bourns mf1602 offline converter protection white paper

Offline Non-Isolated Flyback Converter Protection
WHITE PAPER
ABSTRACT
Multifuse® Polymer PTC
Resettable Fuses
This paper examines the use of resettable polymer fuses for protecting offline flyback converters.
Using a thermal model of the resettable fuse surrounding solder pads and copper to optimize the
trip time so that the converter is protected during overloads, there are two potential positions
considered for polymer Positive Temperature Coefficient (PTC) resettable fuses in the circuit.
One position is directly on the winding and the other position is beyond the control loop.
Results are taken from the converter and compared with a simulation.
Figure 1.
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Photograph of Converter
Offline Non-Isolated Flyback Converter Protection White Paper
INTRODUCTION
L1
D6
Multifuse® Polymer PTC
Resettable Fuses
D3
90-275 Vac
D5
C1
C3 C4
UCC28880
D1
TX1
P1 S1
D4
U1
C5
Figure 2.
R1
D2
SM91047EL
MF-MSMF075
C2 R2
5 Vdc
0.5 A
R3 C6
Circuit Diagram of Offline Non-Isolated Flyback Converter
Polymer PTC resettable fuses are used for protecting circuits from overloads, albeit with the
following drawbacks:
1. Difference between rated hold current and trip current. Typically, the trip current is twice the
hold current with trip times of greater than ten seconds. This paper shows how the trip time
can be reduced significantly.
2. Sensitivity to ambient temperature, leading to significant derating. The resistance of
a resettable fuse at 100 °C can be 220 % of its nominal value at 25 °C. However, high
temperature polymer PTCs, are now available and exhibit an increase of 150 % of their
resistance which is comparable to typical high power MOSFETs at 100 °C as shown in figure 2.
Figure 3.
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Normalized Resistance of a High Temperature PTC and a MOSFET
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Offline Non-Isolated Flyback Converter Protection White Paper
DESCRIPTION OF PTC MODEL
Multifuse® Polymer PTC
Resettable Fuses
The behavior of a polymer PTC can be modelled using the laws of thermal dynamics. Polymer PTCs
react to temperature and will change from low to high impedance at a certain trip temperature. The
polymer PTC time to trip depends on the power generated in the component which increases the rate
of change of temperature as well as the surroundings which can dampen the rate of change. We can
define a polymer PTC as a thermal three-body model consisting of a power source which generates
heat in the PTC chip which in turn dissipates through packaging and surrounding solder pads and
copper tracks.
Figure 4.
PG
B1
B2
Power
Source
PTC
Chip
Packaging
B3
Interface
External
Environment
Three-Body Thermal Model of a PTC
The equations for all three bodies B1, B2 and B3 are as follows:
dtB1
dt
=θ12 k1 pG θ12-k1 (tB1-tB2)
dtB2
dt
= θ k2 (tB1-tB2)-k2 (tB2-tB3)
dtB3
dt
= θ k3 (tB2-tB3)-k3 (tB3-tA)
θ23
12
θ3A
23
(1)
(2)
(3)
Where:
• θ12 is the thermal resistance between bodies B1 and B2
• θ23 is the thermal resistance between bodies B2 and B3
• θ3A is the thermal resistance between body B3 and the environment
• pG is the input power
• ta, tB1, tB2, tB3 is the temperature of the various bodies
• k1, k2, k3 are constants of proportionality
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Offline Non-Isolated Flyback Converter Protection White Paper
DESCRIPTION OF PTC MODEL (Continued)
Multifuse® Polymer PTC
Resettable Fuses
We turn to SPICE models to solve these differential equations. An RC network as shown in figure 5
with a current source IS has the same differential equations. IS represents the power generated in
the circuit. Vcth1 represents the temperature on the chip while Vcth2 represents the temperature on
the packaging and Vcth3 is the temperature on the solder interface. The corresponding differential
equations are now:
=
Is
Cth1
dVcth2
dt
=
(Vcth1-Vcth2)
Rth1Ccth1
-
(Vcth2-Vcth3)
Rth2Ccth2
(5)
dVcth3
dt
=
(Vcth2-Vcth3)
Rth2Ccth3
-
(Vcth3-Vta)
Rth3Ccth3
(6)
Rth1
IS
Figure 5.
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(4)
dVcth1
dt
Cth1
-
(Vcth1-Vcth2)
Rth1Ccth1
Rth2
Cth2
Rth3
Cth3
ta
RC Network Equivalent of Three-Body Model
4
Offline Non-Isolated Flyback Converter Protection White Paper
DESCRIPTION OF PTC MODEL (Continued)
Multifuse® Polymer PTC
Resettable Fuses
We can now use curve fitting to determine the correct vales of Rth1, Rth2, Rth3, Cth1, Cth2 and Cth3. The
thermal resistance of the system is calculated using the power dissipated by the component, as
well as the ambient temperature and the temperature at which the component trips. The thermal
resistance is divided between Rth1, Rth2 and Rth3.
The model can be used for predicting time to trip and for evaluating the effect of thermal resistance
on trip times. Figure 6 shows modelled times for a 0.75 A rated polymer PTC superimposed on the
measured times taken from the actual data sheet.
Figure 6.
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Modelled Times to Trip Compared with Data Sheet
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Offline Non-Isolated Flyback Converter Protection White Paper
DESCRIPTION OF PTC MODEL (Continued)
Multifuse® Polymer PTC
Resettable Fuses
If the polymer PTC is mounted on a circuit board, then the third body (B3) would be the output
solder pad and connecting track drawn as shown in figure 7 where W1, L1, W2, L2, W3 and L3
represent three separate thermal resistances which form Rth3.
L2
W1 W2
W3
L3
L1
Figure 7.
Representation of Rth3 as a Copper Plane
The thermal resistance θcu of a copper plane can be expressed as:
(7)
L
θcu = W*t*β
Where β is the thermal conductivity of copper (4 W / (cm °C) ).
The thermal resistance of a plane as shown in figure 7 of the thickness consisting of a pad plus
copper trace can be represented by the following equation:
L1
L2
L3
+ W2
+ W3
)(
θplane = ( W1
1
βt
)
(8)
Let θplane be Rth3, as this represents the third body in the thermal model. Assuming we adjust L2 and
L1
L3
L2
W2 and assuming W1
and W3
are much smaller than W2
, we can express Rth3 as:
L2
Rth3 = W2**tβ
(9)
Hence, Rth3 can be increased by adjusting W2 downward.
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Offline Non-Isolated Flyback Converter Protection White Paper
DESIGN CONSIDERATIONS FOR THE FLYBACK CONVERTER WITH PTC
Multifuse® Polymer PTC
Resettable Fuses
An offline flyback converter using a UCC28880 monolithic controller was designed to operate in
Continuous Conduction Mode (CCM) with 5 V +/-5 % output and a maximum load of 0.5 A from
an input voltage range of (90 - 275 Vac). The UCC28880 uses a high voltage MOSFET of 700 V.
It switches at 62 kHz and has a typical peak current limit of 0.21 A. The primary inductance was
selected based on the fact that the controller has a maximum current at worst-case -40 °C of 0.3 A.
The minimum inductance required to keep the power supply in CCM is as follows:
Lp=
VDCmin*Dmax
Ipeak*FSW
(10)
Lp was selected as 5 mH based on a minimum input of 90 V and a switching frequency of 62 kHz
as well as a worst-case peak current of 0.3 A.
The turns ratio N is calculated as:
N=
Dmax*VDCmin
Vout*(1-Dmax)
(11)
Being able to operate the controller at the maximum duty cycle of at least 45 % therefore, requires
a higher turns ratio but this also increases the stress on the output diode. Secondary detection of
the current allows for automatic adjustment of the primary current limit. The controller protects
itself from short circuit currents or overloads by entering a “runaway” protection mode, whereby
the switching frequency is reduced, allowing the secondary side more time to discharge. Under
worst-case conditions, the current limit could be 0.3 A. The rms current in the secondary is given
by the following equation:
Imsout= I* √ (1-D) * √ 1+ 13 ( ∆II )2
(12)
Where I represents the DC value of the current.
The duty cycle during overload will be very low so (12) can be simplified as:
Imsout= I*√ 1+ 13 ( ∆II )2
(13)
ΔI is calculated as 0.2 A on the secondary side. This gives Irmsout = 3.25 A. If we ignore the ripple
we can use the following formula:
Iout= Ilimit*N*(1-D)
(14)
Using (14), Iout is 3.4 A.
It is probably excessive to use a diode rated to withstand this short circuit current when the
circuit is designed for 0.5 A. The secondary winding would also have to be chosen so it would not
overheat during such a short circuit.
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Offline Non-Isolated Flyback Converter Protection White Paper
LOCATION OF PTC
A resettable fuse can be placed in two locations as shown in figure 8. In position A, the polymer
PTC could be directly assembled inside the winding. The voltage across the polymer PTC in this
VDC
position will be at least N during the on time and Vout*N during the flyback time where N is the
number of turns and VDCmin is the minimum DC input voltage. Therefore, the polymer PTC must
be rated to this voltage. During an overload, the polymer PTC will reduce the feedback voltage to
zero which in effect creates a potential open loop leaving the output capacitor unprotected.
min
Multifuse® Polymer PTC
Resettable Fuses
An alternative location, position B, is located after the control loop and the output capacitor. In
an overload situation the controller will regulate as before with the polymer PTC acting as a high
ohmic load. Furthermore, the polymer PTC can have a voltage rating equal to the output voltage,
which in this case, was 5 V instead of 32 V if connected in position A. Position B, therefore, is
judged as being the best location for the polymer PTC.
PTC Pos A
PTC Pos B
Load
Secondary
Figure 8.
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Illustration of Two PTC Locations
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Offline Non-Isolated Flyback Converter Protection White Paper
RESULTS AND FINDINGS
Multifuse® Polymer PTC
Resettable Fuses
5/16 • e/MF1602
The polymer PTC in position B was a surface mount device with a resistance of 0.2 ohms on a
board with 70 µm of copper. We can use SPICE to resolve the correct values for Rth3 and Cth3 in
order to reduce the device’s time to trip when it is conducting 1.5 A. Using a track of length of 5
mm, a width of 2.0 mm and the normal thickness for power boards (70 µm), we obtain a value
for Rth3 of 71.4 °C/W. This closely approximates our curve fitting of 69 °C / W for Rth3. The time
to trip at 1.5 A closely matches the simulation. The overload test was repeated with a track width
of 1 mm and the time reduced significantly to 3.5 seconds (figure 9). The thermal resistance was
recalculated to have increased to 178.5 °C/W. By stepping the thermal resistance in increments of
60 °C/W in SPICE, we were able to confirm the same measurements as shown in figure 9.
Figure 9.
Comparison of Simulated Time to Trip Compared with Actual Values
Figure 10.
Time to Trip of PTC at 1.5 Amps of Current (Starting from Continuous 0.5 A)
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Offline Non-Isolated Flyback Converter Protection White Paper
RESULTS AND FINDINGS (Continued)
Multifuse® Polymer PTC
Resettable Fuses
Figure 11.
Thermal Image of Board during Short Circuit Test
The board with the polymer PTC was connected to a 33 mF capacitor. The capacitor charged in
0.3 seconds with a load of 0.5 A (figure 12). A power supply of similar output voltage and current
(5 V at 0.5 A) without a polymer PTC for protection but with integrated secondary overcurrent
protection was also connected to the capacitor. The secondary current limit was set to 0.65 A.
Figure 12 also shows that the protection circuit remains tripped in this condition due to the
very high initial charging currents. These currents are not long enough in duration to trouble
the polymer PTC. This short experiment illustrates one benefit of the polymer PTC for circuits
charging super capacitors.
Figure 12.
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Charging Voltage of 33 mF with PTC and with Secondary Overcurrent Protection
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Offline Non-Isolated Flyback Converter Protection White Paper
CONCLUSION
Multifuse® Polymer PTC
Resettable Fuses
A polymer PTC resettable fuse can be used to provide short circuit protection to a flyback
converter. It is possible, using thermal dynamics, to model the polymer PTC and the environment
to calculate the required external copper traces to obtain the necessary trip time. High temperature
polymer PTCs demonstrate a comparable resistance drift over temperature to MOSFETs and
could be considered for circuits where there is not a secondary overcurrent protection mechanism
or where the initial inrush is too much for the in built short circuit protection circuit. The best
location for the polymer PTC is on the output after the control loop. Putting the polymer PTC on
the secondary, can leave the circuit vulnerable to open circuit conditions.
REFERENCES
Advanced Power Technology. Power Mosfet Tutorial, Application Note APT-0403. Rev B March 2, 2006
Bourns Electronics. MF-RHT Series PTC Resettable Fuses Data Sheet, Rev. H, 11/14
Erickson, Robert W. and Dragan Maksimovic. Fundamentals of Power Electronics Springer Science and Business Media, LLC 2001
Gupta, Milind. Use thermal analysis to predict and IC’s transient behavior and avoid overheating Maxim Engineering Journal, Volume 68, pages 9-15
Texas Instruments. AN-2020 Thermal Design By Insight, Not Hindsight, SNVA419C–April 2010–
Revised April 2013
Texas Instruments. UCCC28880 High Voltage Switcher for Non Isolated AC/DC Conversion, SLUSC05A –
July 2014–Revised October 2014
ADDITIONAL RESOURCES
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www.bourns.com
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