In an AC power source, the voltage is sometimes positive and sometimes negative. For designers who do not use bidirectional thyristors very often, "negative voltage" may sound strange because there is no integrated circuit in the world that operates with a negative voltage. However, in some applications, it is more appropriate to drive the triac with a negative output.
In an AC power source, the voltage is sometimes positive and sometimes negative. For designers who do not use bidirectional thyristors very often, "negative voltage" may sound strange because there is no integrated circuit in the world that operates with a negative voltage. However, as described in this article, a simple solution is required to drive a triac from a positive output, but in some cases it may be more appropriate to drive a triac with a negative output.
Positive and negative power supply principle
If the power semiconductor components can only be controlled by the power supply, it is usually necessary to use a non-isolated power supply when the drive reference point is connected to the mains (line or neutral terminal). For example, the case of an AC switch such as a triac, an ACST, an ACS, or a Silicon Controlled Rectifier (SCR). These components are all controlled by the gate current. The gate current can only be applied to the gate pin and circulate between the gate and the AC switch reference terminal, where the reference terminal refers to the cathode (K) of the thyristor, the A1 or ACST of the triac, and ACS COM. Since the AC switch control circuit and its power supply can only be connected to the component reference terminals (back to line voltage), a non-insulated power supply must be used.
There are two ways to connect this drive reference point to a non-isolated power supply:
Option 1: Connect the control circuit ground (VSS) to the drive reference point.
Option 2: Connect the control circuit supply voltage (VDD) to the drive reference point.
Since the switch drive reference point is also a zero voltage point (VSS), Scheme 1 shown in Figure 1a is most commonly used. Since the supply voltage (VDD) is actually higher than the mains terminal potential (line or neutral) and the supply terminal potential is connected to the drive reference point (VSS), this topology is called a positive voltage. If the power supply is 5V, VDD is 5V higher than the mains reference point (neutral terminal in the example of Figure 1a). As described below, this topology can only be used directly with standard triacs and thyristors, and not with non-standard triacs, ACS, and ACST. However, as described at the end of this article, the user can make some simple modifications to control all components of positive voltage.
Figure 1 Power polarity definition.
Scheme 2 shown in Figure 1b is referred to as a negative voltage. The supply voltage reference point (VSS) is actually lower than A1 or COM connected to the mains reference point. If the supply is 5V, VSS is 5V below the line reference point or –5V compared to the line. According to the description below, this topology can be used with all triacs, ACS and ACST, but not with thyristors.
Power output polarity is consistent with AC switch technology
To turn on an AC switch such as a bipolar device, a gate current must be applied between the switch gate pin (G) and the drive reference terminal. Then there will be several situations.
. For a thyristor, the gate current must be positive (flowing from G to K).
. For triacs and ACST, the gate current can be positive or negative (depending on the voltage applied to the component).
. For ACS, this gate current must be negative (flowing from COM to G).
It is easy to drive a thyristor rectifier with a positive voltage. If the cathode is connected to VSS, as shown in Figure 1a, when the output pin of the control circuit (usually a microcontroller) is at a higher level, the current is sourced from the gate of the thyristor. On the other hand, direct drive of the ACS requires a negative supply, as shown in Figure 1b. Therefore, when the control circuit output pin is at a lower level, the current should come from the gate of the thyristor.
For triacs, ACS, and ACST, four trigger quadrants can be defined based on the gate current polarity and voltage polarity of the pre-open component. When the gate current is derived from the gate, it can be considered a positive current. When the voltage is related to the drive reference point, the voltage can be considered a positive voltage. Different quadrants are
. Quadrant 1: Positive gate current and positive voltage.
. Quadrant 2: Negative gate current and positive voltage.
. Quadrant 3: Negative gate current and negative voltage.
. Quadrant four: positive gate current and negative voltage.
Depending on the triac, ACS, and ACST component technology, these components can be triggered in each quadrant or only in certain quadrants. For thyristors, the component can only be turned on because only a positive gate current can be turned on, and only when a positive voltage is applied across its anode and cathode terminals. These components typically do not consider the trigger quadrant.
Table 1 shows the different quadrants for different component technologies and lists the polarity consistency of the power supplies that make up the direct drive circuit, as shown in Table 1. It can be seen that the negative supply is suitable for all AC switching technologies, with the exception of thyristors. Negative output is preferred because the negative output allows any other technique to change a part number.
Power topology easily affects output polarity
A problem arises if the microcontroller (MCU) supplies a positive voltage and the microprocessor triggers the third quadrant of the triac, ACST or ACS. As shown in Table 1, direct control is not possible in this case. In addition, switching power supplies (SMPS) are often used to accommodate different standby power commands or standards. Since the switching power supply with positive output is the most common topology for low output current offline converters, the selection of switching power supplies is mainly based on the selection of buck converters.
However, in many applications it is only necessary to control the AC switch, so a negative voltage should be applied. The buck-boost converter allows a negative output. This topology is as easy to implement as a buck converter. In addition, for buck-boost converters, there is no need to increase the output load resistor or output regulator because of the requirement to use a buck topology. In fact, for buck, the output capacitor is charged each time the MOSFET is turned on, resulting in an output that is too high under no load or small load.
The buck-boost converter's efficiency and maximum output current should be lower than the buck converter, and the output capacitor should be larger than the buck converter. In fact, for a buck converter, all inductor currents charge the output capacitor, while for a buck-boost converter, the inductor current charges the output capacitor only during freewheeling. However, the 230V AC/12V DC converter has a lower duty cycle and the difference between buck and buck-boost performance is small.
If the two topologies are equipped with the same reactive components, their efficiency is similar.
Although a switching power supply with a negative output is available, we still use positive output as our first choice. In standby mode, the power consumption of the positive output is lower. In fact, we found that the internal power consumption of a positive linear regulator is in the range of 50μA, while the typical power consumption of a negative regulator is about 2mA.
This quiescent current greatly affects the standby power consumption of the switching power supply. Another reason for using positive outputs is the widespread adoption of 3.3V microcontrollers, but it is difficult to find accurate 3.3V negative regulators with lower power consumption.
Therefore, the schematic of Figure 2 should be used to combine the advantages of a negative supply with the advantages of a positive regulator. In this diagram, the ST719M33R positive regulator of STMicroelectronics has a maximum quiescent current of 5.5μA, which is used to display the 3.3V power supply provided by the “negative” 15V output. The “negative” 15V output can be derived from the use of the VIPer06 circuit. Buck-boost converter or flyback converter (Flyback Converter). In this way, the microcontroller can attenuate the current from the T1635T-8 triac and the T series third quadrant components.
Figure 2 is a positive voltage regulator for the negative voltage of the triac control circuit.
