dm00076745

AN4250
Application note
Fishbone diagram for power factor correction
Rosario Costanzo, Gianluca Messina, Antonino Gaito
Introduction
This report aims to show through a Fishbone diagram, all possible causes of failure of the
Power MOSFET mounted on a PFC.
This work is divided into 5 sections:
• The first one describes the power factor
• The second describes the Boost converter
• The third describes the PFC system
• The forth paragraph shows all critical conditions causing the failure of the Power MOSFET
and builds up the Fishbone diagram
• Last paragraph presents a specific Fishbone parallel configuration
March 2014
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www.st.com
Contents
AN4250
Contents
1
Power factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2
Boost converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
Power factor correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4
Fishbone diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5
Fishbone diagram for Power MOSFET in parallel . . . . . . . . . . . . . . . . 12
6
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1
Power factor
Power factor
In the power electrical circuit, the power factor represents an index measuring the available
main power used.
The general power factor definition is given by the ratio between the active power and the
apparent power:
Equation 1
P
P. Active
=
S P.Apparent
P.F . =
Its value varies from 0 to 1. In the efficient system, the value of the power factor should be
equal to 1.
In the classic Graetz rectifier with capacitive filter, the voltage and current waveforms are not
sinusoidal.
Figure 1. Schematics of a single phase diode bridge rectifier
13V
V (n 0 0 2 )
V (n 0 0 1 )
I( V 1 )
12V
D2
1N4148
D3
10V
C1
1N4148
D1
11V
D4
100µ
R1
1K
V1
1N4148
50m A
0m A
8V
-5 0 m A
7V
6V
-1 0 0 m A
-1 5 0 m A
4V
-2 0 0 m A
3V
-2 5 0 m A
2V
SINE(0 12 50 0 0 0)
100m A
9V
5V
1N4148
150m A
1V
0V
-3 0 0 m A
-3 5 0 m A
-4 0 0 m A
-1 V
12m s 15m s 18m s 21m s 24m s 27m s 30m s 33m s 36m s 39m s 42m s 45m s 48m s
;tran 2
.tran 50ms
AM17349v1
The sinusoidal voltage main is expressed by the following equation:
Equation 2
V
mains
= 2 V eff sen (ω t + ϕ )
The current expression, not being sinusoidal, can be represented in fourier series:
Equation 3
I
mains
= 2 I 1 sin(ωt + ϕ1) + 2 I 3 sin(3ωt + ϕ 3) + 2 I 5 sin(5ωt + ϕ 5) + .....
The active power is given by the following formula:
Equation 4
P = V eff
I
1
cos ϕ
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Power factor
AN4250
While the apparent power:
Equation 5
S = V eff I eff
Due to equations 4 and 5, the power factor is:
Equation 6
P.F . =
I
I
1
cos ϕ = cosϑ cos ϕ
eff
The power factor depends on the phase displacement due to the contribute of the factor ϕ
and on the harmonic content due to the contribute of the factor ϑ.
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2
Boost converter
Boost converter
The Boost converter is generally used in the SMPS as a PFC.
Section 3 gives a complete description of the PFC block and the Boost converter.
The Boost circuit is a DC-DC converter providing a higher output voltage than input voltage.
Here below the classic Boost topology.
Figure 2. Boost converter schematic
L1
D1
L
D
V1
C1
SWITCH
R1
R
C
V
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The circuit is composed of:
–
Inductor
–
Switch (Power MOSFET)
–
Boost diode
–
Capacitor and load
The functioning of the circuit can be divided into two steps depending on the conduction
state of the switch: on-state, off-state.
On-state
During the on-state, the switch can be considered as a short-circuit, thus from the initial
topology two meshes can be derived:
Figure 3. Boost converter schematic during the Power MOSFET turn-on
L1
L
V1
Ton
V
C1
C
R1
R
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V =V
L
IN
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Boost converter
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Off-state:
During the off-state the switch is open, thus the electrical schematic of the converter is:
Figure 4. Boost converter schematic during the Power MOSFET turn-of
L1
L
V1
V
C1
Toff
C
R1
R
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Owing to the energy accumulated by the inductance during the previous phase (ON), the
current, through the inductor, cannot go to zero instantaneously. The consequence of this
phenomenon is an extra voltage with a sign, which can oppose to the decreasing current.
Due to the extra voltage, the anode of the Boost diode has a higher voltage than the output
one, allowing the current to flow through it.
Below equation describes the above concept:
Equation 7
V t
IN
on
+ (V IN − V O ) t off = 0
Dividing the equation by the commutation period (Ts) we obtain:
Equation 8
V
V
O
=TS =
IN
t
off
1
1− D
(D is the duty cycle= ton/Ts)
The above two formulas have to be considered valid only if the converter works in the
continuous mode.
In fact, the Boost converter can work in two modes of conduction: CCM (continuous
conduction mode), DCM (discontinuous conduction mode).
The shape of the current, flowing through the switch, distinguishes these two modes:
CCM
In the continuous conduction mode the current, through inductor during an entire period,
does not reach the zero value; the Boost diode starts its conduction phase with a positive
current value.
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Boost converter
Figure 5. CCM Boost converter waveforms
S
OFF
OFF
ON
ON
vD
Vo
iL
I in
Vin / L F
slopes
- (Vo -Vin ) / L F
vS
Vo
iD
Io
Ton
Toff
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DCM
In the discontinuous transition mode the current, through the inductor during an entire
period, reaches the zero value, so the diode and the switch start their conduction phase
from zero.
Figure 6. DCM Boost converter waveforms
S
OFF
ON
OFF
ON
vD
Vo
Vo - Vin
iL
DCM
I in
vS
Vo
Vin
Ton
T'off
Tdcm
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As it is possible to notice, during the discharge phase of the inductor, the current reaches
zero.
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Power factor correction
3
AN4250
Power factor correction
The PFC is a power stage, which can correct the power factor. The circuit target allows the
input current in phase with the main voltage to reduce the harmonic distortion. PF values
are close to 1.
Working on the duty cycle or on the frequency of the switch commutation, the sinusoidal
current shape can be built. The entire system is managed by a microcontroller, which
compares the input main voltage with the output voltage and acts on the duty cycle.
Boost converter features two conduction modes; current waveforms depend on them.
In the DCM, the Power MOSFET of the Boost converter turns on when the inductor current
reaches zero, and turns off when the inductor current meets the desired value.
In the CCM, the Power MOSFET of the Boost converter turns on when the inductor current
is not zero.
Figure 7. Current waveforms on the inductor for the CCM/DCM Boost converter
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Fishbone diagram
Fishbone diagram
In this section most common critical conditions, affecting the Power MOSFET mounted as
switch on the Boost converter, are described. Besides, they are reported in a fishbone
diagram.
Causes linked to the application
Short-circuit of the startup
In the Boost topology above described, the “pre-charge” circuit has not been presented; this
circuit works during the start-up phase when the output capacitor is not charged and the VIN
is greater than VOUT. This condition forces the switch to absorb more current than the
steady-state operation, therefore the switch could work with current exceeding the
maximum ratings.
Using the “pre-charge” circuit, the current doesn't flow through the switch since it flows
through the diode of the “pre-charge” circuit until VIN > VOUT.
Figure 8. Pre-charge circuit
R2
D2
R
D
L1
D1
L
D
V1
SW ITCH
V
C1
C
R1
R
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Choke dimensions
Since coil saturation depends on the current flowing through it, if the coil is not dimensioned
correctly, coil saturation can occur (the coil behaves as a simple short-circuit); a huge
quantity of current can flow through the switch causing an overheating that, in some cases,
causes the failure of the device.
Losses during the switching
If the MOSFET is not driven correctly, an overheating can occur. This phenomenon can
cause the failure of the device.
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Fishbone diagram
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Short-circuit of the load
When the load is in short-circuit condition, if the short protection circuit is not present or if its
operative time is too long, the following failures can happen:
1.
If the “pre-charge” circuit is not present, the switch works in start-up phase with a huge
quantity of current (refer to the start-up critical condition)
2.
the “pre-charge” circuit is present till it works, then current flows onto the switch like the
previous case
Switching diode recovery
In CCM as above written, during the commutation, the current, which flows through the
diode, is not zero. In this case, both the coil current inside the switch and the diode current
flow (current spike depends on the reverse recovery time trr). In relation to the intensity of
the spike, the overall absorbed current could cause the failure.
Figure 9. Recovery current inside a diode
ID
I D nom
25%
T
I RM
I RM
TIRM
TRR
AM17357v1
Causes linked to the material
Due to the intrinsic characteristics of the Power MOSFET device (Vth, Rg, Crss), a spurious
turn-on can happen during the turn-off commutation.
In detail, during the turn-off, spurious noise on the drain (like a fast dv/dt) generates with the
Crss capacitance a spurious current:
Equation 9
i
s
=
C
d
rss
v
DS
dt
High values of Crss can cause the undesired turn-on of the switch.
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Fishbone diagram
The above condition is better described when the intrinsic Power MOSFET Rg is much
lower than the gate driver resistor Rd and the Power MOSFET threshold Vth is close to the
lower datasheet limit.
Causes linked to the method
An improper thermal dissipation due to incorrect driving/heatsink, can cause an overheating
of the device with failure.
Figure 10. Fishbone diagram
APPLICATION
LOSSES DURING THE
SWITCHING
SHORT-CIRCUIT
OF THE LOAD
STARTUP
SHORT-CIRCUIT
WRONG
DIMENSIONED CHOKE
SWITCHING
DIODE
RECOVERY
METHOD
INADEQUATE DRIVING
WRONG
HEATSINK
DIMENSIONING
FAILURE
INFLUENCE OF INTRINSIC
POWER MOSFET
CHARACTERISTICS (Rg, Vth,
Crss)
MATERIAL
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Fishbone diagram for Power MOSFET in parallel
5
AN4250
Fishbone diagram for Power MOSFET in parallel
When the power levels are very high, some PFC makers put in parallel two or more Power
MOSFET devices. In this case, other aspects must be taken in account.
Causes linked to the application
Parasitic inductance
It is generated by interconnection wiring and discrete components. They cause delays and
power losses which affect the balance of the current. It strictly depends on the PCB layout.
Temperature unbalance
A temperature rise causes a decrease of Vth and an increase of RDS(on).
Decrease of Vth -> switching loss rise -> thermal runaway.
Increase of RDS(on) -> conductive loss rise-> current limitation -> unbalance.
Several factors let two Power MOSFETs work according to different temperatures:
–
Another device is mounted on the same heatsink
–
Different air flow according to the fan
Boost diode
During the turn-on, it impacts on the current spike. If devices with different Vth are used
(500 mV), a fast diode or a very fast diode involves a different peak current.
Gate circuitry
Decoupling resistor mismatch causes the current unbalance. The device with lower Rgate
leads more current than other one and its temperature, as well as its RDS(on) increase.
Causes linked to the material
Different Vth
During the switching phase (turn-on, turn-off), the difference of Vth leads the device, with its
lower value, to conduct earlier, causing an unbalance of the currents.
Different RDS(on)
The difference of RDS(on) causes, during the conduction phase, an unbalance between two
currents. In particular, if one of the two Power MOSFETs has a lower RDS(on), an increase of
its ID current is observed, causing an increase of the Power MOSFET temperature.
Gfs influence
Different gfs values, during the commutation, cause substantial differences in relation to the
two currents. This parameter is guaranteed by design and process.
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Fishbone diagram for Power MOSFET in parallel
Causes linked to the method
Different torque on heatsink
Different torque of the two devices causes different heat dissipation and the current
unbalance.
Figure 11. Additional Fishbone diagram for parallel configuration
APPLICATION
METHOD
Circuit component
difference
Parasitic
inductances
Temperature
unbalance
Difference torque of the
mounting on heatsink
Boost diode
Gate circuitry
CURRENT
UNBALANCE
Rdson differences
Vth differences
gfs influence
Difference of intrinsic Power MOSFET
characteristics
MATERIAL
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Revision history
6
AN4250
Revision history
Table 1. Document revision history
14/15
Date
Revision
11-Mar-2014
1
Changes
Initial release.
DocID024230 Rev 1
AN4250
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