Why are the diodes in this circuit forward biased in spite of no current flowing through them?

Question

Considering the following circuit, Normally in the +ve half cycle SCR1 is triggered after it takes its triggering gate pulse and then D2 becomes intrinsically forward biased so it's on and conducting the load current along with the SCR1. The question is Why do we consider D2 to be forward biased in the period between zero time to when SCR1 is triggered in the positive half cycle? Isn't it supposed to be "off" since SCR1 is not triggered yet so the there's no current flowing in the path in which it is connected?

The same scenario is of course applicable to the SCR2, D1 in the negative half cycle.

Any answers are appreciated, Thanks in advance.

Answer 1

Isn't it supposed to be "off" since SCR1 is not triggered yet so the there's no current flowing in the path

Even when blocking, the SCR will allow a small leakage current through. For the first type I checked, this current is limited to 1 uA at room temperature or 100 uA at 125 C.

Even though this current is small, it is enough to develop a forward bias (maybe only a couple 100 mV) across the diode.

Answer 2

1. Both SCR₁ and D₂ will have a little leakage and a tiny current will flow. In addition there is the junction capacitance in both devices. The result is that there is a very high impedance potential divider. If they were equal then the anode of D₂ would be at half supply. Of course, D₂ would conduct and prevent this.

2. Look at D₁ and D₂ in isolation. They are directly across the supply but when the AC supply upper wire is positive D₁ is reverse biased but D₂ is forward biased should the voltage at their junction. Again the leakage of D₁ will cause this.

3. The other reason is that it is convenient in our circuit analysis.

Answer 3

You basically have a 'bridge' rectifier setup except the SCR's can be pulse-width modulated, such as in motor speed control. The reason current is flowing before SCR1 is switched ON is that the diodes D1 and D2 conduct current as soon as AC input has at least -1 volts on one phase.

D2 does not have to wait for a trigger to turn it ON if SCR1 is already forward biased (but not triggered), as D2 and D1 are conventional diodes. D1 and D2 supply the negative part of the current to the load, but the load has very little to no current flow through it until SCR1 or SCR2 is triggered.

EDIT: SCR's often have snubber MOV's or resistor/capacitor across the SCR to protect it from inductive loads, which can have a strong back-EMF when the SCR is switched ON or OFF in mid cycle. These snubbers allow a small leakage current to bypass the SCR at all times, plus any 'OFF' state leakage current the SCR has.

In practice the load always has a trickle of current flowing as long as AC power is applied, even if the SCR's are not triggered. These types of power regulators often have a warning label that warns of a possible shock hazard at the supply outputs as long as AC power is applied, even if the SCR's are in an 'OFF' state.