ST AN441 Application note

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

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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

V

i(t)

MAIN

R

L

T

u(t)

According to Figure 1, the AC load current i(t) is define by Equation 1.

Equation 1

(t)di

L(t)R·iu(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

R

⎛

−

i(t)

=

()

RMS

2U

⎜

2

2

⎜

R·L

+ω

⎝

t·

L

() ()

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:

⎞
⎟

t··sinRt··cos·L·eL··

ω+ωω−ω

⎟
⎠

Equation 3

)ti(t,

0

=

()

RMS

⎛

2U

⎜

2

2

⎜

R·L

+ω

⎝

() ()()()() ()()

R

−00t·

L

00

⎞
⎟

tt··sinRtt··cos·L-·et··sinRt··cos·L·

+ω++ωωω−ωω

⎟
⎠

Where t0 is the triggering delay at the first turn on

2/16 Doc ID 3579 Rev 3

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.

Figure 2. Inrush current difference according to Triac first turn-on delay

400

400

300

200

100

0

0 20406080100

Current (mA)

-100

-200

-300

-400

Turn-on at zero crossing Turn-on at peak mains voltage Mains voltage

Time (ms)

300

200

100

0

-100

-200

-300

-400

Mains voltage (V)

Due to this high peak current, two problems may occur at AC switch level.

● High peak current may be higher than I

value (maximum surge peak current). In this

TSM

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 3/16

Reasons for inrush current in inductive loads AN441

)

Figure 3. Magnetic field H versus induction B (continuous rating)

B(t

B

B

s

B

r

i(t)

t

t

t

H

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 B

in Figure 3.), corresponds to the

r

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.

4/16 Doc ID 3579 Rev 3

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

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 I

Figure 5. Latching current of the AC switch

i(t)

i(t)

i(t)

I

I

L

i(t)

(refer to Section 2.2).

L

I

I

L

B

t

t

t

I

I

G

t

t

Doc ID 3579 Rev 3 5/16

I

I

G

ba

t

t

t