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AN441
Application note
Inductive load control with AC switches
Introduction
AC switches are now commonly used as static switches to drive inductive loads such as
magnetic transformers, valves, induction motors, etc.
This application note describes the particular points to focus on when such loads are
controlled by AC switches like Triac, ACS or ACST. For example, there is an explanation of
just when a Triac has to be triggered to reduce the inrush current at turn on.
Typical examples are given for magnetrons used in microwave ovens, transformers for SELV
halogen lamps, and universal motors used in vacuum cleaners.
March 2010
Doc ID 3579 Rev 3
1/16
www.st.com
Reasons for inrush current in inductive loads
AN441
1
Reasons for inrush current in inductive loads
1.1
Inrush current due to inductive load behavior
Many inductive loads are controlled in full-wave mode. This is the case of valves, pumps,
compressors, etc. For these loads, the inrush current greatly depends on turn-on delay at
start-up.
We do not consider, in this section, the effect of magnetic circuit saturation that could also
lead to inrush current increase. Refer to Section 1.2 for this point.
A typical inductive load, controlled by an AC switch, can be simulated using a standard RL
circuit (Figure 1).
Figure 1.
Inductive load control with Triac
i(t)
R
L
VMAIN
T
u(t)
According to Figure 1, the AC load current i(t) is define by Equation 1.
Equation 1
u(t) = R·i (t) + L
di (t)
dt
Considering the circuit in sinusoidal full-wave mode, with turn on at zero mains voltage, the
value of inrush current is:
Equation 2
i(t) =
R
⎞
− ·t
URMS 2 ⎛⎜
L − L·ω·cos (ω·t )+ R·sin(ω·t )⎟
·
L·
·e
ω
2
2 ⎜
⎟
(L·ω) + R ⎝
⎠
In case of a delay applied between zero voltage and Triac triggering in the first half-cycle
(assuming following cycles are in full-wave mode), the value of inrush current is:
Equation 3
i(t, t 0 ) =
R
⎞
− ·t
URMS 2 ⎛⎜
L - L·ω·cos (ω·(t + t ))+ R·sin(ω·(t + t ))⎟
(
(
)
(
)
)
·
L
·
·cos
·
t
R
·sin
·
t
·e
ω
ω
−
ω
0
0
0
0
2
⎟
(L·ω) + R2 ⎜⎝
⎠
Where t0 is the triggering delay at the first turn on
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AN441
Reasons for inrush current in inductive loads
These two equations show that, when the inductive load is switched on in full-wave, the
transient current depends on the Triac first turn-on delay with respect to the mains voltage
zero point. Figure 2 shows the load current curve for triggering at zero voltage and triggering
at the peak mains voltage. This figure comes from PSPICE simulation for a 150 ohm and
5 H load switched on at a voltage of 230 V rms at 50 Hz.
Triggering at zero voltage brings the highest inrush current which can be up to twice the
peak current reached in case of triggering at peak voltage.
Inrush current difference according to Triac first turn-on delay
400
400
300
300
200
200
100
100
0
0
20
40
60
80
0
100
-100
-100
-200
-200
-300
-300
-400
Mains voltage (V)
Current (mA)
Figure 2.
-400
Turn-on at zero crossing
Time (ms)
Turn-on at peak mains voltage
Mains voltage
Due to this high peak current, two problems may occur at AC switch level.
●
High peak current may be higher than ITSM value (maximum surge peak current). In this
case the component can be damaged.
●
AC switch temperature may exceed the maximum allowed junction temperature (this
will not lead necessarily to device failure but electrical parameters are not anymore
guaranteed if working temperature is above max allowed value).
Inrush currents have also to be checked to fit electromagnetic compatibility standards.
Actually, IEC 61000-3-3 standard make it mandatory to limit inrush currents of appliances
connected to the power network to reduce the flickering effect on lighting.
It should also be noted that reducing inrush current helps to increase the reliability of the
load and other switches or breakers used in series with the load.
1.2
Magnetic core saturation due to remanent induction
In transient operation, the induction can follow a different path and reach the saturation
value BS for which the magnetic field H increases very rapidly even for a low induction
variation (see Figure 3). At saturation level the magnetic material permeability decreases
drastically, down to air permeability. This leads to a lower inductance value. The load current
is then mainly limited by the load resistance, and can increase substantially. Saturation then
leads to a high increase of the coil current.
Doc ID 3579 Rev 3
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Reasons for inrush current in inductive loads
Figure 3.
AN441
Magnetic field H versus induction B (continuous rating)
B(t)
Bs
B
Br
t
H
i(t)
t
According to Equation 2, at start up at zero voltage the current is higher (longer time to
integrate voltage and so higher induction reached in the 1st cycle) and so there is a higher
risk of reaching magnetic core saturation.
Also a second phenomenon can increase risk of saturation. This phenomenon is due to
remanent induction. The remanent induction (refer to Br in Figure 3.), corresponds to the
point where H equals 0. If a positive voltage is applied from a point where there is a positive
remanent induction, the induction will start to increase from a higher initial value, so will
reach saturation faster (refer to appendix 1 for further explanation on this phenomenon).
To avoid this phenomenon in circuits controlled by an AC switch, device switch on has to be
implemented on the reverse polarity according to previous switch off. Figure 4 shows two
different test results carried out on a 200 VA 230 V to 12 V transformer. Curve A shows the
current waveform, recorded after a previous identical current waveform. The particularity of
this waveform is that the first half-cycle conduction is in the same polarity as the previous
one. In this case the transformer reaches saturation very rapidly and the transformer
behaves like a short circuit. The peak current is limited only by the series resistance of the
transformer.
Curve B shows the same recording but here with the first half-cycle conduction in reverse
polarity compared to the last one. These two curves clearly show that saturation is reached
in case A due to previous conduction. Then load current can be approximately eight times
higher than if care is taken to always trigger the device for an integer value of full-cycle
periods.
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AN441
AC switch control with an inductive load
Figure 4.
Last turn-off polarity influence on the next turn on
8.75 A
1.16 A
B
A
2
AC switch control with an inductive load
2.1
Latching current
The latching current IL of a Triac is the minimum value of the load current (circulating
through terminals A2 and A1), to keep the device conducting when the gate signal is
removed. Diagram a in Figure 5 shows a bad Triac turn on due to too short gate pulse width
and Diagram b in Figure 5 shows a good Triac turn on. The pulse width is sufficiently large
so that the current i(t) reach the latching current. See Application note AN303 for more
information.
For inductive loads, as the current rate of increase is limited by the inductance, care has to
be taken to have a large enough gate pulse width to reach IL (refer to Section 2.2).
Figure 5.
Latching current of the AC switch
i(t)
i(t)
IL
IL
t
t
IG
IG
t
t
a
b
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AC switch control with an inductive load
AN441
2.2
Gate control for full-wave operation
2.2.1
Gate current pulse width
Gate current pulse width must be set up to reach the required IL value. To avoid a break in
the current waveform at zero point, holding current also has to be taken into consideration
(see Application note AN302 for a definition of IH). Pulse width is given by Equation 4.
Equation 4
tP >
⎛ IH MAX ⎞ 1
⎛I
⎞
1
⎟ + ⋅ arcsin⎜ L MAX ⎟
⋅ arcsin⎜⎜
⎟ ù
⎜ Ipeak ⎟
Ipeak
ù
⎝
⎠
⎝
⎠
To reduce the pulse width duration, a more sensitive Triac could also be used.
2.2.2
Gate current synchronization with mains voltage
In case of inductive load, the current is lagging regarding to the mains voltage. This is why,
in case of control by gate current pulse, the gate pulse has to be applied with a delay after
each line zero voltage point which approximately equals the current-voltage phase shift (as
shown on Figure 6)
Figure 6.
Triac pulse width synchronization
u(t)
i(t)
IL
IG
Delayed time
In case the zero line voltage event is not sensed by the Triac control circuit or no timer is
available to manage triggering times, one solution is to apply a DC gate current. In this case
the average current consumption for the power supply will be higher. But today more and
more switching mode power supplies (SMPS) are used offering a higher output current
capability.
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AN441
AC switch control with an inductive load
If a power supply with reduced output current capability is used (for example, the case with
capacitive or resistive power supplies), a solution to reduce the gate current average value
is to apply a high-frequency (several kilohertz) burst to the gate. This solution allows
reduction of Triac off time that occurs after current has reached zero and before a new gate
current pulse (coming with the high-frequency burst) is applied. This maximum off time will
then approximately equal (at worst case) to one gate-burst period (see Application note
AN308).
2.3
Inrush current limitation
To limit the inductive load inrush current, triggering at peak mains voltage could reduce the
peak current as previously shown. Also, especially for transformers, it is better to ensure that
the device is switched-on at a different polarity compared to last half-cycle conduction (refer
to Section 1.2).
But this triggering method will only be efficient in applications where the inrush current
comes from inductive part of the load or from core saturation. This is mainly the case with
transformers or valves. The inrush current period usually lasts less than 100 ms.
For other applications, the inrush current could also come from other phenomenon.
●
Low-resistance value at start-up (cold filament of incandescent lamp)
●
High starting torque (induction motors used in compressors)
●
Low back EMF at zero speed, i.e. at start-up (universal motor or permanent-magnets
induction motors used in pump applications)
For these applications, the inrush current can last up to 500 ms.
As the inrush current is not only due to the behavior of the inductive part of the load, the
problem cannot be resolved just by triggering the Triac at peak voltage for the 1st half-cycle
and then control it in full wave-mode. The solution consists of implementing a soft start by
progressively increasing the conduction angle of the AC switch. This solution allows the
maximum current at turn on to be limited to a value near to the steady state. This brings the
following benefits:
●
Lower rate of rise of temperature for the device and so better device reliability
●
Compliance with the IEC 61000-3-3
●
Lower peak current and so better load reliability
Doc ID 3579 Rev 3
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Typical examples of inrush current reduction
AN441
3
Typical examples of inrush current reduction
3.1
Magnetron in microwave oven
The magnetron of a microwave oven is generally supplied by rectified high voltage obtained
through a 50/60 Hz transformer. The power supplied to the oven is controlled by a Triac in
series with the primary winding of the transformer (see Figure 7).
Figure 7.
Typical circuit of a magnetron controlled by a Triac
HV
V
MAINS
The power to be controlled is typically between 1 to 2 KW and the nominal rms current is in
the range of 10 to 20 A according to the line voltage. The inrush current comes from the
magnetizing current through the transformer at turn on.
Due to the high inductance of this transformer this overcurrent can reach a peak value up to
4 to 20 times the steady state value.
The high peak current at turn on as shown in Figure 8 can damage an 8 A Triac as it is close
to ITSM value (max surge peak current, 80 A for example for a BTA08 device).
Figure 8.
Overcurrent at microwave oven switch on
80 A
I T (20A/ div )
21 A
Firing at zero mains voltage
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I T (20A/ div )
Firing at peak mains voltage
AN441
Typical examples of inrush current reduction
Triac turn on at peak mains voltage allows the reduction of the peak inrush current by 75%
compared to a zero voltage switch on as shown in Figure 8.
The reduction of first peak current allows:
3.2
–
Use of a lower Triac range (lower IT(RMS) range),
–
Safer margin for the Triac (ITSM).
Transformer for lighting
A Triac could be used to dim SELV (safety extra low voltage) halogen lamps. In this case the
lamps are connected at the secondary winding of a 220 V to 12 V step-down transformer.
The Triac is then connected on this transformer in series with the primary winding (Figure 9).
The power applied to the lamp is controlled by the variation of the conducting angle of the
Triac and allows dimming the brightness of the lamp.
Figure 9.
Typical schematic of a light dimmer for SELV halogen lamps
V
MAINS
Like microwave oven transformers, transformers for lighting application present an
overcurrent if they are switched on at zero voltage due to magnetizing current.
This inrush current is increased if the lamp filament is cold. Filament resistance decreases
when the temperature decreases.
The high current due to the cold filament effect could cause transformer magnetic core
saturation, which then drastically increases the primary winding inrush current.
To reduce the inrush current, an appropriate triggering needs to be used. Table 1 shows
different halogen lamp test results used with a transformer. The table shows that the
maximum peak current is lower when the load is triggered at zero current.
Indeed the ratio between peak current at zero voltage and at peak voltage triggering is in the
range of 2 to 4. It should also be noted that, in case of triggering at peak voltage, some
transformers (transformer 1 and 2) have a peak inrush current close to 13 times the steady
state peak current whereas others feature a ratio lower than 7 (transformer 3). This is linked
to the low-saturation level of some low-quality transformers.
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Typical examples of inrush current reduction
Table 1.
AN441
Halogen lamp test through a transformer
Turn-on moment
Transformer 1
150 - 300 VA
Transformer 2
100 - 210 VA
Transformer 3
200 VA
3.3
Inrush current
Output power
(12 V halogen lamps)
IPEAK
IRMS
ZVS
275 W
17.52 A
1.26 A
ZCS
275 W
8.65 A
1.26 A
ZVS
200 W
20.32 A
1.3 A
ZCS
200 W
5.32 A
1.3 A
ZVS
200 W
7.48 A
1.12 A
ZCS
200 W
2.83 A
1.12 A
Induction motor
Induction motors or universal motors are today widely used in home appliances for
applications as such as vacuum cleaners, washing machines, blenders, and fridge
compressors. The typical load power is in the range of 100 to 3000 W. These motors
present a very high inrush current due to the inductive current in case of turn on at zero
voltage.
This inrush current is also increased in the case of a universal motor as the back EMF is
zero at zero speed. For zero speed the line current will then be limited only by motor winding
inductance and resistance, but not by the back EMF that can reach 10 to 50% of line
voltage. So applying the whole line voltage at motor start-up will cause a much higher
current than applying the same voltage when the motor is running at full speed.
A better solution is to implement a soft-start. This consists of progressively increasing the
conducting angle of the AC switch.
Figure 10. Over current at Triac turn on for universal motor
80
I(A)
With soft start
60
Without soft start
40
20
0
-20
-40
-60
Time (ms)
-80
0
10/16
100
200
300
400
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500
600
700
800
AN441
Typical examples of inrush current reduction
Figure 10 show an example of Triac switching on a 2200 W universal motor with or without
soft start. The difference is very clear, the current reaches a peak current up to 60 A without
soft start while it reaches 10 A with soft start. This current reduction has a direct impact in
the AC switch maximum junction temperature as shown in Figure 11 with a T1050H Triac.
The use of soft start at motor start-up allows:
●
●
Lower junction temperature
–
Better temperature operation margin leading to increased lifetime for the AC
switch
–
Use of lower AC switch range
Lower peak load current
–
Lower load current stress, so appliance reliability increases
Figure 11. Junction temperature rise for Triac T1050H for universal motor
Tj (°C)
124.2 °C
130
120
Without soft start
100
Tj1a
Tj2a
With soft start
78 °C
80
Time (s)
65
0
0.2
0.4
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0.6
0.8
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Conclusion
4
AN441
Conclusion
AC switches are widely used to drive inductive loads. With inductive loads, triggering
synchronization with line voltage has a high impact on the inrush peak current and needs to
be controlled to minimize the first peak current and reduce the maximum junction
temperature.
Peak-voltage triggering or soft start solution allows inrush current reduction and the
maximum junction temperature to be considerably reduced.
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AN441
BMAX calculation for remanent induction
Appendix A
BMAX calculation for remanent induction
The winding is supplied by an alternating mains voltage “u(t)” given by Equation 5.
Equation 5
u(t) = U 2 ·sin(ù ·t)
According to the laws of Lenz and Faraday, we have Equation 6, Equation 7 and Equation 8.
Equation 6
u(t) = N
dÖ
dB
= N·S
dt
dt
With:
●
Φ = Flux
●
B = Induction
●
N = spires number (spires number of the primary for a transformer)
●
S = section area for magnetic material
Then, induction versus time is:
Equation 7
B(t) =
U 2
N.S
∫ sin(ω·t)dt
Equation 8
ð
U 2
B(t) = B ·sin(ù t − ) + B with Bn =
n
0
2
N·S·ù
We consider the case where the Triac is first triggered at mains zero voltage.
We can distinguish two cases:
Case 1
Winding is switched on whereas a remanent positive induction remains on magnetic core.
According to Equation 8, for t = 0, we have:
Equation 9
B(0) = −Bn + B0 = Br → B0 = Bn + Br
So
Equation 10
ð
⇒ B(t) = B ·sin(ù t − ) + B + B
n
n
r
2
So the maximum induction will be: BMAX = 2·Bn +Br
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BMAX calculation for remanent induction
AN441
Case 2
Winding is switched on whereas a remanent negative induction remains on magnetic core.
According to Equation 8, for t = 0, we have then:
Equation 11
B(0) = −Bn + B0 = −Br → B0 = Bn − Br
So
Equation 12
ð
⇒ B(t) = B ·sin(ù t − ) + B − B
n
n
r
2
So the maximum induction will be: BMAX = 2·Bn - Br
Figure 12 shows the maximum induction difference between two triggering methods
according to previous remanent induction.
When the first half-cycle conduction is the same polarity as the previous one, the induction
reaches a maximum which can be the double of the induction when triggering in reverse
polarity compared to remanent induction.
In the first case saturation can be reached very rapidly and the load behaves like a short
circuit. The peak current is limited only by the series resistance of the load.
Such phenomenon mainly occurs with transformers where the remanent induction can be
very high.
Figure 12. Induction at beginning of conduction according to remanent induction
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Revision history
Revision history
Table 2.
Document revision history
Date
Revision
Changes
May-1992
1
Initial release.
10-May-2004
2
Style sheet update. No content change
09-Mar-2010
3
All technical content revised and material from AN307 included.
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AN441
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