# 2 / 4 quadrant switches - how can current have opposite direction to voltage?

I am learning about 1, 2 and 4 quadrant switches, used in inverters etc. I am however struggling to understand how, in the 2nd and 4th quadrant, the voltage can have an opposite direction to current. Using this picture for example I dont understand here how you can have a positive voltage but current in the opposite direction. For e.g. On the left, current would flow clockwise, and on the right, current will not flow, but if it did (i.e. if diode was replaced with a resistor) it would be anticlockwise. However, the 2nd and 4th quadrants indicate current in opposite direction to voltage - but I can't imagine how this could happen since (conventionally) current goes from positive to negative - the electromotive force is acting in one direction; how could current ever flow the opposite way?

Any help understanding this would be greatly appreciated; thank you.

## 4 Answers

As shown in the diagram posted with the question, four quadrant switches can block voltages of either polarity and allow current to flow either in the direction of supply to load or load to supply.

Blocking voltage in either direction is necessary to use the switch with an AC supply.

Allowing current to flow in either direction is necessary to use the switch allow the supply system to either supply power or absorb power. In other words allow current to flow in the direction supported or opposed by the supply voltage.

In terms of inverter usage, a four quadrant switch is necessary for a grid-tie inverter that can supply power to a load or accept power from the same system that includes an energy source that can supply more energy than is used locally.

Another use is to control an AC motor in four modes, forward motoring, reverse motoring, forward braking and reverse braking.

• So for braking, I am assuming it means taking power from the load, such that the motor acts as a generator and slows down (due to conservation of energy)? I.e. that would be the 2nd and 4th quadrants right? – QuickishFM Dec 31 '19 at 17:18
• Yes, the motor becomes a generator. In terms of motor operation, the four quadrants are forward motoring, reverse motoring, reverse braking and forward braking with speed & rotation direction on the horizontal axis and torque and torque direction on the vertical axis. With a AC motor, the currents and voltages are reversing constantly. The direction of energy flow is governed by the voltage vs current phase relationship or the instantaneous relationships between voltage and current. – Charles Cowie Dec 31 '19 at 18:45

This answer is specific to thyristor / triac operation. I can see from @Cristobol's answer that another meaning can be taken from your question. Figure 1. From Littlefuse's Fundamental Characteristics of Thyristors.

The Littlefuse diagram makes it clear that the voltage in question is the gate voltage relative to MT1. The article goes on to say ...

The most common quadrants for Triac gating-on are Quadrants I and III, where the gate supply is synchronized with the main terminal supply (gate positive -- MT2 positive, gate negative -- MT2 negative). Gate sensitivity of Triacs is most optimum in Quadrants I and III due to the inherent Thyristor chip construction. If Quadrants I and III cannot be used, the next best operating modes are Quadrants II and III where the gate has a negative polarity supply with an AC main terminal supply. Typically, Quadrant II is approximately equal in gate sensitivity to Quadrant I; however, latching current sensitivity in Quadrant II is lowest. Therefore, it is difficult for Triacs to latch on in Quadrant II when the main terminal current supply is very low in value.

They're basically saying that if MT2 is positive then a positive gate voltage will work best (QI). If MT2 is negative then a negative gate voltage will work best (QIII).

The application note is well worth reading.

• I think the quadrants may have been referring to voltage across the switches (there is no mention of gate voltage in what i have seen thus far), as the course i am studying focuses on BJTs and MOSFETs in inverters. Thank you anyhow for your thorough answer – QuickishFM Dec 31 '19 at 13:35
• See if circuitstoday.com/types-of-chopper-circuits is more relevant. – Transistor Dec 31 '19 at 13:52
• Thank you very much for the link - it has helped me understand and cleared up a few misconceptions I had. – QuickishFM Dec 31 '19 at 17:26

how could current ever flow the opposite way?

It could be pulsed switches between a battery, a charger and a load where current can travel in either direction in a positive voltage. (half Bridge)

In order to have current in the opposite polarity of voltage, there just needs to be some external voltage that pulls the load current in the opposite direction. It could be a switched reactive LC load, or motor to generate a back EMF which reduces the current with acceleration or it brakes (-ve acceleration) by design. (Full bridge includes changing directions)

• "some external voltage that pulls the load current" - woudn't this external voltage (however short-lived it may be) need to exceed the normal forward voltage (source voltage) such that there is a net negative voltage to induce this current in the opposite direction? – QuickishFM Dec 31 '19 at 17:25
• No, . Imagine load is at Vcc/2 and drive swings between 0 and Vcc, Is current bipolar? – Tony Stewart EE75 Dec 31 '19 at 17:29
• What do you mean by drive here? – QuickishFM Dec 31 '19 at 17:38
• buffered switch or non-linear current amplifier or active bridge each drive voltage and current depends on load in that case connected to V+/2 thus single polarity supply with bipolar current – Tony Stewart EE75 Dec 31 '19 at 17:42
• Ah, I see, voltage stays positive but current changes direction based on the configuration of the bridge. Thank you for your answer and comments. – QuickishFM Dec 31 '19 at 17:50

Current flows from positive to negative in a load which is dissipating power. In order for current to flow from negative to positive you must be supplying power to the circuit. Look at your voltage sources above...the current is flowing from the negative to the positive, because it's supplying power.

• Say for example in here (imgur.com/a/jPBRnkk), I can see that the switch would be used to supply power (as its an inverter after all) but how can that diode + BJT combination be used to supply a current if the voltage across the diode+BJT in forward direction is negative? Would the diode not block it, making the switch only work in the first quadrant (controlled by bjt only)? – QuickishFM Dec 31 '19 at 13:38