Triacs are now commonly used in microwaveovens as static switches to control the powertransformer, the heating resistor for grill andsometimes the motor for plate rotation.

The conditions of operation of triacs fortransformer control and for heater control areanalysed here after in order to select the suitabledevice, to define the gate drive circuit and toimplement an efficient protection.

I . POWER TRANSFORMER CONTROL :

The magnetron of a microwave oven is generallysupplied by rectified high voltage obtained with a50/60 Hz transformer. The power supplied to theoven is controlled by a triac in series with theprimary (fig.1).

1/ Current stresses :

The power to be controlled is typically 1 to 2 KWand the nominal RMS current is in the range of10 to 16 Amp according to the line voltage but itis necessary to take into account an overloaddue to the magnetizing current through thetransfomer at turn on.Due to the high turns ratio of this transformerthis overcurrent can reach a peak up to 20 timesthe RMS current in steady state !To reduce the stresses at every switching on thecircuit and particularly on the triac it is important

to limit the overcurrent by using a propertriggering synchronization.

Transient operation in inductive circuit with ironcore :During continuous operation, the magnetic fieldH, proportinal to the current in the coil,generatesan induction B in the iron core with a delay asshown by the hysteresis cycle in figure 2A.

In transient operation, the induction can follow adifferent path and reach the saturation value BSfor which the magnetic field H (according to thecoil current) increases very rapidly (Fig.2B).

APPLICATION NOTE

AN441/0592

TRIACS FOR MICROWAVE OVEN

P. RAULT

Figure 1 : Magnetron power supply controlled by a triac.

AC

HV

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In the circuits controlled by a triac, switchingOFF occurs when the current is at zero. Thusthe induction has a remanent value Br (positiveor negative), corresponding to H = 0 (Fig.2A).

When the triac begins to conduct, the transientcurrent depends on the instant ofsynchronization of the control signal with respectto the mains voltage.

FIRING AT ZERO MAINS VOLTAGE :

Peak induction tends to the value :

Bmax = 2 Bn + Br

Thus in most cases B reaches the saturationinduction BS. The amplitude of the currentproportional to the magnetic field H becomesvery high ; this type of control produces thehighest transient overloads (Fig.3A).In this case the transformer behaves like a shortcircuit and peak current is limited only by theseries resistances of this circuit.Nevertheless it is possible to reduce theovercurrent if control at zero voltages done bycomplete cycles i-e the polarity at the moment offiring is the reverse of that at moment the circuitis switched OFF.Peak induction thus reaches the value :

Bmax = 2 Bn - Br.

The overload is lower than previously but stillremains high (Fig.3B).

Figure 2 : A Magnetic field H versus induction B(continous rating)B Saturation induction BS

APPLICATION NOTE

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FIRING AT PEAK MAINS VOLTAGE :

In this case the peak induction takes the value :Bmax = Bn + BrThe level of saturation is not reached andamplitude of the current remains with inacceptable limits (Fig.3c).→This type of synchronization must be used fortransformer control.See in appendix actual oscillogrammes ofcurrent through a transformer for differentsynchronization modes.

Figure 3 : Transient induction and current at beginning of conduction.

Figure 4 : A gate control by single pulsesynchronized with zero currentB gate control by pulse train.

V

Ot

(A)

I

O

t

I

O

t

V

Ot

(B)

MAINS VOLTAGE

TRIAC CURRENT TRIAC CURRENT

IGIG

t tOO

APPLICATION NOTE

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2/ Gate control :

After the first firing the gate pulse should besynchronized with the triac current zero point(Fig.4A).

The pulse duration must be sufficient to be surethe main current through the circuit reaches thetriac latching current.(IL)A pulse train can be used to avoid the problemof misfiring or wrong synchronization (Fig.4b).A frequency of several,kilohertz garantees acorrect operation.

Gate control by DC current is also an efficientmethod to keep the triac on in case of inductiveload.To reduce the consumtion of this kind of triggercircuit a sensitive triac can be used as a driver(Fig.5).

3/ Protection :

It is important to avoid as much as possible allrisk of misfiring of triac specially by overvoltage.Transient voltages can be generated by internalmechanical switches used for security system(door, overbreak protection etc ....) orsupperimposed to the line voltage. These lastones must be attenuated by a filter at the input.An efficient protection at the triac level consistsin use of TRANSIL diode across the triac toclamp the spikes higher than its voltage ratings(VDRM) associed with our RC network to limit theoff state dv/dt under the specified value . (Fig.6)

Nevertheless one accidental misfiring is alwayspossible and in this case the triac must be ableto hold the surge current. Therefore it isnecessary to select a device with a sufficientITSM (max peak current for 10 ms) for example120 to 250 Amp.

4/ Prefered devices :

– Current ratings :The nominal RMS current through thecircuit is not the main criteria but the surgecurrent capability. We suggest the followingcurrent ranges.ITRMS = 12 A with ITSM = 120 AITRMS = 16 A with ITSM = 160 AITRMS = 25 A with ITSM = 250 A

– Voltage rating :VDRM = 600 V provides a good safetymargin for 220 VAC mains.

– Package :Insulated cases are generally used tomake easier mounting on chassis and thusreduce the cost.TO220AB and TOP3 cases are suitable forprinted board assembly. If the triac ismounted close of transformer (far fromelectronics board) RD92 with ”FAST ON”connections is a convenient solution.

– Protection devices :A bidirectional silicon diode suppressor(TRANSIL) with a peak power capability of1500 W (1 ms) is a good compromise ifthere is a filter (coil + capacitor) at the lineinput. In fact it is necessary to have a

Figure 5 : Use of sensitive triac (5 mA) asdriver to reduce gate control circuitconsumtion.

BTA16-600BWBTA06-600TW

(IGT < 5mA)

Figure 6 : TRANSIL diode and RC network arenecessary to prevent spurious firingby overvoltages and/or dv/dt.

AC

C

R

HV

FILTER

APPLICATION NOTE

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current limitation (series indepance) in caseof overvoltage.The breakdown voltage compatible with220 VAC supply is VBR = 440 V.

–Part numbers :triacBTA12-600B/BW(1) 12 Amp TO220 ABBTA16-600B/BW(1) 16 Amp TO220 ABBTA26-600B/BW(1) 25 Amp TOP 3BTA25-600B 25 Amp RD 91

TRANSIL 1.5KE440CP

II . HEATING RESISTOR CONTROL :

1/ Triggering :

In this application the triac is used as a ON/OFFswitch to control the power in the grill heater.It is the case of resistive load. Therefore it isabsolutely necessary to trigger the triac at zerovoltage in order to eliminate the turn on di/dtstresses and RFI problems.To keep the triac on, a single gate pulse can besupplied at every zero crossing of voltage that isalso the zero point of current in this case.The pulse duration must be calculated in orderto allow the load current to reach the latchingcurrent (IL) of the triac.

(1) BW : SNUBBERLESS TRIAC series withhigh switching performance and high dv/dtcapability.

For triacs with max gate triggering current of 50mA the latching current is lower than 120 mA.

2/ Current rating :

For normal operation there is no particularcurrent stresses efficient cooling is required tominimize the thermal fatigue due to the variationof junction temperature and consequentlyincrease the life time of the equipment.

3/ Protection :

With the new generation of triacs,SNUBBERLESS triacs, the commutation(turn-off) is possible without external limitation of(dv/dt)c, that is to say without RC network even

Figure 7 : Triacs in insulated cases(UL approved)

Figure 8 : Controlled by the gate pulse, IG,the triac is fired, and a current ITflows through it. If the gate currentIG is stopped before current ITreaches the value of the latchingcurrent IL, the triac turns off.

TO220 AB TOP 3

RD 91

APPLICATION NOTE

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if the load is inductive.In case of overvoltages the triac could be firedby exceeding its breakover voltage and generallyit is distroyed because of the too high di/dt, thatis limited only by the inductance of the load (verylow for a heating resistor) and of the wiring.A RC network across the triac cannot reduce thespikes because there is not enough inductancein series in the kind of circuit.→RC snubber circuit in useless !

HOW TO PROTECT ?

The solution consists in turning on the triac by agate current as soon as the voltage across itexceeds a certain value which ensures a highlevel of safety.To do it we use a low power (i-e low cost)bidirectional TRANSIL diode according to thediagram. Fig.8When the overvoltage reaches the breakdownvoltage VBR of the TRANSIL the latter conductsand the current flows through the gate and turnsthe triac on. Fig.7

Figure 9 : Protection by TRANSIL the triac is turned on by the gate at the beginning of theovervoltage and continues conducting during the rest of the half cycle.

APPLICATION NOTE

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What part number ?

to protect a 600 V triac generally used for220 VAC operation we recommend :BZW04376B or BT.for a 400 V triac (110 VAC mains) we pro-pose a BZW04188B.

III . TO SUMMARIZE :

Transformer control :

Select triac with high surge current capabilityand with high transient immunity(SNUBBERLESS series).

Triggering :first switching on, firing at peak line voltage to

reduce peak inrush current, then triggering withpulse train synchronized at zero current point.

Protection :TRANSIL and RC network across the triac toavoid all risk of firing by overvoltage and/ordv/dt.

Heating resistor control :

Triggering at zero voltage absolutely necessary.Protection :

* with SNUBBERLESS triac, RC snubber circuitis useless.

* TRANSIL diode connected between gate andA2 achieves an efficient protection againstovervoltages.

ANNEX

TRANFORMER CONTROL BY TRIAC

Triggering at voltage peak ---> peak current = 22 A

CURRENT

5 A / div20 ms / div(unloaded transformer)

TRIAC VOLTAGE

100 V / div20 ms / div

APPLICATION NOTE

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Triggering at zero voltage ---> peak current reaches 130 A !

Steady state (without load)

TRIAC VOLTAGE

100 V / div20 ms / div(line voltage = 220 VAC)

CURRENT

50 A / div20 ms / div(unloaded transformer)

TRIAC CURRENT

50 A / div10 ms / div

Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsability forthe consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from itsuse. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics.Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all informationpreviously supplied.SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems withoutexpress written approval of SGS-THOMSON Microelectronics.

1995 SGS-THOMSON Microelectronics - Printed in Italy - All rights reserved.

SGS-THOMSON Microelectronics GROUP OF COMPANIESAustralia - Brazil - France - Germany - Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco -

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APPLICATION NOTE

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