VIPER27
Off-line high voltage converters
Features
■
800 V avalanche rugged power section
■
PWM operation with frequency jittering for low
EMI
■
Operating frequency:
– 60 kHz for L type
– 115 kHz for H type
SO16 narrow
DIP-7
SO 16
Description
■
Standby power < 50 mW at 265 Vac
■
Limiting current with adjustable set point
■
Adjustable and accurate overvoltage
protection
■
On-board soft-start
■
Safe auto-restart after a fault condition
■
Hysteretic thermal shutdown
The device is an off-line converter with an 800 V
rugged power section, a PWM control, two levels
of over-current protection, overvoltage and
overload protections, hysteretic thermal
protection, soft-start and safe auto-restart after
any fault condition removal. Burst mode operation
and device very low consumption help to meet the
standby energy saving regulations.
Application
■
Auxiliary power supply for consumer and home
equipments
■
ATX auxiliary power supply
■
Low / medium power AC-DC adapters
■
SMPS for set-top boxes, DVD players and
recorders, white goods
Advance frequency jittering reduces EMI filter
cost. Brown-out function protects the switch mode
power supply when the rectified input voltage
level is below the normal minimum level specified
for the system. The high voltage start-up circuit is
embedded in the device.
Figure 1.
Typical topology
+
+
DC input high voltage
wide range
DC Output voltage
DRAIN
DRAIN
BR
VIPER27
GND
Table 1.
VDD
CONT
FB
Device summary
Order codes
Package
VIPER27LN / VIPER27HN
DIP-7
Packaging
Tube
VIPER27HD / VIPER27LD
SO16 narrow
VIPER27HDTR / VIPER27LDTR
July 2010
Tape and reel
Doc ID 15133 Rev 5
1/31
www.st.com
31
Contents
VIPER27
Contents
1
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Typical power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.2
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5
Typical electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6
Typical circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7
Operation descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1
Power section and gate driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.2
High voltage startup generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.3
Power-up and soft-start up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.4
Power down operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.5
Auto restart operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.6
Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.7
Current mode conversion with adjustable current limit set point . . . . . . . 18
7.8
Overvoltage protection (OVP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.9
About CONT pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.10
Feed-back and overload protection (OLP) . . . . . . . . . . . . . . . . . . . . . . . . 20
7.11
Burst-mode operation at no load or very light load . . . . . . . . . . . . . . . . . . 23
7.12
Brown-out protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.13
2nd level over current protection and hiccup mode . . . . . . . . . . . . . . . . . 25
8
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2/31
Doc ID 15133 Rev 5
VIPER27
Block diagram
1
Block diagram
Figure 2.
Block diagram
VDD
VDD
Vcc
BR
DRAIN
+
Vin_OK
VBRth
Internal Supply bus
&
Ref erence Voltages
SUPPLY
& UVLO
HV_ON
I DDch
15uA
UVLO
SOFT
START
OCP
BLOCK
-
CONT
OTP
OCP
BURST
+
.
OVP
LOGIC
+
THERMAL
SHUTDOWN
OSCILLATOR
PWM
TURN-ON
LOGIC
LEB
S
Q
R1
R2
6uA
+
OVP
2nd OCP
LOGIC
Ref
OLP
OVP
OTP
Rsense
BURST-MODE
LOGIC
BURST
FB
GND
2
Typical power
Table 2.
Typical power
230 VAC
Part number
VIPER27
85-265 VAC
Adapter(1)
Open frame(2)
Adapter(1)
Open frame(2)
18 W
20 W
10 W
12 W
1. Typical continuous power in non ventilated enclosed adapter measured at 50 °C ambient.
2. Maximum practical continuous power in an open frame design at 50 °C ambient, with adequate heat sinking.
Doc ID 15133 Rev 5
3/31
Pin settings
3
VIPER27
Pin settings
Figure 3.
Note:
Connection diagram (top view)
The copper area for heat dissipation has to be designed under the DRAIN pins.
Table 3.
Pin description
Pin n.
Name
SO16N
1
1...2
GND
This pin represents the device ground and the source of the power
section.
-
4
N.A.
Not available for user. It can be connected to GND (pins 1-2) or left not
connected.
2
5
VDD
Supply voltage of the control section. This pin also provides the charging
current of the external capacitor during start-up time.
3
4
5
7,8
4/31
Function
DIP7
6
7
8
Control pin. The following functions can be selected:
1. current limit set point adjustment. The internal set default value of the
cycle-by-cycle current limit can be reduced by connecting to ground an
CONT external resistor.
2. output voltage monitoring. A voltage exceeding VOVP threshold (see
Table 8 on page 7) shuts the IC down reducing the device consumption.
This function is strobed and digitally filtered for high noise immunity.
FB
Control input for duty cycle control. Internal current generator provides
bias current for loop regulation. A voltage below the threshold VFBbm
activates the burst-mode operation. A level close to the threshold VFBlin
means that we are approaching the cycle-by-cycle over-current set point.
BR
Brownout protection input with hysteresis. A voltage below the threshold
VBRth shuts down (not latch) the device and lowers the power
consumption. Device operation restarts as the voltage exceeds the
threshold VBRth + VBRhyst. It can be connected to ground when not used.
High voltage drain pin. The built-in high voltage switched start-up bias
13...16 DRAIN current is drawn from this pin too.
Pins connected to the metal frame to facilitate heat dissipation.
Doc ID 15133 Rev 5
VIPER27
Electrical data
4
Electrical data
4.1
Maximum ratings
Table 4.
Absolute maximum ratings
Value
Symbol
Parameter
Unit
Min
VDRAIN Drain-to-source (ground) voltage
800
V
EAV
Repetitive avalanche energy
(limited by TJ = 150 °C)
5
mJ
IAR
Repetitive avalanche current
(limited by TJ = 150 °C)
1.5
A
3
A
IDRAIN
Pulse drain current (limited by TJ = 150 °C)
VCONT
Control input pin voltage (with ICONT = 1 mA)
-0.3
Self limited
V
VFB
Feed-back voltage
-0.3
5.5
V
VBR
Brown-out input pin voltage (with IBR = 0.5 mA)
-0.3
Self limited
V
VDD
Supply voltage (IDD = 25 mA)
-0.3
Self limited
V
IDD
Input current
25
mA
Power dissipation at TA < 40 °C (DIP-7)
1
PTOT
TJ
TSTG
4.2
Max
W
Power dissipation at TA < 60 °C (SO16N)
1.5
Operating junction temperature range
-40
150
°C
Storage temperature
-55
150
°C
Thermal data
Table 5.
Thermal data
Max value
Symbol
Parameter
Unit
SO16N
DIP7
RthJP
Thermal resistance junction pin
(Dissipated power = 1 W)
25
35
°C/W
RthJA
Thermal resistance junction ambient
(Dissipated power = 1 W)
60
100
°C/W
RthJA
Thermal resistance junction ambient (1)
(Dissipated power = 1 W)
50
80
°C/W
1. When mounted on a standard single side FR4 board with 100 mm2 (0.155 sq in) of Cu (35 µm thick)
Doc ID 15133 Rev 5
5/31
Electrical data
4.3
VIPER27
Electrical characteristics
(TJ = -25 to 125 °C, VDD = 14 V(a); unless otherwise specified)
Table 6.
Power section
Symbol
Parameter
VBVDSS
Break-down voltage
IDRAIN = 1 mA, VFB = GND
TJ = 25 °C
OFF state drain current
VDRAIN = max rating,
VFB = GND
60
μA
IDRAIN = 0.4 A, VFB = 3 V, VBR = GND,
TJ = 25 °C
7
Ω
IDRAIN = 0.4 A, VFB = 3 V, VBR = GND,
TJ = 125 °C
14
Ω
IOFF
RDS(on)
COSS
Table 7.
Symbol
Drain-source on state
resistance
Effective (energy related)
output capacitance
Test condition
Min Typ Max Unit
800
VDRAIN = 0 to 640 V
V
40
pF
Supply section
Parameter
Test condition
Min Typ Max Unit
Voltage
VDRAIN_START Drain-source start voltage
IDDch
VDD
VDDclamp
Start up charging current
VDRAIN = 120 V, VBR = GND, VFB = GND,
VDD = 4 V
60
80
100
V
-2
-3
-4
mA
VDRAIN = 120 V, VBR = GND, VFB = GND,
VDD = 4 V after fault.
-0.4 -0.6 -0.8
Operating voltage range
After turn-on
8.5
VDD clamp voltage
IDD = 20 mA
23.5
23.5
mA
V
V
13
14
15
V
7.5
8
8.5
V
4
4.5
5
V
VFB = GND, FSW = 0 kHz, VBR = GND,
VDD = 10 V
0.9
mA
VDRAIN = 120 V, FSW = 60 kHz
2.5
mA
VDRAIN = 120 V, FSW = 115 kHz
3.5
mA
400
μA
270
μA
VDDon
VDD start up threshold
VDDoff
VDD under voltage
shutdown threshold
VDD(RESTART)
VDD restart voltage
threshold
VDRAIN = 120 V, VBR = GND, VFB = GND
IDD0
Operating supply current,
not switching
IDD1
Operating supply current,
switching
IDD_FAULT
Operating supply current,
with protection tripping
IDD_OFF
Operating supply current
with VDD < VDD_OFF
VDRAIN = 120 V, VBR = GND, VFB = GND
Current
VDD = 7 V
a. Adjust VDD above VDDon start-up threshold before settings to 14 V.
6/31
Doc ID 15133 Rev 5
VIPER27
Table 8.
Electrical data
Controller section
Symbol
Parameter
Test condition
Min
Typ
Max Unit
Feed-back pin
VFBolp
Over load shutdown threshold
4.5
4.8
5.2
V
VFBlin
Linear dynamics upper limit
3.2
3.5
3.7
V
VFBbm
Burst mode threshold
Voltage falling
0.6
V
VFBbmhys
Burst mode hysteresis
Voltage rising
100
mV
-150 -200 -280
μA
-3
μA
IFB
RFB(DYN)
HFB
VFB = 0.3 V
Feed-back sourced current
3.3 V < VFB < 4.8 V
Dynamic resistance
VFB < 3.3 V
ΔVFB / ΔID
14
21
kΩ
2
6
V/A
CONT pin
ICONT = -100 µA
VCONT_l Low level clamp voltage
0.5
V
Current limitation
IDlim
Max drain current limitation
tSS
Soft-start time
TON_MIN
td
tLEB
ID_BM
VFB = 4 V,
ICONT = -10 µA
TJ = 25 °C
0.66
0.7
0.74
8.5
Minimum turn ON time
220
400
A
ms
480
ns
Propagation delay
100
ns
Leading edge blanking
300
ns
160
mA
Peak drain current during burst mode
VFB = 0.6 V
Oscillator section
VDD = operating voltage range,
VFB = 1 V
VIPER27L
FOSC
VIPER27H
FD
60
66
kHz
103
115
127
kHz
VIPER27L
±4
kHz
VIPER27H
±8
kHz
250
Hz
Modulation depth
FM
Modulation frequency
DMAX
Maximum duty cycle
Over current protection
IDMAX
54
70
80
%
(2nd OCP)
Second over current threshold
1
A
Overvoltage protection
VOVP
TSTROBE
Overvoltage protection threshold
2.7
Overvoltage protection strobe time
3
2.2
Doc ID 15133 Rev 5
3.3
V
us
7/31
Electrical data
Table 8.
VIPER27
Controller section (continued)
Symbol
Parameter
Test condition
Min
Typ
Max Unit
Brown out protection
VBRth
Brown out threshold
Voltage falling
0.41 0.45 0.49
V
VBRhyst
Voltage hysteresis above VBRth
Voltage rising
50
mV
IBRhyst
Current hysteresis
VBRclamp
VDIS
Clamp voltage
7
IBR = 250 µA
Brown out disable voltage
12
3
50
μA
V
150
mV
Thermal shutdown
TSD
THYST
8/31
Thermal shutdown temperature
150
Thermal shutdown hysteresis
Doc ID 15133 Rev 5
160
°C
30
°C
VIPER27
Electrical data
Figure 4.
Minimum turn-on time test circuit
VDRAIN
14 V
GND
DRAIN
VDD
DRAIN
90 %
TONmin
50 Ω
CONT
10 %
IDRAIN
BR
FB
30 V
3.5 V
Time
IDLIM
Time
Figure 5.
Brown out threshold test circuit
VBR
GND
DRAIN
VBRth+VBRhyst
VBRth
VDD
DRAIN
VDIS
IBR
Time
IDRAIN
Time
10 kΩ
CONT
IBRhyst
IBRhyst
BR
FB
14 V
30 V
2V
Time
Figure 6.
OVP threshold test circuit
VCONT
14 V
GND
DRAIN
VDD
DRAIN
CONT
FB
VOVP
10 kΩ
VDRAIN
BR
Time
30 V
2V
Time
Note:
Adjust VDD above VDDon start-up threshold before settings to 14 V
Doc ID 15133 Rev 5
9/31
Typical electrical characteristics
5
VIPER27
Typical electrical characteristics
Figure 7.
Current limit vs TJ
Figure 8.
Figure 9.
Drain start voltage vs TJ
Figure 10. HFB vs TJ
Figure 11. Brown out threshold vs TJ
10/31
Switching frequency vs TJ
Figure 12. Brown out hysteresis vs TJ
Doc ID 15133 Rev 5
VIPER27
Typical electrical characteristics
Figure 13. Brown out hysteresis current Figure 14. Operating supply current
vs TJ
(no switching) vs TJ
Figure 15. Operating supply current
(switching) vs TJ
Figure 16. current limit vs RLIM
Figure 17. Power MOSFET on-resistance Figure 18. Power MOSFET break down
vs TJ
voltage vs TJ
Doc ID 15133 Rev 5
11/31
Typical electrical characteristics
VIPER27
Figure 19. Thermal shutdown
VDD
VDDon
VDDoff
VDD(RESTART)
time
IDRAIN
time
TJ
TSD
TSD - THYST
Normal operation
12/31
Shut down after over temperature
Doc ID 15133 Rev 5
Normal operation
time
VIPER27
6
Typical circuit
Typical circuit
Figure 20. Min-features flyback application
D3
AC IN
R1
C2
BR
Vout
C1
C5
AC IN
D1
GND
D2
R2
R3
VVcc
DD
OPTO
DRAIN
R5
BR
CONTROL
C3
R4
CONT
FB
C6
GND
SOURCE
U2
C4
R6
Figure 21. Full-features flyback application
D3
Rh
AC IN
BR
C2
Vout
R1
C1
C5
Rl
AC IN
D1
GND
Daux
Rov p
D2
R2
R3
VVcc
DD
DRAIN
OPTO
R5
BR
CONTROL
C3
R4
CONT
FB
C6
GND
SOURCE
U2
Rlim
C4
Doc ID 15133 Rev 5
R6
13/31
Operation descriptions
7
VIPER27
Operation descriptions
VIPER27 is a high-performance low-voltage PWM controller chip with an 800 V, avalanche
rugged Power section.
The controller includes: the oscillator with jittering feature, the start up circuits with soft-start
feature, the PWM logic, the current limit circuit with adjustable set point, the second over
current circuit, the burst mode management, the brown-out circuit, the UVLO circuit, the
auto-restart circuit and the thermal protection circuit.
The current limit set-point is set by the CONT pin. The burst mode operation guaranties high
performance in the stand-by mode and helps in the energy saving norm accomplishment.
All the fault protections are built in auto restart mode with very low repetition rate to prevent
IC's over heating.
7.1
Power section and gate driver
The power section is implemented with an avalanche ruggedness N-channel MOSFET,
which guarantees safe operation within the specified energy rating as well as high dv/dt
capability. The power section has a BVDSS of 800 V min. and a typical RDS(on) of 7 Ω
at 25 °C.
The integrated SenseFET structure allows a virtually loss-less current sensing.
The gate driver is designed to supply a controlled gate current during both turn-on and turnoff in order to minimize common mode EMI. Under UVLO conditions an internal pull-down
circuit holds the gate low in order to ensure that the power section cannot be turned on
accidentally.
7.2
High voltage startup generator
The HV current generator is supplied through the DRAIN pin and it is enabled only if the
input bulk capacitor voltage is higher than VDRAIN_START threshold, 80 VDC typically. When
the HV current generator is ON, the IDDch current (3 mA typical value) is delivered to the
capacitor on the VDD pin. In case of auto restart mode after a fault event, the IDDch current is
reduced to 0.6 mA, in order to have a slow duty cycle during the restart phase.
14/31
Doc ID 15133 Rev 5
VIPER27
7.3
Operation descriptions
Power-up and soft-start up
If the input voltage rises up till the device start threshold, VDRAIN_START, the VDD voltage
begins to grow due to the IDDch current (see Table 7 on page 6) coming from the internal
high voltage start up circuit. If the VDD voltage reaches VDDon threshold (see Table 7 on
page 6) the power MOSFET starts switching and the HV current generator is turned OFF.
See Figure 23 on page 16.
The IC is powered by the energy stored in the capacitor on the VDD pin, CVDD, until when
the self-supply circuit (typically an auxiliary winding of the transformer and a steering diode)
develops a voltage high enough to sustain the operation.
CVDD capacitor must be sized enough to avoid fast discharge and keep the needed voltage
value higher than VDDoff threshold. In fact, a too low capacitance value could terminate the
switching operation before the controller receives any energy from the auxiliary winding.
The following formula can be used for the VDD capacitor calculation:
Equation 1
I DDch × tSSaux
C VDD = ---------------------------------------V DDon – V DDoff
The tSSaux is the time needed for the steady state of the auxiliary voltage. This time is
estimated by applicator according to the output stage configurations (transformer, output
capacitances, etc.).
During the converter start up time, the drain current limitation is progressively increased to
the maximum value. In this way the stress on the secondary diode is considerably reduced.
It also helps to prevent transformer saturation. The soft-start time lasts 8.5 ms and the
feature is implemented for every attempt of start up converter or after a fault.
Figure 22. IDD current during start-up and burst mode
VDD
VDDon
VDDoff
t
VFB
VFBolp
VFBlin
VFBbmhys
VFBbm
t
VDRAIN
t
IDD
IDD1
IDD0
t
IDDch (-3 mA)
START- UP
NORMAL MODE
Doc ID 15133 Rev 5
BURST MODE
NORMAL MODE
15/31
Operation descriptions
VIPER27
Figure 23. Timing diagram: normal power-up and power-down sequences
VIN
VIN < VDRAIN_START
HV startup is no more activated
VDRAIN_START
VDD
regulation is lost here
time
VDDon
VDDoff
VDD(RESTART)
VDRAIN
time
IDD
time
IDDch (3mA)
Normal operation
Power-on
time
Power-off
Figure 24. Soft-start: timing diagram
IDRAIN
IDlim
t
VFB
VFBolp
VFBlin
t
VOUT
t
tSS
16/31
( SOFT START- UP )
DELAY (OLP)
Doc ID 15133 Rev 5
STEADY STATE
VIPER27
7.4
Operation descriptions
Power down operation
At converter power down, the system loses regulation as soon as the input voltage is so low
that the peak current limitation is reached. The VDD voltage drops and when it falls below
the VDDoff threshold (see Table 7 on page 6) the power MOSFET is switched OFF, the
energy transfers to the IC interrupted and consequently the VDD voltages decreases,
Figure 23 on page 16. Later, if the VIN is lower than VDRAIN_START (see Table 7 on page 6),
the start up sequence is inhibited and the power down completed. This feature is useful to
prevent converter’s restart attempts and ensures monotonic output voltage decay during the
system power down.
7.5
Auto restart operation
If after a converter power down, the VIN is higher than VDRAIN_START, the start up sequence
is not inhibited and will be activated only when the VDD voltage drops down the
VDD(RESTART) threshold (see Table 7 on page 6). This means that the HV start up current
generator restarts the VDD capacitor charging only when the VDD voltage drops below
VDD(RESTART). The scenario above described is for instance a power down because of a
fault condition. After a fault condition, the charging current, IDDch, is 0.6 mA (typ.) instead of
the 3 mA (typ.) of a normal start up converter phase. This feature together with the low
VDD(RESTART) threshold ensures that, after a fault, the restart attempts of the IC has a very
long repetition rate and the converter works safely with extremely low power throughput.
The Figure 25 shows the IC behavioral after a short circuit event.
Figure 25. Timing diagram: behavior after short circuit
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7.6
Oscillator
The switching frequency is internally fixed to 60 kHz or 115 kHz. In both case the switching
frequency is modulated by approximately ±4 kHz (60 kHz version) or ±8 kHz
(115 kHz version) at 250 Hz (typical) rate, so that the resulting spread-spectrum action
distributes the energy of each harmonic of the switching frequency over a number of sideband harmonics having the same energy on the whole but smaller amplitudes.
Doc ID 15133 Rev 5
17/31
Operation descriptions
7.7
VIPER27
Current mode conversion with adjustable current limit set
point
The device is a current mode converter: the drain current is sensed and converted in voltage
that is applied to the non inverting pin of the PWM comparator. This voltage is compared
with the one on the feed-back pin through a voltage divider on cycle by cycle basis.
The VIPER27 has a default current limit value, IDlim, that the designer can adjust according
the electrical specification, by the RLIM resistor connected to the CONT see Figure 16 on
page 11.
The CONT pin has a minimum current sunk needed to activate the IDlim adjustment: without
RLIM or with high RLIM (i.e. 100 kΩ) the current limit is fixed to the default value (see IDlim,
Table 8 on page 7).
7.8
Overvoltage protection (OVP)
The VIPER27 has integrated the logic for the monitor of the output voltage using as input
signal the voltage VCONT during the OFF time of the power MOSFET. This is the time when
the voltage from the auxiliary winding tracks the output voltage, through the turn ratio
N AUX
-------------N SEC
The CONT pin has to be connected to the auxiliary winding through the diode DOVP and the
resistors ROVP and RLIM as shows the Figure 27 on page 20 When, during the OFF time,
the voltage VCONT exceeds, four consecutive times, the reference voltage VOVP (see Table 8
on page 7) the overvoltage protection will stop the power MOSFET and the converter enters
the auto-restart mode.
In order to bypass the noise immediately after the turn off of the power MOSFET, the voltage
VCONT is sampled inside a short window after the time TSTROBE, see Table 8 on page 7 and
the Figure 26 on page 19. The sampled signal, if higher than VOVP, trigger the internal OVP
digital signal and increments the internal counter. The same counter is reset every time the
signal OVP is not triggered in one oscillator cycle.
Referring to the Figure 21, the resistors divider ratio kOVP will be given by:
Equation 2
V OVP
k OVP = -------------------------------------------------------------------------------------------------N AUX
-------------- ⋅ ( V OUTOVP + V DSEC ) – V DAUX
N SEC
Equation 3
R LIM
k OVP = --------------------------------R LIM + R OVP
18/31
Doc ID 15133 Rev 5
VIPER27
Operation descriptions
Where:
●
VOVP is the OVP threshold (see Table 8 on page 7)
●
VOUT OVP is the converter output voltage value to activate the OVP set by designer
●
NAUX is the auxiliary winding turns
●
NSEC is the secondary winding turns
●
VDSEC is the secondary diode forward voltage
●
VDAUX is the auxiliary diode forward voltage
●
ROVP together RLIM make the output voltage divider
Than, fixed RLIM, according to the desired IDlim, the ROVP can be calculating by:
Equation 4
1 – k OVP
R OVP = R LIM × ----------------------k OVP
The resistor values will be such that the current sourced and sunk by the CONT pin be
within the rated capability of the internal clamp.
Figure 26. OVP timing diagram
VDS
t
VAUX
0
VCONT
t
V
OVP
t
2 µs
STROBE
0.5 µs
t
OVP
t
COUNTER
RESET
COUNTER
STATUS
t
0
0
0
0 →1
1 →2
2 →0
0
0 →1
1 →2
2 →3
FAULT
3 →4
t
NORMAL OPERATION
TEMPORARY DISTURBANCE
Doc ID 15133 Rev 5
FEEDBACK LOOP FAILURE
t
19/31
Operation descriptions
7.9
VIPER27
About CONT pin
Referring to the Figure 27, through the CONT pin, the below features can be implemented:
1.
Current limit set point
2.
Over voltage protection on the converter output voltage
The Table 9 on page 20 referring to the Figure 27, lists the external components needed to
activate one or plus of the CONT pin functions.
Figure 27. CONT pin configuration
R OV P
CONT
SOFT
START
OCP
BLOCK
-
Daux
+
Auxiliary
winding
OVP
LOGIC
R LIM
OCP
to GATE driver
From RSENSE
OVP
Table 9.
CONT pin configurations
Function / component
RLIM (1)
ROVP
DAUX
IDlim reduction
See Figure 16
No
No
OVP
≥ 80 kΩ
See Equation 4
Yes
IDlim reduction + OVP
See Figure 16
See Equation 4
Yes
1. RLIM has to be fixed before of ROVP
7.10
Feed-back and overload protection (OLP)
The VIPER27 is a current mode converter: the feedback pin controls the PWM operation,
controls the burst mode and actives the overload protection. Figure 28 on page 22 and
Figure 29 show the internal current mode structure.
With the feedback pin voltage between VFBbm and VFBlin, (see Table 8 on page 7) the drain
current is sensed and converted in voltage that is applied to the non inverting pin of the
PWM comparator. See Figure 2 on page 3.
This voltage is compared with the one on the feedback pin through a voltage divider on
cycle by cycle basis. When these two voltages are equal, the PWM logic orders the switch
off of the power MOSFET. The drain current is always limited to IDlim value.
In case of overload the feedback pin increases in reaction to this event and when it goes
higher than VFBlin, the PWM comparator is disabled and the drain current is limited to IDlim by
the OCP comparator, seeFigure 2 on page 3.
20/31
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VIPER27
Operation descriptions
When the feedback pin voltage reaches the threshold VFBlin an internal current generator
starts to charge the feedback capacitor (CFB) and when the feedback voltage reaches the
VFBolp threshold, the converter is turned off and the start up phase is activated with reduced
value of IDDch to 0.6 mA, see Table 7 on page 6.
During the first start up phase of the converter, after the soft-start up time, tSS, the output
voltage could force the feedback pin voltage to rise up to the VFBolp threshold that switches
off the converter itself.
To avoid this event, the appropriate feedback network has to be selected according to the
output load. More the network feedback fixes the compensation loop stability. The Figure 28
on page 22 and Figure 29 show the two different feedback networks.
The time from the over load detection (VFB = VFBlin) to the device shutdown
(VFB = VFBolp) can be set by CFB value (see Figure 28 on page 22 and Figure 29), using the
formula:
Equation 5
V FBolp – V FBlin
T OLP – delay = C FB × ---------------------------------------3μA
In the Figure 28, the capacitor connected to FB pin (CFB) is part of the compensation circuit
as well as it needs to activate the over load protection (see equation 5).
After the start up time, tSS, during which the feedback voltage is fixed at VFBlin, the output
capacitor could not be at its nominal value and the controller interprets this situation as an
over load condition. In this case, the OLP delay helps to avoid an incorrect device shut down
during the start up phase.
Owing to the above considerations, the OLP delay time must be long enough to by-pass the
initial output voltage transient and check the over load condition only when the output
voltage is in steady state. The output transient time depends from the value of the output
capacitor and from the load.
When the value of the CFB capacitor calculated for the loop stability is too low and cannot
ensure enough OLP delay, an alternative compensation network can be used and it is
showed in Figure 29 on page 22.
Using this alternative compensation network, two poles (fPFB, fPFB1) and one zero (fZFB) are
introduced by the capacitors CFB and CFB1 and the resistor RFB1.
The capacitor CFB introduces a pole (fPFB) at higher frequency than fZB and fPFB1. This pole
is usually used to compensate the high frequency zero due to the ESR (equivalent series
resistor) of the output capacitance of the fly-back converter.
The mathematical expressions of these poles and zero frequency, considering the scheme
in Figure 29 are reported by the equations below:
Equation 6
fZFB =
1
2 ⋅ π ⋅ CFB1 ⋅ RFB1
Doc ID 15133 Rev 5
21/31
Operation descriptions
VIPER27
Equation 7
fPFB =
RFB(DYN) + RFB1
2 ⋅ π ⋅ CFB ⋅ RFB(DYN) ⋅ RFB1
(
)
1
2 ⋅ π ⋅ CFB1 ⋅ RFB1 + RFB(DYN)
)
Equation 8
fPFB1 =
(
The RFB(DYN) is the dynamic resistance seen by the FB pin.
The CFB1 capacitor fixes the OLP delay and usually CFB1 results much higher than CFB.
The Equation 5 can be still used to calculate the OLP delay time but CFB1 has to be
considered instead of CFB. Using the alternative compensation network, the designer can
satisfy, in all case, the loop stability and the enough OLP delay time alike.
Figure 28. FB pin configuration
From sense FET
PWM
To PWM Logic
+
PWM
CONTROL
-
Cfb
BURST
BURST-MODE
LOGIC
BURST-MODE
REFERENCES
OLP comparator
To disable logic
+
-
4.8V
Figure 29. FB pin configuration
From sense FET
PWM
To PWM Logic
+
PWM
CONTROL
-
Rfb1
Cfb
BURST
Cfb1
BURST-MODE
REFERENCES
BURST-MODE
LOGIC
OLP comparator
+
4.8V
22/31
Doc ID 15133 Rev 5
-
To disable logic
VIPER27
7.11
Operation descriptions
Burst-mode operation at no load or very light load
When the load decrease the feedback loop reacts lowering the feedback pin voltage. If it
falls down the burst mode threshold, VFBbm, the power MOSFET is not more allowed to be
switched on. After the MOSFET stops, as a result of the feedback reaction to the energy
delivery stop, the feedback pin voltage increases and exceeding the level, VFBbm +
VFBbmhys, the power MOSFET starts switching again. The burst mode thresholds are
reported on Table 8 and Figure 30 shows this behavior. Systems alternates period of time
where power MOSFET is switching to period of time where power MOSFET is not switching;
this device working mode is the burst mode. The power delivered to output during switching
periods exceeds the load power demands; the excess of power is balanced from not
switching period where no power is processed. The advantage of burst mode operation is
an average switching frequency much lower then the normal operation working frequency,
up to some hundred of hertz, minimizing all frequency related losses. During the burst-mode
the drain current peak is clamped to the level, ID_BM, reported on Table 8.
Figure 30. Burst mode timing diagram, light load management
VFB
100
50 mV
hyster.
VFBbm
t
IDRAIN
Normal -mode
7.12
Burst-mode
Normal -mode
t
Brown-out protection
Brown-out protection is a not-latched shutdown function activated when a condition of mains
under voltage is detected. The Brown-out comparator is internally referenced to VBRth
threshold, see Table 8 on page 7, and disables the PWM if the voltage applied at the BR pin
is below this internal reference. Under this condition the power MOSFET is turned off. Until
the Brown out condition is present, the VDD voltage continuously oscillates between the
VDDon and the UVLO thresholds, as shown in the timing diagram of Figure 31 on page 24. A
voltage hysteresis is present to improve the noise immunity.
The switching operation is restarted as the voltage on the pin is above the reference plus the
before said voltage hysteresis. See Figure 5 on page 9.
The Brown-out comparator is provided also with a current hysteresis, IBRhyst. The designer
has to set the rectified input voltage above which the power MOSFET starts switching after
brown out event, VINon, and the rectified input voltage below which the power MOSFET is
switched off, VINoff. Thanks to the IBRhyst, see Table 8 on page 7, these two thresholds can
be set separately.
Doc ID 15133 Rev 5
23/31
Operation descriptions
VIPER27
Figure 31. Brown-out protection: BR external setting and timing diagram
VIN
VINon
VINoff
VBR
t
VBRth
VDD
Vcc
VIN
t
VDIS
Rh
Vin_OK
+
-
t
IBR
AC_OK Disable
IBRhyst
BR
+
VDDon
-
VBRth
VDD
Vin_OK
t
VDDoff
IBRhyst
Rl
t
VDRAIN
t
VOUT
t
Fixed the VINon and the VINoff levels, with reference to Figure 31, the following relationships
can be established for the calculation of the resistors RH and RL:
Equation 9
RL = −
VBRhyst
IBRhyst
+
VINon − VINoff − VBRhyst
VINoff − VBRth
×
VBRth
IBRhyst
Equation 10
RH =
VINon − V INoff − V BRhyst
I BRhyst
×
RL +
RL
V BRhyst
I BRhyst
For a proper operation of this function, VIN on must be less than the peak voltage at
minimum mains and VIN off less than the minimum voltage on the input bulk capacitor at
minimum mains and maximum load.
The BR pin is a high impedance input connected to high value resistors, thus it is prone to
pick up noise, which might alter the OFF threshold when the converter operates or gives
origin to undesired switch-off of the device during ESD tests.
It is possible to bypass the pin to ground with a small film capacitor (e.g. 1-10 nF) to prevent
any malfunctioning of this kind.
If the brown-out function is not used the BR pin has to be connected to GND, ensuring that
the voltage is lower than the minimum of VDIS threshold (50 mV, see Table 8). In order to
enable the brown-out function the BR pin voltage has to be higher than the maximum of
VDIS threshold (150 mV, see Table 8).
24/31
Doc ID 15133 Rev 5
VIPER27
7.13
Operation descriptions
2nd level over current protection and hiccup mode
The VIPER27 is protected against short circuit of the secondary rectifier, short circuit on the
secondary winding or a hard-saturation of fly-back transformer. Such as anomalous
condition is invoked when the drain current exceed the threshold IDMAX, see Table 8 on
page 7.
To distinguish a real malfunction from a disturbance (e.g. induced during ESD tests) a
“warning state” is entered after the first signal trip. If in the subsequent switching cycle the
signal is not tripped, a temporary disturbance is assumed and the protection logic will be
reset in its idle state; otherwise if the IDMAX threshold is exceeded for two consecutive
switching cycles a real malfunction is assumed and the power MOSFET is turned OFF.
The shutdown condition is latched as long as the device is supplied. While it is disabled, no
energy is transferred from the auxiliary winding; hence the voltage on the VDD capacitor
decays till the VDD under voltage threshold (VDDoff), which clears the latch.
The start up HV current generator is still off, until VDD voltage goes below its restart voltage,
VDD(RESTART). After this condition the VDD capacitor is charged again by 600 µA current,
and the converter switching restarts if the VDDon occurs. If the fault condition is not removed
the device enters in auto-restart mode. This behavioral results in a low-frequency
intermittent operation (Hiccup-mode operation), with very low stress on the power circuit.
See the timing diagram of Figure 32.
Figure 32. Hiccup-mode OCP: timing diagram
VDD
Vcc
Secondary diode is shorted here
VDDon
VDD off
VDD (RESTART)
t
IDRAIN
IDMAX
t
VDRAIN
t
Doc ID 15133 Rev 5
25/31
Package mechanical data
8
VIPER27
Package mechanical data
In order to meet environmental requirements, ST offers these devices in different grades of
ECOPACK® packages, depending on their level of environmental compliance. ECOPACK®
specifications, grade definitions and product status are available at: www.st.com.
ECOPACK® is an ST trademark.
Table 10.
DIP-7 mechanical data
mm
Dim.
Min.
Typ.
A
5.33
A1
0.38
A2
2.92
3.30
4.95
b
0.36
0.46
0.56
b2
1.14
1.52
1.78
c
0.20
0.25
0.36
D
9.02
9.27
10.16
E
7.62
7.87
8.26
E1
6.10
6.35
7.11
e
2.54
eA
7.62
eB
L
10.92
2.92
M
N
3.30
3.81
2.508
0.40
0.50
N1
0.60
0.60
O
26/31
Max.
0.548
Doc ID 15133 Rev 5
VIPER27
Package mechanical data
Figure 33. Package dimensions
Doc ID 15133 Rev 5
27/31
Package mechanical data
Table 11.
VIPER27
SO16 narrow mechanical data
mm
Dim.
Min.
Typ.
A
1.75
A1
0.1
A2
1.25
b
0.31
0.51
c
0.17
0.25
D
9.8
9.9
10
E
5.8
6
6.2
E1
3.8
3.9
4
e
0.25
1.27
h
0.25
0.5
L
0.4
1.27
k
0
8
ccc
28/31
Max.
0.1
Doc ID 15133 Rev 5
VIPER27
Package mechanical data
Figure 34. SO16 narrow mechanical data
Doc ID 15133 Rev 5
29/31
Revision history
9
VIPER27
Revision history
Table 12.
30/31
Document revision history
Date
Revision
Changes
16-Jan-2009
1
Initial release
20-Jul-2009
2
Added SO16 narrow package.
22-Oct-2009
3
Updated Table 5 on page 5.
16-Jun-2010
4
Updated Figure 3 on page 4 and Table 3 on page 4.
30-Jul-2010
5
Updated Figure 11, Figure 12 and Figure 13.
Doc ID 15133 Rev 5
VIPER27
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