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Posted on 17 July 2019

Negative versus Positive SMPS for Home Appliance

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There are benefits with a negative supply topology

Home appliances are increasingly using SMPS supplies. Connection of the supply to the mains for non-insulated topologies, as these are widely used in this field, thus needs to be checked to ensure correct AC switch control according to the device’s technology.

By Laurent Gonthier, Application & System Group Manager, Appliance Industrial and Lighting Segments, ASD & IPAD Division, STMicroelectronics

 

SMPS for home appliances

Home appliances such as washing machines, refrigerators or food processors employ a lot of AC loads such as valves, motors, or heating resistors. Since these loads are powered by the mains in on/off mode, they are usually controlled by Triacs or ACS. These devices are in fact the least expensive power switches to operate directly from the 110/240 V mains, due to their smaller silicon area for a given alternating current level. In addition, these devices only require a low driving current and so can be directly triggered by the appliance microcontroller unit (MCU), which considerably simplifies the driving circuit and reduces its cost.

In home appliances, capacitive power supplies have been traditionally used. However, designers are now increasingly implementing switched-mode power supplies (SMPS) to reach higher output current levels and especially lower standby power consumption. This new requirement is more and more stringent as the 2005/32/EC Energy for Using Products directive (EuP) reduces the maximum standby mode consumption for each product to be sold on the European market. The objectives are to bring this consumption to between 1 or 2 W by 2010, and below 0.5 or 1 W by 2013.

This article deals with power supply polarity. This point has to be particularly checked when Triacs or ACSs control circuits are powered with this SMPS.

Triggering quadrants for Triacs and ACS

To switch on a Triac, an ACS or an ACST, a gate current must be applied to its gate pin (G).

For Triacs and ACSTs, the gate current could be positive or negative. Figure 1 illustrates the simplified schematic of a Triac or an ACST and its associated silicon structure. As shown in these figures, a Triac or an ACST could be switched on by a positive or a negative gate current as a result of the two diodes embedded in back-to-back between G and A1.

Simplified equivalent schematic (a) and silicon structure of Triac-ACST (b)

The silicon structure of an ACS is different from a Triac. Here the gate is the emitter of an NPN bipolar transistor. So the gate current can only be sunk from the gate, and not sourced to it.

Four triggering quadrants are then defined as follows:
Quadrant 1 (Q1): VT > 0 and IG > 0
Quadrant 2 (Q2): VT > 0 and IG < 0.
Quadrant 3 (Q3): VT < 0 and IG< 0.
Quadrant 4 (Q4): VT < 0 and IG < 0.

 Quadrants Q2 and Q3 are common to all Triacs and ACS/ACST devices. Control in Q2 and Q3 is then recommended.

Moreover, triggering in Q4 is not recommended because the triggering gate current is the highest. Also the dI/dt capability of Triacs is lower in Q4. Working in Q2/Q3 quadrants is then advisable, even for standard Triacs, to decrease the board consumption and increase the system reliability.

Two types of power supply bias

As the Triac, ACST or ACS drive reference is connected to the line, the supply of the control circuit has to be related to A1 or COM. There are two ways to connect this drive reference:

Solution 1: connect the control circuit ground (VSS) to A1; Solution 2: connect the control circuit voltage supply (VDD) to A1 or COM

Solution 1 is called a positive power supply. The voltage supply VDD is above the drive reference (VSS) which is connected to the mains terminal (line or neutral) as shown in Figure 2. If the supply is a 5 V power supply, then VDD is 5 V above the mains reference.

Triac control with positive or negative power supply

Solution 2 is called a negative power supply. The voltage supply reference (VSS) is below A1 or COM, which is connected to the mains reference (line or neutral) as shown in Figure 2. If the supply is a 5 V power supply, then Vss is 5 V below the line reference.

This topology can be used with all Triacs, ACSs and ACSTs.

Buck or buck-boost converter?

Only a positive power supply can be implemented with the most used step-down converter - the buck converter. Indeed, for buck converters, the chopping MOSFET drive reference is connected both to the output capacitor low end (so supply GND) and the line. Indeed, to be able to control the N-MOSFET, the supply voltage has to be above the source reference. This leads to the fact that only positive supplies can be implemented with a buck converter.

The simplest negative supply that can be implemented comes from buck topology, it is the buck-boost converter. In this converter, the MOSFET stores inductive energy (in inductance L2, see Figure 3) when it is on. When the MOSFET is switched off, the energy is supplied to the output capacitor. As the freewheeling diode anode is connected to the supply reference (GND or VSS), a negative voltage is achieved.

Buck-boost power supply with VIPer16 device

Figure 3 shows the diagram of a negative buck-boost power supply using a VIPer16 device.

The advantage of a buck-boost converter compared to a buck converter is that there is no need for an added load resistance or an output Zener diode. Indeed, the feedback and output capacitors are not discharged symmetrically. This dissymmetry is amplified by the buck topology as the output capacitor is charged during each MOSFET on time, whereas the feedback capacitor is not. So output voltage can increase to too high a value and has to be clamped.

An additional resistance or clamping diode for no load or very light load is then required at the buck output, but not with a buck-boost converter.

Theoretically, the efficiency of a buck-boost converter should be lower than for a buck converter, as the whole inductor current is used to charge the output capacitor for the buck supply. But for 230 V AC/15 V DC, the duty cycle is very low, so there is no great difference between buck and buck-boost performances. Similar efficiency is reached for both topologies, with the same reactive components.

Flyback power supply

The second SMPS topology widely used today by designers is the flyback topology. This converter uses a transformer to store the energy instead of an inductance. The benefit of this solution, compared to a buck-boost converter, is the possibility to insulate the output voltage and also to generate several output voltages by using several secondary windings. A flyback converter can also deliver a higher power with the same monolithic device compared to a buck or buck-boost converter. Also a flyback converter can work with a higher duty cycle than a buck-boost converter. The input peak current is then lower and so are its switching losses. The flyback converter efficiency can then be slightly better.

It is easy to implement a negative supply with a flyback converter as the output voltage is insulated from the mains. So the VDD terminal can either be connected to the neutral or the line. For sure, the VDD voltage is then no longer insulated from the mains. This means that the insulation has to be implemented elsewhere to protect the appliance user from electrical shocks (for example, with an insulated keyboard and display).

Figure 4 gives the diagram of a flyback power supply using a VIPer16 device.

Flyback power supply with VIPer16 device

As the VDD level has to be connected to the mains, there is no interest in implementing an insulated power supply. So only the advantage of implementing a 2nd low-voltage supply (insulated or not from the mains) or having a higher output current will push designers to use such a topology.

Conclusion

It has been shown that a negative power supply is the best topology as it can be used for almost all AC switches.

Even if the first reflex of a designer is to implement a positive power supply, negative power supplies are as easy as implementing positive ones. There are also some benefits with a negative supply topology such as, for example, the removal of output overvoltage protection for non-insulated SMPS (buck-boost topology compared to buck topology), or the opto-transistor (for flyback converters).

 

 

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