 900 V
4.2
Thermal data
Table 4.
Symbol
RthJA
TSTG
TJ
PTOT
Thermal data
Description
Value
Max. thermal resistance junction to ambient (DIP16)
80
Max. thermal resistance junction to ambient (SO16)
120
Unit
°C/W
Storage temperature range
-55 to 150
°C
Junction operating temperature range
-40 to 150
°C
Recommended max. power dissipation @TA = 70 °C (DIP16)
1
Recommended max. power dissipation @TA = 50 °C (SO16)
0.83
W
7/36
Electrical characteristics
5
L6599
Electrical characteristics
TJ = 0 to 105 °C, VCC = 15 V, VBOOT = 15 V, CHVG = CLVG = 1 nF; CF = 470 pF;
RRFmin = 12 kΩ; unless otherwise specified.
Table 5.
Electrical characteristics
Symbol
Parameter
Test condition
Min
Operating range
After device turn-on
8.85
VCC(ON)
Turn-ON threshold
Voltage rising
10
VCC(OFF)
Turn-OFF threshold
Voltage falling
7.45
Typ
Max
Unit
16
V
10.7
11.4
V
8.15
8.85
V
IC supply voltage
VCC
Hys
Hysteresis
VZ
VCC clamp voltage
2.55
Iclamp = 10 mA
16
V
17
17.9
V
Supply current
Start-up current
Before device turn-ON
VCC = VCC(ON) - 0.2 V
200
250
µA
Iq
Quiescent current
Device ON, VSTBY = 1 V
1.5
2
mA
Iop
Operating current
Device ON,
VSTBY = VRFmin
3.5
5
mA
Iq
Residual consumption
VDIS > 1.85 V or VDELAY
> 3.5 V or VLINE < 1.25 V
or VLINE = Vclamp
300
400
µA
5
µA
5
µA
Istart-up
High-side floating gate-drive supply
ILKBOOT
VBOOT pin leakage
current
ILKOUT
OUT pin leakage current VOUT = 562 V
rDS(on)
Synchronous bootstrap
diode ON-resistance
VBOOT = 580 V
Ω
150
VLVG = High
Overcurrent comparator
IISEN
Input bias current
VISEN = 0 to VISENdis
tLEB
Leading edge blanking
After VHVG and VLVG
low-to-high transition
Frequency shift
threshold
Voltage rising (1)
Hysteresis
Voltage falling
Latch OFF threshold
Voltage rising (1)
VISENx
VISENdis
td(H-L)
8/36
Delay to output
-1
250
0.76
0.8
ns
0.84
50
1.44
µA
V
mV
1.5
1.56
V
300
400
ns
L6599
Electrical characteristics
Table 5.
Electrical characteristics (continued)
Symbol
Parameter
Test condition
Min
Typ
Max
Unit
1.2
1.25
1.3
V
15
18
µA
8
V
-1
µA
Line sensing
Voltage rising or falling
Vth
Threshold voltage
IHyst
Current hysteresis
VCC > 5 V, VLINE = 0.3 V
12
Clamp level
ILINE = 1mA
6
IDIS
Input bias current
VDIS = 0 to Vth
Vth
Disable threshold
Voltage rising (1)
Output duty cycle
Both HVG and LVG
Vclamp
(1)
DIS function
1.77
1.85
1.93
V
48
50
52
%
58.2
60
61.8
240
250
260
Oscillator
D
fosc
Oscillation frequency
RRFmin= 2.7 kΩ
kHz
Maximum
recommended
500
kHz
0.4
µs
TD
Dead-time
VCFp
Peak value
3.9
V
VCFv
Valley value
0.9
V
VREF
Voltage reference at
pin 4
KM
Current mirroring ratio
RFMIN
Timing resistor range
Between HVG and LVG
(1)
0.2
1.92
0.3
2
2.08
1
1
V
A/A
100
kΩ
PFC_STOP function
Ileak
VL
High level leakage
current
VPFC_STOP = VCC,
VDIS = 0 V
1
µA
Low saturation level
IPFC_STOP =1mA,
VDIS = 2 V
0.2
V
Open-state current
V(Css) = 2 V
0.5
µA
Discharge resistance
VISEN > VISENx
Soft-start function
Ileak
R
Ω
120
Standby function
IDIS
Input bias current
VDIS = 0 to Vth
Vth
Disable threshold
Voltage falling (1)
Hys
Hysteresis
Voltage rising
1.2
1.25
50
-1
µA
1.3
V
mV
9/36
Electrical characteristics
Table 5.
Symbol
L6599
Electrical characteristics (continued)
Parameter
Test condition
Min
Typ
Max
Unit
0.5
µA
Delayed shutdown function
Ileak
ICHARGE
Open-state current
V(DELAY) = 0
Charge current
VDELAY = 1 V,
VISEN = 0.85 V
100
150
200
µA
Vth1
Threshold for forced
operation at max.
frequency
Voltage rising (1)
1.92
2
2.08
V
Vth2
Shutdown threshold
Voltage rising (1)
3.3
3.5
3.7
V
Vth3
Restart threshold
Voltage falling (1)
0.25
0.3
0.35
V
1.5
V
Low - side gate driver (voltages referred to GND)
VLVGL
Output low voltage
Isink = 200 mA
VLVGH
Output high voltage
Isource = 5 mA
Isourcepk
Peak source current
-0.3
A
Peak sink current
0.8
A
Isinkpk
12.8
13.3
V
tf
Fall time
30
ns
tr
Rise time
60
ns
UVLO saturation
VCC = 0 to VCC(ON),
Isink = 2 mA
1.1
V
1.5
V
High-side gate driver (voltages referred to OUT)
VHVGL
Output low voltage
Isink = 200 mA
VHVGH
Output high voltage
Isource = 5 mA
Isourcepk
Peak source current
-0.3
A
Peak sink current
0.8
A
Isinkpk
13.3
V
tf
Fall time
30
ns
tr
Rise time
60
ns
HVG-OUT pull-down
25
kΩ
1. Values traking each other
10/36
12.8
L6599
Typical electrical performance
6
Typical electrical performance
Figure 4.
Device consumption vs supply
voltage
Figure 5.
IC consumption vs
junction temperature
Figure 6.
VCC clamp voltage vs
junction temperature
Figure 7.
UVLO thresholds vs
junction temperature
11/36
Typical electrical performance
Figure 8.
Oscillator frequency vs junction
temperature
Figure 10. Oscillator frequency vs
timing components
12/36
L6599
Figure 9.
Dead-time vs
junction temperature
Figure 11. Oscillator ramp vs
junction temperature
L6599
Typical electrical performance
Figure 12. Reference voltage vs
junction temperature
Figure 13. Current mirroring ratio vs junction
temperature
Figure 14. OCP delay source current vs
junction temperature
Figure 15. OCP delay thresholds vs junction
temperature
13/36
Typical electrical performance
L6599
Figure 16. Standby thresholds vs junction
temperature
Figure 17. Current sense thresholds vs
junction temperature
Figure 18. Line thresholds vs
junction temperature
Figure 19. Line source current vs junction
temperature
Figure 20. Latched disable threshold vs
junction temperature
14/36
L6599
7
Application information
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 21), 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.
2.
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.
Figure 21. Multi-mode operation
15/36
Application information
7.1
L6599
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 2 V reference with
about 2 mA source capability and the higher the current sourced by the pin is, the higher the
oscillator frequency will be. The block diagram of Figure 22 shows a simplified internal
circuit that explains the operation.
The network that loads the RFmin pin generally comprises three branches:
1.
A resistor RFmin connected between the pin and ground that determines the minimum
operating frequency;
2.
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;
3.
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.
Figure 22. Oscillator's internal block diagram
L6599
2V
KM·I R
+
RFmin
RFmin
R SS
C SS
R Fmax
4
-
3
CF
2·KM·I R
IR
0.9V
1V
KM·I R
CF
+
-
S
+
-
R
Q
3.9V
4V
The following approximate relationships hold for the minimum and the maximum oscillator
frequency respectively:
1
f min = -----------------------------------3 ⋅ CF ⋅ RFmin
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 23. Oscillator waveforms and their relationship with gate-driving signals
CF
HVG
TD
TD
t
LVG
t
HB
t
t
In Figure 23 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
7.2
L6599
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 1.25 V 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 2 V
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 1.25 V by
50 mV.
To implement burst-mode operation the voltage applied to the STBY pin needs to be related
to the feedback loop. Figure 24 shows the simplest implementation, suitable with a narrow
input voltage range (e.g. when there is a PFC front-end).
Figure 24. Burst-mode implementation: narrow input voltage range
RFmin
4
RFmin
RFmax
STBY
RFmin
L6599
RFmax
5
Figure 25. Burst-mode implementation: wide input voltage range
B+
RFmin
4
L6599
RFmin
STBY
D
RC
R
L6599
RFmax
LINE
7
5
RA
C
R
RD
RB
RA + RB >> RC
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:
3 RF min
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 24 the
value of PoutB would change considerably. In this case it is recommended to use the
arrangement shown in Figure 25 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 26 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 ÷ 1 W). 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 27 When the L6599 is in UVLO the pin is kept open, to let the PFC controller start
first.
19/36
Application information
L6599
Figure 26. Load-dependent operating modes: timing diagram
STBY
50 mV
hyster.
1.25V
t
fosc
t
LVG
HVG
t
PFC_STOP
PFC
GATE-DRIVE
Resonant Mode
Burst-mode
Resonant Mode
Figure 27. How the L6599 can switch OFF a PFC controller at light load
ZCD
Vcc
22 kΩ
12
100 kΩ
L6599
L6599
PFC_STOP
BC547
BC547
9
PFC_OK
PFC_STOP
L6563
20/36
9
L6561/2
(AC_OK)
L6599
7.3
Application information
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 28).
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 29).
Figure 28. Soft-start circuit
|Z(f)|
RFmin
4
RFmin
RSS
Css
L6599
1
CSS
21/36
Application information
L6599
Figure 29. Power vs frequency curve in an resonant half-bridge
|Z(f)|
-1
RESONANCE
FREQUENCY
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
–3
⋅ 10
C ss = 3
------------------R SS
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 32 to see some
significant signals during the soft-start phase.
22/36
L6599
7.4
Application information
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 30 and Figure 31 a couple of current sensing methods are illustrated that will be
described in the following. The circuit of Figure 30 is simpler but the dissipation on the sense
resistor Rs might not be negligible, hurting efficiency; the circuit of Figure 31 is more
complex but virtually lossless and recommended when the efficiency target is very high.
Figure 30. Current sensing technique with sense resistor
L6599
6
Cr
ISEN
L6599
ICr
τ≈
10
fmin
Rs
6
Vspk
0
23/36
Application information
L6599
Figure 31. Lossless current sensing technique, with capacitive shunt
τ≈
L6599
6
ISEN
pk
CB
10
fmin
1N4148
RB
VCrpk
CA
RA
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 0.8 V, and to that of a second comparator referenced to 1.5 V. If
the voltage externally applied to the pin by either circuit in Figure 30 or Figure 31 exceeds
0.8 V 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 50 mV; 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 0.8 V; however, if the
voltage on the ISEN pin reaches 1.5 V, 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 30 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
4
R 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 31 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:
C
0.8π
R B = --------------- ⎛⎝ 1 + ------r-⎞⎠
I Crpkx
CA
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
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